Abstract
The scanning electron microscope (SEM) is useful in the identification of biogenic opal. Opaline spheres, cups, and scrolled or convoluted sheets were identified in both soil and vegetative isolates. X-ray diffraction analysis indicates that both alpha quartz and cristobalite were co-associated with the amorphous opaline phase synthesized during life metabolism of deciduous tree leaves. Such crystalline phases were most abundant in the 2–5 μm fraction and many consist of anitsotropic rods with parallel extinction or equidimensional bodies with aggregate extinction. Between 2/3 and 3/4 of the total opal isolate from deciduous tree leaves was solubilized when digested for 2–5-min in boiling 0.5 N NaOH. Rate of dissolution was a function of particle-size and tree species. Biogenic opal of forest origin was about 10–15 times more soluble than grass opal, which probably reflects the higher specific surface of the former.
Résumé
Le microscope électronique à balayage (SEM) est très utile pour l’identification de l’opale biogène. Des sphères d’opaline, des cupules et des feuillets en volutes ou enroulés, ont été identifiés à la fois dans des fractions isolées à partir de sols ou de végétaux. L’analyse par diffraction X indique que du quartz alpha et de la cristobalite sont associés avec la phase opale amorphe synthétisée durant le métabolisme des feuilles d’arbres à feuilles caduques. De telles phases cristallines sont les plus abondantes dans la fraction 2–5 μm, et beaucoup d’entre elles consistent en des bâtonnets anisotropes à extinction parallèle ou en des masses équidimensionnelles avec une extinction d’aggrégat. De 2/3 à 3/4 de l’opale totale isolée des feuilles d’arbres à feuilles caduques est solubilisée lors d’un traitement de 2,5 min dans NaOH 0,5 N bouillant. La vitesse de dissolution est fonction de la taille des particules et de l’espèce de l’arbre. L’opale biogène forestière est environ 10–15 fois plus soluble que l’opale venant de l’herbe, ce qui indique probablement une surface spécifique plus élevée pour la première.
Kurzreferat
Das Rasterelektronenmikroskop (SEM) ist brauchbar für die Identifizierung von biogenem Opal. Kugeln, Schalen und gerollte oder gewundene Blättchen aus Opal wurden sowohl in Fraktionen aus Böden als auch aus Pflanzematerial nachgewiesen. Die Röntgenbeugungsanalyse zeight, daß sowohl α-Quarz als auch Cristobalit mit der amorphen Opalphase vergesellschaftet sind, die im Verlauf des Lebensstoffwechsels von Laubbaumblättern synthetisiert wird. Solche kristallinen Phasen traten am häufigsten in der 2–5 μm-Fraktion auf und bestehen vielfach aus anisotropen Stäbchen mit paralleler Auslöschung oder aus Körpern mit gleichen Achsenlängen und Aggregatauslöschung. Zwischen 2/3 und 3/4 der gesamten aus Laubbaumblättern isolierten Opalfraktionen lösen sich nach 2,5 minütiger Behandlung in kochender 0,5 N NaOH. Die Auflösungsrate war eine Funktion der Korngröße und der Baumart. Biogener Opal forstlichen Ursprungs war etwa 10–15 mal löslicher als Grass-Opal, was wahrscheinlich die höhere spezifische Oberfläche des erstgenannten widerspiegelt..
Резюме
Растровый электронный микроскоп полезен при выяснении природы биогенного опала. Опаловые шарики, чашечки и витковые или свернутые листки найдены как в земляных, так и в растительных выделениях. Рентгенографический анализ показывает, что как альфа квартц, так и кристобалит связаны с аморфной опаловой фазой, образовавшейся во время жизненного метаболизма листьев лиственных деревьев. Эти кристаллические фазы обильнее всего встречаются во фракции 2–5 /хм и могут состоять из анизотропических палочек с параллельной экстинкцией или с телами одинаковой размерности с экстинкцией агрегата. Между 2/3 и 3/4 общего опалового выделения из листьев лиственных пород растворилось при вываривании в течение 2,5 минут в кипящем 0,5 N NaOH. Скорость растворения зависит от размера частицы и от породы дерева. Биогенный опал лесного происхождения растворяется в 10–15 раз скорее, чем травяной опал, что, возможно, отражает более высокую удельную поверхность первого.
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Wilding, L.P., Drees, L.R. Contributions of Forest Opal and Associated Crystalline Phases to Fine Silt and Clay Fractions of Soils. Clays Clay Miner. 22, 295–306 (1974). https://doi.org/10.1346/CCMN.1974.0220311
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DOI: https://doi.org/10.1346/CCMN.1974.0220311