Calcite twin stress and strain indicators for various polymineralic rocks and mortars

This paper proposes new formula to estimate strain and stress of various polymineralic materials in low-temperature elastic condition. Sakaguchi et al. (2011) proposed that the average twin lamellae density (Dtw) (number of twins/mm) depends on the load affecting the entire sample, based on discrete element method simulations and triaxial compression tests in the elastic deformation range. However, their derived relational formula between stress and Dtw, based solely on hard sandstone from the Shimanto accretionary complex in SW Japan, is only applicable to rocks with similar Young's modulus. This study attempted to perform uniaxial compression tests on mortar samples mixed with synthetic calcite grains. The specimens with different Young's modulus from approximately 5.9 to 10 GPa can be made by varying the water/cement ratio using the high strength cement. Calcite synthesized in an autoclave does not contain primary twin lamellae. From the result of the uniaxial compression test, the Dtw increased from 0 to 212/mm. The Dtw depends on the load affecting the entire sample. The rate of increase of Dtw with load varies for specimens with different Young's modulus, but Dtw increases in direct proportion to strain for all specimens regardless of Young's modulus, as represented by:

$\epsilon =0.0094\text{Dtw}–0.20\left(\pm 0.105\right)\text{,}$where, ε represents shortening strain (%). For a hard specimen without plastic deformation and with a known Young's modulus, the paleo-stress experienced by the specimen can be estimated from the strain obtained from the average Dtw, since elastic strain and stress are proportional. This technique can be used for problems ranging from structural geology to building construction ones at low temperature and elastic strain state.