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 Karr, Sweden, ideally NaBaFe42+Ti2(Si2O7)(2)O-2(OH)(2)F, Z = 4; surkhobite (holotype) from the Darai-Pioz massif, Tajikistan, ideally NaBaMn4Ti2(Si2O7)(2)O-2(OH)(2)F, Z = 4; and bobshannonite (holotype) from Mont Saint-Hilaire, Canada, ideally Na2KBa(Mn7Na)Nb-4(Si2O7)(4)O-4(OH)(4)O-2, Z = 2-were refined in space group C (1) over bar to R-1 = 2.73, 2.85, and 2.02% on the basis of 2746, 2657, and 4963 unique reflections [F-o > 4 sigma F-o], 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: alpha = 10.720(5), b = 13.823(7), c = 11.044(6) angstrom, alpha = 108.222(6), beta = 99.28(1), gamma = 89.989(6)degrees, V = 1532.0(2.2) angstrom(3); surkhobite: alpha = 10.728(6), b = 13.845(8), c = 11.072(6) angstrom, alpha = 108.185(6), beta = 99.219(5), gamma = 90.001(8)degrees, V = 1540.0(2.5) angstrom(3); and bobshannonite: alpha = 10.831(7), b = 13.903(9), c = 11.149(8) angstrom, alpha = 108.145(6), beta = 99.215(9), gamma = 90.007(7)degrees, V = 1572.6(3.2) angstrom(3). 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 Fe-4(2+) apfu (jinshajiangite), Mn-4 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 M-[6](H) sites is Ti-2 apfu (jinshajiangite, surkhobite) and Nb-4 apfu (bobshannonite). The four ([4]) Si sites are occupied solely by Si. The M-H octahedra and Si2O7 groups constitute the H sheet. The TS blocks link via common vertices of M-H octahedra. The I block contains A(P) (1,2) and B-P (1,2) cation sites. In the I block of jinshajiangite and surkhobite, the A(P)(1) site is occupied by Ba and the A(P)(2) site by K > Ba; the ideal composition of the two A(P) (1,2) sites is Ba apfu. In the I block of bobshannonite, Ba and K are ordered at the A(P) (1) and A(P) (2) sites, Ba:K similar to 1:1, ideally BaK apfu. The two B-P (1,2) sites are each occupied by Na > Ca, ideally Na apfu (jinshajiangite, surkhobite) and solely by Na, ideally Na-2 apfu (bobshannonite). There is no order of Na and Ca at the B-P(1,2) sites in jinshajiangite and surkhobite [currently defined as a Ca-ordered analogue of perraultite, ideally NaBaMn4Ti2(Si2O7)(2)O-2(OH)(2)F, Z = 4]. The ideal formulae of surkhobite, KBa3Ca2Na2Mn16Ti8(Si2O7)(8)O-8(OH)(4)(F,O,OH)(8) (current IMA formula) and of bobshannonite, Na2KBa (Mn,Na)(8)(Nb,Ti)(4)(Si2O7)(4)O-4(OH)(4)(O,F)(2) (current IMA formula) have been revised as follows: NaBaMn4Ti2(Si2O7)(2)O-2 (OH)(2)F, Z = 4 (surkhobite) and Na2KBa(Mn7Na)Nb-4(Si2O7)(4)O-4(OH)(4)O-2, Z = 2 (bobshannonite). The revised ideal formula of surkhobite is identical to the ideal formula of perraultite and hence surkhobite should be discredited.