Sanidine

Class: Silicates
Group: Feldspar

Most common in felsic volcanic rocks; in volcanic glass; also from mafic, high-temperature contact metamorphic, and hydrothermally altered rocks; from eclogite nodules in kimberlite (Anthony et al., 2001—2005).

Sanidine — specimen 0017, photo © NMNHS

Original description: ‘Sanidin’ Nöggerath, J. J., 1808. Sanidin — Mineralogische Studien über die Gebirge am Niederrhein, Johann Christian Hermann, Frankfurt: 24—39 [view in ‘Library’].

Type locality: Drachenfels, Königswinter, North Rhine-Westphalia, Germany.

Type material: unknown.

Etymology: from the Greek for board, in allusion to the mineral’s common habit.

Distribution: Canada; France; Germany: Drachenfels, Königswinter, North Rhine-Westphalia; Italy; Japan; Mexico: Durango; 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 tabular on {010}, with a square cross section, to 50 cm. Twinning: Carlsbad twins are common, Baveno and Manebach twins rarer (Anthony et al., 2001—2005).

Physical properties

Cleavage: perfect on {001}, distinct on {010}; parting on {100} (Anthony et al., 2001—2005). Fracture: uneven to conchoidal (Anthony et al., 2001—2005). Tenacity: brittle (Anthony et al., 2001—2005). Hardness: 6 (Arem, 1987: 93). Density: 2.54—2.61 g/cm3 (Lazzarelli, 2012). Luminescence: none. Radioactivity: GRapi 152.94.

Optical properties

Colour: colourless to white; colourless in thin section (Anthony et al., 2001—2005). Diaphaneity: transparent (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: weak (Anthony et al., 2001—2005). Pleochroism: weak.

Material from ‘Repository’

1 specimen: 0017 — 1.08 ct, Sri Lanka.

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