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External Crystal Form

The crystal form of a mineral is a set of faces that have a defi-nite geometric relationship to one another. What most people call a "crystal" is a mineral with well-developed crystal faces. To geologists a crystal usually means any homogenous solid that is crystalline (with or without crystal faces). If minerals were always able to develop their characteris-tic crystal forms, mineral identification would be a much sim-pler task. In rocks, however, most minerals grow while competing for space with other minerals. In fact, the orderly sets of faces that make up a crystal form can develop only under rather specialized conditions. Specifically, most miner-als are able to develop their characteristic crystal faces only if they are surrounded by a fluid that can be easily displaced as the crystal grows. On the other hand, a few minerals, notably garnet, are able to overpower and displace surrounding solid material during growth, so that they almost always develop their characteristic crystal faces. Some common minerals with well-developed crystal forms are shown in figure 2.15. Minerals displaying well-developed crystal faces have played an important role in the development of chemistry and physics. Steno, a Danish naturalist of the seventeenth century, first noted that the angle between two adjacent faces of quartz is always exactly the same, no matter what part of the world the quartz sample comes from or the color or size of the quartz. As shown in figure 2.16, the angle between any two adjacent sides of the six-sided "pillar" (which is called a prism by mineralo-gists) is always exactly 120°, while between a face of the "pil-lar- and one of the "pyramid" faces (actually part of a rhombohedron) the angle is always exactly 141°45'. The discovery of such regularity in nature usually has pro-found implications. When minerals other than quartz were stud-ied, they too were found to have sets of angles for adjacent faces that never varied from sample to sample. This observation became formalized as the law of constancy of interfacial angles. Later the discovery of X-ray beams and their behavior in crys-tals confirmed Steno's theory about the structure of crystals. Steno suspected that each type of mineral was composed of many tiny, identical building blocks, with the geometric shape of the crystal being a function of how these building blocks are put together. If you are stacking cubes, you can build a structure having only a limited variety of planar forms. Likewise, stack-ing rhombohedrons in three dimensions limits you to other geo-metric forms (figure 2.17).

I IldIf ler lallUldf I. H WIC/ LS SflOW f IOW UL1Ueb Can Ue SLEICKeCI101" C;UUIC ana dodecahedral (12-sided) crystal forms. C and D show the relationship of stacked rhombohedrons to a "dog-tooth" (scalenohedron) form and a rhombohedral face.

Steno's law was really a precursor of atomic theory, devel-oped centuries later. Our present concept of crystallinity is that atoms are clustered into geometric forms—cubes, bricks, hexa-gons, and so on—and that a crystal is essentially an orderly 3-dimensional stacking of these tiny geometric forms. Halite, for example, may be regarded as a series of cubes stacked in three dimensions (figure 2.18). Because of the cubic "building block," its usually crystal form is a cube with crystal faces at 90° angles to each other.



Cleavage

The internal order of a crystal may be expressed externally by crystal faces, or it may be indicated by the mineral's tendency to split apart along certain preferred directions. Cleavage is the ability of a mineral to break, when struck, along preferred directions. A mineral tends to break along certain planes because the bonding between atoms is weaker there. In quartz, the bonds are equally strong in all directions; therefore quartz has no cleav-microscopes (for example, figure 3.3B). Explaining optical phe-nomena, such as this, is beyond the scope of this book but, if interested, you can go to the Molecular Expressions Microscopy Primer site http://micro.magnet.fsu.edu/primer irtualpolarized.html. Specialized equipment is needed to determine some prop-erties. Perhaps most important are the characteristic effects of minerals on X rays, which we can explain only briefly here. X rays entering a crystalline substance are deflected by planes of atoms within the crystal. The X rays leave the crystal at pre-cise and measurable angles controlled by the orientation of the planes of atoms that make up the internal crystalline structure (figure 2.26). The pattern of X rays exiting can be recorded on photographic film or by various recording instruments. Each mineral has its own pattern of reflected X rays, which serves as an identifying "fingerprint."

 


Chemical Tests

One chemical reaction is routinely used for identifying miner-als. The mineral calcite, as well as some other carbonate miner-als (those containing CO3-2), reacts with a weak acid to produce carbon dioxide gas. In this test, a drop of dilute hydrochloric acid applied to the sample of calcite bubbles vig-orously, indicating that CO2 gas is being formed. Normally this is the only chemical test that geologists do during field research. Accurate chemical analyses of minerals and rocks are done in labs using a wide range of techniques. A chemical analysis can accurately tell us the amount of each element present in a mineral. However, chemical analysis alone cannot be used to conclusively identify a mineral. We also need to know about the mineral's crystalline structure. Diamond and graphite have an identical composition but very different crystalline structures.

microscopes (for example, figure 3.3B). Explaining optical phe-nomena, such as this, is beyond the scope of this book but, if interested, you can go to the Molecular Expressions Microscopy Primer site http://micro.magnet.fsu.edu/primer irtualpolarized.html. Specialized equipment is needed to determine some prop-erties. Perhaps most important are the characteristic effects of minerals on X rays, which we can explain only briefly here. X rays entering a crystalline substance are deflected by planes of atoms within the crystal. The X rays leave the crystal at pre-cise and measurable angles controlled by the orientation of the planes of atoms that make up the internal crystalline structure (figure 2.26). The pattern of X rays exiting can be recorded on photographic film or by various recording instruments. Each mineral has its own pattern of reflected X rays, which serves as an identifying "fingerprint."


Date: 2016-01-03; view: 1214


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