BaFeSi 4 O 10 , refined from data collected at 1 bar and 9, 21, and 45 kbar. Coordination of barium increases from 8 to 10. The almost perfect square-planar coordination group of iron in gillespite I changes to a flattened tetrahedron with two additional long Fe-O bonds 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 14 combined high-pressure, high-temperature conditions. The Clausius-Clapeyron slope of the transition is 27+ or -9 bar/ degrees C, which is similar to the slope of gillespite-I isochores. 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.--Modified journal abstract.