Question

A mineral specimen has very large crystals. Where did it most likely form?

Answer 1

Large open caves or igneous pegmatites are the most likely suspects.

Explanation:

Very large gypsum crystals, larger than a person in fact, have been found in secret caves in Mexico (see pic., note person at bottom right for scale). This site must have had an ongoing source of chemical fluids to grow the crystals and the open cavity, allowed the crystals to grow mega large!

Pegmatites are the other kind of rock that forms large crystals, but in this case we are talking a few centimetres or few inches in size. (pic 2). Pegmatites usually form as the last stage of cooling in a granitic magmatic melt and are usually fluid rich. See pic. 3.