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The difference in stability between aragonite and calcite, as measured by the Gibbs free energy of formation, is small, and effects of grain size and impurities can be important. The formation of aragonite at temperatures and pressures where calcite should be the stable polymorph may be an example of Ostwald's step rule , where a less stable ...
Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratch hardness comparison. Large calcite crystals are used in optical equipment, and limestone composed mostly of calcite has numerous uses. Other polymorphs of calcium carbonate are the minerals aragonite and vaterite.
Vaterite, like aragonite, is a metastable phase of calcium carbonate at ambient conditions at the surface of the Earth. As it is less stable than either calcite, the most stable polymorph, [5] or aragonite, vaterite has a higher solubility than either of these phases.
Limestone is the most common carbonate rock [3] and is a sedimentary rock made of calcium carbonate with two main polymorphs: calcite and aragonite.While the chemical composition of these two minerals is the same, their physical properties differ significantly due to their different crystalline form.
An aragonite sea is the alternate seawater chemistry in which aragonite and high-magnesium calcite are the primary inorganic carbonate precipitates. The Early Paleozoic and the Middle to Late Mesozoic oceans were predominantly calcite seas, whereas the Middle Paleozoic through the Early Mesozoic and the Cenozoic (including today) are ...
Calcite is the least soluble of these carbonates, so the CCD is normally the compensation depth for calcite. The aragonite compensation depth (ACD) is the compensation depth for aragonitic carbonates. Aragonite is more soluble than calcite, and the aragonite compensation depth is generally shallower than both the calcite compensation depth and ...
Some of the calcite polymorphs are further subdivided by relative magnesium content (Mg/Ca ratio), with calcite solubility increasing with increasing Mg. [6] [7] The solubility of various forms of CaCO 3 differs in seawater; specifically, aragonite exhibits greater solubility compared to pure calcite. [7]
The aragonite needles in carbonate mud are converted to low-magnesium calcite within a few million years, as this is the most stable form of calcium carbonate. [28] Ancient carbonate formations of the Precambrian and Paleozoic contain abundant dolomite, but limestone dominates the carbonate beds of the Mesozoic and Cenozoic .