Peterson Field Guides Note of the Week - Moonstone

ORTHOCLASE KalSi3O8 Monoclinic - prismatic

Environment: A mineral of igneous, plutonic, and metamorphic rocks, and occasionally of high-temperature veins. Crystal description: Isolated, 1-in. (3-4 cm) sharp white singles sometimes weather from porphyry dikes (Good Springs, Nevada, etc.) where they can be collected on the outcrop of a crumbling granulating rock. The best crystals are found in porphyries. Often they are elongated on the a-axis, parallel to the base, so the upright (front and back) prism pairs are rather short. Others may be flattened on the side, having large b-faces paralleling the a-c axes. With crystals of this habit, penetrant intergrowths of 2 individuals, Carlsbad twins, are common. Twins are names according to localities where they were first conspicuous: Carlsbad (Czechoslovakia) has the pictured pair side by side and halfway through, with a tent-top of one emerging from the base of the other. Physical Properties: White, flesh, yellow, brown, colorless. Luster glassy (sanidine), translucent (adularia) to porcelaneous; hardness 6; specific gravity 2.6; fracture irregular; cleavage 2 good pinocoidal at 90 degrees, plus occasional prismatic. Brittle; transparent to practically opaque. Composition: Potassium aluminum silicate. Sodium can replace up to 50% of the potassium in sanidine. Tests: Fusible only with some difficulty, insoluble in acids. Sanidine chip may glow blue-white in gas flame but colors the flame only slightly. Fuses only on thin edges. Non-fluorescent even after roasting.

Occurrence: As a constituent of aplite (a granite composed exclusively of orthoclase and quartz), orthoclase is used in the ceramic and glass industry (see microcline, p.279). Transparent varieties have slight gem use. Much microcline has been called orthoclase. Although orthoclase is primarily a rock-making mineral of igneous or plutonic rocks, mineral specimens and free crystals usually come in veins and in porphyritic rocks.

The glassy variety known as sanidine forms tabular crystals embedded in volcanic rocks. It sometimes reflects a bluish sheen in certain crystal directions. This phenomenon is known as adularescence, and gems cut from such feldspar are moonstones. Good, though not commercial, sanidine and moonstone have come from New Mexico. A transparent yellow variety from a single unusual Madagascar pegmatite has been cut into brilliant jewelry stones.

This Peterson Field Guide Note of the Week is from the Peterson Field Guide to Rocks and Minerals. Considered to be the most informative and practical guides available, the Peterson Field Guides cover a broad range of topics, including birds, fish, ecology, plants, earth and sky, reptiles and amphibians, and the seashore.