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Igneous rocks have varieties which can also be altered to form yet more variables. The Greisen is an altered granite rock and is an intrusive igneous rock. It is categorized by the deficiency of biotite and feldspar and mainly consists of muscovite and quartz. Its appearance is usually silvery with a glittery look due to the numerous muscovite crystals in layers, but not all greisens have this look. Some are dull and look like pale granite. Large plates having crystalline outlines that are imperfect are mainly formed by muscovite, which is also referred to as white mica.

The transformation of granite to greisen is a steady process. Granite is usually altered and forms greisen when fluids or vapours mix through the fissures. These rising fluids contain elements like boron, fluorine and lithium and so the new minerals of the granite rock, which are mica, tourmaline and topaz, will contain these elements. The transformation occurs when the granite magma releases the vapours and cools.

The alteration of granite can be found in different greisen forms such as:

· Incipient greisen: This has properties of muscovite, tourmaline, fluorine and chlorite.

· Greisenized granite: This has properties of quartz, muscovite, topaz, fluorite and probably tourmaline because it retains the granite texture.

· Massive greisens: This has properties of the quartz, topaz, muscovite and probably the fluorite and tourmaline.

Greisens can only be formed in intrusions that are high in the earth crust usually just a few meters beneath the surface. Generally, greisens are related to potassic igneous rocks (the S-type granite). They are also potential for mineralisation because some elements, which are contrary, are concentrated due to the last crystallization process of the granite. These elements include tungsten, molybdenum, fluorine and tin, including silver, gold and sometimes copper.

Greisens can be found as deposits in the following areas:

· Cornwall has the tin deposit

· Gold-greisen deposit is found in Timbarra, Australia.

· The tin greisens can be found in Alaska, USA.

Other deposits like the tin, topaz, tin-antimony greisens and beryl are located in other areas around the globe.

Additional Resources

Graphic Texture   Hornblende   Igneous Rock

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