New Mexico Mineral Symposium — Abstracts

Minerals that fluoresce under ultraviolet (UV) light are of interest for several reasons. They are a curiosity and an aesthetic attraction to collectors. The fluorescence can be a useful aid in recognizing and identifying minerals; it may provide otherwise hidden information about a mineral's trace-element content and crystal chemistry. Finally, the existence or distribution of fluorescent minerals in a given area may provide clues to the presence, size, zoning, composition, and genetic history of a mineral deposit.

Fluorescent minerals in New Mexico can be grouped by occurrence: 1) minerals in igneous rocks, especially pegmatites; 2) ore and gangue minerals in base- and precious-metal deposits, including skarns; 3) secondary uranium minerals from the Grants mineral belt-Colorado Plateau area; and 4) low-temperature carbonate, sulfate, and silica minerals that occur as nodules, veins, or coatings in sedimentary and volcanic rocks.

Many of the pale-colored, largely iron-free minerals of igneous rocks fluoresce, though often weakly; these include sodium and potassium feldspars, sodalite, zircon, and apatite. Fluorapatite from the Harding pegmatite, Taos County, is notable for its yellow fluorescence (due to manganese, Mn⁺²) under short¬wave (SW) UV, which us brightest in pale gray to tan apatite but nearly absent in darker, blue apatite. Eucryptite, an uncommon and inconspicuous mineral at the Harding pegmatite, is most easily located by its crimson fluorescence. At the Globe pegmatite near Petaca, Rio Arriba County, pods of green fluorite fluoresce pale violet under long-wave (LW) UV but strong greenish white SW. This differs from the typical europium (Eu⁺²)- activated fluorescence (violet LW, weak violet SW) seen in most fluorite and is due to the greater abundance of rare-earth elements (REE) in addition to Eu in fluorite from the Globe pegmatite (about 1 wt. % Y and about 0.1 wt. % or more each of several other REEs).

Ore minerals that fluoresce include scheelite (blue to yellowish white SW) from numerous localities, such as the Ortiz gold mine, Santa Fe County; the Dalton Canyon area, Sante Fe County (northwest of Pecos); Iron Mountain, Sierra County; and the Victorio district, Luna County. Powellite (yellow SW) is also reported from Iron Mountain. Yellow, cadmium-bearing "turkey-fat" smithsonite from the Empire mine, Grant County, fluoresces orange to red (LW and SW) as described by R. W. and D. L. Graeme at the 1984 New Mexico Mineral Symposium. Pale-colored, iron-poor sphalerite from many worldwide localities fluoresces orange (LW); such sphalerite is uncommon in New Mexico though Northrop (Minerals of New Mexico, 1959) noted "museum specimens of fluorescent sphalerite" from the Black Range. Willemite, common in the oxidation zone of zinc-bearing ore deposits in the southwestern United States, is sometimes fluorescent (green, SW). Occurrences of green-fluorescent willemite include Iron Mountain, Socorro Peak, the Tres Hermanas Mountains, and Hillsboro and elsewhere in the Black Range. Other secondary ore minerals that commonly fluoresce include hydrozincite (blue SW) and cerussite (yellow white, strongest LW); they occur at the Blanchard and Kelly mines, Socorro County, and elsewhere. Hemimorphite commonly fluoresces faintly or not at all, but some (verified by XRD from the Carnahan mine, Santa Fe County, and the Waldo mine, Socorro County) does fluoresce fairly bright blue white (SW). Coarsely crystalline fluorapatite intergrown with quartz, biotite, chalcopyrite, and potassium feldspar in a mineralized quartz-monzonite breccia pipe at the Copper Flat deposit, Sierra County, fluoresces pinkish white SW, indicative of a high REE content. Fluorite, abundant in many districts throughout the southern half of New Mexico, often has a strong violet fluorescence (LW); generally, the darker the color of the fluorite, the weaker the fluorescence. Calcite is a common fluorescent gangue mineral in many mining districts--Magdalena, Kingston, Tres Hermanas, and Luis Lopez, to name a few. Calcite most commonly fluoresces red (SW) due to Mn⁺² but may range to blue, yellow, or white (activators unknown).

Fluorescent uranium minerals reported from the state include uranyl phosphates and sulfates (such as autunite or meta¬autunite, and zippeite) that fluoresce yellow green (strongest SW), and uranyl carbonates (such as andersonite and liebigite) in which the color of luminescence is shifted to bluish green. Many other uranium minerals (carnotite, torbernite, uranophane) are essentially nonfluorescent.

Other minerals of low-temperature origin include many occurrences of green-fluorescent, slightly uraniferous chalcedony (including agate and silicified wood) and opal. Pink chalcedony "roses," which weather out of rhyolite in the Apache Creek area of Catron County, have a particularly bright green fluorescence (strongest SW). Calcite in sedimentary rocks may fluoresce red, white, or other hues, or not at all. Hot-spring or cave calcite deposits (travertine) commonly fluoresce and phosphoresce white or near white (LW and SW). Aragonite or calcite (such as that from the Tres Hermanas Mountains, Luna County) may fluoresce green, but in many (most?) cases, the fluorescence is due to an otherwise imperceptibly thin layer of opal deposited atop the carbonate minerals. Thin coatings of green-fluorescent opal on fracture surfaces are common in many kinds of rock and in many ore deposits. Calcite as caliche, deposited by rising capillary water on loose, fractured, or porous rock in or below the soil, is one of the most widely distributed fluorescent minerals. It forms chalky-white crusts that may fluoresce various shades of orange, yellow, or white; this luminescence is controlled by mechanisms that so far remain a mystery. Other fluorescent minerals of sedimentary or diagenetic origin reported from New Mexico include barite, gypsum, langbeinite, thenardite, and trona.

One example of a locality only recently examined for fluorescent minerals is the Point of Rocks phonolite sill in Colfax County. Fluorescent minerals discovered here include villiaumite (dark red to orange and yellow, SW and LW); sanidine (violet red SW); sodalite (orange LW); polylithionite (dull yellow SW); searlesite (green SW); cancrinite (green, white, violet, or pale orange SW and LW, probably due sometimes to inclusions or coatings); lovdarite (newly identified by XRD and microprobe; brilliant green SW, weaker LW); coatings of green-fluorescent opal on fracture surfaces and in vesicles; secondary calcite and aragonite (blue white to violet white SW and LW); and several additional minerals, recognizable because of their fluorescence but as yet unidentified. A brownish-orange hydrocarbon film, which coats crystals in some vugs, fluoresces orange LW and SW. This locality--as yet only incompletely examined--is an example of an area where close scrutiny of mineralized rock under ultraviolet light is likely to lead to the recognition of additional minerals.