Dolomites ideal

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. 

This model for the system CaC03 MgC03 applies only for ideal ordering of Mg and Ca ions in the dolomite structure. Ideal ordering occurs only in precipitates of dolomite formed at temperatures above about 250°C. Studies in the laboratory (52) show that dolomitization (the development of ordering in the Mg and Ca distribution in the calcite structure) is a very slow process at ordinary temperatures. Therefore, a solid-state chemical model more applicable to precipitated dolomites is ... 

Figure 7.7. Unit cell measurements, in Angstroms, of synthetic (o), Holocene (x), pre-Holocene ( ) and "ideal" (+) dolomites. (After Land, 1985.)...

Figure 7.12. Saturation relations in mixtures of seawater and meteoric water for different solubilities of dolomite. A. Ideal dolomite B. Calcium-rich dolomite. (After Hardie, 1987.)...

Figure 7.13. Conceptual model for the relationship between rock composition, openness of system, and dolomite stoichiometry. Shallow formation of dolomite produces less "ideal" dolomite. More open systems tend to produce more complete dolomitization. (After Sperber et al 1984.)...

The listed chemical formulae are ideal and most of these minerals contain trace and minor elements which undoubtedly affect the CL. Several of these minerals have polymorphic or compositional varieties which also may, or do, show CL (e.g. the silica polymorphs quartz, cristobalite, tridymite phosphate compositional varieties apatite, whitlockite, farringtonite, buchwaldite carbonate compositional varieties calcite, dolomite, magnesite). Glass and maskelynite (shock modified feldspar), although not strictly minerals, are relatively common. Below are described the CL observations for the most common phases including enstatite, feldspar and forsterite and they are related to their use for interpreting the mineralogy of meteorites. The observations for the other minerals are sporadic and many details have yet to be studied. 

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. 

Seawater in the upper regions of the ocean is strongly supersaturated with respect to calcite, aragonite, and ideal dolomite. Why these minerals fail to precipitate directly from it has been a problem of major interest. In experimental studies (e.g., Pytkowicz, 1965, 1973 Berner, 1975), the magnesium ion has been observed to be a strong inhibitor of calcite precipitation and largely responsible for its failure to precipitate... 

Dolomite is the second most abundant carbonate mineral after calcite. In their occurrences, dolomitic rocks are usually associated with limestones. In the crystal structure of ideal (ordered) dolomite, which is the thermodynamically most stable phase, layers of carbonate groups are separated by and coordinated with alternating layers of calcium and then magnesium ions. In disordered dolomite, which is less stable than the ordered form, a significant number of calcium and magnesium ions are mixed throughout the cation layers. Recently formed dolomite tends to be disordered, whereas the dolomite found in older rocks, such as those of Paleozoic age, is usually well-ordered. The molar Ca /Mg ratio in ordered dolomites tends to be close to unity, whereas that ratio in disordered dolomites is usually several percent enriched in calcium. In Table 6.1, solubility products are given for both ordered and disordered dolomite. As expected, ordered dolomite is less soluble than its disordered form. 

Figure 6.14 Idealized, end-member approaches to mineral saturation of groundwater, dissolving calcite or dolomite at 12°C and 1 bar total pressure. Open-system dissolution pathways are labeled with hxed values of PCOj- Closed-system pathways are labeled no CO2 added. Reprinted from Geochim. et Cosmochim. Acta, 35, D. Langmuir, The geochemistry of some carbonate ground waters in central Pennsylvania, 1023-45, 1971, with permission from Elsevier Science Ltd, The Boulevard, Langford Lane, Kidlington 0X5 1GB, U.K.

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 ... 

Trace element substitution in the early marine dolomite from the central basin is relatively Fe rich, with slight excess calcium substitution from the ideal formula Ca/(Mg -i- Fe -h Mn) = 1, and very little Mn substitution (Boles Ramseyer, 1987). Excess Ca is about l-4mol%. Typical compositions have... 

Alkali-carbonate reaction. The alkali-carbonate reaction is different from the alkali-silica reaction in forming different products. Expansive dolomite contains more calcium carbonate than the ideal 50 % (mol) proportion and frequently also contains illite and chlorite clay minerals. 

The value and suitability of tlie mineral for any particular commercial purpose depends on the physical state of the mineral and to a large extent on the amount of CaC03, MgC03 and the amounts and nature of other impurities, for which a complete analysis of the mineral will be ideal. For routine work, however, the analysis of limestone and dolomite includes the following determinations ... 

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