The univariant reactions (1) Та + Fo = 5En + Stm and (2) Та = 3En + Qtz + Stm have been experimentally reversed at PH2O = Ptotal. Smooth curves drawn between the brackets pass through the coordinates 0.5 kbar, 617°C; 1 kbar, 638°C; 2 kbar, 662°C; 3 kbar, 679°C; 4 kbar, 696°C for reaction (1); and 0.5 kbar, 660°C; 1 kbar, 697°C; 2 kbar, 738°C, for reaction (2). The positions of the curves are probably within 5°C of the stated values. Intersections of reactions (1) and (2) with the reversed reactions (3) 9Ta + 4Fo = 5Ant + 4Stm and (4) Ant = 7En + Qtz + Stm, respectively, generate two invariant points. The reaction (7) Ant = Та + 4En is common to both invariant points and together with reactions (3) and (4) serves to bound the stability field of anthophyllite. Reactions (2) and (4) intersect at РH2O = 5 kbar, and their positions are consistent with the P–T diagram proposed by Greenwood. However, the slope of reaction (1) is less positive than the slope of reaction (3), and this is inconsistent with Greenwood's proposed P–T diagram. If reaction (1) is accurately positioned, the conflict can be resolved in one of two ways. Reaction (3) can be rotated until its slope is less positive than the slope of reaction (1), or the P–T diagram can be inverted. The first alternative is most plausible. If Greenwood's proposed P–T diagram is correct, the data presented here limit the stability of pure Mg-anthophyllite with respect to the assemblage Та + En to a maximum pressure of about 5 kbar РH2O.

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