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.
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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.
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| 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). |
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.
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.
 |
| BROKEN SELENITE CRYSTALS LITTER THE SURFACE AT THE ST. DAVID LOCALITY. |
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.
 |
| 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.
REFERENCES
CITED
U. S. Department of Agriculture, 2003, Soil
Survey of Cochise County, Arizona: http://soildatamart.nrcs.usda.gov/Manuscripts/AZ671/0/cochise.pdf
