This study of argentopentlandite from El Charcon, in southeastern Spain, is the first documented report for Spain and also the first occurrence of argentopentlandite associated with hydrothermal systems related to acid-type volcanism. Ag values above 1.00 and M/S values lower than 1.125 can be explained by the presence of defects (mainly vacancies of Fe and Ni or S or both) in the argentopentlandite structure. These defects can also explain: (1) the differences observed between measured M/S and Fe/Ni values and M/S and Fe/Ni values calculated by the "cation addition-omission solid solution" model for natural argentopentlandite, and (2) the fact that most compositions of natural argentopentlandite fall outside the stability fields established for synthetic argentopentlandite at 300 and 400 degrees C. The following modification of the argentopentlandite formula is proposed: (Fe (sub 5+ or -0.77) Ni (sub 3+ or -0.75) ) (sub Sigma 8+ or -x) Ag (sub 1+ or -y) S (sub 8+ or -z) , with 0 <

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