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Richard A. Sheppard

Publications and source records attributed to Richard A. Sheppard.

18 recordsLinked to original sources
USGS2024

Woolly erionite from the Reese River zeolite deposit, Lander County, Nevada, and its relationship to other erionites

Woolly erionite from the Reese River deposit, Nevada, is identical in appearance to that at the type locality, near Durkee, Oregon. Both of these erionites differ in appearance from all other erionite reported in the past 20 years from diverse rocks throughout the world which are described as prismatic or acicular in habit. The non-woolly erionites are especially common as microscopic crystals in diagenetically altered vitroclastic lacustrine deposits of Cenozoic age. The Reese River woolly erionite fills joints in gray to brownish-gray lacustrine mudstone of probably Pliocene age, in a zone about 1 m thick beneath a conspicuous gray vitric tuff. Compact masses of long, curly, woolly erionite fibers are in the plane of the joint and locally are associated with opal. Indices of refraction are ω = 1.468 and ε = 1.472; hexagonal unit-cell parameters are a = 13.186(2) Å, c = 15.055(1) Å, and V = 2267.1(0.9) Å 3 . A chemical analysis of woolly erionite yields a unit-cell composition of: Na 1.01 K 2.84 Mg 0.3 Ca 1.69 Al 8.18 Si 27.84 O 72 · 28.51H 2 O.

Clays and Clay Minerals↗
USGS1978

Chabazite in siliceous tuffs of a Pliocene lacustrine deposit near Durkee, Baker County, Oregon

A relatively pure chabazite deposit of at least 75 X 10 6 metric tons occurs near the Durkee type locality of erionite. This erionite is in a welded tuff interbedded with lacustrine rocks- of a closed Pliocene basin. The zeolites, chabazite, erionite, clinoptilolite, and other authigenic silicate minerals are confined to a 27-square-kilometer area of altered rocks enclosed in relatively freshwater lacustrine beds. About 135 meters of the lower part of the 350-m stratigraphic section at the southeastern part of the lake basin contain zeolitic tuff. The chabazite is an early product of the reaction between glass of rhyolitic or dacitic composition and alkaline (pH>9) saline lake waters. Erionite, clinoptilolite, analcime, and potassium feldspar follow chabazite in the paragenetic sequence. At the erionite type locality, chabazite and clinoptilolite form megascopic crystal clusters within fractures and cavities in unaltered welded tuff. Elsewhere in the basin, most zeolite crystals in the altered airfall or reworked tuffs are less than 80 micrometers long. Chemical analyses, unit-cell determinations, and mean indices of refraction show that fine-grained chabazite varies in composition and physical parameters.

Oregon↗
USGS1974

Chert derived from magadiite in a lacustrine deposit near Rome, Malheur County, Oregon

Nodules and thin beds of chert occur in the upper part of the informally named Rome beds, about 8 11 km southwest of Rome, Oreg. The chert is in green to gray mudstone, about 8 m beneath a conspicuous gray and yellow zeolitic tuff. The bedded chert contains molds of saline minerals and it grades southward and marginward into nodular chert. The nodular chert is of two varieties: relatively large lobate nodules that have greenish-brown interiors and abundant saline crystal molds, and relatively small lobate nodules that have gray interiors and a surface reticulation. The latter variety is the so-called snakeskin agate prized by lapidaries. Both nodular varieties characteristically have a thin white rind. The cherts are similar, if not identical, to cherts reported from Quaternary lacustrine deposits in eastern Africa and from Pleistocene to Jurassic lacustrine deposits in Wyoming. The Rome chert probably formed from a hydrous sodium silicate precursor such as magadiite during diagenesis. Magadiite is generally a primary precipitate from a highly alkaline lake which is capable of attaining a high concentration of silica in solution.

Oregon↗
USGS1973

Boron-bearing potassium feldspar of authigenic origin in closed-basin deposits

Silicic vitric tuffs in saline, alkaline lacustrine deposits are commonly altered to a variety of zeolites and potassium feldspar. The tuffs generally show a lateral gradation, in a basinward direction, of fresh glass to zeolites and then to potassium feldspar. Zeolites were formed early in diagenesis by reaction of the glass with the interstitial water. The feldspar, however, was formed later by reaction of the zeolites with interstitial water, and its formation can be correlated with water of relatively high salinity and alkalinity. Semiquantitative spectrographic analyses for boron in the zeolites and potassium feldspar show that most of the boron resides in the relatively late feldspar. The boron content of the zeolites is commonly less than 100 ppm, whereas the boron content of the potassium feldspar is commonly greater than 1,000 ppm. Boron apparently substitutes for aluminum in the feldspar structure and causes distortion of the monoclinic unit cell such that the b and c dimensions are shortened. These boron-bearing potassium feldspars having anomalous cell parameters seem unique to saline, alkaline lacustrine deposits and could serve as a prospecting aid for locating buried saline minerals.

Journal of Research of the U.S. Geological Survey↗