Abstract

Here we report the crystal structure of zircophyllite, ideally K2NaFe2+7Zr2(Si4O12)2O2(OH)4F, from Mont Saint-Hilaire, Québec, Canada. This is the first determination of the crystal structure of zircophyllite, which was originally described by Kapustin (1972). Zircophyllite is a mineral of the astrophyllite group of the astrophyllite supergroup. Chemical analysis by electron microprobe gave SiO2 31.89, Al2O3 1.54, Na2O 1.91, K2O 6.13, Rb2O 0.19, Cs2O 0.10, CaO 0.62, SrO 0.10, MgO 0.22, ZnO 0.48, FeO 17.60, MnO 17.19, TiO2 4.48, ZrO2 8.65, Nb2O5 2.07, F 1.36, (H2O)calc. 2.53, sum 96.48 wt.%; H2O was calculated from crystal-structure analysis. The empirical formula based on 31 (O + F) pfu is (K1.85Rb0.03Cs0.01Na0.05)Σ1.94(Na0.83Ca0.16Sr0.01)Σ1(Fe2+3.48Mn3.44Zn0.08Mg0.08)Σ7.09(Zr1.00Ti0.80Nb0.22)Σ2.02[(Si7.54Al0.43)Σ7.97O24]O2[(OH)3.98F0.02]Σ4F, Z = 1, Dcalc. = 3.365 g/cm3. Zircophyllite is triclinic, space group Pforumla, a 5.447(2), b 11.966(5), c 11.789(4) Å, α 112.95(1), β 94.688(6), γ 103.161(7)°, V 676.4(7) Å3. The crystal structure has been refined from a twinned crystal to R1 = 3.79% for 3657 unique (Fo > 4σF) reflections. In the crystal structure of zircophyllite, the four [4]T sites, with <T–O> = 1.626 Å, are occupied mainly by Si, with minor Al. The [6]D site is occupied by Zr1.00Ti0.78Nb0.20, ideally Zr2apfu, with <D–φ> = 2.013 Å (φ = O, F). The T4O12 astrophyllite ribbons and D octahedra constitute the H (Heteropolyhedral) sheet. In the O (Octahedral) sheet, the four [6]M(1–4) sites, with <M–φ> = 2.173 Å (φ = O, OH), are occupied by (Fe2+3.48Mn3.44Zn0.04Mg0.04), ideally Fe2+7apfu. The central O sheet and two H sheets form the HOH block, and adjacent HOH blocks link via a common anion (XPD = F) of two D octahedra. In the I (Intermediate) block between adjacent HOH blocks, the two interstitial cation sites, [13]A and [10]B, are ideally occupied by K2 and Na apfu, <A–φ> = 3.338 Å and <B–φ> = 2.650 Å (φ = O, F). Zircophyllite is a Zr-analogue of astrophyllite, K2NaFe2+7Ti2(Si4O12)2O2(OH)4F. Zircophyllite and astrophyllite are related by the substitution: DZr4+DTi4+.

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