Dolomite occurrence

Fig. 3.45 Carbon and oxygen isotope compositions of some recent and Pleistocene dolomite occurrences (after Tucker and Wright 1990)...

The amount and quality of information in these studies varies greatly and only a few of the more recent papers applied a combination of several different methods to decipher the origin of saddle dolomite. We scanned published work for the following information on saddle dolomite occurrences optical mineralogy, cathodoluminescence (CL) characteristics, elemental composition, carbon, oxygen and strontium isotopic compositions, and fluid inclusion analysis. In the following sections we shall address these types of data in detail. 

Table 1. Sources of petrographic and geochemical data for saddle dolomite occurrences (post-1980 publications)... 

Fig. 9. Compilation of Sr isotope ratios of saddle dolomite. Horizontal arrows indicate studies that reported only a range of values for saddle dolomite occurrences in carbonate rocks. The total range of Phanerozoic seawater Sr/ Sr ratios is shown for comparison (Smalley et at.,...

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. 

The occurrence of kaolinite is generally erratic but in the terrigenous sediments (Muffler and White, 1969) it can apparently react with dolomite to form the assemblage calcite + chlorite between 120-180°C. Expandable chlorite was noted in shear zones, and iron-rich chlorite is common in most of the rocks becoming more evident at greater depths. In the terrigenous rocks observed, the apparent alumina content of chlorite decreases with depth. Alkali zeolites have been observed at temperatures up to 100°C in the deeply buried rocks. 

Thornburg, K. and Sahai, N. (2004) Arsenic occurrence, mobility, and retardation in sandstone and dolomite formations of the Fox River valley, eastern Wisconsin. Environmental Science and Technology, 38(19), 5087-94. 

Mineralogically the sediments of the laminite series consist of carbonates (mainly dolomite and calcite), various clay minerals, zeolites, opal, quartz and rare gypsum (8). The occurrence of gypsum, based on our X-ray diffraction (XRD) data, is restricted to the marl unit. Deposition, according to Jankowski (8), occurred in a periodically evaporitic, stagnant lake. The high bitumen concentrations most probably were responsible for the preservation of unusual minerals such as Mg-rich calcites (up to 25% Mg 9) and bituminous smectites (Muller, G., University of Heidelberg, personal communication, 1989). 

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. 

Dolomite rhombs have been observed on smear slides of sediments from several DSDP sites, and the occurrence of dolomite in these sediments has been documented quantitatively by Lumsden (1988). Deep-marine dolomite averages about 1 wt % in all sampled DSDP sediments throughout post-Jurassic time. The dolomite is nonstoichiometric, averaging 56 mole % CaC03 (Figure 8.24), and has a crystal size and appearance similar to that of supratidal dolomite. Lumsden (1988) concludes that most is an early chemical precipitate from seawater. He estimated that about 10% is detrital, but he advocates that more criteria are needed to distinguish dolomite formed in cold, deep marine waters from that formed in supratidal deposits before this estimate can be substantiated. 

Baker P.A. and Burns S.J. (1985) Occurrence and formation of dolomite in organic-rich continental margin sediments. AAPG 69, 1917-1930. 

Compton J.S. and Siever R. (1984) Stratigraphy and dolostone occurrence in the Miocene Monterey Formation, Santa Maria Basin area, California. In Dolomites of the Monterey Formation and Other Organic-Rich Units (eds. R.E. Garrison, M. Kastner and D.H. Zenger), pp. 141-153. Society Economic Paleontologists and Mineralogists Pacific Section 41. 

Composite cements may contain mineral additions other than, or as well as, ones with pozzolanic or latent hydraulic properties. Regourd (R34) reviewed the use of ground limestone, which is widely used in France in proportions of up to 27%. The limestones used consist substantially of calcite, with smaller proportions of quartz or amorphous silica and sometimes of dolomite. They must be low in clay minerals and organic matter because of the effects these have on water demand and setting, respectively. The XRD peaks of the calcite are somewhat broadened, indicating either small crystallite size or disorder or both IR spectra confirm the occurrence of disorder. 

Last W. M. (1990) Lacustrine dolomite—an overview of modem, Holocene, and Pleistocene occurrences. Earth Sci. Rev. 27, 221-263. 

The Ca content of soils varies widely, ordinarily ranging from about 0.07 to 3.60%. Calcium is contained in a number of soil minerals including dolomite, calcite, Ca feldspars, apatite, amphiboles and many others. Coarse-tex-tured soils in humid regions, particularly those formed from rocks low in Ca minerals, are generally low in Ca. In spite of this, Ca deficiencies in crops do not appear to be of widespread occurrence, although a number of other factors affect nutrition in low base status soils. Fertilizers are not generally manufactured specifically to provide Ca as a plant nutrient, since this element is economically supplied by periodic applications of agricultural lime, as discussed subsequently. 

Figure 9. Conceptual diagram of the important paleohydrologic controls on the occurrence and distribution of MVT mineralization, and arsenic enrichment. This figure is simplified from one originally constructed by T. Hayes (written communication, 1989). Sandstone is. shown as a dot pattern, limestone as a rectangular pattern, dolomite as a trapezoidal pattern, and shale as the dash-dot pattern. The arrows show pathways of fluid migration, and the letters show the locations of ore formation discussed in the text.

Limestone (chiefly calcite, CaCOa) and dolomite rocks (chiefly dolomite, CaMg(C03)2) are exposed at about 20% of Earth s surface. Carbonate detritus, fossil shell materials, and carbonate cements are also common in noncarbonate sedimentary rocks and arid-climate soils. The carbonate minerals found in such occurrences, in decreasing order of importance, are calcite, dolomite, magnesian cal-cites (Cai jMgfCOa where jc is usually <0.2), aragonite (a CaCOa polymorph) and, perhaps, magnesite. As a rule of thumb, when such materials are present in silicate or aluminosilicate rocks or soils at a level of about 1 % or more, they will lend to dominate the chemistry of the soil or ground-water. This fact is extremely important when one is concerned about the ability of a rock to neutralize acid mine waters, other acid wastewaters, or acid rain. 

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. 

The carbonates are mainly calcite, dolomite, or siderite. The occurrence of calcite is frequently bimodal. Some calcite occurs as inherent ash, while other calcite appears as thin layers in cleats and fissures. Iron can be present in small quantities as hematite, ankorite, and in some of the clay minerals such as illite. In addition to the more common minerals, silica is present sometimes as sand particles or quartz. The alkalies are sometimes found as chlorides or as sulfates but probably most often as feldspars, typically orthoclase and albite. In the case of lignites, unlike bituminous and subbituminous, sodium is not present as a mineral but is probably distributed throughout the lignite as the sodium salt of a hydroxyl group or a carboxylic acid group in humic acid. Calcium, like sodium, is bound organically to humic acid. Therefore, it too is uniformly distributed in the sample [10]. 

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