Fluorocronite, the natural analogue of beta -PbF (sub 2) , from the Sakha Republic, Russian Federation

Fluorocronite, ideally PbF (sub 2) , is a new mineral (IMA2010-023), from the Kupol'noe deposit, Sarychev range, Sakha Republic, Russian Federation. It occurs intimately mixed with cassiterite and a potentially new Sn oxy-hydroxide with the composition Sn (sub 4) O(OH,F) (sub 6) , and is found in direct association with quartz, anglesite, cerussite, galena, hocartite, bindheimite and chlorargyrite. Fluorocronite forms flattened, leaf-like microcrystals up to about 20 mu m across. The main form observed is {100}, while {111} may also be present. The crystals are translucent and white with a pearly lustre. The streak is also white and Mohs hardness is between 3 and 4 (estimated). No parting or twinning was observed. Fluorocronite has prefect cleavage on {111} by analogy with other minerals with the fluorite structure type. Crystals are optically isotropic; however, the refractive index could not be measured due to the small size of the crystals. The empirical formula (based on 3 apfu) is Pb (sub 0.98) F (sub 2.02) . The simplified formula is PbF (sub 2) . Fluorocronite is cubic, space group Fm3m, with a=5.9306(5) Aa, V=208.59(5) Aa (super 3) and Z=4. Fluorocronite is isostructural with fluorite. The five strongest lines in the powder X-ray diffraction pattern are [d (sub obs) in Aa (I) (hkl)]: 3.437 (100) (111); 2.976 (46) (002); 2.103 (44) (022); 1.794 (42) (311); 1.717 (21) (222). The name is in relation to the composition; fluoro (for fluorine) and cron (kronos, the alchemical name for lead).