Spodumene
(Kunzite, Hiddenite)
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 σποδίος, 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 ἴον, violet, and λίθος, 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).