Axinite: Gemstone Information
Mineralogists now differentiate axinite group minerals into ferro-axinite,magnesio-axinite, manganaxinite and tinzenite. However, the transparent axinite of the gemmologist, as shown below, can most conveniently be described as a calcium borosilicate of a mineral group whose members show considerable compositional variation.Axinite is notable for its strong pleochroism in which the colours olive-green, violet and cinnamon may be seen when transparent specimens are examined in different directions. Axinite is a member of the triclinic crystal system: crystals are characteristically wedge- or axe-shaped. This is a gemstone that can be fairly easily spotted by the unaided eye. The hardness is 7 and the SG in the range 3.27–3.29. The RI for the alpha, beta and gamma rays respectively is 1.674–1.693, 1.681–1.701 and 1.684–1.701 with birefringence 0.010–0.012. The optic sign is biaxial negative. Absorption lines may be seen at 532, 512, 492, 466, 444 and 415 nm. The absorption at 492 and 466 nm is sometimes broad and the 415 nm absorption may be intense.Axinite as a mineral may show variable luminescence though this is not common in gem-quality specimens. Some yellow crystals from Franklin, New Jersey, may show red under SWUV.
A variety of magne-sio-axinite has been found to show an orange-red response to LW. These phenomena are not generally shown by gem-quality material. Pale brown transparent axinite has recently been reported from Khapalu,Ghauche District, Pakistan.While axinite has traditionally been written calcium aluminium borosilicate (Ca2Al2BSi4O15(OH)), there is considerable variation of the composition through replacement of the calcium by iron, magnesium and manganese. Ferro-axinite forms a series with manganaxinite.Axinite is a mineral of contact metamorphism and metasomatism.Magnificent crystal groups have been found at St Cristophe, near Bourg d’Oisans, Isère, France. This material is not often fashioned as crystals are so fine. Axinite is found in the state of Bahia, Brazil.Gem-quality axinite of reddish brown colour, and in quite large crystals, was discovered at Mina la Olivia in Baja California, Mexico, in the 1960s. Magnesio-axinite from Tanzania has RIs of 1.656, 1.660 and 1.668 with the unusually low SG of 3.18. Analysis showed the specimen to have Al2O3SiO2 (44%), MgO (6.9), MnO (0.4), FeO (0.03) and V (0.4). A cinnamon-brown ferro-axinite has been found in Sri Lanka. The SG is 3.178–3.314 and the RIs are 1.656–1.675, 1.660–1.685 and 1.668–1.685. The pleochroism is reddish brown, dark violet,colourless to yellowish.