Saturday, 17 August 2013

Spodumene

Kunzite
Silicate of lithium and aluminium. This is a lithium-rich member of the monoclinic pyroxene group, similar to jadeite. Its name basically means "'ashen," because this mineral is often grayish white or ash gray. In the past, it was also called triphane, a word of Greek derivation meaning "three aspects," due to its clear trichoism.

Crystal system

Monoclinic

Appearance

Spodumene frequently occurs as very long. large unevenly terminated, flat-sided crystals, often with longitudinal striations. Some are among the largest in the mineral world: crystals a few meters in length, weighing over a ton, have been found. The main color is a faintly green, nontransparent whitish gray; whence the name. Violet pink, bright green, yellow green or yellow specimens are much less common and transparent crystals of these are used as gems. Transparent crystals have vitreous luster and marked pleochroism. The semiopaque crystals look almost pearly.

Physical properties

Spodumene has a hardness of 6-7. The density is between 3.17 and 3.23 g/cm3. The refractive indices vary from nα 1.654, n 1.669 to nα 1.664, fly 1.679, with quite obvious birefringence. It has very easy prismatic cleavage.

Genesis Spodumene is a typical mineral of pegmahtes particularly those rich in lithium. Occurrence Pegmatites containing spodumene found in Great Britain, Sweden, some parts of the United States (South Dakota, Connecticut, New Mexico, Massa-chusetts), Brazil, and Madagascar.

Kunzite

This is the violet pink, transparent variety of spodumene named after the American mineralogist G.F. Kunz a noted gem expert active at the turn of the century.

Appearance

SpodumeneThe characteristic color is a violet pink which can be quite intense. It has marked pleochroism seen as a clear difference in depth of color in different crystallographic directions, rather than a color change as such. The crystals used as gems generally have few inclusions and good transparency. Plane surfaces, looking both specular and transparent at the same time, can sometimes be seen on the inside and are the warning signs of deny age. In fact, its easy cleavage makes this gem quite brittle sensitive to knocks, and therefore unsuitable as a ring stone. It is usually given a (sometimes quite elongated) oval mixed cut, a pear-shaped or triangular mixed cut, or evena step cut. Although it is often found as large crystals, the smaller section of the crystal is used for cutting, the strongest, most valued color being perpendicular to this surface Gems several carats in weight are not uncommon and some of 200-300 carats have even been cut as museum pieces.

Distinctive features

Some pink stones look very muds alike: e.g. kunzite; pink topaz, and morganite. Kunzite, however, has the strongest pleochroism, best seen, of course, in larger, richer-colored stones. It has much clearer birefringence than the other two minerals, a factor easily established in larger stones, with the aid of a lens, by the presence of a double image of the back facet edges Kunzite is also somewhat less hard than, the others. This can be established by touch, the less acute facet edges feeling almost oily or soapy if rubbed between forefinger and thumb. Pink tourmaline can sometimes also look like kunzite. In this case, there is no appreciable difference in birefringence and the pleochroism can be vaguely. The physical properties are also quite close, but are nonetheless sufficiently different to establish the distinction.

Occurrence

Kunzite is found in the United States (in various parts of California, source of the largest crystals, Maine and Connecticut). Brazil, and Madagascar.

Value Not very high; as secondary gems go, it is mores, less on a par with good quality red garnets. Gems several carats in weight are common; small stones, especially pale ones, are of very limited value. Simulants and synthetics It is mainly imitated by pale pink corundum. It is not synthesized, at least not courier. cially.

Hiddenite

This is the green variety of spodumene, which has only been known for about a century and is named after AE Hidden, a mine-owner in the United States, where the mineral was first discovered. Some people nowadays maintain that the name hiddenite refers only to the emerald green or rich green variety of spodumene, whereas others apply this name to all gem-quality green spodumene, including pale and yellowish-green specimens; this seems the most Practical definition.

Appearance

The best, and very rare, specimens are a bright, almost emerald green, with quite strong green to blue-green pleochroism; but hiddenite may be a rather dull, pale green or even green with a yellowish tinge. The step cut is the most common. Strongly colored stones are usually small to medium-sized; paler specimens are often a bit bigger, but never as large as kunzite.

Distinctive features

It is hard to generalize about a gem of such rarity and diverse appearance. Depending on the specimen, hiddenite may resemble both pale and strong-colored emeralds, bright green and yellowish tourmalines, chrysoberyl, and diopside. The physical characteristics al. ways have to be measured in order to identify it.

Occurrence

Cut Kunzite The finest gems used to come from North Carolina. The less attractive, paler types, perhaps with a yellowish tinge, come from California, Brazil, and Madagascar.

Value

Its attractive color, rarity, and the difficulty of finding reasonable-sized stones make intensely colored hiddenite one of the most valuable secondary gems. Paler-colored specimens, which are easier to find in a good size, are of quite low value, similar to that of kunzite. Simulants and synthetics Being little known and of re-cent history, hiddenite is not imitated. Nor is it produced synthetically, at any rate not on a commercial scale. Yet large stones, in which the medium-light green color is due to some form of treatment (probably irradiation) of very pale or colorless stones, or even very pale kunzite, do appear on the market from time to time.

Writer – Curzio Cipriani & Alessandro Borelli

1 comment:

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