Thermal dependence of colour and ultraviolet luminescence emission in amazonite feldspar

Abstract

Amazonite is a natural blue-green coloured perthitic Pb-bearing feldspar, structurally hydrous occuring in some pegmatite ore deposits. The irradiation-induced colour in amazonite can develop only in K-feldspar having both, structurally bound H2O and Pb impurities. The different types of colour are linked with a limited presence of Pb (as Pb3+ or Pb1+ colour centres) and structurally bound H2O. The dependence of the colour intensity on both, Pb and H2O concentration suggest that the lead and water occur in a 1:1 ratio in the colour centres. They also point to water plays a catalytic role in the irradiative transformation of Pb2+ to the amazonite chromophores. Recently we have proposed the close association existant between twinning and the UV band in perthite K-feldspar points to charge trapping and light emissions from complexes of structural defects located at the twin-domain boundaries. The high temperature slope of the TL glow curve follows a power law decay, suggesting possible trapping-detrapping dynamics related to cooperative phenomena inside the complexes. Assuming the following published features: (1) some feldspars with as much as 1000 ppm Pb are not coloured, (2) Pb1+ centers are present in amazonites but not in other feldspars, (3) radiation converts Pb2 + to Pb1+ (4) Correlation between loss of colour and weight loss during dehydration of amazonite. Here we analize preheated aliquots (RT, 80, 120, 170, 220ºC) of a Brazilian amazonite sample by electron probe microanalysis, spectra thermoluminesce, and optically stimulated luminescence observing a clear correlation between decolouration, dehydration, dropping of the ultraviolet 3DTL and OSL emission fitting. Consequently, assuming that no monoclinic sanidine have shown amazonitic corolation, a possible relationship between these colour centres and the optically active UV luminescent centres with the twin-domain boundary dynamics can not be disregarded