Mechanical properties of cryolite from Ivigtut, South Greenland

Cryolite made up about 75 per cent of the deposit in Ivigtut, South Greenland. Its mechanical properties seem important for the shaping and for the internal build-up of the deposit of which a major part, siderite­cryolite with about 15 per cent siderite and a few per cent of sulphides, quartz etc., consolidated between 500 and 600'C. Thermal expansion, by volume, of cryolite is shown to be two to five times greater than the expansion of the accompanying minerals and rocks. The elastic constants of cryolite are found to be: E = 6.8 • 1()5 bars, G = 2.7 • 10' bars and Poisson's No. = 0.24. Compressive strength at room temperature 919 bars. Uniaxial compression tests at atmospheric pressure showed strength to increase to 1360 bars at 200"C and strain rate 0.05 mm per minute. Cryolite behaved in the tests up to 300"C as a brittle substance. Duc­tility was noted at temperatures from 400"C and, depending on strain rate, cryolite was found to exhibit unrestricted plastic deformations above 450"C. Ductility of cryolite around 500°C was found to be about the same as found for halite, tested in the same instrument, at 150-200°C. It is to be noted that the plastic deformation observed in these uniaxial compression tests dominantly appeared as bending of lamellae developed parallel to the load axis both in cryolite and in halite. In the siderite-cryolite, representing the earliest formed part of the deposit, siderite exhibits a pronoun­ced cataclastic structure. This is attributed to the high ductility of cryolite at temperatures around 500°C. The contraction on cooling of cryolite appears important in judging joint patterns of the intrusion as well as macro- and microfissures within the deposit.