Thermo-optical detection of defects and decarbonation in natural smithsonite

Abstract

Natural hydrothermal ZnCO3 crystal aggregates are nominally anhydrous phases with interfacial water, with substitutional divalent cations and decarbonation c. 300°C. All these common features must be involved during the experimental heating of a thermoluminescence (TL) glow curve up to 500°C: dehydration-dehydroxylation, phase transition and ion transition of point defects in Zn2+ positions. A representative specimen of natural smithsonite was analysed by X-ray fluorescence spectrometry, field emission scanning electron microscopy (FESEM) with a chemical probe of energy dispersive spectrometer, high temperature in situ X-ray diffraction, differential scanning calorimetry, differential thermal analyses coupled to thermogravimetry, TL, radioluminescence and high resolution spectra thermoluminescence (3DTL), to gain an overview of the spectra emission and defects linkages modified by heating from room temperature up to 500°C. The ZnCO3 specimen contains minor amounts of Ca, Cu, Cr, Cd, Pb, Ce, Co, Ni, Mn and Fe. Under FESEM, it displays CaCO3 clusters and oscillatory zoning distribution with lamellae ranging from Ca0.11Zn0.89 to Ca0.19Zn0.81. The analytical results suggest assignments of defects and processes to measured 3DTL emission bands, as follows: (1) peak at ∼260°C, ∼360 nm bonds, breaking during the thermal decarbonation process; (2) peak ∼120°C, ∼340 nm: non-bridging oxygen centres associated to a complex dehydration-dehydroxylation process and (3) peak at ∼170°C, ∼650 nm, crystal field effects on the thermoluminescence of Mn2+ centres and associated transitional elements in the ZnCO3 phase.