CALCITE SAND CRYSTALS: WYOMING

SAND CALCITE CRYSTALS COLLECTED ~1966 RATTLESNAKE BUTTE, SOUTH DAKOTA.

In a Blog posting dated January 8, 2014, I note information about calcite sand crystals collected several decades ago from an isolated butte in Jackson County, South Dakota.  The crystals are well known to the collecting world and small quantities appear each year on the market.  The specimens are from Rattlesnake Butte (aka Snake Butte or Devil’s Hill), a locality on the Pine Ridge Indian Reservation that is now off limits to most collectors.

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!

In my Blog article I noted that calcite sand concretions are known from only a few other localities around the world, including: the Imperial Valley-Salton Sea area of California; in Monterey County, California, at a locality in Tertiary rocks at Cholame Hills; from Fontainebleau, France; and somewhere in Saudi Arabia.

It then was a complete surprise to see a dealer at the Flatirons Rock and Mineral show in Longmont who was offering a few nice sand calcite specimens, from an “old collection”.  The only collecting locality noted was from near Wheatland, Wyoming.

The mention of Wheatland jogged something in the far recesses of my mind since the area was mapped, back in the 1960’s, by an  acquaintance of mine:  Geology of the Fort Laramie Area, Platte and Goshen Counties, Wyoming (Laura McGrew).  So, I dug out the publication and there it was, in black and white: In some areas, crystalline calcite has formed authigenically around sand grains, resulting in calcite sand crystals. Some of these crystals are as much as three-fourths of an inch in diameter and 2 inches in length… The upper unit of the Arikaree formation, which is exposed in the northwest corner of the map area, is about 200 feet thick and consists of fine- to medium-grained soft massive orange-gray sandstone. Small limy spherical concretions covered with irregular, partly formed calcite sand crystals also are common.

So, it now appears that the calcite sand crystals from Wyoming occur in rocks similar to the “type locality” at Rattlesnake Butte in South Dakota.  The “find” at the show was just a bit of serendipity.

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.

McGrew, L., 1963, Geology of the Fort Laramie Area, Platte and Goshen Counties, Wyoming: US Geological Survey Bulletin 1141-F.
 

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

SAND CALCITE CRYSTALS FROM GOSHEN HOLE, WYOMING.  LARGE CRYSTAL IS ~5.5 CM. 

EXOTIC WATERFALL IN COLORADO

Rifle Falls State Park is located a few miles northeast of the Garfield (CO) County city of Rifle.  The park is a small area, as state parks go, with ~48 acres, but has some spectacular scenery anchored by the falls themselves. The area surrounding the falls is a lush oasis of exotic looking vegetation, cascading waterfalls, and crystal clear streams.  The Leadville Limestone (Mississippian in age) is the bedrock at the falls with East Rifle Creek providing the water.  The Leadville is quite porous (in other parts of the state the Leadville is the host rock for metallic deposits) and numerous small caves may be observed in the Park.  Ground and surface water flowing over and through the Leadville has dissolved parts of the limestone and the water has become saturated with calcium carbonate.  As East Rifle Creek cascades, ~70 feet, over the Leadville Limestone cliff, a gas, carbon dioxide, is released and the chemistry of the water is changed and calcium carbonate is rapidly deposited in the form of travertine, a dense, usually banded, sedimentary rock. It appears that the Creek has changed course many times over the past centuries as the travertine wall extends many hundreds of feet on either side of the current falls.

CONICHALCITE: A PRETTY GREEN ROCK

SPECIMEN OF CONICHALCITE, A GREEN BOTRYOIDAL COPPER ARSENATE.

Back in the 1980’s I was enjoying the geology of western Utah, looking at the rocks, and tracking down rumors of fossils.  In fact, I did find a nice locality of Miocene vertebrates just across the state line in Nevada and described these (Roglove Fauna) in a professional publication.   The fossiliferous  beds were located south of Wendover, Nevada/Utah—the town is located along I-80 in both states with the casinos in Nevada--- and then west of Ibapah, Utah, on roads and trails.  During the excavation of these fossils my crew always liked to explore the countryside and who could resist a town by the name of Gold Hill?  In fact, Gold Hill became even more exciting when we learned it really was a ghost town!  There are a few human occupants left but also a number of old mines and abandoned buildings.  It seems as if travelers heading west to California discovered the precious metal ~1858 and “Gold District Clifton” was established in ~1869.  Gold Hill continued to grow and became a “town” in ~1892 when a stamp mill was constructed.

The information sign at Gold Hill notes that the usual gold, silver, copper and lead were initially mined but when those minerals became depleted, the boom was over and the town became a ghost.  During World War I, arsenic was in demand as insect control for creatures devastating the cotton fields of the southern U.S.  So, Gold Hill opened up again and mined arsenates such as adamite [zinc arsenate hydroxide, Zn₂ (AsO₄)(OH)], austinite [calcium zinc arsenate hydroxide, CaZn(AsO₄)(OH)] and conichalcite [copper calcium arsenate hydroxide, CaCu(AsO₄)(OH)].  After the war, the need for Gold Hill arsenic waned as foreign sources offered less expensive material (sound familiar) and the boom was over. But, in World War II the U. S. had a need for tungsten to help harden the massive amounts of steel needed for the war effort.   So, mining begin for ores such as wolframite [iron manganese tungstate, (Fe,Mn)WO₄], scheelite [calcium tungstate, CaWO₄], ferberite [FeWO₄] and hübnerite [MnWO₄].  Again, after the war, the “need” for tungsten became less as other sources emerged.  Today, most of the action in the Gold Hill area is exploration for its namesake—gold.  And, for mineral collectors since nearly 70 amazing species have been identified from the mines (http://www.rocks4u.com).

Gold Hill is located along the west side of the Deep Creek Mountains where peaks reach 12,000 feet—Ibapah Peak at 12,087 and Haystack at 12,020.  They are major topographic features in western Utah.  The range has a Precambrian core surrounded by Paleozoic sedimentary rocks intruded by Tertiary intrusions—mostly quartz monzonite and granite.  The mineralization at Gold Hill is associated with the intrusions.

A beautiful mineral collected from Gold Hill is conichalcite, a bright green, often botryoidal, copper arsenate that usually appears as an encrustation.  Of course, the green color is due to copper and the mineral usually occurs in the oxidation zone, as an alteration production.   As I understand it, and that may be a big stretch, conichalcite (copper arsenate) is in solid solution with austinate (zinc arsenate) and duftite (lead arsenate), with cobaltaustinate (cobalt arsenate), and with tangeite, AKA calciovolborthite (copper vanadate).  All that I really know is that conichalcite is a “pretty” mineral that I hope to recollect at Gold Hill!  

Gold Hill ca. 2007.  Photo courtesy  Mark Hufstetler.