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Nontronite in a Deep-Sea Core from the South Pacific

Published online by Cambridge University Press:  02 April 2024

A. Singer ,
P. Stoffers ,
L. Heller-Kallai  and
D. Szafranek
A. Singer
Affiliation:
Hebrew University of Jerusalem, Rehovot, Israel
P. Stoffers
Affiliation:
Institut für Sedimentsforschung, Universität Heidelberg, Heidelberg, Germany
L. Heller-Kallai
Affiliation:
Hebrew University of Jerusalem, Rehovot, Israel
D. Szafranek
Affiliation:
Hebrew University of Jerusalem, Rehovot, Israel
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Abstract

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Smectite close to the pure Fe end member of the nontronite-beidellite series was found in the fine clay separated from a 354-cm deep sediment core in the southwestern Pacific Basin. The mineral has a b-axis of 9.09 Å and an unusually low dehydroxylation temperature of 454°C and is composed of sheaves of fibers less than 50 Å wide. Its charge density is 5.09 × 10~4 esu/cm2. The charge originates mainly from the presence of 18% of the total Fe in tetrahedral positions, as determined by Mössbauer analysis. Slight deviations of the infrared spectra from those reported for nontronites are probably due to the presence of more octahedral Mg. The presence of authigenic quartz in the same sample permits some speculation on the concentration of dissolved silicon during nontronite genesis. A δ18O value of 26 ± 0.3‰ indicates a temperature of formation of about 22°C. The Sr isotope ratio suggests that the nontronite formed at least 12 million years ago.

Резюме

Резюме

Смектит, близкий чистому Ре конечному члену серии нонтронит-бейделлит, обнаруживался в мелкой глине, отделенной из 354 цм глубокой колонки осадка в юго-западном районе Тихого Океана. Минерал имеет ось b равную 9,09 Å и необыкновенно низкую температуру дегидроксилирования и состоит из связок волокон менее, чем 50 Å широких. Плотность заряда равна 5,09 х 10−4 эсе/цм2. Заряд возникает, главным образом, в следствие присутствия 18% всех атомов Ре в тетраэдрических местах, как это было определено при помощи анализа Мессбауера. Небольшие отклонения инфракрасного спектра этого минерала от спектра нонронита являются результатом присутствия большого количества октаэдрического Mg. Присутствие аутигенного вкарца в этом же самом образце дает возможность какого-либо предположения концентрации растворенного кремния во время генезиса нонтронита. Значение δ18O, равное 26 ± 0,3‰, указывает на температуру формирования около 22°С. Sr изотопный состав показывает, что нонтронит образовался по крайней мере 12 миллионов лет тому назад. [Е.G.]

Resümee

Resümee

Ein Smektit, der in der Zusammensetzung dem reinen Fe-Endglied der Nontronit-Beidellit-Mischungsreihe sehr nahekommt, wurde in der feinen Tonfraktion, die von einem 354 cm tiefen Sedimentbohrkern von südwestlichen Pazifik Becken abgetrennt wurde, gefunden. Das Mineral hat eine kleine è-Achse von 9,09 Å und eine ungewöhnlich niedrige Dehydroxilierungstemperatur von 454°C und ist aus Faserbündeln zusammengesetzt, die eine Dicke unter 50 Å haben. Seine Ladungsdichte beträgt 5,09 × 10−4 esu/cm2. Die Ladung rührt hauptsächlich von der Anwesenheit von 18% Gesamteisen in tetraedri-scher Koordination her, wie aus der Mössbauerbestimmung hervorgeht. Geringe Abweichungen der Infrarotspektren von denen, die für Nontronit angegeben werden, kommen wahrscheinlich durch die Anwesenheit von mehr oktaedrischem Mg. Die Anwesenheit von autigenem Quarz in der gleichen Probe erlaubt einige Spekulationen über die Konzentration des gelösten Siliziums während der Nontronitent-stehung. Ein δ18O-Wert von 26 ± 0,3‰ deutet auf eine Bildungstemperatur von etwa 22°C hin. Das Sr-isotopen Verhältnis läßt daraufschließen, daß der Nontronit vor mindestens 12 Millionen Jahren gebildet wurde. [U.W.]

Résumé

Résumé

De la smectite proche du membre terminal Fe pur de la série nontronite-beidellite a été trouvée dans l'argile fine séparée d'une carotte sédimentaire de 354 cm de profondeur dans la Basin Pacifique du sud-ouest. Le minéral a un axe-b de 9,09 Å et une température de déshydroxylation inhabituellement basse de 454°C et est composé d'un ensemble de fibres d'une longueur de moins de 50 Å. Sa densité de charge est 5,09 × 10~” esu/cm2. L'origine de la charge est principalement due à la présence de 18% du Fe total en positions tétraèdrales, comme l'a déterminé l'analyse de Mössbauer. De légères déviations des spectres infrarouges par rapport à ceux déterminés pour des nontronites sont probablement dues à la présence de plus de Mg octaèdral. La présence de quartz authigénique dans le même échantillon permet quelque speculation quant à la concentration de silice dissoute pendant la genèse nontronite. Une valeur δ18O de 26 ± 0,3‰ indique une température de formation d'environ 22°C. La proportion d'isotope Sr suggère que la nontronite s'est formée il y a au moins 12 million d'années. [D.J.]

Keywords

AgeCharge densityDeep seaIronMössbauer spectroscopyNontroniteOxygen isotopes
Type
Research Article
Information
Clays and Clay Minerals , Volume 32 , Issue 5 , October 1984 , pp. 375 - 383
Copyright
Copyright © 1984, The Clay Minerals Society

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