Pseudo-cubic trigonal pyrite from the Madan Pb–Zn ore field (Rhodope Massif, Bulgaria): morphology and twinning

[Marco E. Ciriotti]

Referenza:
▪ Moëlo, Y. (2023): Pseudo-cubic trigonal pyrite from the Madan Pb–Zn ore field (Rhodope Massif, Bulgaria): morphology and twinning. European Journal of Mineralogy, 35, 333–346.

Abstract:
A new occurrence of pyrite crystals with rhombohedral habit, up to several centimeters in length, is described from the Madan Pb–Zn ore field (Rhodope Massif, south Bulgaria), where it constitutes a late pyrite generation. As observed in the past in other deposits, the ideal rhombohedron is derived from the pyritohedron by suppression of half of its faces (six “polar faces”) around a ternary axis. In studied crystals, together with six main “equatorial faces”, additional minor faces correspond to cube faces as well as polar faces. Such a dissymmetry indicates that the crystallographic point group of these crystals is -3, a subgroup of the eigensymmetry -32/m of a rhombohedron taken as geometric face form. Twinning by metric merohedry confirms such a symmetry decrease and permits the definition of this type of pyrite as a dimorph of cubic pyrite, i.e., pseudo-cubic trigonal pyrite (pyrite-R). Twin operations belong to the set of symmetry operations absent in point group -3 relative to pyrite symmetry m-3: reflection about the {100} plane or two-fold rotation about the <100> direction. Four twin types have been distinguished (name, chromatic point group): three contact twins (reflection, m′; rotation, 2′; trapezoidal, (m(2)m(2)2(2))(4)), as well as one penetration twin (crossed, 2'/'m). Composition planes always correspond to {100}, but there are two types of twin interfaces. More complex twinned samples may develop erratically during crystal growth. Other twin variations as well as genetic aspects of such a type of pyrite are discussed.

NdR: Trattandosi di un dimorfo potrebbe anche essere proposto per una nuova specie?
A questa domanda risponderei: No.
Secondo me si tratta semplicemente di una variante strutturale distorta.

[Marco E. Ciriotti]

Nella discussione in Mindat Uwe sostiene che invece dovrebbe trattarsi di polimorfo.
Credo sia possibile, ma resto dubbioso e credo anche che tutto si possa accertare solo dopo adeguati studi diffrattometrici.