Cataclastic solution creep of wet polycrystalline

sodium chlorate aggregates

Mohsine Zahid1 and Bas den Brok2

1.Institut für Geowissenschaften, Johannes Gutenberg-Universität, Becherweg 21, D-55099 Mainz, Germany.

2.Geologisches Institut ETH, Sonneggstr 5, CH-8092 Zürich, Switzerland.

(zahid@mail.uni-mainz.de; denbrok@erdw.ethz.ch)

 

Uniaxial dead-weight creep experiments were carried out on wet synthetic polycrystalline aggregates of sodium chlorate (NaClO3) at room tempera-ture and atmospheric pressure. Aim of the experiments was to study the deformation mechanism and resulting microstructures in wet rock analo-gues that are loaded below the plastic limit within the pressure solution de-formation regime. NaClO3 has a solubility and dissolution/precipitation rate comparable to that of NaCl, but it is an elastic/brittle salt. At room temperature and atmospheric pressure dry single crystals of this material fail catastrophically at differential stresses in the range 15-25 MPa. Below this stress it only deforms elastically (strain resolution ~0.05 %).

Cylindrical samples (5 mm in diameter and 10 mm long) of poly-crystalline NaClO3 were loaded axially up to a differential stress of 2 MPa. The samples have a grain size in the range 180-212 Ám and a porosity of 1-2%. There is about 1-2 Vol% saturated NaClO3-solution present at grain boundaries and in the pores. Samples were sealed-off with latex jackets and were deformed undrained.

The mechanical data show rate-dependent steady-state creep at a strain rate of ~10-7 /s up to finite bulk strains of ~6%. The data are in good agreement with mechanical data obtained in compaction experiments on porous NaClO3 aggregates where pressure solution was inferred to be the dominant compaction mechanism. At axial strains higher than about 6%, samples drastically weaken and strain rates increase up to ~2.10-6 /s within 6 hrs (and strains of about 18%). The optical deformation microstructures indicate that this weakening effect is associated with a transition from pres-sure solution creep to bulk (i.e., non-localised) cataclasic creep. Original grains show large numbers of healed micro fractures, mostly oriented sub-parallel to the {100} crystallographic planes. Our poster documents the mechanical data and resulting deformation microstructures.

Poster presented at EUG Conference Strasbourg April 1999.