End-member danalite is compositionally equivalent to a mixture of pyrrhotite, phenakite, and fayalite, but danalite’s natural occurrences with pyrite, magnetite, and quartz suggest that it is stable under somewhat more sulfidizing conditions than pyrrhotite and more oxidizing conditions than fayalite. Danalite tends not to occur with hematite. These facts can be used to construct a tentative log fS2−log fO2 diagram for the stability of the end member. The diagram shows that danalite, if it is stable at all, has an extremely narrow stability field centered near the pyrrhotite/magnetite field boundary. Outside this field, phenakite (or, at low temperatures, bertrandite) is stable with magnetite, pyrrhotite (or pyrite), and quartz. A natural example of these equilibria apparently occurs at Iron Mountain, Bartlett, New Hampshire, where zoned danalite–helvite solid solutions occur as overgrowths on phenakite with magnetite, pyrite, and quartz.

Danalite is the only end member of the helvite group that is appreciably sensitive to hypogene oxidation. The Mn and Zn end members helvite and genthelvite are instead sensitive only to S–O exchange (variation in log fS2/fO2).

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