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Other Minerals
Calcite: CaCO3
Calcite at Stoneham ranges from pale yellow to deep honey-colored; solid, inter-grown layers tend to appear a deeper brownish yellow. Distinct crystals of calcite form rhombohedra, sometimes modified or distorted, and typically 2-4 mm in size. Some of the calcite in pocket #2 appeared white rather than yellow, creating the visual effect of “snow” resting on the blue barite. The white color was later found to be due to a thin layer of rough-surfaced, microbotryoidal opal encrusting the calcite; the underlying calcite is the normal pale yellow color.
Calcite, 2-mm rhombohedral crystals. SEM photograph, 18x.
“White” calcite from pocket #2; 1-2-mm rhombohedral crystals coated with opal. SEM photograph, 35x.
Montmorillonite: [(Na,Ca)0.3(Al,Mg)2Si4O10(OH)2.nH2O]
X-ray diffraction indicates that the principal clay mineral in the host rock is montmorillonite. This clay mineral has formed through alteration by ground water of the glass shards in volcanic ash. The volcanic glass shards in this bed are nearly completely altered to clay, whereas most are quite fresh in the overlying beds.
Opal: SiO2.nH2O
Small amounts of opal are present as a late deposit on the surface of barite crystals and more commonly on calcite. The opal is most noticeable because of its fluorescence under UV radiation; brilliant green under short-wave UV, and faint green under long wave. The “white” calcite in pocket #2 appears as white as it does because of a thin layer of lustrous sparkling opal covering the calcite. The opal (nonprecious, with no opalescent colors) is colorless, glassy, and transparent; it can be termed hyalite. In a microscopic level it has a botryoidal morphology. The green fluorescence of hyalite opal is caused by traces of uranium as the uranyl ion, UO2+2, the presence of uranium in the Stoneham opal on calcite was confirmed by KEVEX EDX-XRF analysis.
Detail of botryoidal surface of opal coating on calcite; opal spherules are 50-100 microns. SEM photograph, 150x.
GEMSTONES:
Stoneham barite can be suitable for cutting into gemstones as a specialty collector’s item; the Denver Museum of Natural History has 4.81- and 9.45-carat light blue stones from Stoneham in its collection, and the American Museum of Natural History has a 5.12-carat Stoneham barite on display in the Morgan Hall of Minerals and Gems (Modreski, 1989). Dennis Wilson has a 17.5-carat round-cut blue stone from barite he collected at Stoneham early in 1989 faceted by Jerrald Hess.
Current Status
The 1989 excavation disturbed relatively little ground. The barite- and calcite-bearing fault zone was the only feature that was prospected. This fault was followed completely down to the contact with the Pierre Shale and – in the area of this excavation only – will probably not yield any specimen material in the future. However east of the excavation, the fault appeared to continue back into the hillside. This posed the problem of extensive overburden removal; it was therefore left undisturbed. The possibility exists that deeper into the hillside, along the continued intersection of the fault with the horizontal plane on which the two major pockets were encountered, still other unexposed pockets may exist.
Removal of about 21 meters of overburden would be necessary, a task for which equipment used on the 1989 dig would be inadequate. Minor pockets, of course, may exist at higher levels along the main fault zone or on subsidiary faults; other areas along the projected strike of the fault to the southeast and northwest might contain additional pockets nearer to the surface.
Many collectors over the years have dug by hand along the slope of the escarpment west of the present dig and have found generally small isolated pockets with individual or inter-grown crystals. This area is near or in the footwall of the projected trend of the fault zone, and barite crystal fragments are commonly found loose on the ground surface. New discoveries may come from this area.
About 45–90 meters to the north and east of the 1989 dig, a projecting “finger” of the ridge, composed of the Chadron Formation lying atop the Pierre Shale, extends to the northeast. This is an area that has produced fine barite specimens since the 1950s. It is suggested that toward the lower part of the Chadron clay beds, near the contact with the Pierre Shale, barite pockets may be more plentiful. A larger lobe of land, northwest of the 1989 dig and forming the western rim of the “blowout”, forms a low hill composed of the Chadron Formation. Some barite crystal float can be seen in parts of this area, but it has been little prospected by collectors.
Farther afield in this region of Colorado, there are many other possible areas for the occurrence of barite crystals in rocks of the White River Group: along both the north and south sides of the Pawnee Creek drainage, eastward and westward of the present site; and along other drainages running southward from the Chalk Bluffs and the Peetz Table, and bordering the South Platte valley. Workman’s thesis (1964a) describes over one hundred occurrences of barite crystals in eastern Weld and western Logan counties, eight of which he considered major localities.
The barite-producing areas near Stoneham extend for many square miles, and it is certain that many fine pockets will continue to be discovered in the future.
**UPDATE**
The collecting areas on the Niklas Ranch are no longer open or available to collecting. The owners ask that collectors do not come to the ranch, as permission to the property will not be granted, and trespassers will be reported.
References
Collector’s Edge Minerals, Inc. recognizes and thanks Rocks & Minerals for permission to use portions or all of their original article from the “Rocks & Minerals”, Volume 65, Number 3, pp. 202-222. Contributing authors were Peter J. Modreski, U.S. Geological Survey, Denver, Colorado; Bryan Lees CEMI, Golden, Colorado; and Dennis Wilson, Lakewood, Colorado.
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