Na-Ca-feldspar

Dissolution reaction of Na Ca-feldspar (Na Ca =1 1 in atomic ratio) is written as... 

The ratios of Si02 (aqueous) to HCO3 and of Na to Ca " dissolved from Na Ca-feldspar by (1.45) are constant. However, these ratios differ for different Na Ca-feldspars (Fig. 1.24). Similarly, the compositional change for the feldspar... 

Na Ca-feldspar is the most common silicate mineral in earth s surface environment. It dissolves by the reaction... 

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... 

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.

Berndt et al. (1989) have indicated that aQ +/a + and aNa+/r H+ of midoceanic ridge hydrothermal fluids is controlled by clinozoisite, Ca-feldspar, and Na-feldspar. In addition to these assemblages, calcite is in equilibrium with fluids. Therefore, we can derive the /CO2 temperature relationship from the following equilibrium relations. 

In a final application of kinetic reaction modeling, we consider how sodium feldspar (albite, NaAlSisOs) might dissolve into a subsurface fluid at 70 °C. We consider a Na-Ca-Cl fluid initially in equilibrium with kaolinite [Al2Si20s (OF )/ ], quartz, muscovite [KAl3Si30io(OH)2, a proxy for illite], and calcite (CaC03), and in contact with a small amount of albite. Feldspar cannot be in equilibrium with quartz and kaolinite, since the minerals will react to form a mica or a mica-like... 

High Potassic Feldspars. A high potassic feldspar from Kingston, South Australia, (mole % K Na Ca - 81 18 1) represents a typical NTL spectrum for members of this group. Figures 2a and 3a show the 3-D and contour plots respectively. An emission band near 400 nm with FWHM of about 40 nm is prominent in the glow-curve. The NTL peaks occur between 250-350°C followed by another high temperature peak around 500°C. 

The Sodic End Members. Figures 2b and 3b show the NTL behaviour of an albite from Amelia, Virginia, U.S.A. which has very low K and Ca content (K Na Ca - 2 98 -). The main emission band is around 560 nm which peaks near 320°C. This broad (in wavelength and temperature) peak is characteristic of low K, low Ca feldspars. There is also a low intensity emission at shorter wavelengths and higher temperatures. The Amelia albite emits two to three times more TL light per mg than the high potassic end-member feldspars and considerably more than those of intermediate composition. 

It has been postulated [10] that silicate minerals as feldspar exposed to atmospheric agents undergo hydration and decay through the polarization and the ensuing dissociation of the water dipole into and OH due to the attractive forces of the free valencies. In this interaction the oxygens are converted to hydroxyl groups and part of the potassium is removed in solution. A partial or total cationic (K, Na, Ca ) depletion decomposes the feldspar. Since the neutral water now reaches an increased pH, introduction of acids neutralizes these alkali and facilitates a further decay of these minerals. 

Figure 6 Representation of chemical compositions of potassic, low-temperature micas in space. The poles represent feldspar, dioctahedral clays, and trioctahedral clays, respectively. M = Na, Ca, and especially K ions, R = Al, Fe R = Fe Mg. The compositional positions of the minerals Mu (muscovite) kaol (kaolinite), smectite, and mixed layer mica/smec-tites are indicated. Initial materials are kaolinite (kaol) and iron oxides. A second step is the production of an iron-aluminous smectite and then the formation of either illite via an iUite/smectite mixed layer mineral or glauconite via a glauconite mica/iron-smectite mixed layer phase.

Feldspar a group of common rock-forming minerals with the general composition MAl(AlSi3)08 where M=K, Na, Ca, Ba, Rb, Sr, or Fe. 

Weathering in the saprolite zone causes the destruction of feldspars and ferromagnesian minerals. Here, the mobile elements Na, Ca, K and Sr are leached and evacuated from the system (although K and Ba can be retained... 

In their study of the Canadian uranium deposit at Cigar Lake, Cramer and Smellie (1994) have plotted data for K, Na+, Ca +, and Mg +, in site waters on log([M"]/[H+]") versus log[H4Si04] diagrams. In Fig. 9.15, the illite phase field is contoured to show the stabilities of different illite fractions in I/S. The plot describes the evolution of water chemistry from atmospheric precipitation and surface-waters (lakes and streams) to infiltrating soil water and groundwater above, and then in contact with, the orebody. In the soil, kaolinite and illite (the dominant clay), quartz, and feldspars are... 

In the above equation, the composition of the montmorillonite was adjusted so that the Na/Ca ratio is the same as the parent-feldspar. This assumption is based upon the observation that the molal ratio of exchangeable Na to Ca in the montmorillonite of the Belly River Formation is 1.5, a value derived from the data reported in Table IV. 

Quartz K-feldspar Na-feldspar (albite) Ca-feldspar (anorthite)... 

The composition of feldspar in a particular rock as calculated from the major element chemistry of the rock using a specific set of rules - the (CIPW) normative calculation. The feldspar content is expressed in terms of the weight % proportion of the Na-, K- and Ca-feldspars (Albite (Ab), Orthoclase (Or) and Anorthite (An)) Basalt from an arc environment which typically has about 20 wt% Al203, rather than the ca. 15 wt% found in oceanic basalts... 

The chemical composition of minerals was determined in 29 polished carbon-coated thin sections using a Cameca Camebax BX50 microprobe equipped with three spectrometers and a back-scattered electron detector (BSE). Operating conditions were 20 kV acceleration voltage, 8 nA (for carbonates and clay minerals) to 12 nA (for feldspars) measured beam current, and a 1-10 pm beam diameter (depending on the extent of homogeneous areas). Standards and count times were wollastonite (Ca, 10 s), orthoclase (K, 5 s), albite (Na, Si, 5 and 10 s, respectively), corundum (Al, 20 s), MgO (Mg, 10 s), MnTiOj (Mn, 10 s) and hematite (Fe, 10 s). Precision of analysis was better than 0.1 mol%. 

Another form of nonstoichiometry arises from the partial replacement of one element by another in a crystal. It is common in natural minerals, such as the aluminosilicate feldspars (Na,Ca)(Al,Si) 408. The notation (Na,Ca) means that Na and Ca can be present in the same crystal sites in varying... 

A second chemical factor affecting mineral weatherability is the position of ions in the structure. The tetrahedra of Ca feldspars contain half Al3+ and half Si4+. At room temperature, Al3+ is more stable in octahedral coordination, The charge deficit created by the Al3+ substitution is made up by Ca2+ ions between the tetrahedra. The structural strain, the charge deficit in the tetrahedra, and concentrated Ca2+ counter charge weaken the anorthite feldspar structure with respect to weathering relative to Na and K feldspars. In Na and K feldspars, only one-quarter of the tetrahedral positions are occupied by A1 and that charge deficit can be locally neutralized by Na+ or K+. Calcium feldspars are, therefore, the least stable feldspars under soil conditions. Potassium feldspars are more stable than Na feldspars, because K fits better between adjacent tetrahedra. 

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