Summary: American Mineralogist, Volume 89, pages 455461, 2004
The transition from tetrahedral to octahedral coordination of
Si cations by O anions is the dominant structural change accom-
panying the increase in pressure with depth in the Earth. It marks
the transition from upper to lower mantle and implies major
changes in density (Dziewonski and Anderson 1981), rheology
(Stolper and Ahrens 1987; Williams and Jeanloz 1988; Karato
et al. 1990), and chemistry (Kellogg et al. 1991; Tschauner et al.
1999).Although the Earths mantle is dominated by mixed oxide
silicates, the pure silica phases provide the basic model of this
coordination change (e.g., Williams and Jeanloz 1988). Previous
work on this transition has focussed on ambient temperatures.
Low-pressure silica phases are initially compressed by tetra-
hedral tilt and collapse to dense, higher-coordinated structures
by multiple reconstructive phase transitions into disordered
sixfold coordinated phases based on corner-sharing octahedra
(Tsuchida and Yagi 1990; Kingma et al. 1996; Haines et al.
2001; Dubrovinsky et al. 2001). This behavior differs from that