Gypsum may form in some environments due to oxidation of pyrite. Gypsum may be confused with barite, but barite has a higher relief in thin section. Barite is colorless and non-pleochroic with moderate relief and weak interference colors. It is prismatic to lenticular in shape with cleavage in one direction. Not too common in soils, but may be found in saline soils or saturated conditions.
Mica occurs as platy grains that often are thin.
All the micas are closely related structurally and crystallographically. In thin section, micas are characterized by distinct relief, strong birefringence, single cleavage, low to moderate relief, positive elongation, and parallel extinction. Twinning is rarely observed. Biotite shows substantial variability in chemical composition, and thus, variable optical properties. The interference color is often masked by the mineral's color.