feldspar

the family

Feldspars are framework silicates of aluminium with potassium, sodium or calcium. Nearly all of them are a mix of three end-members:

• orthoclase KAlSi₃O₈ — potassium. pink, salmon, cream. (microcline and sanidine are the same chemistry in different crystal structures.)
• albite NaAlSi₃O₈ — sodium. white, often tiny tabular crystals.
• anorthite CaAl₂Si₂O₈ — calcium. grey to white; the moon's bright highlands are mostly this.

Albite and anorthite mix freely as the plagioclase series; orthoclase and albite mix as the alkali series at high temperature. Potassium and calcium refuse to mix much at all.

name any feldspar

Click the triangle (or drag) to choose a composition.

field facts

• hardness: 6 on the Mohs scale. a steel knife just fails to scratch it, and it is the reference mineral for 6.
• cleavage: two good planes meeting at about 90°. the name comes from Swedish fältspat, "field spar," for the stones cleared from fields.
• twinning: plagioclase shows fine parallel stripes (polysynthetic twins) on its cleavage faces; microcline shows a tartan grid. a quick way to tell them from quartz, which has neither.
• perthite: a potassium feldspar that cooled slowly and unmixed into sodium-rich threads inside it.
• moonstone and labradorite: the shimmer is light scattering off microscopic layers formed as the crystal unmixed.
• amazonite: blue-green microcline, coloured by traces of lead and water.
• weathering: feldspar rots into clay. kaolin, the stuff of porcelain, is weathered feldspar.
• uses: ceramics and glass fluxes, enamel, tile glazes, and the white in a lot of toothpaste-adjacent powders.

where it is

Granite is mostly feldspar and quartz. Basalt is mostly plagioclase and pyroxene. Anorthosite, a rock that is nearly all plagioclase, forms much of the lunar highlands. Sand beaches are quartz because feldspar weathers away first; a feldspar-rich "arkose" sandstone means the rock was buried before it could rot.