STIBIOCLAUDETITE, A FIRST BRITISH OCCURRENCE, FROM WET SWINE GILL, CALDBECK FELLS, CUMBRIA

摘要

The recent description of stibioclaudetite (AsSbO_3), a mineral which is differentiated from claudetite (As_2O_3) bya site-speciic substitution of Sb for As, prompted a re-examination of 'antimony-rich claudetite' from Wet SwineGill in the Caldbeck Fells, Cumbria. The acceptance of stibioclaudetite as a new species places a constraint on thecomposition field of claudetite that was not present when the 'antimony-rich claudetite' was originally reported. Theboundary between the two minerals is at As_(1.5)Sb_(0.5)O_3: compositions that are more Sb-rich are stibioclaudetite, thosethat are less Sb-rich are claudetite. The empirical formula determined in 1998 for 'antimony-rich claudetite' fromWet Swine Gill was As_(1.2)Sb_(0.8)O_3. Nine further analyses by wavelength-dispersive X-ray spectrometry, reported herein,produced As_(1.05)Sb_(0.95)O_3. Both are close to end-member stibioclaudetite. Qualitative analyses by energy-dispersive X-rayspectrometry indicated the presence of substantial amounts of Sb in every specimen that was examined. No Sb-poorcompositions were found. These data suggest that claudetite should be removed from the species list from Wet SwineGill and the first British occurrence of stibioclaudetite added. Stibioclaudetite occurs in quartz veinstone which isrich in primary antimony minerals. It is restricted to cavities and fractures near the base of the oxidation zone whereit occurs as colourless, transparent, monoclinic crystals with a resinous lustre and perfect cleavage. It overgrowsantimony-bearing sulphides and sulphosalts, is overgrown by senarmontite and sulphur, and rarely contains bright red metastibnite inclusions. It is suggested that stibioclaudetite forms under low-temperature supergene conditions at a relatively low oxidation potential from solutions that are rich in As and Sb in their intermediateAs(III) and Sb(III) oxidation states.