Blue or green? turquoise–planerite species from Carico Lake Valley in Nevada, the United States: Evidence from Raman spectroscopy

Abstract

AbstractBlue and green turquoise–planerite species from Carico Lake Valley in Nevada have the formulae □0.69(Na0.01K0.01Cu0.20Ca0.06Mg0.02Ba0.01 Zn0.01)(Al5.97V0.04 Fe0.03)[PO4]4 (OH)8·4H2O and □0.66(K0.01Cu0.24Ca0.05Mg0.02Ba0.01Zn0.01)(Al5.87Fe0.13)[PO4]4(OH)8·4H2O, respectively. The content of Al3+ and Fe3+ at the B‐site together with slight differences in Cu amount at the A‐site is the main factors that differentiate these two species. Green planerite shows higher crystallinity and contains less water in its structure. The blue species is more porous with a lower degree of structural order and higher water enrichment. The Cu/Fe ratio together with local symmetry around chromophore ions in the structure seem to be responsible for the specific colouration of both phosphates. The species are inhomogeneous and contain a wealth of mineral (goethite, barite, quartz, kaolinite, chloritoid, destinezite, and some vanadate) and organic inclusions (bituminous matter and aliphatic hydrocarbons). Raman microspectroscopy with the application of different exciting radiation, that is, laser lines of 514.5 nm (green) and 442 nm (blue) for blue and green turquoise–planerite species revealed further differences between these two coloured species: (a) various positions of the most intensive band attributed to ν3 (PO4)3‐, (b) shift of ν3 four component bands, (c) various intensity ratios and integral intensity ratios of ν3 component bands, and (d) some diagnostic bands coming from Fe‐O (green species) and Cu‐O vibrations (blue species) obtained with principal component analysis (PCA).