Orthoclase

Class: Silicates
Group: Feldspar

Varieties | Names:

A common feldspar of granites, granite pegmatites, and syenites; in cavities in basalts; in metamorphic rocks (Anthony et al., 2001—2005).

Orthoclase — specimen 0478, photo © NMNHS

Original description: ‘Orthoklas’ Breithaupt, J. F. A., 1823. Feldspath. Orthoklas — Vollständige Charakteristik des Mineral-Systems, Arnoldischen Buchhandlung, Dresden: 271—272 [view in ‘Library’].

Type locality: unknown.

Type material: unknown.

Etymology: from the Greek for straight and fracture, in allusion to the cleavage angle.

Distribution: Austria: Tirol; Germany; Italy; Madagascar; Mexico; Russia: near Yekaterinburg, Urals; Switzerland; USA (Anthony et al., 2001—2005).

Chemistry

KAlSi3O8

Essential elements: oxygen (O), aluminium (Al), silicon (Si), potassium (K).

Crystal data

Crystallography: monoclinic — prismatic. Crystal habit: crystals commonly short prismatic along [100] or [001], tabular on [010], to 20 cm; granular, massive. Twinning: common as simple, contact, or penetration twins according to the Carlsbad, Baveno, or Manebach laws (Anthony et al., 2001—2005).

Physical properties

Cleavage: perfect on {001} and {010}; partings on {100}, {110}, {110}, and {201} (Anthony et al., 2001—2005). Fracture: uneven to conchoidal (Anthony et al., 2001—2005). Tenacity: brittle (Anthony et al., 2001—2005). Hardness: 6 (Lazzarelli, 2012). Density: 2.54—2.61 g/cm3 (Lazzarelli, 2012). Luminescence: none. Radioactivity: GRapi 200.97.

Optical properties

Colour: colourless, white, grey, pale yellow, red, green; colourless in thin section (Anthony et al., 2001—2005). Diaphaneity: transparent to translucent (Anthony et al., 2001—2005). Lustre: vitreous (Anthony et al., 2001—2005). Refractive index: 1.518—1.536 — anisotropic [biaxial (-)] (Lazzarelli, 2012). Birefringence: 0.005—0.008. Dispersion: 0.012. Pleochroism: weak.

Material from ‘Repository’

15 specimens: 0259 — 0.71 ct, Tanzania; 0352 — 0.94 ct, Tanzania; 0478 — 1.67 ct, Brazil; 0131 — 1.00 ct, India; 0132 — 1.06 ct, India; 0162 — 0.85 ct, India; 0163 — 1.00 ct, India; 0106 — 4.04 ct, India; 0095 — 3.90 ct, India; 0099 — 3.06 ct, India; 0092 — 0.82 ct, Sri Lanka; 0093 — 0.76 ct, Sri Lanka; 0096 — 3.50 ct, India; 0366 — 1.37 ct, India; 0360 — 13.30 ct, no precise locality [Africa].

→ Gallery view