Mineralogy and Petrology Notes



Mineralogy and Petrology Notes

Go over syllabus

Play game: Name that rock or mineral

1 point for each mineral identified, 1 point for composition

1 point for each rock identified, 1 point for story it tells.

15-18, including mostly straightforward, but with azurite (striking) and siderite

Physical Properties of Minerals

Crystal faces: faces are planes in the crystal with particular ion/atom densities and arrangements. In general, faces will be the planes with lowest surface energy. However, planes are also affected by growth rates. Are also constrained by the area available for growth.

Faces reflect underlying symmetry of the crystal

Overhead: point out different faces, expressed to different degrees, or growing faster or slower than comparable faces gives more complex appearance, or distorted appearance.

[pic]

[pic]

Habit: Malformations, differential growth rates, restrictions of growth area

euhedral

subhedral

anhedral

Form: group of faces that all have the same relationship to underlying symmetry

Can be either open or closed (e.g. pinacoid open, 2 horizontal planes, prismatic is open with any number of planes parallel to one axis, sphenoid has 2 planes intersecting in “hat”)

Form names (not same as form): prismatic, rhombohedral, cubic, octahedral, pinacoidal (2-sided forms)

Twinning: symmetrical intergrowth of two or more crystals of the same substance, often on mirror plane or axis of rotation.

[pic]

show examples of albite twinning (polysynthetic twinning), show contact twinning (Qz), and penetration twin (carlbad twinning in orthoclase)

State of Aggregation

(overhead, point out styles)

Luster, color, streak

luster: way light reflected, metallic and nonmetallic: vitreous (Qz), resinous (sphalerite), pearly (talc), greasy silky (milky qz), adamantine (refractive index)

color: a few are diagnostic (azurite, malachite, turquoise), some vary according to exposure to air (bornite), some by trace composition (Quartz, sapphire, ruby), some by major composition (pyroxene-talk about effect of amount and color)

streak: color of powder, especially useful for oxides. E.g. hematite always has red streak, but color not always red.

translucency: metallic oxides often opaque, most silicates, carbonates, sulfates, others are transparent or translucent if sliced thin enough.

Diffraction among amorphous hydrated silica spheres in opal. (didn’t get to)

[pic]

Chatoyancy and Asterism

oriented (parallel) elongate inclusions in a mineral can give optical (reflectance) effects.

elongate minerals give reflective line perpendicular to orientation of inclusions (chatoyancy). If have three sets of oriented inclusions, can get 6 sided star (asterism) E.g. “star” sapphire, has elongat

Fluorescence and phosphorescence

If you excite an electron to a higher energy state, it can return to ground state in series of small steps in which energy is transferred e.g. to heat (no fluorescence), by releasing a photon (which may be in visible range or not). If the spin state of the electron is different in the ground state than the excited state, the decay is slower, get phosphorescence.

Fluourescence that produces visible light usually results from excitation in the ultraviolet range. The wavelength of light is proportional to the inverse of the energy of the photon. The energy of the photon depends on the amount of change in the energy level from excited to less excited states.

Cleavage, parting, fracture

Planes of weakness in the crystal,

parting is breaking along other planes of weakness, such as twinning surface, exsolution surface.

fracture-no planes of relative weakness (Qz)

break along planes with weaker bonds, e.g. Van der Waals bonding in graphite, easily cleaves along that plane.

OrthoPyroxene (Mg2Si2O6): cleavage planes is between more ionic Mg-O bonds, not more covalent Si-O bonds.

Hardness

Mohs scale

Related to bond strength. Different bonds in different directions, so hardness may depend on direction (kyanite), or which crystal face (calcite)

In general, in increasing bond strength/hardness

Van der Waals, hydrogen bonds, ionic bonds, covalent bonds

Tenacity

brittle, malleable, sectile, ductile

Specific gravity

depends on how closely packed atoms are and atomic mass of atoms. (compare to mass= like dividing by mass of H2O, makes dimensionless, but because water has a density of 1g/cc, you get the same number)

Magnetism (diamagnetic and paramagnetic)

radioactivity, solubility in HCl

Piezoelectric

non-conductor, otherwise it shorts itself out. Must not have center of symmetry (that is, its atomic arrangement is different in one direction from the other along some axis, or polar). Used in a altimeters, pressure gauges, timer in watches and computers. Hydroxyapatite is piezoelectric, important in bone formation.

Mineralogy and Petrology.

Physical Properties of Minerals, Lab #1

Mineral hardness:

Mohs Hardness scale consists of 10 “standard” minerals, with hardnesses that increase in a roughly exponential fashion.

These are Talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, corundum and diamond.

1) Examine the Mohs sample set with the intent of becoming familiar with these (please don’t scratch these samples)

2) Fingernails are roughly 2.2-2.5, a pocketknife or nail is roughly 5.1, a glass plate is roughly 5.5. For each of the following minerals, test its hardness against each of these to see if it is harder or softer. Then, check the actual hardness for that mineral given in your text book to be sure you got it right.

M2 (gypsum)

M3 (fluorite)

M14 (orthoclase)

49-1652 Wards (Kyanite). This sample has a hardness that is strikingly different in different directions. Parallel to the crystal blades it is about 5 (softer than knife or nail), perpendicular to the blades it is about 7 (harder than knife or nail). Test it in both directions until you ‘get’ it. The difference in hardness is due to differences in bond strength in the different directions.

State of Aggregation:

3) Look at the following samples and think about their growth form

Examine the two varieties of gypsum, alabaster and selenite (M1 and M2). Make sure you know which is which. Also look at the satin spar sample 97 Wards in the Mineral Cabinet (from East Bridgeford England).

Use the Figure in your book to identify which type of aggregation or growth character that each of the following might exhibit.

49-1652 Wards (Kyanite),

46-E-4894 Wards (Malachite) This sample is fragile, be careful with it.

Wards (Hematite)

Asbestos (25 Wards Mineral Cabinet) This sample is in the locked cabinet.

Twinning:

Look at the samples of Staurolite Garnet Schist (R11), Find the twinned staurolite crystals present in a few samples (called cruciform twins).

Look at the samples of albite we looked at in class. Make sure you can see and understand the polysynthetic twinning. (M9)

Look at the samples of orthoclase we looked at in class. Find a carlsbad twin. (M14)

Crystal faces, cleavage, and fracture:

Examine the Quartz crystals (M8). These samples exhibit both crystal faces, and conchoidal fracture. Quartz doesn’t have cleavage because the covalent bond strength is equal in all directions.

Examine the Pyrite crystals (49E3167 Wards). Also look at the bottom where the fracture (no cleavage) is apparent. DON’T DAMAGE THIS SAMPLE, DON’T SCRATCH IT, BASH IT, WHATEVER. Compare this sample with sample (188 in wards mineral cabinent). Notice the different crystal form.

Look at the samples of Orthoclase (M14). These samples exhibit both crystal faces and cleavage plains. Learn to tell them apart. Two cleavage planes are at nearly right angles. Crystal faces are NOT at right angles. Cleavage will appear in areas of breakage. Crystal faces clearly will not have been broken.

Examine three different samples of fluorite (49E1631 Wards; M3, and 49-1645 Wards). One sample shows crystal growth form (cubic), one shows cleavage (octahedral), and one is a more typical sample. Think about the differences. Look at the atomic structure for fluorite shown in your book.

Examine two different samples of Calcite, M16 and 49E1602 Wards (PLEASE, PLEASE, PLEASE DO NOT SCRATCH, PUT ACID ON, OR CRUSH THIS SAMPLE, OR ANYTHING ELSE BAD!!!!!). M16 shows mostly crystal forms, 49E1602 is a cleavage rhomb. Also look at sample 83 Wards Mineral Cabinet, from Chihuahua Mexico. Notice its hexagonal form. Look at the atomic structure diagram for calcite in your book showing the relationship between the hexagonal and rhombic forms. Think about it. Talk about it with someone.

Examine several samples of m11. Notice the stubby prisms of the pyroxene crystals, as they grew in an igneous rock Also, notice the cleavage. Pyroxene is distinguished from amphibole on the basis of its two cleavage planes at nearly right angles to each other. This shows up most obviously as a stairstep appearance to the cleavage surfaces.

Cleavage in pyroxene (Mg2Si2O6) occurs at the ionic bonds between Mg and oxygen, not at the covalent bonds between Si and O. Examine the picture below from your text. lines on this diagram represent covalent bonds between Si and O. Oxygen are represented by the smaller open circles and Si by small black dots. The larger black dots represent octahedrally-coordinated sites in the crystal (called M1 by mineralogists), and the larger open circles represent a structurally-different octahedral site (called M2). Mg goes into both M1 and M2. No lines are drawn for the more ionic bonds between Mg and O. With a pencil, draw how the mineral will break on the right-hand image (don’t cross any covalent bond lines! Remember that cleavage plains are at roughly right angles when seen on the big scale, cleavage planes pass through the M1 sites, not the M2 sites).

[pic]

Color and Streak:

Look at the samples that are referenced.

Color is sometimes diagnostic of a mineral, but usually not. It is diagnostic for

Malachite (49E1553, Wards) and Azurite (49H5760 Wards). It is not diagnostic for Quartz (samples M8, and the following samples from the Mineral Cabinet: 45 amethyst, 46 milky quartz, 47 Smoky Qz, 48 rose quartz.

Observe both the color and streak for two varieties of hematite:

specular hematite (46E3877 Wards)

oolitic hematite (46E3867 Wards)

Density:

Two factors influence density: The molecular weight of the ions in the mineral, and the closeness of the packing of ions. Metamorphic minerals often are more closely packed because higher-density minerals are usually more stable at higher pressure.

Examine the samples that are referenced.

M10 (Galena) Dense due to high molecular weight of Pb (PbS). It has the same atomic structure as Halite (M15). Compare them.

46E1022 (Wards) Barite is dense due to the high molecular weight of Ba (BaSO4). Gypsum (M2) is also a sulfate mineral (CaSO4۰2H2O) but is less dense that Barite. Compare them. Look up the molecular weights for Ba and Ca on a periodic table.

Another sulfate is Anhydrite (CaSO4) Because Ca is so much smaller (as well as less massive) than Ba, the coordination number of Ca (the number of SO42- ions around Ca) is much lower than the coordination number for Ba. Therefore, the structures of the two sulfates, Barite and Anhydrite, are very different, even though Ca and Ba have very similar chemical properties (they are in the same column of the periodic table).

Garnet and Kyanite are Alumino-silicate minerals (contain aluminum and silicon bound with oxygen) that have high-densities due to close packing of the ions. They both form under high-pressure metamorphic conditions. Compare them with a lower-density alumino-silicate mineral, Albite.

Garnet = M18

Kyanite = 49-1652 Wards

Albite=M9

Other Cool Stuff

Birefringence:

Due to the electrical fields generated in crystals by the arrangement of ions, light travels through crystals at different speeds in different directions. When speed changes, the light will bend (refract) just like light bends when it goes from air into water (the “bent pencil” effect when you put it in a glass of water) or like seismic waves refract as they pass through the Earth. This can produce a “double vision” affect.

Calcite (49E1602 Wards): DO NOT DAMAGE THESE SAMPLES!! Place the Calcite rhomb over a sample of text. Rotate the crystal and watch the double images rotate around each other. (Side note: There is one axis in calcite along which this double image effect does not occur: the c axis. This is because light passing in this direction travels the same speed regardless of the direction of light vibration. Trilobites, which had an eye lens made of calcite, had a lens oriented such that it looked parallel to the c axis.)

Exsolution Lamillae: Perthite (46E0514 Wards)

Sometimes a mineral that is stable at high temperature, reacts to form two structurally similar but chemically different minerals at lower temperature. This process is called exsolution (meaning, that the second mineral does not dissolve in the first, but exsolves). Chemically, this is similar to how water will dissolve in air at high temperature, but will condense out at low temperature. We will talk lots more about exsolution later in the course. Feldspar commonly shows an exsolution texture. A composite feldspar at high temperature exsolves to form Perthite, which is a mixture of long, thin laminae of albite (the more Na-rich feldspar) and orthoclase (the more K-rich feldspar). Examine the examples of Perthite until you can spot the whitish stringers of albite and the pinkish stringers of orthoclase.

Labradorescence and Opalescence:

One variety of Plagioclase feldspar, labradorite (if you don’t know what plagioclase is, chemically, and where labradorite falls, then look this up in your book, or in an intro Physical book), often has very tiny exsolution lamilae (too small to see). These laminations form tiny layers in the mineral which will act as a diffraction grating for incoming light. Diffraction is the effect that causes rainbow colors in an oil slick in a wet parking lot.

Look at sample 49-1654 Wards (polished Labradorite), as well as the more typical sample 46E4514 Wards. Find the rainbow colors. You should see really striking yellows, greens, and cobalt blues. Sample 46E4514 also has great polysynthetic twinning! Can you find it?

Sample of Opal in the Ore Mineral Cabinet, 213.

Find an opal that opalesces (Not all of them do)

The rainbow colors of opal are also from diffraction from layers in the sample. Opal is actually not a mineral because it is amorphous. However, layers are made of tiny round beads of hydrated silicate that form a diffraction grating in a similar fashion to the layers in Labrodorite.

Idea of diffraction. When light “reflects” off of multiple layers, some of the light beams will be in-phase and some out-of-phase when the light emerges from the rock. Whether it is in our out of phase depends on both the angle the light enters and the wavelength of the light. The colors of light that are in-phase will show up as brighter, giving the sample a rainbow appearance.

[pic]

Elements of Crystal Chemistry

Spectroscopic lines from elements indicates that energy is discrete. Leads to idea of quantized energy states, that is, electrons can’t exist in any energy state, but only in particular ones. The energy of a particular photon is related to the wavelength by the expression E=hc/λ.

[pic][pic]

Bohr postulated that electrons exist only in particular shells (or orbits). The more distant from the nucleus, the higher the energy, until the electron escapes from the nucleus entirely.

E=-A/n2

where n is the quantum number, related to the mass and charge. Notice that as n goes to infinity, energy goes to 0 (escapes from nucleus). As n goes to 1, E approaches its maximum.

From this, it can be seen that it is easiest to remove the outermost electrons. More and more energy is required to remove inner electrons. Ionization potential: energy to remove easiest-to-remove electron.

[pic]

Notice, the easiest to remove (such as Li and Na) are those that form positive ions. Those hardest to remove (such as Ne, and Kr) don’t generally form ions at all). Ones like F, Cl tend to form negative ions.

valence electrons, are the outer electrons most easily removed. Produces a charged ION.

Elements typically lose a characteristic number of electrons, giving ion a typical valence (e.g.Na+, K+, Cl-, Br-, Ca++, Mg++, Ni++, Sc+++) Some elements may lose a different number of electrons under different conditions, giving it more than one valence (e.g. Fe++, Fe+++, Ti3+, Ti4+,

Go through electromagnetic spectrum and common energy levels absorbed by rotational quantum levels=microwave, vibrational=infrared, electronic (outer electrons=visible, inner electrons = X-ray), nuclear quantum levels = gamma.

Schrodinger model of the atom (briefly)

Electron can be thought of as wave like. Schrodinger equation describes position as a wave, predicting only the probability it is at any particular location. The distance of highest probability corresponds to the Bohr distance from nucleus.

4 quantum numbers for electronic energy levels. one corresponds to Bohr’s energy levels (K, L, M, N, O), others orbital shape (s, p, d, f, g), magnetic (determines number of orientations of and shapes, e.g. s=1, p=3, d=5, f=7), and electron spin (only two values, so only two electrons possible per orbital).

Explain how typical ionic charge relates to the number of ions in the outer shell (K, L, M, etc). Examples of Na, Mg, Al, Cl). Explain how it gravitates toward form of noble gases (most stable configuration). Do electron orbital fill exercise. For each atom, indicate its likely valence (charge).

Types of Bonds

Notice that noble gases are very inert, stable.

ionic bond: Transfer of electrons from one atom to another so that both achieve an electronic configuration like noble gas. This is related to filled s and p orbitals in the outer shell. This gives each a charge, and bonding results from electrostatic interaction.

energy = (AZ+Z-/d) Z = charge, d = interatomic distance, A is madelung constant which depends on crystal structure.

force (strength of bond) = AZ+Z-/d2)

Which will be stronger, bond between Na and Cl or between Na and I?

reflected in melting T: NaCl melts at 801C, NaI at 651 C (melting is when short range order lost).

typical of ionic bonds: Non-conductive (no easy exchange of electrons once noble-gas-like configuration achieved). soluble (many are called ‘salts’), electrons are not shared, but go to one atom, distributed over atom, making bond nondirectional so symmetry of resulting minerals are often high. Moderate hardness (not as strong as covalent bonds, but stronger than other types of bonds). Once dissolved, the free ions provide electrical conductance in the solution.

Often, geochemists approximate energy of crystals from ionic model even when they are not perfectly ionic.

U = N(AZ+Z-/d + sye-d/p)

second term is a repulsion term. If try to cram large ion into too small a space, the electrons bump into each other. Since like charges repel, this results in a repulsion term. Repulsion is shorter-range term. There is some “balance” distance (minimum energy).

[pic]

Side note on my research: Trying to understand and predict how easily trace elements substitute into a particular crystal. I proposed that it could be understood in terms of electrostatic energy:

Relative ease with which different elements substitute into olivine I found that there is a best-fit size about at size of Ni, getting smaller to either side of that, so that both bigger and littler cations fit less well Repulsion energy higher if crammed too tightly, electrostatic energy higher if too large.

[pic]

Goldschmidts rule: Substitution of one element for another in a lattice: will subst. better if of similar size and charge.

covalent bond

Share electrons, such that some electrons do double duty filling outer shells of more than one atom.

e.g. C

Covalent bonds are very strong, very hard (like diamond), high melting T. No free electrons, so do not conduct electricity well.

In reality, all bonds have some ionic and some covalent character.

Some atoms have a strong tendency to attract electrons (electronegativity), others a much weaker ability. The more different two are (one that tends to attract electrons and one that doesn’t), the more ionic the bond. The closer they are, the more covalent the bond.

metallic bond

valence electrons “swim” freely among the nuclei and bound non-valence electrons. The cloud of electrons allows easy movement of atoms (plasticity, tenacity, ductility) and the movement of electrons provides conductivity (both heat and electricity). Weaker bonds yield much softer materials. Only native metals (in nature) exhibit this behavior.

Van der waals bond

These bonds form in neutrally charged atoms or molecules when the motion of electrons becomes synchronized such at one adjacent sides of atoms or molecules gain slightly opposite charges. They are very weak bonds, yielding soft materials and usually low melting temperatures (such as for cooled dimers of Cl2 or O2)

e.g. Graphite

[pic][pic]

has divalent bonds in plane, with planes bound by Van der Waals forces. The mineral easily cleaves in this plane, making graphite an excellent lubricant. Also used as pencil ‘lead’. Layers for many clay minerals are held together by VanderWaals (e.g. kaolinite, gibbsite, pyrophyllite, brucite, talc).

hydrogen bond

Hydrogen, when it loses its electron, becomes an unshielded proton (positive charge). This exposed positive charge can bond with negative ions, or polar molecules that have a negative pole. Polar molecules are ones that are not the same on all sides, and have positive and negative ends. e.g. H2O. This bond is weaker than covalent or ionic, but stronger than Van der Waals.

Mineralogy and Petrology.

Thinking about crystal structure, Lab #2

Coordination number:

For ionic compounds, the number of nearest neighbors is determined by the relative sizes of ions. This number is called the coordination number.

consider the diagram from your book.

[pic]

Consider how the coordination of the alkali atom changes in alkali chloride as the size of the cation changes (keeping the size of the anion the same).

[pic]

Does the coordination number of Cs increase or decrease relative to Na?

Is Cs larger or smaller than Na?

Is this consistent with the diagram from your book?

Draw atoms on the shown faces below for NaCl and CsCl. Use open circles for Cl and filled circles for Na and Cs.

Close Packing:

Another way to think of crystal packing is to consider that the anions (usually O2-, but sometimes Cl- etc) are packed in some type of “closest packing” arrangement, and cations then fit into interstitial areas of various shape (tetrahedral, octahedral).

There are two types of closest-packing: Cubic closest packing and hexagonal closest packing. Both of these represent the most closely-packed that equal-sized spheres can be. Consulting with figures 3.37 and 3.38 from your text book (shown in small form below), Use equal-sized marbles to construct three layers of each type of packing (hint, the first two layers is the same for both of them-only the third layer differs).

[pic][pic]

Polyhedral models:

The interstitial spaces between the close-packed anions often have simple geometric shapes (although sometimes distorted), such as tetrahedral and octahedral. Due to its covalent bonding with 4 oxygens, Si often fills tetrahedral spots. 6-coordinated cations (Al and Mg often) occupy octahedral spots. Examine the shapes below to verify that 4-coordination results in shapes with 4 sides (tetrahedral), and 6-coordination results in shapes with 8 sides (octahedral).

These polyhedra can share corners (left two pictures), edges (middle two pictures) or sides (right two pictures). Make sure you understand this and can see it.

Crystal structure can be shown either by ball-and-stick models, showing atoms and bonds, or by polyhedral models, showing the polyhedra formed by groups of atoms.

NaCl is shown below in both models, from your book.

Unit Cell:

A unit cell is the unit that can be copied over and over to fill up space, thus making up the entire crystal. It reflects the overall symmetry and form of the crystal. On the ball-and-stick picture of NaCl above, draw the boundaries of the unit cell that has the octahedron in it. There is a Na atom at each corner of the unit cell.

How many corners does the unit cell have?

With how many unit cells is each corner Na shared?

How many edges does each unit cell have?

With how many unit cells is each edge Cl shared?

How many sides does each unit cell have?

With how many unit cells is each side Na shared?

There is one atom that is entirely within the unit cell. What is it?

Unit cells for NaCl and CsCl are shown. Notice how they differ. The total number of Na, Cl, and Cs in each are shown. Make sure that you understand how this number is derived by thinking of fractions of atoms shared with more than one unit cell.

ATTENTION: BE VERY CAREFUL WITH THE MODELS. THEY COST OVER $1000!

Halite Model (NaCl)

green = Chlorine (Cl)

gray = Sodium (Na)

What is the coordination number (number of nearest neighbors) for Na?

for Cl?

Find the octahedra around the Na or Cl. Visualize it.

Find the octahedral planes (there are 4 of them). Rotate such that you see the plane of Na atoms and plane of Cl atoms. These are the octahedral planes.

At what angle are these planes to the sides of the cube?

Relate the chemical composition of Halite to the number of Na and Cl you see in the model.

Halite is in the isometric system. This system has very high symmetry. Examine the block models 1, 2, and 3. Think about how the crystal form is related to the planes of the cube and octahedral.

Beta-Quartz Model

black = Silicon (Si)

red = Oxygen (O)

Notice the tetrahedra formed by the four oxygens around each Si. Think about the polyhedral model.

How many sides, corners, edges does each tetrahedron share with adjacent tetrahedral?

(the answer, to be read only when you have thought about it, is..............no sides, no edges, 4 corners)

What is the chemical formula for Beta-Quartz, based on the number of oxygens and Si present in the model? (also, think about how many O there must be for each Si, if every O is shared with one other Si). Check its composition in your book to see if you are right.

Framework silicates, like quartz, have tetrahedral that share all corners, and have a ratio of tetrahedral cations (Si), to anions (O) of 1:2.

Notice the unit cell shown on the model. Each unit cell contains 3 Si, and 6O, many of them shared with adjacent unit cells. Make sure that you can count them up, and figure it out! Think about how many unit cells that a particular atom is shared with.

Alpha-Quartz Model

This form of Quartz is a polymorph of Beta Quartz, that is, it has the same composition but a different atomic structure. Alpha-quartz is the form that occurs at lower temperatures (below 500 C). Also, the alpha quartz is slightly preferred at higher pressure, such that at 8kbars pressure it occurs below about 800C. This is because the alpha-quartz is slightly denser than the beta-quartz (the atoms packed together more tightly).

The two forms are very similar. Notice, as with beta-quartz, the shared tetrahedron corners (all four corners are shared).

As with beta-quartz, there are no shared sides or edges.

The basic shape of the unit cell is similar.

To notice the difference, count the number of Si and O in each unit cell, as you did with beta-quartz, and notice that different fractions of particular atoms are shared with adjacent unit cells.

Rotate the model so that the Si atoms line up. You should be looking at the rhombohedral unit cell from the side. With the model in this orientation, you are looking approximately in the “c” direction of the crystal. When quartz grows into hexagonal prisms, the prisms grow in the c direction, and the hexagonal outline will be perpendicular to the c direction.

Do you see any hints of hexagonal form?

Use the 6 two-dimensional rhombohedrons to think about he relationships between rhombs and hexagons.

Make patterns with the rhombs.

Fill up space with the rhombs by translating (moving without rotating) a rhomb-shape.

Translate with a 180 degree rotation (the blue dot will not always appear in the same corner).

Put two rhombs in a relationship to each other that represents reflection. Imagine that you could draw a line between them such that each side of the line looks like a reflection of the other. There is more than one way to do this.

Make a hexagon with the rhombs. What operations (translation, rotation, reflection), did you have to use?

Make a square and a rectangle with the rhombs (or, can you?)

Forsterite Model (one end of the Olivine solid solution series)

black = silicon (Si)

red = oxygen (O)

silver = magnesium (Mg)

Try to infer the chemical formula for Forsterite based on the proportions of atoms that you see. Check your book to see if you are right.

Find the Si tetrahedra. What is the coordination number for Si?

Find the Mg octahedra. What is the coordination number for Mg?

Try to visualize the polyhedral model for Forsterite.

How many sides, corners, or edges do Si tetrahedral share with other Si tetrahedral?

(And the answer is, not to be read before you think about it.........no edges, no sides, no corners)

Look at the model end-on in such a way that the Oxygen atoms line up. Think about what it would look like if projected onto a flat sheet of paper (the way crystal structures are often shown in books).

Look at the model sideways such that the Mg atoms line up. As above, think about the projection onto a flat page. Notice the “apparent” hexagons around Mg?

Nesosilicates like Forsterite have tetrahedra that do not share any corners, edges, or sides. The ratio of tetrahedral cations (Si) to anions (O) is 1:4.

Illite Model (a mica-like clay mineral, very similar to muscovite and montmorillinite in composition and structure. Use of this mineral name has a problematic history.)

black = silicon (Si)

red = oxygen (O)

silver = aluminum (Al)

aquamarine = hydroxyl group (OH-)

gold = potassium (K)

orange = other large cations maybe Na, Ca

Find the silica tetrahedra.

how many corners, edges, sides do they share with other silica tetrahedral?

Notice that some of the Si has been substituted by Al. Typically, 10-15% of the Si is replaced by Al.

Find the Al octahedrons.

How many corners, edges, sides do octahedral share with other octahedral?

(the answer is, which you shouldn’t read until looking, is share edges, no sides, no corners)

How many corners, edges, sides, do octahedral share with tetrahedral?

(share 4 corners)

With how many octahedrons is any one octahedron-oxygen shared? (we will talk later in the term about dioctahedral and trioctahedral sheet silicates).

It is very common for sheet silicates (micas and clays) to be made of up various sets of tetrahedral and octahedral layers (covalent or strong ionic bonds) that make characteristic “sandwiches” that are in turn bound by much weaker ionic bonds, or even hydrogen bonds. See if you can find the layers of tetrahedral and octahedral and figure out the pattern.

The stacking sandwiches for illite (which is like muscovite) is roughly the following:

Make sure you can find and see these layers in the model.

Think about how the strong cleavage in micas and clays results from this layering.

Montmorillinite (the super-water absorbing clays in bentonite) are similar in structure but lack the K layer, having instead much more weakly bonded water layer between the sandwiches.

The c axis is perpendicular to the sheets. Look into the crystal in the c direction (you won’t see the layers). Notice how any plane cutting through the crystal in this direction must cut across the covalent bonds of the tetrahedral and octahedral. Therefore, there is no good cleavage in these directions.

Look how big a unit cell must be!!! Observe how far you go before the crystal repeats, the entire size of the model!

Mineral Reactions, Stability, and Behavior:

Crystallization:

Concept of phases: phases are macroscopically homogeneous regions bounded by distinct edges.

gases, liquids, solids are the examples of phases you learn in high school. But a particular material can exist in more than one solid or liquid phase. For example, graphite and diamond are two solid phases with the same composition (polymorphs).

In gases, individual molecules or atoms have no long range order, and are not bonded to nearby molecules or atoms.

In liquids, molecules or atoms have no long range order but are bonded to nearby molecules or atoms but those bonds are not strong enough or persistent enough to maintain a regular long-range order, although a short-range order often exists.

In solids, molecules and atoms are bonded to nearby modecules and atoms, most normally establishing both local and long range order (crystals). Some solid materials do not have long range order (although short range order typically exists). These amorphous materials are called glass.

Crystallization occurs when a material goes from a gaseous or liquid state to a solid, ordered state. This occurs when T, P, composition or other properties change in such a way that the solid state is energetically favored over the former state.

For example, evaporating water from salt water increases the concentration of Na and Cl dissolved in the water to the point that salt crystals will form.

Cooling magma will bring the temperature to a value where crystals begin to form in the melt.

Energy of a crystal is related to the bond energy as well as the arrangement of atoms in the crystal.

We can also think of the bulk energy, the energy of a block of essentially infinite size, and the surface energy, the energy of the material where it encounters something else (air, water, another mineral, etc). Generally, the atoms at the very edge of a crystal are less stable (higher energy).

Think about what the effect of surface energy will have on tiny crystals versus big crystals. (think about volume increases by cube, surface area by square: use example sizes e.g. cube 1x1x1 vs cube 2x2x2 what is surface area and volume of each?)

The surface energy makes tiny beginning crystals less energetically favorable. This is what makes crystals tend to grow into a few big ones instead of many small ones. But it also makes the “starting” step of crystal growth difficult. This step is called nucleation.

E.g. of nucleation in weather. Seeding clouds in the 60’s, still done in some countries. Lowers surface energy. Supercooled air then forms ice crystals or water droplets. Sometimes, air can become supercooled. It is below T at which ice crystals should form, but due to surface energy, they crystals don’t form.

Big perfect crystals, usually form from slow growth, lots of space to grow in to, and ideal growing conditions (such as the T, P, composition are held persistently where the crystals grows slowly at a regular rate). They are rare.

Phase Diagrams, graphical illustration of crystallization reactions and phase transitions.

One-component reactions (different phases of a single chemical component)

Primary variables are T and P.

In general, the phase preferred at higher pressure will be the denser phase.

The phase preferred at higher temperature will be the less well-ordered phase and/or the phase with higher energy bonding.

Parameters other than T and P can also affect equilibrium, and could be plotted, but are generally not significant in natural situations (e.g. magnetic field, gravitational field, electrostatic field, etc.).

Water overhead: Phase diagrams illustrate fields of T and P where phases are stable. Lines represent reactions, such as the reaction in which liquid water freezes to ice (find that reaction). The triple point is invariant, meaning there only one T and P where all three (liquid, gas, solid) can coexist. Lines are univariant, curves of T and P where e.g. liquid and gas can coexist. Critical point is T and P beyond which liquid and gas are not separated by a distinct phase transition (they become like each other). Based on the ideas discussed above, which is more random, and/or has higher energy bonding, liquid water or vapor water? Does this make sense? Which is more dense, ice or liquid water? Does this make sense? Below 6 millibars, what happens to ice as you heat it up? This is the state of H2O on most of Mars surface.

[pic]

[pic]

Broader H2O phase diagram, overhead.

Which is denser, Ice I (normal ice that we know), Liquid H2O, Which is denser, Ice I or ice III? How about Ice VI? Which is denser liquid water, or ice VI?

Is ice deep inside a moon of Jupiter likely to have a density of less than, equal to, or greater than 1g/cc?

[pic]

C phase diagram, overhead

Which is more dense, graphite or diamond? Which is less ordered and/or more energy in bonding? Which is more dense, C melt, or diamond? Which is more dense, graphite or C melt? In which will the atoms of C be packed more closely, diamond or Carbon III? Notice the C vapor. What will happen to vapor at a single T if pressure increases?

[pic]

[pic]

SiO2 phase diagram overhead.

At pressure of around 10kbars (about 29 kilometers depth), what will happen to pure SiO2 as T falls from around 1800 (tell the story). What would happen at about 3 Kbar?

Stishovite generally forms in meteorite impacts, is a fingerprint for impact. 80-90 kbar is a pressure 250 km deep or more, where SiO2 generally does not occur as a distinct phase.

[pic]

CaCO3 overhead.

Which is more stable at high P, aragonite or Calcite? Which is the more dense structure? Which is more stable at low pressure? Why does aragonite occur is many gastropod shells?

2-component Phase diagrams (two compositional components)

Can only easily show 2 variables on 2-dimensional page. With only 1 component, you can show both T and P and composition doesn’t change. With 2 components, can’t easily show both T and P as well as composition. So often show a diagram that is valid at only a single pressure (often 1-atm pressure).

Solid-solution series: (e.g. olivine and plagioclase)

[pic]

Same structure, but Fe substitutes for Mg as go from Forsterite to Fayalite.

Above both curves, there is a single phase, melt, that has the composition of the bulk material.

As T decreases, the upper curve is encountered. It is called the liquidus, the temperature at which all solid disappears during melting, or where the first solid appears during cooling.

At this T, solid olivine begins to form. You can determine the composition of that olivine (remember, it’s a solid solution) by drawing a horizontal line at that temperature. The intersection of the horizontal line with the lower curve (called the solidus) indicates the composition of the olivine.

As T continues to fall, the composition of both the residual melt and the olivine solid solution must change. At any T, the equilibrium composition of melt and solid is indicated by the intersection of the horizontal line with the liquidus and solidus respectively.

Eventually, a temperature is reached where the solid olivine has the same composition as the bulk starting material. At this temperature, the last of the liquid material will solidify (or, if we are melting solid material, it is at this temperature that the first melting will occur).

What reaction does the liquidus and solidus lines represent? (liquid olivine = solid olivine).

Notice that, in general, solids in 2 or more component systems will not have the same composition as the bulk liquid. Therefore, as the solid crystallizes, the composition of the residual melt must change since the total of solids+liquids must always equal the initial bulk composition.

How many phases are present above the liquidus?

How many phases below the liquidus but above the solidus?

How many phases below the solidus?

Handout of Plagioclase phase diagram, one for each person.

[pic][pic]

Questions:

Which melts at a higher T, pure anorthite or pure albite?

What happens to the melting T of Albite as you add more Ca-Al to it?

What happens to the melting T of anorthite as you add more Na-Si to it?

Consider 40%An, 60% Ab. At what temperature would such a mineral begin to melt? (about 1229C)

If it was all completely melted, at what temperature would it start to freeze? (about 1413C)

At what temperature would it completely freeze? (about 1229C)

What would be the composition of plagioclase at about 1413C when the first plagioclase crystals start to form? (about 76.2% An)

What would the composition of the melt be at about 1413C when the first plagioclase crystals start to form? (40% An)

Are the plagioclase crystals more Ca rich or more Na rich than the melt? Is that always true? So, if the solid has more Ca in it than the melt, how must the melt change as more plagioclase crystallizes from it?

Will the composition of the plagioclase stay the same once it starts to crystallize, or will it change?

How will it change? (more Na rich at lower T)

what is the composition of the melt at 1300C? (15.3% An)

What would be the composition of plagioclase at about 1229C when the last melt solidifies? (40% An)

What would be the composition of plagioclase at about 1300C? (54.3% An)

Non-solid-solution binary systems: phase diagram overhead.

Pick a couple of compositions and decrease T, showing first phase to appear on liquidus, zone of freezing, and encounter of solidus.

Two different phases on the liquidus, depending on the starting bulk composition.

Last drop of melt will always be at the invariant point where liquid, phase A and phase B all coexist (remember, other invariant point was where three things coexisted).

Albite-Qz phase diagram overhead.

[pic]

What form of quartz if went to even lower T? (high quartz then low quartz) (show other phase diagram if necessary)

What if at higher Pressure? what would be different? (high Qz instead of cristobalite and tridymite).

Didn’t get to the following, but will probably do these when we cover igneous rocks.

Two solid solution series plus a subsolidus exsolution curve. See Albite-Orthoclase overhead, and also draw a simplified schematic version on the blackboard. Note where various phases occur, including polymorphic transitions. High albite, less ordered Si-Al, low albite has more ordered Si-Al.

If slow cooling occurs, microcline occurs in rock. More rapid cooling from higher T results in orthoclase, or even sanidine.

[pic]

Bunny rabbit overhead with simplified schematic on blackboard. Effect of pressure (H2O pressure) on the curve (5 kbar H2O). Explain how this results in a single feldspar at low water pressure, and two feldspars at high water pressure. Perthite forms when crystallizes at low P, then cools below solidus curve. If the rock cools at depth with H2O, 2 feldspars form to start with and perthite does not occur.

Ternary systems (overhead):

plot three components, with temperature plotted as contour lines. Composition is resolved as illustrated.

[pic]

Other types of phase diagrams. phase diagrams in which two compositional variables are shown (at T and P are constant), are called fence diagrams. Often, pH and Eh are the compositional variables.

[pic]

Mineralogy and Petrology.

Mineral identification, occurrence, and properties, Lab #3

For each of the listed minerals (58), you should create a neat record of the mineral’s features (including composition, crystal class, typical crystal form, hardness, color, streak), and its occurrence (what type of rocks it is found in, under what geological conditions it forms). You should also examine the examples of this mineral that we have in the lab, and make notes about your observations. You can use these notes on exams. This lab is due in 3 weeks. At 3 hours of lab per week, this gives you about nine to ten minutes per mineral. Feel free to spend more time on some and less on others. You have already looked at many of these in Lab #1.

You might use a form like the one on the attached page:

All the minerals listed below should be available in the mineral cabinet (mc), the ore mineral cabinets (omc), or the introductory mineral cabinet (imc). You will have to identify the imc samples in order to find them, which is a good exercise anyway. There are multiple samples for many of the minerals. You should look at all the different samples, because they won’t all look the same. Please don’t scratch, streak, or apply acid to any samples other than those from the introductory mineral cabinet.

Oxides:

cuprite (omc 21), corundum (omc 201, 202, mc 54, 55, imc), hematite (omc 46, 47, 48, 49, imc, imc, mc 61, 62, 63, 64), ilmenite (mc 59), chromite (omc60, mc 58), cassiterite (omc 39, 40), magnetite (omc 44, 45, imc, imc, mc 56, 57), pyrrhotite (omc15, mc 70, imc) bauxite (omc 41)

Sulfides:

bornite (omc 17, imc), galena (omc 26, 27, imc), sphalerite (omc 27, 32, 33, imc, imc), covellite (omc18), cinnabar (omc73), stibnite (omc65, imc), arsenopyrite (omc 67, imc-THIS IS A POISON-WASH YOUR HANDS)

Sulfates:

barite (omc 82, 176, imc, mc 99), gypsum (mc 95, 96, 97, imc, imc), anhydrite (mc 98)

Carbonates:

calcite (omc 36, 179, mc 83, 84, 85, 89, imc), siderite (omc 54, mc 93, imc), magnesite (omc 112, mc 94), cerussite (omc 29), dolomite (omc 28, mc 91, 92), malachite (omc 23), azurite (omc 22)

halides:

halite (omc 182, 183, imc, mc 100)

Silicates:

nesosilicates: olivine(forsterite-fayalite) (mc 26), garnet [almandine omc 203, imc, mc27, mc28; andradite mc29; grossularite mc30], andalucite (mc 35, 36), silliminite (omc 110, mc 38), kyanite (omc 111, mc 37), staurolite (mc 34).

sorosilicates: epidote (mc 31)

cyclosilicates: beryl (omc69), tourmaline (omc 181, mc 39, 40, 41)

inosilicates: pyroxene [wollastonite mc29, diopside-hedenburgite (mc 18, 19), augite (mc 20, imc), enstatite-ferrosilite (mc 17)] amphibole (mc 22, 23, 24, 25, imc)

phylosilicates: micas [biotite (omc 172, mc 15, imc), phlogopite (omc 171, mc 16), muscovite (omc 170, mc 13, imc), chlorite (mc 75), lepidolite (omc 86, mc 14, 41)], talc (omc 173, 174, 175, imc, mc 78, 79), kaolinite (omc 99, 100, mc 73), serpentine (mc 76, chrysotile, omc 166, mc 77)

tectosilicates: quartz(omc1,2, 4, 180, 208, 215, 216, mc 44, 45, 46, 47, 48, 49, 50, 51, 52, imc) , feldspars [orthoclase (imc), microcline (omc 107, mc 3, 4), sanidine (mc 1), albite (omc 107, mc 5, imc), anorthite (mc 8)] leucite (mc 12), nepheline (omc 43, mc 10), sodalite (mc 11)

Mineral:

Chemical composition:

crystal system (and Hermann-Maugin symmetry if you choose):

typical form and appearance:

hardness:

color and streak:

Typical occurrence (include rock types and geological environments of formation)

Notes on your observations of this mineral (include how many different types of samples you looked at).

Mineral:

Chemical composition:

crystal system (and Hermann-Maugin symmetry if you choose):

typical form and appearance:

hardness:

color and streak:

Typical occurrence (include rock types and geological environments of formation)

Notes on your observations of this mineral (include how many different types of samples you looked at).

Crystal symmetry:

Crystallography is (arguably) the second oldest science, after astronomy.

Crystals can be thought of as made of motifs (a unit pattern of atoms) which are periodically repeated to construct the entire crystals. The periodic array of points to which the motif is copied is called a lattice. Imaginary lines constructed between these points will enclose only a limited number of shapes. The unit cell is one possible repeating pattern which can fill up space. The environment around each unit cell will be identical to that around all other unit cells. The motif, and the lattice, will have symmetry and dimensions which is characteristic of each crystal.

Symmetry operations (illustrated in 2-D, for 3-D illustrations, see your book)

Rotation:

[pic]

2-D: 1 fold, 2 fold, 3-fold, 4-fold, 6 fold axes of rotational symmetry

Reflection:

[pic]

Can consider mirror planes. Some motifs may have more than one. Consider mirror planes in the 2-D figures:

Inversion (in 2-D it is like rotation, but is more complex in 3-D):

Rotoinversion (in 2-D it is like a rotation, but is more complex in 3-D):

[pic]

The symmetry above can be related to the symmetry of a particular motif. However, to fully understand a crystal, we also have to consider the operations by which the motif is copied through a crystal.

Translation and glide.

[pic]

Translation and screw (only occurs in 3-D)

Consider the following 2-D motifs. What symmetry do they have? (i4; i4mm)

Hermann-Maugin notation (or the international symbols)

numbers refer to axes of rotation. e.g. 222 refers to three separate axes of rotation, each of which is two fold. Show with an orthorhombic box. A bar over the number designates an axis of rotoinversion (rotation and inversion).

m refers to mirror planes. e.g. 4mm refers to a four fold axis of symmetry with mirror planes in two different orientations (it could be more than two mirror planes). 4/m refers to a mirror plane that is perpendicular to the 4-fold axis.

i refers to a center of inversion. It is usually not listed if higher orders of symmetry are present.

Only certain shapes fill up space (lattice systems or crystal systems).

Of those shapes that fill up space, only certain organizations of points within the lattice are possible (lattices).

In addition, only certain types of motif symmetry are possible (point groups, or crystal classes)

Considering the different types of possible lattices and the different point groups, and considering the way that unit cells can be moved by translation, glide, and screw, there are only a limited number of possible types of crystals (plane groups or space groups)

| |2-D |3-D | |

|lattice system or crystal system|4 |6 |these are the basic shapes |

|lattices or Bravais lattices |5 |14 |number of non-identical periodic|

| | | |arrays of points |

|point groups or crystal classes |10 |32 |number of different symmetries |

| | | |possible for motifs. |

|plane groups or space groups |17 |230 |combines the number of point |

| | | |arrays with the different |

| | | |symmetries, and the effects of |

| | | |translation, glide, and screw |

For example, in two dimensions the 4 basic shapes are square (a=b, γ=90º), rectangular (a≠b, γ=90º), oblique (a≠b, γ≠90º), and hexagonal (the shape is a rhombus, a=b, γ=60º). Although other shapes can fill up space, all other possible shapes are equivalent to one of these.

Examples of the different levels of crystal organization for a two dimensional square shape:

lattice system square, has only one lattice type (square)

it has two point groups. (4mm, and 4). 4 refers to axis of rotation, first m to vertical and horizontal mirror planes and second m to diagonal mirror planes: The illustration of possible atom configurations is symbolic, showing possible symmetries.

The square lattice has three plane groups (analogous to space groups in 3-D). These include the ways that the square shape can combine with point groups, including also the operations of translation, glide, and skew.

p4, p4gm, and p4mm. 4 refers to the axis of rotation, the first m to the mirror planes perpendicular to the sides of the square, the second m refers to diagonal mirror planes, either through the corners (for p4mm), or between the corner and center point (for p4gm), g refers to glide planes parallel to the sides but between the center and the sides.

For comparison in 3-D, in the cubic (isometric) system, there are 5 different crystal classes (compared to 2 in the 2-D point groups), and 36 space groups (compared to 3 in 2-D squares).

Consider the highest symmetry example: P432. it has 3 4-fold axes of symmetry, 4 3-fold axes of symmetry, and 6 2-fold axes of symmetry (show with a cube). There are lots of mirror planes, however these are not given in the Hermann-Maugin notation because the mirror plane symmetry is already implied by the rotational symmetry.

The six crystal systems in 3-D

(from least to most symmetry)

[pic]

show lack of symmetry with parallelogram in 2-D (although point out rotation axis).

[pic]

[pic]

[pic]

Crystallographic notation for planes, Miller indices - go through cubic and octahedral examples for isometric system only.

Consider the intersection of the plane of a crystal face (or a planar feature within a crystal or unit cell) and a line drawn perpendicularly through that plane from the origin of the a, b, c axes within the figure. Take the inverse of the point of intersection for each axis, a, b, and c. Adjust the points of intersection such as to yield only integers and only integers with no common denominator other than 1. This is the Miller Index for the plane.

Example in Isometric system: planes of the cube and planes of the octahedral expression of that system. e.g. first figure: intersection of b axis is at 1, a and c are at ∞. 1/1 = 1, 1/∞ = 0. Second figure: intersection of a, b, and c are all at 0.5. 1/0.5 = 2. These have 2 as a common denominator. Dividing by 2 yields (111). A bar over the number indicates it is negative.

[pic][pic]

Example Test Exercise Questions for the section on Symmetry and crystallography.

Illustrate a motif in 2-D that has a three-fold axis of rotation and 3 mirror planes.

Illustrate a motif in 2-D that has a three-fold axis of rotation but no mirror planes.

Illustrate a rectangular figure in 2-D, with motifs at the corners, that has only a single mirror plane.

Illustrate the three 2-D point groups for the square, using motifs different from those we used in class.

Illustrate the similarities and differences between the orthorhombic and tetragonal systems.

Draw a perspective view of a cube (isometric system). Use dashed lines for edges that are hidden. For each of the 2-fold axes of symmetry, put a dot where the axis emerges from the cube. Number the dots such that the two dots associated with each of the axes have the same number (i.e. the first axis has two dots each labeled with a “1”, etc.).

Match the space group (hermann-maugin notation) with the appropriate crystal system. Choose from the following crystal systems for each

isometric, orthorhombic, tetragonal, hexagonal (not rhombohedral), hexagonal (rhombohedral), monoclinic, triclinic. (NOTE: These can be figured out simply from the rules that I discussed in class for each crystal system)

P432

P6/m

Pmm2

P2/m

P422

P4mm

P3m

P622

P23

P32

P4

P1

Pm

P6

Match the indicated faces with the proper miller index (three faces in the isometric system). Presume that the “a” axis is emerging from the figure (negative = going into the figure) and the “c” axis goes upward (negative = downward). Positive “b” axis is toward the right. Possible indices are (100), (111), (110), (211), (222), (010), (001), (101), (110), and (422). Mark each of the lettered faces.

Classification of Minerals:

Crystal structure and symmetry are not the only important characteristics of a mineral. Chemical composition is also important. Minerals are often classified into mineral groups.

Mineral groups are based on the primary anion (not cation) of the crystal. This is because minerals with a common cation usually have more in common in terms of properties than do minerals with common cations (for example, compare cerrusite and siderite to galena and pyrite). Also, the anion more consistently reflects the geological environment of formation. That is, sulfides tend to occur together in one type of environment, whereas carbonates occur together in a different environment, and silicates in a third environment.

Native elements (metals and nonmetals) (no anion)

e.g. Cu, Au, Fe, Fe-Ni

e.g. S, C

bonds are metallic in metals, or covalent or other in nonmetals

Crystal structures for the metals are often based on closest packing structures like hexagonal or cubic closest packing (12 nearest neighbors), or other simple packing structures like body-centered cubic (8 nearest neighbors). Structures in S or C are controlled by covalent bonding angles and typically have 3 or 4 nearest neighbors.

Degree of solid solution is primarily controlled by similarities of atom size, thus Au and Ag have complete solid solution, but the much smaller Cu does not dissolve significantly in Au or Ag. Fe and Ni substitute fairly readily for each other, being of very similar size. etc.

Sulfides (and sulfarsenides, aresenides, antimonides, selenides, and tellurides) (S, As, Sb, Se and Tl are anions)

e.g. FeS2 (pyrite or marcasite), ZnS (sphalerite or wurtzite), arsenopyrite (FeAsS - arsenic substitutes for S), CuS (covellite), Cu2S (Chalcocite), Cu5FeS4 (bornite). Other cations can include the metals cobalt (Co), nickel (Ni), molybdenum (Mo), silver

(Ag), cadmium (Cd), tin (Sn), platinum (Pt), gold (Au), mercury (Hg), tellurium (Tl), and lead (Pb) (for example), or the semimetals arsenic (As), antimony (Sb), and bismuth (Bi).

bonds are mainly ionic, although there are also covalent bonds and metallic bonds.

Usually opaque with distinctive streaks and colors

Structures can often, but not always, be thought of as metals in octahedral or tetrahedral coordination in the interstices between S anions (polygons often are distorted though).

associated with low Eh environments, with high S. Is usually aqueous, often hydrothermal. Is a primary ore-forming mineral group, especially for Cu, Zn, Pb, Ag, Hg, and many others.

Structures of Sphaelerite, Chalcopyrite, and Wurtzite.

Note that Sphaelerite and Wurtzite differ in that the Zn cations have a face-centered cubic arrangement in sphaelerite, but a hexagonal closest packing in Wurtzite (remember that these differ in that the third layer up is different for hexagonal closest packing, but returns to be like layer one in cubic closest packing).

Chalcopyrite differs from sphalerite in that Fe and Cu replace Zn. Because the top and bottom atoms in the sphalerite-sized cell are different (Fe and Cu) and thus adjacent cells wouldn’t have exactly the same environments (criteria for unit cell), the actual unit cell for Chalcopyrite must be twice as big. Notice that you couldn’t stack just half-cells on each other and have it make sense because you would end up with a half-Fe-half-Cu atom between the cells.

Structures of Pyrite and Marcasite

[pic]

Pyrite is isometric (2/m3, most common crystal types are cube, pyritohedron, and octahedron), Marcasite is orthorhombic (2/m2/m2/m, crystals often tabular). Pyrite has structure like Halite with covalently-bonded S pairs occupying Cl positions and Fe in Na positions. The S pairs in pyrite decrease symmetry from 432. The three-fold axis of rotation (in this case, rotoinversion) is the characteristic symmetry of the isometric class). The three axes of binary symmetry are the characteristic of the orthorhombic class.

Sulfosalts As, Sb, or Bi (semimetals) substitute not for the anion S, but onto the metal lattice sites.

Oxides (and hydroxides)

e.g. Fe2O3 (hematite), Al2O3 (corundum), Ilmenite (FeTiO3), Magnetite (Fe3O4), cassiterite (SnO2), goethite (FeO(OH)).

The structure can be understood as deriving from oxygens that take on some close-packing configuration, with metal cations occupying various tetrahedral or octahedral interstitial spaces.

bonds are mostly very strong ionic bonds. These minerals are often very hard. Oxides are usually very stable minerals.

Oxides are important ore minerals, including Fe, Cr, Mn, U, Sn, Al (although the stability means that substantial energy investment must be made to separate the metal from the oxygen). Ruby and Sapphire are members of this group.

May be grouped either as simple oxides (one metal plus oxygen), and complex oxides (more than one metal cation).

Or they may be grouped according to the cation-oxygen ratio (e.g. Divalent cations yield 1:1, trivalent 2:3, or mix of divalent and trivalent 3:4, tetravalent 1:2).

periclase-hematite-corundum structure: Hexagonal closest-packed oxygens, with metal cations occupying octahedra.

Considering all octahedral sites, each oxygen is shared with six octahedra. So, what valence charge is associated with each octahedra? (each oxygen contributes -2, distributed over 6 octahedra, each octahedra has 6 closest neighbor oxygens = 2/6 x 6 = -2). Therefore, each octahedron can be filled with a divalent cation (e.g. periclase = MgO), or 2/3 of the octahedrons can be filled with trivalent cations (e.g. Hematite, corundum). Note: periclase has the same structure as Halite....see if you can count the octahedra around a particular Cl ion.

Brucite-gibbsite structures.

[pic]

OH- groups in place of oxygens. Different charge, but octahedra connect only in a plane, rather than in 3-D like in the oxides discussed above. (the individual planes are connected by Van der Waals bonds). Therefore, each OH- at the corners of the octahedra are shared with only 3 octahedrons. This leaves an anion charge associated with each octahedron of -2. Therefore, all can be filled with Mg (Brucite), or two thirds can be filled with Al (Gibbsite). These layers are called dioctahedral and trioctahedral layers respectively.

Conditions: how will CuS and Cu2S differ in environment of formation? How Cu2O and Cu2S differ?

[pic]

Halides

The halides include F, Cl, Br, I, etc. e.g. NaCl (halite), KCl (sylvite), CaF2 (fluorite)

Bonding is the most completely ionic of any of the mineral groups because the electronegativities of the constituent elements are the most different.

This group has the highest crystal symmetries because ions are spherical and bonds are symmetrical. Symmetry decreases as cations of higher valence than 1 are involved, and the bonds become more covalent.

Have the characteristics of ionic solids: e.g. low hardness, poor conductors

Carbonates: (and nitrates)

e.g. CaCO3 (calcite, aragonite), FeCO3 (siderite), CaMg(CO3)2 (dolomite), Cu2CO3(OH)2 (malachite), Cu2(CO3)2(OH)2 (Azurite).

Triangular anionic complexes bound more strongly than the complexes are bound to other ions. Each oxygen has a residual charge of -2/3. Bonding of the CO3 group is not as strong as CO2 bond, so in presence of H+, the carbonate group becomes unstable, breaking down to form CO2 and water.

Bonds in the complex are covalent, bonds between complex and metal cations are ionic.

Calcite structure: Like halite, but with CO3 groups in place of Cl and Ca in place of Na. Symmetry of the triangular CO3 groups produces a rhombohedral rather than isometric crystal. Pseudohexagonal structure of calcite derives from the near-hexagonal close packing of the Ca cations.

Other two groups: Dolomite is also rhombohedral, aragonite is orthorhombic. Larger cations (or Ca at higher T) tend to organize into the aragonite-type structure.

Ca very different in size from Mg and Fe. Therefore, there is little solid solution. Dolomite and Ankarite result when Ca and Mg or Fe don’t mix, but occupy distinct layers in the crystal.

As T increases (at high CO2 pressure so crystals don’t become unstable), the amount of mixing increases, and a solid solution exists at sufficiently high pressure.

[pic]

Sulfates:

e.g. BaSO4 (Barite), CaSO4 (Anhydrite), CaSO4۰2H2O (Gypsum).

non-polymerizing complexes.

Nitrates, Borates, chromates, tungstates, molybdates, phosphates, arsenates, vanadates.

anionic complexes bound more strongly than the complexes are bound to other ions.

Tidbits: borates can polymerize. Nitrates triangular like carbonates.

Silicates:

Key idea: whether and how the silica tetrahedra are connected to each other by strong covalent bonding, or whether the corners of tetrahedra connect to octahedral or other sites occupied by usually-larger cations by ionic bonds.

Show overhead. Key things to note, the sharing of tetrahedral corners (means that two Si are linked to each other by a single oxygen). The number of oxygens shared between tetrahedra determines the Si-O ratio. For example, with no sharing, then each Si has 4 oxygens, if two Si share one O, then each 2 Si has 7 oxygens, etc.

comment on the unit cell shown for phyllosilicates. Notice how each corner of the rhombus is surrounded by a “motif” of silica tetrahedrons arranged in a hexagon.

Also comment on what I teach in Physical Geology: the idea of tetrahedrons connected in 0, 1, 2, and 3 dimensions corresponding to island, chain, sheet, and framework silicates.

[pic]

Nesosilicates: olivine

remind of model that we looked at. Show overhead. Point out that tetrahedra share corners with octahedra (M1 and M2), not other tetrahedra.

[pic]

Inosilicates: Single Chain Silicates: The pyroxenes.

Draw on board:

show cleavage, cutting down through M1 sites at angles such that cleavage is at nearly right angles (remind of lab activity).

The clinopyroxenes (monoclinic) and orthopyroxenes (orthorhombic):

Draw illustration:

immiscibility gap: Mg and Fe are near the same size, so they mix in solid solution series, but Ca is much different, resulting in a miscibility gap between the Diopside-Hedenbergite series and the Enstatite-Ferrosilite series. Tie lines show coexisting pyroxenes at a particular temperature (Compositions of coexisting pyroxenes can be used as a geothermometer recording temperature of formation).

Primary compositional components:

Diopside (CaMgSi2O6), orthopyroxene ((Fe2+,Mg)2Si2O6)

but can substitute Na, Al, Fe3+, Li for M1 and M2 sites, and Al on tetrahedral sites

Putting Ca in M2 site, much larger than Mg or Fe, distorts lattice resulting in the lower symmetry of clinopyroxenes (including diopside, augite, and pigeonite).

(note: putting Ca in M1 site also distorts chains, resulting in the even lower symmetry, triclinic, of the pyroxenoid, wollastonite)

Augite: is mostly in the series CaMgSi2O6-CaFeSi2O6, but with some substitution of Na and Al.

But must charge balance. So, for example, if Na or Li is substituted for Ca (2+), a trivalent cation, such as Fe3+ or Al must substitute for Mg or Fe2+.

Tschermacks substitution: Can substitute Al for Mg on the M1 site, and charge balance by substituting Al for Si4+ on a tetrahedral site.

M2 is bigger (show picture of jadeite): Na in bigger M2, small Al in M1. Point out shared tetrahedral corners and where chains are connected by octahedra.

Phyllosilicates (sheet silicates)

Structure of micas:

Similar to the structure of illite looked at in lab (triangles represent tetrahedra in 2-D, diamonds represent octahedra in 2-D).

Muscovite KAl2(AlSi3O10)(OH)2 - K between covalent octahedra-tetrahedra sandwiches, OH substituting for some O in octahedra, Al in octahedra, other Al substituting for Si in tetrahedra. Dioctahedral because Al is trivalent, not all octahedral sites are occupied (2 of 3). Is the Si-O ratio correct? Have to compensate for the Al on the tetrahedral sites. So better to think of the T-O ratio.

Phlogopite KMg3(AlSi3O10)(OH)2 - what has changed? Mg in octahedral sites, it is divalent, so this is a Trioctahedral structure, all the octahedral sites are filled.

Biotite K (Mg, Fe)3 (AlSi3O10)(OH)2 - what has changed? Is this dioctahedral or trioctahedral?

Petrology

Sedimentary Petrology

Terms related to deposition

Detrital = transported fragments and particles

Clastic = fragments and particles that may not have been transported.

Chemical and biochemical = precipitated from water

Major rock types

Sandstone (20-25%): clastic rock with particles 0.06 to 2mm.

Mudstone (65% of sedimentary): clastic rock with particles less than 0.062mm

Carbonate (10-15%): usually chemical or biochemical rock made of carbonate minerals, particularly calcite, aragonite, dolomite and some siderite and magnesite.

Evaporites: Chemical rock, usually formed from evaporation of sea water, or terrestrial alkaline or salty waters, in arid, restricted basins.

First three make up >95% of sedimentary rocks.

Problems in classification:

A carbonate might be made of clastic fragments (either transported or not), such as large fossil fragments in a limestone, or wave-worked shell fragments in a coquina.

Variable amounts of clastic clay can mix with carbonate (Marl).

Age distribution of sedimentary rocks

Half of sedimentary rocks are 130myo or younger (Cretaceous or younger).

Exponential decline in exposure as go to progressively older rocks.

[pic]

Does that mean that sedimentary rocks form more commonly today than in the past?

No, is related to a roughly constant probability of destruction by erosion, with a certain fraction of older rocks surviving to later time periods.

Common depositional settings

Most sedimentary rocks are deposited in marine (as opposed to terrestrial) environments. This is because oceans constitute a larger fraction of earth’s surface, because marine environments are more likely to be depositional rather than erosional, and because the sediments are more likely to be covered by later sediments rather than eroded.

Due to fluctuations in sea level, shallow marine water invading continental areas has been more pervasive at various times in the past than they are today. These shallow seas are called epicontinental seas.

Depositional Basins (regions either significantly below base level, or where persistent subsidence provides room for deposition over extended time periods.)

activity: in groups, try to identify key plate-tectonic environments, and the general characteristics of sediments deposited in each.

Oceanic Basins: deposits underlain by oceanic crust (basaltic rather than granitic)

a few key considerations: water depth affects light penetration, fossil materials are often pelagic. Deeper, colder water is more acidic, there is a depth below which carbonates are not stable and an even greater depth below which settling carbonates do not accumulate.

Arc-trench system basins: complex system of basins related plate covergence and subduction.

a few key considerations: extensive tectonism and metamorphism makes these regions complex. Often associated with volcanic input. Basins range from extremely deep to not so deep, and may have either oceanic or continental material base. Sediments include mélanges and turbidites, to more fluvial, deltaic, marine as get closer to the continent.

Continental collision basins: basins that develop where continents converge

a few key considerations: include elements of ocean basins prior to convergence, such as ophiolite, and deposits related to the orogeny such as flysch and molasse deposits.

Basins in displaced terrains (exotic or “suspect” terrain):

Key considerations: are tacked onto the edge of a continent by plate movements and so have structural, stratigraphic, and paleontological discordances with the rest of the continent.

Divergent Grabbens: basins that develop during continental divergence

Key considerations: are often on presently-stable continental margins where past divergence of oceanic-basin-formation occurred. volcanics, intrusives common, interbedded with arkosic red beds. In arid climates, evaporates occur.

Intracratonic basins (regions of subsidence in the interior of stable continents)

Key consideration: Deposited in epicontinental seas (non-orogenic, shallow water), with very thick sediments grading laterally into much thinner sediments of similar type. (e.g. Williston Basin, Michigan Basin).

Sandstones and Conglomerates

Studied a lot because particles are big enough to see and study.

classified by particle size and type (composition) of particle

Particle Size (phi scale = -log base 2 of particle size in mm-so 1mm = 0, 1/2mm = 2, 2mm = -1 etc)):

Gravel = 2mm to 4096 mm (-1 phi to –12 phi) (granule, pebble, cobble, boulder)

Sand = 0.062mm to 2 mm (4 phi to –1 phi) (fine, medium, coarse)

Silt = 0.004mm to 0.062mm (8 phi to 4 phi) (fine, medium, coarse)

Clay = 1? D ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download