Saturday, December 17, 2011



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.

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.


No comments:

Post a Comment