Catalog Search Guide

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Orthoclase

Orthoclase is a potassium-rich member of the alkali feldspar group, chemically KAlSi3O8, and one of the most important rock-forming minerals in the Earth's crust. It is the most common member of the series, being the dominant K-feldspar in most granite, granitic rocks and other plutonic rocks, with an intermediate cooling regime. Structurally it sits between disordered monoclinic sanidine and fully ordered triclinic microcline, sharing the same composition as the latter but stable at higher temperature. Orthoclase got its name because it has two directions of cleavage, which are at right angles to one another, a diagnostic feature collectors use alongside its typical prismatic, often twinned habit — Carlsbad, Baveno, and Manebach twin laws are all named after classic orthoclase occurrences.

Typical crystallized orthoclase comes from granite and pegmatite localities across the Alps and beyond: St. Gotthard and Val Giuv in Switzerland, the Zillertal in Austria, and in Italy the historic twin locality of Baveno in Piedmont together with San Piero in Campo on Elba. Classic twinned forms come from Carlsbad in Czechoslovakia, and well-formed red crystals from Kirkpatrick in Scotland, Flims in Switzerland, Marienburg in Germany, and Striegau in Silesia. Fine adularia-type orthoclase from alpine clefts is also prized, with exceptional crystals from Habachtal in Tyrol. In the United States, notable material comes from the Pala and Mesa Grande pegmatite districts of San Diego County, California, and from Oxford County, Maine.

The finest gem-quality orthoclase, however, is the transparent yellow variety from Madagascar, especially the Itrongay (Itrongahy) and Ampandrandava pegmatites near Betroka in Fianarantsoa Province. Crystals from Itrongahy are very fine, transparent yellow and very large, up to 250 carats, making Madagascar the world's leading source of cuttable orthoclase. Related gem varieties include moonstone, the schiller-effect orthoclase-albite perthite valued in jewelry, with fine material from Sri Lanka, Myanmar, Brazil, and India. Collectors seek orthoclase for its sharp twinned crystal forms, its role as a type example of feldspar structural states, and the rarity of genuinely gemmy, transparent specimens set against the mineral's ubiquity as a common rock-former.