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Nepheline, also referred to as nephelite is a significant rock-forming mineral deposit amongst other minerals in the feldspar group. The feldspathoids are minerals with poor silica content and chemistries similar to the alkali feldspars. Its name was derived from a Greek word which means "cloud" and has the chemical formula Na3KAl4Si4O16.

Nepheline is one of the main constituents of various igneous rocks such as nepheline syenite, nepheline monzonite and nephelinite, with a basic difference in the types of feldspars. In nepheline monzonite, the plagioclase and K-feldspars are present and in almost proportional amounts. The nepheline syenite has K-spars and potassium feldspars as the main feldspars. The nephelinite contains a small amount of feldspars, making it almost similar to the nepheline.

Characteristics of Nepheline

·    Its lustre is either greasy or dull in specimens that have undergone weathering.

·    It has a hexagonal crystal system.

·    It has a poor cleavage with a conchoidal or uneven fracture.

·    It contains a white streak and is associated with minerals like calcite, apatite, albite, biotite, etc.

Nepheline is an undersaturated aluminosilicate that is found in volcanic rocks, intrusive rocks and in related pegmatites. It is usually a constituent of gneiss and mica schist. There are two types of nepheline, which both look, different in their outward appearance, as well as their manner of existence.

The first type called the eleolite had its name derived from Greek words meaning oil and stone by M.H. Warproth in 1809. This greasy lustre is due to the presence of abundant microscopic surroundings of minerals such as the augite and hornblende.

The second type is the glassy nepheline which is a small, transparent crystal that is colourless and has a vitreous lustre. It has similar qualities to that of the volcanic rocks with a high quality alkalis. 

Additional Resources

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