ARSC/GEOL 2110
Physical Science of the Earth System
Assignments
Fall 2001

Lab report is due Thursday, November 7, 2001.

Useful resources: Ch. 6 pp. 112-125, and Ch. 7, pp. 149-151, Blue Planet; Appendix C

1. Summarize your findings from Part I of the crystal growth lab in one or two sentences. (Hint: they will need to be well-crafted, information-packed sentences!)
(example) "Samples cooled quickly, such as over ice which rapidly removed the heat, formed small crystals because the molecules solidified before they had time to organize into extended, ordered, crystalline networks. Samples cooled more slowly, such as at room temperature, formed larger crystals because the molecules retained heat energy longer and could therefore move around and organize into larger crystalline networks."

2. Describe the experiment your group came up with in Part II, using the following format:

a) State your question or hypothesis (1 sentence).
Individual answers will vary. Should have a clear and testable hypothesis or question based on earlier experiments and/or other knowledge. For example: "We hypothesized that a sample cooled by placing it over hot water would form even larger crystals, because it would cool more slowly than either of the two samples already observed."

b) Summarize what you did to investigate your question or hypothesis. (1-3 sentences)
Individual answers will vary. Should state clearly what you did in a way that makes it clear why that process makes sense given your hypothesis. for example: "In order to cool the sample even more slowly, we placed the dish on a bath of hot water and observed the growth of crystals as it slowly cooled to room temperature. We then compared the crystal size to the crystals formed by fast cooling on ice and medium-rate cooling at room temperature."

c) Summarize your results and interpret them (what you found out from your experiment and what it means). (2-4 sentences)
Individual answers will vary. Should state clearly what you found out and whether it agreed or disagreed with your hypothesis, and why you think so. For example: "When cooled slowly over hot water, the crystals that formed were larger than the crystals formed on ice or at room temperature. We could see large, shiny facets that were 1-2 mm across or even larger. The larger crystals formed because the molecules stayed warm longer and had enough energy to move around a little to attach themselves to the ordered crystalline network. Instead of many small crystals, the crystals that formed were fewer in number but larger in size."

3. How do laboratory experiments on growing crystals help interpret whether a natural igneous rock is intrusive or extrusive? That is, what's the connection between the lab observations and observations of field samples? Explain.
By growing crystals in the lab, we can test the idea that large crystals form when the sample is cooled slowly, and small crystals form when the sample is cooled rapidly and compare our findings to real samples found in nature. The observations in the lab are consistent with the differences in field samples. Igneous rocks with small crystals, such as basalt, are cooled more quickly, close or at the Earth's surface where air and water can remove the heat faster. Igneous rocks with large crystals are those that are formed underground, such as granite, where the heat is removed only slowly by the neighboring rocks, and thus there is time for large crystals to grow.

4. Explain why, when the elements are combined into ionic compounds, the element Na tends to form an Na⁺ cation but the element O tends to form a O^2- anion.
Na tends to form Na⁺ ion when it ionizes because it has 11 electrons, one more than the closest noble gas, Ar. By losing one electron to become a sodium cation, the ion takes on the same electron shell arrangement as Ar, with two filled shells, a particularly low-energy and stable arrangement. Oxygen, on the other hand, tends to gain two electrons to form an O^2- anion. O has 8 electrons of its own, so by gaining two electrons it too has 10 electrons in the same shell arrrangement as Ar.

5. Why do cations and anions alternate in space in ionic compounds such as NaCl or Cu₂O?
Cations and anions are attracted to each other by their opposite electrical charges, positive for cations and negative for anions. By alternating in space, these attractive forces are maximized and a stable structure with an overall low energy (therefore favorable) is formed. Anions can't easily be placed next to anions, or cations next to cations, because the repulsive forces between them will tend to push them apart-a high-energy, unstable arrangement. Cations and anions thus naturally "find" the alternating structure as the lowest-energy and thus most stable or most energetically favorable arrangement.

6. How does the microscopic (atomic-scale) crystal structure affect the macroscopic (ordinary scale) physical properties of a mineral? Explain, using the forms of carbon as an example.
Carbon is a good example of how the microscopic structure affects the macroscopic properties. Graphite has a structure of planar layers of covalently bonded carbon atoms in a six-cornered (hexagonal) geometry. The bonds between layers are much weaker than the covalent bonds within the structure, so graphite tends to cleave between layers easily and is soft- it can be rubbed off on paper with light pressure, such as by a pencil. Diamond, on the other hand, is hard and not easily cleaved due to its 3-D network of interlocking covalent bonds. Because the network is the same in all directions, no one direction is easier to cleave than any other. This tightly packed 3-D network also makes diamond more dense than graphite, which has large empty space between layers. Buckyballs, with their spherical network of carbons, would surely have different properties than both diamond and graphite. They might be expected to be cleaved between balls rather than through balls. They are likely not very dense, due to lots of open space within the spheres. Perhaps they might make a good lubricant, as they can 'roll' or spin within the crystal structure (spheres are not locked together). They also might be good at "trapping" other atoms inside them, e.g. absorbing other substances like a sponge.

7. Explain why silicate (SiO₄^4-) ions can form such a variety of mineral structures (chains, sheets, 3-D networks, etc.).
Silicate ions take the shape of tetrahedra, 4-cornered pyramids with silicon in the middle and oxygen atoms at each corner. The Si and O atoms are covalently bonded together. Because any of the oxygen atoms can be shared with another tetrahedron, so that it forms the corner of both tetrahedra, the tetrahedra can be linked together in an extended network of tetrahedra connected by O-Si-O-Si-O chains. If two of the oxygen corners of a given tetrahedron are shared with neighboring tetrahedra, a chain is formed (such as in pyroxene). If some of the tetrahedra share oxygens and form connections at three corners, double chains are formed (e.g. amphibole), and if all of them are connected at three corners, a sheet is formed (such as in mica). If all four corners of the silicate tetrahedron are shared with neighboring tetrahedra, a strong 3-D network of tetrahedra is formed (such as in quartz, crystobalite, or feldspar). The 'open' faces of the silicate ion are where the cations will collect so they can interact with the overall negative charge on the silicate ion. In a structure with the silicate tetrahedra sharing no corners, the silicate anion and metal cations will alternate similar to a simple ionic compound.

8. At the microscopic level, what's the difference between a rock composed of crystalline minerals, such as granite, and a glassy rock such as obsidian? Explain.
Granite is made of small crystals where the minerals are organized into ordered networks with regular spacing and repeating organization of the cations and anions in the mineral. Obsidian is a disordered solid without regular or repetitive crystalline structures-the ions are jumbled together. Obsidian forms when magma solidifies very quickly so that no crystals have time to form.

9. Did using the models help you learn? Describe one or two specific concepts or ideas that were clarified for you by working with the crystal models, or if no ideas were clarified, describe one or two specific concepts that you are having difficulty with.
Answers will vary.