Spodumene

Crystal system Monoclinic
Transparency Transparent
Luster Vitreous to pearly
Fracture Uneven to splintery
Cleavage Perfect prismatic
Specific Gravity 3.18
Hardness 6 to 7
Optical Character Biaxial + Double Refractive
Refractive index 1.660 - 1.676
Birefringence 0.016
Dispersion 0.017
Fluorescence Strong
Pleochroism Kunzite: light purple & near colorless
hiddenite: bluish-green & yellowish-green.
Chemical Formula LiAl(SiO3)2
Comments
Streak

Spodumene (SPOD-you-meen), a lithium-aluminum silicate, expressed by the formula LiAl(SiO3)2, was known for some time before gem-quality specimens were discovered in pegmatite dikes. Despite the fact that it is not used extensively as a gemstone, it has two well-known variety names: kunzite (KOON-zite), which is light red to light violet, and hiddenite (HIDD-en-ite), which ranges in color from yellowish-green to almost emerald green. Hiddenite was named after its discover, William E. Hidden, and kunzite was named for Dr. George Frederick Kunz, one-time gemologist for Tiffany & Company. The name of the species itself comes from the Greek word meaning "burned to ashes", which referred to the appearance of the mineral when it was first known. There is no question but that hiddenite would be an important gemstone, if a supply were available; however, it was discovered in only one mine in 1881, and apparently only one or two pockets within the pegmatite produced a fine green color. Although kunzite is more readily available, it is not a particularly popular stone, principally because its perfect two-directional cleavage makes it rather fragile and difficult to cut. Its delicate color is cherished by many collectors and gem fanciers, nevertheless. A third variety, whish is yellow to light greenish yellow to colorless, is simply called spodumene; very rarely, it occurs also in a blue color.

Spodumene is interesting because it is a member of the pyroxene family, the same family that produces jadeite, one of the toughest of all minerals. The difference is that spodumene occurs in single crystals (which are prismatic in shape and often striated), whereas jadeite is a very closely interwoven mat of fibers. The photograph below of spodumene crystals was reproduced with the permission of the Smithsonian Institute, Washington, D.C. The luster is vitreous to pearly and the fracture is uneven to splintery. It has a variable R.I., but the usual figure for kunzite is 1.660 - 1.676 and the birefringence is usually about .016. The optic character is biaxial positive. The reading for hiddenite may be slightly higher (1.662 and 1.676), but the birefringence is somewhat lower. Dispersion is very weak (.017), the S.G. is usually 3.18, and the hardness ranges from 6 to 7. Kunzite shows very strong light purple to almost colorless dichroism, which is visible to the unaided eye when a stone or crystal is turned from side to side; in the long direction of the crystal the color is deep, whereas at right angles the color is almost lost. Hiddenite shows distinct bluish-green and yellowish-green pleochroism. Spodumene is very strongly fluorescent. No phenomenon is present, nor does it contain inclusions that can be considered diagnostic. It is not attacked by acids, but it fuses easily in the flame of the blowpipe or the jeweler's torch.

Kunzite and the other varieties of spodumene are found in pegmatite dikes, often with tourmaline and beryl, in many areas throughout the world. The most important sources are the Pala area, San Diego County, California, where it was first discovered in 1903; Riverside County, California; Branchville, Connecticut; Maine; Minas Gerais, Brazil; and Madagascar. In recent years spodumene has become an important ore mineral, the principal source of lithium. It sometimes occurs in huge masses. A 42 x 6 x 3 foot crystal weighing 65 tons was once found in the Black Hills of South Dakota.

Spodumene is usually rather easily identified because of its .016 birefringence and positive sign at that point on the refractometer. Rotating a stone on the hemisphere shows definitely that it is biaxial, because both high and low readings vary, and the positive sign is usually readily distinguished because the intermediate index is at 1.666, which is distinctly closer to the lower index than to the higher.

Because of its easy cleavage, this mineral presents problems in fashioning. Only the thinnest and newest sawing blades should be used, and grinding must be done with caution. In addition, it is heat sensitive. Also, because of the strong pleochroism, stones must be oriented carefully for the most attractive color. A satisfactory polish can be produced with Linde A on a tin lap. 43° crown angles and 39° pavilion angles are usually recommended.

In recent years, a considerable supply of light greenish-yellow to yellowish-green spodumene has become available from mines in Brazil; frequently, it is offered under the name hiddenite, in an effort to interest purchasers. Since the name hiddenite was given to a much darker green variety that is no longer available, the use of the name with the pale stones is dubious.