The name sandstone is, of course, based on the fact that this rock is made up largely of sand grains. Sandstones that contain less than 10% clay matrix are called arenites. These can be subdivided based on the percentages of Quartz, Feldspar and unstable lithic fragments (fragments of preexisting rock). Sandstones are composed of mostly quartz, feldspar, and lithic fragments. Other minerals also occur, depending on the mineralogical maturity of the sandstone.
- Quartz grains: Greater than 2/3 of the minerals found in sandstones is quartz because it is one of the most abundant minerals in crystalline rocks like granitoids, schists, and gneisses. Quartz is also mechanically durable due to its high hardness and lack of cleavage. Quartz is chemically stable under conditions present at the earth's surface. It has a very low solubility in water as well.
- Feldspar grains: In fact, feldspars are the most common minerals in igneous and metamorphic rocks. Feldspars are less stable than quartz at conditions near the earth's surface. As a result, feldspars make up only 10-15% of all sandstones. As, feldspars are unstable in the sedimentary environment, most feldspars in sandstones show the effects of alteration. This is usually evident as growths of microcrystalline clay minerals along cleavage planes and on the surfaces of the feldspars.
- Rock fragment grains: With the exception of fragments of quartz, lithic fragments are generally unstable in the sedimentary environment, yet, if present in a sandstone give the best clues to provenance. Any type of rock fragment can be found in a sandstone, but some kinds are more common because of areal extent in the source drainage basin, stability of the rock fragment in the sedimentary environment.
