Geo331 lect5 - Lecture 5 Crystallography I Internal...

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Lecture 5: Crystallography I. Internal Arrangement: It's what's inside that counts All minerals (and therefore most gemstones) are crystalline and have a unique crystal structure. They are composed of an orderly structure of molecules. This internal structure influences the external shape of the crystal. Not all crystals are valuable, however. Halite (table salt) is a crystalline material, as is quartz - the major component of beach sand. The crystalline shape of a mineral depends on its chemical constituents. The chemical composition of a mineral is always the same, although it can have minor variations. The crystal structure of a particular mineral is often the same - some minerals can have multiple crystal shapes, referred to as polymorphs (graphite and diamond, for example). A pseudomorph is a mineral that (usually due to disolution of the original mineral) forms in the shape of another crystal. A hematite pseudomorph after pyrite is a cube of hematite that has predipitated into a cavity that was formed when pyrite dissolved away. Pseudomorphs do not exhibit the true crystal form, but are more like "molds" that fill an available space. The shape of a crystal gives us important clues to the internal composition of a mineral. As an example we will use the relatively cheap mineral Halite (although in many regards, large halite crystals are very pretty). As discussed previously, Halite is made of Sodium (Na+) and Chloride (Cl-) ions. The orderly arrangement of sodium and chloride ions gives halite it's characteristic shape. By stacking many Na and Cl ions together in 3 dimensions, the internal structure of Halite forms a cube pattern. If we look at larger halite crystals, we notice that they form cubes. This is not a coincidence. The internal arrangement of Na and Cl directly influences the large-scale shape of halite crystals. These salt crystals were grown in petri dishes by slowly evaporating water with lots of dissolved table salt. A large halite crystal.
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Table salt crystals under high magnification. Note the cubic shape in all three cases. Not all minerals have such an obvious correlation between molecular appearance and crystal shape. Quartz, for example, is a complex lattice of Silicon and Oxygen ions. While this may not look like much, if left to grow unimpeded, quartz will always form six-sided crystals ending in a point (hexagonal prism).
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