Dolomites problem

General considerations The "dolomite problem" has been one of the most intensely studied and debated topics in geology. The "problem is that modem marine sediments contain only relatively rare and minor occurrences of this mineral, whereas it is a major component of sedimentary rocks. Questions about whether major dolomite formation from seawater could have occurred in the geologic past, the conditions necessary for dolomite formation, and many other aspects of the "dolomite problem" have resulted in a voluminous literature that includes entire books devoted to the topic (e.g., Zenger et al., 1980 Zenger and Mazzullo, 1982). The literature on dolomite formation is typified by the vigor with which contending hypotheses are supported and attacked. Many of the controversies have stemmed from attempts to find "the answer" to how sedimentary dolomite forms. 

The timing of the dolomitization of carbonate rock bodies and emplacement of dolomite cements has been one of the more controversial aspects of the "dolomite problem." Most of the basic factors controlling dolomite formation, where were discussed in Chapters 6 and 7, also apply to dolomite formation during the later stages of diagenesis. However, the extended periods of time, the solution compositions likely to be encountered, and the elevated temperature and pressure that occur during deep burial provide highly favorable conditions for dolomite formation. 

Voluminous research on the "dolomite problem" (Chapters 7 and 8) has shown that the reasons for the high magnesium content of carbonates are diverse and complex. Some dolomitic rocks are primary precipitates others were deposited as CaCC>3 and then converted entirely or partially to dolomite before deposition of a succeeding layer still others were dolomitized by migrating underground waters tens or hundreds of millions of years after deposition. However, the observation that the magnesium content of pelagic limestones that have not been uplifted, or... 

Gunatilaka A. (1987) The dolomite problem in the light of recent studies. Modern Geology 11,311-324. 

Arvidson R. S., Mackenzie F. T., and Guidry M. W. (2000) Ocean/atmosphere history and carbonate precipitation rates a solution to the dolomite problem. In Marine Authigenesis from Global to Microbial, SEPM Spec. Publ. No. 65 (eds. C. R. Glenn, L. Prevot-Lucas, and J. Lucas), SEPM, Tulsa, pp. 1-5. 

Thermal analysis has been widely and usefully applied in the solution of technical problems concerned with the commercial exploitation of natural dolomite including, for example, the composition of material in different deposits, the influence of impurities on calcination temperatures, etc. This approach is not, however, suitable for the reliable determination of kinetic parameters for a reversible reaction (Chap. 3, Sect. 6). 

With time, however, the company encountered problems, including caving of the formation into the wellbore and the loss of permeability in zones that had accepted fluid. In June 1987, a number of sidewall cores were taken from the formation (Mehnert et al., 1990). Mineralogic analysis by x-ray diffraction showed that significant amounts of calcite (CaCCb) and brucite [Mg(OH)2], as well as some amorphous matter, had formed from the original dolomite. In some samples, the dolomite was completely consumed and the rock was found to be composed entirely of a mixture of brucite and calcite. 

A general problem with CaO, limestone and dolomite is the limited lifetime of the C02 acceptor material [32], The capacity for C02 is initially very high, but is depleted to almost zero after several cycles. Although the minerals are relatively cheap, this would imply a very considerable stream of waste material coming out of the hydrogen plant. Novel materials are in development with a higher stability [39]. 

Corrosion and erosion of the tubes immersed in the bed are at a low level, although there is evidence that the addition of limestone or dolomite causes some sulphide penetration. The chief operating problem is corrosion by chlorine. 

As a specific example of the problem, let us calculate the equilibria for an actual case study a deep water from the Sarcidano region (Sardinia, Italy) in equilibrium with Mesozoic dolomites (Bertorino et al., 1981). The compositions in mEq/1 of water sampled in a drilled well are listed in table 8.8. The in situ temperature is 21 °C we assume here that the in situ T is 25 °C at 1 bar, to simplify calculations. We also assume for the sake of simplicity that the main ion species in solution are HCO3, Mg, Ca, CO3, OH, and H, and that all Ca and Mg are in the ionic forms Ca and Mg. ... 

CO, SC ), or the occurrence of the minerals is sufficiently rare to represent a special case—the various halide salts, for example. However, dolomite presents a special problem in that the existence of Mg is important to silicate equilibria under consideration. The main trouble here is that the conditions of crystallization and stability of dolomite in sediments and sedimentary rocks is imperfectly known, thus leaving a question as to its influence on silicates or the influence of silicates on its presence. One is forced more or less to ignore the importance of dolomite at present. This does not mean that it can be ultimately excluded from a complete discussion of clay mineral stability. 

Dolomite Dolomite is one of the most abundant sedimentary carbonate minerals, yet 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 seems to be that its formation is kinetically hindered by its complex and well-ordered structure. Another problem in understanding dolomite may be as Lynton Land says "there are dolomites and dolomites and dolomites". The topic of dolomite genesis will be dealt with in several later sections of this book. 

The many problems that plague partition coefficients in calcite and aragonite (see Chapter 3) probably also are important for dolomite, but comparable experimental data are not available for dolomite at near Earth surface temperatures. Additional complications are likely to arise in dolomite in because of its degree... 

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