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Geology topics

P.J. Modreski

Publications and source records attributed to P.J. Modreski.

14 recordsLinked to original sources
USGS2001

Geochemical and mineralogical studies of dinosaur bone from the Morrison Formation at Dinosaur Ridge

The dinosaur bones first discovered in 1877 in the Upper Jurassic Morrison Formation at Morrison, Colorado were the first major find of dinosaur skeletons in the western U.S. and led to the recognition of four new dinosaur genera (Apatosaurus, Allosaurus, Diplodocus, and Stegosaurus). Eight articles dealing with these bones which appeared as research reports in the annual reports of the Friends of Dinosaur Ridge from 1990-1999 are condensed and summarized with some additional comments. Two of the articles are about the mineralogy and preservation of the bones; two are about the physical description of the bone occurrence; two are about the history of the site, and two are about use of novel instrumental methods (ground-penetrating radar and a directional scintillometer) to search for new bones.

Mountain Geologist↗
USGS1990

Mineralogy and geochemistry of altered rocks associated with Lemitar carbonatites, central New Mexico, U.S.A.

The intrusion of more than 100 Cambrian-Ordovician carbonatite dikes caused minor alteration of Proterozoic granitic and mafic rocks in the Lemitar Mountains, although hematization, carbonatization and fenitization caused extensive alteration locally. Ampibolites within 15-20 m of the carbonatite dike contacts were highly altered by carbonatization. Locally the Lemitar diorite/gabbro adjacent to some carbonatites were altered in a thin, discontinuous zone by sodic-potassic fenitization. The granite at Polvadera Peak was locally altered by potassic fenitization. Most of the altered rocks have been further altered by hematization and carbonatization. The altered rocks show increases in loss on ignition and in one or more the elements Ca, K, Na and Al. Only granitic fenites in the Lemitar Mountains lost silica and some diorite/gabbro fenites gained silica. Petrologic and geochemical studies clearly indicate that the types and degree of alteration differ notably in different rock types. The fenitizing fluids were oxidizing, slightly acidic, high in volatiles (especially CO2) and possibly fairly low in alkali elements. ?? 1990.

LITHOS↗
USGS1987

Shocked quartz in the Cretaceous-Tertiary boundary clays: Evidence for a global distribution

Shocked quartz grains displaying planar features were isolated from Cretaceous- Tertiary boundary clays at five sites in Europe, a core from the north-central Pacific Ocean, and a site in New Zealand. At all of these sites, the planar features in the shocked quartz can be indexed to rational crystallographic planes of the quartz lattice. The grains display streaking indicative of shock in x-ray diffraction photographs and also show reduced refractive indices. These characteristic features of shocked quartz at several sites worldwide confirm that an impact event at the Cretaceous-Tertiary boundary distributed ejecta products in an earth-girdling dust cloud, as postulated by the Alvarez impact hypothesis.

Science↗
USGS1984

Copper and cobalt in aquatic mosses and stream sediments from the Idaho Cobalt Belt

Samples of stream sediments and aquatic mosses were collected from nine sites across several mineralized zones at the southeasternmost extension of the Idaho Cobalt Belt. Because the steepness of the terrain and the attendant high flow rate of the streams made it difficult to obtain adequate sediment samples, mosses were considered as an alternative sampling medium. The results not only showed that the Cu and Co content of the mosses correlated almost perfectly with that of the sediments, but that the contrast between samples taken from mineralized and background areas was greater in mosses, especially for Co. Maximum concentrations of 35,000 μg/g Cu and 2000 μg/g Co were observed in the ash of mosses, compared to maximum concentrations of 1700 μg/g and 320 μg/g, respectively, in the associated sediments. Species identification was considered unimportant, which should dispel some reluctance to use mosses in mineral exploration.

Journal of Geochemical Exploration↗
USGS1984

Mineralogic evidence for an impact event at the Cretaceous-Tertiary boundary

A thin claystone layer found in nonmarine rocks at the palynological Cretaceous-Tertiary boundary in eastern Montana contains an anomalously high value of iridium. The nonclay fraction is mostly quartz with minor feldspar, and some of these grains display planar features. These planar features are related to specific crystallographic directions in the quartz lattice. The shocked quartz grains also exhibit asterism and have lowered refractive indices. All these mineralogical features are characteristic of shock metamorphism and are compelling evidence that the shocked grains are the product of a high velocity impact between a large extraterrestrial body and the earth. The shocked minerals represent silicic target material injected into the stratosphere by the impact of the projectile.

Science↗
USGS1982

Margaritasite: a new mineral of hydrothermal origin from the Pena Blanca uranium district, Mexico.

Margaritasite, (Cs,K,H3O)2(UO2)2V2O8.nH2O (where Cs > K, H3O and n approx 1), a 10.514, b 8.425, c 7.25 A, beta 106.01o, P21/a, Z = 2, is a newly recognized uranium ore mineral named for the Margaritas deposit, Pena Blanca uranium district, Chihuahua, Mexico, at which it was discovered. A Cs-rich analogue of carnotite, margaritasite is the natural equivalent of synthetic Cs-uranyl vanadate (A.M. 43- 799, 50-825). A fine-grained yellow mineral, it is most easily distinguished from carnotite by XRD; X-ray powder patterns (CuKalpha radiation) show that the (001) reflection of margaritasite lies at 12.7o (2theta ), while that of carnotite is found at 13.8o (2theta ). The shift of the (001) reflection in margaritasite reflects the structural changes caused when Cs occupies the sites filled by K in carnotite. Synthesis experiments indicate that margaritasite also differs from carnotite in a higher-T hydrothermal origin. Chemical analyses and XRD data for margaritasite and synthetic Cs- carnotite, and chemical analyses for rocks from Sierra Pena Blanca and vicinity, are tabulated.-J.A.Z.

American Mineralogist↗