George Frederick Herbert Smith

ENSTATITE, DIOPSIDE, KYANITE, ANDALUSITE, IDOCRASE, EPIDOTE, SPHENE, AXINITE, PREHNITE, APATITE, DIOPTASE

Enstatite

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 ??st?t??, an opponent, referring to the infusibility of the mineral before the blowpipe, and hypersthene comes from ?pe?s?????, 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.

Diopside

This species, which is also known as malacolite and alalite, provides stones of a leaf-green colour which have occasionally been cut. It is a silicate of calcium and magnesium corresponding to the formula MgCa(SiO₃)₂, but usually contains in place of magnesia some ferrous oxide, to which it owes its colour; with increase in the percentage amount of iron the colour deepens and the physical constants change. The double refraction is large in amount, 0·028, biaxial in character, and positive in sign. The least and greatest of the refractive indices corresponding to the stones suitable for jewellery range about 1·671 and 1·699 respectively, but they may be as high as 1·732 and 1·750 in the two cases. The specific gravity varies from 3·20 to 3·38, and the hardness from 5 to 6 on Mohs’s scale. Dichroism is noticeable in deep-coloured stones, but is not very marked.

The name diopside comes from d??, double, and ????, appearance, in allusion to the effect resulting from the double refraction; malacolite is derived from a?a???, soft, because the mineral is softer than the felspar associated with it; and alalite is named after the principal locality, Ala Valley, Piedmont, Italy.

Kyanite

Kyanite, also known as disthene, is interesting for two reasons. Its structure is so grained in character that the hardness varies in the same stone from 5 to 7 on Mohs’s scale; it can therefore be scratched by a knife in some directions, but not in others (p. 79). It has the same chemical composition as andalusite, both being silicates of aluminium corresponding to the formula Al₂SiO₅, but possesses very different physical characters, a fact which shows how large a share the molecular grouping has in determining the aspect of crystallized substances. It is biaxial with small negative double refraction, the least and greatest of the refractive indices being 1·72 and 1·73 respectively; the specific gravity is 3·61. It occurs in sky-blue prismatic crystals, whitish at the edges, in schist near St. Gothard, Switzerland. It is seldom cut.

Kyanite is derived from its colour, ??a??? blue, and disthene, from its variable hardness, d??, twice, and s?????, strong.

Andalusite

Andalusite bears no resemblance whatever to kyanite, although, as has been stated above, the composition of the two species is essentially the same. It is usually light bottle-green in colour, and more rarely brown and reddish. Its extreme dichroism is its most remarkable character, the twin colours being olive-green and red. The reddish gleams that are reflected from the interior are in sharp contrast with the general colour of the stone, and impart to it a weird effect (Plate XXIX, Fig. 15). Cut stones are often confused with tourmalines, and can, indeed, only be distinguished from the latter with certainty by noting on the refractometer the smaller amount of double refraction and the difference in its character. The least and greatest of the refractive indices are 1·62 and 1·643 respectively, and the double refraction, 0·011, about half that of tourmaline, is biaxial and negative; the specific gravity is 3·18, and hardness 7½ on Mohs’s scale.

Good stones are found at Minas Novas, Minas Geraes, Brazil, and in the gem-gravels of Ceylon. It was first known from the province of Andalusia, Spain, whence is the origin of its name.

Idocrase

Idocrase, also known as vesuvianite, is occasionally found in the form of transparent, leaf-green, and yellowish-brown stones which, when cut, may be mistaken for diopside and epidote respectively, but are distinguishable from both by the extreme smallness of their double refraction. Californite is a compact variety which has all the appearances of a jade; its colour is green, or nearly colourless with green streaks.

In composition idocrase is a silicate of aluminium and calcium, the precise formula of which is uncertain, but may be—

The double refraction, which is uniaxial in character and negative in sign, may be less than 0·001, and never exceeds 0·006, so that it is not easily detected with the refractometer, even in sodium light. The refractive indices vary enormously in value, from 1·702 to 1·726 for the ordinary, and from 1·706 to 1·732 for the extraordinary ray. The specific gravity varies from 3·35 to 3·45, and the hardness is about 6½ on Mohs’s scale.

The name idocrase, from e?d??, form, and ???s??, mixture, was assigned to the species by HaÜy, but his reasons have little meaning at the present day. The other names are taken from the localities where the species and the variety were first discovered.

Bright, green crystals come from Russia, and also from Ala Valley, Piedmont, and Mount Vesuvius, Italy. Californite is found in large masses in Siskiyon and Fresno Counties, California.

Epidote

Epidote often possesses a peculiar shade of yellowish green, similar to that of the pistachio-nut—hence the origin of its alternative name—which is unique among minerals, though scarcely pleasing enough to recommend it to general taste. Its ready cleavage renders it liable to flaws; nevertheless, it is occasionally faceted. The name epidote, from ?p?d?s??, increase, was given to it by HaÜy, but not on very precise crystallographical grounds.

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

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 sf??, 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—?????, 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.

Prehnite

This species, which is named after its discoverer, Colonel Prehn, is found in nodular, yellow and oil-green stones, of which the latter have very occasionally been cut. It is a little soft, the hardness being only 6 on Mohs’s scale. The double refraction is large in amount, 0·033, biaxial in character, and positive in sign, the least and the greatest of the refractive indices being 1·616 and 1·649 respectively; the specific gravity varies from 2·81 to 2·95. In composition prehnite is a silicate of aluminium and calcium corresponding to the formula H₂Ca₂Al₂(SiO₄)₃.

The best material has been found at St. Cristophe, Bourg d’Oisans, DauphinÉ, France.

Apatite

This interesting mineral is found occasionally in attractive green, blue, or violet stones, but is unfortunately too soft for extensive use in jewellery, the hardness being only 5 on Mohs’s scale. In composition it is a fluo-chloro-phosphate of calcium, corresponding to the formula Ca₄[Ca(F,Cl)](PO₄)₃. When pure, it is devoid of colour, the tints being due to the presence of small amounts of tinctorial agents. The double refraction is uniaxial in character and negative in sign, the ordinary index being 1·642 and the extraordinary 1·646; the specific gravity varies from 3·17 to 3·23. The dichroism is usually feeble, but sometimes is strong; for instance, in the stones from the Burma ruby mines (yellow, blue-green). A cut stone might be mistaken for tourmaline, but is distinguished by its softness, or, when tested on the refractometer, by its inferior double refraction. It received its name from ?pat?e??, deceive, because it was wrongly assigned to at least half a dozen different species in early days. Moroxite is a name sometimes given to blue-green apatite.

Beautiful violet stones are found at Ehrenfriedersdorf, Saxony; Schlaggenwald, Bohemia; and Mount Apatite, Auburn, Androscoggin County, Maine, U.S.A.; and blue stones come from Ceylon.

Dioptase

Though of a pretty, emerald-green colour, dioptase has never been found in large enough crystals for gem purposes, and it is, moreover, rather soft, the hardness being only 5 on Mohs’s scale, and has an easy cleavage. In composition it is a hydrous silicate of copper corresponding to the formula CuH₂SiO₄. The double refraction, which is large in amount, is uniaxial in character, and positive in sign, the ordinary refractive index being 1·667 and the extraordinary 1·723. Its colour and softness distinguish it from peridot or diopside, which have about the same refractivity. The name was assigned to the species by HaÜy, from d??, through, and ?pt?a?, see, because the cleavage directions were distinguishable by looking through the stone.

Dioptase has been found near Altyn-TÜbe in the Kirghese Steppes, at RezbÁnya in Hungary, and Copiapo in Chili, and at the mine Mindouli, near Comba, in the French Congo.