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

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 ¼ 4; surkhobite (holotype) from the Darai-Pioz massif, Tajikistan, ideally NaBaMn4Ti2(Si2O7)2O2(OH)2F, Z ¼ 4; and bobshannonite (holotype) from Mont Saint-Hilaire, Canada, ideally Na2KBa(Mn7Na)Nb4(Si2O7)4O4(OH)4O2, Z ¼ 2-were refined in space group C1 to R1 ¼ 2.73, 2.85, and 2.02% on the basis of 2746, 2657, and 4963 unique reflections [Fo. 4rFo], 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 ¼ 10.720(5), b ¼ 13.823(7), c ¼ 11.044(6) Å, a ¼ 108.222(6), b ¼ 99.28(1), c ¼ 89.989(6)8, V ¼ 1532.0(2.2) Å3; surkhobite: a ¼ 10.728(6), b ¼ 13.845(8), c ¼ 11.072(6) Å, a ¼ 108.185(6), b ¼ 99.219(5), c ¼ 90.001(8)8, V ¼ 1540.0(2.5) Å3; and bobshannonite: a ¼ 10.831(7), b ¼ 13.903(9), c ¼ 11.149(8) Å, a ¼ 108.145(6), b ¼ 99.215(9), c ¼ 90.007(7)8, V ¼ 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]M O sites is Fe2+ 4 apfu (jinshajiangite), Mn4 apfu (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]M H sites is Ti2 apfu (jinshajiangite, surkhobite) and Nb4 apfu (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 Na2 apfu (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 ¼ 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 ¼ 4 (surkhobite) and Na2KBa(Mn7Na)Nb4(Si2O7)4O4(OH)4O2, Z ¼ 2 (bobshannonite). The revised ideal formula of surkhobite is identical to the ideal formula of perraultite and hence surkhobite should be discredited. © 2019 Mineralogical Association of Canada. All rights reserved.