New Mexico Mineral Symposium — Abstracts

Gahnite, the zinc spinet, has not been known from New Mexico in noteworthy specimens. Margarite, a brittle calcium mica has not been previously reported from New Mexico. One of the authors (JK) in 1995 located both of these minerals in the Harding mine (Picuris) district. The find was made in spite of the fact that this area has been one of the most extensively studied areas of the state. Although gahnite has been found in the Harding mine district (Jahns, 1953), euhedral crystals larger that 1-2 mm have been unknown. The new find was made on the dumps of a small unnamed pegmatite prospect pit about three-quarters of a mile north of the Harding mine. The dominant crystal form is the octahedron (111) modified by smaller dodecahedron (110) faces. The octahedral faces are marked by raised triangular growth patterns. Crystals are a dark grass green and lustrous. They average about 5 mm with the largest crystals about 1 cm in size. They are in miarolitic cavities in a groundmass of albite, white muscovite, and anhedral spessartine garnet. Euhedral white beryl crystals and some small blue anhedral apatite crystals were also found on the dumps of the pit, and light-green-blue slender crystals of beryl up to 5 mm in length have been found at the margins of the prospect. Gahnite was not found in the exposed walls of the pit.

Margarite was found in a shear zone between amphibolite and schist about 1.5 mi northwest of the Harding mine. It occurs as loose spherical nodules and botryoidal masses up to 3.5 cm in contact with quartz veins. The nodules are very fine grained and buff to silver although most are stained darker by iron oxides. A nodule that was sawed in half was found to be margarite throughout (P. J. Modreski, personal communication). Identification was confirmed by x-ray diffraction (Bart Cannon) and by optical methods (P. J. Modreski). Corundum, often associated with margarite, has not been found at this location. Less than 200 m north of this area, pink zoisite (thulite) was found in exceptional euhedral crystals. Free-standing, gemmy, twinned crystals to 4 mm with an orange-pink color were found in a vuggy coarse-grained amphibolite. Larger translucent to sub-translucent crystals with prominent prism faces were also found along with sharp dodecahedral red-purple garnets.

The Pilar cliffs of the Glenwoody district lie adjacent to the Rio Grande just south of the village of Pilar. The upper portion of the Precambrian Vadito Group (Glenwoody Formation) exposed in the steep-walled cliffs have produced noteworthy specimens of piemontite, idocrase (cyprine), zoisite (thulite), and dravite. A new occurrence of the chromian muscovite, fuchsite, has recently been found high up the cliffs by one of the authors (JK). The fuchsite, which is light grass green, occurs in a seam about 10-12 cm thick that seems to extend laterally only a few meters. Exact dimensions are difficult to discern because of the steepness of the terrain. Small (less than 0.5 mm) brown octahedral gahnite crystals are found with the fuchsite. The gahnites are similar to those found in a fuchsite occurring at La Madera Mountain in Rio Arriba County. A dravite-bearing schist is exposed about 15 m south of the fuchsite location. Dark-brown dravite crystals, which commonly are 2 cm x 2 mm in size, are abundant in this area. A pegmatite with pink-purple muscovite occurs about 500 m south of the fuchsite location. Also high up on the cliff, the pegmatite is about 20 cm thick and dips steeply eastward. Specimens have a superficial resemblance to material from the Harding mine but upon closer examination, the muscovite has a distinct purple appearance unlike the Harding mine muscovite.

These recent mineral discoveries show that the persevering collector can expand the mineralogical knowledge of the Harding mine, Glenwoody and Picuris districts, by continued scrutiny of the less-accessible area of the region.

References:

1. Bauer, P. W., and Helper, M. A., 1994, Geology of Trampas quadrangle, Picuris Mountains, Taos and Arriba Counties, New Mexico: New Mexico Bureau of Mines and Mineral Resources, Geological Map 71.
2. Jahns, R. H., 1953, The genesis of pegmatites, II, Quantitative analysis of lithium-bearing pegmatites, Mora County, New Mexico: American Mineralogist, v. 38, pp. 1078-1112.
3. Montgomery, A., 1953, Pre-Cambrian geology of the Picuris Range, north-central New Mexico: New Mexico Bureau of Mines and Mineral Resources, Bulletin 30, 89 pp.
4. Schilling, J. H., 1960, Mineral resources of Taos County, New Mexico: New Mexico Bureau of Mines and Mineral Resources, Bulletin 71, 124 pp.