The role of magnesium in the crystallization of calcite and aragonite in a porous medium

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SEPM (Society for Sedimentary Geology)
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Morphological development of calcite crystals is related to supersaturation conditions during growth. Crystallization of calcium carbonate (calcite and aragonite l as well as Mg-calcite was studied under controlled supersaturation conditions by the counter diffusion of Ca2+ and co,2 - ions through a porous transport medium (a column of silica gel). U~der our experimental conditions, where ion transport is constrained to be diffusion controlled, nucleation and growth take place under conditions of high supersaturation, the actual threshold value of the supersaturation depending on the supersaturation gradient. In the pure CaC03 system, calcite grows at lower supersaturation than aragonite. The calcite develops relatively simple rhombohedra while the aragonite grows as spherulites. Presence of Mg2+ in the interstitial fluid inhibits nucleation, increasing the threshold supersaturation at which crystallization begins. The resulting Mg-calcite crystals show a range of morphologies depending on the Mg content and the upersaturation at the point of crystallization. At high values of supersaturation, up to 15 mol % MgC03 is incorporated into the calcite and the crystals form spheres. At lower supersaturations, Mg content decreases and morphologies change progressively through a well-defined and reproducible sequence from spheres to dumbbell-like forms to wheat-sheaf-like bundles and eventually single crystals with steep rhombohedral faces. The crystals are compositionally zoned, showing both sector and oscillatory zoning. The compositional evolution is related to the supersaturation and interface roughness during crystal growth.
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