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Sericite, the fine-grained form of muscovite or ‘white mica’, occurs as a result of hydrothermal alteration processes in rocks throughout the Cheviot Hills. It is commonly seen in altered feldspar crystals, particularly plagioclase, associated with secondary minerals that give host crystals a cloudy appearance.
Al-Hafdh’s analyses showed that sericitic alteration resulted in a net gain of potassium and the silicate and hydroxyl groups and a net loss of sodium, these gains and losses promoting the formation of sericite.
He argues for two distinct phases of sericite-forming alteration. Sericite formed in the first phase occurs in rocks lying close to veins of quartz and chlorite or calcite. The heat and pressures associated with the hydrothermal fluids that created these veins led to intense alteration resulting in the destruction of the original textures of the host rocks and gave rise to a fleshy coloration in them. In the second phase of sericitic alteration, tourmaline as well as sericite was produced.

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Identification

Sericite grains are too small to see in hand specimens and it is transparent with only moderate relief when viewed in thin-section in plane polarised light.
With polars crossed, its medium to high order interference colours make it plainly visible, commonly appearing as minute grains scattered throughout the host feldspar crystals and occasionally in larger patches.

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Photomicrographs of Cheviot Sericite

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Return to list of minerals in Cheviot rocks

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