Dioxides – Cassiterite, Tin Stone Composition and Structure

Composition. Tin dioxide, SnO2 = Oxygen 21.4, tin 78.6.

Crystallization. Tetragonal. Common forms are prisms and pyramids of first and second orders (Fig. 267). Frequently in elbow. Shaped twins; twinning plane being a pyramid of the second order (Fig. 268).

Structure. Usually massive granular; often in reniform shapes with radiating fibrous-like structure (wood tin), crystallized.

Physical Properties. H. = 6.7.  G. = 6.8-7.1 (unusually high for a mineral with nonmetallic luster). Nonmetallic, adamantine luster to submetallic and dull. Color usually brown or black; rarely yellow or white. Streak white.

Test. Infusible. Gives globule of in with coating of white tin oxide when finely powdered is fused on charcoal with a mixture of sodium carbonate and charcoal powder. Insoluble. When fragments of cassiterite are placed in dilute hydrochloric acid together with a little metallic zinc the cassiterite becomes coated with a dull gray deposit of metallic zinc which becomes bright on friction. Recognized by its high specific gravity, its color and light streak.

Occurrence. Cassiterite is widely distributed in small amounts but is only produce on a commercial scale In a few localities. Casiterite has been note as an original constituent of igneous rocks, but it is more commonly to be found in veins associated with quartz. As a rule tin-bearing veins are found in or near pegmatites or granitic rocks. Tin veins usually have minerals which contain fluorine and boron, such as tourmaline, topaz, fluorite, apatite, etc., and the minerals of the wall rocks are commonly much altered. It is thought, therefore, that the tin veins have been formed through the agency of vapors which carried tin with boron and fluorine. Cassiterite is at times a minor constituent of pegmatite veins. Also it is found in the from of rolled pebbles in placer deposits.

Cassiterite is not found in large quantities in the United States, the most important locality being on the Seward Peninsula, Alaska. Found also in the pegmatites of North and South Carolina; in the Black Hills, South Dakota. The world’s supply of tin ore comes from Tasmania, from New South Dakota. The world’s supply of tin ore comes from Tasmania, from New South Wales, Queensland and other states of Australia, from Bolivia and from the Malay States. Cornwall, England, has produced large amounts of tin ore in the past.

Use. only ore of tin. Chief use of tin is in coating or “tininning” metals, particularly iron, to form what is known as sheet tin. Tin is also used in various alloys: solder, containing tin and lead; bell-metal and bronze, containing copper and tin.