New Mexico Mineral Symposium — Abstracts

The exact nature of the banded-massive to radiating-fibrous black manganese oxides from the Luis Lopez district, Socorro County, has long been and continues to be a mineralogical riddle. The mineral has been described variously as psilomelane, "pseudo-psilomelane", pyrolusite, pseudomorphs of psilomelane after pyrolusite, hollandite, coronadite, and intergrowths of hollandite + romanechite.

The difficulties arise because the material is mineralogically complex and does not fall into simple mineral categories. Much of it is poorly crystalline and gives diffuse X-ray diffraction patterns. Its chemical composition is variable, with the proportions of large metal cations (Ba, Pb, K, Sr), the water content, and the proportions of manganese in different oxidation states all varying between ideal end-members such as hollandite, Ba(Mn⁺⁴,Mn⁺²)₈O₁₆; coronadite, Pb(Mn⁺⁴,Mn⁺²)₈O_16; cryptomelane, K(Mn⁺⁴,Mn+2)₈O₁₆; and romanechite, Ba(Mn⁺²Mn⁺⁴)₈O₁₆,(OH)₄. Banded fibrous material from the vicinity of the Tower and Nancy mines at the head of Black Canyon, Luis Lopez 71/2' topographic quadrangle, gives X-ray powder diffraction patterns that are inconclusive as to its exact mineralogical identity. Electron micro¬probe analysis shows this material to be chemically zoned; individual layers range from nearly pure barium manganese oxide (hollandite or romanechite) to material containing as much as 20 weight percent PbO, close to coronadite in composition. Turner and Buseck (1979, Science, v. 203, p. 456-458), using high-resolution transmission electron microscopy, have shown that individual crystal fibers from the Rattlesnake mine in the Luis Lopez district are composed of a random mixture of submicroscopic regions, each of which is just a few unit-cells thick, alternating between hollandite and romanechite crystal structures. This variably constituted mineral only marginally satisfies the criteria of crystal structure, fixed chemical composition, and homogeneity that are used to define a specific mineral species. The material might be considered a mineraloid rather than a particular mineral species, and the old general term, psilomelane, may be the best name to use after all.