Feldspar

Feldspar is not one mineral but a family, and that family makes up more of the Earth's crust than everything else combined — roughly six parts in ten. Any rock you pick up in most of the world is likely to be mostly feldspar by volume, whatever else it contains.

The group divides on which large cation sits in the structure. Potassium gives the alkali feldspars, of which orthoclase and microcline are the common members; sodium and calcium give the plagioclase series, which runs continuously between the two. All of them share the same framework of linked silicon-aluminium tetrahedra, which is why they share a hardness, a density range and two cleavage directions meeting at close to a right angle.

Two good cleavage directions meeting at or near ninety degrees, giving flat reflective faces when a specimen is turned in the light. That is the family trait, and it is what separates feldspar from quartz, its constant companion in granite: quartz has no cleavage and breaks in curved conchoidal fractures.

Telling the members apart is harder and often needs more than a hand lens. Fine parallel striations on a cleavage face indicate plagioclase and are absent in orthoclase. A cross-hatched pattern under polarised light is diagnostic of microcline. Colour is suggestive at best — pink usually means potassium feldspar, white or grey usually means plagioclase, and both have exceptions.

Feldspar crystallises from cooling magma and is a major constituent of nearly every igneous rock. Which member forms depends on the melt's composition and on temperature: calcium-rich plagioclase crystallises first and hottest, and the crystals grow progressively more sodium-rich as the melt cools, often leaving a single zoned crystal recording its own history.

At the surface feldspar is unstable. Water and dissolved carbon dioxide convert it slowly to clay minerals, releasing potassium, sodium and calcium into solution. That reaction is among the principal ways the continents weather, it produces most of the world's clay, and over geological time it consumes atmospheric carbon dioxide.

Glass and ceramics take almost all commercially mined feldspar. It acts as a flux — melting at a lower temperature than the silica it accompanies and pulling the rest of the batch into a workable melt — and it supplies the alumina that gives the finished body its strength and durability. Porcelain is impossible without it.

Ground feldspar is also a mild abrasive in scouring powders and a filler in paint, plastics and rubber. Its indirect importance is larger still: as the parent of clay it stands upstream of brick, pottery and every ceramic there is.