Evaporite minerals weathering

The efficiency of the weathering of rocks in using carbonic acid produced in the carbon cycle is affected by various hydrologic, environmental, and cultural controls. The fact that the principal anion in fresh surface water worldwide almost always is bicarbonate attests to the overriding importance of this process. Exceptions are systems in which evaporite minerals are available for dissolution by groundwater or where human activities are major sources of sulfate or chloride inflow. 

Metals are temporarily attenuated as evaporite minerals on the surface or in secondary oxides and hydroxides in the tailings. The evaporites will re-dissolve in wet weather conditions and the secondary minerals become unstable with acidification of the tailings releasing these metals into the environment. 

Degree of terrestrial weathering is an additional classihcation parameter commonly applied to meteorite hnds. Two classihcation schemes are used one for hand specimens of Antarctic meteorites (commonly used), and one for meteorites as they appear in polished sections (rarely used). Weathering categories for hand specimens are A—minor rustiness B—moderate rustiness C—severe rustiness and e—evaporite minerals visible to the naked eye (e.g., Grossman, 1994). 

The occurrence of highly reactive minerals, such as evaporitic minerals, pyrite and even calcite, in low proportions—a percent or less— in a given rock, e.g., calcareous sandstone, pyritic shale, marl with traces of anhydrite, granite with traces of calcite, may determine the chemical character of stream water (Miller, 1961 Drever, 1988). In a study of 200 streams from monolithologic catchments underlain by various rock types under similar climatic conditions in France, the relative weathering rate based on the cation sum (Meybeck, 1986) ranges from 1 for quartz sandstone to 160 for gypsiferous marl. 

Compared with the parent rock, mineral products of weathering are usually more hydrated (as in the clays), have lower metal cation concentrations relative to A1 and Si (as in the clays and quartz), are oxidized (as in the ferric oxyhydroxides), or are chemical precipitates such as the carbonates or evaporite minerals, stable at earth-surface temperatures. 

Their contribution to the total dissolved load in rivers can be estimated by considering the mean composition of river water and the relative importance of various rocks to weathering. Estimates (18) indicate that evaporites and carbonates contribute approximately 17% and 38%, respectively, of the total dissolved load in the wodd s rivers. The remaining 45% is the result of the weathering of siUcates, underlining the significant role of these minerals in the overall chemical denudation of the earth s surface. 

The chemical weathering of crustal rock was discussed in Chapter 14 from the perspective of clay mineral formation. It was shown that acid attack of igneous silicates produces dissolved ions and a weathered solid residue, called a clay mineral. Examples of these weathering reactions were shown in Table 14.1 using CO2 + H2O as the acid (carbonic acid). Other minerals that undergo terrestrial weathering include the evaporites, biogenic carbonates, and sulfides. Their contributions to the major ion content of river water are shown in Table 21.1. 

From the above observations, it is certain that corrensite is a mineral which will form in normal sedimentary rocks and weathering environments. Thus it will be stable throughout the full range of clay mineral physical environments. It forms in sedimentary rocks which contain important quantities of iron (divalent) or magnesium but is not necessarily related to evaporite deposits and thus alkaline conditions of formation. 

In the dissolved load, the preferential dissolution of the different types of lithology or mineral can cause large variations in trace-element abundances in rivers. A classical tracer of rock dissolution is strontium. The isotopic ratio of strontium in river waters is, to a first approximation, explained by mixing between different sources. Rain, evaporite dissolution, and carbonate and silicate weathering are the predominant sources of strontium in rivers (see Chapter 5.12). Examples of the use of Ca/Sr, Na/Sr, and Sr isotopic ratios to quantify the proportion of strontium derived from carbonate weathering in large basins can be found in Negrel et al. (1993)... 

---