Calcite buffering

Thus, dissolution of 0.25 mol of K-feldspar buffers sufficient acid to allow (potentially) precipitation of I mol of calcite. Buffering by silicates has been postulated as the primary control on subsurface pH (Hutcheon and Abercrombie, 1989,1990 Smith and Ehrenberg, 1989 Hutcheon etal, 1993), consistent with the observation that dissolution affecting feldspars typically leaves adjacent calcite cements and skeletal debris unaffected (Siebert et al, 1984). 

Fig. 4. Comparison of regional trends among pH and FcojCg) Tono groundwaters with calculated variations in these parameters assuming simple calcite buffering in a closed system (line) (modified from Arthur et al. [20] with permission from Geological Society Publishing House).

Calcite, with its vast reservoirs on the ocean floor, plays a major role in the oceanic CO2 balance, acting as an effective buffer against pH changes. Consequently the change in pH is small as CO2 or other acids dissolve in the ocean or fresh waters with calcite buffer. The growth and erosion of coral material (calcium carbonate) is sensitive to pH fluctuations [5]. The lowering of pH in the sea causes dissolution of coral material. Inorganic chemical acidification, introduced in natural aqueous systems, causes dissolution of solid calcium carbonate [6, 7]. 

For Further Reading J. P. Grime, Biodiversity and ecosystem function The debate deepens, Science, vol. 277, 1997, pp. 1260-1261. C. K. Fajcwski and H. T. Mullins, Historic calcite record from the Finger Fakes, New York Impact of acid rain on a buffered terrane, Geological Society of America Bulletin, vol. 115, 2003, pp. 373-384. J. Raloff, Pollution helps weeds take over prairies, Science News, vol. 150, 1996, p. 356. Environment Canada, Acid rain, http //www.ec.gc.ca/acidrain/. 

The H2S concentration of hydrothermal solution is plotted in Fig. 2.33. Based on these data, we can estimate the temperature of hydrothermal solution buffered by alteration mineral assemblages such as anhydrite-pyrite-calcite-magnetite and pyrite-pyrrhotite-magnetite for Okinawa fluids. 

For example, assuming anhydrite-magnetite-calcite-pyrite-pyrrhotite buffers redox in sub-seafloor reaction zones and a pressure of 500 bars, dissolved H2Saq concentrations of 21 °N EPR fluid indicate a temperature of 370-385°C. However, the estimated temperatures are higher than those of the measurement. This difference could be explained by adiabatic ascension and probably conductive heat loss during ascension of hydrothermal solution from deeper parts where chemical compositions of hydrothermal solutions are buffered by these assemblages. 

Fig. 2.33. H2Saq concentration.s as a function of temperature for hot spring fluids at midocean ridges as a function of redox. Assuming AMPC (anhydrite-magnetite-pyrite-calcite) and PPM (pyrite-pyrrhotite) buffers redox in sub-seafloor reaction zones and a pressure of 500 bars, dissolved H2Saq concentrations indicate temperatures of approximately 370-385°C. Solid star Okinawa. (Modified after Seyfried and Ding, 1995.)...

The X-ray diffraction results showed that peaks of calcite disappeared after extraction of the carbonate (CARB) fraction by the NaOAc-HOAc solution at pH 5.0 from the soils with 10-30% of CaC03, such as soils H13, B10 and J3. This indicates that the buffer solution at pH 5.0 is able to... 

The fugacities of gases such as CO2 and O2 can be buffered (Fig. 2.1 see Chapter 14) so that they are held constant over the reaction path. In this case, mass transfer between the equilibrium system and the gas buffer occurs as needed to maintain the buffer. Adding acid to a C02-buffered system, for example, would be likely to dissolve calcite,... 

In the calculation results (Fig. 14.6), increasing the CO2 fugacity decreases the pH to about 6, causing calcite to dissolve into the fluid. The fugacity increase drives CO2 from the buffer into the fluid, and most of the CO2 (Fig. 14.7) becomes C02(aq). The nearly linear relationship between the concentration of C02(aq) and the fugacity of C02(g) results from the reaction... 

In a first example of how minerals can buffer a fluid s chemistry, we consider how a hypothetical groundwater that is initially in equilibrium with calcite (CaCC F) at 25 °C might respond to the addition of an acid. In REACT, we enter the commands... 

Fig. 31.3. Variation in pH as pyrite reacts at 25 °C with a groundwater held in equilibrium with atmospheric 02, calculated assuming that the reaction occurs in the absence of buffering minerals (line line, from Fig. 31.1) and in the presence of calcite (bold line).

In contrast to the previous calculation, the fluid maintains a near-neutral pH (Fig. 31.3), reflecting the acid-buffering capacity of the calcite. 

In the calculation, reaction of 1 cm3 of pyrite consumes about 3.4 cm3 of calcite, demonstrating that considerable quantities of buffering minerals may be required in mineralized areas to neutralize drainage waters. 

The simulated C02 fugacity matches the initial reservoir C02 content and indicates that the pH is buffered by C02-calcite equilibrium. Further modelling was carried out using the Geochemists Workbench React and Tact modules with the thermodynamic database modified to reflect the elevated P conditions and kinetic rate parameters consistent with the Waarre C mineralogy. The Waarre C shows low reactivity and short-term predictive modelling of the system under elevated C02 content changes little with time (Fig. 1). 

As described in Chapter 21.7, a system of biogeochemical feedbacks act to stabilize the major ion composition of seawater. Some operate on short time-scale cycles, such as calcite compensation, and others operate over longer periods, such as the basalt-carbonate buffer. The linkages in the crustal-ocean-atmosphere fectory that act on the major ions also influence atmospheric CO2 levels and seawater s pH and alkalinity. 

Soils of lower pH (high H " molar concentration) have mobilized the Ca and therefore the soil slurry will be relatively unbuffered. The addition of HCI to the solution will immediately drop the pH in these samples where calcite has been removed, but will have little effect where calcite been precipitated. These buffered soils should be on the edge of the low pH (high H" ). The pattern from an oxidizing sulfide should therefore be an H" high and surrounded by a small or no change in H" concentration when HCI has been added. 

A third problem with the mitochondrial theory of biomineralization is that many mineralized tissues contain carbonate rather than phosphate. Since bicarbonate ions do not pass across mitochondrial membranes with any ease, it has now been shown that in phosphate-free buffers, calcium will enter mitochondria if dissolved carbon dioxide is available. It appears that some mitochondria possess carbonic an-hydrase activity on the inner membrane or in the mitochondrial matrix and are thus able to synthesize bicarbonate within the organelle. In such cases, inhibitors of carbonic anhydrase block the accumulation of calcium and carbonate ions622) since crystals of calcite have been identified in the mitochondria of earthworms calci-ferous glands623. These cells freqently showed spherical granules in the cytoplasm and lumen of the glands during phases of mineral secretion and it was suggested that they were aspects of cellular breakdown which occurred at these times. 

Each compartment of the cell contains a mixture of solid PbC03 ( sp = 7.4 X 10 l4) and either calcite or aragonite, both of which have Ksp 5 X 10-9. Each solution was buffered to pH 7.00 with an inert buffer, and the cell was completely isolated from atmospheric C02. The measured cell voltage was — 1.8 mV. Find the ratio of solubility products, Kif> (for calcite)/ (for aragonite)... 

The buffer capacity may be considered as defined for the incremental addition of a constituent to a closed system at equilibrium—e.g., adding a strong acid to a carbonate solution, C02 to a water solution in equilibrium with calcite, a strong acid to a sea water solution in equilibrium with both kaolinite and muscovite, etc. In general,... 

Buffering of pH during the early heavier precipitation of calcite and sepiolite is clear and is reflected in a near constancy of HCCV and CO32. However, after Ca2+ and Mg2+ are substantially reduced, the pH again rises with further concentration. Because the Sierra waters are so low in sulfate, gypsum does not precipitate abstraction of Ca2+ as calcite never permits the solubility product of gypsum to be exceeded. 

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