SOLID-SOLUTIONS IN THE ANNABERGITE-ERYTHRITE-HORNESITE SYNTHETIC SYSTEM

Previous crystal-structure determination of magnesian annabergite (Ni, Mg)(3)(AsO4)(2) . 8H(2)O indicated that ordering of Ni and Mg occurs i n nonequivalent octahedral sites; such ordering supports the possibili ty of miscibility gaps on the Ni-3(AsO4)(2) . 8H(2)O - Mg-3(AsO4)(2) . 8H(2)O (annabergite - hornesite) join. In a predecessor study based o n compositions of natural specimens, it was proposed that solid soluti on is incomplete both for annabergite - hornesite and annabergite - er ythrite [Co-3(AsO4)(2) . 8H(2)O]. In the current study, syntheses by a slow-addition technique at 70 and 95 degrees C indicated that the ann abergite - erythrite solid solution is complete, that no miscibility g ap is present on the annabergite - hornesite join to at least 58 mol% Mg, and that no miscibility gap occurs on the erythrite - hornesite jo in to at least 53 mol% Mg. Furthermore, no miscibility gaps are eviden t in the low-Mg portion of the annabergite - erythrite - hornesite ter nary system. Thus, interpretation of the compositions of these phases in oxidized mill-tailings impoundments can be done with the normal pre cautions for homogeneity, and without the added concern of the possibl e presence of miscibility gaps in the ternary system.