T. A. Ivanovskaya

Mössbauer characteristics, mineralogy and isotopic age (Rb-Sr, K-Ar) of Upper Riphean glauconites from the UK Formation, the southern Urals

Comprehensive mineralogical analysis, Mössbauer spectroscopy and isotopic-geochronological study have been carried out for globular phyllosilicates (GPS) of glauconite group from the Uk Formation, the second one below the top of the Upper Riphean stratotype in the southern Urals. Glauconites have been sampled in the Kurtaza and Kulmas sections remote from each other in the Alatau anticlinorium that corresponds to western facies zone of the Bashkirian meganticlinorium. As is shown, size and density monomineral fractions of globules are represented by Al-glauconite according to established structural formula. The isotopic (Rb-Sr and K-Ar) dating of glauconites from the Uk Formation is performed for the first time along with computer simulation of cation arrangements in their crystal lattice and comparison of the results obtained with data of Mössbauer spectroscopy. It has been assumed by simulation that origin and transformation of the Rb-Sr and K-Ar systems in glauconite are concurrent to stages in structural evolution of this mineral, which have been controlled by geological and geochemical events in the history of sedimentary successions. The approach has been aimed at recognition of stratigraphically meaningful isotopic dates corresponding to the glauconite formation at the stage of the early diagenesis close to sedimentation time and the “rejuvenated” dates characterizing ages of subsequent geological events. The comparison of simulated cation arrangements with data of Mössbauer spectroscopy shows that the Rb-Sr (663 ± 9 Ma) and K-Ar (669 ± 16 Ma) dates established for glauconites correspond to the time of early diagenesis in their host sediments, being suitable for age assessment of the Uk Formation. The dates obtained are of interregional and wider significance, as they must be taken into consideration when constructing the general curve of Sr isotope variations in the Late Riphean oceans.

Nature of the structural and crystal-chemical heterogeneity of the Mg-rich glauconite (Riphean, Anabar Uplift)

The detailed mineralogical and structural-crystal-chemical characteristics are reported for the first time for glauconite grains extracted from the fine-platy silty-sandy dolomites at the roof of the lower subformation of the Yusmastakh Formation (Riphean, Anabar Uplift, North Siberia). Based on the complex study (X-ray diffraction, classical chemical analysis, microprobe analysis, IR-spectroscopy, thermogravimetric analysis, scanning electron microscopy with microprobe analysis, and Mössbauer spectroscopy), it was demonstrated that the studied glauconite sample is characterized by unique chemical and structural heterogeneity.The mineral structure consists of micaceous (90%), smectite (6%), and di-trioctahedral chlorite (4%) layers. Mica is classed with Al-glauconite (Al > Fe3+) with elevated Mg content. The elevated Mg mole fraction of the mineral is caused by the presence of Mg-bearing brucite-type interlayers of di-trioctahedral chlorite and the high Mg content in the octahedral sheets of 2: 1 layers. It was first discovered that glauconites are characterized by the heterogeneous distribution of cations over the available trans- and cis-octahedra due to the coexistence of trans- and cis-vacant octahedra and small trioctahedral clusters in octahedral sheets. The distribution of isomorphic cations over the accessible octahedral sites is also heterogeneous due to the tendency of Fe, Mg and Al, Mg cations to segregation and formation of corresponding domains.It was found that structure of the studied glauconite has a specific stacking defect: in addition to the predominant subsequent layers of similar azimuthal orientation according to 1M type (∼77%), some layer fragments are rotated at 180° (∼15%) and ±120° (8%). The structural-crystal-chemical heterogeneity of the mineral is explained by the fact that its microcrystals grew in the dolomitic sediment under nonequilibrium conditions of the reduction zone of a shallow-water basin with a sufficiently high content of Mg cations, which significantly contributed to the glauconite formation.

Glauconite from lower cretaceous marine terrigenous rocks of England: A concept of biochemogenic origin

Detailed mineralogical characteristics of various forms of glauconite occurrence in Lower Cretaceous marine terrigenous rocks of the White Island (Binnel Bay, southern England) are discussed. It has been shown that glauconite was formedin situ due to the transformation of fine-dispersed and sandy-silty terrigenous materials. The influence of bacterial activity on glauconite formation is supported by the study of dissolution zones on quartz and feldspar grains, which revealed biomorphic structures akin to fossilized bacteria.

Rb-Sr, K-Ar, H-and O-Isotope systematics of the Middle Riphean shales from the Debengda Formation, the Olenek Uplift, North Sibera

Clay subfractions (SFs) of <0.1, 0.1–0.2, 0.2–0.3, 0.3–0.6, 0.6–2 and 2–5 μm separated from Middle Riphean shales of the Debengda Formation are studied using the TEM, XRD, K-Ar and Rb-Sr isotopic methods. The oxygen and hydrogen isotope compositions in the SFs are studied as well. The low-temperature illite-smectite is dominant mineral in all the SFs except for the coarsest ones. The XRD, chemical and isotopic data imply that two generations of authigenic illite-smectite different in age are mixed in the SFs. The illite crystallinity index decreases in parallel with size diminishing of clay particles. As compared to coarser SFs, illite of fine-grained subfractions is enriched in Al relative to Fe and Mg, contains more K, and reveals higher K/Rb and Rb/Sr ratios. The Rb-Sr age calculated by means of the leachochron (“inner isochron”) method declines gradually from 1254-1272 Ma in the coarsest SFs to 1038-1044 Ma in finest ones, while the K-Ar age decreases simultaneously from 1225–1240 to 1080 Ma. The established positive correlation of δ18O and δD values with dimensions of clay particles in the SFs seems to be also consistent with the mixing systematics. The isotopic systematics along with data on mineral composition and morphology lead to the conclusion that mixedlayer illite-smectite was formed in the Debengda shales during two periods 1211–1272 and 1038–1080 Ma ago. The first period is likely close to the deposition time of sediments and corresponds to events of burial catagenesis, whereas the second one is correlative with the regional uplift and changes in hydrological regime during the pre-Khaipakh break in sedimentation.

Mixed-layer corrensite-chlorites and their formation mechanism in the glauconitic sandstone-clayey rocks (Riphean, Anabar Uplift)

The paper presents the first detailed mineralogical, structural, and crystal-chemical characteristics of the mixed-layer corrensite-chlorites from the glauconitic sandy-clayey rocks that make up the bottom (0.10 m) of a basal member (1.50 m) of the lower subformation of the Yusmastakh Formation (Riphean, Anabar Uplift, North Siberia). Like the overlying mudstones (1.40 m) in the basal member, these rocks are generally transformed up to the deep catagenesis level and included in a thick dolomite sequence. In mudstones represented by the dioctahedral micas, the corrensite-type minerals are observed as traces.The real structure of the studied mixed-layer phases represents an alternation of corrensite (Cor) and chlorite (Ch) layers with a distinct tendency of segregation (R = 1) and ratio of layers Cor: Ch = 0.60: 0.40 in the well-crystallized corrensite-type minerals and 0.70: 0.30 in the poorly crystallized varieties.Among these mineral formations, the high-Mg (hereafter, Mg) and magnesian-ferruginous (Mg-Fe) mixedlayer corrensite-chlorites prevail, whereas the high-Fe (Fe) and ferruginous-magnesian (Fe-Mg) varieties are rare. Synthesis of the corrensite-type microcrystals of different sizes took place after the formation of glauconite globules, probably, at the reductive stage of late diagenesis.Two possible structural mechanisms are proposed for the formation of the mixed-layer corrensite-chlorites, preference being given to the two-stage version. The first stage is marked by the formation of corrensite proper with a structure periodic along the layer normal; the second stage, by the successive replacement of smectite interlayers by the brucite-type sheets, probably, due to the further evaporitization that promotes the replacement of a part of smectite interlayers in the corrensite component of the structure and the stabilization of the newly formed brucite-type sheets.

Crystal-chemical microheterogeneity of Precambrian globular dioctahedral mica minerals

The paper presents a novel method for determining the crystal-chemical heterogeneity of finely dispersed dioctahedral 2: 1 mica mineral phases based on the modeling of their powder diffraction patterns. We used three samples, which were taken from Lower-Middle Riphean rock sections of the Anabar and Olenek uplifts in northern Siberia, and one sample from the Upper Riphean Inzer Formation in the southern Urals. Choice of globular samples was determined by contrast features of their chemical composition and different lithological types of the host terrigenous rocks that are commonly transformed at the level of deep catagenesis.Based on structural formulas, unit cell parameters, and coordinates of atoms occupying the cells, we modeled powder diffractograms that made it possible to determine the sizes of coherent scattering domains and probability parameters, which characterize the type, content, and distribution of stacking faults in each of the studied samples. The modeling results demonstrated that each sample represents a physical mixture of individual micaceous phases of different compositions. The paper discusses scales and modes of the crystalchemical heterogeneity of micaceous varieties, which make up globules in the coarse- and fine-grained terrigenous sediments, as well as their various specific crystal-chemical characteristics. The probable physicochemical settings, which were responsible for specific features of the structural and crystal-chemical heterogeneity of micaceous varieties in each of the studied four samples, are also discussed.

Crystal-chemical peculiarities of globular layer silicates of the glauconite-illite composition (Upper Proterozoic, Northern Siberia)

The paper presents an overview of crystal-chemical peculiarities of the previously studied globular dioctahedral 2 : 1 layer silicates of the glauconite-illite compositions from the Upper Proterozoic sections in northern Siberia (Anabar and Olenek uplifts). Lithomineralogical peculiarities of the glauconite-bearing rocks are discussed. Geochronological data on some samples are given. Monomineral fractions of grains were studied with the modern chemical and physical methods (X-ray diffraction, oblique-texture electron diffraction (OTED), scanning electron microscopy, IR and Mossbauer spectroscopy, classical chemical and microprobe analyses, and others). Low-charge dioctahedral 2 : 1 layer silicates were classified with consideration of the IMA NC and AIPEA NC recommendations (Rieder et al., 1998; Guggenheim et al., 2006). This classification showed that the studied globular Al- and Fe-bearing varieties include a continuous isomorphic glauconite-illite series. Intermediate layer silicates beyond the IMA NC and AIPEA NC classification are assigned to Al-glauconites (Fe-illites). True illites should be supplemented with adjectives “globular” or “platy,” because green globules and the fine-dispersed Al-bearing clay minerals are traditionally recognized as “glauconite” and “illite,” respectively.

Phyllosilicates (glauconite, illite, and chlorite) in terrigenous sediments of the Arymas Formation (Olenek High)

It is shown that globular phyllosilicates subjected to deep categenesis are abundant in Middle Riphean sandstones and siltstones (lower subformation of the Arymas Formation) of the Olenek High. Their detailed structural-crystallochemical characteristics are given. Secondary alterations of globules and the associated pelletal minerals at different stages of lithogenesis (illitization, chloritization, ferrugination, disintegration, and so on) are considered. Structural-crystallochemical characteristics of Fe-illite and Fe2+-Mg-chlorite in globules, pellets, separate clayey strata among glauconite-bearing sandstones and siltstones (hereafter, sandy-silty rocks), and mudstone interlayers are also presented. Possible mechanisms of the formation of these minerals are discussed.

Glauconite at Different Stages of Lithogenesis in Lower Cambrian Rocks of Western Lithuania

It is shown that glauconite was mainly formed under diagenetic conditions in terrigenous–clayey rocks (Rausven Unit) of the Lower Cambrian Virbalis Formation in western Lithuania. This was preceded by bioturbation at some levels of geological column and local short-term reworking at other levels. Different forms of glauconite and its interrelation with ambient minerals in diagenesis and epigenesis are considered. A two-phase micaceous (glauconite–illite) composition of globules has been revealed and crystallochemical characteristics of each phase is presented.

Pseudomorphous Replacement of Globular Glauconite by Mixed-Layer Chlorite–Berthierine in the Outer Contact of Dike: Evidence from the Lower Riphean Ust'-Il'ya Formation, Anabar Uplift

Globular, platy, and fine-dispersed phyllosilicates of chloritic composition were studied in the outer contact of dike with glauconite-bearing rocks. It is shown that the globular Al-glauconite is replaced by pseudomorphs of mixed-layer Mg- and Fe-bearing chlorite–berthierine containing 5% berthierine layers in this zone. The crystallochemical characteristics and microstructure are reported for the globular, platy, and fine-dispersed chlorites. The possible models of chlorite–berthierine formation are discussed.