• journal_title：Mineralogical Magazine
• Contributor：E. Sokolova ; F. C. Hawthorne
• Publisher：Mineralogical Society of Great Britain and Ireland
• Date：2013-08-01
• Format：text/html
• Language：en
• Identifier：10.1180/minmag.2013.077.6.05
• journal_abbrev：Mineralogical Magazine
• issn：0026-461X
• volume：77
• issue：6
• firstpage：2753
• section：CNMNC Newsletter 16

The crystal structure of mosandrite, ideally (Ca₃REE)[(H₂O)₂Ca_0.5□_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 (1İ 0 0, 0 1İ 0, 1 0 1): a 7.4222(3), b 5.6178(2), c 18.7232(7) Å_, β 101.4226(6)°, V = 765.23(9) ų, space group P2₁/c, D_calc. = 3.361 g.cm⁻³, R₁ = 3.69% using 1347 observed (F_o > 4σF) 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: [(Ca_2.89Ba_0.01)_Σ2.90 (Ce_0.39La_0.18Nd_0.14Sm_0.02Gd_0.03 Y_0.16Th_0.03)_Σ1.01Zr_0.09]_Σ4[(H₂O)_2.00Ca_0.32Na_0.17Al_0.10Mn_0.04 □_0.35]_Σ3(Ti_0.87Nb_0.09Zr_0.04)_Σ1 (Si₂O₇)₂[(OH)_1.54F_0.46]_Σ2[(H₂O)_1.50F_0.50]_Σ2, 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 Å_, [7]-coordinated M^H and A^P sites occupied by Ca and REE in the ratio ~ 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□_0.33Na_0.17], ideally [(H₂O)_0.5□_0.5] p.f.u. The M^O(3) site has ideal composition [(H₂O)_1.5Ca_0.5] p.f.u. In the O sheet, the and anion sites have compositions [(OH)_1.54F_0.46] ( ) and [(H₂O)_1.50F_0.50] ( ), 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 [M^O(3)] and F₄ [ and anion sites]; (2) 2 H₂O [M^O(2)] and M^O(3)] and (OH)₂ and (H₂O)₂ [ and ]. 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)F₂. Mosandrite and rinkite are related by the following substitution at the M^O(2,3) and sites in the O sheet: ^M[(H₂O) + □_0.5] + ^X[ + (H₂O)₂] → ^M[ + ] + ^X[(OF)³⁻ + (F₂)²⁻].