I don't know well this group. Sample is from Imiter. This fishtail twinning is a classic for monoclinic minerals and is possible that it is xanthoconite, but the crystals are striped as is well know for proustite. I need help. Grazie
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Proustite xx geminati, Imiter Marocco, ex Daniele Alberti.jpg (55.7 KB)
There are two twin planes common for Proustite: (10-14) and (10-11). The axial lengths are a=10.79 and c=8.69 So, since the angle between the twins is rather acute I would guess that the twin plane is (10-11).