Speciation and phase separation of water in quartz (A review)

Abstract

Speciation, supersaturation and phase separation of water in hydrothermal grown (synthetic) quartz cryst-als have been examined as fundamental issues determining clustering, nucleation and bubble formation, which have profound influence on hydrolytic weakening, dislocation-bubble interactions and high temperature deformat-ion of quartz. Details of water speciation are necessary for explaining cluster dynamics and dislocation motion in quartz, and are also relevant in understanding magma evolution and eruption dynamics, since quartz and other sil-icates are important minerals of the earth crust. Infrared absorption studies strongly suggest that water bubbles in quartz contain hydrous species of hydroxyl ion (OH^-) and molecular water (H₂O_m), originating from phase separat-ion of supersaturated water vapour after prolonged heat treatment of quartz at temperatures in excess of 500 °C. leading to decomposition of the hydrolyzed bonds. The hydrous species interconnect through a homogeneous spe-ciation reaction, condense, cluster and nucleate to form water bubbles, either homogeneously by thermal fluctuati-ons or heterogeneously on defects and other grown-in occlusions. The nucleation is a first order phase transition of creating liquid nucleus within the vapour phase, and chemical potential difference (related to the level of super-saturation) is the thermodynamic driving force.

The unique properties of water that affect the mechanical deformation of quartz is due not only to the di-pole character, but even more to the geometrical structure of the molecules which form extended four coordinated networks. Incorporation of water tetrahedra in silicate framework [SiO₄] produce electron-unsaturated chemical bonds. By sp³-hybridization of electron bonds, the water molecule exhibits a tetrahedral charged structure, leading to various water point defects in the form of substitution of SiO₂ by two H₂O substitutional defects (4H)_Si, interstit-ial water molecules in channels parallel to the c-axis, hydrolyzed Si-O-Si bonds, and H-like alkali ions associated with substitutional impurities (such as Al³⁺ or Fe³⁺) to maintain charge balance. Water-related defects in quartz have complex structures and compositions, and the aqua-complexes are either bonded with different cations or are structurally bonded in the quartz matrix. The formation of combined defect [SiO₄]-H₂O-M⁺ [M³⁺O₄] upon water speciation (M⁺ is metallic ion), is an indication of the presence of hydrous species.

Keywords: Speciation, Supersaturation, Phase separation, Phase transition, Infrared absorption, Nucleation