Tuesday, December 28, 2010

Tin

Tin is a metallic element (number 50 on the periodic table) and abbreviated Sn.  The word tin is of Germanic origin and the Sn for the symbol comes from its Latin name stannum.  Tin is directly above lead (Pb) on the periodic table with which it shares some chemical properties.

Tin has two oxidation states, +2 and +4 with the +4 being more stable.  This leads to a common oxide ore mineral of tin, cassiterite, with the formula SnO2 (oxygen has a -2 oxidation so the tin is +4 in cassiterite).  The name derives from the Greek kassiteros (tin) derived from the Phoenician word for Britain (Cassiterid), an important source of tin in the ancient world (a topic we'll return to tomorrow).


Cassiterite has a high density and its specific gravity, around 7.0, means its 7 times heavier than an equal volume of water.  This leads to cassiterite occuring as placer deposits in many areas.  The cassiterite weathers out of rocks and is concentrated by streams into alluvial placer deposits much like gold.

Placer mining was very important in the state of Perak in northwestern peninsular Malaysia near the city of Ipoh (see map).  In a very labor-intensive practice, pits were dug into alluvial gravels and the sediments were then washed over boards with riffle traps catching the heavy cassiterite grains (similar to the way placer gold mining is done, see diagram at left as a simple example).

Malaysian tin mining began in the 1840's, provided over 30% of the world's tin around 1980, but is only a minor source today since the price of tin on the world market is too low to make these mines economical.

Tin is not mined in the United States.  Most tin today comes from China and Indonesia.  In China, deposits occur in an area called the Southeast Yunnan Tin Belt.  The Yunnan Province in on China's southern boundary and the tin belt is south of the provincial capital of Kunming and just north of China's border with Vietnam (see map).  Tin here occurs in quartz veins associated with Mesozoic-aged granitic intrusions.  In Indonesia, tin is mined on Bangka Island ("Tin Island") off the southeastern coast of Sumatra (see map).  Much of the mining here are alluvial placer deposits derived from Mesozoic-aged intrusives as well.

Most tin produced today (over 50%) is used as an alloy in solders (metal pipe and electrical connections).  Much of the rest is used in tin plating of various metals for corrosion resistance and in various chemicals.  Tin was formerly used a lot more in the production of tin cans and tin foil but has now been replaced by aluminum.

Tin is historically interesting because it can be alloyed with copper to create bronze.  I'll leave that for tomorrow's post.

2 comments:

  1. I happened across your blog today and can't tell you how much I have enjoyed it. I loved the article about Ozzy, particularly the story about his blowing out the candles of the satanists and singing Happy Birthday. That was a pretty unusual response to his wedding proposal too. Thanks so much for the wonderful things you've written about. All so very interesting. You're making a world a whole lot better place. :-)

    Thanks so much and Merry Christmas! Alena

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