Upper thermal stability of tourmaline + quartz in the system MgO-Al (sub 2) O (sub 3) -SiO (sub 2) -B (sub 2) O (sub 3) -H (sub 2) O and Na (sub 2) O-MgO-Al (sub 2) O (sub 3) -SiO (sub 2) -B (sub 2) O (sub 3) -H (sub 2) O-HCl in hydrothermal solutions and siliceous melts

The upper thermal stability of Mg-Al tourmaline (both Na-bearing and Na-free) in the presence of H (sub 2) O, SiO (sub 2) , H (sub 3) BO (sub 3) and HCl was studied experimentally at 200 MPa between 680 and 850 degrees C as a function of the B content of the fluid. In the Na-free system, tourmaline + quartz breaks down to cordierite + sillimanite + B-bearing fluid at > or = 750 degrees C, at B (sub 2) O (sub 3) contents in the fluid of approximately 5-9 wt.%. In the Na-bearing system, the reaction to cordierite + melt occurs at > or = 730 degrees C, at B (sub 2) O (sub 3) contents of approximately 5-8 wt.%; the melt contains approximately 2 wt.% B (sub 2) O (sub 3) . At lower B (sub 2) O (sub 3) concentrations in the hydrous fluid, tourmaline + quartz react to give cordierite + albite + B-bearing fluid at approximately 700 degrees C. The composition of tourmaline changes systematically as T increases. In the Na-bearing system, an increasing proportion of vacancies on the X site of tourmaline was found as a result of the substitution Na (sub -1) Mg (sub -1) []Al in addition to some Al incorporation by Mg (sub -1) H (sub -1) Al. In the Na-free system, the latter substitution leads to Al-enriched tourmaline.