Dolomite precipitation

Finally, common ion effects link many mineral precipitation reactions, so the reactions do not operate independently. In the seawater example, dolomite precipitation consumed magnesium and produced hydrogen ions, significantly altering the saturation states of the other supersaturated minerals. 

The mixing calculation is interesting in that it demonstrates a common ion effect by which dolomite precipitation drives feldspar alteration. In the model, dolomite forms because the saline water is rich in Ca++ and Mg++, whereas the fresh water... 

Dolomites compose a significant portion of ancient evaporites. This mineral is essentially a Mg-rich calcite, having variable Mg content. Its empirical formula is commonly given as MgCa(C03)2. Having no modern examples of dolomite precipitation beyond those of a few salinas and sabkhat, geochemists have been unable to determine conclusively how the ancient deposits must have formed. 

To convert calciiun carbonate to dolomite, some of the calcium must have been replaced by magnesiiun, requiring the partial dissolution of the carbonate. This process is promoted by contact with acidic pore water, such as occurs in organic-rich sediments because remineralization produces carbon dioxide. This is probably why dolomites are presently forming in detrital algal mats buried beneath sabkhat. The restricted extent of these modern dolomites reflects a kinetic hindrance to precipitation. Apparently dolomite precipitation in this setting is too slow to form substantial deposits when sea level is rapidly fluctuating. 

Vasconcelos C, Mackenzie JA, Warthmann R, Bemasconi S (2005) Cahbration of the 6 0 pale-othermometer for dolomite precipitated in microbial cultures and natural environments. Geology 33 317-320... 

Morrow D.W. (1982a) Diagenesis 1. Dolomite—Part 1. The chemistry of dolomitization and dolomite precipitation. Geoscience Canada 9,5-13... 

Table 42 Dependence of calcite dissolution and dolomite precipitation on temperature...

Rodriguez Navarro, C., Sebastian, E. Rodriguez Gallego, M. (1997). An urban model for dolomite precipitation authigenic dolomite on weathered building stones. 

Observations of dolomite formation in natural systems have been used for defining additional factors that may influence the rate of dolomite formation. These include catalysis by certain clay minerals (e.g., Wanless, 1979) and production of organic by-products by bacteria (e.g., Gunatilaka et al, 1985). Mg transport to sites of dolomite precipitation can inhibit the reaction in hemi-pelagic sediments (e.g.. Baker and Burns, 1985). However, the true influence of reaction rates is largely speculative, because the kinetic factors are... 

Vasconcelos C. and McKenzie J. A. (1997) Microbial mediation of modem dolomite precipitation and diagenesis under anoxic conditions (Lagoa Vermelha, Rio de Janeiro, Brazil). J. Sediment. Res. 67, 378—390. 

The lack of understanding of the dolomite precipitation process is reflected in the discrepancy of solubility products reported by different investigators. Published figures range from 10 to 10 . As mentioned before, solubility equilibrium can be reached (under atmospheric conditions) only from undersaturation. The time of approaching equilibrium is unknown. Thus it is very difficult to ascertain equilibrium in laboratory experiments. 

For the Ca VNlg " ratio of seawater, ideal, stoichiometric and well-crystallized dolomite is more thermodynamically stable than calcite. Why then do calcite and aragonite, rather than dolomite, precipitate from seawater ... 

Sediments were also extensively cemented by near-surface and pre-compactional eogenetic siderite (8 0 = -16.9%o to -8.1%o), calcite (5 0 = -10.2%o to -4.8%o) and dolomite (5 0 = -4.3%o to -1.5%o). The anomalously low5 0 values of siderite are attributed to microbial oxygen isotopic fractionation, as it would otherwise indicate unreasonably high precipitation temperatures (up to 120°C). The eogenetic caleites and dolomites precipitated from such meteoric waters at temperatures of 10-30°C. 

Ferroan dolomite precipitation is also prominently localized around partially dissolved detrital K-feldspars (Fig. 7B). Localization is not strictly within the volume formerly occupied by the K-feldspar, but rather is crudely centred on the feldspar, extending also into the surrounding pore space. Ferroan dolomite clearly postdates quartz cementation. In a few samples there is petrographic evidence suggesting that ferroan dolomite pre-dates the formation of the late calcite. 

Volumetrically minor authigenic ferroan dolomite is present locally as overgrowths on distinctive cores of partially dissolved detrital dolomite (Fig. 5). Prominent zoning of Fe and Mg in these overgrowths is readily observed on back-scattered electron images. Dolomite precipitation clearly precedes the formation of calcite cement. 

Table 4. Bulk-rock carbon and oxygen isotope results for dolomite cement in the Angel Formation, Angel Field. Precipitation temperatures for dolomite cement were calculated using the fractionation factor of Northrop Clayton (1966) and assuming a marine composition for the original 8 0 pore water (6 0 = 0%o). When integrating the calculated dolomite precipitation temperatures (93-97 °C) with the geohistory plot for Angel-2 (Fig. 4) an Eocene to Late Miocene age for the dolomite cement is suggested, closely matching the seismic evidence (Fig. 22c)...

In contrast, if the intergranular dolomite precipitated during pore fluid flow associated with cataclasis (Hippier, 1993 Fowles Burley, 1994), it becomes difficult to account for the medial position of the subsequent veins. The greatest rheological contrast would be expected at the margins of the dolomite cementation rather than within the ce-... 

Dolomite precipitation probably took place at relatively low temperatures (<100°C), on the basis of planar crystal fabrics (Sibley Gregg, 1987) and the dominance of monophase aqueous fluid inclusions (Goldstein Reynolds, 1994). Under such conditions, the precipitation of well-ordered dolomite is favoured by high (Mg " " + Fe " )/Ca ratios, low salinities and high carbonate alkalinity in pore fluids (Folk Land, 1975 Machel Mountjoy, 1986). Low salinity reduces ion pairing, and high COf activities facilitate dehydration of Mg " " ions. Dolomite exhibits retrograde solubility. 

Dolomite cement components were all sourced from outside the sand body, most probably from local or basinal mudrocks. Stable isotope data indicate a mixed organogenic-marine carbonate source, and precipitation at relatively low temperatures (s70°C, if pore fluids were sourced from clay mineral dehydration reactions during deep burial of Carboniferous mudrocks in the Rathlin basin 55°C if they were locally sourced). Thermobaric mass transfer was enhanced by tectonic pulsing and dolomite precipitation was driven by CO2 degassing. 

Many authors reported paragenetic relationships that allow the relative timing of saddle dolomite precipitation within a given rock unit to be determined. There is general agreement among these... 

The 6 0 values of saddle dolomite range from -20.1 to -1.6%o, with a mean of -9.4 2.9%o (n = 838 Fig. 7). No relationship exists between the oxygen isotopic composition of saddle dolomite and host-rock lithology, suggesting that saddle dolomite precipitation occurred over a similar temper-... 

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