1986 Volume 36 Issue 197 Pages 219-230
The stability relationships of zunyite, AL13Si 5O 20F 2(OH, F) 16Cl, were determined at Pf=1kb and T=300-600°C in solutions with constant HCl=1M but different HF=0.1 to 20M. The products of stoichiometric oxide mix seeded with natural zunyite in respective solutions at 400°C were used as starting materials. The stability field of zunyite is rather limited; it is stable in a solution with HF concentrations no more than 2M. In 1M HF solution, zunyite is stable up to 450(5)°C and is replaced by topaz at higher temperatures. It appears to remain stable at the same temperature range in solutions with HF as low as 0.1M. In solution with HF concentration of 3M, topaz is stable at temperature above 470(5)°C. At lower temperatures topaz is accompanied by AlF3 phase. This assemblage is also stable at higher temperatures in solutions with HF concentrations up to 10M. In solutions with high HF con-centrations, quartz+AlF 3 become stable. The rare occurrence of zunyite compared to topaz is the consequnce of stringent chemical conditions aside from temperature. Both F and Cl are indispensable for the mineral and yet higher F activities destabilize it. The occurrence of zunyite as a massive body in altered felsic volcanics reported elsewhere is thus indicative of unusual solution behavior and physico-chemical condition during the alteration.
