X-ray, microscopic, microchemical, contact-print, and mineral synthesis techniques have been applied to the problem of the relationships of the higher natural arsenides of cobalt, nickel, and iron. The data obtained have been correlated with a review of existing analyses.

Both the isometric and “orthorhombic” groups exhibit limited isomorphism in regard to the three metals, and the two groups appear to be essentially complementary. The isometric members form a single group of triarsenides with the skutterudite structure. The long-held assumption that published analyses indicate the existence of an isometric diarsenide series is not substantiated by a critical review of the analyses. Extensive arsenic deficiency shown by published analyses of isometric arsenides is due largely to mechanical inhomogeneity involving “orthorhombic” diarsenides, niccolite, and other minerals. Isomorphous substitution of metal for arsenic may contribute to the arsenic deficiency, but evidence for this was not obtained.

The lattice constants of the isometric arsenides are related to the ratio between the three metals, varying directly with increasing nickel, iron, or nickel and iron content. Variations in arsenic ratio do not appear to influence the lattice constants appreciably. The lattice constants range from 8.187 Å to 8.311 Å, an increase over that previously recorded (8.18Å to 8.27 Å).

Isometric arsenide crystals are almost invariably zoned and inhomogeneous, containing both isometric and nonisometric constituents. Fine granular aggregates of “orthorhombic” diarsenides frequently occur in these complex cubic crystals and have probably often been overlooked since the anisotropism is weak. The inhomogeneous character of these zonal crystals could account for many of the reported compositional peculiarities of the isometric arsenides such as extensive arsenic deficiency and extreme variation in the cobalt-nickel-iron ratio.

Since there is no valid basis for an isometric diarsenide series, it is urged that the names smaltite, chloanthite, chathamite, and arsenoferrite, long associated with an alleged series of isometric diarsenides, be dropped. The single term skutterudite should be applied to all isometric arsenides of cobalt, nickel, and iron and in a restricted sense to those in which cobalt is dominant; the appropriate modifiers, nickelian and ferrian, would indicate dominance of nickel and iron respectively. Such a terminology is believed adequate since the isomorphism between cobalt, nickel, and iron in the isometric group is clearly limited, and only a single end member (cobalt) appears to exist.

The “orthorhombic” diarsenides exhibit both discontinuous and limited isomorphism between cobalt, nickel, and iron; those high in nickel are completely separated from the iron-bearing members by a broad wedge of isometric arsenides. The term rammelsbergite is retained for the common high-nickel member, and pararammelsbergite for the rarer nickel—rich form. The term loellingite is applied to all iron-rich orthorhomic diarsenides and in a restricted sense to those with an iron ratio greater than 85. The group of iron-rich orthorhombic diarsenides with an iron ratio of less than 85 and with cobalt dominant over nickel may be termed cobaltian loellingite. The corresponding group with nickel dominant over cobalt is designated nickelian loellingite. Safflorite has been rather generally accepted for the cobalt end member of an allegedly unlimited isomorphous series of orthorhombic diarsenides, the other end members being loellingite and rammelsbergite. There is no evidence for the existence of such a cobalt end member of the “orthorhombic” series, either from the available data on natural material or from the experiments in synthesis. The plotting of the published analyses indicates a clustering of “orthorhombic” analyses in the mid-portion of the cobalt-iron edge of the diagram. The symmetry of these iron-cobalt arsenides is monoclinic according to (Peacock, 1944), justifying the retention of the name safflorite as a species distinct from loellingite. However it should be redefined as a monoclinic diarsenide of cobalt and iron in which the proportions of the two metals are approximately equal

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