What is Spodumene? its chemistry and varieties and mining

 Spodumene 

spodumene pronunciation

Spodumene, a member of the pyroxene group, is lithium aluminium silicate LiAlSi₂O₆ and forms highly characteristic flattened and striated prismatic transparent crystals of the monoclinic system with good cleavages in two directions intersecting nearly at right angles and a notably brittle tendency. The hardness is 6.5–7 and the SG 3.18. The RI for the alpha, beta and gamma rays is 1.648–1.663, 1.655–1.669 and 1.662–1.679 respectively with biaxial positive DR 0.014–0.027. Crystals are strongly pleochroic with lilac, green or yellow in one direction and colourless in the other. The varieties include a yellow to pale green spodumene with no varietal name, the lilac-pink kunzite and a very rare chrome-green hiddenite.

Kunzite shows no useful absorption in the visible but the chrome-green hiddenite shows a characteristic chromium spectrum with the customary strong doublet in the deep red at 690.5 and 686 nm and weaker lines at 669 and 646 nm. A broad absorption band has its centre near 620 nm but no Cr lines are visible in the blue. The iron band is strong and well-defined at 437.5 nm and is accompanied by a weaker band at 433 nm. A narrow line at 505 nm is reported from a yellow spodumene from Myanmar. The band at 437.5 nm is seen in a number of pyroxenes, including jadeite and is due to iron.

 

Kuzite (pink)

Luminescence

Under LWUV kunzite glows golden-pink or orange with a weaker effect under SW; it shows a very strong response to X-rays with a very strong orange with persistent phosphorescence. After this irradiation the speci- men may have changed from lilac-pink to bluish green. Kept away from

fluorescence in spodumene

 

strong light sources this induced colour will be maintained. If such a specimen is exposed to strong sunlight for some hours or if it is heated to around 200 °C the colour will discharge with emission of orange luminescence. Yellow-green spodumene may give a weak orange-yellow under LWUV; a much weaker though similar effect is seen under SW.

Yellow-green spodumene gives a fairly strong glow under X-rays but with no phosphorescence or change of colour.

A brownish orange spodumene was found to have been irradiated, primarily by the isotope scandium-46. A Geiger counter should be used routinely on spodumene of citrine-like colour. Gamma ray spectroscopy also detected radio-active isotopes. Wearing is not recommended.

 

Occurrence

Spodumene is a common constituent of lithium-rich granite pegmatites. Fine kunzite is found in southern California, in particular at the White Queen mine, Hiriart hill, San Diego County, and elsewhere in the area. It is also found in many Brazilian pegmatites, particularly at Urupuca, at Lavra do e Mario, Corrego do Urucum, Galileia and as thick, pale pink and pale green crystals at Resplendor. The pegmatites of Kunar, Lagham and Nuristan, Afghanistan, provide good-quality crystals. Yellow-green spo- dumene is found in the Brazilian pegmatites and also at locations in Afghanistan. In the early days of its appearance yellow spodumene was thought to be yellow chrysoberyl. Various colours of spodumene have been found in the pegmatites of Madagascar, in particular Anjanabonoina, Mt Bity and Tilapa. Hiddenite spodumene has been found only at Stony Point, Alexander County, North Carolina. The variety was identified in the late nineteenth century. Spodumene has been reported from the Mogok area of Myanmar. A blue spodumene weighing 35 carats has been reported. The stone had RI in the spodumene range.

 

Note on the Cutting of Kunzite

Owing to the strong cleavage, brittleness and heat sensitivity, spodumene is a difficult stone to facet. Further, owing to the thin nature of some of the crystals it is not possible to cut well-coloured stones, since to get stones of good colour advantage should be taken of the strongest pleochroic colour. The deepest colour is seen approximately parallel to the length of the crys- tal, so for the best result the stones should be cut with the table facet at right angles to this direction. This is, of course, more important in the case of kunzite, which is usually cut very deep in order to enhance the colour.

Spodumene

 

Some kunzites have a tendency to fade on exposure to strong sunlight. Some yellowish brown spodumenes turn purple after heat treatment.

 

Simulation

Kunzite is simulated by synthetic pink spinel and by a suitably coloured glass, but distinction is easy as both of these stones are singly refractive, whereas in kunzite the DR is strong enough for the doubling of the oppos- ing facet edges to be seen with a hand lens in appropriate directions. Further, glass and spinel are not dichroic. What may be a more convincing simulator of kunzite is a bluish pink amethyst, but even here, apart from the differences in the RI and SG, the lustre of the amethyst is not nearly that of kunzite.

Kunzite Mining at the Oceanview Mine, Part 2 by GIA