Orthoclase

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Orthoclase is a polymorph of other minerals that share the same chemistry, but have different crystal structures. If positive identification between these minerals can not be made by field methods, then the specimen may simply be referred to as a potassium feldspar or K-spar. Plagioclase feldspars lack potassium, are light coloured and are usually striated. The other k-spar minerals are sanidine, microcline and anorthoclase. Orthoclase is the more common of the k-spars. Orthoclase does not show the lamellar twinning that is common in microcline and is occasionally present as striations on cleavage surfaces.

Orthoclase is the main k-spar of granites and syenites that cooled moderately quickly. Sanidine and anorthoclase are common constituents in extrusive igneous rocks such as rhyolites, where the rock cooled quickly. Optical properties and x-ray techniques are the only sure ways to distinguish orthoclase from sanidine, microcline and anorthoclase.

Twinning is common in all feldspars and follow certain twin laws such as the Albite Law, the Pericline Law, the Carlsbad Law, the Manebach Law and the Baveno Law. In orthoclase, only the Carlsbad Law, the Manebach Law and the Baveno Law are seen. The Carlsbad Law twin produces what appears to be two intergrown crystals growing in opposite directions.