Characteristics of some corundum from songea deposit, Tanzania

Abstract

Recently, many gem corundums have been imported from around the world. The most important localities are Myanmar, Madagascar and Sri Lanka where are well known sources of beautiful ruby and blue sapphire. However the large mass productions of gem corundums are mostly from East Africa. Geological setting of East Africa, appears to have been effected by Pan-African event, generating Mozambique Orogenic Belt. Rocks in this belt consist of metamorphic rocks, mainly grouped as granulite facies. The potential gem deposits in Tanzania are related to Mozambique belt. New large corundums deposits have been discovered in the southern part of the country, so called Songea areas. Songea corundums appear to have formed during prograde and retrograde metamorphism between granulite and amphibolite facies. Corundum samples from Songea have wide range of colors (e.g. red, red orange, orange, violet, violet blue, blue, purple, yellow, yellow green, green and colorless); many of them show color patch or zoning. Corundum samples available for this study can be divided into 5 main color varieties including red, purple, blue, yellow and colorless. Color-change effects from blue/purple blue in daylight to purple/purple red in incandescent light can be observed in purple and blue color samples. Luminescence under ultraviolet lamp is weak to moderate or inert under long wave and mostly inert under short wave; in addition the red variety shows weak red under cathodoluminoscope. Mineral inclusions are characterized by apatite, epidote, plagioclase, garnet, hematite, mica, paragonite and rutile. Trace element analyses reveal rather high iron content (1.01-4.39 %wt). Although Songea corundum samples could not be distinctively separated from metamorphic and basaltic corundums from elsewhere; they have some specific features (e.g. mineral inclusions and trace compositions) that would be useful for gemological laboratory. These corundums have low potential for heat treatment, except for red and purple stones that can be purified their red shades by heating at 1,200°c or lower temperature in oxidizing atmosphere.