Crystals

[sheilah14]

What is a type of quartz crystal that has taken on the cubic form of fluorspar crystals by a process of substitution?

[gen2]

pseudomorph

[brachiopod]

Cristobalite is the rare, cubic, high temperature polymorph of quartz, though it is meta-stable below something like 1400�C (off the top of my head).

Below these temperatures, it will tend to revert to the lower T� polymorphs, hexagonal �-quartz, or trigonal quartz ('low quartz').

Where a reversion to, say, a low T� polymorph occurs, but the mineral retains the crystal system of the high T� polymorph, it, as gen2 says, is called a pseudomorph.