An alkali feldspar series

A series of single-phase alkali feldspars in the system Na₂O-K₂O-Al₂O₃-SiO₂-H₂O were crystallized from dehydrated gels at 500 bars and 800° C, in the presence of an aqueous vapor phase, quartz, and corundum. Twenty of these bulk samples were analyzed for K₂O and Na₂O by atomic absorption spectrophotometry. The unit cell parameters for 19 of these samples and 8 others in the series were obtained by least squares refinement of x-ray powder diffraction data collected with a focusing camera. Polynomial expressions giving the variation in unit cell parameters as a function of composition, in mol fraction potassium feldspar, were obtained, and compared with similar polynomial expressions developed from the avilable data on the sanidine-high albite series.This series of alkali feldspars is distinct from the previously studied series, but is closely related to the sanidine-high albite series. The differences illustrated by this series are principally in the higher values of the b and c unit cell edges, for a given N_or, and consequently higher unit cell or molar volume for a given N_or.Although the source of these differences may be related to crystalline solution with a hydronium (or oxonium) feldspar molecule, H₃OAlSi₃O₈, a more probable source of these differences is that this feldspar series is less ordered in terms of Al/Si distribution than the sanidine-high albite series.