Cassiterite

Cassiterite which is also known as tinstone is a heavy, metallic hard tin dioxide(SnO₂) that is the major ore of tin. Cassiterite is colorless when it is pure but turns to brown or black when the iron impurities are present. It is generally opaque but is also translucent in thin crystals. Throughout ancient history, cassiterite was the chief tin ore and even to the present day, it remains the most important source of tin. Cassiterite also occurs in granites and pegmatites.In the fifteenth century, the cassiterite veins present in Saxony and Bohemia were mined for tin and the production of tin was at its peak at the same place in the seventeenth century. In the eighteenth century and the nineteenth century, the very large vein deposits of Cornwall were the major source of tin. In the present day, most of the world’s cassiterite has been mined in Indonesia, Bolivia, Malaysia, Nigeria, Myanmar, Thailand, and other major parts of China. In this article on cassiterite, we are going to discuss what is cassiterite, cassiterite ore, its occurrence, and its physical properties.

What is Cassiterite? 

Cassiterite name has been derived from the term Cassiterides which in pre-Roman times was applied for ‘islands off the western coast of Europe’.Cassiterite is a tin oxide mineral and it is considered to be the most significant source of tin and most of the world’s supply of tin is derived by mining out cassiterite. Cassiterite chemical composition is SnO₂. Throughout the world, small amounts of primary cassiterite are found in igneous and metamorphic rocks. Cassiterite is also the residual mineral that is found in solid and sediments. It has been shown that cassiterite is more resistant to weathering when compared to other minerals and as a result of this reason, cassiterite in nature is concentrated in the streams and shoreline sediments. Cassiterite is the most important ore of tin and despite that its concentration is high in only a few locations. Cassiterite has been found in the hydrothermal veins and pegmatites which are associated with granite intrusions. 

Occurrence of the Cassiterite Ore 

•  The primary source of cassiterite ore which is worth mining is found in the high-temperature hydrothermal veins that companies granitic intrusions. 

• The alluvial or the placer deposits which contain the resistant weathered grains is considered to be the main source of cassiterite in today’s world. 

• Along with the deposits of cassiterite, there are also deposits of fluorite, topaz, apatite, and tourmaline. 

• The most important deposits of primary cassiterite ore are found in Brazil, Indonesia, Australia, Bolivia, China, England, The Democratic Republic of Congo, Peru, Portugal, Russia, Spain, and other countries of South Asia. 

• The secondary placed deposits are responsible for producing the world’s most cassiterite. These are sediment-hosted concentrations of cassiterite in stream valleys and along the shorelines. 

•  The hardness of cassiterite allows it to survive the stream transport and its property of high specific gravity causes it to concentrate in the deposits that are large and rich enough for mining. 

• The tin mines of Bolivia are the best source of the primary cassiterite, in the tin mines, the cassiterite is found in the hydrothermal veins. Rwanda has a nascent cassiterite mining industry.

• There are also other high specific gravity minerals that occur in the tin deposits. Deposits if cassiterite is being found in Burma, China, Indonesia, Malaysia. Myanmar, Nigeria, and Rwanda.

• There has been fighting over the cassiterite deposits particularly in Walikale and it is considered to be the major cause of the conflict waged in the eastern part of the Democratic Republic of Congo. Due to this reason cassiterite has been considered as the conflict mineral. 

• The United States of America does not have any major deposits of cassiterite or tin minerals and is heavily dependent on other countries for it. There are small deposits in Alaska, South Dakota, and other states.

• In the present day, the major production of tin comes from placer or the alluvial deposits in Malaysia, Thailand, Indonesia, Russia, and the Maakhir region of Somalia.

Physical Characteristics of Cassiterite

• The Colour of cassiterite is black, brownish-black, reddish-brown, brown, red, yellow, gray, white and  it is rarely colorless

• Transparency: Cassiterite is a very transparent mineral and they are transparent in the thin crystals otherwise are opaque. 

• Cassiterite has a luster that is either greasy or adamantine. 

• The crystal structure of cassiterite is tetragonal. 

• Cleavage is good but in two directions which forms prisms but it is poor on the third side(basal).

• Mohs Hardness of cassiterite is between 6-7.

• Specific Gravity of cassiterite mineral is between 6.8-7.1 which is very heavy for nonmetallic minerals.

• Cassiterite mineral has a high refractive index which is approximately close to 2. 

• Diagnostic properties of cassiterite include high specific gravity, bright metallic to adamantine luster, light streak, and fibrous appearance. 

• The chemical composition of cassiterite is SnO₂, Tin oxide. 

• The cassiterite mineral is used as an ore of tin, a collector’s gem, and a mineral specimen.

• The other minerals that are associated with cassiterite are molybdenite, bismuthinite, topaz, fluorite, arsenopyrite, tourmalines, and wolframite.

• The cassiterite mineral is either infusible or soluble with any other compounds. 

• The primary cassiterite ore is found in places such as England; Durango, Mexico; Malaya; Indonesia; Russia,  China, La Paz, and Colquiri areas of Bolivia and Cornwall

Cassiterite - The Gemstone

Cassiterite is a gem that is very rare to be found. Cassiterite must be transparent, free of fractures, have attractive color, and should have a high clarity to be gem-quality cassiterite. When the cassiterite mineral is cut properly it can be a beautiful gemstone. Cassiterite as gemstone occurs in various colors such as yellow, brown, orange, red, and green. Some gemstones of cassiterite have a strong fire that even rivals that of a diamond. 

Cassiterite as the gemstone is not found in the jewelry stores as it is very rare and that is the reason that there is no demand for it. The cassiterite gemstone is so rare that the adequate amounts required to support a marketing campaign are not available and as a result of this cassiterite is cut mainly for collectors and museum exhibits. 

High dispersion is the one property of cassiterite that makes it an eye-catching gemstone. Dispersion is the ability of any material to separate the white light into separate spectral colors. High dispersion is the property that is responsible for producing the colorful “fire” of a diamond. The dispersion of diamond is about 0.044 whereas the dispersion of cassiterite is 0.71 which is considerably higher than that of a diamond. Due to the high dispersion property of cassiterite, it enables it to produce a fire that exceeds that of a diamond. Cassiterite gemstones which have light colors show a strong fire whereas in many cases the cassiterite with dark color partially masks the fire.