Fluorocronite, the natural analogue of β-PbF2, from the Sakha Republic, Russian Federation

Abstract

Fluorocronite (Фторокронит), ideally PbF 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 4 O(OH,F) 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 μm 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 0.98 F 2.02 . The simplified formula is PbF 2 . Fluorocronite is cubic, space group Fm 3 m , with a = 5.9306(5) A, V = 208.59(5) A 3 and Z = 4. Fluorocronite is isostructural with fluorite. The five strongest lines in the powder X-ray diffraction pattern are [ d obs in A ( 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 (κρόνος, the alchemical name for lead).