Abstract
A crystal-chemical study of historical specimens as well as new ones belonging to the jordanite–geocronite series from the Pollone baryte + pyrite ± (Pb-Zn-Ag) ore deposit (Valdicastello Carducci, Apuan Alps, Tuscany, Italy) has been performed. These crystals were collected in quartz extension veins embedded in three different occurrences: (i) baryte + pyrite orebodies; (ii) schist layers interbedded between baryte + pyrite orebodies; and (iii) schists at the contact with pyrite-poor baryte orebodies. Electron-microprobe data indicated the occurrence of three distinct groups of compositions within the sample suite. These correspond to As-bearing geocronite, Sb-rich jordanite, and Sb-bearing jordanite, with mean compositions Pb14Sb3.8As2.2S23, Pb14Sb2.9As3.1S23, and Pb14Sb2.6As3.4S23, respectively. Crystals representative of these different compositions have been investigated through single-crystal X-Ray diffraction studies and their crystal structures have been solved to R1 = 0.078, 0.069, and 0.033, respectively. The unit-cell volume decreases passing through As-bearing geocronite (V = 2149.5(3) Å3) to Sb-bearing jordanite (V = 2132.3(3) Å3). The As-to-Sb substitution takes place preferentially at the Sb4 site; through the increasing of the Sb content, Sb can substitute As also at the As6 site. According to the structural study of the ore deposit, formation of jordanite–geocronite is subordinated to a late Alpine deformative D2 stage, which permitted in situ remobilization of preexisting sulfide ore in small quartz extension veins. Such a local recrystallization would explain the variability of the As/(As + Sb) ratio of the members of the jordanite series, reflecting the heterogeneity of the orebody.
Highlights
IntroductionThe two isotypic minerals jordanite, Pb14 (As,Sb) S23 , and geocronite, Pb14 (Sb,As) S23 , are the phases having the most PbS-rich composition in the ternary system PbS–As2 S3 –Sb2 S3 [1], comprising
The two isotypic minerals jordanite, Pb14 (As,Sb)6 S23, and geocronite, Pb14 (Sb,As)6 S23, are the phases having the most PbS-rich composition in the ternary system PbS–As2 S3 –Sb2 S3 [1], comprising22 approved mineral species (Table 1).Minerals 2016, 6, 15; doi:10.3390/min6010015 www.mdpi.com/journal/mineralsName
The refinements reported in this study suggest that the increasing Sb content from jordanite to geocronite is accompanied by the complete filling of Sb4 by antimony and the progressive substitution of As by Sb at the As6 site
Summary
The two isotypic minerals jordanite, Pb14 (As,Sb) S23 , and geocronite, Pb14 (Sb,As) S23 , are the phases having the most PbS-rich composition in the ternary system PbS–As2 S3 –Sb2 S3 [1], comprising. Members of the jordanite–geocronite series have been collected in sub-vertical quartzbarytesulfides veins, trending N140-170 and embedded in the country rocks as well as, more rarely, in the orebodies (Figure 1) These veins clearly postdate the formation of the main orebodies, crosscutting at high angle the metamorphic foliations (S2 and S1 ) and ore layering. Type-2 represents the most common occurrence for jordanite–geocronite specimens at the concentrated in the overstepping domain between the two main SW–NE faults (Pizzone-Pozzo areType-2 hosted in schist layersthe (1–5 m thick). Veins are predominantly coarse-grained multiple baryte lenses, rarely propagate into thefilled ore by bodies They are mainly massive quartz but large cavities lined by beautiful crystals of smoky quartz and baryte are quite concentrated in the overstepping domain between the two main SW–NE faults
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