major minerals can be categorized into subdivisions based on their resistance to chemical decomposition. those that possess a great resistance to decomposition are categorized as stable, while those that do not are considered less stable. the most common stable mineral in siliciclastic sedimentary rocks is quartz (sio). quartz makes up approximately 65 percent of framework grains present in sandstones and about 30 percent of minerals in the average shale. less stable minerals present in this type of rocks are feldspars, including both potassium and plagioclase feldspars. feldspars comprise a considerably lesser portion of framework grains and minerals. they only make up about 15 percent of framework grains in sandstones and 5% of minerals in shales. clay mineral groups are mostly present in mudrocks (comprising more than 60% of the minerals) but can be found in other siliciclastic sedimentary rocks at considerably lower levels.