IMA 2018-108a = fluorapophyllite-(Cs)

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Marco E. Ciriotti
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IMA 2018-108a = fluorapophyllite-(Cs)

Messaggio da Marco E. Ciriotti » gio 07 nov, 2019 10:56

Prossima pubblicazione.

Referenza:
▪ Sokolova, E., Agakhanov, A.A., Pautov, L.A., Kasatkin, A.V., Karpenko, V.Yu., Day, M.C., Hawthorne, F.C., Muftakhov, V.A., Pekov, I.V., Cámara, F., Britvin, S.N. (2019): Fluorapophyllite-(Cs), CsCa₄(Si₈O₂₀)F(H₂O)₈, a new apophyllite mineral from the Darai-Pioz Massif, Tien-Shan, Northern Tajikistan. Canadian Mineralogist, 57, (in press).
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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Marco E. Ciriotti
Messaggi: 26138
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
Contatta:

Re: IMA 2018-108a = fluorapophyllite-(Cs)

Messaggio da Marco E. Ciriotti » lun 18 nov, 2019 19:19

Referenza:
▪ Sokolova, E., Agakhanov, A.A., Pautov, L.A., Kasatkin, A.V., Karpenko, V.Yu., Day, M.C., Hawthorne, F.C., Muftakhov, V.A., Pekov, I.V., Cámara, F., Britvin, S.N. (2019): Fluorapophyllite-(Cs), CsCa₄(Si₈O₂₀)F(H₂O)₈, a new apophyllite mineral from the Darai-Pioz Massif, Tien-Shan, Northern Tajikistan. Canadian Mineralogist, 57, (in press).

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).
Marco E. Ciriotti

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