Chert should not be confused with siltsone Siltstone will generally show a cementing of individual grains. Chalcedony is cryptocrystalline quartz having birefringence about that of quartz. It is invariably fibrous in character, often including radiating or spherulitic aggregates. Chalcedony and chert are usually colorless, but may exhibit weak color without pleochroism. Feldspars are common minerals, but due to weathering, are not often observed in abundance in soils, and usually absent in strongly weathered soils.
Feldspars are usually colorless in thin section, have low relief and lack pleochroism.
Cleavage is good. Lath and prismatic shapes are common yielding parallel or inclined extinction. Perthite is the intergrowth of K and Na feldspars, commonly albite in microcline or orthoclase. Perthite may give the appearance of twinning, but the intergrowths often are irregular and not as sharp as twins. Since this is an intergrowth of two feldspars, differential weathering may result in one phase being altered.