Abstract

Fluorocronite (Фторокронит), ideally PbF2, 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 Sn4O(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 Pb0.98F2.02. The simplified formula is PbF2. Fluorocronite is cubic, space group Fmm, with a = 5.9306(5) Å, V = 208.59(5) Å3 and Z = 4. Fluorocronite is isostructural with fluorite. The five strongest lines in the powder X-ray diffraction pattern are [dobs in Å (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).

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