Orthoclase

Orthoclase: potassium aluminosilicate (KAlSi₃O₈) belonging to the Alkali Feldspar Group. It crystallizes in the Monoclinic System as prismatic crystals, which can be colorless, white, gray, yellow, pink, or green. It cleaves easily along two directions at right angles. Twin formations are common and characteristic, including Karlsbad, Manebach, and Baveno twins. The Karlsbad twin, the most frequent, is a penetration twin in which the “C” axis (001) serves as the twin axis.

(Photo: Rob Lavinsky, iRocks.com – CC-BY-SA-3.0/Wikimedia Commons, License CC BY-SA 3.0)

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