Arizona is the home of the various venues associated with the Tucson Gem and Mineral Show and is certainly a reason for visiting the state. However, there are numerous other localities that may afford the recreational collector opportunities to add specimens from the field. One of the better known localities is the St. David Desert Rose area where clusters of the selenite variety of gypsum are available. This classic locality is west and south of the hamlet near the Apache Powder Plant and has usually excellent access (some rumors? float around about the Plant closing access—but it was open during my visit). To get there, go south from Benson on Highway 80 for 5 miles and then turn onto Apache Powder Plant Road. Follow Apache Powder Plant Road for 3.5 miles, passing the Apache Powder Plant on the west side of the road. At 3.5 miles down Apache Powder Plant Road, look for ruts to the west leading towards the hills about a mile away. Follow these ruts to the hills (directions courtesy of Bob’s Rock Shop at http://www.rockhounds.com/rockshop/azsites.html.
Gypsum is a very soft mineral falling at “2” on Mohs Scale of Hardness and you can scratch specimens with your fingernail. Gypsum is a hydrated calcium sulfate with a chemical formula of CaSO4-2H2O meaning that water is present in the mineral. The non-hydrated form of calcium sulfate is a mineral termed anhydrite (CaSO4). Gypsum is well known to most people as “plasterboard”, “drywall” or “plaster of Paris”. The latter term has an interesting name derivation. It seems that Parisians mined gypsum near their city and then heated it to several hundred degrees F (a process they evidently learned from the Romans). Water in the gypsum was released as steam and the resulting residue was in powder form and known as plaster of Paris. This powder was then recombined with water, heat was released, and “new gypsum” was formed. Of course, the “new gypsum” could be molded into a variety of forms before hardening, or could be used as a fire-resistant building material (as in the pre-plasterboard era of lathe and plaster).
Gypsum (massive variety) commonly results from evaporation of saline waters and often is associated with other evaporates such as halite (rock salt, NaCl). Since it has a very low solubility, gypsum is often the first mineral to form from these ocean or lake brines. Other types of gypsum can form near hydrothermal veins or as disseminated crystals precipitated by groundwater. The Permian Period was a great time of continental “drying” as the large oceans of the Paleozoic were disappearing from North America. As a result, massive deposits of evaporatic gypsum are found in several states and form the basis of a quite large “drywall” industry that feeds our appetite for construction.
Gypsum is also quite soluble in water and this fact makes for two interesting features---sinkholes and sand dunes (maybe three---cramped stomachs if you drink it). Because of its easy solubility, gypsum usually does not linger for long periods of geologic time on the surface of the earth—it dissolves by chemical weathering. In dryer climates, sand-size grains are sometimes formed by physical weathering—particles break down into smaller and smaller sizes. With this reasoning in mind, one understands that sand-size particles of gypsum are somewhat rare in the rock record—climate changes are always around the corner. Geologists just cannot locate sandstones composed of gypsum particles; however, there are some modern examples of gypsum sand dunes.
In south-central New Mexico, in the Tularosa Basin, lies White Sands National Monument and the adjacent military reservations, about 275 sq. miles of gypsum sand dunes. The source of the original gypsum: rocks of Permian age (~250 Ma) exposed and eroding in the adjacent mountain ranges (San Andres and Sacramento Mountains). The gypsum chemically weathers from the rocks and is then transported to the Basin via streams running off the mountains. In a more normal situation, streams in the Basin would transport the dissolved gypsum downstream to larger and larger rivers until finally an ocean/sea is reached. However, the Tularosa Basin is an internal basin and does not have an external water outlet. So, the water seeps into the ground in the Basin or flows into shallow lakes such as Lake Lucero (or in the Pleistocene, Lake Otero). Both of these lakes have/had a high rate of evaporation and so the gypsum precipitates/precipitated out along the lake edges. During the late Pleistocene, a very wet time period, Lake Otero filled almost the entire Tularosa Basin but as the climate dried and lake water evaporated, gypsum was deposited. Over the millennia the deposited gypsum physically weathered in this arid climate into sand-sized grains, was picked up by the prevailing western winds, and moved eastward. As the wind moved up the eastern slopes of the Sacramento Mountains, it lost velocity, and therefore carrying power, and deposited the gypsum grains. This wind then, and does, blows the grains into dunes---the largest exposure of gypsum sand dunes in the world. Most likely, even parts of this massive dune field will never make it into the rock record as sandstone. White Sand Dunes was established in 1933 by President Herbert Hoover.
The second interesting geomorphic feature associated with gypsum is the formation of solution features, sinkholes, and flow structures. The Paradox Basin of southwestern Colorado (southwest of the Uncompahgre Range of the Ancestral Rockies) has massive salt anticlines where halite and gypsum have flowed upwards and bent overlying rocks into a dome. On the northeastern side of the range is the Central Colorado Basin which received several hundred feet of evaporates in the late Paleozoic. Near Gypsum, CO, the Eagle Valley Formation of Pennsylvanian age is highly contorted and outcrops are very visible along I-70. The massive gypsum is mined at several localities in the area.
CONTORTED BEDS IN THE LATE PALEOZOIC SEQUENCE OF THE CENTRAL COLORADO BASIN EXPOSED ALONG I-70 NEAR GYPSUM, COLORADO.|
Crystalline varieties of gypsum are most often referred to as selenite (tabular, often colorless and watery crystals), satin spar (fibrous and prismatic crystals), gypsum flowers (needle-like crystals forming bends and curves), alabaster (massive and fine grained), and desert roses (bladed crystals forming rose-like [rosette] patterns, with sand inclusions). Sometimes authors just prefer to combine everything except massive gypsum and alabaster into selenite.
Desert Roses are found at several localities in the U. S. but near St. David they occur as individual clusters and are disseminated in certain sedimentary deposits. The Arizona Roses occur in soft lake bed deposits associated with Pliocene and Pleistocene lakes in what is now the San Pedro River Valley.
The Roses seem to occur in paleosols (ancient soil
horizons) that formed along these lakes and are related to a high water table
that occurred at a time during the development of the soils (USDA, 2003). It
appears that groundwater (the water table) concentrated the calcium sulfate,
which was common in the lake sediments, and these concentrates crystallized
into the mineral gypsum. These crystals
continued to grow in the pore space between the clay and sand particles and in
the process incorporated some of the particles into the crystals. Desert Roses seem to take on the color of the
clay/sand particles—in this case a pink to light reddish orange.
|GYPSUM DESERT ROSE COLLECTED FROM NEAR ST. DAVID, ARIZONA, IN THE COLLECTION, AND COURTESY, OF ROGER WELLER AT COCHISE COLLEGE (WHOSE ROSE IS MUCH BETTER THAN WHAT I COLLECTED).|
Since the rosettes seem to make their way to the surface over long periods of time, the ground surface of the collecting site is littered with tens of thousands of broken gypsum crystals that sparkle brilliantly in the sunshine. To locate unbroken Roses the collectors must dig in the soft sediments. I found that digging in the areas of intense distribution of surface crystals seems to be the most productive; however, it also seemed to be a crap shoot with unpredictable results! The newly dug crystals are quite fragile until they dry and collectors must be extremely careful in packing specimens.A visit to the collecting site also gives the traveler additional opportunities for observing some fantastic geological features. The San Pedro River Valley (the river itself flows north from Mexico) is a graben or fault-block down dropped valley seemingly associated with Basin and Range faulting. The terrace deposits (the benches) represent ancient lake and stream sediments that are exposed due to a series of fairly recent (perhaps last ~2.5 my) episodes of stream down cutting.. The rock and sediments contain a wide variety of Pleistocene (Ice Age) vertebrate fossils.
|BROKEN SELENITE CRYSTALS LITTER THE SURFACE AT THE ST. DAVID LOCALITY.|
|SAN PEDRO RIVER GRABEN SHOWING ANCIENT LAKE AND STREAM SEDIMENTS/ROCKS EXPOSED BY RECENT DOWN CUTTING BY THE RIVER. THE DESERT ROSES ARE FOUND IN THESE SEDIMENTS. THE DRAGOON MOUNTAINS, A HORST OR FAULT-UPLIFTED BLOCK, BOUND THE VALLEY ON THE EAST.|
St. David is a watery oasis in the middle of the desert and numerous springs and lakes are quite evident; irrigated agriculture is noticeable. Certain layers of the ancient lake and stream sediments in the graben seem to confine the ground water running into the valley and it reappears as artesian wells in the bottom of the San Pedro River Valley. Members of the LDS church (Mormons) found the town in the 1870’s because of the abundance of water for growing crops.
U. S. Department of Agriculture, 2003, Soil Survey of Cochise County, Arizona: http://soildatamart.nrcs.usda.gov/Manuscripts/AZ671/0/cochise.pdf