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 (NH 4 ) 0.20 ) Σ0.92 Na 0.08 Ca 4 [Al 0.04 Si 7.96 O 20 ]F·8H 2 O from Nidym and K 0.87 Na 0.06 Ca 4 [Si 8 O 20 ]F·8H 2 O from Akhaltsykhe localities are characterized by the deficiency of large cations K + and (NH 4 ) + in the A site; the chemical composition of Talnakh sample (K 0.94 (NH 4 ) 0.06 ) Σ1.00 Ca 4 [Si 8 O 20 ]F·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 Å away from the A position. The planar coordination of Na is formed by four H 2 O 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 H 2 O molecules in the crystal structure.