K-feldspar albitized

Three MER diagrams (Figs. 2, 3, 4) collectively illustrate the mineralogical controls observed in each of the Meguma Supergroup formations. Because these metamorphosed rocks derive from proximal and distal flysch sediments, they likely once contained quartz, K-feldspar, albite, muscovite, illite, smectite (montmorillonite-beidellite), chlorite... 

A consideration of natural occurrence and chemical composition of alkali zeolites allows a certain refinement of the zeolite facies concept previously proposed. The key factor is the grouping of the alkali zeolites into a continuous solid solution series. Other possible coexisting phases of similar composition are sodium and potassium feldspar, natrolite and analcite. The extent of solid solution decreases with temperature, possibly also with pressure. This effect allows the sequential series zeolite-K feldspar, zeolite-analcite-K feldspar, analcite-K feldspar-albite and eventually two feldspars to the exclusion of analcite, the alkali zeolite with the highest stability limits. 

Following the keyword RATES, reactions rates and mathematical equations are listed from different references to describe the kinetics of K-feldspar, albite, calcite, pyrite, organic carbon, and pyrolusite reactions. Again, these entries have to be seen as examples that often have to be replaced or adjusted by site-specific data. 

Further examples can be found as already quoted in chapter 2.1.4.2 for K-feldspar, albite, calcite, pyrite, organic carbon and pyrolusite in the data set PHREEQC.dat or WATEQ4F.dat with the keyword RATES. There, all parameters are marked as comments by means of the sign in the block KINETICS. 

A - petalite (white) k-feldspar quartz albite B - petalite (pink) k-feldspar albite C - petalite (blue-grey and pink) albite mica D - petalite lepidolite albite albitite wali zone... 

Few data on the chemical compositions of feldspars (albite, K-feldspar) are available. Fuji (1976) indicated that K-feldspar and albite in the propylite of west Izu Peninsula, middle Honshu are of nearly end member composition. Nagayama (1992) showed that K-feldspars in the Hishikari Au-Ag vein and in the host andesitic rock have different composition Na/K ratio of K-feldspars from the vein is lower than that from the host rocks. 

Figure 1.140. The dependence of concentration of K+, Na, Ca + and HaSiOa in equilibrium with common alteration minerals (K-feldspar, Na-feldspar, quartz) with temperature (Shikazono, 1988b). Thermochemical data used for the calculations are from Helgeson (1969). Calculation method is given in Shikazono (1978a). Chloride concentration in hydrothermal solution is assumed to be 1 mol/kg H2O. A-B Na+ concentration in solution in equilibrium with low albite and adularia. C-D K+ concentration in solution in equilibrium with low albite and adularia. E-F H4Si04 concentration in solution in equilibrium with quartz. G-H Ca " " concentration in solution in equilibrium with low albite and anorthite.

Alteration minerals at the greatest depth in areas of relatively high temperatures (200-300°C) are quartz, chlorite, mica, anhydrite, K-feldspar, calcite, pyrite, albite, wairakite, laumontite, and minor amounts of epidote and prehnite. 

Hence, it is assumed that the minerals in equilibrium with geothermal waters are albite, K-feldspar, muscovite, quartz, calcite, anhydrite, chlorite and wairakite. 

Albite and K-feldspar are commonly observed to coexist. If the following reaction is in equilibrium,... 

Fig. 2.3. Relation between the K+ and CI concentration of geothermal waters and inclusion fluids. The solid line defines the equilibrium condition between the solution and the assemblage albite-K-feldspar at 250°C. For symbols used, see caption to Fig. 2.2. (Shikazono, 1978a).

The above discussions are based on the assumption of constant temperature. However, temperature varies widely. The chemical compositions of geothermal waters intimately relate to temperature. For example, the correlation between Na/K ratio in geothermal waters and temperature has been interpreted as indicating that this ratio is controlled by albite and K-feldspar (White, 1965 Ellis, 1969, 1970). 

Helgeson (1967) constructed an activity diagram depicting chemical equilibrium points (albite-sericite-K-feldspar and albite-sericite-Na-montmorillonite) of NazO-K20-Si02-Al203-H20 system at elevated temperatures. At these points,... 

This equation shows that activity of Ca + is related to pH, concentration of H2CO3 and temperature. Because pH is related to the concentration of Cl for the equilibrium curves 1 and 2 in Fig. 2.14, the relationship between the concentrations of Ca " " and Cl" can be derived for calcite-albite-sericite-K-feldspar-quartz equilibrium (curves 4 and 7 in Fig. 2.14) and calcite-albite-sericite-Na-montmorillonite-quartz equilibrium (curves 5 and 8 in Fig. 2.14) with constant w2h2C03- The range of zh2C03 in the solution in equilibrium with calcite is assumed to be 10 to 10 . The other equilibrium curves for the assemblage including Ca minerals are also drawn (Fig. 2.14). These assemblages are wairakite-albite-sericite-K-feldspar-quartz (curve 3), Ca-montmotillonite-albite-sericite-Na-montmorillonite-quartz (curve 6), Ca-montmorillonite-albite-sericite-K-feldspar-quartz (curve 9) and anhydrite (curve 10). The effect of solid solution on the equilibrium curves is not considered because of the lack of thermochemical data of solid solution. 

Fig. 2.14. The variation of concentration of with concentration of CP in aqueous solution in equilibrium with a given mineral assemblage at 250°C. I Equilibrium curve based on albite-sericite-Na-montmorillonite-quartz-aqueous solution equilibrium and Na-K-Ca relationship obtained by Fournier and Truesdell (1973). 2 Equilibrium curve based on albite-K-feldspar-aqueous solution equilibrium and Na-K-Ca relationship obtained by Fournier and Truesdell (1973). 3 Wairakite-albite-sericite-K-feldspar-quartz. 4 Calcite-albite-sericite-K-feldspar-quartz (/jjhjCO, = 10 ). 5 Calcite-albite-sericite-Na-montmorillonite-quartz (mH2C03 = 10 ). 6 Ca-montmorillonite-albite-sericite-Na-montmorillonite-quartz. 7 Calcite-albite-sericite-K-feld-spar-quartz (mnjCOj = 10 ). 8 Calcite-albite-sericite-Na-montmorillonite-quartz (mHjCOj = 10 ). 9 Ca-montmorillonite-albite-sericite-K-feldspar-quartz. 10 Anhydrite = 10 ). (Shikazono, 1976)...

These results indicate that the chemical composition of geothermal water at 250°C is largely controlled by such minerals commonly occurring in geothermal area as albite, K-feldspar, sericite, calcite, wairakite and quartz. 

The main alteration minerals surrounding Kuroko ore body are K-mica, K-feldspar, kaolinite, albite, chlorite, quartz, gypsum, anhydrite, and carbonates (dolomite, calcite, magnesite-siderite solid solution), hematite, pyrite and magnetite. Epidote is rarely found in the altered basalt (Shikazono et al., 1995). It contains higher amounts of ferrous iron (Fe203 content) than that from midoceanic ridges (Shikazono, 1984). 

Shikazono (1978) theoretically derived that the concentrations of alkali and alkali earth elements in chloride-rich hydrothermal solution are nearly in equilibrium with hydrothermal alteration minerals such as albite, K-feldspar, K-mica, quartz, calcite, wairakite, and Mg-chlorite. If we use 500 mmol/kg H2O as the average Cl concentration of hydrothermal solution from the back-arc basin, which is in equilibrium with... 

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