Sericite

Sericite is a fine grained mica, similar to muscovite, illite, or paragonite. Sericite is a common alteration mineral of orthoclase or plagioclase feldspars in areas that have been subjected to hydrothermal alteration typically associated with copper, tin, or other hydrothermal ore deposits. Sericite also occurs as the fine mica that gives the sheen to phyllite and schistose metamorphic rocks.

The name comes from Latin sericus, meaning "silken" in reference to the location from which silk was first utilized, which in turn refers to the silky sheen of rocks with abundant sericite.

...
Wikipedia