Chukanovite, Fe2(CO3)(OH)2, a new mineral from the weathered iron meteorite Dronino

The new mineral chukanovite, Fe-2(CO3)(OH)(2), occurs in cavities of weathered fragments of the Dronino ataxite iron meteorite found near the Dronino village, Kasimov district, Ryazan' Oblast, Russia. It is a product of terrestrial alteration of meteorite iron. Associated minerals are goethite, akaganeite, hematite, hibbingite, reevesite, honessite, etc. Chukanovite forms acicular to fibrous individuals (up to 0.5 mm long and up to 2 - 3 mu m thick) combined in spherulites up to I mm in diameter, botryoidal spherulitic clusters and parallel- or radial-columnar aggregates which form crusts up to I mm thick. Unaltered chukanovite is transparent, pale-green or colourless. The surface of aggregates is brownish-green. Streak is white. Lustre is vitreous. Cleavage is perfect, probably on {0 - 21}, fracture is uneven. The mineral is brittle, the Mobs' hardness is 3.5 - 4, the calculated density is 3.60 g/cm(3). It is optically biaxial (-) with alpha 1.673(3), beta 1.770(5), gamma 1.780(5), 2V(meas.) 10(5)degrees. Average chemical composition (wt.%; electron probe, H2O by modified Penfield method, CO2 by selective sorption) is: MgO 0.1, FeO 68.8, NiO 0.6, CO2 19.8, H2O 10.9, total 100.2. The empirical formula calculated on the basis of two metal atoms is (Fe1.972+Ni0.02Mg0.01)(Sigma 2.00)(CO3)(0.93)(OH)(2.14)center dot 0.18H(2)O, ideally Fe-2(CO3)(OH)(2). Chukanovite is monoclinic P2(1)/a, with a= 12.396(1) angstrom, b=9.407(1) angstrom, c=3.2152(3) angstrom,beta=97.78 degrees. The strongest lines of the X-ray powder pattern [d(angstrom), I, (hkl)] are: 6.14, 40, (200); 5.15, 60, (231); 3.73, 80, (310); 2.645, 100, (230); 2.361, 40, (510); 2.171, 40, (520). The structure of chukanovite was refined on synchrotron data by the Rietveld method up to Rp=3.43%, wRp=4.51%, R-Bragg=2.48%. Chukanovite is closely related to the minerals of the malachite-rosasite group. It was named in honour of Nikita V. Chukanov (b. 1953), Russian physicist and mineralogist. The holotype specimen is deposited in the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow.