Synthetic staurolites in the system Mg-Fe-Al-Si-O-H

Staurolites were synthesized at 6 different compositions (M = 100Mg/(Mg + Fe (sub tot) ) = 0, 20, 40, 60, 80, 100) on the join (Fe,Mg) (sub 4) Al (sub 18) Si (sub 7.8) O (sub 46.6+x/2) H (sub x) , using oxide-gel mixtures as starting material. The experiments were performed at 2 GPa, 720-800 degrees C, with or without oxygen buffers (WM, NNO or MH). Nearly single-phase staurolites crystallized on all bulk compositions, which suggests complete Fe-Mg solid solution. The presence of trace amounts of additional phases in most run products points to small deviations of the staurolite compositions from the bulk, which is confirmed by electron-microprobe analyses. Although the mean Si/Al ratio for all samples is nearly identical with the average of 0.43 proposed by Holdaway et al. (1993) for natural and synthetic staurolites, there is a significant scattering of the single mean values of the different samples (0.40-0.46). The (Fe + Mg) contents range from 12.3 cat.% in an Fe-endmember to 14.2 cat.% in a staurolite with M approximately 80. The cell volumes of the synthetic staurolites decrease with increasing M, due to a linear decrease of the b and c parameters. In contrast, a plot of the a parameter against M yields an upwards convex curve, with a maximum at M approximately 60 to 80. This may reflect an increase in the Mg-content in the M(1, 2, 3) sites up to this M value, but a concentration of Mg in the T(2) and M(4) sites at M = 100.