埼玉県秩父鉱山産セムセイ鉱の再研究 <Bull. Natn. Sci. Mus., Tokyo>Re-study of Semseyite from the Chichibu Mine, Saitama Prefecture, Japan

Semseyite from the contact metasomatic ore deposits of the Chichibu mine, Saitama Prefecture, Japan, previously identified after the X-ray powder and optical studies, is analysed by microprobe. The ranges and average (parenthesized) of 23 analyses are : Pb 51.29-53.09 (52.24), Sn 0.12-0.21 (0.17), Cd 0.03-0.14 (0.10), Ag 0.00-0.12 (0.07), Fe 0.00-0.05 (0.02), Zn 0.00-0.08 (0.01), Mn 0.00-0.04 (0.01), Cu 0.00-0.03 (0.01), Sb 27.39-28.37 (27.99), Bi 0.00-0.22 (0.09), S 19.77-20.09% (19.92%), and the total of the average is 100.58%. The calculated S content is 19.26%, giving the total 99.92%. The empirical formula calculated on the basis of the latter figure of S=21 is : (Pb_<8.81>Sn_<0.05>Cd_<0.03>Fe_<0.01>Ag_<0.01>Zn_<0.01>Mn_<0.01>Cu_<0.01>)_<Σ8.94> (Sb_<8.03>Bi_<0.01>)_<Σ8.04>S_<21> (S_<21.71> if uncorrected), where Sn is handled as divalent. Neither discrete Sn nor Bi mineral has been found not only in the associated minerals but also in the whole ore deposits. Semseyite coats galena grains, which is the unique direct associate, and the galena perches on coarse-grained pyrite forming hollow hemisphere aggregates with an iron-rich sphalerite. Minor associates are chalcopyrite, tetrahedrite, bournonite, arsenopyrite, quartz, and hedenbergite. No boulangerite is found therein despite its compositionally intermediatary nature between galena and semseyite and its occurrence in the other part of the same orebody. An interval of sulphide formation seems to have existed between galena and semseyite, which was preferentially precipitated on galena at the latest stage of sulphide formation, or under lower temperature conditions, where many other Pb-Sb sulphosalts have been synthesized. Some of them are structurally out of the fuloppite-semseyite homologous series despite their chemical compositions between the members thereof.