Pressure solution compaction of sodium chlorate

Bas den Brok*, Mohsine Zahid, Cees W. Passchier

Sodium chloride (NaCl) has been extensively used as a material to develop, test and improve pressure solution (PS) rock deformation models. However, unlike silicate and carbonate rocks, NaCl can deform plastically at very low stresses (~0.5 MPa). This could mean that NaCl is less suitable for use as an analogue for rocks that do not deform plastically at conditions where PS is important. In order to test the reliability of NaCl as a rock analogue, we carried out a series of uniaxial compaction experiments on sodium chlorate (NaClO3) at room pressure and temperature (P-T) conditions and applied effective stresses of 2.4 and 5.0 Mpa. NaClO3 is a very soluble, elastic-brittle salt, that cannot be deformed plastically at room P-T conditions. The results were compared with experiments on NaCl at similar conditions and show that NaClO3 behaves in a strikingly similar way to NaCl, despite its brittleness. Like NaCl, it most likely compacts by a grain boundary diffusion controlled PS mechanism. Mechanical data were fitted to a power law in the form:

(with volumetric strain rate , volumetric strain e, effective stress s and grain size d). A reasonable fit was obtained, with a = 2 - 4, n = 1.6 ± 0.5, and m = 2.8 ± 0.5. The similarity in mechanical behaviour of the two materials (NaCl plastic, NaClO3 brittle) suggests that plasticity does not play a key role in PS compaction deformation of NaCl. This means that its plasticity is not a drawback for its use as a PS analogue for rocks or for deriving PS creep laws for salt from compaction experiments.