Orthoclase and smoky quartz Gebel quarry – Cuasso al Monte – Ceresio Valley – Varese prov. – Lombardy – Italy Specimen weight:315 gr. Crystal size:mm. 23 Overall size: 112mm x 103 mm x 41 mm © minservice

Chemical Formula: KAlSi3O8
Locality: Common world wide occurrences.
Name Origin: From the Greek orthos – “right” and kalo -” I cleave” in allusion to the mineral’s right angle of good cleavage.

Orthoclase (endmember formula KAlSi3O8) is an important tectosilicate mineral which forms igneous rock. The name is from the Greek for “straight fracture,” because its two cleavage planes are at right angles to each other. Alternate names are potassium feldspar and K-feldspar. The gem known as moonstone  is largely composed of orthoclase.

Formation and Subtypes

Orthoclase is a common constituent of most granites and other felsic igneous rocks and often forms huge crystals and masses in pegmatite.

Typically, the pure potassium endmember of orthoclase forms a solid solution with albite, the sodium endmember (NaAlSi3O8), of plagioclase. While slowly cooling within the earth, sodium-rich albite lamellae form by exsolution, enriching the remaining orthoclase with potassium. The resulting intergrowth of the two feldspars is called perthite.

The higher-temperature polymorph of orthoclase is sanidine. Sanidine is common in rapidly cooled volcanic rocks such as obsidian and felsic pyroclastic rocks, and is notably found in trachytes of the Drachenfels, Germany. The lower-temperature polymorph of orthoclase is microcline. Adularia is found in low temperature hydrothermal deposits, in the Adula Alps of Switzerland. The largest documented single crystal of orthoclase was found in Ural mountains, Russia. It measured ~10×10×0.4 m and weighed ~100 tons.


Discovery date : 1823

Optical properties

Optical and misc. Properties : Transparent – Translucide – fluorescent- Gemme, pierre fine
Refractive Index: from 1,51 to 1,52
Axial angle 2V: 35-75°

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Physical properties

Hardness : from 6,00 to 6,50
Density: from 2,55 to 2,63
Color: colorless; white; grey; yellow; grey yellow; pink red; reddish; green; greenish; pink
Luster : vitreous; nacreous
Streak : white
Break : irregular; conchoidal; splintery
Cleavage : Yes


Orthoclase 4.1×3.8×2.8 cm Bear Lake Diggings Gooderham Ontario, Canada Copyright © David K. Joyce Minerals
Orthoclase Montecatini quarry, Baveno, Piedmont, Italy Specimen weight:181 gr. Crystal size:40 mm Overall size: 78mm x 68 mm x 48 mm © minservice
Orthoclase Montecatini quarry, Baveno, Piedmont, Italy Specimen weight:135 gr. Crystal size:35 mm Overall size: 78mm x 62 mm x 55 mm © minservice