The crystal structure of mosandrite, ideally (Ca3REE)[(H2O)2Ca0.5□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) Å3, space group P21/c, Dcalc. = 3.361 g.cm−3, R1 = 3.69% using 1347 observed (Fo > 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: [(Ca2.89Ba0.01)Σ2.90 (Ce0.39La0.18Nd0.14Sm0.02Gd0.03 Y0.16Th0.03)Σ1.01Zr0.09]Σ4[(H2O)2.00Ca0.32Na0.17Al0.10Mn0.04□0.35]Σ3(Ti0.87Nb0.09Zr0.04)Σ1 (Si2O7)2[(OH)1.54F0.46]Σ2[(H2O)1.50F0.50]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 -coordinated Si sites with <Si–O> 1.623 Å, -coordinated MH and AP sites occupied by Ca and REE in the ratio ~ 3:1, and one -coordinated Ti-dominant MO(1) site. There are two H2O-dominant H2O-alkali-cation sites. The partly occupied MO(2) site has composition [(H2O)0.5□0.33Na0.17], ideally [(H2O)0.5□0.5] p.f.u. The MO(3) site has ideal composition [(H2O)1.5Ca0.5] p.f.u. In the O sheet, the and anion sites have compositions [(OH)1.54F0.46] () and [(H2O)1.50F0.50] (), ideally (OH)2 and (H2O)2 p.f.u. The MH and AP polyhedra and Si2O7 groups constitute the H sheet that is completely ordered. In the O sheet, MO(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 [MO(2)], Ca [MO(3)] and F4 [ and anion sites]; (2) 2 H2O [MO(2)] and MO(3)] and (OH)2 and (H2O)2 [ and ]. Linkage of H and O sheets occurs mainly via common vertices of MH polyhedra and Si2O7 groups and MO (1) octahedra. Two adjacent TS blocks are related by the glide plane cy. 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 MO(2,3) and sites in the O sheet: M[(H2O) + □0.5] + X[ + (H2O)2] → M[ + ] + X[(OF)3− + (F2)2−].