This is a great tennantite – it is a piece that detached from the surrounding rock during the late stages of formation and has recrystallized on the back/underside, and this explains the specimen's overall shape and aspect. The large tennantite crystal faces which are featured across the left and up to the middle exhibit super traingular growth patterns. The tennantite crystals are sharp and mirror-bright. In excellent condition – minor chipping – not distracting. This is a great specimen.
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About these Tennantites
Mundo Nuevo has been best known for its hubnerite, quartz, fluorite and augelite. In recent years there have been occasional excellent finds of tennantite.
These tennantites, mined during 2014-15 specimen recovery work, are part of a find that has now been analyzed quite extensively. Of those (over 40 specimens tested) all were tennantite except one that was a borderline tennantite-tetrahedrite. Accordingly, specimens from this find are now labelled tennantite. I have not had each of these analyzed, given the pervasive tennantite results (and not wanting to add to their prices by incurring unnecessary cost). These tennantites were originally posted on this website labelled "tetrahedrite", as they were sold to me under that label – they were then removed from the site until the analysis work on the find had been completed.
They are excellent sharp crystals. After the crystals had formed, a late-stage fragmentation and recrystallization event seems to have occurred – the specimens have uneven naturally-fractured surfaces on the back/underside, and these surfaces are recrystallized.