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Igneous and  metamorphic rocks don't just form by magic. Some minerals do all the background work and biotite are one such example. The biotite is a widespread rock-forming natural resource with the formula K (Mg, Fe)3 AlSiO10 (F, OH). It is a widespread phyllosilicate mineral amongst other minerals in the mica group. The name "Biotite" was derived in 1847 by J.F.L. Hausmann to honour Jean-Baptiste Biot, a French scientist who, through research discovered various, peculiar properties of mica.


Biotite has a structure of silicate sheets in layers and is occasionally referred to as "Iron Mica" due to its richness in iron, unlike the phlogopite that is poor in iron. Whenever there is an increase in iron content in Biotite, its colour gets darker and its density increases. Potassium ions form a weak force that binds the constituents of biotite which include iron, aluminium, hydrogen, magnesium and oxygen.

Although Biotite has desirable features, it is hardly considered a precious mineral and it helps compliment other minerals. In places like Bancroft, huge crystals are formed by Biotite, having hornblende and green coloured apatite. A specimen of this mineral can appear exquisite when huge, attractive sizes of biotite are formed from large plates. It has sometimes been mistaken as gold due to its crystals, which produce a, sparkling golden yellow colour when weathered.

Other physical properties of Biotite include:

· It has a vitreous luster. Its crystals are either transparent or translucent.

· Its crystal system is monoclinic, and the crystals could be prismatic or tubular, often terminating at a pinnacoid.

· Due to its two pinnacoid and four prism faces, its crystal forms a pseudo hexagonal shape.

· When biotite is weathered, it can be yellow, brown or black in colour and has a streak, which is grey.

· It has an uneven fracture due to the sheets and cleavage.

· When seen as large plates, they are usually referred to as "books" because they appear as books having many pages.


Additional Resources

Basalt Uses   Black Granite   Calcite

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