GEO 2301: Mineralogy                                             Name:

Fall 2003

Whitney

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Information related to this key:

The answers I have given below are examples of ones I accepted for full credit. There are other possibilities for some questions. I also give partial credit if what you wrote/drew made some sense in relation to that particular question. I have indicated for each question below some additional information about the question or topic, including the context/class in which each topic was discussed.


Quiz #1

 

Please write neatly and concisely in the space provided. Your answers will be graded based on correctness and clarity. This quiz is closed-book, closed-notes, closed-electronic devices, but you may ask me questions during the quiz if you need help understanding a question.

 

1. (a) In the list below, cross out anything that is NOT a mineral. Next to each crossed out word, briefly explain WHY it is not a mineral.

 

Diamond

 

Graphite

 

Glass : does not have an ordered internal structure (not crystalline)

 

Quartz

 

Wood: not inorganic (or, you could write that it is organic)
    wood does have a definite internal structure and a definite composition (within certain limits, must like minerals)

 

Oil: not a solid (also it is organic)

 

Clam shells (aragonite)

 

 

(b) In the list above, circle the minerals that have polymorphs (that is, that may experience the process of polymorphism). Define polymorph:

 

I put the polymorphs in bold type above. All the minerals in the list are polymorphs: graphite and diamond are polymorphs of each other. Quartz has many polymorphs (see the handout ). Calcite and aragonite are polymorphs (see the handout ).


Polymorphs are minerals that have the same composition but different internal structures.

 

 

2. For each environment below, describe the most important driving force for crystallization of minerals. Be specific about what variable changes and how it changes to create crystals.


see review questions from 9/4 webpage


(a) a shallow lake in a desert

 

  evaporation of water, increasing the concentration of dissolved solids
(or something about saturation/supersaturation)

 

(b) a hydrothermal vent at an oceanic ridge

 
minerals precipitate when heated seawater containing dissolved minerals cools quickly as it encounters cold seawater

 

(c) lava erupting from a volcano


minerals crystallize when hot lava cools quickly (you could also mention the decrease in pressure, but the cooling is the main driving force)

 

3. Draw and compare the crystallographic axes for the (a) orthorhombic and (b) tetragonal crystal systems. Label your drawings well enough so that I know what the angles and relative lengths of axes are in each system.

 

(a) orthorhombic: all axes (a, b, c) of different lengths and all angles at 90 degrees


(b) tetragonal: a = b but not equal to c, and all angles at 90 degrees

 

see 9/11 web review page (" What are crystallographic axes? Know how the axes are defined for each crystal system.")


4. All of these questions have the same general answer:

Write your general answer (just one answer that covers all those questions, not 4 separate answers):

 

  Possible answers:
- There are only so many ways that the symmetry elements can combine to create ordered structures
- There are a limited number of stable structures than can be created by the ordering (stacking, arrangement) of unit cells
- Or, you could make reference to the principle of parsimony (Pauling's 5th rule), if you described why nature is parsimonious.


There were review questions similar to this one on 9/9, 9/11, and 9/30

 

 

5. What is the symmetry of a perfect tetrahedron , expressed in Hermann-Mauguin symbols? If you have trouble picturing this, draw a tetrahedron and list as many symmetry elements as you can (drawing is optional).

 

A tetrahedron is an isometric shape: all axes are of equal length. Think about the Si-O tetrahedron, which you have now seen many times; for example,the handout illustrating coordination , or the handout  illustrating Pauling's rule about corner sharing , or the handout from October 3 showing Al-Si ordering in tetrahedra. The tetrahedron is an important shape in geology because it is the geometric shape that is the framework for silicate minerals.


I gave partial credit for drawings of a tetrahedron and/or for recognition of some symmetry elements (4-fold axis, mirror planes) -- if you got most of these but just didn't get the Hermann-Mauguin symbols correct, you only lost 1 point.


The correct answer is bar4 3m.

 

6. Using the symbol below, show how it would move on the page if affected by a glide that is oriented in this direction: à (that is, parallel to the short dimension of the page). Repeat the pattern enough times to show the action of the glide.

 

 

                     

 

  a glide is a combination of translation and reflection, so the triangles below the glide must be flipped over, as if reflected in a mirror

 

 

7. What is a Bravais lattice? (in general, you do not need to describe any particular lattice)

 

Possible answers:


- 3-d unit cell
- 3-d unit cell that can be stacked to create crystal structures
- 3-d structure produced from stacking the 2-d plane lattices
- one of the 14 unique crystal lattices


see web review page for 9/16 : "What is a Bravais lattice?"

 

 

8. If a crystal face intersects only the c-axis of an isometric mineral, what are the Miller indices for that face? Put your answer in the parentheses (the convention for reporting Miller indices).

 

                                    (  001  )

 

 

9. (a) Why is the metallic radius of an ion different from its effective ionic radius ?

 

  The metallic radius is the radius of an ion bonded to the same kind of ion (optional: mentioning that the ions are in a close-packed structure, CN = 12); the ionic radius is the radius of an ion bonded to a different ion (optional: mentioning that the actual radius will vary with coordination number).

 
see Tuesday, 9/23 web review page: "What is the relation between "metallic" radii for elements and "ionic" radii? (know how and why they vary from element to element."

 

(b) What is the most common coordination number for silicon ions?

 

              4  (think silicon-oxygen tetrahedron) - this is an important relationship to know in geology; you need to know that Si is almost always in 4 coordination

for (b) and (c); see 9/25 web review: What are the most common coordination numbers in minerals? (know what geometrical arrangement) corresponds to each coordination number 

(c) What is the shape of a coordinating polyhedron for a cation with a coordination number of 8?

(you don’t need to draw it, just write the word describing the shape)

 

                cube (cubic)

 

10. (a) Explain which would have higher symmetry (that is, be more symmetrical): a high-temperature feldspar showing Al-Si disorder, or a low-temperature feldspar with Al-Si ordering.

 
The disordered, high-T feldspar will have higher symmetry because disorder creates more possibilities for symmetry, whereas order restricts the possibilities. In a disordered state, elements are more randomly distributed -- that is, elements show less of a preference for particular sites. Refer to the handout illustrating Al-Si order-disorder, and see how the 2-fold axis is present in the disordered and partially ordered examples, but disappears in the ordered example.

 

 

(b) If kyanite (high pressure polymorph) and andalusite (low pressure polymorph) are related to each other by a reconstructive transformation, will you ever find both minerals in the same rock, preserved at the Earth’s surface? Explain your answer.


I accepted a variety of answers for this question, depending on how you interpreted the question.


One possible answer was to treat the question as asking whether it is possible to have a high-pressure polymorph in a rock at the Earth's surface (that is, a mineral stable under high pressures present in a low pressure environment): the answer to this is yes, it can exist metastably because the kyanite to andalusite transformation is reconstructive and therefore requires breaking bonds and lots of energy to do this. The situation is analagous to why we have diamonds (high pressure polymorph of graphite) at the Earth's surface.


I accepted a variety of other answers, with partial credit amount depending on whether your answer made geological sense. The best answers used the characteristics of reconstructive transformations (breaking bonds, energy required to do this) to interpret the above description of a possible rock with two polymorphs.


See 9/30 web review