Feldspar anorthite

Kalk-erz, n. limy ore. -failimg,/. precipitation of lime, -farbe, /. lime color, lime paint, -fass, n. (Leather) lime pit. -feldspat, in. lime feldspar, anorthite. kalk-frei, a. free from lime, -fiihrend, a. limebearing, calciferous. 

Positive Eu anomalies were found first in a few grains of Apollo 11 soil35) and later on in various anorthosite samples where calcic feldspar (anorthite) is the dominant mineral. 

The chemistry of hydrothermal fluids indicates that basalt-seawater interactions are a source of some elements that have been stripped from ocean crust and injected into seawater. Data from hydrothermal fluids show that both Ca2+ and dissolved silica are concentrated in the hydrothermal waters compared with seawater (Table 6.6). Calcium is probably released from calcium feldspars (anorthite) as they are converted to albite by Na+ uptake, a process called albitization. Silica can be leached from any decomposing silicate in the basalt, including the glassy matrix of the rock. Globally, basalt-seawater interaction seems to provide an additional 3 5 % to the river flux of Ca2+ and silica to the oceans. 

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

The feldspars are subdivided into two groups the alkali feldspars (including mi-crocline, orthoclase, and sanidine), in which potassium is dominant with a smaller proportion of sodium and negligible calcium and the plagioclase feldspars, which vary in composition in a series that ranges from pure sodium feldspar (albite) through to pure calcium feldspar (anorthite) with negligible potassium. Feldspars form colourless, white, or pink crystals with a hardness of 6 on the Mohs scale. 

Most igneous and metamorphic rocks are composed predominantly of alurninosiHcate minerals, including feldspar such as albite (NaAlSi Og) or anorthite (CaAl2Si20g) and crystalline forms of siHca such as quartz (Si02). Various mixed metal-plus-siHcon oxides such as oHvine [(Mg,Fe)2(SiO ] and... 

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.

Figure 13.4. Phase diagram of the system feldspar, a solid solution of albite (NaAlSisOg) and anorthite (CaAlSi20g). Data from Ref. 4.

In Chapter 13 we discussed briefly the solid-liquid equilibrium diagram of a feldspar. Feldspar is an ideal, solid solution of albite (NaAlSiaOg) and anorthite (CaAlSi20g) in the solid state as well as an ideal, liquid solution of the same components in the molten state. The relationships that we have developed in this chapter permit us to interpret the feldspar phase diagram (Figure 13.4) in a quantitative way. 

Ca (aq), Mg (aq), and HCOjCaq). Silicate weathering is an incongruent process. The most important of these reactions involves the weathering of the feldspar minerals, ortho-clase, albite, and anorthite. The dissolved products are K (aq), Na (aq), and Ca (aq), and the solid products are the clay minerals, illite, kaolinite, and montmorillonite. The weathering of kaolinite to gibbsite and the partial dissolution of quartz and chert also produces some DSi,... 

Table 5.62 Chemical analyses of some natural feldspars (Deer et al., 1983). (1) orthoclase (Mogok, Burma) (2) Adularia (St. Gottard, Switzerland) (3) anorthoclase inclusion in augite (Euganean Hills NE Italy) (4) sanidine from a nepheline-leucitite (5) albite from a pegmatite (6) anorthite from a calc-silicatic rock. ...

Table 5.63 lists the structural characters of the various feldspar polymorphs, according to Smyth and Bish (1988). Note duplication of cell edge c in anorthite and celsian and the progressive increase in volume that accompanies the lowering of symmetry in KAlSi30g. 

Fleer V. N. (1982). The dissolution kinetics of anorthite (CaAl2Si20g) and synthetic strontium feldspar (SrAl2Si20g) in aqueous solutions at temperatures below 100°C With applications to the geological disposal of radioactive nuclear wastes. Ph.D. diss., Pennsylvania State University University Park. 

Kaolin - Kaolinite 4, 5, 6, Dickite 16. 27 Mica - Biotite, Phologopite, Muscovite Illite - Illite 36, Illite-Bearing Shale Mixed-Layer Clays - Metabentonite 37, 42 Montmorillonite - 21. 22A, 22B, 24, 25, 26. 31 Feldspars - Albite, Anorthite, Orthoclase Chlorite - Chlorite... 

The feldspars are widely distributed and comprise almost two-thirds of all igneous rocks. Orthoclase and albite (NaAlSisOg) are feldspars in which one-fourth of the silicon atoms are replaced by aluminum and anorthite (CaAl2Si20g) and has one-half of the silicon atoms replaced by aluminum. Because the ionic radius of Na+ (0.095 nm) and Ca" " (0.1 nm) are the same, solid solutions are often formed between albite and anorthite. Good stones of albite and orthoclase are known as moonstones. 

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. 

Plagioclase - feldspar solid solution between CaAl2Si208 (anorthite) and NaAlSi30s (albite). Maskelynite is a diaplectic glass of plagioclase composition. 

---