Calcite and aragonite transformation to dolomite has been accomplished in a few days in hydrothermal experiments using artificial seawater without sulfate and in MgCl₂ + CaCl₂ + NaCl solutions of seawater ionic strength, at 200° and 150°C. Calcium carbonate transformation to dolomite is retarded and frequently inhibited, depending on the concentration of SO₄^2- in solution. The mechanism of this reaction is being investigated. Preliminary results indicate that it is a surface-controlled mechanism.

These results explain: (1) the formation of either primary or replacement dolomite in organic-rich sediments, especially in sedimentary environments or rapid sedimentation, in which microbial sulfate reduction prevails, and diffusive communication of the interstitial water with seawater is precluded; (2) the observed large variations--from negative to strongly positive

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^dgr¹³C values (PDB)--of dolomites in the Monterey Formation, California, and in recent sediments in the Gulf of California, controlled by the CH₄/CO₂ ratio; and (3) the often observed similar oxygen isotope values of coexisting calcites and dolomites in limestones which were dolomitized by seawater mixed with large volumes of fresh water.

Thus, in low or sulfate-free environments, dolomitization of CaCO₃ and the formation of primary dolomite are limited by supply of alkalinity, calcite or aragonite, or dissolved Ca²⁺ or Mg²⁺. Even in the absence of sulfate, dolomitization may be retarded or inhibited by the transformation of opal-A to opal-CT, a reaction which can compete with dolomite for available Mg²⁺.

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