Structural refinements of dolomite and a magnesian calcite and implications for dolomite formation in the marine environment Available to Purchase

The structures of dolomite and a calcite containing 10 mole percent MgCO₃ were an-isotropically refined, using single-crystal X-ray techniques. Cell constants and bond lengths were determined as: a = 4.8033(9), с = 15.984(4) A, V = 319.3(1) A³, Ca-O = 2.378(1), Mg–O = 2.081(1), C–O = 1.2835(15) A for dolomite; and a = 4.941(2), с = 16.864(2) A, V = 356.60(22) A³, M–O = 2.331(1), C–O = 1.276(3) A for magnesian calcite. Interatomic bond lengths and angles in the dolomite structure are more ideal than those in the calcite, magnesite, and magnesian calcite structures. Therefore the octahedra are less distorted and provide better cation shielding for both Ca and Mg in dolomite than in the other carbonate structures. Substitution of Mg in the calcite structure distorts the octahedra, diminishing cation shielding. This results in exaggerated thermal motion of ions and bond weakening.

In the marine environment, hydration of Mg and the problem of cation ordering combine to prevent dolomite precipitation. Metastable magnesian calcite forms instead. Once magnesian calcite is formed, pressure increases (less than 1 kbar) in the marine environment may cause sufficient additional disequilibrium to produce dolomite. Increased pressure favors dolomitization of magnesian calcite because dolomite is denser, occupies less volume, and closely approaches an ideal atomic arrangement. Dolomite can form from magnesian calcite by ordering of the Mg already in the structure, and by cation exchange of Mg from sea water for Ca in the structure. Weakened bonding in magnesian calcite facilitates these processes. With increasing pressure, 10 mole percent MgCO_a in calcite is sufficient to favor the formation of dolomite rather than low-magnesium calcite.