Limestone is a type of carbonatesedimentary rock which is the main source of the material lime. It is composed mostly of the mineralscalcite and aragonite, which are different crystal forms of calcium carbonateCaCO3. Limestone forms when these minerals precipitate out of water containing dissolved calcium. This can take place through both biological and nonbiological processes, though biological processes, such as the accumulation of corals and shells in the sea, have likely been more important for the last 540 million years. Limestone often contains fossils which provide scientists with information on ancient environments and on the evolution of life.
About 20% to 25% of sedimentary rock is carbonate rock, and most of this is limestone. The remaining carbonate rock is mostly dolomite, a closely related rock, which contains a high percentage of the mineral dolomite, CaMg(CO3)2. Magnesian limestone is an obsolete and poorly defined term used variously for dolomite, for limestone containing significant dolomite (dolomitic limestone), or for any other limestone containing a significant percentage of magnesium. Most limestone was formed in shallow marine environments, such as continental shelves or platforms, though smaller amounts were formed in many other environments. Much dolomite is secondary dolomite, formed by chemical alteration of limestone. Limestone is exposed over large regions of the Earth's surface, and because limestone is slightly soluble in rainwater, these exposures often are eroded to become karst landscapes. Most cave systems are found in limestone bedrock.
Marble is a metamorphic rock consisting of carbonate minerals (most commonly calcite (CaCO3) or dolomite (CaMg(CO3)2) that have recrystallized under the influence of heat and pressure. It has a crystalline texture, and is typically not foliated (layered), although there are exceptions.
In geology, the term marble refers to metamorphosed limestone, but its use in stonemasonry more broadly encompasses unmetamorphosed limestone.
Carbonate rocks are also crucial components to understanding geologic history due to processes such as diagenesis in which carbonates undergo compositional changes based on kinetic effects. The correlation between this compositional change and temperature can be exploited to reconstruct past climate as is done in paleoclimatology. Carbonate rocks can also be used for understanding various other systems as described below.
Dolomite (mineral) in the context of Dolomite (rock)
Dolomite (also known as dolomite rock, dolostone or dolomitic rock) is a sedimentarycarbonate rock that contains a high percentage of the mineraldolomite, CaMg(CO3)2. It occurs widely, often in association with limestone and evaporites, though it is less abundant than limestone and rare in Cenozoic rock beds (beds less than about 66 million years in age). One of the first geologists to distinguish dolomite from limestone was Déodat Gratet de Dolomieu, a French mineralogist and geologist after whom it is named. He recognized and described the distinct characteristics of dolomite in the late 18th century, differentiating it from limestone.
Most dolomite was formed as a magnesium replacement of limestone or of lime mud before lithification. The geological process of conversion of calcite to dolomite is known as dolomitization and any intermediate product is known as dolomitic limestone. The "dolomite problem" refers to the vast worldwide depositions of dolomite in the past geologic record in contrast to the limited amounts of dolomite formed in modern times. Sulfate-reducing bacteria living in anoxic conditions can precipitate dolomite suggesting that some past dolomite deposits might be due to microbial activity. Recent laboratory research focused on the crystal growth of dolomite at the microscopic scale has revealed that multiple cycles of precipitation/dissolution can promote the growth of dolomite crystals.
German geologist Ernst Friedrich Glocker discovered sphalerite in 1847, naming it based on the Greek word sphaleros, meaning "deceiving", due to the difficulty of identifying the mineral.
Dolomitization conditions are present in Abu Dhabi, the Mediterranean Sea, and some Brazilian hypersaline lagoons (most notably Lagoa Vermelha Lagoon). The areas where dolomitization take place are limited, as modern seawater is less suited to dolomite formation. This is evident in the noticeable decrease in modern dolomite depositions compared to older depositions. Dolomitization involves substantial recrystallization which can be described by the following equation: