One of the most abundant minerals on earth--quartz--occurs in an incredible number of varieties. Picture the luxurious violet of an amethyst, or the colorless quartz crystals as familiar as they are common. One may just as accurately picture beach sands, granite, and chert. These too are composed of mineral quartz, though we may not as readily associate such structures with the mineral. With an exploration into the Museum's collection of geology specimens, it becomes apparent that many quietly contain quartz.
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This microcrystalline quartz formation will
be on display for the month of February. |
Take, for example, one of our microcrystalline quartz formations. This Specimen of the Month might not inspire immediate awe but in reality it's a rather interesting item. The coarse, grainy outside obscures an intriguing inner surface. The specimen formed by quartz precipitating in layers--and hollow for reasons currently unknown. Perhaps it filled in an odd cavity or replaced a stalactite. A closer look reveals bands of agate, and much, much closer, a microscopic crystal structure.
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A close look at our hollow specimen, but not close
enough to view its microscopic crystalline qualities! |
In contrast, an amethyst is visually recognizable as a macrocrystalline quartz form--the unaided eye can pick up on noticeable crystals. The terms "microcrystalline" and "cryptocrystalline" both refer to formations, such as our aforementioned specimen, with aggregates of crystals only visible under higher magnification. A number of common formations are comprised of such quartz structures.
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| Amethyst--a macrocrystalline quartz. |
Two main groups of these microcrystalline formations are chert and chalcedony. Chert, flint, and jasper are all terms used for similar specimens, however "chert" is most widely used by geologists as a blanket term. The blanket term "chalcedony" refers to another category of quartz formations, such as agates. Oftentimes with agates we can observe banding--beautiful layers of different colors and thicknesses. Banded agate may be viewed on parts of the Specimen of the Month.
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Two examples of microcrystalline quartz. Above, bedded chert
with jasper. Below, an agate specimen from the collection. |
Specimens of quartz have captured our attention since early human history and being that quartz occurs in most rocks, it's nearly impossible to overlook. Whether we find value in the macro- or microcrystalline varieties, the many formations are truly intriguing.
