Henrymeyerite, a new hollandite-type Ba-Fe titanate from the Kovdor complex, Russia

A new hollandite-type titanate, henrymeyerite, occurs in a vein of tetra-fe rriphlogopite - calcite - dolomite carbonatite of the Kovdor alkaline ultra mafic complex, Kola Peninsula, in Russia. The mineral was found in a single mineralized vug as acicular crystals less than 0.2 mm in length. The cryst als represent a combination of two tetragonal prisms and a bipyramid. Henry meyerite is opaque, black, and has an adamantine luster. In reflected light , the mineral is greyish brown and has a strong bireflectance. Reflectance values R-epsilon and R-omega are given for lambda over the interval 420-720 nm. Henrymeyerite is associated with dolomite, fluorapatite, tetra-ferriph logopite, rimkorolgite, catapleiite, collinsite, and pyrite. The compositio n of henrymeyerite, as determined by electron microprobe, is (wt.%): BaO 18 .25, TiO2 67.78, FeO 9.20, Nb2O5 1.00, Ce2O3 0.56, La2O3 0.50, Na2O 0.40, S iO2 0.37, K2O 0.05, CaO 0.02, sum 98.13. Stoichiometrically, this compositi on closely corresponds to the Ba-Fe end-member of the cryptomelane group, B aFe2+Ti7O16. Henrymeyerite is tetragonal, space group I4/m, Z = 1, a 10.219 (3), c 2.963(1) Angstrom, V 309.4(3) Angstrom(3). The crystal structure of the mineral was refined to R-1 = 0.027 for 173 unique reflections with \F-o \ greater than or equal to 4 sigma(F) using single-crystal X-ray-diffractio n data. The structure of henrymeyerite corresponds to the undistorted holla ndite archetype; no indication of splitting of the Ba site along [001] was observed. The available single-crystal data also indicate the absence of lo ng-range order of the Ba2+ cations within the structural tunnels. The miner al is named for Prof. Henry O.A. Meyer (1937-1995) in honor of his contribu tions to the petrology and mineralogy of mantle-derived xenoliths and kimbe rlitic rocks.