Rock-dominated chemistry

Wolery (1978) and Reed (1982, 1983) have indicated based on a computer calculation of the change in chemistry of aqueous solution and mineralogy during seawater-rock interactions that epidote is formed under the low water/rock ratio less than ca. 50 by mass. Humphris and Thompson (1978), Stakes and O Nell (1982) and Mottl (1983) have also suggested on the basis of their chemical and oxygen isotopic data of the altered ridge basalts that epidote is formed by seawater-basalt interaction at elevated temperatures (ca. 200-350°C) under the rock-dominated conditions. If epidote can be formed preferentially under such low water/rock ratio, the composition of epidote should be influenced by compositions of the original fresh rocks. 

Elevated pH values occur in waters whose chemistry is dominated by minerals, most of which, as noted above, are salts of strong bases and weak acids. Thus, in the absence of sources of acidity, mineral carbonate and silicate and alumino-silicate minerals tend to raise the pH to values of 9 to 10 or even higher. Such high pH s are found in arid soils and in some deep groundwaters that are not exposed to fresh recharge and so can be said to be rock dominated as opposed to water dominated. In such systems the minerals present can exist stably, without weathering. Water pH values above 10 are exceptional and may reflect contamination by strong bases, such as NaOH or Ca(OH)2. 

As part of a study of the secondary chemistry of members of Cistus (the rock-rose) in France, Robles and Garzino (1998) examined the essential oil of C albidus L. Plants were sampled from two areas in Provence characterized by different soil types, calcareous sites west of Marseille, and siliceous sites near Pierrefeu-du-Var and Bormes les Mimosas (PF and BM, respectively, in Fig. 2.23), which lie about 60 km and 80 km to the east, respectively, in the Massif les Maures. Regardless of the soil type, a-zingiberene [88] (Fig. 2.24) was the dominant component. Concentrations of other major components of the plants varied between the two soil types, as summarized in Table 2.6. Many other compounds were present in lesser amounts, but varied little between the two areas. A more recent paper by the same workers (Robles and Garzino, 2000) described an analysis of C. monspeliensis L. leaf oils, the results of which are summarized in Table 2.7. 

A wide range of groundwater chemistry has been recorded in crystalline rock environments. Shallow groundwaters (usually <200 m) are dominantly Ca-Na-HCOa formed by the interaction of atmospherically recharged meteoric water with the soil and shallow bedrock. These waters are fresh with dilute dissolved loads and young, as indicated by the presence of tritium. Occasionally, saline intrusions from adjacent seawater bodies or upwelhng of deeper saline fluids can influence the chemistry of shallow groundwaters. 

Terrestrial environments consist of solid (rocks, sediments and soils), liquid (rivers, lakes and groundwater) and biological (plants and animals) components. The chemistry of terrestrial environments is dominated by reactions between the Earth s crust and fluids in the hydrosphere and atmosphere. 

Carbonic acid is the most abundant acid in natural water systems and is the acid most responsible for rock weathering. Bicarbonate ion is generally the dominant anion in fresh surface- and ground-waters. Bicarbonate and carbonate ions are also the chief contributors to total alkalinity in natural waters (see below). For such reasons we will consider carbonate-solution chemistry in some detail. 

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. 

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