Sand calcite crystals from
These interesting specimens are thought to have been formed by the action of ground water or by spring deposition and are composed of calcite (CaCO3) and coarse wind-blown sand from an ancient dune deposit field. The absence of mud and silt and the well-rounded sand grains, along with wind-etched surfaces, indicates dune origin. The crystals are composed of about 37 % calcite and the rest is mainly sand inclusions. The sand is composed of quartz, orthoclase, microcline, albite, muscovite, biotite, hornblende, augite, tourmaline, zircon, garnet, staurolite, and magnetite (Wanless, 1922). Additionally, volcanic ash, fragments of volcanic glass, and schist can be found found. Some of the minerals present in the sand are from Precambrian pegmatites (tourmaline) and schists (garnet, staurolite, and schist fragments) of the Black Hills (Wanless, 1922).
The process forming these sand calcite crystals is likely a fluctuating water source that floods the unconsolidated sand on an annual basis. That water comes from a source that provides calcium carbonate in solution. This mechanism could be as simple as groundwater, natural springs, or a combination of snowmelt and/or spring rains flowing on and through a limestone that then flows into the unconsolidated sand exposure. This process fills the pore space within the sand completely with water that is close to being saturated with calcium carbonate, having dissolved it at a relatively low temperature (taking advantage of the carbonates inverse solubility with respect to heat).
Then, as summer comes, solar energy heats the pore water while also causing some evaporation. Concentration of calcium carbonate follows and the pore water becomes super saturated. The surface tension of water would hold pores full of solution in some areas instead of allowing a partial emptying of pores. Also the permeability factor is an important part of the mechanism of sand calcite crystal formation. High permeability would allow the pore water to easily flow through an unconsolidated and very loosely compacted sand. Once the well-sorted sand is saturated with water, subsequent evaporation will—at one point—set off precipitation.
Since there are many nucleation sites available around the sand grains, crystallization may proceed rapidly, and as it proceeds it will do so with many growths in parallel orientations. With calcite there are several chances of forming orientations that will lead to a single crystal. When chance creates a different orientation an intergrowth results, and through time—as the crystals grow outward—those orientations really begin to come into play so that intergrowths of interlocked crystals occur. The process stops for each crystal group when the water pockets run out. Then the process can begin again next year. This is likely a rapid process—hours rather than days, or at maximum a few days.
Today the Snake Butte calcite sand crystal locality in South Dakota is a National Natural Landmark and is managed by the Oglala Sioux Parks and Recreation Authority.
View of Snake Butte, a National Natural Landmark designated in 1967. National Park Service Photo.
Wanless, H.R., 1922. Notes On Sand Calcite From South Dakota, American Mineralogist, v. 7, p. 83-86.