Tancaite-(Ce), a new molybdate from Italy

Tancaite-(Ce), ideally FeCe(MoO4)3•3H2O, is a new mineral occurring within cavities in the quartz veins which cut the granite at Su Senargiu, Sarroch (CA), Sardinia, Italy. It is a secondary mineral formed in the oxidation zone of a sulphide ore vein. Associated minerals are quartz, muscovite, molybdenite, pyrite and mendozavilite. Tancaite-(Ce) is red or pale brown in colour, with a vitreous to adamantine lustre. Electron microprobe analyses give (wt.): SiO2 0.34, CaO 0.09, Fe2O3 11.29, SrO 0.02, La2O3 5.04, Ce2O3 10.35, Pr2O3 1.07, Nd2O3 3.66, Sm2O3 0.19, ThO2 2.58, UO2 0.17, MoO3 58.62, H2O* 7.43, sum 100.85. It gives the formula: Fe3+1.03(Ce0.46La0.23Nd0.16Pr0.05Sm0.01U0.01Th0.07)Σ0.99(Mo2.96 Si0.04)Σ3.00O12•3H2.00O, which can be simplified as Fe3+(REE)(MoO4)3•3H2O. The calculated density is 3.834 g/cm3. The presence of H2O was confirmed by micro-Raman spectrometry (stretching and bending vibrations of O-H). The X-ray diffraction pattern of tancaite is characterised by a set of strong reflections, which point to a cubic unit cell, with a 6.80 Å and space group Pm(-3)m, plus a set of weaker reflections. By measuring only the strong reflections with a 4-circle diffractometer, it was possible to solve and refine the “average structure” of tancaite (R 0.032 for 149 unique reflections). The crystal structure consists of FeO6 octahedra centred at the origin of the cubic cell and linked together through MoO4 tetrahedra by corner sharing. The molybdate tetrahedra are statistically distributed in four symmetry-related positions, with ¼ occupancy. In the centre of the unit cell the REE cations exhibit 6+3 coordination, bonding six oxygen atoms and three H2O molecules. The refined structure is clearly an average structure, characterised by statistical disorder which involves the Mo cations, the H2O molecules and the ligands of the REE cations at the centre of the unit cell. Two intensity data collections were performed in order to measure the weaker reflections and to solve the real crystal structure of tancaite. The former was performed at the synchrotron facility Elettra, at the XRD1 beamline; the latter was carried out by means of a CCD-equipped Oxford diffractometer, operating with MoKα radiation. Both sets of data indicated a larger super-cell with respect to the previous cubic cell with a 6.80 Å. One of the possible super-cells showed rhombohedral symmetry, with a 19.2, c 47.4 Å in the hexagonal setting. The new mineral has been approved by the IMA CNMNC (# 2009-097). The name is after Giuseppe Tanca, mineral collector