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

Abstract

Three samples of fluorapophyllite with different cation composition were studied by the X-ray diffraction and IRspectroscopy methods. The samples (K 0.72 (NH 4 ) 0.20 ) Σ0.82 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 M 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 isclose 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 H 2 O 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 H 2 O molecules in the apophyllite crystal structure.

Comparative study of the HP‐HTbehavior of a layered silicate apophyllite in water and paraffin oil

“…A minor amount of (NH 4 ) + was revealed by IR spectroscopy and estimated by the structure analysis based on the single‐crystal X‐ray diffraction data. [ 25 ] The resulting composition is (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. [ 25 ] Taking into account the predominance of K + and F − in the chemical formula, our sample can be classified as (KF) apophyllite.…”

“…[ 25 ] The resulting composition is (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. [ 25 ] Taking into account the predominance of K + and F − in the chemical formula, our sample can be classified as (KF) apophyllite. The X‐ray single‐crystal diffraction data are consistent with the space group P 4/ mnc and the unit cell parameters a = 8.97567(8) Å, c = 15.7920(2) Å, V = 1272.25(3) Å 3 .…”

“…Water, with kinetic diameter of its molecules of about 2.7 Å, [ 26 ] is regarded as a penetrating medium for microporous silicates, such as zeolites [ 7 ] having the minimal entrance aperture (formed by the 8‐fold silica rings) comparable with that of the interlayer distance in the apophyllite structure (around 2.5 Å). [ 13,25 ] Paraffin oil is a mixture of high molecular weight (C n H 2n + 2 , 18 ≤ n ≤ 35) alkane derivatives [ 27 ] with a much larger molecular size (roughly 4.7 Å in the smallest section); it is used as a nonpenetrating medium in the H P studies of microporous compounds. [ 28 ] In addition, paraffin oil is a low‐reactivity material and is expected to have no chemical interaction with silicates at high P–T parameters.…”

Ammonium-Bearing Fluorapophyllite-(K) in the Magnesian Skarns from Aleului Valley, Pietroasa, Romania

“…Considerable interest was devoted to the study of both infrared ( [3,8,11,40,54,[63][64][65][66][67]) and Raman ( [3,11,65,66,[68][69][70]) factors of fluorapophyllite-(K). Therefore, the attribution of the bands in the spectra in Figures 8 and 9, which depict representative FTIR and Raman spectra, respectively, was considerably facilitated and is attempted in Table 8.…”

“…fluorapophyllite-(K) contain low amounts of ammonium as a substitute for K + (e.g., [54,67]). Apparently, the presence of NH 4 F in solution accelerates the crystallization and decreases the number of crystal defects, as in the case of zeolites [74].…”