Potassium feldspars

Mixed-layer clays, particularly lUite—smectite, are very common minerals and illustrate the transitional nature of the 2 1 layered siHcates. The transition from smectite to iUite occurs when smectite, in the presence of potassium from another mineral such as potassium feldspar, or from thermal fluids, is heated and/or buried. With increasing temperature smectite plus potassium is converted to iUite (37,39). 

In our second example, we calculate the same ratio for the reaction between muscovite and potassium feldspar (KAlSiaOs maximum microcline in the database) in the presence of quartz ... 

We consider as an example the hydrolysis of potassium feldspar (KAlSisOg), the first reaction path traced using a computer (Helgeson et al., 1969). We specify the composition of a hypothetical water... 

Fig. 13.1. Mineralogical results of reacting potassium feldspar into a hypothetical water at 25 °C, plotted in linear, semilog (a spaghetti diagram ), and log-log coordinates.

The predicted saturation states of the formation minerals (Fig. 23.5), furthermore, no longer identify a unique formation temperature. Whereas the temperatures suggested by albite, quartz, and potassium feldspar are quite close to the 250 °C formation temperature, those predicted by assuming that the fluid was in equilibrium with muscovite and calcite are too low, respectively, by margins of about 25 °C and 100 °C. To avoid error of this sort, we would need to determine the amount of gas lost from the sample and reintroduce it to the equilibrium system before calculating saturation indices. 

The analysis for Namafjall well 8 is similar to that of the Hveragerdi well in that a number of Ca, Mg, and Fe-bearing minerals appear supersaturated over the temperature range of interest. Again, this result probably reflects mixing or contamination. The equilibrium temperatures for quartz, albite, potassium feldspar, potassium clinoptilolite, and muscovite (Fig. 23.10) bracket a relatively broad temperature range of 205 °C to 250 °C, which can be compared to the well s inflow temperature of 246 °C. In this case, the equilibrium temperature is notably less well... 

To test whether the mixing hypothesis might explain the diagenetic alteration observed, we begin by equilibrating the fresh water, assuming equilibrium with the potassium feldspar ( maximum microcline in the database), quartz, and muscovite (a proxy for illite) in the formation. In REACT, we enter the commands... 

To prepare the initial system, we use the analysis in Table 25.1 for the saline water, which we assume to be in equilibrium with potassium feldspar, quartz, muscovite, and dolomite ( dolomite-ord is the most stable variety in the database). The commands... 

Strangely, Reaction 25.2 proceeds backward in the early part of the calculation (Fig. 25.1), producing a small amount of potassium feldspar at the expense of muscovite and quartz. This result, quite difficult to explain from the perspective of mass transfer, is an activity coefficient effect. As seen in Figure 25.2, the activity coefficient for K+ increases rapidly as the fluid is diluted over the initial segment of the reaction path, whereas that for H+ remains nearly constant. (The activity coefficients differ because the a parameter in the Debyc-IIuckcl model is 3 A for K+ and 9 A for H+.) As a result, aK+ increases more quickly than aH+, temporarily driving Reaction 25.2 from right to left. 

As a second example, we construct a simple model of how minerals might dissolve and precipitate as rainwater percolates through a soil (Bethke, 1997). The soil, 1 m thick, is composed initially of 50% quartz by volume, 5% potassium feldspar (KSiAEOfO, and 5% albite (sodium feldspar, NaSiAEOx). The remaining 40% of the soil s volume is taken up by soil gas (15% of the bulk) and water (25%). 

Minerals in the soil can dissolve or, if they become supersaturated, precipitate according to the kinetic rate law in the previous section (Eqn. 27.2). We take a rate constant of 4.2 x 10-18 mol cm-2 s-1 for quartz, as before, and of 30 x 10-18 mol cm-2 s-1 for potassium feldspar and 100 x 10-18 mol cm-2 s-1 for albite, from Blum and Stillings (1995). We assume a specific surface area of 1000 cm2 g-1, typical of sand-sized grains (Leamnson el al., 1969), for each of the minerals. 

The ore used in this example contained a mixture of pyrochlore and columbite as the major niobium minerals. The tantalum is mainly associated with columbite. The major gangue minerals present in this ore were soda and potassium feldspars with small amounts of mica and quartz. Beneficiation of this ore using cationic flotation, normally employed for flotation of niobium, was not applicable for this particular ore, since most of the mica and feldspar floated with the niobium and tantalum. The effect of amine on Ta/Nb flotation is illustrated in Figure 23.9. The selectivity between Ta/Nb and gangue minerals using a cationic collector was very poor. 

Potassium feldspar (orthoclase) Muscovite mica Quartz Felsic (rhyolite granite)... 

Desborough G. A. (1975). Authigenic albite and potassium feldspar in the Green River formation, Colorado and Wyoming. Amer. Mineral, 60 235-239. 

Harrison T. M. and McDougall I. (1982). The thermal significance of potassium feldspar K-Ar ages inferred from " °Ar/ Ar age spectrum results. Geochim. Cosmochim. Acta, 46 1811-1820. 

Hovis G. L. (1974). A solution calorimetric and X-ray investigation of Al-Si distribution in monoclinic potassium feldspars. In The feldspars, W. S. MacKenzie and J. Zussman, eds. Manchester Manchester University Press. 

Yund R. A. and Anderson T. F. (1974). Oxygen isotope exchange between potassium feldspar and KCl solution. In Geochemical Transport and Kinetics, Hofmann, Giletti, Yoder, and Yund, eds. New York Carnegie Inst. Washington and Academic Press. 

Microlithofacial classification of the sandstones is based on Dott s classification modified by Pettijohn et al. (1972). They are mostly arenites and subarkose and quartz wackes (rare sublithic, sporadically lithic and arkosic). Quartz is the main component of the sandstones (about 60-70 vol. percent). Feldspars (6 vol. percent) are mostly represented by potassium feldspars with plagioclases in lesser amounts. Some micas (muscovite and biotite) and chlorites are observed. Mica content of arenites reaches 3 vol. %, but is higher in the wackes. Heavy minerals present include zircon, sphene, rutile and apatite. Magmatic rocks (volcanic more than Plutonic) are predominant among lithoclasts (about 2 vol. %), but some metamorphic and sedimentary clasts being present too. 

Biotite and magnetite are also usually present and visible in hand specimen, muscovite may be present, and more rarely other oxides may be seen. Field estimates of modes ranged from 20-35 vol.% quartz, 15-35 vol.% plagioclase, 30-50 vol.% potassium feldspar, and 1-10 vol.% biotite. Accessory minerals include magnetite, muscovite, monazite, xenotime, zircon, apatite, epidote, ilmenite, titanite, allanite, molybdenite, and galena. The major U and Th minerals are uraninite and uranothorite. 

The pale yellow color in feldspar is due to Fe " in a tetrahedral Si/Al site. This color is often masked by the pervasive turbidity of common feldspars. A smoky color, the result of radiation damage from the decay of K-40, is also common but often masked. The blue color in the amazonite variety of potassium feldspar (and pale-blue albite) is from the interaction of trace amounts of Pb " in the feldspar with ionizing radiation. Lead-containing feldspars with a higher... 

Feldspars may be refractory as well or crystallized from partial melts. Whereas potassium feldspars are found to be mostly refractory, anorthite rich plagioclase may be newly formed. Pyroxene and spinel were identified in varying amounts but no olivine could be detected. Metals and iron oxide minerals (magnetite-hematite) are always present in BA, but have not been quantified yet. In Table 4 the amount and the ranges of measured mineral contents are summarized. 

A wide variety of zeolites are known to form in saline lakes where the species present is dependent upon the chemistry of the solutions. Rapid zeolite formation is aided by the existence of the volcanic glass and high water salinities. Potassium feldspar occurs with the common alkali zeolites (Hay and Moiola, 1963 Hay, 1964 Hay, 1966 Sheppard and Gude, 1969, 1971), however, albite is not evident as a diagenetic mineral in saline lakes. 

Many investigations have reported the presence of zeolites at the deep ocean bottom (Biscaye, 1965 Heath, 1969 Bonatti, 1963 Sheppard and Gude, 1971 Jacobs, 1970 Morgenstein, 1967 among others). Most of the alkali zeolites are represented except the silica-poor species natrolite and analcite. Rex and Martin (1966) indicate that detrital potassium feldspar is not stable under ocean floor conditions. Zeolites are found in most ocean basins where wind-carried volcanic ash predominates over detrital river-born clay mineral sediments. In these sediments phillipsite is particularly evident and it is known to continue to grow in the sediment column to depths of more than a meter (Bernat, t al.,... 

Figure 34. Alkali zeolites projected into a portion of the Na-K-Si coordinates. Anal = analcite Ph = phillipsite solid solution Ze = alkali zeolites undifferentiated Alb = albite KF = potassium feldspar Q = quartz Si = amorphous silica, a) low, b) medium, and c) high temperature facies. Shaded areas are two-phase fields.

Finally, when analcite is the only alkali zeolite stable, one finds the association analcite- albite-potassium feldspar (Figure 34c). This is the highest temperature zeolite paragenesis. It might be added that natro-lite may be stable also but one cannot decide, using the available data, how it fits into zeolite parageneses. From experimental studies, the analcite-albite-K feldspar paragenesis exists up to 180°C at low pressures (Pu = P total). However, Iijima (1970) indicates that its limit is... 

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