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. Grain growth of quartz was investigated using two quartz samples(powder and novaculite) with water under pressure and temperatureconditions of 1.0–2.5 GPa and 800–1100  ∘ C. The compacted powderpreserved a substantial porosity, which caused a slower grain growth than inthe novaculite. We assumed a grain growth law of d n - d 0 n = k 0 f H 2 O r exp⁡ ( - Q / R T ) t with grain size d ( µ m) at time t (seconds), initial grain size d0 ( µ m), growth exponent n , a constant k0 ( µ m n  MPa −r  s −1 ), water fugacity f H 2 O (MPa) with the exponent r ,activation energy Q (kJ mol −1 ),gas constant R , and temperature T inKelvin. The parameters we obtained were n = 2.5 ± 0.4 , k 0 = 10 - 8.8 ± 1.4 , r = 2.3 ± 0.3 , and Q = 48 ± 34 for the powder and n = 2.9 ± 0.4 , k 0 = 10 - 5.8 ± 2.0 , r = 1.9 ± 0.3 , and Q = 60 ± 49 for the novaculite. The grain growth parameters obtained forthe powder may be of limited use because of the high porosity of the powderwith respect to crystalline rocks (novaculite), even if the differencesbetween powder and novaculite vanish when grain sizes reach ∼70 µ m. Extrapolation of the grain growth laws to natural conditionsindicates that the contribution of grain growth to plastic deformation in themiddle crust may be small. However, grain growth might become important fordeformation in the lower crust when the strain rate is −12  s −1 .

Experimental grain growth of quartz aggregates under wet conditions and its application to deformation in nature