Compositional variation of turquoise group minerals from 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, back-scattered 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 come from Sinai (Egypt), Santa Fè S. Miguel (New Mexico), Saxony (Germany), Montebras Creuse (France) and Nishapur (Khorassan, Iran) and display different mineralogical compositions and various mineral arrangements. The analyses have 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 aims to show the high mineralogical and geochemical variability of the samples under investigation, with particular attention to the mineralogical association, the textural analyses and the trace-element concentrations.