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Gramaccioliite-(Y), a new mineral of the crichtonite group from Stura Valley, Piedmont, Italy

¹ Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 1–56126 Pisa, Italy
² Istituto di Geoscienze e Georisorse, CNR, Via La Pira 4, 1–50121 Firenze, Italy
³ Dipartimento di Chimica Strutturale e Stereochimica Inorganica, Università di Milano, Via G. Venezian 21, 1–20133 Milano, Italy
⁴ Laboratoire de Chimie des Solides, Institut des Matériaux J. Rouxel, UMR CNRS 6502, 2 rue de la Houssinière, F-44322 Nantes Cedex 3, France
^** Present address: Laboratorium fur Kristallographie, Universitat Bayreuth, Universitätsstrasse 30, D-95440 Bayreuth, Germany

^* Corresponding author, e-mail: orlandi{at}dst.unipi.it

Gramaccioliite-(Y) is a new mineral within the crichtonite group. It was found in the gneiss of the Hercynian Massif of Argentera, at Sambuco (Stura Valley, Piedmont, Italy). It occurs as black lamellar mm-sized crystals. Associated minerals are quartz, albite, muscovite, anatase, brookite, rutile, fluorapatite, xenotime, pyrite, a mineral of the synchysite series, dessauite-(Y) and senaite. Optically, gramaccioliite-(Y) is opaque and shows very weak anisotropy and low bireflectance; pleochroism is not observed. Chemical analysis carried out by means of EPM led to the following simplified formula: (Pb, Sr)(Y, Mn)(Ti, Fe)₁₈Fe₂O₃₈. Gramaccioliite is trigonal, space group R3, = 9.186(4) Å, = 68.82(4)°. The strongest five reflections in the X-ray powder diffraction pattern [d in Å (I) (hkl)] are: (3.002) (100) (300), (1.606) (95) (1.3.10), (2.892) (70) (116), (2.258) (70) (134), (1.809) (60) (318) (indices are referred to the hexagonal setting). The crystal structure of gramaccioliite-(Y) has been refined (R = 0.086) from single crystal X-ray diffraction data. The structure corresponds to that of the other members of the crichtonite group. During the study of gramaccioliite-(Y), a mineral described in literature as a Re-rich member of the crichtonite group from an unknown locality has been re-examined. New EPM data indicated the complete lack of rhenium in this mineral and allowed to correctly identify it as a second finding of gramaccioliite-(Y).

Key-words: gramaccioliite-(Y), new mineral, crichtonite group, Stura Valley, crystal structure, chemical analysis.

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