Geos 306, Mineralogy Final Exam, Dec 12, pts

Transcription

1 Name: Geos 306, Mineralogy Final Exam, Dec 12, pts 1. (9 pts) What are the 4 most abundant elements found in the Earth and what are their atomic abundances? Create a reasonable hypothetical charge-balanced chemical formula that uses these four elements in roughly the same proportions as found in earth.

2 2. (5 pts) Suppose the illustrated figures represent closest-packed arrangements of anions. Identify the different locations where cations could be coordinated with 2, 3, 4, 6, or 12 anions. In the position of the cation, write its coordination number. (4 pts) Label the packing type for each of the following diagrams.

3 3. (20 pts) Magnetite, Fe 3 O 4, is an important spinel, both because it is an important geological phase on earth, mars, and other parts of the universe, but also because of its high-tech industrial properties useful in magnetic storage devices, for instance. Remember that the spinel structure is ccp anions with both octahedral and tetrahedral cation. Assume the following radii, r(o) = 1.35 Å, r(fe 2+ ) = 0.88 Å, r(fe 3+ ) = 0.54 Å a) Write a charge balanced chemical formula for magnetite by inserting the valence states into the formula Fe 3 O 4. b) Using Pauling s rules, argue the relative cases for Fe 3+ and Fe 2+ in octahedral or tetrahedral coordination. (ie. Which cation prefers tetrahedral and which prefers octahedral coordination). c) What are the relative occupancies of the octahedral and tetrahedral sites by Fe 3+ and Fe 2+. d) What is the tetrahedral FeO bond length? e) What is the octahedral FeO bond length?

4 4. (10 pts) From the microprobe analyses of a mineral collected from an alpine deposit in the Coastal Mountains of British Columbia, the following oxide weight percent were measured. Determine the chemical composition of this sample. Weight % SiO Al 2 O K 2 O Na 2 O 1.43 * 2 pt bonus, name the mineral.

5 5. (10 pts) Make a careful drawing and use it to derive Bragg s law for X- ray diffraction.

6 6. (20 pts) A basalt rock was found to be composed of 65% olivine, Mg 1.8 Fe 0.2 SiO 4, and 35% pyroxene Mg 1.4 Fe 0.5 Ca 0.1 Si 2 O 6. Construct a table of oxide weight percents that represent the total composition of the rock. Oxide Weight percent FeO MgO CaO SiO 2

7 7. (15 pts) Enstatite, MgSiO 3, is an orthorhombic mineral found in meteorites and as a major component of the upper mantle. The cell parameters from a fragment of the Bishopville meteorite were carefully measured as a = Å, b = Å, c = Å and Z = 8. Recall that Avogadro s number is mole -1 and that 10 8 Å = 1 cm. a) Determine the cell volume of this mineral. b) Determine the molecular weight of MgSiO 3. c) Determine its density in units of gm/cm 3.

8 8. (7 pts) Pyrite is a cubic sulfide mineral, FeS 2. The (200) peak was located in a diffraction experiment using Cu radiation, λ = Å, at 2θ = a) determine the d-spacing of (200) b) determine the cell parameters of pyrite

9 9. (20 pts) Sketch a diagram showing the structure of the earth. Include the surface and the center of the earth in your answer. Make a table and define the boundaries of each region by a) depth, b) temperature, and c) pressure. d) List the important mineral phases that are found in each of the different regions.

10 10. (20 pts) The image provided below is an electron microscopic picture of a sample of feldspar, of bulk composition ~50% K-spar, ~30% albite, ~20% anorthite. a. Identify each of the 3 feldspar phases in the image. b. Describe and explain their mixing properties. Provide your explanation at the atomic scale in terms of cation radii. Draw a ternary mixing diagram of the alkali cations that also shows the temperature effect. c. Name each of the different phases defined by Al/Si ordering in both the K- spars and albite. Describe the geologic conditions in which you would expect to find these phases.

11 11. (20 pts) For each illustration, provide representative mineral name and chemical formula for each.

12 5. The following diagram is the phase diagram for silica. a) (10 pts) Label the various regions. b) (10 pts) Suppose a diamond crystal was found with a coesite inclusion. Plot the geotherm onto the silica phase diagram and estimate the minimum depth that the diamond may have originated. Use the following table for your estimate of the geotherm: Depth (km) P (GPa) T (K) (note: 1 GPa = 10 kbar)

13 12. (5 pts) Perovskite exhibits the structure of an important high-tech material. What is the material, and describe a potential application of the property? 13. (5 pts) What is the geotherm?

14 14. (5 pts) What is the most abundant mineral in Earth? 15. (5 pts) Give the names and formula for 5 different pyroxenes.