New Mexico Mineral Symposium — Abstracts

Solid solutions between end members of isomorphous series are responsible for most of the variations seen in mineral compositions and are useful for recording genetic conditions. Indeed, by determining the exact composition of a specimen the mineralogist may be able to tell much about the temperature, pressure, and chemical conditions present when the mineral formed. In the case of zoned crystals the varying compositions also indicate the trends in conditions with time. Recent microprobe examinations of two specimens from different places in New Mexico not only revealed some interesting and unusual zoning patterns but also identified four uncommon mineral species and gave information about the solid solubilities of the mineral pairs.

The first specimen of interest came from a cavity in altered andesite found in some old mine workings in the Bear Mountains north and a bit west of Magdalena. These crystals are bright, lustrous tablets with a rhombic outline and vary from a bright emerald-green to a very dark green color. Microprobe analyses of the crystal surfaces indicated major amounts of Cu, Ca, V, and As, a composition not represented by any known mineral species. In cross section, the zoned nature of the crystals became obvious; bands rich in either As or V alternated with constant amounts of Ca and Cu. Quantitative analyses confirmed that the crystals were intermediate between conichalcite CaCu(AsO₄)OH and calciovolborthite CaCu(VO₄)0H. The oscillatory- and sector-zoning records repeated major changes in the chemistry of the fluids from which the crystals formed. These may represent influxes of fresh As-rich fluid from the weathering of primary minerals. Continuous zoning within each sector or band indicates that the As-rich species is less soluble than the V-rich and that there is complete solid solubility between the two even if they do occur in different crystallographic space groups.

The other specimen comes from the Tyrone mine of Phelps Dodge Corp. near Silver City. These crystals are modified rhombohedra with a poor luster and a strange whitish-yellow color. Analyses of the surfaces indicated a confusing mix of Ca, Al, Si, P, and S--again not easily recognizable as any known species. Analyses of a cross-sectioned crystal revealed the outer layer to be quite different from the bulk, which turned out to be the mineral corkite PbFe₃³(PO₄)(SO₄)(OH)₆. Corkite is one member of a large family of isomorphous minerals that also contains a Ca-Al-P-S member, woodhouseite, and a third member in which Si is substituted for P. By integrating all of this information it appears that this specimen consists of a thin layer of siliceous woodhouseite on a core of corkite. These crystals contain minor amounts of K and Ba that, together with the major elements, create a complex pattern showing (again) oscillatory, sector, and continuous zoning. In this material the oscillations are mainly between Pb and K + Ba. Continuous zoning involves the enrichment from core to rim of P, Al, Pb, and Ca at the expense of S, Fe, and K. Sector zoning is exhibited by the sudden disappearance of Pb and Fe, the appearance of Si, and the abrupt enrichment in Ca and Al. The suddenness of the change suggests that the two minerals do not form a continuous solid solution even though they are isomorphous. Another interesting observation is that the corkite seldom contains S and P in a perfect one-to-one ratio indicating that the formula ratios are somewhat flexible and not rigid. In fact, these non-stoichiometric ratios in the corkite seem to indicate that there may be solid solutions between the three groups.

*This work was performed at Sandia National Laboratories supported by the U.S. Department of Energy under Contract #DE-AC04-76DP00789.