In composition this species is a silicate of calcium and aluminium, with some ferric oxide in place of alumina, corresponding to the complex formula, Ca2(Al,Fe)2[(Al,Fe)OH](SiO4)3. It occurs in monoclinic, prismatic crystals richly endowed with natural faces. The colour deepens with increase in the percentage amount of iron, and the stones become almost opaque. The double refraction is large in amount, 0·031, biaxial in character, and negative in sign. The dichroism is conspicuous in transparent stones, the twin-tints corresponding to the principal optical directions being green, brown, and yellow. The values of the least and greatest of the refractive indices given by transparent stones are 1·735 and 1·766 respectively; the specific gravity varies from 3·25 to 3·50, and the hardness from 6 to 7 on Mohs’s scale.
Transparent crystals have come from Knappenwand, Untersulzbachtal, Salzburg, Austria; Traversella, Piedmont, Italy; and Arendal, Nedenäs, Norway. Magnificent, but very dark, crystals were discovered about ten years ago on Prince of Wales Island, Alaska.
Sphene
(Titanite)
The clear, green, yellow, or brownish stones provided by this species would be welcomed in jewellery because of their brilliant and almost adamantine lustre, but, unfortunately, they are too soft to withstand much wear, the hardness being only 5½ on Mohs’s scale. In composition sphene is a silico-titanate of calcium corresponding to the formula CaTiSiO5, and in this respect comes near the recently discovered gem-stone, benitoite. The refractive indices lie outside the range of the refractometer, the values of the least and the greatest of the refractive indices varying from 1·888 and 1·917 to 1·914 and 2·053 respectively. It is to this high refraction that it owes its brilliant lustre. The double refraction, which is biaxial in character and positive in sign, is so large that the apparent doubling of the opposite edges of a cut stone when viewed through one of the faces is obvious to the unaided eye (cf. [p. 41]). Cut stones have additional interest on account of the vivid dichroism displayed, the twin-tints, colourless, yellow, and reddish yellow, corresponding to the three principal optical directions, being in strong contrast. The specific gravity ranges from 3·35 to 3·45. The negative test with the refractometer (cf [p. 26]), the softness, and the large amount of double refraction suffice to distinguish this species from gem-stones of similar appearance.
The name sphene, from σφήν, wedge, alludes to the shape of the natural crystals. The alternative name is obviously due to the fact that the species contains titanium.
Good stones have come from the St. Gothard district, Switzerland.
Axinite
Called axinite from the shape of its crystals—ἀξίνη, axe—this species supplies small, clear, clove-brown, honey-yellow, and violet stones which can be cut for those who care for a stone out of the ordinary. The composition is a boro-silicate of aluminium and calcium, with varying amounts of iron and manganese, corresponding to the formula (Ca,Fe)3Al2(B.OH)Si4O15. Axinite is interesting on account of its strong dichroism, the twin-tints corresponding to the principal optical directions being violet, brown, and green. The double refraction is biaxial in character and negative in sign, the least and greatest of the refractive indices being 1·674 and 1·684; the specific gravity is 3·28, and hardness about 6½ to 7, or rather under that of quartz.
The best examples have been found at St. Cristophe, Bourg d’Oisans, in the Dauphiné, France. Violet axinite is a novelty that has come within recent years from Rosebery, Montagu County, Tasmania.