Feldspar composition

Furthermore, a Pearce element ratio (PER) scatterplot testing feldspar compositional control (Fig. 3) illustrates that some hypogene rocks have compositions controlled by feldspar (primary igneous or potassic alteration). 

Feldspar Unlike enstatite and forsterite, feldspar is present in most meteorites. The meteoritic feldspars are dominated by plagioclase which are those feldspar compositions in the continuous solid solution series ranging from albite (NaAlSi308) to anorthite (CaAl2Si2C>8) compositions near the ends of this series are most common. The earliest CL studies of feldspar were prompted by the lunar program about 1970 and a series of papers compared the CL of lunar, meteoritic and doped synthetic plagioclase. Later observations were made for anorthite in the carbonaceous chondrites. 

There are insufficient data for us to determine exactly the dependence of r " on feldspar composition, but we find that the assumption ... 

Development of alteration layers on minerals dissolved under neutral and alkaline conditions has not been thoroughly investigated, but some work has been completed, especially on feldspar compositions (Chou and Wollast, 1984 Hellmann et al, 1989, 1990a Muir et al, 1990 Nesbitt et al, 1991 Hellmann, 1995, Hamilton et al, 2000). Under neutral conditions, the leached layer thickness (tens of angstroms to a few hundred angstroms) is generally less than that observed for more acid dissolution, with variability reported in the composition and thickness of the layer i.e., sodium depletion is generally observed, but both aluminum depletion and enrichment (with respect to silicon) have been reported. Variations in solution chemistry (see Section 5.03.7) and feldspar composition may explain some of these differences for... 

Type of feldspar Composition (wt.%) Si02 M2O3 CaO Na20 K2O Density (g cm ) Melting temperature (°C)... 

Shotyk, W., Nesbitt, H.W., 1992. Incongruent and congruent dissolution of plagioclase feldspar effect of feldspar composition and ligand complexation. Geoderma 55, 55-78. 

The softening range of the body decreases, and refractoriness increases with increased KjO, at the expense of NaO and CaO, in the feldspar used. With semi vitreous bodies which do not contain any CaO, soda feldspar compositions mature slightly sooner than potash feldspar. When hme is added the situation is reversed. The reason lime causes a sharp increase in the fusibUity of potash feldspar but not of soda spar. The glassy content of the body produced with soda feldspar is less viscous than that produced by potash spar, and thus the soda spar bodies deform more readily. 

Clays are composed of extremely fine particles of clay minerals which are layer-type aluminum siUcates containing stmctural hydroxyl groups. In some clays, iron or magnesium substitutes for aluminum in the lattice, and alkahes and alkaline earths may be essential constituents in others. Clays may also contain varying amounts of nonclay minerals such as quart2 [14808-60-7] calcite [13397-26-7] feldspar [68476-25-5] and pyrite [1309-36-0]. Clay particles generally give well-defined x-ray diffraction patterns from which the mineral composition can readily be deterrnined. 

Typical ranges of enamel compositions are Hsted in Table 2. Raw materials (Table 1) for the glass batch include minerals, such as feldspars and quartz, because these are inexpensive sources of Si02 and AI2O2 (see Clays). The batch composition for cover coats is comprised primarily of manufactured chemicals of known, controlled levels of purity to maintain reproducible, clean colors. 

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. 

The dependence of concentration of K+, Na+, Ca + and H4Si04 in equilibrium with common alteration minerals (K-feldspar, Na-feldspar, quartz) on temperature is shown in Fig. 1.140 (Shikazono, 1988b). This figure demonstrates that (1) chemical compositions of hydrothermal solution depend on alteration minerals, temperature and chloride concentration, and K" " and HaSiOa concentrations increase and Ca + concentration decrease with increasing of temperature. In this case, it is considered that potassic alteration adjacent to the gold-quartz veins occurs when hydrothermal solution initially in... 

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). 

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. 

Fig. 2.19. Reservoir temperature versus saturation indices (logQ/K) for calcite, anhydrite, K-feldspar and K-mica based on the estimated composition of reservoir fluid (Seki, 1991). Estimation based on gas results of Seki (1990), with saturation calculations carried out by PECS (Takeno, 1988). Gas concentrations were assumed to be 1 wt% of CO2 and 250 mg/kg for H2S for all wells (Seki, 1991).

The compositions of the Sumikawa discharge fluids are controlled by K-feldspar, illite, calcite and Ca-aluminosilicates (epidote, prehnite, wairakite). 

The composition of the particles is related to that of the source rocks. Quartz sand [composed of silica (silicon dioxide)], which makes up the most common variety of silica sand, is derived from quartz rocks. Pure quartz is usually almost free of impurities and therefore almost colorless (white). The coloration of some silica sand is due to chemical impurities within the structure of the quartz. The common buff, brown, or gray, for example, is caused by small amounts of metallic oxides iron oxide makes the sand buff or brown, whereas manganese dioxide makes it gray. Other minerals that often also occur as sand are calcite, feldspar and obsidian Calcite (composed of calcium carbonate), is generally derived from weathered limestone or broken shells or coral feldspar is an igneous rock of complex composition, and obsidian is a natural glass derived from the lava erupting from volcanoes see Chapter 2. 

Once the initial equilibrium state of the system is known, the model can trace a reaction path. The reaction path is the course followed by the equilibrium system as it responds to changes in composition and temperature (Fig. 2.1). The measure of reaction progress is the variable , which varies from zero to one from the beginning to end of the path. The simplest way to specify mass transfer in a reaction model (Chapter 13) is to set the mass of a reactant to be added or removed over the course of the path. In other words, the reaction rate is expressed in reactant mass per unit . To model the dissolution of feldspar into a stream water, for example, the modeler would specify a mass of feldspar sufficient to saturate the water. At the point of saturation, the water is in equilibrium with the feldspar and no further reaction will occur. The results of the calculation are the fluid chemistry and masses of precipitated minerals at each point from zero to one, as indexed by . 

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... 

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