New Mexico Mineral Symposium — Abstracts

Chalcedony is cryptocrystalline quartz, composed of minute, fibrous microcrystals. Agate is chalcedony with a distinct banded or other pattern. Thus, all agate is chalcedony, but the term chalcedony is generally used for material that is translucent but which has only indistinct or limited banding. Chalcedony and agate most often occur as fillings of gas cavities within volcanic rocks, though they can also form in sedimentary rock and as a replacement of fossil wood. In “thundereggs,” agate fills angular, often star-shaped cavities in rhyolite, generally within zones of spherulitic devitrification of the rhyolite.

This talk will describe cryptocrystalline quartz from four localities for chalcedony nodules and “roses”:

• Locality “A,” an occurrence of chalcedony nodules and geodes in rhyolite near Geronimo Pass in the Peloncillo Mountains, Hidalgo County, New Mexico.·
• Locality “B,” an occurrence of chalcedony nodules in Luna County, New Mexico.·
• Locality “C,” of chalcedony “roses” near Apache Creek, Catron County, New Mexico.·
• Locality “D,” a well-known rockhounding area for chalcedony, agate, and fire agate nodules near Round Mountain and Engine Mountain, in the Peloncillo Mountains south of Duncan, Greenlee County, Arizona.

The mode of formation of the chalcedony at these localities is curious and not well understood. Rather than forming concentric layers coating the entire interior surface of a cavity, the chalcedony has been deposited only on a portion of the cavity, appearing to originate from one or several “feeder” sources where silica-bearing water seems to have seeped into the cavity and immediately deposited the chalcedony. Sometimes just a small amount of chalcedony has formed, which may erode out of the rhyolite to form an individual “button”; or, chalcedony originating from several sites within a cavity may grow and coalesce to fill or almost fill it, leaving a small mouth-like opening and a hollow center.

Nodules from the Hidalgo County locality “A” tend to form cup or bowl shapes resembling partial geodes, while those from the Luna County locality more often form flattened “chalcedony roses”. Many of the nodules from all these localities fluoresce green under shortwave ultraviolet light, indicating a trace content of uranium; the fluorescence tends to be quite bright at the Hidalgo County locality, less so and more subdued at the Luna County locality. There is no known explanation for the pink coloration—more common at the Luna County and Catron County localities—though it appears to be more evident in the portions of nodules that were most exposed to sunlight as they rested on the ground after weathering out of the host lava.

We seem to be in a time of extreme rockhound interest in agate and other forms of chalcedony; many books on the topic are being publishing, ranging from “coffee table” picture books of spectacular agates, to those that try to elucidate the origin of agates, nodules, and thundereggs. Recent books by Brzys (2010), Colburn (2008), Lynch and Lynch (2011), Kasper (2012, 2013), and Woerner (2011) have tried to address the origin of agates; Dunbar and McLemore (2000, 2002) described the agate-bearing spherulites at Rockhound State Park, New Mexico; and the present author discussed the nodules from the Hidalgo County locality in several previous presentations (Modreski, 1995, 2005a,b).

References:

1. Brzys, K. A., 2010, Agates Inside Out: Grand Marais, MI, Gitchee Gumee Agate and History Museum, 246 pp.
2. Colburn, R., 2008, The formation of thundereggs (lithophysae), (privately published on CD-ROM), Deming, NM; see http://www.zianet.com/GEODEKID/CDInfo.html
3. Dunbar, N. W., and McLemore, V. T., 2000, The origin of rhyolitic spherulites at Rockhound State Park, New Mexico: 21st Annual New Mexico Mineral Symposium, Socorro, NM, pp. 7–8, and New Mexico Geology, 2001, v. 23, no. 1, p. 22.
4. Dunbar, N., and McLemore, V., 2002. The origin of rhyolitic spherulites at Rockhound State Park: in Gemstone Deposits of Colorado and the Rocky Mountain Region, Abstract Volume, pp. 29–32. See: http://geoinfo.nmt.edu/staff/dunbar/publications/abstracts/Dunbar_gem_and_mineral_ symposium_abstract-2002.pdf
5. Kasper, D., 2012, A Student's Guide to the Genesis of the Lithophysae—Geodes and Amygdules: Kasper Jasper Press, 40 pp.
6. Kasper, D., 2013, A Southwestern Field Guide to the Agates, Jaspers, and Opals, Volume I, The Geology of their Formation, Inclusions, and Structures (revised edition): Kasper Jasper Press, 469 pp.
7. Lynch, B., and Lynch, D. R., 2011, Agates of Lake Superior: Stunning Varieties and How They Are Formed: Cambridge, MN, Adventure Publications, 223 pp.
8. Modreski, P. J., 1995, Origin of chalcedony nodules in rhyolite from the Peloncillo Mountains, Hidalgo County, New Mexico [abs.]: 16th Annual New Mexico Mineral Symposium, 1995, Socorro, NM, pp. 18–19, and New Mexico Geology, 1996, v. 18, no. 1, pp. 18–19.
9. Modreski, P. J., 2005a, Fluorescence of agate and related minerals from New Mexico and the World [abs.]; 26th Annual New Mexico Mineral Symposium, 2005, Socorro, NM, pp. 7–8.
10. Modreski, P. J., 2005b, Luminescence of cryptocrystalline quartz and opal: in Symposium on Agate and Cryptocrystalline Quartz, Program and Abstracts, Golden, Colorado, pp. 98–102.
11. Woerner, Peter, 2011, Geodes and Thundereggs in Rhyolite: Kasper Jasper Press, 420 pp.