Lidia G. Dainyak

Illite-smectite structural changes during metamorphism in black Cambrian Alum shales from the Baltic area

Abstract Illite-smectite (I-S) from Cambrian black shale of both early diagenetic and anchimetamorphic grade was investigated to determine the mechanism of the clay transformation. The layer sequences, the distribution of thicknesses of coherent scattering domains (CSDs), and the three-dimensional ordering were determined by X-ray diffraction (XRD). The proportions of cis-vacant (cv) and transvacant (tv) 2:1 layers were determined by thermal analysis and the proportion and distribution of interlayer ammonium by XRD and by infared spectroscopy (IR). The structural formulae were determined from total chemical analysis, and Mössbauer and 27Al NMR spectroscopies, and the particle shape and size investigated by atomic force microscopy (AFM). In the early diagenetic samples, the I-S is composed of two phases, one of which contains 0.05 and the other 0.25 smectite (S) interlayers. The first phase does not change during metamorphism. In the second phase, 0.20 S are converted to tobelite (T) layers through fixation of NH4+, but the I layers are not changed. Simultaneously, the proportion of cv layers changes from 0.18 to 0.02, and the tetrahedral substitution of Al for Si is parallel to the increase in T layers. All I interlayers contain 0.75K per O10(OH)2. Furthermore, the metamorphism results in increasing mean thickness of CSDs from 5.1-6.8 nm for the lowdiagenetic samples to 6.7-8.4 nm for the anchimetamorphic samples. We conclude that the tobelitization was accompanied by transformation of cv to tv 2:1 layers adjacent to the smectite interlayers, and formation of tv layers adjacent to the newformed tobelite interlayers in otherwise intact crystallites. This mechanism only partly resembles the tobelitization previously observed in the Upper Jurassic North Sea oil source rocks. I-S in these rocks contained tv 2:1 layers and T interlayers formed through solid-state Al for Si substitution in the tetrahedral sheet and by ammonium fixation in the corresponding interlayers. These different mechanisms are probably because the North Sea I-S originated from weathered illite, like the Cambrian high-illitic phase, whereas the Cambrian low-illitic phase undergoing the transformation originated from cv smectite of volcanic origin. The results indicate that the illitization in oil source rocks is linked to oil generation, and that it deviates from the illitization in other rocks because of the supply of ammonium formed during oil generation and the fixation of this ammonium in the former smectite interlayers.

Structural and chemical heterogeneity of illite-smectites from Upper Jurassic mudstones of East Greenland related to volcanic and weathered parent rocks

Illite-smectites (I-S) in one Upper Jurassic mudstone core from East Greenland were investigated to determine their structural and crystal-chemical features and to find the relation between these features and source rocks. The phase composition and layer sequences were determined by X-ray diffraction (XRD), the distribution of octahedral cations over trans - and cis -octahedra by thermal analysis, the structural formulae by XRD, Mossbauer spectroscopy, and total chemical analysis, and the short-range order in isomorphous cation distribution by infrared (IR) and Mossbauer spectroscopies. For all samples (except one having maximum degree of ordering for R = 1), simulation of the experimental XRD patterns led to two different I-S models having indistinguishable diffraction patterns. For the first, single-phase model, expandability ( w S) is 0.45–0.60. For the second, two-phase model, two randomly interstratified I-S having w S equal to 0.40 and 0.85, respectively, are present in different proportions in different samples. The single-phase model was selected. A new approach for simulating the two-dimensional distributions of the isomorphous octahedral cations using IR and Mossbauer parameters revealed a tendency for Fe segregation into edge-shared octahedra that may form zigzag chains. Almost identical IR and Mossbauer parameters found for the I-S having different amounts of trans -vacant ( tv ) and cis -vacant ( cv ) layers (ranging from 0.08 to 0.80) demonstrate that these parameters are largely determined by local cation environments around Fe3+ and OH groups. Different levels in the Upper Jurassic Kimmeridgian core contain I-S having different structures. I-S of hemipelagic mudstones at the bottom (37 m depth) and in the middle (13 m depth) of the core, with a high proportion of cv layers and of smectite layers ( w S ~ 0.60), probably formed from volcanic material. The other four samples have a high proportion of tv layers and probably formed by weathering of micaceous material. One of these I-S, from a mudstone turbidite (at 27 m depth), having maximum degree of ordering for R = 1, probably originated from one type of parent rock, and three mudstones (at depths of 4, 12, and 13 m) with segregated I-S, probably originated from a second rock type.

TOBELITIZATION OF SMECTITE DURING OIL GENERATION IN OIL-SOURCE SHALES. APPLICATION TO NORTH SEA ILLITE-TOBELITE-SMECTITE-VERMICULITE

Illite-smectite (I-S) minerals isolated from Upper Jurassic oil-source rock shales from Denmark and the North Sea have been investigated by X-ray diffraction, thermal analysis, infrared, Mössbauer, and solid-state nuclear magnetic resonance spectroscopies and chemical analysis. Detailed structures have been determined in order to reveal the diagenetic transformation mechanism in these shales. Generally, in oil-source rocks of sedimentary basins, oil generation takes place simultaneously with the diagenetic transformation of I-S. We demonstrate a link between the two reactions: NH3 released from kerogen during maximum oil generation is fixed as NH4+ in the NH4-bearing mica or tobelite layers formed from smectite or vermiculite layers in I-S, in a diagenetic interval which we name the ‘tobelitization window’. Due to this solid-state transformation, mixed-layer structures have been formed consisting of interstratified illite, tobelite, smectite and vermiculite layers (I-T-S-V) and having maximum ordering of illite + tobelite and smectite layers for R = 1. The tobelitization of smectite in I-S is probably typical for all oil-source rock shales.

Interpretation of the nontronite-dehydroxylate Mössbauer spectrum using EFG calculations

Dehydroxylated Garfield nontronite has been studied using Mossbauer spectroscopy. According to literature, in dehydroxylated Fe 3+ -rich dioctahedral 2:1 phyllosilicates (celadonites, glauconites and nontronites), the octahedral cations migrate from cis -into trans -sites with the formation of five-fold coordination of the former cis- and trans- octahedra. Therefore, the two main fitted doublets with quadrupole splittings, Δ, of 0.906 and 1.392 mm/s are supposed to be related to Fe 3+ in the former cis- and trans -octahedra. To assign these doublets to this or that positions, analysis of Pauling’s bond strength (PBS), structural modeling and EFG calculations were performed. The calculated quadrupole splittings for Fe 3+ located in the former cis- and trans- octahedra in the nontronite-dehydroxylate structure are equal to 1.295 and 1.026 mm/s, respectively. On the basis of the calculation results, the quadrupole doublet with smaller Δ should be assigned to the former trans -octahedra whereas the doublet with larger Δ should be assigned to the former cis -octahedra. The calculated EFG parameters proved to be independent of the mode in the layer stacking, which confirms the major role of the Fe 3+ nearest environment in the formation of the different EFGs at Fe nuclei. Possible disruptions of two-dimensional continuity of dehydroxylated octahedral sheets that may be a reason for superparamagnetic effects in nontronite-dehydroxylates at low temperatures are discussed in terms of the structural model of dehydroxylated Fe-rich dioctahedral 2:1 phyllosilicates.

Cation redistribution in the octahedral sheet during diagenesis of illite-smectites from Jurassic and Cambrian oil source rock shales

Abstract During diagenesis of Jurassic and Cambrian oil source rock shales illite-smectite(-vermiculite) [I-S(-V)] is transformed to illite-tobelite-smectite(-vermiculite) [I-T-S(-V)]. This transformation of S layers to T layers takes place by an increase in tetrahedral charge through Al for Si substitution and subsequent fixation of interlayer NH4, accompanied by an increase in Al and a decrease in Fe and Mg in the octahedral sheet. In the present investigation, the distribution of isomorphous cations in octahedral sheets of trans-vacant I-S(-V) and I-T-S(-V) was studied by Mössbauer and Infrared (IR) spectroscopies. Mössbauer spectra have been modeled using numerical values of the Fe3+ and Fe2+ quadrupole doublets corresponding to local cation arrangements around Fe3+ and Fe2+ in octahedral sheets of micaceous minerals. To interpret IR spectra in the OH-stretching region, frequencies for each pair of cations bonded to OH groups determined for micas and I-S are used. Combination of Mössbauer and IR data by computer simulation provides two-dimensional cation distributions of octahedral cations. The Jurassic and Cambrian I-S(-V) and I-T-S(-V) have clustered octahedral sheets. Ordered clusters of mixed cation composition (Mg, Al, Fe3+, and Fe2+) with regular alternation of di- and trivalent cations and Fe3+-clusters dispersed over an Al-matrix are found in detrital samples. In diagenetically transformed samples, ordered clusters persist while Fe3+-clusters degenerate to either short chains consisting of two Fe-Fe pairs or to isolated Fe-Fe pairs oriented along the b, b1, and b2 directions. The release of Fe and Mg during diagenesis occurs from Fe3+ clusters and through partial destruction of ordered clusters and of b1, b2-oriented Mg-Mg pairs. However, as the cation composition and the short-range cation order within the clusters are preserved and the Al for Fe and Mg substitution occurs at cluster edges, the diagenetic transformation of S (and V) to T layers in both the Jurassic and Cambrian I-S(-V) proceeds through a solid-phase transformation and not through dissolution-reprecipitation.

Computer simulation of octahedral cation distribution and interpretation of the Mössbauer Fe2+ components in dioctahedral trans-vacant micas

Cation distributions (CDs) in the representative collection of trans-vacant dioctahedral celadonites, glauconites and ferriillites were investigated using Mossbauer and IR spectroscopies and the original approach. The approach is based on individual quadrupole splittings Δ i pred for Fe 3+ in possible local cation arrangements around Fe 3+ , and on a computer simulation of two-dimentional CDs using the IR data in the OH stretching vibration region. The resulting CDs were next used to make correlations between Δ j of Fe 2+ derived from computer fits to the corresponding spectra and cation composition of local cation arrangements around Fe 2+ with their occurrence probabilities. Basing on this correlations, a total of eight individual Δ tent for Fe 2+ referred as “tentative” have been derived. The order of local cation arrangements in terms of increasing quadrupole splitting was found to be the same both for Fe 3+ and Fe 2+ and implies a direct dependence of the Fe 2+ quadrupole splitting on the structural distortion at Fe 2+ site. The set of Δ i tent for Fe 2+ combined with Δ i pred for Fe 3+ and with the new CD simulation program provide an additional means for controlling the CD reconstruction.

Formation and transformation of mixed-layer minerals by tertiary intrusives in cretaceous mudstones, West Greenland

In the Nuussuaq Basin, West Greenland, a thick succession of Tertiary dolerites has penetrated Upper Cretaceous mudstone. The mixed-layer minerals of mudstone core samples have been analyzed by X-ray diffraction, solid-state 29Si and 27A1 magic-angle spinning nuclear magnetic resonance, Mössbauer and infrared spectroscopies, thermal analysis, chemical analysis, stable isotopes (18O/16O), and K/Ar dating. The mixed-layer minerals include for each sample two mixed-layer phases consisting of pyrophyllite, margarite, paragonite, tobelite, illite, smectite and vermiculite layers. The main, 80 m thick intrusion resulted in the formation of pyrophyllite, margarite, paragonite and tobelite layers. However, the tobelite layers are absent in samples <21 m from this intrusion. Furthermore, chlorite was formed and kaolinite destroyed in samples adjacent to minor intrusions and at distances <60 m from the large intrusion. For the first time, the detailed, complex mixed-layer structures formed during contact metamorphism of kaolinitic, oil-forming mudstones have been investigated accurately. The formation of tobelite layers reveals that oil formation has taken place during contact metamorphism. Furthermore, K/Ar dating of mixed-layer minerals from shale indicates that the intrusives are of early Eocene age. The 80 m thick intrusive is responsible for the main mixed-layer transformations, whereas two thin (3 m and 0.5 m thick) intrusions contribute little. Thus, the detailed mixed-layer investigation has contributed significantly to the understanding of the regional geology and the contact metamorphic processes.

An improved model for the interpretation of Mössbauer spectra of dioctahedral 2:1 trans-vacant Fe-rich micas: refinement of parameters

Two Upper Riphean glauconite samples of the Southern Urals with unusually low Fe3+/Fe2+ ratios were studied by 57Fe Mossbauer and IR spectroscopy. Room and liquid-nitrogen temperature Mossbauer spectra were analyzed with quasi-continuous model-independent quadrupole splitting distributions (QSDs). The obtained QSDs were interpreted using a crystal-chemical model taking account of the effects of the local structural and chemical heterogeneity of layer silicates. The main parameters of the crystal-chemical model are individual quadrupole splittings, Δ i pred and Δ i tent, for Fe3+ and Fe2+, respectively, in different local cationic arrangements (LCAs) with their occurrence probabilities, w i , provided by two-dimensional simulation of the cation distribution (CD). Using the parameters of the crystal-chemical model, QSDsim profiles were simulated and compared with the experimental QSDs. To obtain agreement between QSDexp and QSDsim profile, w i and Δ i values were adjusted using the method of successive approximations. As a result, the tentative Δ i tent values for Fe2+ contributions to Mossbauer spectra were refined. The sequence of refined Δ i ref values corresponding to specific LCA types remained the same as that for the Δ i tent values. For the studied glauconites, the CD reconstructions satisfying the experimental QSDs are characterized by similar local distribution of cations around Fe3+ and by different local cation distribution around Fe2+.

Octahedral cation distribution in glauconites from Southern Urals by combination of crystal-chemical model and quasi-continuous model-independent quadrupole splitting distributions (QSD) fitted to their Mössbauer spectra

Mossbauer spectra of two glauconites, BSH-11 and KUL-1, from the Southern Urals are interpreted by simulation using crystal-chemical modelling and by fitting of quadrupole splitting distribution (QSD). The crystal-chemical modelling is based on X-ray diffraction (XRD) data and includes, as a priori information, individual quadrupole splittings for Fe 3+ and for Fe 2+ that correspond to specific local cation arrangements (LCA) having certain occurrence probabilities. The latter are provided by two-dimensional octahedral cation distribution using integrated infrared optical densities for bands corresponding to the cation pairs bound to the OH groups. Combination of crystal-chemical modelling and quasi-continuous model-independent QSDs fitted to Mossbauer spectra resulted in cation distributions for which R 2+ (Mg, Fe 2+ ) and R 3+ (Al, Fe 3+ ) cations occupy one of two structurally independent cis- sites with equal probabilities. The cation distributions obtained are characterized by Fe-clustering, domain structure, and different trends to homogeneous charge dispersion.

A model for the interpretation of Mossbauer spectra of muscovite

The literature Mossbauer parameters for a representative collection of muscovites, which were interpreted assuming the possible coexistence of trans- and cis- vacant sheets, were used to prove an alternative interpretation in terms of the local structure. Crystal-chemical models including the assignment of the local cation arrangements (LCA) around Fe 3+ and Fe 2+ to predicted and tentative quadrupole splitting values, respectively, and the LCA occurrence probabilities provided by the simulation of two-dimensional cation distribution (CD) were used. The obtained CD reconstructions in dioctahedral trans -vacant sheets correspond to the two types of muscovites revealed by Mossbauer spectroscopy. The main contributors into type-I muscovite are the 2R 2+ Al LCAs around Fe 2+ (R 2+ stands for Fe 2+ and Mg) whereas in type-II muscovite the 3Al and 2AlFe 3+ LCAs are more important. This implies a more ordered CD in type-II muscovites than that in type-I muscovites from the point of view of Pauling’s principle of the homogeneous dispersion of charge.