DOLOMITE

type of limestone the carbonate fraction of which is dominated by the mineral dolomite, calcium magnesium carbonate [CaMg(CO3)2]. For detailed physical properties of the minerals, see carbonate mineral (table). Dolomite is generally formed from limestone when the calcite (calcium carbonate, CaCO3) in the limestone is replaced by calcium magnesium carbonate volume for volume. This process, known as dolomitization, may occur either soon after limestone deposition by exchange with sea water or after lithification by exchange with magnesium-bearing solutions. In most cases, textures coarsen during dolomitization and original structures (and textures) are eradicated. In a few instances, however, the structural and textural features of the parent limestone are preserved, resulting in a finely crystalline dolomite. Such rocks are considered primary dolomites and are typically associated with evaporites. The mineral dolomite occurs in marbles, talc schists, and other magnesium-rich metamorphic rocks; textural studies have shown that at least some of this dolomite has formed on cooling from highly magnesian calcite, stable at higher temperatures. It occurs in hydrothermal veins, in cavities in carbonate rocks, and less commonly in various sedimentary rocks as a cement. It is most common as a rock-forming mineral in carbonate rocks. In dolomitized limestones it is a secondary replacement, and individual dolomite crystals cut across preexisting fossil outlines and other textural features; fossil coral reefs are often replaced in this fashion. Dolomitization is very selective, giving rise to interbedded limestone and dolomite. Modern dolomite forms in supratidal flats such as the Bahamas or in hypersaline lagoons associated with the precipitates of calcite, aragonite, magnesium calcite, gypsum, and anhydrite. Hydrothermal dolomite occurs in veins associated with lead, zinc, or copper ores, as well as with fluorite and barite. type of limestone, the carbonate fraction of which is dominated by the mineral dolomite, calcium magnesium carbonate [CaMg(CO3)2]. R.V. Dietrich Additional reading Terence P. Scoffin, An Introduction to Carbonate Sediments and Rocks (1987), is a well-illustrated, easy-to-read book about limestones and dolostones. A review article by Laurence A. Hardie, Dolomitization: A Critical View of Some Current Views, Journal of Sedimentary Petrology, 57(1):166183 (1987), emphasizes the problems involved with accepting the most commonly embraced models of dolomitization and concludes that direct precipitation models (among other things) should receive more attention and study. Vijai Shukla and Paul A. Baker, Sedimentology and Geochemistry of Dolostones (1988), a collection of symposium papers, covers organogenic dolomites and the geochemistry of dolomite textures. John K. Warren, Evaporite Sedimentology (1989), includes a chapter on dolomites and dolomitization, a good recent summary of the pertinent data and hypotheses; the bibliography includes most of the noteworthy articles published in English. R.V. Dietrich