Single-crystal X-ray diffraction and IR-spectroscopy studies of K-deficient fluorapophyllite

Abstract

Abstract Three samples of fluorapophyllite with different cation composition were studied by the X-ray diffraction and IR-spectroscopy methods. The samples (K0.72(NH4)0.20)Σ0.82Na0.08 Ca4[Al0.04Si7.96O20]F·8H2O from Nidym and K0.87Na0.06 Ca4[Si8O20]F·8H2O from Akhaltsykhe localities are characterized by the deficiency of large cations K+ and (NH4)+ in the M site; the chemical composition of Talnakh sample (K0.94(NH4)0.06)Σ1.00 Ca4[Si8O20]F·8H2O is close to the stoichiometric one. The large cations deficiency in the structure of Nidym and Akhaltsykhe samples is compensated by the presence of medium-sized Na+ cations, located 1.12–1.31 Å away from the M position. The planar coordination of Na is formed by four H2O molecules. The occurrence of ammonium ions is found for the Nidym and Talnakh fluorapophyllites and their concentrations are estimated from the IR spectra and structure refinement. Polarized infrared measurements performed for apophyllite samples oriented along the c-axis reveal pleochroic behavior of the OH absorption bands. The observed pleochroism is explained in terms of absorption from two OH vectors of the H2O molecules in the apophyllite crystal structure.