Compositional variation of turquoise-group minerals from the historical collection of the Real Museo Mineralogico of the University of Naples

Five turquoise samples, belonging to the XVII century historical collection of the Real Museo Mineralogico (University of Naples Federico II), were investigated by a multi-methodological approach based on powder X-ray diffraction, electron microprobe analysis in wavelength-dispersive spectroscopy mode, backscattered electron images from scanning electron microscopy in energy-dispersive spectroscopy mode, laser ablation inductively coupled plasma mass spectrometry, Fourier transform infrared and Raman spectroscopy. The samples originate from Sinai (Egypt), Santa Fe S. Miguel (New Mexico), Saxony (Germany), Montebras Creuse (France) and Nishapur (Khorassan, Iran) and display different mineralogical compositions and various mineral associations. The study has shown the presence of: (1) four minerals of the turquoise group: turquoise, faustite, chalcosiderite and planerite; (2) other phosphates from different groups: wavellite, crandallite, goyazite, gorceixite, variscite, metavariscite, fluorapatite; and (3) other minerals: voltaite, adularia and quartz. The present investigation is intended to show the mineralogical and geochemical variability of the samples with particular attention to the mineralogical parageneses, textural analyses and trace-element concentrations.