The Crystal Structure of Zircophyllite, K2NaFe2+7Zr2(Si4O12)2O2(OH)4F, An Astrophyllite-Supergroup Mineral From Mont Saint-Hilaire, QUÉBEC, CANADA

Abstract

Abstract Here we report the crystal structure of zircophyllite, ideally K 2 NaFe 2+ 7 Zr 2 (Si 4 O 12 ) 2 O 2 (OH) 4 F, from Mont Saint-Hilaire, Quebec, 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 SiO 2 31.89, Al 2 O 3 1.54, Na 2 O 1.91, K 2 O 6.13, Rb 2 O 0.19, Cs 2 O 0.10, CaO 0.62, SrO 0.10, MgO 0.22, ZnO 0.48, FeO 17.60, MnO 17.19, TiO 2 4.48, ZrO 2 8.65, Nb 2 O 5 2.07, F 1.36, (H 2 O) calc. 2.53, sum 96.48 wt.%; H 2 O was calculated from crystal-structure analysis. The empirical formula based on 31 (O + F) pfu is (K 1.85 Rb 0.03 Cs 0.01 Na 0.05 ) Σ1.94 (Na 0.83 Ca 0.16 Sr 0.01 ) Σ1 (Fe 2+ 3.48 Mn 3.44 Zn 0.08 Mg 0.08 ) Σ7.09 (Zr 1.00 Ti 0.80 Nb 0.22 ) Σ2.02 [(Si 7.54 Al 0.43 ) Σ7.97 O 24 ]O 2 [(OH) 3.98 F 0.02 ] Σ4 F, Z = 1, D calc. = 3.365 g/cm 3 . Zircophyllite is triclinic, space group P , a 5.447(2), b 11.966(5), c 11.789(4) A, α 112.95(1), β 94.688(6), γ 103.161(7)°, V 676.4(7) A 3 . The crystal structure has been refined from a twinned crystal to R 1 = 3.79% for 3657 unique ( F o > 4σ F ) reflections. In the crystal structure of zircophyllite, the four [4] T sites, with = 1.626 A, are occupied mainly by Si, with minor Al. The [6] D site is occupied by Zr 1.00 Ti 0.78 Nb 0.20 , ideally Zr 2 apfu , with = 2.013 A (φ = O, F). The T 4 O 12 astrophyllite ribbons and D octahedra constitute the H (Heteropolyhedral) sheet. In the O (Octahedral) sheet, the four [6] M (1–4) sites, with = 2.173 A (φ = O, OH), are occupied by (Fe 2+ 3.48 Mn 3.44 Zn 0.04 Mg 0.04 ), ideally Fe 2+ 7 apfu . The central O sheet and two H sheets form the HOH block, and adjacent HOH blocks link via a common anion (X P D = 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 K 2 and Na apfu , = 3.338 A and = 2.650 A (φ = O, F). Zircophyllite is a Zr-analogue of astrophyllite, K 2 NaFe 2+ 7 Ti 2 (Si 4 O 12 ) 2 O 2 (OH) 4 F. Zircophyllite and astrophyllite are related by the substitution: D Zr 4+ ↔ D Ti 4+ .