Most of the silt-sized fraction is quartz, but very little of the clay sized fraction is quartz. Single crystal of quartz may originate as detrital grains produced by abrasion of larger grains, may be diagenetically produced during the smectite to illite transformation, or may be recrystallized remains of silica secreting organisms like diatoms and radiolaria. Feldspars also occur in the silt-sized fraction of most mudstones, but in very small amounts. The lack of feldspars in mudrocks in comparison to river sediment is due to the unstable nature of feldspar - it continues to weather even after deposition, and due to diagenetic changes that take place in the sediment. Carbonate Minerals .
Carbonate minerals occur in mudrocks with about the same abundance as feldspars.
Most is probably calcite and is produced either by diagenesis or as detrital grains produced by foraminefera or other microorganisms. Organic Matter . As discussed above, deposition in waters with restricted circulation or rapid burial can result in reducing conditions and the preservation of carbon and carbon compounds in mudrocks. In some cases, like oil shales, the carbon compounds undergo further transformations that convert them to hydrocarbons like petroleum. Otherwise, the carbon or carbon compounds generally tend to give the rock a dark gray to black color.