1st order birefringence, 'tartan' twinning is diagnostic, with tapering twin lamellae. Fairly common in sediments. Most plagioclase in sediments appear to be Na-rich. It is possible to distinguish tiny crystals of clay minerals. Extinction is characteristically speckled and flakes are often deformed in sediments.
Authigenic muscovite has a fresh appearance and cross-cuts other textures.
This mica is generally brown, but may also be green. Biotite is less stable than muscovite, especially when deformed, and may alter to chlorite or iron oxides. Less common than muscovite in sediments. There is much intergradation due to base exchange and mixed-layer clays. Commonly forms as alteration product of silicate minerals and rock fragments.