From structure topology to chemical composition. XIV. Titanium silicates: refinement of the crystal structure and revision of the chemical formula of mosandrite, (Ca3REE)[H2O)2Ca0.5□0.5]Ti(Si2O7)2(OH)2(H2O)2, a Group-I mineral from the Saga mine, Morje, Porsgrunn, Norway

The crystal structure of mosandrite, ideally (Ca3REE)[(H2O)(2)Ca-0.5 square(0.5)]Ti(Si2O7)(2)(OH)(2)(H2O)(2), from the Saga mine, Morje, Porsgrunn, Norway, has been refined as two components related by the TWIN matrix ((1) over bar 0 0, 0 (1) over bar 0, 1 0 1): a 7.4222(3), b 5.6178(2), c 18.7232(7) angstrom, beta 101.4226(6)degrees, V = 765.23(9) angstrom(3), space group P2(1)/c, D-calc. = 3.361 g.cm(-3), R-1 = 3.69% using 1347 observed (F-o > 4 sigma F) reflections. The empirical formula of mosandrite (EMPA) was calculated on the basis of 4 Si a.p.f.u., with H2O determined from structure refinement: [(Ca2.89Ba0.01)(Sigma 2.90) (Ce0.39La0.18Nd0.14Sm0.02Gd0.03 Y0.16Th0.03)(Sigma 1.01)Zr-0.09](Sigma 4)[(H2O)(2.00)Ca0.32Na0.17Al0.10Mn0.04Fe0.022+square(0.35)](Sigma 3)(Ti0.87Nb0.09Zr0.04)(Sigma 1)(Si2O7)(2)[(OH)(1.54)F-0.46](Sigma 2)[(H2O)(1.50)F-0.50](Sigma 2), Z = 2. The crystal structure of mosandrite is a framework of TS (titanium silicate) blocks; each TS block consists of HOH sheets (H-heteropolyhedral, O-octahedral). In the TS block, there are five fully occupied cation sites, two [4]-coordinated Si sites with < Si-O > 1.623 angstrom, [7]-coordinated M-H and A(P) sites occupied by Ca and REE in the ratio similar to 3:1, and one [6]-coordinated Ti-dominant M-O(1) site. There are two H2O-dominant H2O-alkali-cation sites. The partly occupied M-O(2) site has composition [(H2O)(0.5)square Na-0.33(0.17)], ideally [(H2O)(0.5)square(0.5)] p.f.u. The M-O(3) site has ideal composition [(H2O)(1.5)Ca-0.5] p.f.u. In the O sheet, the X-M(O) and X-A(O) anion sites have compositions [(OH)(1.54)F-0.46] (X-M(O)) and [(H2O)(1.50)F-0.50] (X-A(O)), ideally (OH)(2) and (H2O)(2) p.f.u. The M-H and A(P) polyhedra and Si2O7 groups constitute the H sheet that is completely ordered. In the O sheet, M-O(1) octahedra are long-range ordered whereas H2O and OH groups and alkali cations Na and Ca are long-range disordered. Two SRO (short-range ordered) arrangements have been proposed for the O sheet: (1) Na [M-O(2)], Ca-2 [M-O(3)] and F-4 [X-M(O) and X-A(O) anion sites]; (2) 2 H2O [M-O(2)] and M-O(3)] and (OH)(2) and (H2O)(2) [X-M(O) and X-A(O)]. Linkage of H and O sheets occurs mainly via common vertices of M-H polyhedra and Si2O7 groups and M-O(1) octahedra. Two adjacent TS blocks are related by the glide plane c(y). Mosandrite is an H2O- and OH-bearing Na- and Ca-depleted analogue of rinkite, ideally (Ca3REE)Na(NaCa) Ti(Si2O7)(2)(OF)F-2. Mosandrite and rinkite are related by the following substitution at the M-O(2,3) and X-(M,A)(O) sites in the O sheet: (M)[(H2O)(2) + square(0.5)] + (X)[(OH)(2)(-) + (H2O)(2)] <-> M[Na-2(+) + Ca-0.5(2+)] + X[(OF)(3-) + (F-2)(2-)].