Dolomite recrystallization

XENOTOPIC. The fabric of a crystalline sedimentary rock in which the majority of the constituent crystals are anhedral. Fabric found in evaporites. chemically deposited cement, and recrystallized limestone or dolomite. 

The precipitation of protodolomite under 100°C. from solutions of calcium and magnesium chlorides has been reported, and it seems plausible to suggest that, given sufficient time, dolomite would result from recrystallization of protodolomite. Siegel (35) found that protodolomite was produced at 25 °C. when calcium and magnesium ions were entrapped in activated charcoal and could react with carbonate ion at a low rate he also found that sulfate ion was involved in dolomite formation. 

Both MgO and CaO have high melting temperatures and do not combine into compounds. The only eutectic in the system CaO — MgO melts at about 2300 C. In a pure binary system, very high temperatures are required for sintering. However, natural dolomite always contains impurities (FCiOs, AI2O3, Si02) which facilitate formation of clinker at lower temperatures. The impurities partly produce a liquid phase above 1300 °C, and form silicates, ferrites and aluminates of calcium either directly or by recrystallization from the melt. However, most of the CaO and MgO remain uncombined. 

Folk, R.L. (1965) Some aspects of recrystallization in ancient limestones. In Pray, L.C. Murray, R.C. (Eds) Dolomitization and Limestone Diagenesis. Special Publication 13. Tulsa, OK Society of Economic Paleontologists and Mineralogists, pp. 14-48. 

In addition to dissolution, the destabilization of carbonate cements may result in recrystallization and replacement by other carbonates. Microcrystalline calcite and dolomite are sensitive to recrystallization at various burial depths. The recrystallization of dolomite has been reviewed by Mazzullo (1992). Burial recrystallization of micritic/microsparitic cements in sandstones may result in the formation of poikilotopic calcite (Saigal Bjorlykke, 1987). However, poikilotopic calcite is also a common primary cement in calcretes (e.g. Knox, 1977 Tan-don Narayan, 1981). Recrystallized calcite and dolomite are recognized as patchily distributed, coarsened crystals. In contrast, precipitational vari-... 

Gregg, J.M., Howard, S.A. Mazzullo, S.J. (1992) Early diagenetic recrystallization of Holocene (< 3000 years old) peritidal dolomites. Ambergris, Belize. Sedi-mentology, 39, 143-160. 

Kupecz, J.A. Land, L.S. (1994) Progressive recrystallization and stabilization of early-stage dolomite Lower Ordovician Ellenburger Group, west Texas. In Dolomites (Eds Purser, B., Tucker, M Zenger, D.), Spec. Publ. int. Ass. Sediment. 21, 255-279. Blackwell Scientific Publications, Oxford. 

Malone, M.J., Baker, P.A. Burns, S.J. (1994) Recrystallization of dolomite evidence from the Monterey Formation (Miocene), California. Sedimentology, 41, 1223-1239. 

In marine turbidite sandstones of the central basin, following minor siderite precipitation, dolomites formed early in the zone of methanogenesis. These have Ca-rich compositions similar to dolomites reported from contemporaneous fine-grained rocks of the Monterey Formation, coastal California. The dolomites are an example of young (< 6 Ma) dolomite formation at shallow burial depth in marine pore water, and they may have undergone some recrystallization during shallow burial without resetting their initial Sr/ Sr values. 

In the upper part of the succession in drill hole HM-i the carbonate carbon isotope composition fluctuates between well-defined limits of -1-4.5 and -7.4%o (Table 7.1, Fig. 7.2). In this case, recrystallization of carbonates and neoformation of calcite or dolomite have taken place under participation of CO which in certain instances contained a large portion of isotopically light carbon ( C) whereas in other instances there was a predominance of heavy carbon ( C). The results obtained, together with other geochemical data, show that in the zones in which the sedimentary rocks have been affected by infiltration waters the migrating hydrocarbons have been subjected to oxidation and other changes, to be discussed in detail below. 

For samples taken up-dip of the Setif and Medjounes areas the values range from +2.9 to -5.8%o (Tables 7.2,7.3 Fig. 7.2). The light carbon makes itself felt in the carbonates where we find traces of recrystallization or of other mineralogical neoformations and in particular the appearance of mixed-layer clay minerals with perfectly ordered crystal structures. In the same samples microfissures are filled by secondary calcite or dolomite with detrital carbonate cement frequently being present between the crystals. In other cases we observe entire fields of neoformed minerals (dolomite... 

Relatively pure carbonate rocks when subjected to regional metamorphism simply recrystallize to form either calcite or dolomite marble with a granoblastic texture. Any silica present in a limestone tends to reform as quartz. The presence of micas in these rocks tends to give them a schistose appearance, schistose marbles or calc-schists being developed. Where mica is abundant, it forms lenses or continuous layers, giving the rock a foliated structure. 

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