Best known as an ore of the soft metal tin, cassiterite (SnO2) has an elemental magic all its own. Cassiterite occurs in igneous rocks such as granite and metamorphic rocks such as greisen, a thermally altered type of granite.
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Significant sources of cassiterite come from placer deposits dated as far back as c. 4500 BCE. Cassiterite is also found in some rock veins.
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As a crystal, cassiterite has a captivating dark glow. It's usually dark brown or black. Other shades include dark golden brown, brownish black, reddish brown, red and yellow. It may occur in white but is never clear. Most sources of cassiterite are placer or alluvial deposits.
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They're formed as streams and rivers erode shoreline and rocks. As running water uncovers deposits, sediment and granules are carried downstream. When the water slows, as at a widening of the river, rocky pools or a curve, heavier elements fall to the riverbed.
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Placer deposits can include gold, cassiterite, platinum, diamonds, magnetite, and some gemstones such as sapphire. Alluvial placers can carry sediment a long way. Placers associated with basalt, an igneous rock, are the most commercially viable sapphire sources in ancient times and today.
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Placer deposits can also happen on beaches. Famous gold deposits in Nome, Alaska, as well as zircon in South America, and South African diamonds are from beach placers.
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Repeated wave action sifts through the sediment, moving elements of various weights into concentrated areas. Beach placer finds include gold, diamonds, cassiterite, zircon as well as ilmenite, rutile and monazite.
The Bolivian source and the c. 2200 BCE workings of Cornwall and Devon, England, are concentrated in high temperature quartz veins and pegmatites (coarse granite or other igneous rock). The veins often contain tourmaline, topaz, fluorite, apatite, wolframite, molybdenite, and arsenopyrite.
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Current major cassiterite mining sites include placer deposits in China, Malaysia, Myanmar (Burma), Thailand, Indonesia, parts of Somalia, and Russia.
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