Rockhound State Park
Rockhound State Park is a state park of New Mexico United States, located 7 miles (11 km) southeast of Deming, it is named for the abundance of minerals in the area, and visitors can search for quartz crystals, geodes, jasper, perlite, and many other minerals. The park is in the Little Florida Mountains a range of low mountains that have been said to be sky islands due to the blue sky and arid desert between the peaks.
It was established in 1966 as the first park in the United States that allowed collecting of rocks and minerals for personal use. Each visitor can collect as much as 15 lb. of rocks and minerals from the 1,100-acre park; mineral dealers are not allowed to collect for sale. The Florida and Little Florida Mountains are typical of the mountain desert throughout southern New Mexico and Arizona. Elevations range from 4,400 ft along the foothills, where the state park is located, to 7,448 ft at Florida Peak in the Florida Mountains. (Links below for fields guides for rockhounding new Mexico below)
Geology
Manganese-oxide and fluorite veins that cut the fanglomerate of Little Florida Mountains in the northeastern part of the Little Florida Mountains were formed. These deposits are present where various manganese- and iron-oxide minerals, along with fluorite, barite, calcite, and quartz, are found in the fanglomerate of Little Florida Mountains
Hydrothermal fluids that contained high concentrations of manganese and fluorite, along with silica, also formed these mineral deposits. The Manganese mine was one of the larger producing mines in the district. The mines are extremely unsafe, and you should not enter the adits. Care is needed around the shafts and prospect pits as well. Fluorite production from epithermal fluorite veins is estimated as 13,428 short tons, mostly from the Spar mine (McAnulty, 1978). Manganese production from epithermal manganese veins is reported as 19,527 long tons of ore and 21,393 long tons of concentrate (Farnham, 1961; McLemore et al., 1996). Production of manganese ceased in 1959 when the Federal government ended its buying program.
Formation of thundereggs and geodes
Gray perlite, thundereggs, geodes, jasper, onyx, agate, crystalline rhyolite, Apache tears (obsidian), and quartz crystals are among the more common rocks and minerals found in the park. Thompsonite, a zeolite, is found in amygdules in quartz latite (Northrop and LaBruzza, 1996). Agate is present in a wide range of colors and is one of the minerals that you may collect at Rockhound State Park. Some thundereggs and geodes found at Rockhound contain multicolored agate in addition to well-formed quartz crystals.
Many thundereggs found at Rockhound State Park are spherical and consist of two distinct parts: a dark gray to pinkish outer part and a white, blue, or gray inner part, or core, which is recognizable as agate, chalcedony, and quartz crystals, all forms of the compound SiO. In many samples, these two parts can be described as a shell and a filling. However, some thundereggs, or spherulites, do not contain the filling; they are composed of solid dark gray to pinkish shell material or are partly hollow.
In order to better understand the processes by which the thundereggs form, samples from Rockhound State Park were examined using a specialized microscope called an electron microprobe.
Microprobe examination of the "shell" portion of Rockhound spherulites show that they are composed of intergrown crystals of quartz (SiO2), alkali feldspar (K, Na) [AlSi3O8], plagioclase feldspar Na [AlSi3O8]-Ca [Al2Si2O8], and magnetite (Fe3O4). The images from a microprobe show that the spherulites are formed either of intimately intergrown quartz, feldspar, and magnetite or of bands of quartz systematically interspersed with bands of intergrown feldspar and quartz. This banding produces the concentric structure that is apparent in some spherulites. Quartz veinlets crosscutting the banded structure are also observed.
Another type displays feathery and non-equant shapes demonstrated by the crystals in the spherulites. Many spherulites found within the rhyolitic lava are hollow or partially hollow and/or contain a filling of banded agate, chalcedony, and quartz crystals surrounded by the dark gray to pinkish outer shell. The agate, chalcedony, and quartz veins and open-space-fillings within voids in the spherulites formed later by multiple cycles of hydrothermal fluids. The hydrothermal fluids move through fractures in the rocks, which crosscut the igneous textures, and form veins or banded agate, chalcedony, and quartz. Some of these fluids seep through microscopic pores and into spherulites and gas pockets in the volcanic rocks, and they precipitate crystals along the walls of the cavity, forming geodes and geode-like spherulites. Different temperatures and fluid compositions would account for the variety of textures found within any given thunderegg or geode. The banding found within some spherulites and geodes consists of multiple layers of different colored agate, chalcedony, and locally quartz, and may have been formed by fluids supersaturated in silica (Fournier, 1985a). These supersaturated fluids are unstable and quickly deposit thin layers of chalcedony or amorphous silica, typically at lower temperatures (<200° C). The different colors of the bands are a result of trace amounts of impurities, such as iron (red), manganese (black, pink), cobalt (blue, violet-red), copper (green, blue), chromium (orange red), nickel (green), etc. Faceted quartz crystals indicate that the fluids were somewhat supersaturated with silica and that precipitation occurred under relatively slow-changing conditions (Fournier, 1985a).
Not all geodes are spherulites formed by magmatic processes; other natural processes form some geodes. Some of the processes form the hard-outer shell that is characteristic of geodes. The outer shell may be strengthened by the precipitation of some ions that were excluded during crystallization of agate, chalcedony, and quartz and concentrated in the remaining fluid.
Figure 5 – Schematic cross section of formation of tiltage spherulite (modified from Colburn, 1999).
The presence of calcite in some thundereggs and geodes indicates another episode of physical and chemical conditions (Fournier, 1985b). Tiltage thundereggs, locally found at Rockhound State Park, are filled with horizontal layers of agate and chalcedony that are overlain by concentric-banded agate and chalcedony; the contact between the layered and banded agate resembles an angular unconformity.
Other events can modify or even destroy thundereggs and geodes. As the spherulite forms, the expanding magmatic vapor can rupture and break the spherulite, forming fractures that may later fill with agate, chalcedony, or quartz and form vein-like features instead of thundereggs (Colburn, 1999).
Lapidary
The beauty of a thunderegg or geode is locked inside the hard, rough-textured outer shell. Lapidary is done by cutting the geode in half, some cut with the seam some cut against it, ask around or use your own judgement. A flat lap or sanding disk starting with 100 grit then 200, 400, 600 and then polish is the standard practice. Some have used a hard block of would glued with white glue or carpenters glue to have more to grip the vice with, I have not tried this on geodes, but it has worked for me on other stones with a flat side to glue. I usually cut my geodes as close to center as I can get with a good grip in the vise. Thundereggs and geodes cannot be distinguished from one another until they have been cut or broken apart. All it takes to look for thundereggs and geodes is a rock hammer, pick, shovel, brush, chisel, collecting bag or pack, and persistence and patience.
“Remember Rockhounds don’t Die They petrify”
Join me on the Radical Rocks Podcast, our Blog or social media below, also see our affiliate links below that help support our mission to keep Rockhounding and lapidary arts alive and thriving. Feel free to contact me with any suggestions on this blog or any other ideas you may have. If you would like to sponsor in some way, or have your club, book, or mine, spotlighted in Radical Rocks, Blog, Podcast or Video’s feel free to contact Shane with Radical Rocks.
Affiliate/referral links
A Guide to RockHounding New Mexico https://amzn.to/2SxVni2
Rockhounding New Mexico https://amzn.to/2vkO2ub
New Mexico’s Rock’s and Minerals https://amzn.to/2HdrpKQ
Fundrise, become a real estate investor for only $500 I have for months now and it’s been earning an average of 10% dividends. https://fundrise.com/i/33o5g
Reference
Picture credits and referances. https://geoinfo.nmt.edu/tour/state/rockhound/home.html
Akizuki, M., 1983, An electron microscopic study of anorthoclase spherulites: Lithos, v. 16, no. 4, pp. 249–254.
Clemons, R. E., 1982, Geology of Florida Gap quadrangle, Luna County, New Mexico: New Mexico Bureau of Mines and Mineral Resources, Geologic Map 52, scale 1:24,000.
Clemons, R. E., 1984, Geology of Capitol Dome quadrangle, Luna County, New Mexico: New Mexico Bureau of Mines and Mineral Resources, Geologic Map 56, scale 1:24,000.
Clemons, R. E., 1998, Geology of the Florida Mountains, southwestern New Mexico: New Mexico Bureau of Mines and Mineral Resources, Memoir 43, 112 pp.
Clemons, R. E., and Brown, G. A., 1983, Geology of Gym Peak quadrangle, Luna County, New Mexico: New Mexico Bureau of Mines and Mineral Resources, Geologic Map 58, scale 1:24,000.
Clemons, R. E., Christiansen, P. W., and James, H. L., 1980, Southwestern New Mexico: New Mexico Bureau of Mines and Mineral Resources, Scenic Trips to the Geologic Past, no. 10, 119 pp.
Colburn, R., 1999, The formation of thundereggs (lithophysae): Robert Colburn, CD-ROM, 385 pp.
Davis, B. K., and McPhee, J., 1996, Spherulites quench fractures and relict perlite in a Late Devonian rhyolite dike, Queensland, Australia: Journal of Volcanology and Geothermal Research, v. 71, pp. 1–11.
Dunbar, N. W., Jacobs, G. K., and Naney, M. T., 1995, Crystallization processes in an artificial magma: Variations in crystal shape, growth rate, and composition with melt cooling history: Contributions to Mineralogy and Petrology, v. 120, pp. 412–425.
Farnham, L. L., 1961, Manganese deposits of New Mexico: U.S. Bureau of Mines, Information Circular 8030, 176 pp.
Fournier, R. O., 1985a, The behavior of silver in hydrothermal solutions; in Berger, B. R., and Bethke, P. M. (eds.), Geology and geochemistry of epithermal systems: Reviews in Economic Geology, v. 2, pp. 45–62.
Fournier, R. O., 1985b, Carbonate transport and deposition in the epithermal environment; in Berger, B. R., and Bethke, P. M. (eds.), Geology and geochemistry of epithermal systems: Reviews in Economic Geology, v. 2, pp. 63–72.
Jacobs, G. K., Dunbar, N. W., Naney, M. T., and Williams, R. T., 1992, In-situ vitrification: Observations of petrological processes in a man-made magmatic system: EOS, Transactions of the American Geophysical Union, v. 73, pp. 401–411.
Kiely, J. M., and James, W. C., 1988, Diagenesis of the fanglomerate of Little Florida Mountains (Miocene), southwestern New Mexico; in Mack, G. H., Lawton, T. F., and Lucas, S. G. (eds.), Cretaceous and Laramide tectonic evolution of southwestern New Mexico: New Mexico Geological Society, Guidebook 39, pp. 175–184.
Lasky, S. G., 1940, Manganese deposits in the Little Florida Mountains, Luna County, New Mexico; a preliminary report: U.S. Geological Survey, Bulletin 922-C, pp. 55–73.
Lofgren, G., 1970, Experimental devitrification rate of rhyolitic glass: Geological Society of America, Bulletin, v. 81, pp. 553–560.
Lofgren, G., 1971, Spherulitic textures in glassy and crystalline rocks: Journal of Geophysical Research, v. 76, no. 23, pp. 5635–5648.
McAnulty, W. N., 1978, Fluorspar in New Mexico: New Mexico Bureau of Mines and Mineral Resources, Memoir 34, 64 pp.
McLemore, V. T., 1996, Copper in New Mexico: New Mexico Geology, v. 18, pp. 25–36.
McLemore, V. T., Sutphin, D. M., Hack, D. R., and Pease, T. C., 1996, Mining history and mineral resources of the Mimbres Resource Area, Doña Ana, Luna, Hidalgo, and Grant Counties, New Mexico: New Mexico Bureau of Mines and Mineral Resources, Open-file Report 424, 252 pp.
Northrop, S. A., and LaBruzza, F. A., 1996, Minerals of New Mexico: University of New Mexico Press, Albuquerque, New Mexico, 356 pp.
Shaub, B. M., 1979, Genesis of thundereggs, geodes, and agates of igneous origin: Lapidary Journal, v. 32, pp. 2340–2354, 2548–2566.
Taylor, B. E., 1986, Magmatic volatiles: isotopic variation of C, H, and S; in Valley, J. W., Taylor, H. P., and O'Neil, J. R. (eds.), Stable isotopes in high temperature geological processes; Reviews in mineralogy: Chelsea, Michigan, Mineralogical Society of America, pp. 185–225.
Weber, R. H., 1980, Rockhound: New Mexico Geology, v. 2, pp. 59–60.
Comments
Post a Comment
Thank you for your comment. I will review content for spam and such.