From structure topology to chemical composition. XXVII. Revision of the crystal chemistry of the perraultite-type minerals of the seidozerite supergroup: Jinshajiangite, surkhobite, and bobshannonite

Abstract

ABSTRACT The crystal structures of the three perraultite-type minerals (bafertisite group, seidozerite supergroup)—jinshajiangite from Norra Kärr, Sweden, ideally NaBaFe2+4Ti2(Si2O7)2O2(OH)2F, Z = 4; surkhobite (holotype) from the Darai-Pioz massif, Tajikistan, ideally NaBaMn4Ti2(Si2O7)2O2(OH)2F, Z = 4; and bobshannonite (holotype) from Mont Saint-Hilaire, Canada, ideally Na2KBa(Mn7Na)Nb4(Si2O7)4O4(OH)4O2, Z = 2—were refined in space group C to R1 = 2.73, 2.85, and 2.02% on the basis of 2746, 2657, and 4963 unique reflections [Fo > 4σFo], respectively. Refinement was done using data from twinned crystals (jinshajiangite: three twin components; surkhobite and bobshannonite: two twin components). The parameters of a C-centered triclinic unit cell are as follows: jinshajiangite: a = 10.720(5), b = 13.823(7), c = 11.044(6) Å, α = 108.222(6), β = 99.28(1), γ = 89.989(6)°, V = 1532.0(2.2) Å3; surkhobite: a = 10.728(6), b = 13.845(8), c = 11.072(6) Å, α = 108.185(6), β = 99.219(5), γ = 90.001(8)°, V = 1540.0(2.5) Å3; and bobshannonite: a = 10.831(7), b = 13.903(9), c = 11.149(8) Å, α = 108.145(6), β = 99.215(9), γ = 90.007(7)°, V = 1572.6(3.2) Å3. New electron microprobe data are reported for the holotype surkhobite and new IR data for jinshajiangite. In the perraultite-type structure (structure type B1BG, B – basic, BG – bafertisite group), there is one type of TS (Titanium-Silicate) block and one type of I (Intermediate) block; they alternate along c. The TS block consists of HOH sheets (H-heteropolyhedral, O-octahedral). In the O sheet, the ideal composition of the five [6]MO sites is Fe2+4apfu (jinshajiangite), Mn4apfu (surkhobite), and (Mn7Na) (bobshannonite). There is no order of Fe2+ and Mn in the O sheet. In the H sheet, the ideal composition of the two [6]MH sites is Ti2apfu (jinshajiangite, surkhobite) and Nb4apfu (bobshannonite). The four [4]Si sites are occupied solely by Si. The MH octahedra and Si2O7 groups constitute the H sheet. The TS blocks link via common vertices of MH octahedra. The I block contains AP(1,2) and BP(1,2) cation sites. In the I block of jinshajiangite and surkhobite, the AP(1) site is occupied by Ba and the AP(2) site by K > Ba; the ideal composition of the two AP(1,2) sites is Ba apfu. In the I block of bobshannonite, Ba and K are ordered at the AP(1) and AP(2) sites, Ba:K ∼ 1:1 , ideally BaK apfu. The two BP(1,2) sites are each occupied by Na > Ca, ideally Na apfu (jinshajiangite, surkhobite) and solely by Na, ideally Na2apfu (bobshannonite). There is no order of Na and Ca at the BP(1,2) sites in jinshajiangite and surkhobite [currently defined as a Ca-ordered analogue of perraultite, ideally NaBaMn4Ti2(Si2O7)2O2(OH)2F, Z = 4]. The ideal formulae of surkhobite, KBa3Ca2Na2Mn16Ti8(Si2O7)8O8(OH)4(F,O,OH)8 (current IMA formula) and of bobshannonite, Na2KBa(Mn,Na)8(Nb,Ti)4(Si2O7)4O4(OH)4(O,F)2 (current IMA formula) have been revised as follows: NaBaMn4Ti2(Si2O7)2O2(OH)2F, Z = 4 (surkhobite) and Na2KBa(Mn7Na)Nb4(Si2O7)4O4(OH)4O2, Z = 2 (bobshannonite). The revised ideal formula of surkhobite is identical to the ideal formula of perraultite and hence surkhobite should be discredited.