a mineral consisting of CaCO3. Aragonite is less stable than calcite under pressures at the surface of the Earth. Calcite precipitation is often inhibited, however, and both modern and ancient seawater are supersaturated with respect to calcite. With high enough supersaturations, aragonite can also precipitate. Aragonite is common in many modern carbonate sediments as shells and as crystals that precipitated from sea water.
adjective implying conditions between 2.5 billion years ago and 4.0 billion years ago. The oldest rocks are about 4.0 billion years old defining the older boundary. The younger boundary is defined by an apparent difference in geological attributes of rocks older than 2.5 billion years and younger than 2.5 billion years. These differences may be due to our incomplete knowledge of the rock record rather than to a dramatic change in the Earth.
a mineral consisting of CaCO3. Sometimes Mg2+ substitutes for Ca2+ forming Mg-calcite. Sea water is supersaturated with respect to calcite, and calcite is common in many modern and ancient carbonate sediments as shells and as crystals that precipitated from sea water.
a group of minerals usually consisting of a divalent cation and CO3. Shells and corals are made of carbonate. The two most abundant carbonate minerals in today's oceans are calcite and aragonite, two different arrangements of CaCO3. Dolomite is another common carbonate mineral in ancient sediments.
carbonate mineral consisting of MgCa(CO3)2. It is the most stable carbonate mineral in the oceans, but it precipitates very slowly and is almost never present. Instead, it is found predominantly in ancient sediments where it forms when the sediments are buried. "Dolomite" also refers to rock consisting of mostly of the mineral dolomite.
very fine (1-5 microns) carbonate crystals (either calcite or aragonite). It composes carbonate mud. Micrite can precipitate from seawater or form from the breakdown of larger carbonate grains. Most modern micrite is the result of calcifying algae.
a grain consisting of multiple coatings of carbonate (usually calcite and/or aragonite) that precipitated on a nucleus. They are usually round and smooth. They form in shallow water depositional environments.
adjective describing time before the Cambrian Era which was the time in Earth history when the first macroscopic life was found in the fossil record (in England when historical geology was first developing). The Precambrian-Cambrian boundary has recently been shifted in absolute time by Sam Bowring, John Grotzinger, and Beverly Saylor at MIT. It is now placed at about 540 million years ago. Thus, Precambrian time covers all rocks older than 540 million years old.
The most common usage is: a laminated, relief-forming structure of a biogenic, specifically microbial, origin. Usually they are found in carbonate sediments, particularly Precambrian sediments. It is usually very difficult to demonstrate a biological origin, and care should be taken not to assume a biological origin for all "laminated, relief-forming structures" in carbonate rocks.