Fluorapophyllite-(Cs), CsCa4(Si8O20)F(H2O)8, a new apophyllite-group mineral from the Darai-Pioz Massif, Tien-Shan, northern Tajikistan

Abstract

Fluorapophyllite-(Cs) (IMA 2018-108a), ideally CsCa4(Si8O20)F(H2O)8, is an apophyllite-group mineral from the moraine of the Darai-Pioz glacier, Tien-Shan, Northern Tajikistan. Associated minerals are quartz, pectolite, baratovite, aegirine, leucosphenite, pyrochlore, neptunite, fluorapophyllite-(K), and reedmergnerite. Fluorapophyllite-(Cs) is a hydrothermal mineral. It is colorless and has a vitreous luster and a white streak. Cleavage is perfect; it is brittle and has a stepped fracture. Mohs hardness is 4.5–5. Dmeas. = 2.54(2) g/cm3, Dcalc. = 2.513 g/cm3. Fluorapophyllite-(Cs) is unixial (+) with refractive indices (λ = 589 nm) ω = 1.540(2), ε = 1.544(2). It is non-pleochroic. Chemical analysis by electron microprobe gave SiO2 48.78, Al2O3 0.05, CaO 22.69, Cs2O 10.71, K2O 1.13, Na2O 0.04, F 1.86, H2Ocalc. 14.61, –O=F2 –0.78, sum 99.09 wt.%; H2O was calculated from crystal-structure analysis. The empirical formula based on 29 (O + F) apfu, H2O = 8 pfu, is (Cs0.75K0.24)Σ0.99(Ca3.99Na0.01)Σ4(Si8.01Al0.01)Σ8.02O20.03F0.97(H2O)8, Z = 2. The simplified formula is (Cs,K)(Ca,Na)4(Si,Al)8O20F(H2O)8. Fluorapophyllite-(Cs) is tetragonal, space group P4/mnc, a 9.060(6), c 15.741(11) Å, V 1292.10(19) Å3. The crystal structure has been refined to R1 = 4.31% based on 498 unique (Fo > 4σF) reflections. In the crystal structure of fluorapophyllite-(Cs), there is one [4]T site occupied solely by Si, <T–O> = 1.615 Å. SiO4 tetrahedra link to form a (Si8O20)8– sheet perpendicular to [001]. Between the Si–O sheets, there are two cation sites: A and B. The A site is coordinated by eight H2O groups [O(4) site], A–O(4) = 3.152(4) Å; the A site contains Cs0.75K0.24□0.01, ideally Cs apfu. The Cs–O bond length of 3.152 Å is definitely larger than the K–O bond length of 2.966–2.971 Å in fluorapophyllite-(K), KCa4(Si8O20)F(H2O)8. The [7]B site contains Ca3.99Na0.01, ideally Ca4apfu; <B–φ> = 2.417 Å (φ = O, F, H2O). The Si–O sheets connect via A and B polyhedra and hydrogen bonding; two H atoms have been included in the refinement. Fluorapophyllite-(Cs) is isostructural with fluorapophyllite-(K). Fluorapophyllite-(Cs) is a Cs-analogue of fluorapophyllite-(K).

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

“…, whereas the distance H12•••O2 implies a significantly weaker bond or even its absence. The same tendency is also observed in the structure data available in the literature(Chao 1971;Prince 1971;Matsueda et al 1981;Agakhanov et al 2019;Števko et al 2020…”

“…A general formula of apophyllite can be written as M(Ca,Na) 4 [(Si,Al) The literature data on the chemical composition of apophyllite show it is nearly stoichiometric. According to the structure determinations (Colville et al 1971;Chao 1971;Price 1971;Matsueda et al 1981;Stahl et al 1987;Agakhanov et al 2019;Števko et al 2020), all the atomic positions are 100% occupied. This work is devoted to the study of K-deficient fluoraphophyllite with the M sites partially occupied, as well as fluorapophyllite with the stoichiometric composition.…”

Fluorapophyllite-(NH4), NH4Ca4(Si8O20)F⋅8H2O, a new member of the apophyllite group from the Vechec quarry, eastern Slovakia

“…Unfortunately, it was not possible to localise H atoms associated with the NH 4 moiety from the difference-Fourier maps, probably due to some degree of freedom (rotation) of NH 4 in the structure, bonded by the relatively weak H bonds (corresponding Donor⋅⋅⋅Acceptor distance, N⋅⋅⋅O, ∼3 Å). (Dunn and Wilson, 1978;Hatert et al, 2013;Agakhanov et al, 2019). Marriner et al (1990) discussed the existence of an NH 4 -enriched variety ('ammonian apophyllite') of fluorapophyllite-(K) in samples from Mexico, with up to 25% of stoichiometric K + replaced by NH 4 + .…”

“…Fluorapophyllite-(NH 4 ), NH 4 Ca 4 (Si 8 O 20 )F⋅8H 2 O, is the NH 4 dominant analogue of fluorapophyllite-(K), KCa 4 Si 8 O 20 F⋅8H 2 O, fluorapophyllite-(Na), NaCa 4 Si 8 O 20 F⋅8H 2 O and fluorapophyllite-(Cs), CsCa 4 Si 8 O 20 F⋅8H 2 O, and belongs to the apophyllite group (Dunn and Wilson, 1978; Dunn et al , 1978; Hatert et al ., 2013; Agakhanov et al ., 2019). Marriner et al (1990) discussed the existence of an NH 4 -enriched variety (‘ammonian apophyllite’) of fluorapophyllite-(K) in samples from Mexico, with up to 25% of stoichiometric K + replaced by NH 4 + .…”

“…Fluorapophyllite-(NH 4 ) is named according to the current nomenclature scheme of apophyllite-group minerals (Hatert et al , 2013), and is an NH 4 dominant analogue of fluorapophyllite-(K), KCa 4 Si 8 O 20 F⋅8H 2 O, fluorapophyllite-(Na), NaCa 4 Si 8 O 20 F⋅8H 2 O and fluorapophyllite-(Cs), CsCa 4 Si 8 O 20 F⋅8H 2 O (Dunn and Wilson, 1978; Dunn et al , 1978; Hatert et al , 2013; Agakhanov et al ., 2019). The new mineral and the name has been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2019-083, Števko et al ., 2020).…”