Dravite

Chemistry

Na(Mg3)Al6(Si6O18)(BO3)3(OH)3(OH)

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

Crystal data

Crystallography: trigonal — ditrigonal pyramidal. Crystal habit: crystals equant or short to long prismatic, to 20 cm; striated; cross sections typically triangular with curved convex sides; commonly radiating, granular, and massive. Twinning: rare, on {1011} or {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: 3.03—3.18 g/cm3 (Anthony et al., 2001—2005). Luminescence: may weakly fluoresce under SW UV (Anthony et al., 2001—2005). Other: pyroelectric and piezoelectric (Anthony et al., 2001—2005).

Optical properties

Colour: brown to black, red, yellow, blue, green, colourless, white; colourless to yellow in thin section (Anthony et al., 2001—2005). Diaphaneity: transparent to translucent (Anthony et al., 2001—2005). Lustre: vitreous to resinous (Anthony et al., 2001—2005). Refractive index: 1.612—1.661 — anisotropic [uniaxial (-)] (Anthony et al., 2001—2005). Birefringence: 0.014—0.032. Dispersion: 0.017. Pleochroism: very strong; O = pale yellow; E = colourless, yellowish, greenish, brownish (Anthony et al., 2001—2005).