Wednesday, January 8, 2014

CALCITE SAND CRYSTALS: SOUTH DAKOTA



In looking at a couple of Oklahoma barite roses the other day, I was reminded of some very interesting crystals collected many decades ago.  As a student at the University of South Dakota I had the opportunity to observe some great geology in the western part of the State; we were always traveling “West River” to examine rocks of virtually every geological age.  During a summer tenure with the State Geological Survey, I was involved in chasing some high elevation stream gravels coming from the Black Hills and trending eastward.  This ancient river is represented today by sporadic gravel deposits found on top of ridges and mesas.  Since these gravels are most likely earliest Pleistocene (Pliocene?) in age (~2 Ma), their current location on ridge tops indicates much erosion has taken place since deposition.  At any rate, I had the opportunity to explore and collect (with permission) at some very isolated localities in the southwestern part of the state, including Rattlesnake Butte (aka Snake Butte or Devil’s Hill) in Jackson County.
Sandstone layer containing calcite sand crystals on the summit of Rattlesnake (Snake) Butte.  Photo courtesy of U. S. National Park Service.

The western part of South Dakota is unglaciated, is part of the Great Plains Physiographic Province, and has three very distinct regions—the gently undulating plains with a cover of short grasses, the Black Hills, and the eroded badlands.  The Hills are a Laramide anticline and the easternmost range of the Rocky Mountains.  Geologists often refer to the Hills as a “vest pocket” range since their footprint is small (as compared to something like the Sawatch Range in Colorado), the rocks are well- exposed, and large scale thrust faulting is absent.  Precambrian rocks are exposed in the center of the anticline and successively younger Paleozoic, Mesozoic, and Tertiary rocks dip away from the dome.  The best known features of the Hills include Mt. Rushmore National Park and the Homestake Gold Mine (no longer functioning as an active mine).  They are also home to Wind Cave National Park and Jewel Cave National Monument while Devil’s Tower National Monument lies just outside their perimeter.  Harney Peak at 7244 feet is the highest point in the Hills.

Surrounding the Hills on the plains of South Dakota, Colorado, Nebraska and Wyoming are a series of badly eroded Cretaceous and Tertiary badlands.  The exposed Cretaceous rocks are primarily represented by the Pierre Shale (deep water marine), the Fox Hills Formation (near shore to beach representing the final regression of the vast Western Interior Seaway), and then a series of formations representing fluvial to deltaic to swamp to brackish water deposits such as the Hell Creek, Fort Union, and Lance. Some of these units span the Cretaceous-Tertiary (K-T) boundary and several have significant reserves of coal.  The overlying Tertiary rocks include the well-known White River Group of Eocene and Oligocene age overlain by several units of Miocene and Pliocene age.
Rocks of the White River Group exposed at Badlands National Park in South Dakota.  Photo courtesy of The Virtual Fossil Museum.
The best known Tertiary rocks in South Dakota are those of the White River Group exposed along the White River at Badlands National Park south of Kadoka (#152 on I-90) and Wall (#109 on I-90), South Dakota.  However, to me the most interesting areas of Tertiary rocks are found around the perimeter, and external, of the park at a series of tables and buttes such as Porcupine Butte, Sheep Mountain Table and Cuny Table.  Most of these tablelands have rocks of the White River Group at or near the base overlain by prominent escarpments of Miocene rocks.  These cliffs are ledge forming and contain a high percentage of volcanic ash blown in from the west.
Red Shirt Table, Shannon County, South Dakota.  The White River Group forms the “badlands” in the background while Miocene age rocks cap the table.  Photo courtesy of “freyawin” at  http://www.panoramio.com/
One of these buttes in southwestern South Dakota, Rattlesnake Butte, has been known since the 1800’s as a source for calcite sand crystals, a really interesting “member” of the calcite family.  Essentially, the crystals are composed of sand, perhaps ~60%, and calcite, approximately ~40%.  They occur as double terminated, hexagonal scalenohedradon crystals sometimes modified by rhombohedrons.  Crystals range in length from less than one inch to perhaps 15-20 inches (these are rare).  The crystals are found in a bed of coarse sandstone, about three to four feet in thickness, at the top of the butte, probably in the Miocene Arikaree Group.  Wanless (1922) believed the crystals were formed post-deposition by the action of ground water in eolian (wind blown) sands, perhaps in a spring environment with pressure from overlying rock.  Newer studies using petrographic and scanning electron microscopes (Cirone and Law, 2005) indicated that the compaction of the sand grains was minimal and that presumed fluid pressure was higher than normal groundwater flow.  In other words, they seemed to rule out the spring water theory of Wanless but failed to provide an adequate explanation for the crystal formation!  I have searched the recent literature but have been unable to locate additional new information concerning these amazing crystals.  calcite sand crystals collected from Rattlesnake Butte several decades ago.  Small crystal at bottom of photo is ~1.9 cm.
Calcite sand crystals collected from Rattlesnake Butte several decades ago.  Small crystal at bottom of photo is ~1.9 cm.
Other calcite sand concretions are known from only a few localities around the world, including the Imperial Valley-Salton Sea area of California.  Here, the secondary growth of large calcite crystals has cemented together grains of the surrounding sand and “spikes” (non crystal shape) are formed.  Similar sand crystals have recently appeared in Colorado rock and mineral shows from an "old collection" (according to the seller) from a locality in eastern Wyoming (see Blog posting 12-17-2011).   IMonterrey County, California, a locality in Tertiary rocks at Cholame Hills produces very poor calcite sand crystals, many of which are twinned.  Better formed calcite sand crystals are known from Fontainebleau, France; however, these crystals are rhombohedral in nature.  And finally, the folks at Minertown Minerals (www.minertown.com) have for sale a nice specimen of calcite sand crystals from Saudi Arabia that appear to be clones of the South Dakota specimens.  However, I could not find additional information concerning the geology of these crystals.

Now, this added disclosure statement: The Rattlesnake (Snake) Butte calcite sand crystal locality in South Dakota was designated as a National Natural Landmark (NNL in 1967).  The National Park Service acts as a steward for the National Natural Landmarks Program.   Rattlesnake (Snake) Butte is managed by the Oglala Sioux Parks and Recreation Authority and is located on the Pine Ridge Indian Reservation.  It is illegal to collect or sell fossils, artifacts and minerals on reservation land without a permit from the tribe. 

So here is my suggestion---since collecting the South Dakota calcite sand crystals has been illegal since 1967 it might be wise to purchase a specimen on the open market or at your favorite rock shop.  They make quite interesting display specimens and are very unique.  Get them while they last!

REFERENCES CITED

Cirone, A. and E. Law, 2005, Microstructure of Calcite Sand Crystals and Implication on its Crystallization Process (abst.):  Geological Society of America Abstracts with Programs, v. 37, no. 1, p.59.

Wanless, H. R., 1922, Notes on Sand Calcite from South Dakota; American Mineralogist, v. 7, pp. 83-86.

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