Water-related defects in quartz

Abstract

Themode of incorporation of water and trace elements into silica matrices was studied in natural quartz of various origins: hydrothermal, magmatic, pegmatitic, and generations of vein quartz related to ore mineralization. Several methods were used toward identifying different manifesta- tions of water-trace element defects. Transmission electron microscopy (TEM), as a unique direct tool for the imaging and identification of lattice defects, played an important role. IRS, TG, chemical analysis (AAS), X-ray microprobe, X-ray powder diffraction, and EPR were used for obtaining additional in- formation on the nature of the complex defects. As a result, we propose a model for the combined silica-water-trace element defect (2SiO3 -O H 2 - M n+ 2M m+ O'4) (where is a vacancy, O' is O and other volatiles such as S, Cl, etc.), called an aqua-complex. Aqua-complexes are units of the min- eral-forming medium. Quartz (SiO2), which corresponds to the left part of aqua-complex, is crystallized as a result of disintegration of the whole species. Non-disintegrated aqua-complexes become trapped during the formation of quartz. Single aqua-complexes are crystallographically accommodated into the SiO4 framework, whereas their unordered segregations, which are actually micro-portions of the mineral-forming medium, are incorporated into the silica bulk as glass-like inclusions (gel-defects). Aqua-complexes are metastable species in quartz. During post-crystallization cooling of the mineral they may disintegrate into water-containing bubbles saturated with basic radicals, and acidic and volatile elements. As the thermodynamic parameters in the mineral system change, the aqua-complex constituents H (H2O) and foreign cations M n+ and M m+ migrate through the SiO4-lattice, which also be- comes rearranged in this process. Such properties of the aqua-complex explain the elusive character of water-related defects in quartz.