Water-related defects in quartz


Authors: Stenina NG

Published in: Bulletin of Geosciences, volume 79, issue 4; pages: 251 - 268; Received 29 April 2004; Accepted in revised form 30 July 2004;

Keywords: natural quartz, water speciation, transmission electron microscopy, aqua-complex,

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The mode 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 manifestations 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 information on the nature of the complex defects. As a result, we propose a model for the combined silica-water-trace element defect [2SiO3 ∅ - OH2 -Mn+2Mm+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 mineral-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 Mn+ and Mm+ migrate through the SiO4-lattice, which also becomes rearranged in this process. Such properties of the aqua-complex explain the elusive character of water-related defects in quartz.