Kihlmanite-(Ce), Ce2TiO2[SiO4](HCO3)(2)(H2O), is a new rare-earth titanosilicate carbonate, closely related to tundrite-(Ce). It is triclinic, P (1) over bar, a = 4.994(2), b = 7.54(2), c = 15.48(4) angstrom, alpha = 103.5(4), beta = 90.7(2), gamma = 109.2(2)degrees, V = 533(1) angstrom(3), Z = 2 (from powder diffraction data) or a = 5.009(5), b = 7.533(5), c = 15.407(5) angstrom, alpha = 103.061(5), beta = 91.006(5), gamma = 109.285(5)degrees, V = 531.8(7) angstrom(3), Z = 2 (from single-crystal X-ray diffraction data). The mineral was found in the arfvedsonite-aegirine-microcline vein in fenitized metavolcanic rock at the foot of the Mt Kihlman (Chil'man), near the western contact of the Devonian Khibiny alkaline massif and the Proterozoic Imandra-Varzuga greenstone belt. It forms brown spherulites (up to 2 cm diameter) and sheaf-like aggregates of prismatic crystals, flattened on {010} and up to 0.5 mm diameter. Both spherulites and aggregates occur in interstices in arfvedsonite and microcline, in intimate association with golden-green tundrite-(Ce). Kihlmanite-(Ce) is brown, with a vitreous lustre and a pale yellowish-brown streak. The cleavage is perfect on {010}, parting is perpendicular to c and the fracture is stepped. Mohs hardness is similar to 3. In transmitted light, the mineral is yellowish brown; pleochroism and dispersion were not observed. Kihlmanite-(Ce) is biaxial (+), alpha = 1.708(5), beta = 1.76(1), gamma = 1.82(1) (589 nm), 2V(calc) = 89 degrees. The optical orientation is Y <^> c = 5 degrees, other details are unclear. The calculated and measured densities are 3.694 and 3.66(2) g cm(-3), respectively. The mean chemical composition, determined by electron microprobe, is: Na2O 0.13, Al2O3 0.24, SiO2 9.91, CaO 1.50, TiO2 11.04, MnO 0.26, Fe2O3 0.05, Nb2O5 2.79, La2O3 12.95, Ce2O3 27.33, Pr2O3 2.45, Nd2O3 8.12, Sm2O3 1.67, Gd2O3 0.49 wt.%, with CO2 15.0 and H2O 6.0 wt.% (determined by wet chemical and Penfield methods, respectively), giving a total of 99.93 wt.%. The empirical formula calculated on the basis of Si + Al = 1 atom per formula unit is (Ca0.16Na0.11Mn0.02)(Sigma 0.29) [(Ce0.98La0.47Pr0.09Nd0.29Sm0.06Gd0.02)Sigma(1.91) (Ti0.82Nb0.12)(Sigma 0.94)O-2 (Si0.97Al0.03)Sigma O-1(4.02)(HCO3)(2.01)](H2O)(0.96). The simplified formula is Ce2TiO2(SiO4)(HCO3)(2)center dot H2O. The mineral reacts slowly in cold 10% HCl with weak effervescence and fragmentation into separate plates. The strongest X-ray powder-diffraction lines [listed as d in angstrom (I) (hkl)] are as follows: 15.11(100)(00 (1) over bar), 7.508(20)(00 (2) over bar), 6.912(12)(0 (1) over bar1), 4.993(14)(00 (3) over bar), 3.563(15)(0 (2) over bar1), 2.896(15)(1 (2) over bar(2) over bar). The crystal structure of kihlmanite-(Ce) was refined to R-1 = 0.069 on the basis of 2441 unique observed reflections (MoK alpha, 293 K). It is closely related to the crystal structure of tundrite-(Ce) and is based upon [Ce2TiO2(SiO4)(HCO3)(2)] layers parallel to (001). Kihlmanite-(Ce) can be considered as a cation-deficient analogue of tundrite-(Ce). The mineral is named in honour of Alfred Oswald Kihlman (1858-1938), a remarkable Finnish geographer and botanist who participated in the Wilhelm Ramsay expeditions to the Khibiny Mountains in 1891-1892. The mineral name also reflects its occurrence at the Kihlman (Chil'man) Mountain.
