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Plagioclase

plagioclase

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The feldspar family is one of the broadest families of minerals. The Plagioclase is an essential series amongst the tectosilicate minerals, a subclass of the silicates. Its name was gotten from the Greek word meaning "oblique fracture" due to the angle of its cleavage which was first discovered by Johann Friedrich Hessel, a German mineralogist in 1826. It is more appropriately called plagioclase feldspar with members from the albite to the anorthite.

The plagioclase feldspars are six in number and include the albite, oligoclase, andesine, labradorite, anorthite and bytownite. The composition of these feldspars is determined by the percentage of albite or anorthite that they contain.

·    Albite: This feldspar has a pure white colour and so was named from the Latin word "albus". It has 100-90% albite and 0-10% anorthite. It is a sodium aluminium silicate and is a widespread rock-forming mineral.

·    Oligoclase: This name was obtained from the Greek word for little and fracture due to its cleavage angle which is not 90 degrees. It is a sodium calcium aluminium silicate and can be found in syenite, granite, gneiss and diorite. It has 90-70% albite and 10-30% anorthite.

·    Andesine: This is a sodium calcium aluminium silicate with 70-50% albite and 30-50% anorthite. It is formed in rocks like diorite which have a fair amount of silica.

·    Labradorite: This is related to either the amphiboles or the pyroxenes. It is a calcium sodium aluminium silicate with 50-30% albite and 50-70% anorthite. The refraction of light through the crystal causes it to display many bright colours.


·    Bytownite: This mineral was termed after "Bytown" which was the former name for Ottawa, Canada. It is a calcium sodium aluminium silicate with 10-30% albite and 70-90% anorthite.

·    Anorthite: Due to its triclinic crystallization, it was termed after a Greek word meaning "oblique" by Rose in 1823. It is a calcium aluminium silicate with 10-0% albite and 90-100% anorthite.

The plagioclase is the main constituent of the earth crust. It is used in petrology to identify the origin, composition and formation of igneous rocks. The presence of plagioclasais not only restricted to the earth alone because it is also found in large quantities in rocks on the moon.

Additional Resources

Phanteritic   Porphyry   Potassium Oxide

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