Luminescence behavior of turquoise [CuAl6(PO4)4(OH)8·4H2O]

Abstract

We, herein, study the thermoluminescence (TL) and cathodoluminescence (CL-SEM) emissions of a commercial turquoise to determine its possible use as an emergency dosimeter. CL spectrum of bulk sample displays an intense broad emission from ∼260 to ∼650 nm together with a weaker narrow band at ∼710 nm. Through EDS and EMPA chemical analyses, an important amount of rare earth elements (REE) such as Ce, La, Y, Nd, Dy, Yb, Er, Pr, Sm, Gd, Ho, Tb, and Tm have been identified associated with phosphate phases as well as in turquoise itself. Apatite [Ca5[OH(PO4)3]], monazite [(Ce,La,Nd,Th)PO4], and xenotime [YPO4] have been detected in the turquoise matrix, as well as very small amounts of quartz [SiO2]. Turquoise has a high content in Zn2+ substituting for Cu2+, together with small amounts of Cr3+ substituting for Al3+. The well defined peak observed at ∼710 nm can be due to Cr3+ activation centers in the turquoise lattice with the rest of the emissions associated with a REE activators and intrinsic defects. Regarding the blue TL, two emission bands (190° and 330 °C) appear in the natural aliquot whereas the irradiated sample displays no response at all. This glow emission could be due to the losses of structural water molecules in the turquoise lattice