Analysis of the inverse dependence between the Ca content in alkaline feldspars and the d_(100) parameter of their albite phase structure, as determined by the monocrystal X-ray surveying of their oriented specimens, allows concluding that at the very low contents of calcium in albite (near 0.1-0.4 percent) there is, in this mineral, the isomorphic replacement of sodium along the following scheme: 2Na~+<-Ca~(2+)+ square, where square- is the cation vacancy. The most principal factor of such isomorphic replacement is the elevated pressure, which may be appraised by the depth of formation of the muscovite-bearing pegmatites; it was just during their investigation that the phenomenon has been detected.
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