Thermal Studies on Mackinawite

On mackinawite mistaken for valleriite, the specimens obtained from eleven localities have been investigated especially concerning its thermal behaviors. Those discriminated under the reflection microscope on the basis of their optical characters revealing abrupt change at 130°C～250°C are classifiable into A) Kawayama type (including the specimen from Mackinaw) losing its extraordinary pleochroism and anisotropism at 130°C～160°C, B) Kômori type (including those from Muskox, Tsumo, Akagane and Ohmine) showing the similar variation at intermediate temperature and C) Outokumpu type (including those from Makimine, Besshi and Shimokawa doing so above 200°C, among which the third type only discloses transformation from mackinawite to pyrrhotite. A considerable difference in transformation temperature is commonly observable even in the specimens collected from the same locality.
True valleriite, on the other hand, is stable at the temperature lower than about 500°C.