Dolomite weathering

Plate 10 Karst Landscape in Minerve, Herault, France 2005 Karst minerve by Hugo Soria. Karst topography is a geologic formation caused by the dissolution of carbonate rocks (limestone or dolomite). Weathering resistant rocks such as quartzite also forms Karst topography (see Chap. 1)... 

Barite [13462-86-7], natural barium sulfate, BaSO, commonly known as barytes, and sometimes as heavy spar, tiU, or cawk, occurs in many geological environments in sedimentary, igneous, and metamorphic rocks. Commercial deposits are of three types vein and cavity filling deposits residual deposits and bedded deposits. Most commercial sources are replacement deposits in limestone, dolomitic sandstone, and shales, or residual deposits caused by differential weathering that result in lumps of barite enclosed in clay. Barite is widely distributed and has minable deposits in many countries. 

The action of carbonic acid on limestone produces a calcium bicarbonate solution that is exceedingly soluble in water. (For comparison, at 20°C the solubility of calcium carbonate in water is only 0.0145 g per liter while the solubility of calcium bicarbonate is 166 g per literJ ) Magnesium ions from dolomite are also released into aqueous solution according to the same mechanism. The weathering of gypsum, calcium sulfate, also releases calcium ions into natural water supplies. 

Chukhrov et al. (1963) bluish glassy aggregates on weathered rocks between a pyrite-quartz ore body and an underlying dolomite bed, central Aldan, U.S.S.R. analyst V.M. Senderova. 

Rodriguez Navarro, C., Sebastian, E. Rodriguez Gallego, M. (1997). An urban model for dolomite precipitation authigenic dolomite on weathered building stones. 

For example, lakes in drainage basins of easily weathered soils such as calcareous rocks (calcite, dolomite, gypsum, halite) generally have high values of pH, alkalinity, total dissolved solids, conductivity, and hardness. Such is the case for lakes located in cavities formed through the gradual dissolution of water-soluble rocks (solution or Karst lakes). On the other hand if the drainage basin is in an acidic rock basin (i.e., where silicates predominate and are difficult to weather), the water has an acidic pH, low alkalinity, and low total dissolved solids. 

Capo R. C., Whipkey C. E., and Chadwick O. A. (2000) Pedogenic origin of dolomite in a basaltic weathering profile, Kohala Peninsula, Hawaii. Geology 28, 271—274. 

Other molar weathering ratios can be devised to reflect leaching (Ba/Sr), oxidation (Fe0/Fe203), calcification (CaO + MgO/AlaOs), and salinization (Na20/K20). Two of these ratios reflect differential solubility of chemically comparable elements, but calcification ratio quantifies the accumulation of pedogenic calcite and dolomite, and the ratio of iron of different valence gives reactant and product of iron oxidation reactions. In the Precambrian paleosol illustrated (Figure 4), these molar ratios indicate that the profile was oxidized and well drained, but little leached, calcified or salinized. 

The most abundant anion delivered by rivers to the oceans is bicarbonate ion (HCO ), and most of the bicarbonate alkalinity in rivers comes from the weathering of carbonate rocks (Meybeck, 1987). The chemical weathering of limestones and dolostones by dissolved CO2 can be represented by the reactions for dissolution of calcite and dolomite ... 

World annual production of natural diamonds, the cubic form of carbon, is about 110 million carats (1 carat = 200 mg). Almost all is derived from kimberlite or its weathered remnants, but Australian production is from the Argyle mine, at which the host rock is lamproite. Kimberlites are olivine- and volatUe-rich potassic ultrabasic rocks of variable geological age that typically form near-vertical carrot-shaped pipes intmded into Archean cratons. The volatile-rich component is predominantly CO2 in the carbonate minerals calcite and dolomite, and the texture is characteristically inequigranular, with large grains (macrocrysts), usually of olivine [Mg2Si04], in a fine-grained, olivine-rich matrix. 

Mineralised breccia pipes occur in Palaeozoic sedimentary rocks of the Colorado Plateau in northern Arizona (Weinrich, 1985). These pipes are usually circular or oval at suboutcrop, have horizontal dimensions that are typically a few tens of metres and vertical dimensions that may extend to 1000 m. They appear to have developed over solution-collapse structures resulting from karstic weathering of Lower Mississippian carbonate sediments between the Upper Mississippian and Triassic. The mineralisation, comprising pitchblende and sulphides of Fe, Cu, Mo, Pb and Zn, is found beneath a massive pyrite cap, several hundreds of metres below surface. Some of the ore bodies are, or have been, mined for uranium. Gangue minerals include calcite and dolomite. 

The rates of dissolution of carbonates and aluminosilicates as a function of pH are generalized in Fig. 2.11. Calcite and dolomite dissolution rates are generally 10 to 1 O -fold faster than rates for the silicates and decrease with pH up to saturation with the carbonates, usually between pH 8 and 10. Dissolution rates among the silicates range widely and are greatest for rapidly weathered minerals such as nepheline and olivine and slowest for quartz, muscovite (illite) and kaolinite, important products of chemical weathering in soils, discussed in more detail in Chap. 7. 

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