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Clays and Clay Minerals

, Volume 28, Issue 2, pp 92–96 | Cite as

Effect of SiO2/Al2O3 Ratio on the Thermal Reactions of Allophane

  • Teruo Henmi
Article

Abstract

Differences were found in the differential thermal analysis curves and in the temperatures of new-phase development between allophanes of high (1.91–1.99) and low (1.47–1.53) SiO2/Al2O3 ratios. The endothermic peak due to continuous dehydration and dehydroxylation was at higher temperatures (153°-185°C) for allophanes with high SiO2/Al2O3 ratios and at lower temperatures (148°–165°C) for those with low SiO2/Al2O3 ratios. The temperature of the exothermic peak was lower and the height affected more by the exchangeable cation content for allophanes with high ratios than for those with low ratios. New phases did not develop in allophanes having high Si02/Al2O3 ratios even after they were heated to 1000°C, above the temperature of the exothermic peak. In contrast, a symptomatic development of new phases was noted in allophanes with low SiO2/Al2O3 ratios at 900°C, below the temperature of the exothermic peak. The effect of SiO2/Al2O3 ratio in the thermal behavior of allophane strongly suggests that differences in the structure are closely associated with the chemical composition of this material.

Key Words

Allophane Dehydration Dehydroxylation DTA Silica/alumina ratio Thermal reactions 

Резюме

Были обнаружены различия в кривых дифференциального термического анализа и в температурах развития новой фазы между аллофанами высоких (1,91-1,99) и низких (1,47-1,53) отношений SiO2/А12O3. Эндотермический пик из-за постоянной дегидротации и дегидроксилиро-вания наблюдался при высоких температурах (153°–185°С) для аллофанов с высокими отношениями SiO2/Аl2O3 и при низких температурах (148°–165°С) для аллофанов с низкими отношениями SiO2/А12O3. Для аллофанов с высокими отношениями температура эндотермического пика была ниже и высота более обусловлена содержанием обменных катионов, чем для аллофанов с низкими отношениями. Новые фазы не развились в аллофанах с высокими отношениями §Ю2/А1203 даже после подогрева до 1000°С, выше температуры экзотермического пика. Наоборот, симптоматическое развитие новых фаз было замечено в аллофанах с низкими отношениями SiO2/Аl2O3 при 900°С, ниже температуры экзотермического пика. Влияние отношения SiO2/А12O3 на термическое поведение аллофанов указывает на то, что различия в структуре тесно связаны с химическим составом этого материала. [N. R.]

Resümee

Bei den DTA-Kurven und bei den Bildungstemperaturen neuer Phasen wurden Unterschiede zwischen Allophanen mit hohen (1,91 bis 1,99) und niedrigen (1,47 bis 1,53) SiO2/Al2O3-Verhältnissen gefunden. Der endotherme Peak, verursacht durch die ständige Dehydrierung und Dehydroxylierung, war für Allophane mit hohem SiO2Al2O3-Verhältnis bei höheren Temperaturen (153° bis 195°C). Für Allophane mit niedrigem SiO2/Al2O3-Verhältnis war er bei niedrigeren Temperaturen (148° bis 165°C). Bei Allophanen mit großen Verhältniszahlen war die Temperatur des endothermen Peaks niedriger, und die Höhe mehr durch die austauschbaren Kationen beeinflußt, als bei solchen mit kleinen Verhältniszahlen. In Allophanen mit hohen SiO2/Al2O3-Verhältnissen wurden keine neuen Phasen gebildet, selbst wenn sie auf über 1000°C, über die Temperatur des exothermen Peaks, erhitzt wurden. Im Gegensatz dazu wurde bei Allophanen mit niedrigen SiO2/Al2O3-Verhältnissen die symptomatische Bildung neuer Phasen bei 900°C, unter der Temperatur des exothermen Peaks, beobachtet. Die Wirkung des SiO2/Al2O3-Verhältnisses auf das thermische Verhalten von Allophan legt die Annahme nahe, daß Unterschiede in der Struktur eng mit der chemischen Zusammensetzung dieses Materials zusammenhängen. [U. W.]

Résumé

Des différences ont été trouvées dans les courbes d’analyse thermale différentielle et dans les températures de développement de nouvelle phase entre les allophanes aux proportion hautes (1,91–1,99) et basses (1,47–1,53) de SiO2/Al2O3. Le sommet endothermique dû à la déshydration et à la déshydroxy-lation continuelles était à des températures plus hautes (153°–185°C) pour des allophanes avec des proportions élevées de SiO2/Al2O3 et à de plus basses températures (148°–165°C) pour celles avec les proportions les plus basses. De nouvelles phases ne sont pas développées dans les allophanes ayant des proportions élevées de SiO2/Al2O3, même après échauffement à 1000°C, au-delà de la température du sommet exothermique. Par contraste, un développement symptomatique de nouvelles phases a été noté dans les allophanes ayant de basses proportions de SiO2/Al2O3 à 900°C, en dessous de la température du sommet exothermique. L’effet de la proportion de SiO2/Al2O3 dans le comportement thermal de l’allophane suggère fortement que les différences dans la structure sont associées de près avec la composition chimique de ce matériel. [D. J.]

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Copyright information

© Clay Minerals Society 1980

Authors and Affiliations

  • Teruo Henmi
    • 1
  1. 1.Faculty of AgricultureEhime UniversityMatsuyamaJapan

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