Synthetic staurolites in the system Mg-Fe-Al-Si-O-H
Lattard, Dominique; Bubenik, Wolf
European Journal of Mineralogy Volume 7 Number 4 (1995), p. 931 - 948
published: Aug 1, 1995
manuscript accepted: Mar 7, 1995
manuscript received: May 31, 1994
ArtNo. ESP147050704019, Price: 29.00 €
Abstract Staurolites were synthesized at 6 different compositions (M = 100Mg/(Mg + Fetot)= 0, 20, 40, 60, 80, 100) on the join (Fe,Mg)4Al18Si7.8O46.6+x/2Hx, using oxide-gel mixtures as starting material. The experiments were performed at 2 GPa, 720-800°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 ~ 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 ~ 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.