Wednesday, May 5, 2010

May 2010 Mineral of the Month


The name Spinel comes from the Latin word ‘spina’ meaning ‘thorn’ in reference to its sharp sided crystals. Spinel is a member of the oxides mineral group. Spinel crystallizes in the isometric system; common crystal forms are octahedral, usually twinned. It has an imperfect octahedral cleavage and a conchoidal fracture. Its hardness is 8, its specific gravity is 3.5-4.1 and it is transparent to opaque with a vitreous to dull luster. It may be colorless, but is usually various shades of red, blue, green, yellow, brown or black. There is a unique natural white spinel, now lost, that surfaced briefly in what is now Sri Lanka. Some spinels are among the most famous gemstones: Among them is the Black Prince's Ruby and the 'Timur Ruby' in the British Crown Jewels, and the 'cote de Bretagne' formerly from the French Crown jewels. The Samarian Spinel is the largest known spinel in the world, weighing 500 carats. True spinel has long been found in the gemstone-bearing gravel of Sri Lanka and in limestones in Afghanistan and in Mogok, Burma. Recently gem quality spinels were found in the marbles of Luc Yen in Vietnam, the Mahenge and Matombo regions in Tanzania, the Tsavo region in Kenya and in the gravels of Tunduru in Tanzania and Ilakaka in Madagascar. Spinel is found as a metamorphic mineral, and also as a primary mineral in igneous rocks. In these igneous rocks, the magmas are relatively deficient in alkalis relative to aluminum. Aluminum oxide may form as the mineral corundum or may combine with magnesium to form spinel. This is why spinel and ruby are often found together.