Jan Srodon

X-RAY POWDER DIFFRACTION IDENTIFICATION OF ILLITIC MATERIALS

The 10-Å clay components of sedimentary rocks (“illites”) are commonly mixtures of 100% nonexpandable illite and an ordered illite/smectite mixed-layer mineral. If the proportion of the illite/ smectite in a mixture is sufficient to produce a measurable reflection between 33–35°2θ (CuKα radiation) that is noncoincident with an illite reflection, the ratio of component layers and type of interstratification for the mixed-layer mineral can be determined. The identification technique developed in this study rests upon the following experimental findings for ordered illite/smectites of diagenetic origin: (1) the thickness of the illite layer in illite/smectites is 9.97 Å; (2) the thickness of smectite-ethylene glycol complex ranges from 16.7 to 16.9 Å; (3) illite/smectites form a continuous sequence of interstratification types—random, random/IS, IS, IS/ISII, ISII—and each type is related to a specific range of expandability.The new technique broadens the computer simulation method developed by R. C. Reynolds and J. Hower to include those sedimentary materials which are dominated by the presence of discrete illite, are low in illite/smectite, and, as such, have been described previously only by an “illite crystallinity index.”Резюме10-Å глинистые компоненты осадочных пород (“иллиты”) обычно являются смесями 100% нерасширяемого иллита и упорядоченного минерала типа смешанно-слойного иллита/смектита (ИС). Отношение составляющих слоев и тип переслаивания для смешанно-слойного минерала могут быть определены, если пропорция иллита/смектита в смеси достаточна, чтобы вызвать измеряемое отра¬жение между 33–35°2θ (излучение СиКα), которое не совпадает с отражением иллита. Техника иден¬тификации, разработанная в этой статье, основывается на последовательных экспериментальных данных для упорядоченных иллитов/смектитов диагенетического происхождения: (1) толщина ил-литового слоя в иллите/смектите равна 9,97 Å; (2) толщина комплекса смектита с этиленовым гликолом изменяется в диапазоне от 16,7 до 16,9 Å; (3) иллиты/смектиты образовывают непрерывный ряд типов прослоев—беспорядочный, беспорядочный/ИС, ИС, ИС/ИСЦ, ИСП-и каждый тип связан со специфическим диапазоном расширяемости.Эта новая техника расширяет метод компьютерного моделирования, развитый Рейнольдсом и Гоуером и включает такие осадочные материалы, в которых находится отдельный ил лит, которые имеют малые количества иллита/смектита и которые, как таковые, предварительно описывались только при помощи “индекса кристальности иллита.” [Е.G.]ResümeeDie 10-Å Tonkomponenten von sedimentären Gesteinen (“Illite”) sind gewöhnlich Mischlingen aus 100% nicht expandierbarem Illit und einem regelmäßigen Illit/Smektit-Wechsellagerungsmineral. Wenn das Verhältnis von Illit/Smektit in einer Mischung ausreicht, um einen meßbaren Reflex zwischen 33 und 35°2θ (CuKα-Strahlung) zu erzeugen, der nicht mit einem Illitreflex zusammenfällt, dann kann das Verhältnis der Komponentenschichten und die Art der Wechsellagerung für das Wechsellagerungsmineral bestimmt werden. Die Identifikationstechnik, die in dieser Untersuchung entwickelt wurde, beruht auf den folgenden experimentellen Ergebnissen für geordnete Illit/Smektit-Wechsellagerungen diagenetischen Ursprungs: (1) Die Dicke der Illitlagen in den Illit/Smektit-Wechsellagerungen beträgt 9,97 Å; (2) die Dicke des Smektit-Äthylenglykolkomplexes reicht von 16,7–16,9 Å; (3) Illit-Smektitwechsellagerungen bilden eine kontinuierliche Abfolge von Wechsellagerungstypen—unregelmäßige, unregelmäßige/IS, IS, IS/ISII, ISII—und jeder Typ gehört zu einem bestimmten Bereich von Expandierbarkeit.Die neue Untersuchungsmethode baut die Computersimulationsmethode aus, die von R. C. Reynolds und J. Hower entwickelt wurde, um solche sedimentären Materialien mit einzuschließen, bei denen diskreter Illit vorherrscht, die wenig Illit/Smektit enthalten, und die, als solche, früher nur durch einen “Illit-Kristallinitätsindex” beschrieben wurden. [U.W.]RésuméLes composés argile de 10 Å de roches sédimentaires (“illites”) sont communément des mélanges d’illite 100% non expansible et d’un minéral ordonné à couches mélangées illite/smectite. Si la proportion d’illite/smectite dans un melange est suffisante pour produire une reflection mesurable entre 33-35°2θ (radiation CuKα) qui ne coïncide pas avec une reflection illite, on peut déterminer la proportion de couches du composé et le genre d’interstratification du minéral à couches mélangées. La technique d’identification développée dans cette étude est basée sur les trouvailles expérimentales suivantes pour des illite/ smectites d’origine diagénétique: (1) l’épaisseur de la couche illite dans les illite/smectites est 9,97 Å: (2) l’épaisseur du complexe glycol smectite-éthylène s’étend de 16,7 à 16,9 Å; (3) les illite/smectites forment une séquence continuelle de types d’interstratification—au hasard, au hasard IS, IS, IS/ISII, ISII—et chaque type est apparenté à une étendue spécifique de pouvoir de dilatation.La nouvelle technique élargit la méthode de simulation à l’ordinateur développée par R. C. Reynolds et J. Hower pour inclure les matériaux sédimentaires qui sont dominées par la présence d’illite discrète, ont un bas contenu en illite/smectite, et, en tant que tels, n’ont jusqu’à présent été décrits que par un “indexe de cristallinité d’illite.” [D.J.]

Precise identification of illite/smectite interstratifications by X-ray powder diffraction

The thickness of the two-layer ethylene glycol complex of dioctahedral smectites varies under room conditions between 17.3 and 16.5 Å because of such factors as layer charge density, type of interlayer cation, and relative humidity. Neglecting this variability can give up to 30% error in the X-ray powder diffraction estimation of the smectite:illite ratio of the mixed-layer structures. Three methods have been developed for the interpretation of X-ray powder diffraction patterns of glycolated mixed-layer illite/smectite which take layer-spacing variability into account. The methods include a technique for quantifying the degree of layer ordering. In addition, the proposed techniques minimize the error due to the influence of domain size on positions of reflections. The experimental error can be kept below 5% or below 1% smectite layers, depending on the method applied, provided that the peak positions are measured with the accuracy of ± 0.02°2θ.РезюмеТолщина двух-слойного этиленгликолевого комплекса с диоктаэдрическим смектитом изменяется, при комнатных условиях, в пределах 17,3 и 16,5 А, вследствие таких факторов как плотность заряда слоя, тип межслойного катиона, и относительная влажность. Неучет этой изменчивости может вызвать ошибку до 30% в оценке отношения смектит:иллит смешанно-слойных структур при использовании порошкового метода рентгено-структурного анализа. Были разработаны три метода для интерпретации картин результатов исследования гликолированного смешанно-слойного иллита/смектита порошковым методом рентгено-структурного анализа, которые учитывают изменчивость расположения слоев. Методы включают прием для количественного определения степени упорядочения. Кроме того, предложенные методы доводят до минимума ошибку, связанную с влиянием размера домена на положение отражений. Экспериментальная ошибка может быть меньше 5% или меньше 1% смектитовых слоев в зависимости от используемого метола при условии, что позиции пиков измерены с точностью ±О,О2°20. [Ы.Е.]ResümeeDie Dicke eines Zweischicht-Äthylen-Glykol-Komplexes mit dioktaedrischen Smektiten variiert bei Raumtemperatur Zwichen 17,3 und 16,5 Å aufgrund von Faktoren wie Dichte der Schichtladung, Art des Zwischenschichtkations, und relative Feuchtigkeit. Eine Vernachlässigung dieser so hervorgerufenen Schwankung kann bei der Abschätzung des Verhältnisses von Smektit:Illit in Wechsellagerungsstrukturen mittels Röntgenpulverdiffraktometrie zu einem Fehler bis zu 30% führen. Es wurden drei Methoden für die Interpretation von Röntgendiffraktometeraufnahmen von mit glykol behandelten Illit/ Smektit-Wechsellagerungen entwickelt, die die Variation des Schichtabstandes berücksichtigen. Diese Methoden beinhalten eine Methode für die Quantifizierung des Ördnungsgrades. Zusätzlich reduzieren die vorgeschlagenen Methoden den Fehler, der durch den Einfluß der Domänengröße auf die Peaklage herrührt, auf ein Minimum. Der experimentelle Fehler kann kleiner als 5% bzw. als 1% der Smektitlagen gehalten werden, je nach der verwendeten Methode, vorausgesetzt, daß die Peaklagen mit einer Genauigkeit von ±0,02°2θ gemessen werden. [U.W.]RésuméL’épaisseur du complex glycol ethylene à 2 couches avec des smectites dioctaèdrales varie sous des conditions ambiantes entre 17,3 et 16,5 Å, à cause de facteurs tels que la densité de charge de couche, le genre de cation interfolaire, et l’humidité relative. Si l’on néglige cette variabilité, une erreur de 30% peut être introduite dans l’estimation à la diffraction poudrée aux rayons-X de la proportion smectite:illite de structures à couches mélangées. Trois méthodes qui tiennent compte de la variabilité de l’espacement de couches ont été développées pour l’interprétation de clichés de diffraction poudrée aux rayons-X d’illite/ smectite glycolatée à couches mélangées. Les méthodes comprennent une technique pour quantifier le degré de rangement. De plus, les techniques proposées minimisent l’erreur due à l’influence de la taille du domaine sur les positions des réflections. L’erreur expérimentale peut être maintenue sous 5% ou sous 1% couches de smectite, dépendant de la méthode utilisée, à condition que les positions des sommets sont mesurées avec une exactitude de ±0,02°2θ. [D.J.]

XRD Measurement of Mean Crystallite Thickness of Illite and Illite/Smectite : Reappraisal of the Kubler Index and the Scherrer Equation

The standard form of the Scherrer equation, which has been used to calculate the mean thickness of the coherent scattering domain (CSD) of illite crystals from X-ray diffraction (XRD) full width data at half maximum (FWHM) intensity, employs a constant, Ksh, of 0.89. Use of this constant is unjustified, even if swelling has no effect on peak broadening, because this constant is valid only if all CSDs have a single thickness. For different thickness distributions, the Scherrer “constant” has very different values.Analysis of fundamental particle thickness data (transmission electron microscopy, TEM) for samples of authigenic illite and illite/smectite from diagenetically altered pyroclastics and filamentous illites from sandstones reveals a unique family of lognormal thickness distributions for these clays. Experimental relations between the distributions’ lognormal parameters and mean thicknesses are established. These relations then are used to calculate the mean thickness of CSDs for illitic samples from XRD FWHM, or from integral XRD peak widths (integrated intensity/maximum intensity).For mixed-layer illite/smectite, the measured thickness of the CSD corresponds to the mean thickness of the mixed-layer crystal. Using this measurement, the mean thickness of the fundamental particles that compose the mixed-layer crystals can be calculated after XRD determination of percent smectitic interlayers. The effect of mixed layering (swelling) on XRD peak width for these samples is eliminated by using the 003 reflection for glycolated samples, and the 001, 002 or 003 reflection for dehydrated, K-sa-turated samples. If this technique is applied to the 001 reflection of air-dried samples (Kubler index measurement), mean CSD thicknesses are underestimated due to the mixed-layering effect.The technique was calibrated using NEWMOD©-simulated XRD profiles of illite, and then tested on well-characterized illite and illite/smectite samples. The XRD measurements are in good agreement with estimates of the mean thickness of fundamental particles obtained both from TEM measurements and from fixed cations content, up to a mean value of 20 layers. Correction for instrumental broadening under the conditions employed here is unnecessary for this range of thicknesses.

Chemistry of illite/smectite and end-member illite

Chemical data from three different series of diagenetic illite/smectites (I/S), analyzed statistically by two regresion techniques, indicate that the content of fixed-K per illite layer is not constant, but ranges from ~0.55 per O10(OH)2 for illite layers in randomly interstratified I/S (R=0; >50% smectite layers) to ~ 1.0 per O10(OH)2 for illite layers formed in ordered I/S (R>0; <50% smectite layers). By extrapolation of the experimental data, the following chemical characteristics were obtained for end-member illite derived from the alteration of smectite in bentonite: average fixed-K per illite layer = 0.75 per O10(OH)2; total charge = about -0.8; cation-exchange capacity = 15 meq/100 g; surface area (EGME) = 150 m2/g.

XRD measurement of mean thickness, thickness distribution and strain for illite and illite-smectite crystallites by the Bertaut-Warren-Averbach technique

A modified version of the Bertaut-Warren-Averbach (BWA) technique (Bertaut 1949, 1950; Warren and Averbach 1950) has been developed to measure coherent scattering domain (CSD) sizes and strains in minerals by analysis of X-ray diffraction (XRD) data. This method is used to measure CSD thickness distributions for calculated and experimental XRD patterns of illites and illite-smectites (I-S). The method almost exactly recovers CSD thickness distributions for calculated illite XRD patterns. Natural I-S samples contain swelling layers that lead to nonperiodic structures in the c* direction and to XRD peaks that are broadened and made asymmetric by mixed layering. Therefore, these peaks cannot be analyzed by the BWA method. These difficulties are overcome by K-saturation and heating prior to X-ray analysis in order to form 10-Å periodic structures. BWA analysis yields the thickness distribution of mixed-layer crystals (coherently diffracting stacks of fundamental illite particles). For most I-S samples, CSD thickness distributions can be approximated by lognormal functions. Mixed-layer crystal mean thickness and expandability then can be used to calculate fundamental illite particle mean thickness. Analyses of the dehydrated, K-saturated samples indicate that basal XRD reflections are broadened by symmetrical strain that may be related to local variations in smectite interlayers caused by dehydration, and that the standard deviation of the strain increases regularly with expandability. The 001 and 002 reflections are affected only slightly by this strain and therefore are suited for CSD thickness analysis. Mean mixed-layer crystal thicknesses for dehydrated I-S measured by the BWA method are very close to those measured by an integral peak width method.

K-Ar dating of illite fundamental particles separated from illite-smectite

Abstract Fundamental particles of illite-smectite from bentonites were separated into classes by high-speed centrifugation after infinite osmotic swelling of mixed-layer crystals, achieved by Naexchange and dispersion in distilled water. In samples free of detrital contamination, the thinnest fundamental particles yield older K-Ar ages than the thicker fundamental particles. This implies that they do not preferentially lose radiogenic 40Ar due to size, and that the illitization process is a crystal growth mechanism (not nucleation plus growth). As a result, any K-Ar age of fundamental illite particles from bentonites is an integral over longer or shorter periods of time, depending on the thermal history of the rocks. In thick bentonite beds, the measured age difference between the beginning of the illitization process at the contact with the host rocks and the end in the centre of the bed records extremely slow K diffusion in these well compacted rocks. These data explain why measured K-Ar ages of illite-smectite from bentonites are younger than the corresponding age of shale illitization, inferred from the burial history of the basin. The finest technically separable size-fractions of associated shales (<0.02 μm) yield K-Ar dates* greater than the stratigraphic age. This observation points to incomplete recrystallization of detrital illite during burial diagenesis.

EVOLUTION OF FUNDAMENTAL-PARTICLE SIZE DURING ILLITIZATION OF SMECTITE AND IMPLICATIONS FOR REACTION MECHANISM

Area-weighted thickness distributions of fundamental illite particles for samples of illite and illite-smectite from seven locations (including bentonites and hydrothermally altered pyroclastics) were measured by Pt-shadowing technique, by transmission electron microscopy. Most thickness distributions are described by lognormal distributions, which suggest a unique crystallization process. The shapes of lognormal distributions of fundamental illite particles can be calculated from the distribution mean because the shape parameters α and β2are interrelated: β2= 0.107α − 0.03. This growth process was simulated by the mathematical Law of Proportionate Effect that generates lognormal distributions. Simulations indicated that illite particles grow from 2-nm thick illite nuclei by surface-controlled growth, i.e., the rate of growth is restricted by how rapid crystallization proceeds given a near infinite supply of reactants, and not by the rate of supply of reactants to the crystal surface. Initially formed, 2-nm thick crystals may nucleate and grow within smectite interlayers from material produced by dissolution of single smectite 2:1 layers, thereby transforming the clay from randomly interstratified (Reichweite, R = 0) to ordered (R = 1) illite-smectite after the smectite single layers dissolve. In this initial period of illite nucleation and growth, during which expandable layers range from 100 to 20%, illite crystals grow parallel to [001]* direction, and the dimensions of the (001) plane are confined to the size of the original smectite 2:1 layers. After nucleation ceases, illite crystals may continue to grow by surface-controlled growth, and the expandable-layer content ranges from 20 to 0%. This latter period of illitization is characterized by three-dimensional growth. Other crystal-growth mechanisms, such as Ostwald ripening, supply-controlled growth, and the coalescence of smectite layers, do not produce the observed evolution of α and β2and the observed shapes of crystal thickness distributions.

SURFACE AREA AND LAYER CHARGE OF SMECTITE FROM CEC AND EGME/H2O-RETENTION MEASUREMENTS

The total specific surface area (TSSA) and smectitic layer charge (Qs) calculated from the structural formulae and unit-cell dimensions of 12 pure smectite samples were used as a reference in the design and evaluation of TSSA and Qs measurement techniques based on cation exchange capacity (CEC), H2O retention at 47% RH, and ethylene glycol monoethyl ether (EGME) retention. A thermogravimetric analysis-mass spectrometry (TGA-MS) technique was used to study the release of H2O from smectite on heating, and to introduce a correction for H2O remaining in the smectite after heating to 110°C, because the sample weight at this temperature has been used routinely as a reference in CEC and EGME sorption measurements. A temperature of 200°C was found to be the optimum reference for such measurements.A good agreement between Qs from the structural formula and from CEC was obtained when this correction was applied. The TSSA of smectite was measured with similar accuracy (mean error of ±5–7%) by three techniques: (1) using mean H2O coverage; (2) using mean EGME coverage; and (3) using a combination of H2O coverage and CEC. A reduction of the mean error from 5–7% to 4% can be obtained by averaging these measurements, and a further reduction to 3% by introducing corrections for the dependence of H2O and EGME coverage on layer charge. The study demonstrates that Ca2+-smectite samples at 47% RH have H2O contents corresponding to 88–107% of the theoretical mass of a monolayer and offers an explanation of this variation.

Direction high-resolution transmission electron microscopic measurement of expandability of mixed-layer illite smectite in bentonite rock

Samples of mixed-layer illite/smectite were investigated from a single bentonite bed zoned with respect to expandability from 90 to 30%. Chips of natural rocks were embedded in a resin, using a procedure designed to preserve the original fabric, cut with an ultramicrotome, and observed by high-resolution transmission electron microscopy (HRTEM). These observations confirmed the X-ray powder diffraction (XRD) model of mixed-layer clays, i.e., that illite/smectite grains in natural rocks are built of mixed-layer crystals, from 1 to as many as 15 silicate layers thick (4–6 interlayers per crystal on average). These crystals are present either as individual particles (loose crystals) or, typically, they form nearly parallel face-to-face groupings called here quasi-crystals. Free fundamental smectite and illite particles as defined by Nadeau and coworkers were essentially absent.Illite and smectite interlayer spacings were 10 and 13.5 Å, respectively. Crystal thickness and number of interlayers were measured for 35–100 mixed-layer crystals per sample. Illite/smectite expandabilities were calculated from these data in two ways: either neglecting the crystal edges or accounting for them. The former determinations agree well with XRD estimates of expandability and the latter, with expandabilities calculated from the distributions of fundamental particle thickness measured by a shadowing technique in the TEM. This result explains the systematic discrepancy between XRD and TEM measurements of illite/smectite expandability.

Diagenetic history of the Podhale-Orava Basin and the underlying Tatra sedimentary structural units (Western Carpathians) : evidence from XRD and K-Ar of illite-smectite

Abstract Diagenesis in the Podhale and Orava Paleogene flysch basins and in the underlying Mesozoic structural units was studied by XRD measurement of the percent smectite in the mixed- layer illite-smectite from shales and K-Ar dating of the illite-smectite from bentonites, supported by XRD quantitative mineral analysis, grain density, and porosity measurements of the bulk shales. The diagenetic mineral reactions identified in the flysch shales include illitization of smectite (> 60 to < 5%S), dissolution of K-feldspar and kaolinite, crystallization of quartz, albite and chlorite. An unusually large amount of basin history information was obtained by combining the illite-smectite data from wells and from the present erosional surface of the basin. The rocks underwent burial diagenesis at a stable geothermal gradient similar to the present-day value of 21 ± 2℃/km. The maximum burial temperatures were reached very quickly (high sedimentation rate) close to the basin inversion time, at ~17 Ma in the western part and 18 Ma in the eastern part. The basin floor, which included the present-day Tatra Mts., was inclined towards the East. The thickness of the sedimentary filling of the basin ranged from 3.5-4.5 km in the western Tatra (removed entirely), to 5-6 km in the western Podhale (<3-4 km removed), to 6.5-7.5 km in the eastern Podhale (>4-5 km removed), and even more in the eastern Tatra and Spisska Magura close to the Ruzbachy Fault. These data imply a major subsidence followed by uplift of the Podhale plus Tatra block along the Ruzbachy Fault and the deposition of a thick sequence of Lower Miocene sediments over the entire area (latter removed by erosion). The Mesozoic rocks of all the structural units underlying the flysch basin underwent advanced diagenesis (maximum palaeotemperatures of 160-270°C) during an Upper Cretaceous tectonic burial event at ~80-90 Ma. The tectonic overload was removed before the Eocene transgression (49-42 Ma).