Cassiterite

cassiterite

ferberite

molybdenite

arsenopyrite

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Formula: SnO2 simple oxide, rutile group
Specific gravity: 6.98 to 7.01
Hardness: 6 to 7
Streak: Brownish white, white, greyish
Colour: Black, yellow, brown, red or white.
Solubility: Slightly soluble in hydrochloric and nitric acid; insoluble in sulphuric acid
Environments:

Plutonic igneous environments
Pegmatites
Placer deposits
Hydrothermal environments

Cassiterite is widely distributed. It occurs as a primary mineral in igneous rocks and pegmatites but it is more commonly found as an unaltered primary mineral in hypothermal (high temperature) hydrothermal quartz veins of tin deposits in or near granitic rocks. Because of its durability it is also found frequently in placer deposits.
In high temperature quartz veins associated with granitic intrusions cassiterite is often associated with ferberite, molybdenite and arsenopyrite.

Localities

At Llallague, Bolivia, cassiterite in hydrothermal veins crystallises at about 300oC (Mineralogy and Petrology 111.547-568).

The Needle Hill Mine, Needle Hill, Sha Tin District, New Territories, Hong Kong, China, is a tungsten mine, abandoned in 1967. The principal ore is wolframite, and the principal gangue mineral is quartz. Molybdenum also occurs. The mineralisation consists of a series of parallel fissure veins that cut through granite. Wolframite and quartz are the main minerals, but galena, sphalerite, pyrite, molybdenite and fluorite have also been found here (Geological Society of Hong Kong Newsletter 9.3.29-40). The quartz-wolframite veins are of high-temperature hydrothermal formation, and grade into wolframite-bearing pegmatites. Wolframite is almost always associated with molybdenite. Other associated minerals found occasionally include pyrite, chalcopyrite, arsenopyrite, sphalerite, bismuth, fluorite, topaz and cassiterite (Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)

At Wheal Coates, St Agnes, Cornwall, Engand, UK, a cassiterite pseudomorph after a Carlsbad twin of orthoclase has been found (KL p141).

At the Emmons pegmatite, Greenwood, Oxford county, Maine, USA, cassiterite has been found as crystals to several centimetres and as masses to 60 cms. Sometimes it is found in intimate intergrowth with columbite group species. The Emmons pegmatite is an example of a highly evolved boron-lithium-cesium-tantalum enriched pegmatite (R&M 94.6.505-506).

Common impurities: Fe,Ta,Nb,Zn,W,Mn,Sc,Ge,In,Ga

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