Authigenic minerals

(mineralogy) A mineral which has not been transported but has been formed in place. Also known as authigenic mineral.

Minerals that are formed in sediment or a sedimentary rock. Their in-place origin distinguishes them from minerals that are formed elsewhere and transported to the site of deposition (detrital minerals). Authigenic minerals form at the Earth's surface as well as during subsequent burial. The postdepositional processes are referred to as diagenesis, and the resulting minerals are important clues to postdepositional physical and chemical changes in the rock. See also Diagenesis; Sedimentary rocks.

Authigenic minerals precipitate from the overlying water column, pore fluids in the sediment, recrystallization or alteration of preexisting minerals, and structural transformation of one mineral to another. The minerals change in an attempt to equilibrate to the physical and chemical conditions present at any given time. Critical factors in their formation are initial mineral assemblage, temperature, pressure, ionic concentration, pH, electron availability, and the fluid flux through the rock.

In sedimentary rocks it is common to find a record of multiple diagenetic events based on the authigenic minerals. For example, in sediments near the surface, meteoric water may displace original marine pore water, resulting in distinct types of cements. Iron oxide can result from oxidizing fluid. Depletion of oxygen by bacteria may result in the formation of iron sulfides, such as pyrite. During burial, the sediments respond to increasing temperature (up to 200°C; 390°F), pressure (up to 2.5 kilobars; 250 megapascals), and fluid movement from compaction-driven waters or influx of water from the basin flanks. As a result, the sedimentary rock may contain authigenic minerals that record a sequence of events ranging from processes occurring near the sediment-water interface to those forming during deep burial. Unlike metamorphic rocks, the preexisting (detrital) mineral assemblage is at least partially retained, in part due to the sluggish reaction rates at diagenetic conditions. Early cementation processes often seal up the rock, preventing subsequent diagenetic reactions and preserving the original detrital mineral assemblage.

Authigenic minerals occur in all sedimentary rock and can vary from trace amounts to virtually the total rock (see table). The carbonate minerals calcite, dolomite, and siderite are some of the most common types. They form in a wide range of depositional environments and at varying burial depths. Calcite and dolomite form the principal minerals in limestones and dolostones, respectively, as well as cements in sandstones or shales. Carbonate cements result from recrystallization of detrital carbonates and from dissolution of other calcium, iron, and magnesium minerals with carbon dioxide from organic reactions. Much of the calcite in limestones initially consisted of aragonite or magnesium-rich calcite, whereas most dolomite has been formed by the chemical alteration of calcite. Recrystallization may change aragonite to calcite. Aragonite (orthorhombic) is a naturally unstable form of calcium carbonate. With the passage of geologic time, aragonite normally inverts to the more stable calcite (hexagonal). The substitution of magnesium for calcium is responsible for the conversion of calcite or aragonite to dolomite, and it has been shown that dedolomitization (replacement of magnesium by calcium) is also possible. See also Aragonite; Calcite; Carbonate minerals; Cement; Dolomite.

*Group of minerals characterized by considerable chemical variation.