THE MN ISOTYPE OF ANDORITE AND UCHUCCHACUAITE

Phase relations in the system Ag2S-MnS-Sb2S3 Were studied between 300- degrees and 500-degrees-C using evacuated and sealed glass capsules. T wo ternary phases, Ag2Mn2Sb6S12 (Ag2S.2MnS.3Sb2S3) and Ag1.41Mn2.82Sb5 .65S12 (Ag2S.4MnS.4Sb2S3) are stable in the system, and both form equi librium assemblages with stibnite and alabandite. The phase Ag2Mn2Sb6S 12 is a Mn isotype of andorite, with a 12.79(1), b 19.58(1) and c 4.00 (1) angstrom. The limits of Pb-for-Mn substitution along the join ando rite and its Mn isotype, Ag2(Pb1-xMnx)2Sb6S12, are 0.40 greater-than-o r-equal-to x greater-than-or-equal-to 0.95. Uchucchacuaite is readily synthesized from its reported composition AgMnPb3Sb5S12. It has an and orite-type structure and calculated unit-cell dimensions a 13.08(1), b 19.46(1), and c 4.27(1) angstrom. Uchucchacuaite forms a complete sol id-solution series with andorite, whereas the solid-solution is incomp lete between uchucchacuaite and the Mn isotype of andorite.