Axinite-(Mg)

Class: Silicates
Group: Axinite

Typically a mineral formed during contact metamorphism and boron metasomatism (Anthony et al., 2001—2005). Faceted specimens are extremely rare.

Axinite-(Mg) — specimen 0084, photo © NMNHS

Original description: ‘Magnesioaxinite’ Jobbins, E. A., Tresham, A. E., Young, B. R., 1975. Magnesioaxinite, a new mineral found as a blue gemstone from Tanzania — Journal of Gemmology 14: 368—375 [view in ‘Library’].

Type locality: Merelani, Lelatema Mts, Tanzania.

Type material: Natural History Museum, London, England, MI 34610 (Anthony et al., 2001—2005).

Etymology: from the Greek αξίνα (axina), for ‘axe’, in allusion to its crystal habit, and the magnesium in the formula.

Distribution: Australia: New South Wales; Tanzania: Arusha; USA: Nevada, Santa Fe district (Anthony et al., 2001—2005).

Chemistry

Ca4Mg2Al4[BSi8O30](OH)2

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

Crystal data

Crystallography: triclinic — pedial. Crystal habit: as crystals, to 3 cm, with the axe-head-shaped morphology typical of axinites (Anthony et al., 2001—2005).

Physical properties

Cleavage: good on {100}, poor on {001}, {110}, and {011} (Anthony et al., 2001—2005). Fracture: uneven to conchoidal (Anthony et al., 2001—2005). Tenacity: brittle (Anthony et al., 2001—2005). Hardness: 6.5 (Anthony et al., 2001—2005). Density: 3.178 g/cm3 (Anthony et al., 2001—2005). Luminescence: fluoresces red-orange in LW UV, duller red in SW UV (Anthony et al., 2001—2005).

Optical properties

Colour: pale blue to pale violet; light brown to light pink (Anthony et al., 2001—2005). Diaphaneity: transparent to translucent (Anthony et al., 2001—2005). Lustre: vitreous (Anthony et al., 2001—2005). Refractive index: 1.656—1.678 — anisotropic [biaxial (+/-)] (Anthony et al., 2001—2005). Birefringence: 0.009—0.011. Dispersion: strong (Anthony et al., 2001—2005). Pleochroism: pale blue to pale violet and pale grey (Anthony et al., 2001—2005).

Material from ‘Repository’

2 specimens: 0084 — 0.73 ct, Tanzania; 0378 — 0.23 ct, Tanzania.

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