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Clays and Clay Minerals

, Volume 30, Issue 5, pp 327–336 | Cite as

Origin of Magnesium Clays from the Amargosa Desert, Nevada

  • Hani N. Khoury
  • Dennis D. Eberl
  • Blair F. Jones
Article

Abstract

Deposits of sepiolite, trioctahedral smectite (mixed-layer kerolite/stevensite), calcite, and dolomite, found in the Amargosa Flat and Ash Meadows areas of the Amargosa Desert were formed by precipitation from nonsaline solutions. This mode of origin is indicated by crystal growth patterns, by the low Al content for the deposits, and by the absence of volcanoclastic textures. Evidence for low salinity is found in the isotopic compositions for the minerals, in the lack of abundant soluble salts in the deposits, and in the crystal habits of the dolomite. In addition, calculations show that modern spring water in the area can precipitate sepiolite, dolomite, and calcite following only minor evaporative concentration and equilibration with atmospheric CO2. However, precipitation of mixed-layer kerolite/stevensite may require a more saline environment. Mineral precipitation probably occurred during a pluvial period in shallow lakes or swamps fed by spring water from Paleozoic carbonate aquifers.

Key Words

Dolomite Kerolite Mixed layer Precipitation Sepiolite Smectite Stevensite 

Резюме

Резюме—Отложения сепиолита, трехоктаэдрического смектита (смешанно-слойного керолита/стевенсита), кальцита, и доломита, найденные в районах Амаргоской низменности и пепловых лугов Амаргоской пустыни, были образованы путем осаждения из несоленых растворов. На этот способ происхождения указывают структуры роста кристаллов, низкое содержание Аl в отло-жениях ш отсутствие вулканокластических текстур. Свидетельством низкой солености являются изотопные составы минералов, отсутствие обильных растворимых солей в отложениях и кристал-лические черты доломита. Кроме того, расчеты показывают, что современная источниковая вода в районе может осаждать сепиолит, доломит и кальцит после незначительной испарительной кон-центрации и равновесия с атмосферическим CO2. Однако, осаждение смешанно-слойного керо-лита/стевенсита может потребовать более соленой среды. Минеральные осаждения, вероятно, выступили в течение плювиального периода в мелких озерах или топях, питаемых источниковой водой из палеозойских карбонатных водоносных пластов. [Е.С.]

Resümee

Ablagerungen von Sepiolith, trioktaedrischem Smektit (Wechsellagerung Kerolit/Stevensit), Calcit, und Dolomit, die in den Gebieten Amargosa Flat und Ash Meadows der Amargosa Wüste gefunden werden, bildeten sich durch Ausfallung aus nicht-salinen Lösungen. Diese Entstehungsart ergibt sich aus der Art des Kristallwachstums, dem niedrigen Al-Gehalt der Ablagerungen und durch das Fehlen von vulkanoklastischen Strukturen. Der Beweis für eine niedrige Salinität ergibt sich aus den einheitlichen Zusammensetzungen der Minerale, aus dem Fehlen überschüssiger löslicher Salze in den Ablagerungen und aus den Arten des Kristallhabitus von Dolomit. Außerdem zeigen Berechnungen, daß aus rezentem Quellwasser dieses Gebietes Sepiolith, Dolomit und Calcit ausfallen kann, was nur von einer geringen Konzentrierung durch Evaporation und von der Gleichgewichtseinstellung mit dem CO2 der Luft abhängt. Die Ausfallung der Wechsellagerung Kerolit/Stevensit könnte jedoch höher saline Bedingungen erfordern. Diese Mineralausfällung geschah wahrscheinlich während einer pluvialen Periode in seichten Seen odor Mooren, die von Quellwasser aus paläozoischen, karbonathaltigen wasserführenden Schichten gespeist wurden. [U.W.]

Résumé

Des dépôts de sepiolite, de smectite trioctaèdre (kérolite/stevensite à couches mélangées), de calcite, et de dolomite, trouvés dans les régions de l’Amargosa Fiat et d’Ash Meadows du désert d’Amargosa ont été formés par la précipitation de solutions non-salines. Ce mode d’origine est indiqué par les dessins de croissance cristallographique, par le bas contenu en Al des dépôts, et par l’absence de textures volcanoclastiques. L’évidence pour la basse salinité est trouvée dans les compositions isotopiques pour les minéraux, dans le manque d’abondants sels solubles dans les dépôts, et dans les habitudes cristallographiques de la dolomite. De plus, des calculs montrent que de l’eau de source moderne dans la région peut précipiter la sépiolite, la dolomite, et la calcite suivant une concentration evaporative mineure et l’équilibration avec du CO2 atmosphérique. La précipitation de kérolite/stevensite à couches mélangées peut cependant exiger un environement plus salin. La précipitation des minéraux s’est probablement passée pendant une période pluviale dans des lacs peu profonds ou dans des marais nourris d’eau de source provenant des aquifères carbonates paléozoiques. [D.J.]

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Copyright information

© The Clay Minerals Society 1982

Authors and Affiliations

  • Hani N. Khoury
    • 1
  • Dennis D. Eberl
    • 2
  • Blair F. Jones
    • 3
  1. 1.Department of Geology and MineralogyUniversity of JordanAmmanJordan
  2. 2.U.S. Geological SurveyDenver Federal CenterDenverUSA
  3. 3.U.S. Geological SurveyNational CenterRestonUSA

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