From Structure Topology To Chemical Composition. Xxix. Revision Of The Crystal Structure Of Perraultite, Nabamn4ti2(Si2o7)(2)O-2(Oh)(2)F, A Seidozerite-Supergroup Ts-Block Mineral From The Oktyabr'Skii Massif, Ukraine, And Discreditation Of Surkhobite

The crystal structure of perraultite from the Oktyabr'skii massif, Donetsk region, Ukraine (bafertisite group, seidozerite supergroup), ideally NaBaMn4Ti2(Si2O7)(2)O-2(OH)(2)F, Z = 4, was refined in space group C (1) over bar to R-1 = 2.08% on the basis of 4839 unique reflections [F-o > 4 sigma F-o]; a = 10.741(6), b = 13.841(8), c = 11.079(6) angstrom, alpha = 108.174(6), beta = 99.186(6), gamma = 89.99(1)degrees, V = 1542.7(2.7) angstrom(3). Refinement was done using data from a crystal with three twin domains which was part of a grain used for electron probe microanalysis. In the perraultite structure [structure type B1(BG), 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 M-[6](O) sites is Mn-4 apfu. There is no order of Mn and Fe2+ in the O sheet. The M-H octahedra and Si2O7 groups constitute the H sheet. The ideal composition of the two M-[6](H) sites is Ti-2 apfu. 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. The A(P)(1) site is occupied by Ba and the A(P)(2) site by K > Ba; the ideal composition of the A(P)(1,2) sites is Ba apfu. The B-P(1) and B-P(2) sites are each occupied by Na > Ca; the ideal composition of the B-P(1,2) sites is Na apfu. We compare perraultite and surkhobite based on the work of Sokolova et al. (2020) on the holotype sample of surkhobite: space group C (1) over bar, R-1 = 2.85 %, a = 10.728(6), b = 13.845(8), c = 11.072(6) angstrom, alpha = 108.185(6), beta = 99.219(5), gamma = 90.001(8)8, V = 1540.0(2.5) angstrom(3); new EPMA data. We show that (1) perraultite and surkhobite have identical chemical composition and ideal formula NaBaMn4Ti2(Si2O7)(2)O-2(OH)(2)F; (2) perraultite and surkhobite are isostructural, with no order of Na and Ca at the B-P(1,2) sites. Perraultite was described in 1991 and has precedence over surkhobite, which was redefined as "a Ca-ordered analogue of perraultite'' in 2008. Surkhobite is not a valid mineral species and its discreditation was approved by CNMNC IMA (IMA 20-A).