Abstract
Lucasite-(Ce), Ce0.51La0.27Nd0.11Pr0.11Ti2(O,OH)6, has been found in a heavy-mineral concentrate derived from an olivine lamproite tuff at Argyle, in the east Kimberley region of Western Australia. It occurs as brown subhedral grains, 0.5 to 1 mm in diameter, and is associated with calcite, talc, titanite, dolomite, amphibole, and manganoan ilmenite. The mineral is translucent with a resinous luster and has a white streak, prominent {001} cleavage, and a conchoidal fracture. Dcalc is 5.00 g/cm3. It is optically biaxial with extremely high birefringence. Reflectivity values are 16.6% at 470 nm, 15.6% at 546 nm, 15.3% at 589 nm, and 15.0% at 650 nm. VHN10 is 761-900. The symmetry is monoclinic, space group I2/a, with a = 5.178(1), b = 8.756(4), c = 9.768(5) Å, β = 93.52(4)°, Z = 4. The strongest eight lines in the X-ray powder pattern are 3.376 (10) (112), 3.257 (6) (022), 3.203 (8) (112, 121), 2.584 (7) (200), 2.541 (4) (130), 2.225 (5) (202, 220), 2.029 (4) (213), 1.8306 (6) (204).
The structure has been determined and refined to R = 0.054. It is related to the PbSb2O6-type structure, with ordering of Ce and Ti atoms in alternate {001} cation layers in a hexagonal closest-packed anion arrangement. The Ti-centered octahedra share edges to form gibbsite-like fused hexagonal rings. The Ce atoms are sandwiched between pairs of hexagonal rings and are displaced by 0.5 Å along [010] from the centers of octahedral sites to adopt eightfold coordination.
