Behavior of alkali feldspars; crystallographic properties and characterization of composition and Al-Si distribution

Abstract

Ansrnlcr Four new alkali feldspar ion-exchange series have been synthesized, three with topochemically monoclinic Al-Si distributions ranging from sanidine to relatively ordered orthoclase, the other a microcline-low albite series. Parent materials for three series have previously been studied by single-crystal techniques, so Al-Si distributions are well characterized. Unit-cell dimensions and volumes have been measured for all series members and, with data for a previously reported fifth series, have been analyzed as a function of both composition and Al-Si distribution. The a unit-cell dimension, cornmonly used as a measure of composition in alkali feldspars, is affected to a small but significant extent by Al-Si distribution and also apparenfly does not vary linearly with composition, as has previously been assumed. Variation of the b and c unit-cell dimensions with composition is best analyzed separately for triclinic and monoclinic parts of the topochemically monoclinic series. Furthermore, b is not a linear function of c, an assumption that until now has been made in using these dimensions to calculate Al-Si distributions in alkali feldspars. Variation of c with mole fraction KAlSirO8 (N*) is linear in both the triclinic and the monoclinic parts of the topochemically monoclinic alkali feldspar series, as well as in the potassic region of the low albite-microcline series. In addition, Ac/AN* slopes are virtually constant for all series. This provides an ordering parameter, c*, which for alkali feldspars with No. > 0.32 (feldspars with No. < 0.32 require other equations given in the text) may be computed from the observed c value of an alkali feldspar (c.J and its composition as cr: cot + 0.038(l - N*). For natural feldspars, c* varies from about 7.180 A for high sanidine to approximarely 7.222 A for maximum microcline. The distribution of Al and Si between the T, and T, tetrahedral sites of an alkali feldspar (measured by the ordering parameter Z, wherc Z vaies from 1.0 for a perfectly ordered feldspar to lower values with disordering) may be calculated from c* as Z : - 138.575 + 19.3153 c*. Mole fractions ofAl in T, and T, are given by Nrur,r : -34.3939 + 4.82884c and N^,'r, :34.8939 - 4.82884c. These equations are valid for both topochemically monoclinic and triclinic feldspars and improve the precision with which Al-Si distributions can be calculated.