Albite

Albite is sodium feldspar and one end of the plagioclase series. The name is from the Latin for white, which describes most specimens fairly, though it is the commonest feldspar to occur colourless and glassy as well.

It sits at a boundary in the classification that is genuinely awkward: albite is both the sodium end of plagioclase and, at low temperature, a member of the alkali feldspar series with potassium. Which family it belongs to depends on what it formed alongside, and mineralogists have simply accepted that it belongs to both.

Albite crystallises late from cooling granitic magma, once earlier minerals have removed the calcium, and it is abundant in pegmatites and in the sodium-rich rocks that result. It also forms during low-grade metamorphism, where it replaces calcium-rich plagioclase as a rock is altered by hot fluids.

That replacement is common enough to have a name — albitisation — and it is a useful marker: a rock whose plagioclase has been albitised has had sodium-bearing fluid pass through it, which usually means it sat near a hydrothermal system.

Albite is mined with the other feldspars as a ceramic and glass flux, and its sodium content is preferred for some glazes because it melts at a slightly lower temperature than potassium feldspar.

It has no significant use distinct from the group. Gem-quality material occurs but is rare, and the iridescent varieties of the wider plagioclase series are more sought after.