The Effect of Heat Treatment on the Magnetic Properties of the Old Red Sandstone

Magnetic methods of analysis have been used to identify and determine the grain size of minerals deposited in rocks of the Old Red Sandstone from South Wales. Changes in magnetic properties occur when the rock is heated in air to temperatures between 500 and 1000 °C. Three distinct processes, deduced from measurements of the low field reversible susceptibility, operate in successively higher temperature ranges. The first process is characterized by an increase in susceptibility over the range 500–700°C; the second involves a decrease in susceptibility over the range 700–850°C; the third process is responsible for a very large susceptibility increase above 850°C. The first process, operating over geological time, may be largely responsible for the production of a relatively high initial susceptibility in the virgin rock and of a secondary magnetization superposed on the original. Production of magnetite on heating is the interpretation given to the mechanism responsible for the increases in susceptibility.