25. Arsenates, arsenites, phosphates, and vanadates

Although the number of arsenate, arsenite, phosphate, and vanadate species is second only to that of the silicates, specimens are for the most part rare, and many exceedingly so. Few occur as primary high-temperature minerals; at both Franklin and Sterling Hill, the primary members of this group are largely in the apatite group.

	Table 25. Chemical analyses of arsenate and arsenite minerals.

Almost all are secondary minerals occurring in veins and fissures. These species are almost entirely arsenates except magnussonite, nelenite, and schallerite, which are vein-filling arsenites; pyrobelonite and descloizite, which are the only known local vanadate minerals; and three anomalous and aberrant phosphates, meta-ankoleite, newberyite, and niahite.

The relative paucity of zinc arsenates in these deposits, which are so overwhelmingly rich in Zn, is enigmatic. There are only 13 zinc-bearing species. Of these, only three (koettigite, legrandite, and adamite) occur as simple zinc arsenates without other cations. The rare arsenates of copper are known only from Sterling Hill, and the arsenates of Mg are more common at Sterling Hill than at Franklin, consistent with the greater amount of Mg there.

The locally dominant group is the manganese arsenates. It is this group of species which has, in part, called attention to apparent but overstated relations to the mine at Långban, Sweden; many of these exceedingly rare minerals are otherwise known only from there or Pajsberg, Sweden. Most of the manganese arsenates are well-crystallized, and fine specimens are prized. A group of six arsenosilicates of manganese is unique to Franklin and Sterling Hill; they are discussed under silicates.

Parker and Troy (1982) were the first to discuss a chemical partitioning of the Sterling Hill arsenates. Those with Zn and Fe³⁺ occur primarily in the black-willemite ore, and those with Mn and Mg occur primarily in the red-willemite ore. Some arsenates of Ca and Mg are post-mining minerals.

The preponderance of the Sterling Hill arsenates are associated with zincite-bearing ore, the red-willemite ore of Parker and Troy (1982), in which no arsenic-bearing primary mineral is found; arsenic may have been introduced to this ore from the black- willemite zone, the enclosing marble, or some other source. The arsenates of Mn and Mg found in this red- willemite ore likely are of higher temperature than those found in the black-willemite ore and are not hydrated.          

A suite of significant Sterling Hill arsenates was found in the black-willemite zone, between the 180 and 340 levels; zincite is absent. The species are principally arsenates of Zn or of Zn and Fe³⁺, and many are hydrated. Fe and As are likely derived from locally abundant loellingite. The species unique to the black ore are noted with an asterisk (*) in the list given below.

The abundance of arsenate species at Sterling Hill, relative to Franklin, is partially unexplained. The arsenates in the black-willemite ore belong uniquely to Sterling Hill; black-willemite ore is uncommon at Franklin and hosts no known arsenate minerals. Many of the Mn/Mg arsenates occur at Franklin, and many others might have occurred there, but were not found, possibly because the deposit was worked long before fine microscopes were available to collectors.

The locally occurring arsenates, arsenites, phosphates, and vanadates are listed below. Symbols indicate: * - arsenate from the black-willemite zone at Sterling Hill;  ** - phosphate; *** - vanadate.

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