Coexisting orthoamphiboles (high Al–Na gedrites and low Al–Na anthophyllites) from the Post Pond Volcanics, Vermont, display a variety of textures that are interpreted to be equilibrium growth textures. These textures include: (1) discrete grains; (2) bladed intergrowths; (3) patchy intergrowths; (4) overgrowths; and (5) lamellar intergrowths with lamellae parallel to (120). Coexisting orthoamphiboles display exsolution parallel to (010); exsolution is much more pronounced in gedrite than in anthophyllite, which suggests that the solvus is steeper on the anthophyllite limb.
The solvus is defined by discontinuities in the amount of Si, AlIV, AlVI, and Na(A) in the orthoamphiboles, which can be described as gaps in the edenite (NaAl⇄☐ Si) and tschermakite (AlVIAlIV⇄MgSi) substitutions. The width of the solvus is a function of Fe/(Fe+Mg), with Fe-rich samples showing the widest gap. Distribution of elements between anthophyllite and gedrite is systematic, with gedrite enriched in AlIV, AlVI, Fe2+, Na, Ti, Ca, and Mn and anthophyllite enriched in Si and Mg. The composition limits of orthoamphiboles at this metamorphic grade are controlled by coexistence with phases more enriched in certain elements: the most Si-rich orthoamphiboles coexist with quartz, the most Al-rich with cordierite or staurolite, the most Fe-rich with garnet, the most Mg-rich with talc, chlorite or cordierite, the most Ca-rich with hornblende, and the most Ti-rich with ilmenite or rutile.
Evaluation of published orthoamphibole analyses reveals that the crest of the solvus probably lies at approximately 600±25°C.