Determination of amphibole fracture strength for quantitative palaeostress analysis using microboudinage structures

The fracture strength of amphibole is estimated with the aim of extending the applicability of the microboudin method to palaeostress analysis. Before estimating the fracture strength of amphibole, it is necessary to evaluate the influence of the size effect on fracturing. Among the three models of size effect (effective-length, effective-area, and effective-volume), the effective-length model is found to be the most suitable in our microboudinage analysis of a metachert from Turkey that contains numerous microboudinaged amphibole grains of variable width embedded within a quartz matrix. Taking into account the influence of the size effect on fracturing, we undertook a comparative microboudinage analysis of the metachert sample, which also contains coexisting microboudinage structures of tourmaline and epidote. The analysis revealed an instantaneous-fracture strength for amphibole of 80 MPa for a 1 mm cube. The far-field differential stress (σ0),(σ0), calculated based on amphibole microboudinage structures and considering the effect of the fatigue limit, is σ0=8λ(1w¯)1/2, where λ   is the dimensionless stress parameter determined by microboudinage analysis and w¯ is the mean width of amphibole grains.