Direct Evidence That Well-Ordered, Stoichiometric (Ideal) Dolomites are the Product of Recrystallization

Geochemical, structural, and mineral surface data from a broad assemblage of synthetic and natural dolomites suggest that stoichiometric and well ordered (ideal) dolomites form through recrystallization of a nonstoichiometric and/or poorly ordered (nonideal) dolomite precursor. Synthetic dolomite is formed by replacement of calcite in Mg-Ca-Cl fluids at high-temperature (>200°C). The first synthetic dolomites to form are invariably poorly ordered with stoichiometries that strongly reflect the initial Mg/Ca of the fluids. Near reaction completion, when calcite reactants are nearly consumed (>95% dolomite), nonideal dolomite is rapidly replaced by a stoichiometric and relatively well-ordered, ideal dolomite.

Scanning electron (SEM) and atomic force microscope (AFM) observations of synthetic dolomite crystal surfaces reveal that nonideal dolomites are covered with round growth mounds and ideal dolomite surfaces are characterized by flat growth layers separated by elongate steps. When synthetic dolomite crystals are etched in dilute acid, the surfaces of nonideal dolomites remain covered with rounded mounds, whereas ideal dolomites are characterized by flat layers with deep, euhedral etch pits. Comparable surface features are also observed on the surfaces of natural dolomite crystals. Chemically etched nonideal dolomite crystal surfaces have mounds and ideal dolomite crystal surfaces are characterized by flat surfaces with euhedral etch pits.

Mineral surface textures, integrated with dolomite stoichiometry and cation order data, are consistent with a stepwise growth model in which nonideal dolomite first forms by precipitation of growth mounds, and ideal dolomite forms by a spiral growth mechanism only through replacement of a nonideal dolomite precursor. Because layers and etch pits dominate the surfaces of etched ideal dolomites, and ideal dolomites form by replacement of a nonideal dolomite precursor, flat layers with euhedral etch pits are interpreted as direct physical evidence of recrystallization. An independent evaluation of recrystallization in natural dolomites is valuable because geochemical data are commonly used to interpret the chemistry of dolomitizing fluids. If recrystallization can be established using an independent test, chemical analyses can be more accurately interpreted as to whether they reflect the original dolomitizing fluid or later diagenetic solutions.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California