Richard A. Sheppard

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) Å3. A chemical analysis of woolly erionite yields a unit-cell composition of: Na1.01K2.84Mg0.3Ca1.69Al8.18Si27.84O72·28.51H2O.РезюмеШерстеподобный эрионит из осадков реки Рииз в Неваде являлся тождественным по виду к тому, который находится около Дуркии в Орегоне. Оба эти эриониты отличаются по виду от всех остальных найденных в последние 20 лет в породах в различных частях мира эрионитов, которые описываются как призматические по поведению. Нешерстеподобные эриониты встречаются особенно часто как микроскопические кристаллы в диагенетически измененных витрокластических озерных осадках кайнозойской эпохы. Шерстеподобный эрионит из реки Рииз выполняет соединения в серых до коричнего-серых озерных иллистых породах, вероятно из плиоценовой эпохы, в зоне около 1 метра толщины под заметным серым стекловипным туфом. Компактные массы длинных закрученных шерстеподобных волокон эрионита находятся в плоскости соединений и местно ассоциируются с опалом. Показатели рефракции равны: ω = 1,468 и ε = 1,472; параметры гексагональной элементарной ячейки: a = 13,186(2) Å, c = 15,055(1) Å, и V = 2267,1 (0,9) Å3. Химический анализ шерстеподобного эрионита дает следующий состав элементарной ячейки: Na1,01K2,84Mg0,3Ca1,69Al8,18Si27,84O72 · 28,51H2O. [E.C.]ResümeeWolliger Erionit von der Reese River Lagerstätte, Nevada, entspricht in der Erscheinungsform dem der Typlokalität in der Nähe von Durkee, Oregon. Diese beiden Erionite unterscheiden sich in der Erscheinungsform von allen anderen Erioniten, die in den letzten 20 Jahren in verschiedenen Gesteinen in der ganzen Welt beschrieben wurden. Alle diese Erionite weisen prismatische oder nadelige Formen auf. Die nichtwolligen Erionite treten vor allem als mikroskopisch kleine Kristalle in diagenetisch umgewandelten, vitroklastischen, lakustrischen Lagerstätten aus dem Känozoikum auf. Der wollige Erionit von Reese River füllt Gänge in einem grauen bis braungrauen lakustrischen Tonstein von wahrscheinlich pliozänem Alter in einer etwa 1 m dicken Zone unter einem auffälligen grauen, glasigen Tuff. Kompakte Massen aus langen, gekräuselten, wolligen Erionitfasern liegen parallel zu den Kluftebenen, gelegentlich zusammen mit Opal. Die Brechungsindizes sind ω = 1,468 und ε = 1,472; die hexagonalen Zellparameter sind a = 13,186(2) Å, c = 15,055(1) Å, und V = 2267,1(0,9) Å3. Eine chemische Analyse des wolligen Erionit ergibt eine Zusammensetzung der Elementarzelle von Na1,01K2,84Mg0,3Ca1,69Al8,18Si27,84O72 · 28,51H2O. [U.W.]RésuméL’érionite laineuse du dépôt de la rivière Reese, Nevada, est identique en apparence à celle de la localité type près de Durkee, Oregon. Ces deux érionites sont différentes en apparence de toute autre érionite rapportée dans les dernières 20 années de roches diverses à travers le monde, et qui est prismatique ou aciculaire de constitution. Les érionites non-laineuses sont particulièrement courantes en tant que cristaux microscopiques dans des dépôts vitroclastiques lacustrins altérés. L’érionite laineuse de la rivière Reese remplit des joints dans une argilite lacustrine grise, probablement d’âge pliocène, dans une zone d’épaisseur d’ environ 1 m, sous un tuff gris vitrique remarquable. Des masses compactes de longues fibres bouclées, et laineuses sont dans le plan du joint et sont localement associeés avec de l’opal. Les indices de réfraction sont ω = 1,468 et ε = 1,472, les paramètres hexagonaux de maille sont a = 13,186(2) Å, c = 15,055(1) Å, et V = 2267,1(0,9) Å3. Une analyse chimique de l’érionite laineuse a rendu une composition de maille de: Na1,01K2,84Mg0,3Ca1,69Al8,18Si27,84O72 · 28,51H2O. [D.J.]

PHILLIPSITE FROM SILICIC TUFFS IN SALINE, ALKALINE-LAKE DEPOSITS

Phillipsite is a common constituent in diagenetically altered rhyolitic vitric tuffs of Cenozoic saline, alkaline-lake deposits where it occurs as spherulites or aggregates of prismatic crystals. This phillipsite has chemical and physical properties that differ from phillipsites that occur in rocks of other compositions and depositional environments, but previously it had not been completely characterized. Published analyses of five samples from four deposits in the southwestern United States show that the phillipsite is very siliceous and alkalic; the Si/(Al + Fe3+) ratio is 3.08–3.37, alkalis greatly exceed the divalent exchangeable cations, and the Na/K ratio is 1.05–3.05. The mean index of refraction (1.451–1.470) is much lower than that generally reported for the phillipsite group and can be correlated with the relatively high Si and alkali contents. Monoclinic unit-cell parameters show the following ranges: a = 9.931–9.991 Å, b = 14.142–14.155 Å, c = 8.650–8.706 Å, ß = 124.57–125.07°, and V = 1000.3–1007.4 Å3. The especially small b dimension is consistent with the high Si content.

Zeolitic Alteration of Lacustrine Tuffs, Western Snake River Plain, Idaho, Usa

Abstract The Miocene Chalk Hills Formation of southwestern Idaho consists mainly of interbedded mudstone, siltstone, diatomite, and vitric tuffs that were deposited in a chiefly freshwater lake. Throughout most of the outcrop belt, the tuffs are unaltered, but south of Oreana, the tuffs are completely altered to clinoptilolite with or without smectite and opal-CT. The zeolitic zone extends parallel to the southern margin of the Snake River Plain for about 21 km. The zeolites and associated silicate minerals undoubtedly formed during diagenesis by hydrolysis and dissolution of rhyolitic glass by pore water that was trapped in the tuffs during lacustrine sedimentation.