Gem-quality Turkish purple jade: Geological and mineralogical characteristics

Abstract

In the Harmancık–Bursa region of the western Anatolia (Turkey), an extensive contact metamorphic aureole at the border between the Late Mesozoic coherent metaclastic rocks of blueschist facies and the Early Senozoic intrusive granodiorite stock hosts an interesting and unique gem material with a mineral assemblage consisting mainly of jadeite, quartz, orthoclase, epidote, chloritoid, and phlogopite as identified by X-ray diffraction spectroscopy and polarized-light microscopy. In addition, chemical analyses performed with X-ray fluorescence and inductive-coupled plasma-atomic emission spectroscopy show that the mass of the metamorphic aureole has a silica-rich, calc-alkaline chemical content. Therefore, some rock building elements (such as Al, Ca, Na, K, P, Sr, and B of which characterize an acidic–neutral rock formation) and trace elements (such as Fe, Cr, Mn, Be, Cu, Ga, La, Ni, Pb, and Zn) are remarkable high ratios. Pale purple-colored gem material of this composition appears to be unique to Turkey, also is only found in one narrow provenance in Turkey. Therefore, it is specially called “Turkish (and/or Anatolian) purple jade” on the worldwide gem market. Even though the mineral jadeite is the principal constituent, 40% by volume as determined with petrographic thin-section examination under a polarized-light microscope, the material cannot be considered pure jadeite. Specific gravity measurements of the jade using a hydrostatic balance confirm that it has a heterogeneous structure. The measured average specific gravity of 3.04, is significantly lower than the normal range for characterized jadeites of 3.24–3.43. Turkish purple jade samples were examined in detail using dispersive confocal micro-Raman spectroscopy (DCμRS) as well as other well-known analytical methods. The resulting strong micro-Raman bands that peaked at 1038, 984, 697, 571, 521, 464, 430, 372, 326, 307, 264, and 201 cm −1 are characteristics of the Turkish purple jade. The first most intensive and widest Raman band that peaked at 697 cm −1 can be interpreted as the ν 2 doubly symmetric bending mode of (SiO 4/M) centers. The “M” includes the some cationic substitutions of Si by Fe, Cr, Mn, Be, Cu, Ni, Pb, and Zn, and also K and Na. The second most intensive and widest Raman band that peaked at 372 cm −1 can be interpreted as the ν 2 single symmetric bending mode of (SiO 4/M) centers. The third most intensive and widest Raman band that peaked at 201 cm −1 can be interpreted as translational libration. Finally, the fourth distinctive Raman bands that peaked at 1038 and 984 cm −1 can be interpreted as the ν 1 doubly symmetric stretching modes of (SiO 4/M) centers. In addition, both sides of these bands were also barricaded with relatively unimportant Raman bands produced by some structural imperfections. The measurements of these all analytical parameters are the most trustworthy method to distinguish the purple jade from the other well-known kinds of natural, synthetic, and/or color-enhanced jades. Finally, these parameters provide positive identification of the provenance (geographic origin) of the original Turkish purple jade. The data obtained in this study for dispersive confocal micro-Raman bands, specific gravity values, and trace element contents provide a unique fingerprint for this kind of jadeite-jade gem material.