1. An electron microprobe analysis shows that a mineral contains one Ti atom and four oxygen atoms for every two iron atoms.
(a) What is the valence of the Fe in this mineral?
(b) Which small circle represents its composition on the triangular diagram?
(c) On the triangular diagram above, identify the small circles that mark the positions of any three additional minerals and provide their names and formulas.
(d) Use a line to link two compositions on the diagram that show solid solution at relatively high temperatures.
2. Color is a somewhat unreliable clue to the identity of a mineral. Explain in a couple of short sentences (use a diagram, if necessary) one possible origin of red color in a mineral. Be sure to identify one possible structural or chemical reason for the color as well as the way in which specific wavelengths of light are affected.
3. (a) Neatly draw I-beam representations of:
(i) a 2:1 layer silicate
(ii) a pyroxene
(iii) an amphibole
(b) mark onto your diagrams the locations of the cleavages for these structures
(c) give the formula and name of a 2:1 layer silicate that contains only Mg, Si, O, H.
Formula:
Name:
(d) On the I-beam representation of the amphibole, identify the portions of the structure that are essentially common to layer silicates.
4. The 3 polymorphs of Al2SiO5 are kyanite, sillimanite, and andalusite. What property of andalusite and sillimanite can cause the transition of andalusite <=> sillimanite to have a negative slope on an equilibrium phase diagram of pressure vs. temperature? What geographic area(s) was(were) mentioned in class where one can collect andalusite and sillimanite but not kyanite?
5. Name two quartz (SiO2) polymorphs that are related by a reconstructive transformation.
6. (a) Briefly explain how X-rays are generated in an X-ray tube. Use a diagram to illustrate, if needed.
(b) Sketch two horizontal planes of atoms. Show the basic geometry needed to satisfy Bragg's law. Include two incoming X-rays, showing that they are in phase. Be sure to mark the interplanar spacing, q, l, and the path difference.
(c) Bragg's law is:
7. (a) How does a pyroxenoid differ from a pyroxene?
(b) Name one pyroxenoid:
(c) and give its chemical formula:
8. (a) What is the general formula for garnet?
(b) Name 5 elements other than Si and O commonly present in garnets?
9. On the triangular (ternary) diagram below, fill in the appropriate carbonate minerals (with their names) and indicate, schematically, the ranges of compositions that you would expect to find in nature. Remember to use what you know about other mineral groups (i.e. amphiboles, pyroxenes) to help you here.
10. What charge would you assign to the following atoms or species in common rock-forming minerals?
What type of coordination / site do the cations occupy in common silicate minerals? (if an element occurs in more than one type of site, give the most common)
CHARGE SITE
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(a) Si=
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(b) Al=
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(c) Mg=
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(d) Fe=
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(e) Li=
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(f) K=
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(g) Ca=
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(h) Ti=
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11. (a) What is the chemical formula for olivine?
(b) What type of silicate is it?
(c) What elements are commonly found in the octahedral site(s)?
12. (a) Complete the pattern below to generate a two dimensional array of atoms that would be seen looking down [100] of a face centered cubic mineral (this should be a projected view of the crystal structure, not a three dimensional representation. Make sure you draw this carefully and neatly, as you are going to have to add a lot of information!
(b) Indicate the face-centered unit cell. Label the axes and indicate the angle between them (is it a, b, or g?)!.
(c) Mark onto the left hand part of your diagram the location of two different symmetry elements that involve rotational axes and the location of a mirror and a glide plane (where the plane is normal to the page).
(d) Onto the right hand part of your diagram, show the following:
(a) (100) planes (indicate the family of planes by marking several)
(b) (210) planes ( " " " " " ")
(c) [120]
13. In a couple of sentences AND a phase diagram explain the consequence of cation ordering on the symmetry and phase relations of the alkali feldspars.
14. Identify each of the following series of interference figures (at different positions of stage rotation) as biaxial (Bxa, Bxo, optic normal, or optic axis) or as uniaxial (centered O.A., off-center O.A., or flash). Also determine if the mineral is positive or negative. The + and -’s indicate that upon insertion of the accessory plate, retardation colors add (+) or subtract (-).