Low-stress pressure solution experiments on halite single-crystals

Pressure solution experiments on halite single-crystals in saturated soluti on were carried out at atmospheric pressure under uniaxial stress ranging f rom 0.1 to 2.0 MPa and at temperatures of 303 and 323 K. The experiments we re performed in ceramic loading rigs with damp-proofed sample chambers. The low uniaxial stress is applied by loading the piston with steel weights ra nging from 0.5 to 5.0 kg. The position of the piston is measured by an elec tronic displacement transducer, connected to a data acquisition system. Dev iations caused by fluctuations of temperature and output voltage of the pow er supply are corrected after data acquisition. The halite cubes {100} with edge dimensions of 3-9 mm are prepared by cleaving and placed with a (100) cleavage face on the (001) face of a muscovite single-crystal (10 x 10 x 0 .1 mm), a polished quartz (0001) plate, or another halite crystal oriented to form a 45 degrees twist boundary. The four free (100) faces of the halit e cube are in contact with the surrounding NaCl solution. The initial displ acement rate of the piston after flooding of the system and loading is up t o 50.0 mu m/day, attributed to smoothing of the halite face and elimination of point contacts with high stress concentration. Within 2 to 3 days this stage grades into steady-state displacement with rates of 0.1-2.0 mu m/day. In some experiments stages of higher displacement rates (2.0-5.0 mu m/day) lasting for 3-5 days are observed episodically, with intervals of 10-15 da ys. These cycles appear not to be triggered by external events. Experiments with a dry mica-halite interface, carried out for comparison at the same t emperature and at an uniaxial stress of 2 MPa, result in a displacement rat e below the limits of detection. This rules out a significant contribution of crystal plastic deformation in the wet experiments. The experimental res ults show no simple correlation between displacement rate and magnitude of uniaxial stress, crystal size, type of the interface, and temperature. At t he given conditions, convergence at a single interface due to pressure solu tion is apparently not a steady-state process. The alternating stages of lo wer and higher displacement rates observed in many experiments suggest that the mechanisms of transport or dissolution may change spontaneously during the experiment. It is possible that the process itself leads to an unstabl e configuration causing episodic changes. (C) 1999 Elsevier Science B.V. Al l rights reserved.