Analyses of natural Co-Ni-Fe diarsenides have indicated two distinct compositional groups: the rammelsbergites (and pararammelsbergites), which are nickel-rich, and the loellingite-safHorite series, which are nickel-poor. The synthetic diarsenides show extensive solid solution at 800° C. A solvus interrupts the FeAs2-NiAs2 series near the NiAs2 end and extends into the ternary solid solution toward CoAS2. This solvus probably interrupts the NiAs2-CoAs2 series at temperatures well below 800° C. and hence explains the separation of rammelsbergites from the loellingite-safflorite series in nature.

At 800° C., synthetic rammelsbergite appears to be stoichiometric with the formula NiAs2,00. Loellingite is homogeneous from FeAs2 to FeAs1.98. Homogeneous cobalt safflorite was made with the compositions CoAs1.99 and CoAs1.98 and appears to have monoclinic symmetry, in contrast to rammelsbergite and loellingite which are orthorhombic. Although the cell edges of the Co, Ni, Fe diarsenides vary as complex functions of the composition, the cell volumes vary as a linear function.

The cell edges of nine analyzed specimens of loellingite and safflorite show fair agreement with synthetic diarsenides of equivalent composition, after corrections are made for the effect of nickel and sulfur on the cell edges.

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