Patterns in the compositions, properties, and geochemistry of carbonate minerals

Abstract

The diversity of carbonate minerals is remarkable, if largely unappreciated. For example, 277 carbonate-bearing minerals have been recognized, and among them are 158 pure carbonates of cations with valences from 1+ to 6+. The other 119 minerals additionally contain chloride, fluoride, borate, sulfate, phosphate, arsenate, arsenite, antimonate, or silicate groups, or combinations of those anions. However, combinations of anions with cations are not uniformly distributed, so that there are no bicarbonates or simple carbonates of highly-charged cations, few hydrated or OH-bearing minerals of monovalent cations, and few U-bearing carbonates with anions other than CO3 2, OH−, and O2−. On the other hand, simple carbonates of divalent cations, OH-bearing Al carbonates, and fluoride-bearing carbonates of rare-earth elements are remarkably numerous. Many of these trends can be related to the coordination chemistry of cations in the solutions from which these minerals form. Among nearly all the carbonate-bearing minerals, ionic potential of the cations is a major control on the extent of hydration. Degree of hydration is in turn a major control on hardness, density, and solubility. Among the simple carbonates, hardness, density, and positions of spectroscopic peaks vary linearly with cation radius or mass, although such trends usually exist only within crystallographic groups or only within cation groups defined by the periodic table. In contrast, geochemical parameters, such as solubility and fractionation of oxygen isotopes, vary with degree ofcation fit in the 6-fold or 9-fold site of the rhombohedral and orthorhombic simple carbonates, so that there is not a linear variation with cation size. The same is true for the distribution coefficients of cations in calcite and aragonite. Patterns thus emerge among the compositions, properties, and geochemistry of the carbonate minerals, with cationic potential and type as a major influence on composition, with degree of hydration and cation radius or mass as a control on physical and spectroscopic properties, but with cation fit as the major control on geochemical parameters. These patterns allow qualitative prediction of mineral properties and help explain the origins of some of the major problems in carbonate petrology.