TANZANITE
Tanzanite is a rare variety of zoisite, which
is calcium hydroxy-aluminosilicate (Ca2Al3Si3O12(OH)).
Tanzanite is zoisite with vanadium (V) impurity. Most tanzanite crystals
are not physically distinguishable from ordinary zoisite. What makes
tanzanite distinctive? Upon heating, tanzanite becomes richly purplish-blue
colored. Occasionally, an untreated tanzanite has a purplish-bluish
color.
Tanzanite is trichroic, meaning it shows three
different colors, depending on the orientation of the crystal. Most
tanzanites only show two of the three colors - bluish and purplish.
Occasionally, a high-quality tanzanite will show the third color - red (e.g.,
see photo on p. 68 of Keller, 1992, Gemstones of East Africa).
Tanzanite (CMNH public display, Cleveland Museum of Natural History, Cleveland,
Ohio, USA)
Tanzanite - two views of same crystal (artificially heated) (~8 to 9 mm
across). The specimen on left shows a
purplish color & the specimen on right shows a dark bluish color.
Tanzanite (vanadiferous zoisite) - large purplish specimen (probably
heat-treated) near center is ~8 mm across. All other crystals shown here
are in their natural, rough state (not heat treated).
Tanzanite is used as a gemstone, but unlike diamonds and rubies and emeralds, tanzanite was discovered late in human history, during the 1960s. Practially all known tanzanite comes from East Africa's Merelani Tanzanite Deposit, a 0.5 to 6 meter wide & 9 km long zone of hydrothermal quartz-calcite-tanzanite-grossular-graphite-pyrite veins and pockets within high-grade metamorphic rocks. Merelani tanzanite formed during the late Neoproterozoic, at about 585 million years, during the late stages of the Pan-African Orogeny. Structurally, the deposit occurs in the crest of the Lelatema Anticline of the Mozambique Collision Belt.
Locality:
mine in the Merelani Hills, near Shambarai, just west of Kimongan Mountain,
northern flank of the Lelatema Mountains, Lelatema District, eastern Arusha
Province, far-northeastern Tanzania, southeastern Africa.