Abstract
Four new alkali feldspar ion-exchange series have been synthesized, three with topo-chemically monoclinic Al-Si distributions ranging from sanidine to relatively ordered or-thoclase, 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, commonly used as a measure of composition in alkali feldspars, is affected to a small but significant extent by Al-Si distribution and also apparently 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 KAlSi3O8 (Nor) 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, Δc/ΔNor slopes are virtually constant for all series. This provides an ordering parameter, cK, which for alkali feldspars with Nor > 0.32 (feldspars with Nor < 0.32 require other equations given in the text) may be computed from the observed c value of an alkali feldspar (cobs) and its composition as cK = cobs + 0.038(1 - Nor). For natural feldspars, cK varies from about 7.180 Å for high sanidine to approximately 7.222 Å for maximum microcline.
The distribution of A1 and Si between the T1 and T2 tetrahedral sites of an alkali feldspar (measured by the ordering parameter Z, where Z varies from 1.0 for a perfectly ordered feldspar to lower values with disordering) may be calculated from cK as Z = -138.575 + 19.3153 cK. Mole fractions of A1 in T1 and T2 are given by NAi(T1) = – 34.3939 + 4.82884 cK and NAi(T2) = 34.8939 - 4.82884cK. These equations are valid for both topochemically monoclinic and triclinic feldspars and improve the precision with which Al-Si distributions can be calculated.
