Mineral stability lines activity

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).

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). 

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).

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 . ... 

The boundary that expresses the relative stability of bornite (bn) and chalcopyrite (ccpy) is shown as a fine dashed line. The field of bornite stability decreases in size with decrease in dissolved sulphur in Fig. 12 total sulphur equals lOOOppm. Chalcopyrite and pyrite are commonly associated with uranium mineralization, but the diagram suggests that the /02 and pH conditions for this assemblage to occur are limited. In these deposits the total dissolved sulphur content may be significantly less than lOOOppm, and thus this assemblage puts an upper limit on sulphur activity. 

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