Atsuyuki Inoue

Chemical and morphological evidence for the conversion of smectite to illite

The continuous conversion of smectite to illite in samples from the Shinzan hydrothermal alteration area of Japan has been examined by X-ray powder diffraction (XRD) and transmission (TEM) and analytical transmission electron microscopy (AEM). TEM shows that randomly interstratified illite/ smectite (I/S) containing 100-50% expandable layers exhibits a flakey shape, whereas regularly and partially ordered interstratified I/S having 50-0% expandable layers exhibits a lath-like habit. An early- formed lath of regularly interstratified I/S is typically <35 Å in thickness and 300–500 Å in width; these dimensions gradually increase with decreasing percentage of expandable layers. XRD shows that the lathshaped I/S has a 1M polytype mica structure. AEM shows that the interlayer K content of flakey I/S increases monotonously with decreasing percentage of expandable layers in the range 100-50% expandable layers, whereas the interlayer K content of lath-shaped I/S increases along a different trend from that for the flakey I/S in the range 50-0% expandable layers. These observations suggest that randomly interstratified I/S is fundamentally smectite that is undergoing K-fixation and dissolution and that regularly and partially ordered interstratified I/S are immature illite which is still growing. Consequently, they suggest a mechanism for the hydrothermal smectite-to-illite conversion that is based on the K-fixation in and dissolution of smectite and the precipitation and growth of thin illite particles. Furthermore, these data suggest that the kinetics of smectite dissolution and illite growth are the most important factors controlling the smectite-to-illite conversion.

FURTHER INVESTIGATIONS OF A CONVERSION SERIES OF DIOCTAHEDRAL MICA/SMECTITES IN THE SHINZAN HYDROTHERMAL ALTERATION AREA, NORTHEAST JAPAN

A complete conversion series for mica/smectites was found in a hydrothermal alteration envelope around Kuroko-type ore deposits at the Shinzan area, Akita Prefecture, Northeast Japan. The minerals are an alteration product of volcanic glass of Miocene age and are commonly associated with zeolites and silica minerals. Degrees of ordering of interstratification of the minerals change discontinuously from Reichweite g = 0 (100–55% expandable layers) to g = 1 (45–20% expandable layers), and from g = 1 to g = 2 (<20% expandable layers). This pattern of conversion differs from the behavior of mica/smectites during burial diagenesis which undergo a continuous change in ordering type, and from the behavior of rectorite which displays a constant expandability and ordering (45–55%) over a wide range of conditions. Differences between these minerals were also found in the relationships between expandability and total layer charge, and between expandability and number of non-exchangeable interlayer cations. In mica/smectites from the Shinzan area, chemical changes in the interlayers and tetrahedral and octahedral sites are consistent with a reaction in which K-enrichment and K-fixation in the interlayers are controlled by an increase in negative layer charge. This conversion occurred in response to a steep geothermal gradient and migrating hydrothermal solutions.РезюмеПолные серии видоизменения для слюд/смектитов были найдены в кармане гидротермического изменения вокруг Куроко-типа залежей руды в Шинзан области, Акита префектура, Северно-восточная Япония. Минералы явдяются продуктом видоизменения вулканического стекла миоценовой эпохи и обычно связываются с цеолитами и кремнеземными минералами. Степени упорядочения переслаивания минералов изменяются прерывисто от числа Рейхвейта g = 0 (100-55% расширяющихся слоев) до g = 1 (45-20% расширяющихся слоев) и от g = 1 до g = 2 (<20% расширяющихся слоев). Этот образец превращения отличается от поведения слюды/смектитов в течение периода диагенеза погребений, которые претерпевают непрерывное изменение типа упорядочения, и от поведения ректорита, который проявлет постоянную способность к расширению и упорядочению (45-55%) в широким диапазоне условий. Различия межлу этими минералами были также найдены и в области соотношений между расширяемостью и полным зарядом слоя, и между расширяемостью и числом необмениваемых межслойных катиогов. Химические изменения в прослойке и в четырехгранных и восьмигранных местах в слюдах/ смектитах из Шинзан области согласуются с реакцией, в которой обогащение прослойки калием и фиксация калия в прослойке контролируется увеличением отрицательного заряда слоя. Это превращение происхоит как реакция на большой геотермальный градиент и миграцию гидротермальных растворов. [Е.С.]ResümeeEine vollständige Umwandlungsserie von Glimmer/Smektite wurde in einer hydrothermalen Umwandlungszone um Erzlagerstätten vom Kuroko-Typ im Shinzangebiet, Akita Prefecture, NO Japan, gefunden. Die Minerale sind Umwandlungsprodukte von vulkanischem Glas aus dem Miozän und treten im allgemeinen zusammen mit Zeolithen und SiO2-Mineralen auf. Der Ordungsgrad der Wechsellagerung in den Mineralen ändert sich diskontinuierlich von der Reichweite g = 0 (100–55% expandierbare Lagen) bis g = 1 (45–20% expandierbare Lagen), und von g = 1 bis g = 2 (<20% expandierbare Lagen). Dieses Umwandlungsschema unterscheidet sich vom Verhalten Glimmer/Smektite, das durch Versenkungsdiagenese hervorgerufen wird, in dem eine kontinuierliche Veränderung im Ordnungszustand stattfindet. Es unterscheidet sich auch vom Verhalten von Rektorit, der über einen großen Bedingungsbereich eine konstante Expandierbarkeit und Ordnung (45–55%) zeigt. Unterschiede zwischen diesen Mineralen wurden auch in der Beziehung zwischen Expandierbarkeit und Gesamtladung der Schichten und zwischen Expandierbarkeit und Anzahl der nichtaustauschbaren Zwischenschichtkationen gefunden. In den Glimmer/Smektit-Wechsellagerungen aus dem Shinzangebiet hängen chemische Veränderungen in den Zwischenschichten sowie auf den tetraedrischen und oktaedrischen Plätzen mit einer Reaktion zusammen, bei der die K-Anreicherung und K-Fixierung in den Zwischenschichten durch eine Zunahme der negativen Schichtladung kontrolliert wird. Diese Umwandlung ist die Folge eines steilen geothermischen Gradienten und wandernder hydrothermaler Lösungen. [U.W.]RésuméUne série de conversion complète pour des mica/smectites a été trouvée dans une enveloppe d’altération hydrothermique autour de dépôts de minéraux du type Kuroko dans la région Shinzan, Préfecture d’Akita, Japon du Nord-Est. Les minéraux sont un produit de l’altération de verre volcanique d’age Miocène, et sont souvent associés avec des zéolites et des minéraux silices. Les degrés d’ordonnement d’interstratification des minéraux changent de manière discontinue de Reichweite g = 0 (100–55% de couches expansibles) à g = 1 (45–20% de couches expansibles) et de g = 1 à g = 2 (<20% de couches expansibles). Ce procédé de conversion diffère du comportement de mica/smectites pendant la diagénèse d’ensevelissement; celles-ci subissent alors un changement de type d’ordonnement continuel; et aussi du comportement de la rectorite qui montre un potentiel d’expansion et un ordonnement (45–55%) constants sur une large gamme de conditions. Des différences entre les relations de potentiel d’expansion et la charge totale de couche, et du potentiel d’expansion et du nombre de cations intercouche non-échangeables de ces minéraux ont aussi été trouvées. Dans les micas/smectites de la région Shinzan, des changements chimiques dans les intercouches et dans les sites tétraèdraux et octaèdraux sont compatibles avec une réaction dans laquelle l’enrichissement de K et la fixation de K dans les intercouches sont contrôlés par une augmentation de la charge négative de couche. Cette conversion s’est produite en réaction à un raide gradient géothermique et à des solutions hydrothermiques émigrantes. [D.J.]

APPLICATION OF CHEMICAL GEOTHERMOMETRY TO LOW-TEMPERATURE TRIOCTAHEDRAL CHLORITES

Low-temperature chlorites formed in diagenetic to low-grade metamorphic environments generally have greater Si contents and larger numbers of octahedral vacancies, and smaller Fe+Mg contents than higher-grade metamorphic chlorites. The compositional variations are characterized approximately by four end-member components: Al-free trioctahedral chlorite, chamosite, corundophilite, and sudoite. The solid solution is considered to be a random mix of cations and vacancies in the octahedral sites. Using the compositions of chlorites from Niger, Rouez, and Saint Martin diagenetic-hydrothermal series, a new, more convenient geothermometer, applicable to low-T chlorites is proposed and comparison made with geothermometers proposed previously. The chlorites studied contain appreciable amounts of Fe(III) (>14% of the total Fe), determined by Mössbauer spectroscopy. The calculations under which all Fe was regarded as ferrous gave considerable overestimates for the formation temperature, irrespective of the geothermometer used. This problem was reduced by taking into account the presence of Fe(III) in the octahedral sites. The geothermometer from this study gave more reasonable estimates than the geothermometers proposed by Walshe (1986) and Vidal et al. (2001), particularly in the case of the Niger chlorites which crystallized in the lowest-temperature conditions. The ordered-site substitution model of solid solution developed by Vidal et al. (2001) predicted satisfactorily the formation temperature of the Rouez chlorites and of some of the Saint Martin chlorites, suggesting that the chlorite compositions are controlled by the

Morphology of clay minerals in the smectite-to-illite conversion series by scanning electron microscopy

{\rm{Si}}\square R_{ - 2}^{2 + }

Potassium fixation by clay minerals during hydrothermal treatment

Si□R−22+ exchange at low-T conditions while they are controlled by Tschermak exchange at higher temperatures. The decreasing number of vacancies with temperature are poorer in Fe-rich than in Fe-poor chlorites. Furthermore, the ordered-site occupation of cations and vacancies in trioctahedral chlorite occurs concomitantly with the compositional changes ruled by increasing temperature conditions.

Smectite-to-illite conversion in natural hydrothermal systems

The morphology of illite/smectite (I/S) from deeply buried bentonites and hydrothermally altered Tertiary volcanic rocks from Japan changes in parallel with the proportion of expandable layers in the I/S. As viewed by scanning electron microscopy, the morphologies range from the typical “cornflake,” “maple leaf,” or “honeycomb” habit of smectite to the typical platy or scalloped (with curled points) habit of illite. Although the changes are more subtle near either end member, at a composition of 60-70% illite layers, the morphology changes from sponge-like or cellular to platy or ribbon-like. The change of morphology at this composition correlates with a change in layer stacking from turbostratic to rotational ordering of the 1Md type. Turbostratic stacking can be thought of as randomly distributed translations of successive layers by any magnitude and in any direction. The rotationally ordered structure, which allows nearly precise juxtaposition of quasihexagonal oxygen surfaces from adjacent layers, probably permits more crystalline regularity in the a-b plane, which promotes a more plate-like or sheet-like habit.

Characterization of illitization of smectite in bentonite beds at Kinnekulle, Sweden

The amount of K fixed in K- and Ca-saturated montmorillonite, vermiculite (trioctahedral), rectorite-type and IMII-ordered mica/montmorillonites was measured as a function of time (1–64 days), temperature (25o-300°C), pH (6.0, 9.7, and 10.7), and K-concentration (0.02 and 1.0 M) in solution. The amount of K fixed by the clays generally increased with increasing temperature, pH, and K-concentration and reached saturation in response to each experimental condition in 5 or 6 days. The K-montmorillonite and K-vermiculite fixed considerable amounts of K even at 25°C. Fixed K in montmorillonite increased with an increase of the layer charge which is also influenced significantly by the interlayer cation. In detail, the behavior in K-fixation was specific to each clay.The type of structural transformation with K-fixation was different for each clay. In montmorillonite, especially, the type of transformation was related to the cationic composition of the system; in K homoionic system, montmorillonite transformed rapidly into illite/montmorillonite with about 40% expandable layers at 300°C and in a mixed cation system with Ca and K, it reacted gradually to random illite/montmorillonites with increasing temperature. These data indicate that the cation-exchange process of a natural pore solution plays an important role in the gradual transformation of detrital montmorillonite to illite.РезюмеКоличества К, фиксированного в К- и Ca-насыщенных монтмориллоните, вермикулите (трехоктаэдрическом), и слюде/монтмориллоните типа ректорита и IMII-упорядоченной, измерялось в зависимости от времени (от 1 до 64 дней), температуры (от 25° до 300°C), рН (6,0, 9,7, и 10,7) и концентрации К (0,02 и 1,0 M) в растворе. Количество К, фиксированного глинами, в основном увеличивалось с увеличением температуры, рН, и концентрации К и достигало насыщения в каждых экспериментальных условиях в течение 5 или 6 дней. К-монтмориллонит и К-вермикулит фиксировали значительные количества К даже при 25°C. Количество К, фиксированного в монтмориллоните, увеличивалось с увеличением слойного заряда, который также подвергался значительно влиянию межслойного катиона. В особенности поведение процесса фиксации К было специфическое для каждой глины. Особенно, в монтмориллоните, тип трансформации зависел от катионного состава системы; в одноионной системе К монтмориллонит быстро трансформировался в иллит/монтмориллонит с около 40% расширяющихся слоев при 300°C, а в системе со смещанными катионами Ca И К, монтмориллонит видоизменялся постепенно при увеличении температуры в неупорядоченный иллит/монтмориллонит. Эти данные указывают на то, что процесс обмена катионов естественных поровых растворов играет важную роль в постепенной трансформации детритального монтмориллонита в иллит. [E.C.]ResümeeDie K-Menge, die an K- und Ca-gesättigten Montmorillonit, Vermiculit (trioktaedrisch), und Montmorillonit vom Rektorit-Typ und IMII-geordneten Glimmer/Montmorillonit gebunden ist, wurde in Abhängigkeit von Zeit (1–64 Tage), Temperatur (25°C–300°C), pH-Wert (6,0, 9,7, und 10,7), und K-Konzentration (0,02 und 1,0 m) der Lösung untersucht. Die K-Menge, die an Tone gebunden wird, steigt im allgemeinen mit zunehmender Temperatur, steigendem pH, und steigender K-Konzentration und erreicht den Sättigungszustand, in Abhängigkeit von den jeweiligen experimentellen Bedingungen, nach 5 bis 6 Tagen. K-Montmorillonit und K-Vermiculit binden beachtliche K-Mengen selbst bei 25°C. Die an Montmorillonit gebundene K-Menge wächst mit zunehmender Schichtladung, die ebenfalls beachtlich durch die Zwischenschichtkationen beeinflußt wird. Im einzelnen ist die K-Bindung für jeden Ton spezifisch.Die Art der strukturellen Umwandlung durch die K-Bindung war bei jedem Ton anders. Bei Montmorillonit war die Art der Umwandlung von der Zusammensetzung der Kationen im System abhängig; im reinen K-System wandelte sich der Montmorillonit rasch in Illit/Montmorillonit um mit etwa 40% bei 300°C expandierbaren Lagen. In einem gemischten Kationensystem mit Ca und K wandelte sich der Montmorillonit allmählich mit zunehmender Temperatur in unregelmäßige Illit/Montmorillonit-Wechsellagerungen um. Diese Ergebnisse deuten darauf hin, daß der Ionenaustausch mit einer natürlichen Porenlösung eine wichtige Rolle bei der allmählichen Umwandlung von detritischem Montmorillonit in Illit spielt. [U.W.]RésuméLa quantité de K fixée dans la montmorillonite saturée de K et de Ca, dans la vermiculite (trioctaèdrale), et dans les montmorillonites/mica du type rectorite et ordonnées-IMII a été mesurée en fonction du temps (1–64 jours), de la température (25°–300°C), du pH (6,0, 9,7, et 10,7) et de la concentration en K (0,02 et 1,0 M) en solution. La quantité de K fixée dans les argiles a généralement augmenté proportionnellement à la température, au pH et à la concentration en K, et a atteint la saturation vis á vis de chaque condition expérimentale en 5 ou 6 jours. La montmorillonite-K et la vermiculite-K ont fixé des quantités considérables de K même à 25°C K fixé dans la montmorillonite a augmenté proportionnellement à la charge de couche qui est aussi influencée de manière significative par le cation intercouche. En détait, le comportement dans la fixation de K ètait spécifique pour chaque argile.Le genre de transformation structurale avec la fixation de K était différent pour chaque argile. Dans la montmorillonite, spécialement, le genre de transformation était apparenté à la composition cationique du système; dans un système homoionique K, la montmorillonite s’est rapidement transformée en illite/montmorillonite avec à peu près 40% de couches expansibles à 300°C, et dans un système melangé avec Ca et K, elle a reagi graduellement en des illite/montmorillonites quelconques proportionnellement à une augmentation de température. Ces données indiquent que le procédé d’échange de cations d’une solution naturelle de pores joue un rôle important dans la transformation graduelle de montmorillonite detritique en illite. [D.J.]

HRTEM evidence for the process and mechanism of saponite-to-chlorite conversion through corrensite

Abstract Smectite-to-illite (S-to-I) conversion in natural hydrothermal systems including active and fossil geothermal fields can be characterized from the viewpoints of mode of conversion, kinetics, and mechanism and this can be related to the durability of buffer materials of high level radioactive waste. From the viewpoint of conversion, in the active geothermal field which can be used as a case study, the S-to-I conversion occurs in different ranges of present day temperature from drill hole to drill hole, which is related to the thermal history of the field up to two or three discrete illite/smectite (I/S) phases having different percents of illite layers (%I) coexist metastably at intermediate stages of the conversion. This is particularly visible in drill holes where the conversion occurs at high temperatures. The S-to-I conversion from a fossil geothermal field suggests that the greater the thermal gradient, the more the smectite illitization is facilitated. By having the average rate of entire S-to-I conversion, which was calculated from the data of K-Ar and % I in I/S, the activation energy required to complete the S-to-I conversion was estimated to be approximately 30 ± 5 kcal/mol in natural hydrothermal systems, where the S-to-I conversion was assumed as a first order reaction and the pre-exponential factor is the same as Eberl and Hower (1976). The estimated value is close to those deduced previously from laboratory experiments. This suggests that the experiments made by using specific geologic materials and conditions may be applicable to evaluate the kinetics of S-to-I conversion in hydrothermal systems from the viewpoint of an empirical equation. Nevertheless, since the S-to-I conversion in natural hydrothermal systems is in fact controlled by mechanisms of smectite dissolution and recrystallization of illite, it will be necessary in the future to construct the real reaction kinetics based on a proven mechanism.

CHEMIOGRAPHIC ANALYSIS OF TRIOCTAHEDRAL SMECTITE-TO-CHLORITE CONVERSION SERIES FROM THE OHYU CALDERA, JAPAN

Structure, morphology, and chemical composition of illite/smectite (I/S) containing 30–50% smectite layers (% S) from Kinnekulle bentonites, Sweden, of diagenetic origin were examined using X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). Interlayer arrangements of I/S changed from random interstratification to short-range ordered at about 40% S. The transition from random to ordered structure proceeded continuously as reflected by the gradual decrease in probability of two smectite neighbors (Pss) towards zero. TEM observations of water-dispersed samples that had not been cation-exchanged showed that the I/S consisted dominantly of flakes coexisting with laths having a length/width ratio of about 4, regardless of % S. The thickness of the I/S particles ranged from 30 to 100 Å, and no systematic variation in thickness was detected with decreasing % S. The chemical composition of the I/S also changed continuously with decreasing % S. These observations suggest no dissolution of smectite layers and no recrystallization of illite layers during the formation of the I/S in these bentonites; rather, cationic substitutions occurred within a smectite precursor (termed a solid-state transformation mechanism). A comparison of interlayer order, particle texture, and chemistry of the I/S from various types of rocks suggests that the mechanism of smectite-to-illite conversion in the range 100% S-30% S was related to the porosity and permeability of original rocks. The solid-state transformation mechanism appears to have predominated in rocks of low porosity and permeability.

ILLITE-SMECTITE MIXED-LAYER MINERALS IN THE HYDROTHERMAL ALTERATION OF VOLCANIC ROCKS: I. ONE-DIMENSIONAL XRD STRUCTURE ANALYSIS AND CHARACTERIZATION OF COMPONENT LAYERS

Abstract To elucidate the process and mechanism of the prograde conversion of saponite to chlorite through corrensite, the microstructures of a series of chlorite-smectite (C-S) mixed-layer samples from Kamikita, northern Japan were examined by high-resolution transmission electron microscopy using both lattice and structure imaging. Corrensite grows epitaxially as domains of 5-20 nm thick mainly within homogeneous saponite domains, without forming randomly interstratified C-S. Then, chlorite domains grow outside homogeneous corrensite domains without forming randomly interstratified C-S or chloritecorrensite (C-Co), and finally are predominant. An atomic resolution image suggests that corrensite essentially consists of the 1 M stacking of alternating chloritic and smectitic layers. The structure and occurrence suggest corrensite is mineralogically a unique species. Comparison of the stacking vectors of corrensite along [001] to those of chlorite reveals that the stacking sequence is not inherited during the process from corrensite to chlorite. We rarely observed layer terminations of the hydroxide sheets both in corrensite and chlorite domains, and the layer terminations that do exist can be explained as defects rather than the formation of corrensite or chlorite. Our data strongly suggest that the saponitetochlorite conversion series progresses stepwise from saponite to corrensite and from corrensite to chlorite, and that the dominant reaction mechanisms are dissolution and precipitation.