Order and anti-order in olivine; I, Structural responses to temperature

The crystal structure of Fe (sub 0.48) Mg (sub 0.52) [SiO (sub 4) ] olivine from the Boseti volcano, Ethiopia, was investigated by single-crystal XRD at between 20 and 900 degrees C. For temperatures up to 600 degrees C, data were collected on crystals equilibrated at 600 degrees C; the data can therefore be assumed to reflect structural changes caused purely by thermal effects, whereas data collected between 600 and 900 degrees C carry additional information about the Fe (super 2+) ,Mg equilibrium distribution. The in situ experiments at elevated temperatures were complemented by ambient temperature data on quenched crystals to check for possible Fe (super 2+) and Mg distributions during quenching. Such effects were absent in crystals quenched from 800 degrees C or below. The temperature dependence derived for the Fe (super 2+) ,Mg distribution is ln K (sub D) = 0.4422(+ or - 0.0070) - 140.0(+ or - 6.5)/T (K). indicating that Fe (super 2+ ) progressively 'anti-orders' into the M1octahedral site as temperature is raised. A reverse ordering reaction at approximately 650 degrees C leading to a strong segregation of Fe (super 2+) into the other octahedral site M2 could not be detected. Both the M1-O and M2-O bond distances increase with temperature, with a change in the rate of increase at approximately 600 degrees C, which conforms with an enrichment of the larger Fe (super 2+ ) cations on the M1 site and depletion on M2. The opposite is true for the distortion defined in terms of the angles subtended at the cation site.