Temperature and field dependent susceptibilities of some Libyan desert glass samples

Magnetic susceptibilities of samples of Libyan desert glass have been measured by the Faraday technique. The samples were cut from specimens in which neither striae, nor spherulites, nor bubbles could be observed with the unaided eye. They were transparent with a pale yellow-green colour. The temperature dependent susceptibility can be explained as primarily due to a 'Brillouin' system of non-interacting' paramagnetic centres in a diamagnetic matrix. Assuming that Fe 3+ ions are the source of the para- and ferromagnetic susceptibilities, an analysis of the data shows that a fraction, about 0.4. of the Fe ions are single ions isolated in the matrix and the remainder are in pairs, with the exception of aggregates. The concentration of Fe atoms, determined by the susceptibility data, indicates that in the samples a fraction of the iron ions are anti-ferromagnetic coupled. Less than 2% of the Fe ions are in aggregates that are ferromagnetic at room temperature. Based on our analysis of the data we suggest that, in the samples, the distribution of iron ions is approximately random, with the ions with the smallest separations having ferromagnetic or anti-ferromagnetic interaction at temperatures <300 K.