Thallium and manganese complexes involved in the luminescence emission of potassium-bearing aluminosilicates

Abstract

The luminescence emission at 285 nm in natural K-feldspar has been studied by Russian groups and associated with thallium ions in structural positions of K+ sites as artificially thallium-doped feldspars display the same emission band. Here attention is focussed on spectra of CL emission bands centered near 285 and 560 nm from paragenetic adularia, moscovite and quartz micro-inclusions. With accesorial thallium they show clear resemblances to each other. Associated sedimentary and hydrothermal aluminosilicate samples collected from Guadalix (Madrid, Spain) were analyzed with a wide range of experimental techniques including Environmental Scanning Electron Microscopy (ESEM) with an attached X-Ray Energy-Dispersive Spectrometer (EDS) and a cathodoluminescence probe (CL) and Electron Probe Microanalysis (EPMA), X-Ray Fluorescence Spectrometry (XRF), Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES), Differential and Thermogravimetric Analyses (DTA-TG), radioluminescence (RL), Mössbauer spectroscopy and X-Ray Photoelectron Spectrometry (XPS). The luminescence emission bands at 285 and 560 nm seem to be associated with hydrous thallium–manganese complexes bonded to potassium-bearing aluminosilicates since various minerals such as K-feldspar, moscovite and quartz micro-inclusions display similar CL spectra, accesorial thallium and hydroxyl groups. The presence of iron introduces a brown color which is attributed to submicroscopic iron oxides detectable in the optical and chemical microanalysis, but this does not contribute to the luminescence emission. The XPS Mn 2p spectrum of the adularia sample at room temperature is composed of a spin–orbit doublet plus clear shake-up satellite structure ~4 eV above the main photoemision lines and is consistent with Mn2+ in good agreement with the observed luminescence emission at 560 nm for aluminosilicates produced by a 4T1(4G)→6A1(6S) transition in tetrahedrally coordinated Mn2+. Moscovite samples display spectral CL bands at 285 and 560 nm but only when the electron-beam is directed along the (0 1 0) orientation and not along the (0 0 1) orientation. The Tl+ versus K+ cation isomorphism anchors the luminogenous hydrous thallium–manganese complexes to the potassium-bearing aluminosilicate surfaces under analyses. The CL emission bands at 285 and 560 nm of these complexes together with the EDS detection of thallium are a fast analytical measurement detecting the presence of thallium in further studies involving this toxic element.