The Mineral CASSITERITE

Chemistry: SnO2, Tin Oxide
Class: Oxides and Hydroxides
Group: Rutile
Uses: major ore of tin
Specimens

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.

PHYSICAL CHARACTERISTICS:

Color is black or reddish brown or yellow.
Luster is adamantine or greasy.
Transparency crystals are transparent in thin crystals otherwise opaque.
Crystal System is tetragonal; 4/m 2/m 2/m
Crystal Habits include eight-sided prisms and blocky or stubby crystals terminated by a blunt four-sided or complex pyramid. The prisms are composed of two four sided prisms with one of the prisms being dominant. Also thin acicular needles or blades are common. Can be massive, granular, fibrous and botryoidal. A concretionary form combined with quartz and hematite is called "wood-tin".
Cleavage is good in two directions forming prisms, poor in a third (basal).
Fracture is conchoidal to uneven.
Hardness is 6 - 7
Specific Gravity is 6.6 - 7.0+ (very heavy for non-metallic minerals)
Streak is white, but at times brownish.
Associated Minerals include, but are not limited to, tourmalines, molybdenite, bismuthinite, topaz, fluorite, arsenopyrite and wolframite.
Other Characteristics: high refractive index of approximately 2.0.
Notable Occurences include the La Paz and Colquiri areas of Bolivia; Cornwall, England; Durango, Mexico; Malaya; Indonesia; Russia and China.
Best Field Indicators are crystal habit, hardness, twinning and high index of refraction (luster).