Why calcite can be stronger than quartz

In the Neves area (Eastern Alps), calcite forms asymmetric centimeter-scale single-crystal porphyroclasts in quartz mylonites developed during hydrous amphibolite facies metamorphism at ~550°C. Under these conditions, coarse calcite was clearly stronger than the surrounding polycrystalline, dynamically recrystallized, quartz matrix. Experimental results indicate that coarse calcite is less strain rate sensitive than wet quartzite, consistent with an inversion in strength on extrapolation to natural strain rates. For this to occur, wet quartzite must be weak, flowing at differential stress of <10 MPa. The lack of high-temperature twins (showing bulging or recrystallization) in calcite clasts is consistent with such low stresses during shear zone development under near peak metamorphic conditions. The maximum effective viscosity ratio of coarse calcite to quartzite for these conditions is probably not large (<10). However, numerical modeling shows that ratios of around 2 are sufficient to maintain near rigid calcite clast behavior for power law rheology with stress exponents appropriate to quartz (n ~ 3–4) and coarse calcite (n ≥ 6). The inversion in relative strength reflects the difference in influence of water on the crystal plastic flow of calcite and quartz: water has a dramatic effect for quartz but little or no effect for calcite. Quartz-rich rocks under hydrous amphibolite facies conditions in the middle to lower crust are therefore relatively weak (in fact, weaker than coarse calcite) and flow at much lower stresses than dry quartz-rich rocks at similar crustal levels.