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Moelo, 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.
Added by: Richard Baschera (2023-06-16 10:22:43) Last edited by: Richard Baschera (2023-06-16 10:31:20) |
| Type de référence: Article DOI: 10.5194/ejm-35-333-2023 Numéro d'identification (ISBN etc.): 0935-1221 Clé BibTeX: Moelo2023a Voir tous les détails bibliographiques  |
Catégories: IMN Créateurs: Moelo Collection: European Journal of Mineralogy |
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| Résumé |
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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 3?2/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 m3?: reflection about the {100} plane or two-fold rotation about the 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.
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