Ab initiostudies on water related species in quartz and their role in weakening under stress

Abstract

Ab initio calculations on the modes of incorporation of water in quartz suggest that, apart from the incidental inclusion of molecular water as fluid inclusions, the most prevalent mode of uptake of water in a-quartz is structural and involves the replacement of Si by four protons in a process formally described as the [4H]si substitution. Existing ub initio computer calculations were used as a basis on which to determine the equilibrium concentration of [4H]si defects in quartz as a function of relevant temperatures and pressures. These calculations indicate that [4H] si concentrations are in the range lo-'-10-4/Si atom increasing rapidly with increasing water pressure at constant temperature, while above 300°C the defect concentration decreases monotonically with increasing temperature in all cases, implying that the defects must react on heating to produce molecular water. Ab inirio results have also been used to establish the concentration of interstitial water molecules in quartz under the same conditions. In this case the concentrations are vanishingly small and of the order of IO-''/Si atom at 500°C and l.OGPa, implying that the diffusion coefficient for water on interstitial sites in quartz must also be extremely low.It has recently become apparent that the results of the ab initio based calculations for the concentration of both [4H]si defects and interstitial water molecules are directly relevant to the weakening behaviour in wet quartz on heating. Since the reaction in which [4H]si defects produce molecular water on heating involves a very substantial volume increase and, since the diffusion coefficient for the migration of molecular water on interstitial sites is now known to be prohibitively small, excess water produced on heating cannot escape from the crystal which therefore behaves locally as a. totally closed system, at least within the time frame of conventional laboratory experiments. Here the reaction of [4HIsi defects to give molecular water on heating leads to the development of water bubbles with the concomitant growth of edge dislocations with b = $u(1120), the dislocations taking up the material transfer from the bubble volume in the closed system. At concentrations of [4HIsi defects of the order of IOOppm, the development of a dislocation densities of the order of 108/cm2 can be shown to be reasonable, a figure which is also confirmed from existing electron optical studies of heated wet quartz.

Dislocation creep of dry quartz

“…One potential solution to the discrepancy between experimental and natural deformation of quartz is that intragranular water contents in nature need not be as large as those in deformation experiments. Very low concentrations of hydrogen defects and complexes were suggested to promote dislocation creep by influencing the rate limiting process, e.g., by nucleation of double kinks [ McLaren et al ., ; Cordier et al ., ], increasing the dislocation density [ Griggs , ; McConnell , ], increasing dislocation velocities [ Kirby and McCormick , ; Hirsch , ; Cordier et al ., ; Mainprice and Jaoul , ], and facilitating climb by nucleation of dislocation jogs [e.g., Hobbs , ; Heggie and Jones , ; Paterson , ; Cordier and Doukhan , ; Cordier et al , ]. Synthetic quartz with water contents as low as ~100 H/10 6 Si can be deformed experimentally by dislocation glide as long as strain remains small (~1% [ Cordier and Doukhan , ]).…”

“…Synthetic quartz with water contents as low as ~100 H/10 6 Si can be deformed experimentally by dislocation glide as long as strain remains small (~1% [ Cordier and Doukhan , ]). Equilibrium concentrations of simple and extended hydrogen point defects in quartz at PT conditions of the TGM are calculated in the range of <20 H/10 6 Si [ Paterson , ] to around 400 H/10 6 Si [ Doukhan and Paterson , ; McConnell , ]. These concentrations must be regarded as crude estimates, and neither the assumed hydrogarnet defect (4 H + substitute Si 4+ ) has been definitively demonstrated for quartz [ Cordier et al ., ], nor have the larger concentrations been observed in natural quartz.…”

Atomic Scale Chemo-mechanics of Silica: Nano-rod Deformation and Water Reaction

“…The interpretation given was that the synthetic crystals contained water which hydrolyzed the silicon-oxygen bonds, forming SiOH groups which became mobile to promote dislocation mobility [24]. This problem has been investigated actively in the ensuing years [49,63], with work including molecular calculations at the empirical potential [25,27] and ab initio [26,35,48] levels. It is fair to say that at present the issue of the dominant mechanism for the observed plasticity is still indeterminate.…”

Chemomechanics of complex materials: challenges and opportunities in predictive kinetic timescales

“…Silica is a naturally occurring material, comprising silicate tetrahedra (SiO 4 ) in the crystalline form of quartz; it is the most abundant material in the Earth's crust and the key component of fiberoptic and dielectric-thin-film communication platforms. It is well known that water reduces the strength of silica through several mechanisms, including hydrolytic weakening of quartz due to interstitial water [24][25][26] and stress corrosion cracking of amorphous silica due to surface water [17,[19][20][21]. This interaction is representative of a broader class of chemomechanical degradation of material strength, including stress corrosion and hydrogen embrittlement in metals, and enzymatic biomolecular reactions.…”