Iron thio-arsenide, FeAsS

Iron thio-arsenide, FeAsS, or FeS₂.FeAs₂, occurs in nature as the mineral mispickel, arsenical pyrites or arsenopyrite. If cobalt is also present, the mineral is known as glaucodote, (Fe, Co)AsS. When the arsenide is in excess, the mineral is called pacite, FeS₂.4FeAs₂.

Many suggestions have been made as to the constitution of mispickel. The structural formula suggested by Beutell may be written



This, however, offers no explanation for the fact that the percentage of sulphur in the crystals obtained from various localities shows considerable variation, as little as 18.05 per cent, and as much as 22.47 per cent, having been found.

Chemical methods for determining the constitution lead to conflicting conclusions, precisely as in the case of iron pyrites, owing to uncertainty as to the state of valency of the iron in the mineral. The formula usually accepted is FeS₂.FeAs₂.

When heated in the absence of air, mispickel loses much of its arsenic. This is explained on the assumption that the iron disulphide decomposes into free sulphur and ferrous sulphide: -

FeS₂ = FeS + S.

The sulphur then attacks the di-arsenide, causing the expulsion of the arsenic: -

FeAs₂ + S = FeS + 2As. Combining the above reactions, the equation may be represented as FeAs₂.FeS₂ = 2FeS + 2As.

Loczka therefore favours the formula FeAs₂.FeS₂ for mispickel.

On the other hand, when mispickel is heated in hydrogen, the whole of the sulphur is evolved as hydrogen sulphide, whereas both pyrites and mareasite under similar conditions lose only half their sulphur, being converted into ferrous sulphide, FeS. These facts suggest that the pyrites molecule is absent, and that if iron disulphide is actually present, it is in some other form than ordinary pyrites.

The formula



is suggestive.