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Quartz is the chemically simple mineral, silica. It consists of silicon and oxygen in the formula, SiO₂. It is familiar to people as a gemstone as rock crystal, cairngorm or amethyst.
In the Cheviot pluton it mainly occurs as tiny grains in the groundmass of the rock, and is not usually visible to the naked eye. It is a remainder product. Most of the silica had already been used up making silicate compounds such as feldspar or the mafic minerals. The quartz is what has been left over.

Quartz also occurs as the main constituent of hydrothermal veins which have been injected as superheated fluid or gas into the pluton. These are typically found at points of weakness where two different types of granitic rock have been intruded. It sometimes forms small hexagonal crystals but is usually a hard massive stone. Its veins may be accompanied by tourmaline.

Identification

When visible in hand specimens, it is milky white in colour.
In thin section in plain polarised light, it appears as clear, white amorphous grains. It has no cleavage but may develop cracks under pressure. It is not easily weathered or chemically altered. It can usually be easily distinguished from feldspars which often show cleavage and have a dusty or turbid appearance. Any doubt can be easily dispelled by examining a thin section in plain polarised light in which the quartz appears as much clearer than the surrounding feldspars.

Photomicrographs of Quartz

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Return to list of minerals in Cheviot rocks

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