Carbonate minerals reactivity

The tailings comprise 5-10 wt. % pyrrhotite a highly reactive sulfide mineral that releases protons and Fe3+ into adjacent pore waters on oxidation. Further, the concentration of carbonate minerals in the tailings is low providing little buffering capacity above pH 5. Therefore, the tailings continued to acidify until they reach the pH of AI(OH)3 (pH 4-4.5) and Fe(OH)3 (pH 2.5-3.5) buffering. 

Carbonate minerals are among the most chemically reactive common minerals under Earth surface conditions. Many important features of carbonate mineral behavior in sediments and during diagenesis are a result of their unique kinetics of dissolution and precipitation. Although the reaction kinetics of several carbonate minerals have been investigated, the vast majority of studies have focused on calcite and aragonite. Before examining data and models for calcium carbonate dissolution and precipitation reactions in aqueous solutions, a brief summary of the major concepts involved will be presented. Here we will not deal with the details of proposed reaction mechanisms and the associated complex rate equations. These have been examined in extensive review articles (e.g., Plummer et al., 1979 Morse, 1983) and where appropriate will be developed in later chapters. 

A major contribution of this paper was pointing out the importance of bioturbation and bioirrigation on chemical processes associated with carbonate dissolution. In the movement of sulfidic sediment from depth to near the interface by biological processes, oxidation of the sediment produces sulfuric acid which ends up titrating alkalinity, lowering pH, and thus lowers saturation state (e.g., Berner and Westrich, 1985). Actually this process is very complex, involving many reactive intermediate compounds such as sulfite, thiosulfate, polythionates, etc. Aller and Rude (1988) demonstrated an additional complication to this process. Mn oxides may oxidize iron sulfides by a bacterial pathway that causes the saturation state of the solution to rise with respect to carbonate minerals, rather than decrease as is the case when oxidation takes place with oxygen. 

Figure 7.38. A schematic representation illustrating the differences between diagenetic processes affecting a carbonate composed of calcite and one composed of a metastable polymineralic assemblage of magnesian calcite, aragonite and calcite. Water-controlled alteration (WCA) processes are driven principally by invasion and evasion of CO2, whereas mineral-controlled alteration (MCA) is mainly governed by the differences in the chemical reactivity of the carbonate minerals. (After James and Choquette, 1984.)...

Methods need to be developed to measure and determine the influences of factors such as strain, crystal geometry, dislocation density, low concentrations of coprecipitates, etc. on subtle solubility and reactivity differences of carbonate minerals. 

In the lower reaches of the Genesee River, the results of the extractions suggest that substances other than hydrous oxides are phosphorus sinks. This is evident where the amount of sediment phosphorus extracted by hydrochloric acid steadily increases down river, while the oxalate extractable phosphorus remains relatively constant. Schwertmann (2 ) emphasized that the results of such procedures are best considered as a measure of the relative amount of a phase or, more generally, a measure of an element s reactivity in a sediment under carefully controlled conditions. Laboratory experiments (Figure 8) show that phosphorus uptake by calcium carbonate, under simulated natural conditions, proceeds slowly. The large hydrochloric acid extractable component observed at Rochester may arise from slow uptake and subsequent mineralization of dissolved inorganic phosphorus by carbonate minerals. 

Over the last several decades, the decline in alkalinity in many streams in Europe and in northeastern USA as a result of acid deposition has been a subject of much concern (Likens et al., 1979). The concentration of bicarbonate, the major anion buffering the water chemistry of surface waters and the main component of dissolved inorganic carbon (DIC) in most stream waters, is a measure of the reactivity of the watersheds and reflects the neutralization of carbonic and other acids by reactions with silicate and carbonate minerals encountered by the acidic waters during their residence in watersheds (Garrels and Mackenzie, 1971). Under favorable conditions, carbon isotopes of DIC can be valuable tools by which to understand the biogeochemical reactions controlling carbonate alkalinity in groundwater and watersheds (MUls, 1988 Kendall et al., 1992 see Chapter 5.14). 

These metal sulfides are stable below the water table within the reactive barrier. Sulfate reduction also releases dissolved inorganic carbon, which neutralizes the pH and favors the precipitation of metal carbonate minerals, e.g., FeCOs and MnC03 (Waybrant et al., 2002). 

For a variety of practical reasons, great effort has been expended in recent years to understand controls on and to predict rates of chemical weathering in soils. A principle reason has been a need to assess the effects of acid precipitation on the chemistry of soils and thus on the health of affected plants and trees. Soil acidification from acid precipitation has been a serious problem in industrialized areas where soils are thin or absent and bedrock and resultant soils lack carbonate or reactive silicate minerals (cf. Likens et al. 1977 Berner and Berner 1996). Soil acidification in such areas has caused the acidification of adjacent streams and even underlying groundwaters (cf. Bottcher et al. 1985 Hansen and Postma 1995). 

The rate of the oxidation process is determined by the reactivity of the starting carbon and oxidizer. The greater the reactivity of the substrates the lower the temperature of the process in which uniform formation of the pores in the granules is observed. In the case of carbonaceous materials the cokes of brown coals show the greatest reactivity, and the cokes of hard coals the smallest activity. The cokes of pit coals show an intermediate reactivity. This is connected with the earlier mentioned ordering of the crystallographic structure of carbon, which is of significant importance in the case of modification of carbon deposits contained in the carbon-mineral adsorbents in which the carbonaceous compound may be characterized by a differentiated chemical and physical structure. Thus the surface properties of hydrothermally modified complex adsorbents are defined by the course of three processes ... 

The reactive surfaces of carbonate minerals are potentially an additional reservoir for G dilution (12, JJ, J ). Exchange of G to carbonate surfaces was estimated to represent 5 to 15 percent of total G02 losses in several batch experiments (14). 

The vast majority of carbon is found in sedimentary rocks as inorganic carbonate minerals and solid organic compounds, and in the deep interior as carbon dioxide and methane. The global carbon cycle can therefore be envisioned as the long-term transfer of deep crustal and mantle carbon to shallow crustal sediment reservoirs via the ocean/atmosphere system, where carbon is parsed into several reservoirs through reactive pathways that modify the initial carbon isotopic value of the outgassed carbon. The... 

Intrinsically fillers can be divided into two types, reactive and inert. Reactive fillers will react with their environment. A good example of this is gibbsite (aluminium hydroxide), which will react with both acidic and basic substances. Aluminium hydroxide also loses its water of crystallisation at around 200 °C and this enables it to provide fire retardancy in polymer formulations. The silicate minerals (kaolin, mica, talc, quartz, etc.), are, in classical chemical terms, virtually inert, only being attacked by very strong acids and alkalis. The carbonate minerals and the hydroxide minerals are very reactive to acids. 

For sandstones with high carbonate mineral content (>15%-20%), HCl-HF mixtures should be avoided. HCl alone should be used in such cases. However, carbonates are often present in sandstones as grain cementation. Removal of carbonates with acid can diminish rock competence. Modern HF-containing buffered acid systems, which have low total acidity (relatively mild pH) and thus low reactivity with carbonates, are a good option in such cases. 

In the matrix acidizing of carbonates, HCl is so rapidly and completely reactive with carbonate minerals that macroscopic channels, or wormholes, are formed through the rock matrix. The formation of wormholes is also... 

H ). The hydrogen ion, not the acid molecule, is the reactive species with carbonate minerals. The add ionization reactions are as follows ... 

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