MULTI-METHODOLOGICAL CHARACTERIZATION OF TURQUOISE GROUP MINERALS

Turquoise is a hydrated copper aluminum phosphate and belongs to a group of minerals, the turquoise group, consisting of at least six isostructural end-members. Turquoise is common in the American Southwest, Iran and Egypt, where most of its value is associated with art, jewelry and ancient culture. When affected by near-surface conditions and extended exposure to sunlight and meteoric (rain) water, turquoise weathers to chalky white clay minerals. This alteration affects identification of the provenance regions of archaeologically recovered turquoise artifacts, the mineralogical properties of turquoise, and the value of turquoise as a semi-precious gemstone. However, this alteration process is poorly understood. Therefore, to improve the characterization of turquoise provenance for archaeological and gemological purposes, it is important to understand the alteration processes that affect turquoise. The general formula for the turquoise group can be written as A0–1B6(PO4)4(OH)8∙4H2O with Cu2+ or Fe2+ as the most common constituents at the A position and Al3+ and Fe3+ at the B position. However, Ca2+ or Zn2+ can occur at the A position in some of the more rare members of the turquoise group. The range in chemical composition (different concentrations of Cu, Fe, and Al) of turquoise (latu sensu) produces a wide range of colors. Foord and Taggart (1998) suggest that differ¬ences in the Cu and Fe ratio are responsible for the differences in color of the samples: blue turquoise has Cu at the A position and Al at the B position, whereas green turquoise (chalcosiderite) largely contains Fe3+ at the B position. The crystal structure of turquoise is triclinic, space group P¯1. Different site nomenclatures have been used in many of the structural and spectroscopic studies of the minerals of the turquoise group, depending on the chemical compositions under study. The turquoise samples belong to the collection of the Real Museo Mineralogico (RMM), Centro Musei delle Scienze Naturali, Università degli Studi di Napoli Federico II. Five turquoise samples covering a wide range of compositions in the turquoise group and in provenance were analyzed by X ray diffraction (XRD), electron microprobe analysis (EMPA), BSE-SEM-EDS analysis, LA-ICP-MS analysis and Fourier transform infrared (FTIR) spectroscopy. The provenances of the turquoise samples used in this study are: Tur 1 (RMM catalogue E5683) Sinai, Egitto; Tur 2 (E7666) Santa Fè S. Miguel, New Mexico; Tur 3 (E1675) Sassonia, Germania; Tur 4 (E7272) Montebras Creuse, Francia; Tur 5 (E1670/13131) Nischapir, Khorassan, Iran. The colors of Tur 1 sample are blue-green and light blue. The EMPA analysis show the presences in Tur 1 of faustite, altered turquoise, crandallite, quartz and magnetite. The turquoise samples display evidence of alteration from weathering processes, showing deficiencies in both cations and anion groups, indicated by EMPA, but preserving the crystal structure of turquoise, as verified by XRD. They also show large amounts of Si and Ca in their microprobe data, due to the presence of quartz and Ca crandallite, respectively, which are identified by XRD analyses. The chemical analysis show the presences in Tur 2, that is blue-green and white, of: turquoise, turquoise ferrian, ferrian variscite, variscite. The XRD analyses indicate the presence also of metavariscite and planerite. Locally in the fracture the EDS-BSE analyses show the presence of iron sulfates and potassium rich silcates. For Tur 3 sample, apple green and light green coloured, the EMPA analysis indicate the presences of Chalcosiderite and variscite; the XRD analyses show the presence also of metavariscite. The colors of Tur 4 sample is turquoise blue, pale blue and white. The EMPA analysis show the presences of turquoise, variscite and wavellite, the XDR pattern also of metavariscite, planerite and quartz. Blue-green, turquoise blue and pale blue Tur 5, on the basis of EMPA analysis is composed by turquoise and ferrian turquoise, the XRD also by metavariscite, planerite and quartz. The EDS-BSE analyses confirm the presence of quartz and cuprian planerite in the fracture of the sample. The pattern of REE elements indicate similarities between Tur 4 and Tur 1, and between Tur 2 and Tur 5. The trace elements particularly V, Ti, Mn, Cr, Ga for the turquoise minerals group, would be useful to be able to distinguish the samples from different localities. Powder samples for FTIR study were prepared as KBr pellets. The spectra are different on the bases of the mineralogical composition of the turquoise samples analyzed. From what has been published spectra closer to the typical turquoise are those Tur 1, Tur 2 and Tur 5. The Tur 4 begins to differentiate more clearly, and finally the Tur 3 is clearly different and seems to be similar to that of variscite. Particularly the absorption frequencies of OH and H2O show marked variations in both the width of the bands in the presence of different components. High-frequency components appear at least 4 (3509, 3494, 3467 and 3448 cm-1) due to the OH groups. They are well resolved in Tur 1 but less so in Tur 5 and Tur 2. In Tur 4 and then definitively in Tur 3 these components are absent altogether, while appears a new component shifted to higher frequency (3584 cm-1). Still in Tur 1, Tur 5 and Tur 2 are two bands rather wide low frequency (3300 and 3104 cm-1) due to the absorption of the molecules of H2O. In Tur 4 Tur 3 and these components are not obvious, but seem to be replaced by components centered at different frequencies, due to the presence of H2O. For the absorption frequencies in the region of the polyhedra Tur 1, Tur 5, Tur 2, Tur 4 and Tur 3 is observed a gradual decrease of the components that constitute the strong absorption centered around 1100 cm-1 (due to the vibrations of the groups PO43-). Substantial differences can still be seen in the region between 800 and 400 cm-1 between samples Tur 1, Tur 5, Tur 2 and samples Tur 4 and Tur 3.