magnesium calcite

magnesium calcite

[mag′nē·zē·əm ′kal‚sīt]
(mineralogy)
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In the Lockhart Limestone conversion of micrite to microspar is a common phenomenon and is characterized by the development of isolated patches of inequi-granular microspar selectively converting high magnesium calcite (micrite) into low magnesium calcite with heterogeneous texture (Plate 2G 2H and 1H).
The presence of abundant secondary porosity due to possible subsurface dissolution potential of predominantly aragonitic and high magnesium calcite skeletal constituents and tectonic deformation (indeed fracture porosity) of Galiat area the Lockhart Limestone can serve as a good reservoir for hydrocarbons.
Three naturally occurring forms of calcium carbonate are used by marine organisms to build shells, plates or skeletons: calcite, aragonite and high magnesium calcite. Microscopic plants called coccolithophores (common alga) surround themselves with protective calcite plates; aragonite is used by pteropods (snail-like molluscs) to build their shells and corals use it to make their skeletons which helps form reefs; while some echinoderms, starfish, sea urchins, brittle stars, utilise magnesium calcite to form their exoskeletons.
Living on a steady diet of copper and tin, the organisms on the statue had "digested" the metals to produce shells with unusual ratios of magnesium calcite and aragonite, for example, as well as traces of feldspar and quartz.
Being a highly organized network of microscopic channels, the magnesium calcite shell of the sea urchin has a configuration similar to that of a photonic crystal.
X-ray diffraction analysis of the residue shows quartz as the dominant residue component, with scarcely more than trace quantities of magnesium calcite (Ca, Mg)[CO.sub.3] and the clay mineral illite ([KAl.sub.2]([OH.sub.2]).
In the Margala Hill Limestone conversion of micrite to microspar is a common phenomenon and is characterized by the development of isolated patches of inequigranular microspar selectively converting high magnesium calcite (micrite) into low magnesium calcite with heterogeneous texture of middle shelf facies (Plate 1C) and inner shelf facies (Plate 2E).
The mineral phase is mainly composed of magnesium calcite (Chave, 1952) with a MgC[O.sub.3] content ranging from 3.0 to 43.5 mol% (Schroeder et al., 1969; Weber, 1969; McClintock et al., 2011).
In the latest of these studies, the scientists found that the sea urchins' teeth contain crystals of magnesium calcite, which are smaller, harder and denser than those of pure calcite.
Dissolution of carbonate rocks occur as a result of undersaturation of pore fluid that lead to dissolution of metastable carbonate grains and cement (aragonite/high- magnesium calcite; FlA1/4gel 2004).
Microdolomite inclusions in cloudy prismatic calcites: a proposed criterion for former high magnesium calcites. Journal of Sedimentary Petrology, 47, 1078-1088.