Quartz (SiO2) is the most abundant mineral in terrigenous sedimentary rocks and is exceedingly durable (surviving multiple generations of weathering and deposition). Quartz and silica occur in many varieties—true quartz in the form of megaquartz, chert, microquartz, or chalcedony and various other forms of silica, mainly opal (opal-A and opal-CT [cristobalite]).
Feldspars (XAl(1-2)Si(2-3)O8) are the most common rock- forming minerals in the Earth’s crust, and they occur in many varieties — ranging from sodium- and calcium-rich (plagioclase) to potassium-rich (K-feldspar or alkali feldspar). K-feldspars may also contain significant amounts of sodium in their crystal lattices. Feldspars are far less resistant than quartz to chemical and physical destruction and thus are altered or removed by weathering, transport and diagenesis, yielding secondary pores or alteration products (illite, white mica/sericite, albite or kaolinite). Even so, they are the second most abundant grains in sandstones, and identifying their mineralogy is crucial for accurate sandstone classification and provenance studies.