Sedimentary rocks chemistry

Syn-sedimentary chemical deposits form by chemical and biochemical precipitation of valuable metal components carried in solution, concomitant with the formation of the enclosing sedimentary rock. The manner of such deposition depends on the concentration of the metal in the solvent, the solubility of the precipitating product, the solution chemistry, and the deposition environment. Iron, manganese, phosphorus, lead, zinc, sulfur and uranium are some of the elements that have formed economically valuable deposits by chemical precipitation during sedimentation. 

G.L. Grandjean, E. (eds.) Mdssbauer spectroscopy applied to inorganic chemistry. Plenum Publ. Corp., 3 417-444 Webb, J. Macey, D.J. Mann, S. (1989) Biomineralization of iron in molluscan teeth. In Mann, S. Webb, J. Williams, R.J.P. (eds.) Biomineralization Chemical and biochemical perspectives. VCH Weinheim, 345-387 Webster, J.G. Swedlund, P.J. Webster, K.S. (1998) Trace metal adsorption onto an acid mine drainage iron(lll) oxy hydroxy sulfate. Environ. Sci.Techn. 32 1361-1368 Wedepohl, K.H. (1969) Composition and abundance of common igneous rocks. In Wedepohl, K.H. (ed.) Handbook of geochemistry. Springer, Berlin, 1 227-249 Wedepohl, K.H. (1969a) Composition and abundance of common sedimentary rocks. 

EVAPORITE. A sedimentary rock formed hy precipilalion from waters ai the earth s surface. As described by Lnwenstcin I.Science. 1090, September 8. 1989), ancient evaporiles have been used to track the chemistry of ancient surface waters, particularly seawaler. Study ol marine evaporiles has led to Ihe general inol unanimous) conclusion that ihe major elemental chemistry of seawater has not changed significantly during the lust 600 million years,... 

The procedure used to define an equilibrium model is to (1) define all the variables and (2) define independent equilibria as a function of phase equilibria. The variables are defined as the chemical parameters typically measured in water chemistry. For the major constituents and some of the more important minor constituents, these are calcium, magnesium, sodium, potassium, silica, sulfate, chloride, and phosphate concentrations as well as alkalinity (usually carbonate alkalinity) and pH. To this list we would also add temperature and pressure. The phase equilibria are defined by compiling well-known equilibria between gas-liquid phases and solid-liquid equilibria for the solids commonly found forming in nature in sedimentary rocks. Within this framework, one can construct different equilibrium models depending upon the mineral chosen actual data concerning the formation of specific minerals therefore must be ascertained to specify a particular model as valid. 

As might be anticipated, the trends observed for lithologic types versus age are reflected in trends in the chemistry and mineralogy of carbonates and other sedimentary rocks. Discussion of some of these latter trends, emphasizing first the overall trends for the past 3.8 billion years, then those of the Phanerozoic, are presented below. 

The nature of the rock record from the time of the first sedimentary rocks ( 3.8 billion years ago) to about 1 to 2 billion years ago suggests that the amount of oxygen in the Earth s atmosphere was significantly lower than today, and that there were continuous chemical trends in the sedimentary rocks formed and, more subtly, in hydrosphere composition. Figure 10.6 illustrates how the chemistry of rocks shifted dramatically during this transitional period. The source rocks of sediments during this time may have been more basaltic than later ones ... 

James H. L. Chemistry of the iron-rich sedimentary rocks, U. S. Geol. Survey Prof. Paper 440-W, 61 p. (1966). 

What is the chemistry of the deterioration of marble by sulfuric acid Marble is produced by geological processes at high temperatures and pressures from limestone, a sedimentary rock formed by slow deposition of calcium carbonate from the shells of marine organisms. Limestone and marble are chemically identical (CaC03) but differ in physical properties because limestone is composed of smaller particles of calcium carbonate and is thus more porous and more workable. Although both limestone and marble are used for buildings, marble can be polished to a higher sheen and is often preferred for decorative purposes. 

Figure 7 Ca/Na ratio versus Sr/ Sr ratio for waters and minerals from the granitoid Loch Vale, Colorado watershed. High Ca/Na ratios are indicative of a significant contribution from calcite dissolution to the dissolved cation chemistry of the waters. Sr isotopes allow differentiation of inputs from bedrock calcite versus eolian carbonate dust derived from sedimentary rocks in surrounding areas (source Clow et al., 1997).

Sedimentary rocks of the McArthur Basin in Northern Australia provide one of the best windows on the chemistry of the Mesoproterozoic ocean. Some 10 km of 1.6-1.7Ga sediments accumulated in this intracratonic basin (Southgate et al., 2000). In certain intervals, they contain giant strata-bound Pb-Zn-Ag mineral deposits (Jackson et al., 1987 Jackson and RaisweU, 1991 Crick, 1992). The sediments have experienced only low grades of metamorphism. 

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. 

Similarly, banded iron formation (BIF), a sedimentary rock produced by chemical precipitation, is extremely rare in the Phanerozoic but common in the Archaean and Proterozoic record. Explaining its origin in terms of the atmospheric or ocean chemistry of the early Earth is an important part of recovering the history of early Earth. This is discussed in Chapter 5 (Section 5.4.3.2). 

Another example of weathering reversal is salt accumulation due to impeded soil drainage or seawater inundation. Weathering under these conditions reverses in the sense that the secondary minerals formed are chemically similar to igneous and sedimentary rock minerals and are unstable under well-drained, oxidative conditions. The chemistry of soil development deals with the degradation of parent minerals and the formation of secondary minerals over a wide range of chemical conditions. 

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