Mineral stability lines

Figure 1. Mineral stability lines for solid phases potentially controlling Ca activity at log pCOj = -2.95 (error bars fall within the areas of the circles).

Tb better understand the magnesium and silicate chemistry of the rock-fluid system, the solubilities of talc, sepiolite, and diopside are compared in Figure 4. These three minerals were plotted because they have been reported to be stable in high silica, alkaline systems similar to the one under study (19-23). The mineral stability lines were developed using the following equations ... 

Figure 4. Mineral stability lines for talc, sepiolite, and diopside after the 5-day reaction period (error bars fall within the area of the black circle).

Fig. 12.2. Redox-pH diagram for the Fe-S-H20 system at 100 °C, showing speciation of sulfur (dashed line) and the stability fields of iron minerals (solid lines). Diagram is drawn assuming sulfur and iron species activities, respectively, of 10-3 and 10-4. Broken line at bottom of diagram is the water stability limit at 100 atm total pressure. At pH 4, there are two oxidation states (points A and B) in equilibrium with pyrite under these conditions.

Figure 3 log/o -pH diagram for the solubility of gold as Au(HS)2, at 200 °C and saturated water vapor pressure. Solid lines delineate mineral-stability fields dotted lines delineate the fields of dominance for dissolved sulfur species (total dissolved sulfur = 0.01 m), and dashed lines show gold solubility contours. Drawn for an activity of water equal to unity (after Wood and Samson, 1998). 

This is the equation of a straight line with an intercept of -0.9 and slope of -2 on a plot of log ([K /[H+]) versus log [1448104] (Fig. 9.8). Note that the intercept is determined by the equilibrium constant, whereas the slope is defined solely by the reaction stoichiometry and is independent of relative or absolute mineral stabilities. 

Because three-dimensional diagrams of mineral stability planes are difficult to show and interpret visually, the number of axes is usually reduced from three to two either by fixing one of the variables or, more commonly, by representing two variables on one axis. For aluminosilicates, the combined variable, pH — l pAl, is often plotted against pSi OH)S. In that case, each mineral s stability field is represented by a straight line. A typical diagram of this sort is shown in Figure 6.15, where the sol-... 

Based on a knowledge of the mineralogy of the Uinta Sandstone, the mineral phases most likely to be controlling the solubility of Ca in the sandstone - L2 leachate system are calcite, dolomite, gypsum, and fluorite. Stability lines and saturation indices calculated for these minerals are present in Figure 1 and Thble III, respectively. The observed data point (black circle) plotted in Figure 1 represents the measured pH and log Ca + activity in the L2 leachate after reaction with the Uinta Sandstone. The log CO2 gas partial pressure of -2.95 atmosphere is based on the measured pH and alkalinity of the reacted solution. The open circle represents the log Ca + activity and pH calculated by MINTEQ for the raw leachate recarbonated to a log CO2 partial pressure of -2.95 atmosphere. The leachate apparently developed a CO2 gas overpressure because equilibrium with calcite was attained in sealed containers at the relatively low pH of 7.91 (18). The calcite-dolomite line shown in the figure represents the pH-dependent activity of Ca + in equilibrium with both calcite and dolomite. 

Figure 1.96. Log /oj-pH diagram constructed for temperature = 200°C, ionic strength = 1, ES = 10 m, and EC = 10 m. Solid line represents aqueous sulfur and carbon species boundaries which are loci of equal molalities. Dashed lines represent the stability boundaries for some minerals. Ad adularia. Bn bomite, Cp chalcopyrite, Ht hematite, Ka kaolinite, Mt magnetite, Po pyrrhotite, Py pyrite, Se sericite. Heavy dashed lines (1), (2), and (3) are iso-activity lines for ZnCOs component in carbonate in equilibrium with sphalerite (1) 4 co3=0-1- (2) 4 ,co3=0-01- (3) 4 co3 =0-001 (Shikazono, 1977b).

Figure 5.24 Phase stability relations for end-members of pyroxene quadrilateral. Melting curves refer to anhydrous conditions. Solidus curves for CaMgSi206 in saturated vapor phase conditions are also shown for various CO2/H2O ratios in the vapor phase. Dashed lines are extrapolated. From Lindsley (1982). Reprinted with permission of The Mineral-ogical Society of America.

Figure 11,42 contours superimposed on the stability fields of Fe-S-O minerals and barite dX T = 250 °C and = 0%o Solid lines and their dashed extensions con-... 

The combined effects of /02. nd pH variation on the sulfur isotopic compositions of ore-forming minerals are shown in figure 11.42 for a temperature of 250 °C and 0%o. The dashed lines delineate the stability limits of con-... 

Fig.1. Eh-pH diagram for the system Fe-U-S-C-H2O at 25 °C showing the mobility of uranium under oxidizing conditions, the relative stability of iron minerals, and the distribution of aqueous sulfur species. Heavy line represents the boundary between soluble uranium (above), and insoluble conditions (below), assuming 1 ppm uranium in solution.

The cations Fe and Al " are both considered hard acids by Pearson (3. Thus, it is not surprising that a plot comparing stabilities of the isostructural complexes and minerals gives a strong correlation. The oxygen-donor species, in particular, plot nearly on a straight line. The almost exact linearity of the OH... 

Fig. 80. Diagrams of mineral equilibria in carbonate iron-formations in the absence of water (isothermal sections) / = actual boundaries of stability fields of minerals J = boundaries that are metastable or unrealistic under the given conditions J = line of graphite stability 4 = isobars of fluid pressure (P, = Pqq -I- Pqq, in kbar) J = isobars of log /o,- Field of metastable states is hatched.

Therefore for an analysis of mineral equihbria it is necessary to plot sections of the diagrams along the line of graphite stability (see Figs. 79, 80). If the fugacities of COj and H2O are used as independent variables such a... 

An example of such a plot is a log([Na /[H+]) versus log[H4Si04] diagram (Bricker and Garrels 1965), in Fig. 9.10. In order to show montmorillonite in such a figure, its composition has obviously been idealized. Also given is the stability field of the zeolite mineral anaicite. As in Fig. 9.9, most water analyses plot in the kaolinite field. The montmorillonite/ kaolinite (mont/kaol) equilibrium line is an upper bound for most of the analyses. The slope of this boundary in Fig. 9.10 is defined by the stoichiometry of the mont/kaol reaction. This is determined by the formula that Bricker... 

This equation defines the stability plane for beidellite, intersecting that for kaolinite, as shown in Figure 6.14. The points on the line of intersection between these planes represent solution compositions (pH, pAF", pS OH) ) that are consistent with beidellite and kaolinite coexisting at equilibrium in soils. This assumes, of course, that both minerals dissolve congruently and reversibly and that some process such as ion exchange buffers the Mg activity at 10 . ... 

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