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Formula: SnO2
Simple oxide, rutile group
Crystal System:
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 Llallagua, Bolivia, cassiterite in hydrothermal veins crystallises at about 300oC
(Mineralogy and Petrology 111.547-568).
Cassiterite from Llallagua - Image
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 Chongyi County, Ganzhou, Jiangxi, China, large opaque, black, vitreous, striated cassiterite crystals occur
(AESS).
Cassiterite from Jiangxi - Image
At Johanngeorgenstadt, Erzgebirgskreis, Saxony, Germany, tin was first mined as
early as the 15th century. Besides the placer deposits, where
cassiterite which had weathered out of the ore veins was recovered as
eluvium, many primary veins were successfully mined. Some of
the granite and schist
bordering the tin veins contained tin
contents as high as those of the veins themselves. Cassiterite was also present as nests in the upper-lying
silver - bismuth -
cobalt ore veins of Johanngeorgenstadt. Highly elongated nests of this type
were exploited in the Neujahr, Gottes Segen, Frisch Glück and other veins. Large cassiterite crystals were
fairly rare; cassiterite appeared most commonly as small grains and as fine disseminations in
quartz and country rock
(MinRec 55.5.591).
At Wheal Coates, St Agnes, Cornwall, Engand, UK, a cassiterite pseudomorph
after a Carlsbad twin of orthoclase has been found
(KL p141).
Cassiterite from Wheal Coates - Image
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).
Cassiterite from the Emmons Quarry - Image
Common impurities: Fe,Ta,Nb,Zn,W,Mn,Sc,Ge,In,Ga
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