New Mexico Mineral Symposium — Abstracts

Agate, chalcedony, and opal from many localities in New Mexico, as well as worldwide, often have a characteristic green or yellow-green fluorescence under shortwave ultraviolet light.

The green fluorescence is known to be caused by uranium, present as its oxidized, hexavalent form, U6+, and more specifically as the hydrated uranyl molecular ion, (UO₂•nH2O)²⁺ (see Gorobets et a1.1977). The fluorescence is strongest under shortwave ultraviolet light (wavelength about 254 nm), and is much weaker under the lower energy longwave ultraviolet light (about 360 nm) produced by less expensive "black lights." The fluorescence has a distinctive, recognizable spectrum of multiple bands due to vibrational structure of the triatomic UO₂²⁺ molecular ion, which makes it easy to distinguish from other activators that may produce green fluorescence, such as the green fluorescence of Mn²⁺ seen in the zinc silicate, willemite.

The most outstanding occurrences of brightly fluorescent agate and chalcedony are in nodules and geodes formed in volcanic rocks. The Apache Creek area, Catron County, is known for agate and for flattened, sometimes faintly pink colored chalcedony "roses" that weather out of rhyolite, and that can be strongly fluorescent. One is pictured as Fig. 14 of Modreski (1987). Agate that occurs in basalt in this same general area is variably fluorescent, with bands ranging from very weak to moderately fluorescent. Similar occurrences of fluorescent agate are present throughout the Mogollon—Datil volcanic field, as well as adjacent parts of Arizona and Mexico. Chalcedony (grayish-white to faintly pink) lining the interiors of gas cavities in rhyolite lava in the Peloncillo Mountains near Geronimo Pass, Hidalgo County, New Mexico, often fluoresces bright green (Modreski 1996). Some of the agate and chalcedony found in the Deming area, including Rockhound State Park, is similarly fluorescent; Colburn (1999) and Dunbar and McLemore (2000) discuss the origin of these spherulitic agatized nodules.

Agate and chalcedony in petrified wood also commonly show green fluorescence; the latest-formed chalcedony in veins and fractures within the wood often appears to have the highest uranium concentration and the brightest green fluorescence. Agate, chalcedony, and silicified wood in terrace gravels in the Los Lunas, New Mexico, area often show this green fluorescence, though usually not exceptionally bright.

Small blebs, layers, and coatings of chalcedony or hyalite opal occur in many localities throughout New Mexico and elsewhere, so that such green-fluorescing mineral material is a common sight both in sedimentary and igneous rocks and in ore deposits. Green-fluorescent patches of hyalite opal can be seen on fractures and in gas cavities in the phonolite sill at Point of Rocks, New Mexico, a well-known locality for microminerals. Minerals such as calcite, aragonite, and gypsum often appear to fluoresce green, but the fluorescence is often, or perhaps always, not actually in the host mineral, but from inclusions or thin coatings of opal; such material has been observed at, for example, the Stevenson—Bennett and Kelly mines (see Figs. 5 and 15 in Modreski 1987) and in the Luis Lopez manganese district, Socorro County.

The intrinsic fluorescence of uranium minerals such as autunite has the same distinctive spectrum and color as that of the uranyl-bearing silica minerals. Some uranyl minerals, such as carnotite, tyuyamunite, and torbernite, are not fluorescent at all. Many, such as autunite and zippeite, have a fluorescence that appears visually as the same yellow-green color as uraniferous agate and opal. Others, such as andersonite and liebigite, have a fluorescence spectrum that is shifted slightly to higher energies (shorter wavelengths) and visually appear blue-green rather than yellow-green.

Worldwide, probably the most brightly fluorescent silica mineral known is hyalite opal from Spruce Pine, North Carolina, which is reported to contain as much as 3,000 ppm (= 0.3 weight percent) uranium (deNeufville et al. 1981). Other examples of chalcedony or opal that fluoresce with moderate brightness (such as "common" opal from Virgin Valley, Nevada) contain several hundred ppm U, and more weakly fluorescent examples typically contain less than 100 ppm. Published analyses were cited and summarized by Modreski (2005); the symposium volume containing that abstract contains many other papers on the occurrence, genesis, and characteristics of silica minerals worldwide. Other general sources of information about the occurrence of fluorescent minerals and quartz minerals from New Mexico include Modreski (1987), Wilbur and Lueth (2000), and Northrop (1959, 1996).

References:

1. Colburn, R., 1999, The formation of thundereggs (lithophysae), privately published on CD-ROM, Deming, NM; see http://www.zianet.com/GEODEKID/CDlnfo.html
2. Dunbar, N. W., and McLemore, V. T., 2000, The origin of rhyolitic spherulites at Rockhound State Park, New Mexico (abs.): New Mexico Mineral Symposium, Nov. 11 & 12, 2000, Socorro, NM, pp. 7-8, and New Mexico Geology, 2001, v. 23, no. 1,p. 22.
3. Gorobets, B. S., Engoyan, S. S., and Sidorenko, G. A., 1977, Investigation of uranium and uranium-containing minerals by their luminescence spectra: Soviet Atomic Energy, v. 42, March 1977, pp. 196-202 [translated from Atomnaya Energiya, 1977, v. 42, no. 3, pp. 177-182).
4. Modreski, P. J., 1987, Ultraviolet fluorescence of minerals: New Mexico Geology, v. 9, no. 2, pp. 25-30, 42. Modreski, P. J., 1996, Origin of chalcedony nodules in rhyolite from the Peloncillo Mountains, Hidalgo County, New Mexico (abs.): New Mexico Geology, v. 18, no. 1, pp. 18-19.
5. Modreski, P. J., 2005, Fluorescence of cryptocrystalline quartz and opal; in Kile, D., Michalski, T., and Modreski, P. (eds.), Symposium on agate and cryptocrystalline quartz, Sept. 10-13, 2005, Golden, Colorado, pp. 98-102.
6. Northrop, S. A., 1959, Minerals of New Mexico, 2nd ed.: University of New Mexico Press, Albuquerque, 665 PP.
7. Northrop, S. A., 1996, Minerals of New Mexico, 3rd ed., revised by F. A. LaBruzza: University of New Mexico Press, Albuquerque, 346 pp.
8. Wilbur, D. E., and Lueth, V. W., 2000, An update on the fluorescent minerals of New Mexico (abs.): New Mexico Mineral Symposium, Nov. 11 & 12, 2000, Socorro, NM, pp. 13-14, and New Mexico Geology, 2001, v. 23, no. 1, pp. 23-24.
9. deNeufville, J. P., Kasdan, A., and Chimenti, R. J. L., 1981, Selective detection of uranium by laser-induced fluorescence: a potential remote-sensing technique. 1: Optical characteristics of uranyl geologic targets: Applied Optics, v.20, no. 8, pp. 1279-1296.