As its name suggests, this mineral also contains beryllium, being a soda phosphate corresponding to the formula NaBePO4. Clear, colourless stones, which occur at Stoneham, Maine, U.S.A., have been cut, but the lack of ‘fire,’ the easy cleavage, and comparative softness, the symbol being 5½ on Mohs’s scale, unfit it for use in jewellery. The double refraction is biaxial in character and negative in sign, the least and the greatest of the refractive indices being 1·553 and 1·565 respectively.
CHAPTER XXXVI
ENSTATITE, DIOPSIDE, KYANITE, ANDALUSITE, IDOCRASE, EPIDOTE, SPHENE, AXINITE, PREHNITE, APATITE, DIOPTASE
Enstatite
(‘Green Garnet’)
THE small green stones which accompany the diamond in South Africa have been cut and put on the market as ‘green garnet.’ They are, however, in no way connected with garnet, but belong to a mineral species called enstatite, which is a silicate of magnesium corresponding to the formula MgSiO3; the green colour is due to a small amount of ferrous oxide which replaces magnesia. The double refraction is biaxial in character and positive in sign, the least and greatest of the refractive indices being 1·665 and 1·674 respectively; the specific gravity ranges from 3·10 to 3·13, and the hardness is only about 5½ on Mohs’s scale. The dichroism is perceptible, the twin-colours being yellowish and green, and, as usual, is more pronounced the deeper the colour of the stone. There is also a good cleavage in two different directions.
With increasing percentage amount of iron enstatite passes into hypersthene. The colour becomes a dark brownish green, and an increase takes place in the physical constants, the least and greatest of the refractive indices attaining to 1·692 and 1·705 respectively, and the specific gravity ranging from 3·4 to 3·5. Hypersthene is never sufficiently transparent for faceting, but when spangled with small scales of brookite it is sometimes cut en cabochon.
The name enstatite is derived from ἐνστάτης, an opponent, referring to the infusibility of the mineral before the blowpipe, and hypersthene comes from ὑπερσθένος, very tough.
An altered enstatite, leek-green in colour and with nearly the composition of serpentine ([p. 289]), has been cut en cabochon. It has much lower specific gravity, only 2·6, and lower hardness, 3½ to 4 on Mohs’s scale. It is named bastite from Baste in the Harz Mountains, where it was first discovered.