Crystal structures of gillespite, BaFeSi4O10, which has a reversible first-order phase transition at 18 kbar, were refined from data collected at 1 bar and 9, 21, and 45 kbar. Silicate-layer topology is constant through the transition, but coordination of barium increases from 8 to 10. The almost perfect square-planar coordination group of iron in gillespite I (Fe-0 = 1.98Å) changes to a flattened tetrahedron with two additional long Fe-O bonds (∼3Å) in gillespite II. The volume discontinuity at the gillespite I—II transition is best described as a consequence of the increased coordination of barium and iron.

Unit-cell parameters of gillespite were measured under nine high-pressure and fourteen combined high-pressure, high-temperature (PT) conditions. The Clausius-Clapeyron slope of the transition is 27±9 bar/°C, which is similar to the slope of gillespite-I isochores. On the basis of these observations, the behavior of the gillespite I—II transition is consistent with a geometrically controlled phase transformation, in which the size of the barium site is the critical dimensional factor.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.