Fluorite on Quartz ( #3)

Lustrous gemmy purple dodecahedrons to 1 1/2" on crystalline white massive quartz.
Asia De'an, Jiangxi Province, China 5" x 3 ¾" x 1 ¾"


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This is the arrangement of atoms - the crystal structure - of fluorite. It is a very simple structure. The formula is CaF2. The blue spheres represent calcium and the green ones represent fluorine. Because fluorine is one of the elements known as halogens (Group VII in the Periodic Table), fluorite is in the family of halide minerals. Halides include halite, NaCl - common table salt. Fluorite and halite, however, have completely different crystal structures - same chemical family, different structures.

Fluorite Crystal Structure


Quartz Crystal Structure


The crystal structure of quartz, SiO2, at approximately the same scale as the one for fluorite above. Note that even though both fluorite and quartz have formulas of the sort XY2, their structures look nothing alike. The small blue spheres represent silicon atoms, and the large red ones represent oxygen. Notice how different the relative sizes of Si and O atoms are, compared to the relative sizes of Ca and F. The relative sizes of atoms in a mineral have a lot to do with the kind of crystal structure that forms.