Stresses and pressures at the quartz-coesite transition in shear experiments

Abstract:

Experiments on quartz (qtz) gouge were performed in a Griggs-type deformation apparatus at displacement rates of ~1.3 x 10^-5 mms^-1 or ~1.3 x 10^-4 mms^-1, at P_c= 1.0 GPa or 1.5 GPa and T = 600°C to 800°C. The starting material is a natural hydrothermally grown single crystal that was crushed to a powder with grain size d < 100 µm.

Coesite (coe) is found if the maximum principle stress (σ₁) is in the coe stability field. In general P_c and the mean stress (P_m) of these samples are below the quartz-coesite phase transition (QCT). Coe is not found if σ₁ is below the QCT. At T = 600 °C, σ₁is always in the coe stability field. But coe is only present in the high strain experiment, indicating slow transformation kinetics.

In one sample we observed that σ₁crosses the QCT during the loading part and after progressive weakening crosses the QCT back into the qtz stability field. The microstructure of this sample shows the formation of coe and the reverse transformation from coe to qtz.

The coe growth penetrates the sample and coe grows around and in between larger qtz clasts. At high stresses, where P_m is also above the QCT, coe often forms radiating aggregates. At lower stresses, where only σ₁ lies in the stability field of coe, and at low strain the coe grains have a preferred orientation of the b-axes (sub-) parallel to σ₁. With increasing strain, the rigid coe grains rotate and align with the preferred qtz fabric.

For coe to be found, it is sufficient that σ₁ reaches values above the transformation pressure. If σ₁ drops back into the qtz stability field during an experiment, a back-reaction from coe to qtz is observed. It appears therefore that the pressure that defines the QCT is not P_c or P_m, but σ₁.