方解石の謎 : なぜ多種類の不純物を取りこむか<総説マイレビュー>

概要

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Calcite has a distorted NaCl structure, where Na+ ion is replaced by Ca2+ ion, and Cl- by planar CO32-. The structure of calcite can be regarded as consisting of CO3 layers linked together by Ca2+ ions located between the layers. This arrangement leaves some open space in the structure, because the Ca2+ ion is too large to give a compact calcite structure. To minimize such space, calcite favors the substitution of Ca2+ ion by divalent cations smaller than the host cation. The representation of partition coefficient of impurity ions in a function of ionic radius, which is known as Onuma diagram, for calcite-solution system showed that Mn2+ is most suitable in size for calcite structure. The incorporation of divalent anions larger than CO32- ion is also effective in filling the space. Thus calcite takes up different impurities to stabilize its structure. Compositional variation of naturally occurring calcites supports this conclusion.
埼玉大学分析センターの論文