L. Heller-Kallai

Dehydroxylation of dioctahedral phyllosilicates

Mössbauer spectroscopy of dioctahedral phyllosilicates showed that on dehydroxylation iron which originally occupied M(2) and M(l) sites became, respectively, 5- and 6-coordinated. The 6-coordinated sites are very distorted. No migration of cations occurs in the course of heating the specimens for 1–3 hr at 600°–700°C.By using a combination of several physicochemical methods, different successive stages of the dehydroxylation process could be distinguished: (1) migration of protons; (2) localized dehydroxylation of individual associations without significant change in the overall configuration of the octahedral sheets; and (3) loss of most of the hydroxyl groups with concomitant changes in the cell dimensions. Penetration of Li into the octahedral sheets does not affect the course of the reaction, but reduces the dehydroxylation temperature and the stability of the products.Dehydroxylation was preceded by or associated with the oxidation of any divalent iron present. Fe3+ derived from Fe2+ was indistinguishable by Mössbauer spectroscopy from iron initially present in the trivalent form. High concentrations of Fe lower the dehydroxylation temperature and reduce the stability of the dehydroxylate to the extent that partial disintegration may precede complete dehydroxylation.РезюмеМоссбауероская спектроскопия диоктаэдрических листовых силикатов показала, что при дегидроксилировании железо, которое первоначально занимало места М(2) и М(1), становилось соответственно 5- и 6-координированным. 6-координированные места являются очень деформированными. Не происходит никакой миграции катионов во время прокаливания образцов в течение от 1 до 3 часов при 600°–700°С.Используя комплекс нескольких физикохимических методов, можно выделить несколько последующих фаз процесса дегидроксилирования (1) миграция протонов; (2) локализованное дегидроксилирование индивидуальных ассоциаций без значительных изменений в общей конфигурации октаэдрических листов; и (3) потеря большинства гидроксильных групп с сопутствующими изменениями размеров ячеек. Проникновение Li в октаэдрические листы не влияет на ход реакции, но понижает температуру дегидроксилирования и устойчивость продуктов.С дегидроксилированием было связано или предшествовало ему окисление любого присутствующего двухвалентного железа. Fe3+, полученное, из Fe3 было неотличимо Моссбауеровской спектроскопией от железа первоначально Присутствующего в трехвалентной форме. Высокие, концентрации Fe понижают температуру дегидроксилирования и уменьшают устойчивость дегидроксилата до такой степени, что частичная дезинтеграция может предшествовать полному дегидроксилированию. [N.R.]ResümeeDie Mössbauerspektroskopie von dioktaedrischen Phyllosilikaten zeigte, daß das Eisen, das ursprünglich die M(2) und M(1) Plätze besetzte, bei der Dehydroxylierung 5- bzw. 6-koordiniert wird. Die 6-koordinierten Plätze sind stark verzerrt. Beim Erhitzen der Proben auf 600°–700°C für 1–3 Stunden findet keine Wanderung der Kationen statt.Mehrere aufeinanderfolgende Stadien des Dehydroxylierungsprozesses können durch die Verwendung einer Kombination von mehreren physikochemischen Methoden unterschieden werden: (1) eine Wanderung von Protonen; (2) eine lokalisierte Dehydroxylierung einzelner Bereiche ohne wesentliche Änderung in der gesamten Konfiguration der oktaedrischen Schichten; (3) Verlust der meisten Hydroxylgruppen mit daraus folgenden Veränderungen der Zelldimensionen. Das Eindringen von Li in die oktaedrischen Schichten beeinflußt den Reaktionsverlauf nicht, reduziert jedoch die Dehydroxylierungstemperatur und die Stabilität der Produkte.Vor der Dehydroxylierung oder gleichzeitig damit oxidiert das vorhandene zweiwertige Eisen. Fe3+, das aus Fe2+ hervorgegangen ist, konnte mittels Mössbauerspektroskopie nicht von Eisen unterschieden werden, das schon immer in dreiwertiger Form vorlag. Hohe Fe-Konzentrationen erniedrigen die Dehydroxylierungstemperatur und können die Stabilität des Dehydroxylats bis zu einem Ausmaß reduzieren, daß es vor der vollständigen Dehydroxylierung teilweise zerfällt. [U.W.]RésuméLa spectroscopie de Mössbauer de phyllosilicates dioctaédrales montre que lors de la déshydroxylation, le fer qui occupait originalement les sites M(2) et M(1) devenait, respectivement coordonné-5 et -6. Les sites coordonnés-6 sont très déformés. Il n’y a aucune migration d’ions pendant l’échauffement des specimens pour 1–3 heures à 600°–700°C.Par l’utilisation de plusieurs méthodes physicochimiques, différentes étapes successives du procédé de déshydroxylation pouvaient être distinguées: (1) la migration de protons; (2) la déshydroxylation localisée d’associations individuelles sans que des changements significatifs ne se produisent dans la configuration générale des feuillets octaédraux; (3) la perte de la plupart des groupes hydroxyles avec des changements concomitants dans les dimensions des mailles. La pénétration de Li dans les feuillets octaédraux n’affecte pas la suite des réactions mais réduit la température de déshydroxylation et la stabilité des produits.La déshydroxylation était précedée ou accompagnée de l’oxidation de n’importe quel fer divalent présent. On ne pouvait distinguer par spectroscopie de Mössbauer Fe3+ derivé de Fe2+, du fer initialement présent dans la forme trivalente. De hautes concentrations de Fe abaissent la température de déshydroxylation et réduisent la stabilité du déshydroxylate à tel point qu’une désintégration partielle peut précéder une déshydroxylation complète. [D.J.]

Reduction and oxidation of Fe (super 3+) in dioctahedral smectites; III, Oxidation of octahedral iron in montmorillonite

Structural Fe2+ in montmorillonite is readily oxidized by contact with water, salt solutions or on mild heating. This is shown clearly by the Mössbauer spectra and is associated with a sharpening of the infrared absorption near 880 cm−1. It was inferred that this band comprises the Fe2+—OH—Al and Fe3+—OH—Al deformations. The rate at which oxidation occurs depends on the exchangeable cations. High acidity of the interlayers is conducive to oxidation, as is contact with Cu2+-containing solutions or concentrated H2O2 solutions.The results show clearly that any chemical treatment of montmorillonite causes changes in the oxidation state of structural iron.РезюмеСтруктурное Fe2+ в монтмориллоните было легко окислено в результате контакта с водой, соляными растворами или умеренного нагревания. Это ясно показано путем использования спектров Моссбауэра и связано с более резко выраженной инфракрасной абсорбцией вблизи 880cm−1. Было сделано заключение что эта связь обусловлена деформациями Fe2+ -OH-Al и Fe3+-OH-Al. Скорость, с которой происходит окисление, зависит от обменных ионов. Высокая кислотность в межслойной среде ведет к окислению, также как контакт с Cu2+ содержащими растворами или концентрированными растворами H2O2.Результаты ясно показывают, что любая химическая обработка монтмориллонита вызывает изменение в состоянии окисления структурного железа.KurzreferatStrukturelles Fe(III) in Montmorillonit ist leicht oxydierbar durch Kontakt mit Wasser, Salzlösungen oder durch leichtes Erhitzen. Mössbauerspektra zeigen das deutlich und es ist verbunden mit einer Verschärfung der Infrarotabsorption bei 880 cm−1. Der Schluß wurde gezogen, daß dieses Band auf die Verformungen von Fe(II)-OH-Al und Fe(III)-OH-Al zurückzuführen ist. Die Geschwindigkeit, mit der Oxydation stattfindet, hängt von den austauschbaren Kationen ab. Hohe Azidität der Zwischenschichten, sowohl wie Kontakt mit Lösungen, die Cu(II) enthalten, oder konzentrierte H2O2 Lösungen fördern die Oxydation.Resultate zeigen deutlich, daß jede chemische Behandlung des Montmorilloniten, eine Veränderung in der Oxydationsstufe des strukturellen Eisens verursacht.

MOSSBAUER STUDIES OF PALYGORSKITE AND SOME ASPECTS OF PALYGORSKITE MINERALOGY

Fe3+ ions in palygorskite occupy sites at the edges and in the interior of the alumino-silicate chains. The Mössbauer parameters of the doublets associated with Fe3+ ions in edge sites indicate that the sites have a regular 6 coordination. Fe3+ ions in the interior of the chains occupy M(1) sites in three of the samples examined and M(2) sites in the fourth. Fe3+ ions in edge positions of palygorskite become 5-coordinated when water is lost on heating. They maintain this coordination on dehydroxylation, probably by cross-linking of the chains. The temperatures at which changes occur in the X-ray powder diffraction patterns and the Mössbauer and infrared (IR) spectra differ from sample to sample. The intermediate stages observed also vary, either due to different reaction paths or to different stabilities of the intermediate phases. The deduced distribution of cations in the octahedral sheets is in good qualitative agreement with the observed IR hydroxyl absorptions.РезюмеИоны Fe3+ в палыгорските занимают места по краям и внутри алюмосиликатных цепей. Мессбауэровские параметры дуплетов связанных с краевыми ионами Fe3+ указывают на то, что эти места имеют шестикратную регулярную координацию. Ионы Fe3+ внутри цепей занимают места М(1) в трёх исследованных образцах и места М(2) в четвёртом образце. Ионы Fe3+, находящиеся на краевых местах палыгорскита, становятся 5-координированными в результате потери воды вследствие нагрева. Оин сохраняют эту координацию при дегидроксилировании, вероятно благодаря образованию поперечных связей в цепях. Температура, при которой наблюдаются изменения в порошковых рентгенограммах и в мессбауэровских и инфракрасных спектрах, различна в зависимости от образцов. Наблюдаемые промежуточные стадии также изменялись либо вследствие различных путей протекания реаций, либо вследствие различной устойчивости промежуточных фаз. Выведенное распределение катионов в октаэдрических слоях находится в хорошем количественном согласии с данными, наблюдаемыми при инфракрасной абсорбции гидроксила. [Е.С.]ResümeeDie Fe3+-Ionen im Palygorskit besetzen Plätze an den Ecken und im Innern der Aluminiumsilikatketten. Die Mössbauerparameter der Dubletts, die von den Fe3+-ionen auf den Eckplätzen herrühren, deuten darauf hin, daß diese Plätze eine regelmäßige 6-Koordination haben. Die Fe3+-Ionen im Innern der Ketten besetzen in drei der untersuchten Proben M(1)-Pläitze und in der vierten Probe M(2)-Plätze. Bei Wasserabgabe durch Erwärmen werden die Fe3+-Ionen in den Eckpositionen von Palygorskit 5-koordiniert. Sie behalten diese Koordination nach der Dehydroxilierung bei, was wahrscheinlich auf eine Querverbindung von Ketten zurückzuführen ist. Die Temperaturen, bei denen Veränderungen in den Röntgenpulverdiffraktometerkurven sowie in den Mössbauer- und Infrarot(IR)-Spektren auftreten, sind in den einzelnen Proben verschieden. Die beobachteten Zwischenstadien variierten entweder aufgrund unterschiedlicher Reaktionswege oder unterschiedlicher Stabilität der Übergangsphasen. Die abgeleitete Verteilung der Kationen in den Oktaederschichten stimmt gut mit der beobachteten IR-Hydroxil-Absorption überein. [U.W.]RésuméLes ions Fe3+ dans la palygorskite occupent les sites périphériques et à l’intérieur des chaînes alumino-silicates. Les paramètres de Mössbauer des doublets associés avec les ions Fe3+ dans les sites périphériqnes indiquent que ces sites ont une coordination-6 régulière. Les ions Fe3+ à l’intérieur des chaînes occupent les sites M(1) dans trois des échantillons examinés et les sites M(2) dans un quatrième. Les ions Fe3+ dans les positions périphériques de palygorskite deviennet coordonés-5 lorsque l’eau est perdue à l’échauffement. Ils gardent cette coordination lors de la déshydroxylation, probablement par la liaison transverse des chaînes. Les températures auxquelles des changements se passent dans les clichés de diffraction poudrée aux rayons-X et dans les spectres de Mössbauer et d’infrarouge (IR) varient d’échantillon en échantillon. Les étapes intermédiaires observées varient aussi, soit à cause de voies de réaction différentes, soit à cause de stabilités différentes des phases intermédiaires. La distribution déduite de cations dans les feuillets octaèdraux s’accordent qualitativement bien avec les absorptions hydroxyl IR observées. [D.J.]

Protonation–deprotonation of dioctahedral smectites ☆

Abstract This paper reviews protonation–deprotonation reactions and loss of structural hydroxyls which change the total anionic charge of the layers of dioctahedral smectites. Protons for protonation of the layers may be derived from interlayer water or from organic material. Deprotonation is caused by proton acceptors. It may also occur on heating in the course of migration of small interlayer cations into the clay layers. Reduction reactions of structural Fe 3+ in iron-rich smectites cause drastic changes in the anionic framework. Oxidation of structural Fe 2+ is readily effected, but it has not been established whether this causes a change in the anionic composition of the layers. The ease with which changes in anionic composition of dioctahedral smectites can be brought about by fairly mild pretreatment is a warning against the tendency to regard this feature as a constant material characteristic.

POTASSIUM HALIDE-TREATED MONTMORILLONITE (KTM) AS A SOLID PHASE IN LIQUID CHROMATOGRAPHY

Columns for high pressure liquid chromatography were prepared from spray dried samples of montmorillonite that were heated with potassium halides (KTM) to increase both the cation density on the clay surfaces and the expandability of the interlayers. Some of the clay samples were exchanged with Cu before and/or after the potassium halide treatment.Retention of nitrobenzene and its chloro and methyl derivatives, of methyl substituted phenols and of nitrophenols on these columns was studied, using eluents ranging in polarity from hexane to isopropanol. The retention of the aromatic molecules depends on their specific interaction with active sites on the clay surfaces and on steric effects which limit access to the clay interlayers. Both penetrability and surface interaction are controlled by the composition and method of preparation of the solid phase and by the polarity of the mobile phase. Very strong adsorption of some eluates and efficient chromatographic separations between related substituted benzenes were achieved. Mixtures of the three isomers of cresol, chloronitrobenzene, nitrotoluene or nitrophenol were completely resolved by a judicious combination of solid phase and eluent.

Reactions of clay volatiles with n-alkanes

Volatiles formed on heating clay minerals contain a variety of cations and anions and are highly reactive. Gas chromatograph-mass spectrometer analysis of long-chain n-alkanes exposed to such volatiles showed that some decomposition occurred even < 250°C. Reactions with anions donated by the volatiles led to compounds containing atoms other than C and H, e.g., Cl. The products obtained from n-alkanes heated in a stream of clay volatiles to 500°C resembled those produced by corresponding alkane-clay mixtures heated to 250°C. At higher temperatures contact between the alkanes and montmorillonite or halloysite led to changes in the assemblages formed, whereas kaolinite or sepiolite had no effect. Thus, the first reactions that occurred when clays were heated with n-alkanes were apparently induced by the volatiles; at higher temperatures catalysis due to direct contact came into play with some of the clay minerals.

Reactions between clay volatiles and calcite reinvestigated

Calcite crystals exposed to clay volatiles react with some components of these volatiles, giving rise to a variety of surface morphologies. F, Cl, and S in different proportions were detected by electron microprobe analysis of the calcite surfaces. Under identical experimental conditions, volatiles from every clay mineral examined caused a specific morphology and chemical composition of the calcite surfaces, but these varied with temperature of the calcite. Changes in pH values and mass spectra of the volatiles after passage through calcite demonstrate that even on rapid heating some clay volatile-calcite reactions occur at temperatures as low as 150°C. Species other than those detectable by electron microprobe analysis also participate in the reactions in which CO2 is liberated.

Reactions of clay condensates with n-alkanes: comparison between clay volatiles and clay condensates

Abstract Long-chain n-alkanes were heated in the presence of clay volatiles or clay condensates, to temperatures up to 250°C in an open and 160°C in a closed system. The product assemblages obtained, which were identified by gas chromatography and gas chromatography-mass spectrometry, resembled each other and those formed on direct contact with the clay minerals. The composition of the assemblages indicates that catalytic cracking occurred. It appears that reactive species, which were previously detected and identified in the volatiles and condensates, act as acid catalysts. Under the mild conditions of the experiments the n-alkanes remained unchanged in the absence of a catalyst and catalysis by clay mineral surfaces had not yet come into play. The persistence of the reactivity of clay volatiles in the condensates implies that fluids derived from clays may catalyse processes that are removed in space and time from the immediate vicinity of the minerals.

An unusual kaolinite-calcite interaction

Abstract In a CO 2 atmosphere, one particular sample of kaolinite from Cornwall, England, when mixed with calcite in the proportions 4:1 or greater gave a very small, sharp endotherm at 840–845°C on curves obtained on three differential thermal analysis instruments of different design. Scanning electron microscopy suggests that this endotherm is associated with surface melting or sintering of the calcite rhombs, but X-ray diffraction and infrared absorption spectrometry reveal that other changes also occur. It is suggested that volatiles evolved from the kaolinite initiate a series of complex reactions.