George Frederick Herbert Smith

SPODUMENE, IOLITE, BENITOITE

Spodumene

TILL a few years ago scarcely known outside the ranks of mineralogists, spodumene suddenly leaped into notice in 1903 upon the discovery of the lovely lilac-coloured stones (Plate XXIX, Fig. 10) at Pala, San Diego County, California; they shortly afterwards received the name kunzite after the well-known expert in gems, Dr. G. F. Kunz. The stones were found here in a pegmatite dyke, and were of all shades, ranging from pale pink to deep lilac, and at times as much as 150 carats in weight. Paler kunzite occurs with beryl and tourmaline at Coahuila Mountain in Riverside County, California, and colourless stones have recently come to light in Madagascar. Kunzite is remarkable for its wonderful dichroism; the beautiful violet tint that springs out in one direction comes with greater surprise because of the uninteresting yellowish tints in other directions. Unlike spodumene in general, kunzite is phosphorescent under the influence of radium.

The emerald-green variety (Plate XXIX, Fig. 11), named hiddenite after Mr. W. E. Hidden, who discovered in 1881 the only known occurrence, in Alexander County, North Carolina, would no doubt have become popular had the supply of material not been so very limited; few stones were found, and the variety has never come to light elsewhere. The colour is supposed to be due to chromic acid. Hiddenite being also dichroic, the tint varies with the direction.

Spodumene is ordinarily rather a pale yellowish in hue, and, as its name (which is derived from sp?d???, ash-coloured) suggests, is not very attractive. Clear, lemon-yellow stones (Plate XXIX, Fig. 9) are found in Brazil and Madagascar.

The species is interesting scientifically because it contains the rare element lithium; it is a silicate of aluminium and lithium, corresponding to the formula LiAl(SiO₃)₂. The double refraction is biaxial in character and positive in sign, the least and greatest of the refractive indices being 1·660 and 1·675; the specific gravity is 3·185, and hardness 6½ to 7 on Mohs’s scale. Spodumene has an easy cleavage, and the cut stones call therefore for careful handling, lest they be flawed or fractured. Two faceted stones, a beautiful kunzite and a fine hiddenite, weighing 60 and 2½ carats respectively, are exhibited in the British Museum (Natural History).

Iolite

Known also by various other names—cordierite, dichroite, and water-sapphire (saphire d’eau)—this species owes its interest to the remarkable dichroism characterizing it, the principal colours—smoky-blue and yellowish white—being in such contrast as to be obvious to the unaided eye. The stones that are usually worked have intrinsically a smoky-blue colour, and are found in waterworn masses in the river-gravels of Ceylon, whence is the origin of the name water-sapphire. Iolite, from ???, violet, and ?????, stone, refers to the colour; cordierite is named after Cordier, a French geologist, who first studied the crystallography of the species; and dichroite, of course, alludes to the most prominent character of the species.

Iolite is a silicate of aluminium and of magnesium and iron corresponding to the formula H₂(Mg,Fe)₄Al₈Si₁₀O₃₇. The double refraction is small in amount, biaxial in character, and negative in sign, the least and greatest of the refractive indices being 1·543 and 1·551; the specific gravity is 2·63, and hardness 7 on Mohs’s scale. Iolite, if used, is worked and polished; it is seldom faceted. A large worked piece, weighing 177 grams, which was formerly in the Hawkins Collection, is exhibited in the British Museum (Natural History).

Benitoite

The babe among gem-stones, benitoite first saw the light of day a few years ago, early in 1907. It occurs with the rare mineral neptunite, which was previously known only from Greenland, in narrow veins of natrolite in Diablo Range near the head-waters of the San Benito River, San Benito County, California. Despite careful search the species has not been found except within the original restricted area. To science it is interesting both because of its composition, a silico-titanate of barium, corresponding to the formula BaTiSi₃O₉, and because its crystals belong to a class of crystalline symmetry which has hitherto not been represented among minerals. The double refraction is uniaxial, and since the ordinary index of refraction is 1·757 and the extraordinary 1·804, it is positive in sign and large in amount, namely, 0·047. The stones are characterized by strong dichroism, the colour corresponding to the ordinary ray being white, and to the extraordinary greenish blue to indigo depending upon the tint of the stone. To obtain the best effect the stone must therefore be cut with the table-facet parallel to the crystallographic axis. The specific gravity is 3·65, and hardness 6½ on Mohs’s scale. When first discovered the species was supposed to be sapphire, and many stones were cut and sold as such. It is, however, much softer than sapphire, and is readily distinguished by its optical characters, since it possesses greater double refraction and of differing sign, so that, when tested with the refractometer, the shadow-edge corresponding to the lower index of refraction remains fixed in the case of benitoite, whereas the contrary happens with sapphire. Benitoite also, unlike sapphire, fuses easily to a transparent glass. Its blue colour, which is supposed to be due to a small amount of free titanic acid present, appears to be stable. Several stones as large as 1½ to 2 carats in weight have been found. The largest of all, perfectly flawless, weighs just over 7 carats, and is remarkable because it is about three times the next largest in point of weight; it is the property of Mr. G. Eacret, of San Francisco.