MUDROCKS

Mudrocks when viewed closely under the ore microscope are fine grained sedimentary rocks made up mostly of silt and clay size fragments. They are otherwise known as argillites. Since they are grain size, they are hard to examine even with the petrographic microscope. Mudrocks are the most abundant sedimentary rocks, comprising of over sixty five percent of all sedimentary rocks. Known to be the likely the source rocks for petroleum and natural gas, and are sometimes valuable ore deposits, they are the protoliths or precursor rock for aluminous metamorphic rocks, called pelitic metamorphic rocks. Mudrocks can be classified in various ways. As to the texture, mudrocks can be identified through the shapes of the grains, fissibility and laminations. In relation to the grain shape, clay materials are basically angular and sheet like revealing their crystal structure. Silt size quartz grains are angular or platy shaped.

Fissility is caused by the tendency of clay minerals to be deposited with their sheet structures parallel to the depositional surface. As observed, a fissile rock tends to break along sheet-like planes that are nearly parallel to the bedding planes. Fissibility is dependent on many factors like abundance of clay mineral, degree of preferred orientation, and bioturbation of organisms. Laminations are known as parallel layers less than one centimeter thick. These laminations can be seen as differences in grain size of the clasts in different laminae. These could be due to changes in current velocity of the depositing medium, or could be due to changes in the organic content and oxidation conditions at the site of deposition of the different layers. Most mudstones fall into two categories with regards to their color. Usually gray to black color of mudrocks signifies the presence of more than one percent organic matter in the form of carbon or some carbon compound. When organic matter is present in the sediment and there is enough oxygen, the carbon would quickly be oxidized to carbon dioxide. The presence of reduced carbon shows a reducing condition at the time of deposition.

Red, brown, yellow, and green colorations are indications of the oxidation state of iron in the sediments. For instance limonite gives the sediment a yellow color. The red, brown, and yellow colored mudrocks reflect deposition in an oxidizing environment. It requires very little hematite in a rock to give the rock a red stain or coloration. These varying colors of mudstone can be best appreciated when viewed under the ore microscope. In mineralogy clay minerals are the most abundant in mudstones. Other minerals like quartz, feldspar, carbonate minerals, organic compounds sulfides, and hematite also exist.

There are three groups of clay minerals in sedimentary rocks. These are most easily distinguished using the ore microscope. Kaolinites are formed in warm moist climates where calcium, sodium and potassium ions are leached and removed in solution during the weathering process. Its presence indicates a source in a humid tropical climate. Smectites are expanding clays which are formed from weathering of iron and magnesium rich igneous and metamorphic rocks in temperate climates. They are the most abundant clays in modern sediment. Illites clays are formed by weathering of feldspars in temperate climates and by alteration of smectite clays during diagenesis. They have a structure similar to muscovite. Mixed layer clays show the interlayering between smectites like layers and illite like layers in the same crystal. These are known common clays in the modern sediment. Majority of the quartz that occurs in mudrocks is single crystals. Most of the silt-sized fraction is quartz. Feldspars can be found in the silt-sized fraction of most mudstones, but in very small amounts. The lack of feldspars in mudrocks is due to its unstable nature. Carbonate minerals occur in mudrocks with about the same abundance as feldspars. Bentonite layers usually represent pyroclastic fall deposits. They make excellent marker beds.Here is a link to the article