ESEEM of industrial quartz powders: insights into crystal chemistry of Al defects

Abstract

A set of raw industrial materials, that is, pure quartz and quartz-rich mixtures, were investigated through electron paramagnetic resonance and electron spin echo-envelope modulation spectroscopies, with the aim of evaluating the effective role played by defect centres and of assessing whether they can be used to monitor changes in the physical properties of quartz powders with reference to their health effects. The obtained results point to two interactions of the Al defect centres with H+, hosted in sites within the channels parallel and perpendicular to the c axis of quartz, respectively. These two Al/H+ (hAl) centres exhibit a weak chemical bond, and their relative amounts appear to be modified/controlled by the thermo-mechanical processes underwent by powders. Indeed, a mechanically promoted inter-conversion between the two kinds of site is suggested. As a consequence, the hAl centres are effective in monitoring even modest activations of powders, through thermal or mechanical processes, and they are also supposed to play a specific, relevant role in quartz reactivity during the considered industrial processes

ESEEM of industrial silica-bearing powders: reactivity of defects during wet processing in the ceramics production

“…1 cw-EPR spectra, registered at X-band on the I 5 and I 6 samples at a room temperature and b 35 K. In the inset, an exemplar spectrum of the raw quartz materials, I 1 sample, also registered at 35 K, is shown (modified after Romanelli et al 2012). Magnetic field values are expressed in milliTesla (mT).…”

“…I 1 and I 2 are the starting raw materials for the process, and they consist of nominally pure quartz microcrystalline powders. These two samples were the object of an extensive spectroscopic characterisation in a previous study (Romanelli et al 2012). I 5 and I 6 represent two wet mixtures of quartz and silicates (eventually combined with other minor mineral phases).…”

“…Owing to the fact that the h Al radical species of quartz (Fig. 4b) exhibits a parallel component at 2.0602 (Ikeya 1993;Romanelli et al 2012), a contribution due to both species (the A-centre in kaolinite and h Al in quartz) in the spectrum from the I 6 sample can be considered. However, the eventual co-presence of multiple radical species superimposed in the experimental spectrum cannot be excluded nor fostered only on the basis of the EPR and Echo-EPR data.…”

“…Ubiquitous defect species in quartz can be represented by the form [AlO 4 ] 0 or [AlO 4 /M + ] + (M being Li, Na, H) and are characterised by a marked Zeeman anisotropy, and a clear hyperfine structure due to the interaction with the Al nucleus. Recently, Pan et al (2009) found further peroxy radicals (D and E centres) in natural quartz, whereas Romanelli et al (2012) have registered an ubiquitous presence of the h Al centre in most of the industrial raw quartz.…”

“…Some recent studies have been focussed on the determination of the radical speciation associated with quartz and with other common industrial mineralogical species, as clays Jones et al 1974;Ikeya 1993;Clozel et al 1994;Pan et al 2009;Allard et al 2012), especially considering materials sampled in the industrial production lines Capacci et al 2010;Romanelli et al 2012). Most of these studies are associated by a common spectroscopic approach, i.e.…”