Cassiterite
Tin
Oxide
SnO2
Cassiterite
is a mineral that has ornately faceted specimens with high luster. It
is generally opaque, but its luster and multiple crystal faces cause a
nice sparkle. Cassiterite has been an important ore of tin for eons
and is still the greatest source of tin today. Most sources of
cassiterite today are not primary deposts but alluvial deposits
containing weathered grains. The best source of original-formation
cassiterite is at the tin mines of Bolivia, where it is found in
hydrothermal veins. Although found throughout the world in many
igneous rocks, cassiterite is usually only a minor constituent. The
Bolivia veins and those worked and nearly exhausted in Cornwall,
England, somehow concentrated the tin in a way not fully understood by
geologists.
Twinning is common in cassiterite and most aggregate specimens show
crystal twins. The typical twin is bent at a near-60-degree angle,
forming an "Elbow Twin". Multiple twinning can continue to
bend the crystal around and possibly form a cyclic twin. However,
cassiterite does not form this type of twin as often as its mineral
cousin, rutile.
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