Sedimentary Rocks: Detrital, Organic & Chemical Types
Detrital (Mechanical) Origin
Detrital or mechanical origin: mineral particles formed by the mechanical disintegration of other rocks. These particles are transported by agents such as water and accumulate as sand, gravel, blocks, ash and other clastic material.
Sandstone
Sandstone: an intermediate clastic rock formed from mechanically derived granules. It consists of consolidated masses of sand held together by a cement. Sandstone is essentially composed of quartz sand.
Cement (Matrix)
Cement: the material that binds grains in sedimentary rocks. It is usually silica, calcium carbonate, or iron oxide. The color of a sandstone (or other clastic rock) is strongly influenced by the cement: iron oxides produce red or brown tones, while other cements can give grayish or whitish colors.
Conglomerate
Conglomerate: composed of clasts larger than 2 mm united by a cement (e.g., calcite or silica). Clasts are embedded in a generally fine sandy matrix. There are two main types:
- Rounded-clast conglomerate (pebble conglomerate)
- Breccia: composed of angular fragments
Clay
Clay: a slightly consolidated clastic rock composed of hydrated aluminum silicates. Pure kaolinite (kaolin) clay is soft and white, has variable plasticity, and retains its color when fired (used in porcelain). Clays that contain impurities change color, and their plasticity can be more or less malleable. Clays are pliable when moistened with water.
Organic Origin
Organic origin: rocks formed by the accumulation of animal and plant remains. These are commonly divided into calcareous (limestone-related) and carbonaceous (coal-related) types.
Limestone
Limestone: composed primarily of calcite. It forms in both marine and freshwater environments by: a) chemical precipitation of calcite, and b) accumulation of organisms with calcareous skeletons.
Chalk
Chalk: a variety of fine-grained limestone of marine origin. It is typically white and formed from the accumulation of microscopic calcareous shells (foraminifera, coccoliths) and is composed of almost pure calcite.
Coral Limestone
Coral limestone: formed by the accumulation of the tiny skeletal remains of coelenterates (corals). These skeletons are often bound together by substances produced by algae, forming reef structures.
Peat
Peat: organic material formed from the decomposition and accumulation of plant matter in swamps and marshes. It is dark brown, earthy and mossy in appearance, with little visible structure to the naked eye. Peat represents the first stage in the transformation of vegetation into coal. It has a heating value similar to wood, lights and burns easily, and produces a lot of smoke.
Lignite
Lignite: a brown coal of relatively low calorific value. It is dark brown to black, contains high moisture, and often preserves woody structures. Lignite is mainly used as fuel in power plants.
Coal
Coal: a highly carbonized black organic rock produced by further transformation of lignite. Coal originates from the accumulation and burial of plant material (often beginning in Carboniferous environments). There are many varieties of coal depending on carbon content and rank.
Anthracite
Anthracite: the highest rank of coal, with the highest carbon content. It is pitch black with a resinous, almost metallic luster. Anthracite burns with difficulty but releases a large amount of energy and produces little smoke or soot.
Oil (Petroleum)
Oil: liquid organic hydrocarbons produced by the anaerobic decomposition of marine plankton and other organic matter. These hydrocarbons become concentrated and trapped in porous sedimentary rocks, forming petroleum reservoirs.
Siliceous Rocks
Siliceous rocks: composed primarily of silica. Their formation is commonly due to the accumulation of: 1) siliceous sponge spicules, 2) diatom and radiolarian shells, and 3) chemical precipitation of silica.
Chemical Origin
Chemical origin: rocks formed by the precipitation of dissolved minerals from water, often as the water evaporates or when chemical conditions change.
Halite
Halite: the mineral form of common salt (sodium chloride). It precipitates from evaporating saline waters.
Gypsum
Gypsum: formed by precipitation of calcium sulfate from water, frequently in marine evaporitic settings. Gypsum is often associated with halite and anhydrite. It can be clear, colorless, white or gray, has a vitreous luster and relatively low hardness. Gypsum contains water of crystallization; if heated to around 128 ºC it loses water and converts to a plaster (hemihydrate). Impurities affect its color.
Anhydrite
Anhydrite: the anhydrous form of calcium sulfate. It is denser and harder than gypsum, crystallizes in the orthorhombic (rhombic) system, and may be white, blue or gray with a vitreous luster. On hydration, anhydrite converts to gypsum.