Schorl

Chemistry

Na(Fe2+3)Al6(Si6O18)(BO3)3(OH)3(OH)

Essential elements: hydrogen (H), boron (B), oxygen (O), sodium (Na), aluminium (Al), silicon (Si), iron (Fe).

Crystal data

Crystallography: trigonal — ditrigonal pyramidal. Crystal habit: crystals prismatic to acicular, or may be flattened along [0001], with prominent trigonal prism and pyramid, to 1.5 m; commonly hemimorphic, and striated; also radial, fibrous, and massive. Twinning: rarely on {1010}, {4041} (Anthony et al., 2001—2005).

Physical properties

Cleavage: on {1120}, {1011}, very poor (Anthony et al., 2001—2005). Fracture: uneven to conchoidal (Anthony et al., 2001—2005). Tenacity: brittle (Anthony et al., 2001—2005). Hardness: 7 (Anthony et al., 2001—2005). Density: 3.18—3.22 g/cm3 (Anthony et al., 2001—2005). Luminescence: none. Other: pyroelectric and piezoelectric (Anthony et al., 2001—2005).

Optical properties

Colour: black, brownish black, bluish black; in thin section, bluish yellow (Anthony et al., 2001—2005). Diaphaneity: transparent to nearly opaque (Anthony et al., 2001—2005). Lustre: vitreous to resinous (Anthony et al., 2001—2005). Refractive index: 1.635—1.672 — anisotropic [uniaxial (-)] (Anthony et al., 2001—2005). Birefringence: 0.025. Dispersion: no data. Pleochroism: very strong; O = yellow-brown; E = pale yellow, pale brown (Anthony et al., 2001—2005).