KARASUGITE, SRCAAL[F,(OH)]7, A NEW MINERAL SPECIES FROM THE KARASUG FE REE BARITE FLUORITE DEPOSIT, TANNU-OLA RANGE, SOUTH SIBERIA, RUSSIA

Karasugite is a new mineral species from the Karasug Fe-REE-barite-flu orite deposit, Tannu-Ola Range, South Siberia, Russia. It occurs as ro settes and fan-shaped aggregates of 0.05 to 0.25x0.03 to 0.10x0.001 to 0.003mm crystals. The crystals are bladed on {100} and elongated afte r [001], extensively twinned with (100) as both twin and composition p lane; the crystals show perfect cleavage or parting after (100). Karas ugite is colourless and transparent, the lustre is vitreous; the measu red density is between 3.06 and 3.11 g/cm3. It occurs in a supergene a ssociation of ''limonite'' - hematite, fluorite, quartz, barytocelesti ne, celestine and gearksutite. Karasugite is optically biaxial positiv e; the indices of refraction are: n(alpha) = 1.4240+/-0.0005, n(beta) = 1.4320+/-0.0005, n(gamma) = 1.4415+/-0.0005, 2V(alpha) (meas.) = 94. 5+/-2-degrees, 2V(alpha) (calc.) = 94-degrees. The optical orientation is: beta = b, alphaLAMBDAc = 7-degrees; if alpha is in the obtuse ang le beta then gamma almost-equal-to a if in the acute angle beta then g ammaLAMBDAa = 13-degrees. The unit cell parameters are: a = 8.215+/-0. 005, b = 11.989+/-0.003, c = 6.076+/-0.003 angstrom and beta = 96.22+/ -0.01-degrees, space group P2(1)/c, Z = 2. The microprobe analyses gav e, with standard deviations in brackets: Na = 0.4 (.2), Al = 9.1 (.4), K = 0.3 (.1), Ca = 13.1 (.3), Sr = 29.1 (1.4), F = 40.2 (1.5), O = 7. 1 (.8), SIGMA = 99.3 (1.7). The empirical formula was calculated, assu ming 3.000 metal atoms and 7 anions in the formula, to be Sr1.05 Ca0.9 2 K0.02 Na0.05 Al0.96 [F5.8(OH)1.2]; the ideal formula of karasugite i s SrCaAl[F,(OH)]7, with F/OH - 5.8/1.2 or within the limits of accurac y SrCaAl[F6(OH)]. The density calculated from the empirical formula is 3.206 g/cm3, the compatibility index is 0.058, i.e. good. Karasugite has not been found to be closely related to any other mineral. The typ e material is preserved in the collections of the Geological Museum, C openhagen, Denmark and the Fersman Mineralogical Museum, Moscow, Russi a.