SYNTHETIC STAUROLITES IN THE SYSTEM MG-FE-AL-SI-O-H

Staurolites were synthesized at 6 different compositions (M = 100Mg/(M g + Fe-tot)= 0, 20, 40, 60, 80, 100) on the join (Fe,Mg)(4)Al18Si7.8O4 6.6+x/2Hx, using oxide-gel mixtures as starting material. The experime nts 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 solutio n. The presence of trace amounts of additional phases in most run prod ucts points to small deviations of the staurolite compositions from th e bulk, which is confirmed by electron-microprobe analyses. Although t he mean Si/Al ratio for all samples is nearly identical with the avera ge of 0.43 proposed by Holdaway et al. (1993) for natural and syntheti c staurolites, there is a significant scattering of the single mean va lues of the different samples (0.40-0.46). The (Fe + Mg) contents rang e from 12.3 cat.% in an Fe-endmember to 14.2 cat.% in a staurolite wit h M similar to 80. The cell volumes of the synthetic staurolites decre ase with increasing M, due to a linear decrease of the b and c paramet ers. In contrast, a plot of the a parameter against M yields an upward s convex curve, with a maximum at M similar to 60 to 80. This may refl ect 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 = 10 0.