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.