Feldspar

Feldspar — the crust's default mineral

Feldspars make up more of Earth's continental crust than any other mineral family. The group splits into two branches: the alkali feldspars — orthoclase and microcline, including the turquoise-green variety amazonite — and the plagioclase series running from sodium-rich albite to calcium-rich anorthite.

The collector varieties

Collectors mostly meet feldspar through its show-off varieties: amazonite's saturated blue-green, moonstone's billowy adularescence, and labradorite's electric flash of blue-gold iridescence (labradorescence) caused by light interference within microscopic exsolution layers.

Identification notes

Two cleavage directions at roughly right angles and hardness 6 are the quick field tests; plagioclase often adds fine parallel striations on cleavage faces that alkali feldspars lack.