Feldspar is the principle ground mineral of granite, monzonite and syenite. It makes up approximately 60-percent of these igneous rocks and gives granite its porphyritic texture (a mix of large grains with intersticial smaller grains). Feldspars are further divided into two types. They are easily identifiable in both weathered and fresh granite by their color. Plagioclase feldspar is clear or gray, and orthoclase feldspar is light pink or orange.
Plagioclase and Orthoclase
In terms of its chemical composition, feldspar is an aluminosilicate. Plagioclase is a sodium or calcium aluminosilicate, occuring widely in rocks as free crystals. Under a polarising microscope, plagioclase crystals are shown to be triclinic in nature. Orthoclase feldspar is a silicate of aluminium and potassium and occurs homogenised in rocks or as free crystals. Orthoclase crystals are monoclinic and tend to weather more quickly.
Feldspar is formed in underground geological temperature and pressure regimes. In these conditions, it is chemically stable. It only begins to chemically weather when exposed to water or acid environments on the Earth's surface. When this happens, it is chemically weathered by hydrolysis. This is the reaction between a water molecule and an ion in the feldspar that releases a hydrogen molecule, which becomes attached to a separate product. The result in solution is Kaolinite.
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Kaolinite is aluminum silicate hydroxide. It is a white or gray clay mineral, which is the chief constituent of kaolin clay. The precise chemical nature of the kaolin will be defined by the nature of the original feldspar, that is, whether it was aluminum, sodium, calcium or potassium rich, as these are the ions that will have been dissolved in solution.
The word "Kaolin" comes from an area in China, where it was first found. This fine, white clay has been used for more than a thousand years to make porcelain and china. Potassium feldspar is also used in the manufacture of glass.