Dolomites protodolomites

Dolomite is one of the most abundant sedimentary carbonate minerals but its mode of formation and its surface properties are less well known than for most other carbonate minerals. As we have mentioned, the nucleation of dolomites and its structural ordering is extremely hindered. There is a general trend for the "ideality" of dolomite to increase with the age of dolomite over geological time (Morse and Mackenzie, 1990). Most dolomites that are currently forming in surfacial sediments and that have been synthesized in the laboratory are calcium-rich and far from perfectly ordered. Such dolomites are commonly referred to as "protodolomites . Morse and Mackenzie (1990) have reviewed extensively the geochemistry (including the surface chemistry of dolomites and Mg-calcites. 

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. 

Dolomite is one of the most abundant sedimentary carbonate minerals. However, after years of intense study its mode of formation remains controversial, and its properties under Earth surface conditions are less well known than for most other carbonate minerals. The primary reason for this is that its formation is kinetically hindered by its well-ordered structure. Another problem in understanding dolomite may be as stated by Land (1985) there are dolomites and dolomites and dolomites. Most recently formed marine dolomites are classified as protodolomites. They are far from being perfectly ordered stmcturally and usually contain a few percent excess calcium. Attempts to measure their solubility have been frustrating, but it does appear that they can be one or two orders of magnitude more soluble than ideal dolomite. Thus, seawater may not be supersaturated with respect to protodolomites. 

Dolomites found in nature seldom have exact stoichiometric composition and are frequently structurally rich in calcium (protodolomite). Dolomite, as well as calcite, has a tendency to form solid solutions with many metal ions. Calcite has a tendency to accommodate Mg in its structure to form magnesian calcite. Kineticall>, the deposition of magnesian calcite may be more favorable than the deposition of dolomite. 

Fe-Mn oxides Mixed clays (+) Dolomite Oxalates Nontronite Palygorskite Fe-Mn oxides Amphiboles Pyroxenes Pyrite Apatite Protodolomite Protodolomite Dolomite Magnesian silicate Sodium silicate Fluorides Bromides Natron Trona Sulphides Fluorides Palygorskite Sepiolite Nontronite Bentonite Dolomite... 

Metasomatic dolomitization is a common process, which actively affects the composition of both rocks and ground water. Three major types of dolomite are distinguished primary, precipitated together with the rock, secondary in the form of cement and the third one at dolomitization of limestones. In the latter case it forms due to the substitution of Mg instead of Ca. Because of this the very substitution process is considered as a number of sequential reactions of dissolution and precipitation. However, total substitution of dolomite instead of calcite is rare. Usually form limestones at various dolomitization stages (magnesian calcite, protodolomite). In its substance this is isomorphism of cations with the same valence, which may be presented by equation... 

With irregular dolomite, Ca and Mg in cation positions are not ordered, which is accompanied by a positional disorder in the placement of. Such disordered modification of dolomite is protodolomite, which usually contains more Ca and less Mg compared with the theoretical formula. 

Figure 2.57 Diagram of thermodynamical stability for CaCO -Ca MgfCOD, -H O system (Machel, and Mountjoy, 1986 Braithwaite et al, 2004). Lines (1-3) are experimentally calculated equilibrium between 1. calcite and completely regular dolomite 2. calcite and regular dolomite with small excess of Ca 3. calcite and completely irregular protodolomite.

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