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 (K0.72(NH4)0.20)Σ0.92Na0.08 Ca4[Al0.04Si7.96O20]F·8H2O from Nidym K0.87Na0.06 Ca4[Si8O20]F·8H2O Akhaltsykhe localities are characterized deficiency large cations K+ (NH4)+ in A site; chemical Talnakh sample (K0.94(NH4)0.06)Σ1.00 is close to stoichiometric one. structure compensated presence medium-sized Na+ cations, located 1.12–1.31 Å away position. planar coordination Na formed four H2O molecules. occurrence ammonium ions found for their concentrations estimated IR spectra refinement. Polarized infrared measurements performed oriented along c-axis reveal pleochroic behavior OH absorption bands. observed pleochroism explained terms two vectors molecules crystal structure.