Orthoclase
The potassium feldspar of granite — the most abundant mineral group in the Earth's crust, and the raw material of porcelain.
Feldspar is not one mineral but a group, and that group makes up more of the Earth's crust than anything else. Orthoclase is the potassium end of it, and it is the pink or cream mineral that gives much granite its colour.
Its hardness of 6 is the reference point on the Mohs scale, which is a fair reflection of how ordinary it is: hard enough to scratch glass, soft enough to be scratched by a steel file. The name means straight fracture, describing cleavage planes that meet at right angles — the property that most reliably separates it from quartz, which has no cleavage at all.
How to identify it
Two cleavage directions meeting at ninety degrees, giving flat reflective faces when a specimen is turned in the light. Quartz, its usual companion in granite, breaks in curved conchoidal fractures instead and never shows a flat face of that kind.
Colour is suggestive rather than diagnostic: orthoclase is commonly pink, cream or white, where plagioclase feldspar tends to grey and often shows fine parallel striations that orthoclase lacks.
Uses
Feldspar is consumed in large tonnage by the ceramics and glass industries, where it acts as a flux — it melts at a lower temperature than the silica it accompanies and pulls the rest of the batch into a melt at a workable temperature. Porcelain depends on it.
Ground feldspar is also a mild abrasive in scouring powders, and a filler in paint and plastics. Weathered feldspar is the origin of most clay, which makes it the indirect ancestor of brick, pottery and porcelain alike.
How it forms
Orthoclase crystallises from cooling magma, and it is one of the last major minerals to do so, filling the space between earlier crystals. It is characteristic of granite and of the coarse-grained pegmatites where individual crystals can reach a metre.
It is also unstable at the surface. Water and dissolved carbon dioxide slowly convert feldspar to clay minerals, releasing potassium and silica — a reaction that is one of the principal ways the crust weathers, and which consumes atmospheric carbon dioxide over geological time.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
is a component of
is a source for
- Clay — material · clay is chiefly what feldspar becomes when it weathers
contains
belongs to the group
- Feldspar — mineral · the common potassium feldspar of granite
has the same composition as
- Microcline — mineral · identical composition; microcline is the ordered low-temperature form and is triclinic where orthoclase is monoclinic Wikidata
is commonly confused with
- Microcline — mineral · identical composition and indistinguishable in hand specimen; separated by cross-hatched twinning under crossed polarisers
- Plagioclase — mineral · the commonest feldspar identification problem; fine parallel striations on a cleavage face indicate plagioclase
has variety
- Moonstone — mineral variety · an intergrowth of orthoclase and albite in layers the thickness of a wavelength of light, which is what produces the glow
Sources
- Material WorldOur own writing
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- COD Advisory Board / Vilnius University · CC0 — contributors place data in the public domain