SPODUMENE, IOLITE, BENITOITE
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(SiO_{3})_{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 H_{2}(Mg,Fe)_{4}Al_{8}Si_{10}O_{37}.
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 BaTiSi_{3}O_{9}, 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.