Jan Środoń

Ostwald ripening of clays and metamorphic minerals.

Analyses of particle size distributions indicate that clay minerals and other diagenetic and metamorphic minerals commonly undergo recrystallization by Ostwald ripening. The shapes of their particle size distributions can yield the rate law for this process. One consequence of Ostwald ripening is that a record of the recrystallization process is preserved in the various particle sizes. Therefore, one can determine the detailed geologic history of clays and other recrystallized minerals by separating, from a single sample, the various particle sizes for independent chemical, structural, and isotopic analyses.

Interpretation of K-Ar dates of illitic clays from sedimentary rocks aided by modeling

Abstract K-Ar dates of illitic clays from sedimentary rocks may contain “mixed ages,” i.e., may have ages that are intermediate between the ages of end-member events. Two phenomena that may cause mixed ages are: (1) long-lasting reaction during the burial illitization of smectite; and (2) physical mixing of detrital and diagenetic components. The first phenomenon was investigated by simulation of illitization reactions using a nucleation and growth mechanism. These calculations indicate that values for mixed ages are related to burial history: for an equivalent length of reaction time, fast burial followed by slow burial produces much older mixed ages than slow burial followed by fast. The type of reaction that occurred in a rock can be determined from the distribution of ages with respect to the thickness of illite crystals. Dating of artificial mixtures confirms a non-linear relation between mixed ages and the proportions of the components. Vertical variation of K-Ar age dates from Gulf Coast shales can be modeled by assuming diagenetic illitization that overprints a subtle vertical trend (presumably of sedimentary origin) in detrital mineral content.

Identification and Quantitative Analysis of Clay Minerals

Abstract This chapter reviews in a comprehensive manner the basic techniques of clay separation, identification, and quantification with particular emphasis on the XRD method. Fundamental textbooks on the topic are shortly characterized. The chapter is thus aimed at introducing the fundamentals of clay mineral research. The techniques of clay separation and sample treatment are presented in detail, mostly following the classic approach of M.L. Jackson. The criteria for distinguishing pure from mixed-layer clay minerals are given. The general logic of identifying pure clay minerals is presented and the techniques specific for different clay groups are discussed in more detail. The reviewed techniques of identification of mixed-layer clay minerals include the peak position techniques, the peak broadening techniques, the expert systems, and the computer modelling of entire XRD patterns. The techniques of clay mineral crystal size measurements are also presented. Quantitative XRD mineral analysis of bulk rocks containing clay minerals is covered in detail, including sample preparation, data acquisition, and data processing by various approaches. Quantitative XRD mineral analysis of clay fractions based on oriented preparations is also discussed.

Thermal history of the Podhale Basin in the internal Western Carpathians from the perspective of apatite fission track analyses

Abstract The thermal history of the Paleogene Podhale Basin was studied by the apatite fission track (AFT) method. Twenty four Eocene-Oligocene sandstone samples yielded apparent ages from 13.8 ± 1.6 to 6.1 ± 1.4 Ma that are significantly younger than their stratigraphic age and thus point to a post-depositional resetting. The thermal event responsible for the age resetting is interpreted as a combination of heating associated with mid-Miocene volcanism and variable thickness of Oligocene and potentially also Miocene sediments. Extending the mid-Miocene thermal event found in the Inner Carpathians into the Podhale Basin as a likely heat source suggests that the amount of denudation in the Podhale Basin determined only on the basis of heat related to the thickness of sedimentary sequence might have be significantly overestimated. Two samples from the western part of the basin that yielded 31.0 ± 4.3 and 26.9 ± 4.7 Ma are interpreted as having mixed ages resulting from partial resetting in temperature conditions within the AFT partial annealing zone. This observation agrees very well with reported vitrinite reflectance and illite-smectite thermometry, which indicate a systematic drop of the maximum paleotemperatures towards the western side of the basin.

Weathering, sedimentary and diagenetic controls of mineral and geochemical characteristics of the vertebrate-bearing Silesian Keuper

Abstract Mudstones and claystones from the southern marginal area of the European Upper Triassic, midcontinental Keuper basin (Silesia, southern Poland) were investigated using XRD, organic and inorganic geochemistry, SEM, K-Ar of illite-smectite, AFT, and stable isotopes of O and C in carbonates in order to unravel the consequent phases of the geological history of these rocks, known for abundant fossils of land vertebrates, and in particular to evaluate the diagenetic overprint on the mineral composition. The detected and quantified mineral assemblage consists of quartz, calcite, dolomite, Ca-dolomite, illite, mixed-layer illite-smectite, and kaolinite as major components, plus feldspars, hematite, pyrite, chlorite, anatase, siderite, goethite as minor components. Palygorskite, gypsum, jarosite and apatite were identified in places. The K-Ar dates document a post-sedimentary thermal event, 164 Ma or younger, which resulted in partial illitization of smectite and kaolinite. The maximum palaeotemperatures were estimated from illite-smectite as ~125°C. Apatite fission track data support this conclusion, indicating a 200-160 Ma age range of the maximum temperatures close to 120°C, followed by a prolonged period of elevated temperatures. These conclusions agree well with the available data on the Mesozoic thermal event, which yielded Pb-Zn deposits in the area. Organic maturity indicators suggest the maximum palaeotemperatures <110°C. Palygorskite was identified as authigenic by crystal morphology (TEM), and calcite by its accumulation in soil layers and by its isotopic composition evolving with time, in accordance with the sedimentary and/or climatic changes. Dolomite isotopic composition indicates more saline (concentrated) waters. Palygorskite signals a rapid local change of sedimentary conditions, correlated with algal blooms. This assemblage of authigenic minerals indicates an arid climate and the location at the transition from a distal alluvial fan to mudflat. Fe-rich smectite, kaolinite, and hematite were products of chemical weathering on the surrounding lands and are therefore mostly detrital components of the investigated rocks. Kaolinite crystal morphology and ordering indicates a short transport distance. Hematite also crystallized in situ, in the soil horizons. A large variation in kaolinite/2:1 minerals ratio reflects hydraulic sorting, except of the Rhaetian, where it probably signals a climatic change, i.e. a shift in the weathering pattern towards kaolinite, correlated with the disappearance of hematite. Quartz, 2M1 illite, and minor feldspars and Mg-chlorite were interpreted as detrital minerals. The documented sedimentation pattern indicates that in more central parts of the Keuper playa system, where an intense authigenesis of the trioctahedral clays (chlorite, swelling chlorite, corrensite, sepiolite) took place, illite and smectite were the dominant detrital clay minerals. Cr/Nb and Cr/Ti ratios were found as the best chemostratigraphic tools, allowing for the correlation of all investigated profiles. A stable decrease of these ratios up the investigated sedimentary sequence is interpreted as reflecting changes in the provenance pattern from more basic to more acidic rocks.

Thickness distribution of illite crystals in shales. II: Origin of the distribution and the mechanism of smectite illitization in shales

The distributions of illite crystal (‘fundamental particle’) thickness in <0.2 µm fractions of 13 shale samples (from the Carpathian Foredeep, Poland), obtained using the Bertaut-Warren-Averbach X-ray diffraction method (MudMaster computer program), were analyzed and interpreted in terms of the mechanism of smectite illitization. All illite crystal thickness distributions in the analyzed shales are characterized by an ‘asymptotic’ shape. The origin of the asymptotic-type distributions is explained by the heterogeneity of illite crystals in shales, i.e. superposition of different populations of crystals, those included in illite-smectite (I-S) interstratifications, and those which occur as discrete illite. The analysis of the distributions in shales shows that the most frequent thickness class of illite crystals forming I-S is 2 nm. Discrete illite is composed of thicker crystals; though crystals as thin as 2 nm can also contribute to this population. The modeling of asymptotic distributions in shales by using a number of theoretical lognormal distributions shows that with progressive diagenesis, the mean thickness of illite crystals forming the I-S component increases gradually, whereas the discrete illite does not show a clear trend.The diverse origins of illite crystals in shales do not permit determination of the mechanism of smectite illitization from the evolution of the shape of the overall crystal-thickness distribution during diagenesis. Therefore, in order to understand the mechanism of smectite illitization in shales, an attempt was made to trace the relative gains and losses of crystals of different thicknesses during illitization. This approach indicates the following mechanism of smectite illitization in the investigated shales: dissolution of smectite monolayers accompanied by growth of 2 nm crystals which are largely of detrital origin. Nucleation of 2 nm illites seems to be very limited.

Qualitative and quantitative mineralogical composition of the Rupelian Boom Clay in Belgium

Abstract The Boom Clay Formation of early Oligocene age, which occurs underground in northern Belgium, has been studied intensively for decades as a potential host rock for the disposal of nuclear waste. The goal of the present study is to determine a reference composition for the Boom Clay using both literature methods and methods developed during this work. The study was carried out on 20 samples, representative of the lithological variability of the formation. The bulk-rock composition was obtained by X-ray diffraction using a combined full-pattern summation and singlepeak quantification method. Siliciclastics vary from 27 to 72 wt.%, clay minerals with 25-71 wt.% micas, 0-4 wt.% carbonates, 2-4 wt.% accessory minerals (mainly pyrite and anatase) and 0.5-3.5 wt.% organic matter. This bulk-rock composition was validated independently by majorelement chemical analysis. The detailed composition of the clay-sized fraction was determined by modelling of the oriented X-ray diffraction patterns, using a larger sigma star (σ*) value for discrete smectite than for the other clay minerals. The <2 μm clay mineralogy of the Boom Clay is qualitatively homogeneous; it contains 14-25 wt.% illite, 19-39 wt.% smectite, 19-42 wt.% randomly interstratified illite-smectite with about 65% illite layers, 5-12 wt.% kaolinite, 4-17 wt.% randomly interstratified kaolinite-smectite and 2-7 wt.% chloritic minerals (chlorite, ‘‘defective’’ chlorite and interstratified chlorite-smectite). All modelled clay mineral proportions were verified independently using major-element chemistry and cation exchange capacity measurements. Bulkrock and clay mineral analysis results were combined to obtain the overall detailed quantitative composition of the Boom Clay Formation.

Role of clays in the diagenetic history of nitrogen and boron in the Carboniferous of Donbas (Ukraine)

Abstract The evolution of the nitrogen and boron content of shales during diagenesis and anchimetamorphism was studied in the Carboniferous rocks of Donbas by means of the bulk rock chemical analysis and the XRD quantification of mineral composition, aided by CEC and EGME sorption measurements. The contributions of the organic-bound N and B were taken into account, based on the literature data (B) and on the relationship established in the course of this study (N). 28–98% of the total N and 80–100% of the total B are contained in the mineral fraction of the investigated shales. The mineral nitrogen is located mostly in illite and accounts for 3 to 64% of the sites available for its fixation in 1Md illite. The fixation of N by illite seems to diminish in the course of diagenesis, but additional fixation occurs in newly formed 2M1 illite during the anchimetamorphic stage. The volume of N contained in illite in most samples greatly surpasses the N available locally from the organic matter contained in shale. Modelling indicates that the measured level of N for K substitution in illite corresponds to the capture in 30 vol.% of shale of all N released in a basin containing 5 vol.% of coal. Boron is held predominantly by the 1Md fraction (illite + smectite). During diagenesis boron is redistributed into the 1Md illite, and during anchimetamorphism it is released from the 1Md illite and incorporated into new 2M1 illite. No indication of the net enrichment of pore water in B due to the clay alteration process was found.

THE ILLITE–ALUMINOCELADONITE SERIES: DISTINGUISHING FEATURES AND IDENTIFICATION CRITERIA FROM X-RAY DIFFRACTION AND INFRARED SPECTROSCOPY DATA

Al-rich K-dioctahedral 1M and 1Md micas are abundant in sedimentary rocks and form a continuous compositional series from (Mg,Fe)-poor illite to aluminoceladonite through Mg-rich illite. The complexity and heterogeneity of chemical composition and structural features, as well as the lack of reliable diagnostic criteria, complicate the identification of these mica varieties. The objectives of the present study were to reveal the structural and crystal-chemical variability in the illite—aluminoceladonite series, and to define the composition ranges and identification criteria for the mica varieties in the series. A collection of illite and aluminoceladonite samples of various compositions was studied by X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectroscopy. Analysis of the relationships between unit-cell parameters and cation composition showed that the series includes three groups, (Mg,Fe)-poor illites, Mg-rich illites, and aluminoceladonites, each characterized by a unique combination of unit-cell parameter variation ranges. The distinctive features of aluminoceladonite are reduced values of csinβ and |ccosβ/a| in combination with b parameters that are smaller than those for Mg-rich illites, and slightly greater than those of (Mg,Fe)-poor illites. The compositional boundary between illite and aluminoceladonite occurs at Si = ~3.7 and Mg + Fe2+ = ~ 0.6 atoms per O10(OH)2.A new approach to the interpretation of the FTIR spectroscopy data involving new relationships between band positons and cation composition of (Mg,Fe)-poor illites, Mg-rich illites, and aluminoceladonites provides additional diagnostic features that include the band positions and profile in the regions of Si—O bending, Si—O stretching, and OH-stretching vibrations. A sharp maximum from the AlOHMg stretching vibration at ~3600 cm−1, the presence of a MgOHMg stretching vibration at 3583–3585 cm−1, as well as characteristic band positions in the Si—O bending (435–, 468–472, and 509–520 cm−1) and stretching regions (985–1012 and 1090–1112 cm−1), are typical of aluminoceladonite.

EXTRACTION OF DIAGENETIC AND DETRITAL AGES AND OF THE 40Kdetrital/40Kdiagenetic RATIO FROM K-Ar DATES OF CLAY FRACTIONS

AbstractIllite age analysis (IAA) is a classical method for extracting diagenetic and detrital ages from mixed ages measured by K-Ar. This approach is based on measuring the masses of diagenetic and detrital illitic components in a few different grain-size fractions of one rock sample and measuring the mixed ages of these fractions. The 1Md illitic polytype is usually considered to be diagenetic, while 2M1 is considered detrital. A plot of the function: exp(λt)−1 (where t is time and λ is the decay constant) vs. weight percent of the detrital fraction is constructed. On the basis of linear extrapolation to end-member fractions, the diagenetic and the detrital age is obtained. This approach does not take into account various K contents in different polytypes (%Kdetrital and %Kdiagenetic). In order to do that, the detrital mass fraction (wt.%detrital) should be recalculated into the percentage of detrital K (%Id(K)):