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Clays and Clay Minerals

, Volume 32, Issue 1, pp 36–44 | Cite as

The Influence of Aluminum on Iron Oxides. VIII. Unit-Cell Dimensions of Al-Substituted Goethites and Estimation of Al from them

  • D. G. Schulz
Article

Abstract

The unit-cell dimensions of synthetic, Al-substituted goethites showed that the c dimension is a linear function of Al substitution in the range 0–33 mole % Al, but that the a dimension is variable over this same range. The b dimension is also linearly related to Al substitution but is slightly more variable than the c dimension for Al substitutions of 20–33 mole %. The variability of the a dimension is postulated to be the result of structural defects. An improved procedure for estimating Al substitution from x-ray powder diffraction positions requires (1) calculation of the c dimension from the positions of the 110 and 111 diffraction lines using the formula: c = (1/d(111)2 − 1/d(110)2)−1/2, and (2) estimation of Al substitution from the relationship: mole % Al = 1730 − 572.0c. The 95% confidence interval of the estimate is ±2.6 mole % Al when using this procedure, in contrast to ±4.0 mole % Al when the position of the 111 reflection alone is used.

Key Words

Aluminum Goethite Iron Unit-cell dimensions X-ray powder diffraction 

Резюме

Размеры элементарной ячейки синтетических, Al-замещенных гетитов указывали на то, что размерность c является линейной фуркцией замещения Al в диапазоне от 0 до 33 молярных % Al, тогда как размерность a является переменной на том же самом диапазоне. Размерность b также являлась линейно зависимой от замещения Al, но проявляла слегка большую изменчи-вость чем с для замещения Al в диапазоне 20 до 33 молярных %. Предлагается, что изменчивость размерности является результатом структурных дефектов. Улучшенная процедура для оценки замещения Al на основе положений линий рентгеновской порошковой дифракции требует: (1) вычисления размерности c на основе положений 100 и 111 дифракционных линий используя формулу: c = (1/d(111)2 − 1/d(110)2)−1/2 и (2) оценки замещения Al из соотношения: молярные % Al = 1730 − 572,0 c. 95% уровень статистической достоверности этой оценки равен ±2,6 молярных % Al при использовании этой процедуры, в противоположность 4,0 молярных % Al когда только используется положение отражения 111. [E.G.]

Resümee

Die Größen der Einheitszellen von synthetischen, Al-substituierten Goethiten zeigten, daß die c-Dimension eine lineare Funktion der Al-Substitution im Bereich von 0–33 Mol.-% Al ist, daß aber die a-Dimension in diesem Bereich variiert. Die b-Dimension zeigt ebenfalls eine lineare Abhängigkeit von der Al-Substitution, variiert aber etwas mehr als c bei Al-Substitution zwischen 20–33 Mol.-%. Es wird vorgeschlagen, daß die Variation der a-Dimension das Ergebnis von Gitterfehlern ist. Eine verbesserte Vorgangsweise zur Abschätzung der Al-Substitution aus der Lage der XRD-Linien erfordert (1) die Berechnung der c-Dimension aus der Lage der 110 und 111 Linien, wozu die Formel c = (1/d(111)2 − 1/d(110)2)−1/2 zu verwenden ist und (2) die Abschätzung der Al-Substitution aus der Beziehung: Mol.-% Al = 1730 − 572,0 c. Das Konfidenzintervall der Abschätzung beträgt ±2,6 Mol.-% Al, wenn man diese Vorgangsweise anwendet, im Gegensatz zu ±4,0 Mol.-% Al, wenn die Lage des 111 Reflexes allein berücksichtigt wird. [U.W.]

Résumé

Les dimensions de la maille-mère de goethites synthétiques substituées par Al ont montré que la dimension c est une fonction linéaire de la substitution par Al sur l’étendue 0–33 mole % d’Al, mais que la dimension a est variable sur cette même étendue. La dimension b était aussi apparentée linéairement à la substitution par Al, mais s’est montrée quelque peu plus variable que c, pour la substitution par Al de 20–33 mole %. On a proposé que la variabilité de la dimension a est un résultat de défauts structuraux. Un procédé amelioré pour estimer la substitution par Al à partir de positions de droite XRD exige (I) le calcul de la dimension c à partir des positions des droites de diffraction 110 et 111 en employant la formule: c = (1/d(111)2 − 1/d(110)2−1/2, et (2) l’estimation de la substitution par Al à partir de la relation: mole % d’Al = 1730 − 572,0 c. L’interval de confiance 95% de cette estimation est ±2,6 mole % d’Al en employant ce procédé, en contraste avec ±4,0 mole % d’Al lorsque seule la position de la réflection 111 est utilisée. [D.J.]

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Copyright information

© The Clay Minerals Society 1984

Authors and Affiliations

  • D. G. Schulz
    • 1
  1. 1.Institut für BodenkundeTechnische Universität MünchenFreising-WeihenstephanFederal Republic of Germany

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