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

Three samples of fluorapophyllite-(K) with different cation composition were studied by the X-ray diffraction and IR-spectroscopy methods. The samples (K-0.72(NH4)(0.20))(sigma 0.92)Na-0.08 Ca-4[Al0.04Si7.96O20]F center dot 8H(2)O from Nidym and K0.87Na0.06 Ca-4[Si8O20]F center dot 8H(2)O from Akhaltsykhe localities are characterized by the deficiency of large cations K+ and (NH4)(+) in the A site; the chemical composition of Talnakh sample (K-0.94(NH4)(0.06))(sigma 1.00) Ca-4[Si8O20]F center dot 8H(2)O 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 angstrom away from the A position. The planar coordination of Na is formed by four H2O molecules. The occurrence of ammonium ions is found for the Nidym and Talnakh samples and their concentrations are estimated from the IR spectra and structure refinement. Polarized infrared measurements performed for fluorapophyllite-(K) 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 crystal structure.

Automated summary

Three samples of fluorapophyllite-(K) with different cation composition were studied. The deficiency of large cations K+ and (NH4)(+) in the A site in Nidym and Akhaltsykhe samples is compensated by the presence of medium-sized Na+ cations. The occurrence of ammonium ions is found in Nidym and Talnakh samples and their concentrations are estimated.