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Vanadiocarpholite, Mn²⁺V³⁺Al(Si₂O₆)(OH)₄, a new mineral from the Molinello mine, northern Apennines, Italy

¹ Dipartimento per lo Studio del Territorio e delle sue Risorse, Università di Genova, Corso Europa 26, I-16132 Genova, Italy
² LAMIA-INFM, Corso Perrone 24, I-16152 Genova, Italy
³ Dipartimento di Chimica e Chimica Industriale, Università di Genova, Via Dodecaneso 31, I-16146 Genova, Italy

^* E-mail: mineral{at}dipteris.unige.it

Vanadiocarpholite, Mn²⁺V³⁺Al(Si₂O₆)(OH)₄, occurs at the Molinello mine (Liguria, Italy) in mm-thick veins and in open fissures in a silicified wood sample from Mn-ore bearing cherts. Vanadiocarpholite is found as millimetric aggregates of acicular crystals associated with coatings and crystals of dark-green volborthite and quartz; rarely strongly elongated [001] prismatic crystals up to 400 µm are also found. The crystals vary in colour from honey yellow-brown and brown (prismatic crystals) to pale straw-yellow (acicular crystal aggregates); they are brittle (prismatic crystals) to flexible (acicular crystals), transparent and non-fluorescent, with vitreous to silky lustre (prismatic crystals and acicular crystal aggregates, respectively) and nearly white streak; they show a perfect {010} cleavage; parting and twinning were not observed. The empirical formula of vanadiocarpholite, derived from microprobe analyses and structural refinement, approaches the ideal formula, Mn²⁺V³⁺Al(Si₂O₆)(OH)₄; however, a wide compositional range is detected, mainly due to a solid solution with carpholite (V³⁺ vs Al substitution). X-ray single crystal data give the refined cell parameters a = 13.830(2) Å, b = 20.681(3) Å, c = 5.188(1) Å and V = 1483.86 ų in the space group Ccca. Micrometric crystals of vanadiocarpholite were also investigated by transmission and analytical electron microscopy. TEM analyses show a good agreement with WDS and XRD data, but disordered layer stacking sequences are observed. The crystal structure refinement indicates vanadiocarpholite to be isotypic with carpholite, therefore it belongs to the carpholite group together with carpholite, magnesiocarpholite, ferrocarpholite, balipholite and potassic-carpholite.

Key-words: vanadiocarpholite, new mineral, carpholite group, solid-solution series, physical and chemical data, X-ray powder pattern, crystal structure, TEM analysis.