Substitution of ferric iron for aluminum in the sapphirine structure extends up to an approximate composition Mg3.5Fe0.73+Al8.3Si1.5O20 at 1150°C, 2 kbar and high oxygen fugacities. The sapphirines coexist with Fe3+-bearing corundum, Fe3+-bearing spinels and cordierite. By comparing the hyperfine parameters of 57Fe in sapphirine with those reported for other crystal structures, Fe3+ is assigned to the tetrahedral positions in sapphirine. It is distributed largely statistically over the different tetrahedral sites, possibly with the exception of the small T2 site. Given high oxygen fugacities, ferric iron contents in sapphirines are expected to increase up to a limiting pressure on the order of 10 kbar and then decrease with greater pressures.

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