The role of weak phases on strength and strain localization in quartz aggregates

We are investigating the role of weak second phases (oxides) on the rheology and microstructural evolution of quartz aggregates. Specifically, to evaluate if progressive dynamic recrystallization of quartz (qtz) grains promotes strain localization, we are conducting deformation experiments on samples of Banded Iron Formation (BIF) and synthetic aggregates composed of qtz, hematite (hem) and magnetite (mgt). Composite aggregates made up of different weight percents of qtz, hem and mgt were deformed in a Griggs Solid Medium Apparatus at T = 900°C, P = 1.5 GPa and strain rates from 10^-4 to 10^-6/s, in both axial compression and general shear. Axial compression of BIF cores, reaching up to 40% of shortening, showed strain localization into hem rich layers. In addition, we find that qtz grains are more deformed near hem than in regions away from hematite-rich bands. At a strain rate of 10^-5/s, our experiments show that the strongest BIF (maximum stress: 320 MPa) contains the highest qtz content. Sample strengths decrease with increasing hem fraction; the specimen with the highest hem content had a strength of 94 MPa. General shear experiments on pure qtz, hem and mgt aggregates showed qtz is significantly stronger than the iron oxides. Hem and mgt aggregates have flow strength of ~110 MPa and ~145 MPa, respectively, while qtz aggregate showed peak strength of 620 MPa. Sheared isotropic qtz plus hem synthetic aggregates show that only a small amount of hematite (2-10% by weight) promotes pronounced weakening (decrease in strength of 76%). Increasing hem content above ~10% has only a minor effect on strength; e.g., a sample with 75 wt% hem is only 10% weaker than samples with 10% hem. Comparison of general shear experiments on synthetic aggregates with those on BIF samples demonstrate that microstructural evolution associated with dynamic recrystallization of quartz promotes significant weakening even at very small hem contents (7-8%). In this case, the weakening is not only related to interconnection of hem. Rather, dynamic recrystallization of quartz in regions with dispersed hem appears to promote localization and weakening. In this experiment the sample strain hardens to ~ 620 MPa (similar to the strength of pure qtz) and then weakens rapidly to a stress of ~140 MPa (similar to the strength of the qtz/hem isotropic aggregates).