Aragonite (#32)

Lustrous to dull opaque white botryoidal and crystal plate of aragonite.
North America Mercur Mine, Tooele County, Utah, USA 6 ¼" x 4 ¼" x 2"

 

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Aragonite Crystal Structure


Calcite Crystal Structure

Of course, at this scale you can't discern many details, but you can certainly see that they are different. The arrangements of atoms are different, and the unit cells (outlined in black) are different. Yet the chemical formula for both aragonite and calcite is CaCO3, calcium carbonate. Minerals that have the same composition but different crystal structures are called polymorphs, which literally means "many forms." Many minerals have polymorphs, and which form occurs depends on the specific conditions of pressure, temperature, and perhaps sometimes other variables. Thus the presence of one polymorph or another in a rock gives geologists a clue about the conditions under which the rock formed. Aragonite is a high pressure form of CaCO3, but it takes so much energy to rearrange those atoms from one structure to the other that aragonite persists at the earth's surface. It is thus not stable, but is called "metastable."