Illite-smectite (I-S) minerals from the Upper Jurassic oil-source shales of Denmark and the North Sea were studied by a complex of diffraction and spectroscopic methods. Detailed structures were identified to reveal the mechanism of postsedimentary transformations of these shales. Usually, oil is generated in the oil-source rocks of sedimentary basins simultaneously with the diagenetic and catagenetic I-S transformations. The results obtained demonstrate the relationship between these two reactions: NH3 molecules released from kerogen during the maximum oil formation are fixed as NH4 cations in smectite or vermiculite interlayers, forming mica or tobelite structural fragments. This solid-phase transformation produces the mixed-layer structures consisting of illite, tobelite, smectite, and vermiculite (I-T-S-V) layers.

Illite-smectite (I-S) minerals from the Upper Jurassic oil-bearing rock shales of Denmark and the North Sea were studied by a complex of diffraction and spectroscopic methods. Detailed structures were identified to reveal the mechanism of diagenetic transformation of these shales. Usually, in oil-bearing rocks of sedimentary basins, oil generation occurs simultaneously with the I-S diagenetic transformation. The results obtained demonstrate the relationship between these two reactions: NH3 molecules released from kerogen during ma-ximum oil formation are fixed as NH4 ammonium cations in smectite or vermiculite interlayers, forming mica or tobelite structural fragments. As a result of this solid-phase transformation, mixed-layer structures are formed, consisting of layers of illite, tobelite, smectite and vermiculite (I-T-S-V).

To resolve the existing ambiguities in the interpretation of the OH-stretching vibrations of kaolinites, relationships were, for the first time, established between the structural and Fourier-transform infrared (FTIR) spectroscopic features for a set of kaolinite samples which differed in terms of their relative amounts of coexisting high- and low-ordered phases. For this purpose, a representative collection of kaolinites differing in origin, particle size, and degree of disorder was studied by powder X-ray diffraction (XRD) and FTIR spectroscopy. Modeling of the experimental XRD patterns based on the orthogonal layer unit cell having a mirror plane showed each sample to be a mixture of nearly defect-free high-ordered (HOK) and low-ordered (LOK) kaolinite phases, with HOK varying from 86 to 4%. The wavenumbers, heights, areas, and full widths at half-maximum (FWHM) were determined for the OH-stretching bands at ~3697 (ν1), ~3670 (ν2), ~3652 (ν3), and 3620 cm–1 (ν4) by decomposition and fitting of the FTIR spectra. The FWHM(ν1)/FWHM(ν4) and FWHM(ν3)/FWHM(ν2) values were related linearly to the HOK content, which may be associated with the in-phase and out-of-phase character of the corresponding pairs of vibrations, respectively. A novel interpretation was suggested for the variations in the relative integrated intensities of the OH bands with the amount of the HOK phase. The intensity distribution of the ν2 and ν3 bands is controlled by the triclinic structure symmetry in the defect-free kaolinite and the mirror symmetry of the layers in low-ordered structures, in agreement with the observed evolution of the corresponding band intensities. The ν1 and ν2 band positions for the low-ordered samples are within the wavenumber range for the high-ordered samples. In contrast, the ν3 and ν4 band positions for the low-ordered samples are shifted toward higher wavenumbers, indicating that some of the low-ordered kaolinites should contain dickite-like structural fragments distributed among kaolinite layers.

The mineralogical, structural, crystal-chemical, and isotope-geochronological data were obtained for the first time in two samples of globular phyllosilicates (GPS) of the glauconite–illite series collected from terrigenous rocks in two sections of the lower part of the Lower Vendian Maastakh Formation (Khorbusuonka and Ulakhan-Sololi rivers, northwestern slope of the Olenek Uplift). Simulation of experimental X-ray diffraction patterns for both samples in the air-dried and ethylene glycol-solvated states was used to determine the expandable layer contents (9, 10%), types of expandable layers (smectite- and vermiculite-type), their ratios in a three-component mixed-layer structure, parameter csinβ (9.98 Å) of mica layers, and the short-range order factor describing the alternation of different layer types (R = 2, 3). Values of the unit-cell parameter b (9.027, 9.039 Å) correspond to the dioctahedral Al, Fe-bearing micas. The mica components in the studied mixed-layer minerals are shown to be represented by Fe-illites (KAl = VIAl/(VIFe3+ + VIAl) = 0.71, 0.82) with the K2O content of 7.77 and 8.40%. The Rb–Sr dating of the Maastakh GPS was carried out for the first time in combination with the calculation of theoretical patterns in the cation distribution in the mineral structure and comparison of the calculation results with the Mössbauer and IR spectroscopy data using the Optima software package supplemented with the new Irmes software. The Rb–Sr data obtained for two Fe-illites (Khorbusuonka and Ulakhan-Sololi rivers: 1033, 913 ± 12 Ma, respectively) are “older” relative to the Vendian Maastakh Formation (~640 ± 5), indicating a terrigenous origin of the studied grains. Globular phyllosilicates from the Maastakh Formation are similar in age and composition to the previously studied Lower Khaipakh (Middle Riphean) GPS (1172 ± 18 and 1112 ± 24 Ma, respectively). The age of GPS samples from the Maastakh Formation is younger (1033–913 Ma), probably, due to the initial cationic disordering in the GPS structure and partial loss of radiogenic elements during secondary alterations (rewashing and redeposition of Fe-illite grains in sediments of the Maastakh Formation) and, probably, during their catagenetic changes as well.

For the first time, mineralogical, structural, crystal-chemical and isotope-geochronological data were obtained for a glauconite sample from the Lower Riphean Ust-Il’ya Formation on the Magan River (western slope of the Anabar Massif, northern Siberia). It has been established that glauconite from the lenticular layer of friable glauconitite (glauconite grains ≥80%) contains two populations of mixed-layer crystals, in which mica layers (93%) alternate with either smectite-type (7%) or vermiculite-type layers (7%) with the short-range order factor R = 3. The ratio KAl = (VIAl / (VIFe3+ + VIAl) equal to 0.45 allows identifying the micaceous mineral as glauconite with the unit-cell parameter b = 9.065 Å and K2O content of 8.29%. Comparison of new data with the previously obtained mineralogical and crystal-chemical characteristics of Ust-Il’ya glauconites from the Kotuikan River section (2.5 km above the Il’ya River mouth), located 65 km south of the Magan River section, showed both similarities and differences. Analysis of the crystal-chemical heterogeneity of Kotuikan glauconite samples from rocks of different lithological types revealed peculiarities in the distribution of individual mica varieties composing the glauconite globules. Isotopic dating of glauconite from the Magan River section was performed in combination with simulation of the distribution of octahedral cations and comparison of the results obtained with Mössbauer and infrared (IR) spectroscopy data. Such an approach combined with the mineralogical and crystal-chemical analyses contributes to correct interpretation of the stratigraphic significance of isotope data. The results obtained provide grounds for the conclusion that isotopic dates of glauconite from the Magan section (1474 ± 21 Ma) coincides within the error limit with the earlier Rb–Sr and K–Ar dating of the Ust-Il’ya Formation (1485 ± 13 and 1459 ± 20 Ma, respectively) based on the glauconite in the Kotuikan section (Zaitseva et al., 2016). The former value marks the stage of early diagenesis of sediments and is suitable for estimating the age of this formation.

A representative collection of K-dioctahedral 1M micas ranging in composition from (Mg, Fe)-poor illites to aluminoceladonites through Mg-rich illites (Fe-poor varieties) and from Fe-bearing, Mg-rich illites to celadonites through Fe-illites, Al-glauconites and glauconites (Fe-bearing varieties) was studied by Fourier-transform infrared (FTIR) spectroscopy in the middle-infrared region. Analysis and comparison of the relationships between the band positions and cation compositions of Fe-poor and Fe-bearing K-dioctahedral micas provided a generalized set of FTIR identification criteria that include the band positions and profiles in the regions of Si–O bending, Si–O stretching, and OH-stretching vibrations. FTIR data allow unambiguous identification of illites, aluminoceladonites, and celadonites, as well as distinction between Fe-illites and illites proper, as well as between Al-glauconites and glauconites. Specifically, 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 (435–439, 468–472 and 509–520 cm−1) and stretching regions (985–1012 and 1090–1112 cm−1) are clearly indicative of aluminoceladonite. The distinction between Fe-illites and Al-glauconites, which have similar FTIR features, requires data on cation composition and unit-cell parameters.

Modelling of experimental X-ray diffraction (XRD) patterns is used to determine the phase composition of partially dehydroxylated kaolinite samples. To identify unambiguously the presence of two or three phases in the heated kaolinite samples, the full range of their XRD patterns has to be analysed. Two different kaolinites, from Imerys (UK) and from Georgia (USA; KGa-2(1)), were studied. The heating temperatures were selected to cover the entire range of dehydroxylation for both kaolinites (400-550 degrees C for Imerys and 400-495 degrees C for KGa-2(1)). Two different dehydroxylation pathways were observed. At each stage of partial dehydroxylation, the kaolinite from Imerys consisted of the original, non-dehydroxylated kaolinite and of a fully dehydroxylated phase, metakaolinite. During partial dehydroxylation of kaolinite KGa-2(1), each product formed at a given heating temperature consisted of three phases: the original kaolinite; a dehydroxylated phase, metakaolinite; and a phase with diffraction features corresponding to a defective kaolinite-like structure. To determine the content of metakaolinite in a partially dehydroxylated specimen, its experimental XRD pattern was reproduced by the optimal summation of the diffraction patterns of the initial kaolinite and metakaolinite. A procedure that reveals the basic diffraction features of the third phase is suggested. The XRD patterns and thus the structures of the metakaolinites formed after dehydroxylation of the Imerys and KGa-2(1) samples differ substantially. The conventional determination of the initial kaolinite and metakaolinite contents in partially dehydroxylated kaolinite based on the analysis of basal reflections and weight losses may lead to overlooking the formation of the intermediate phases.

To reveal the factors that determine the different ranges of compositional variations in high- and low-temperature Al-rich K-dioctahedral micas, relationships between structural parameters and cation composition were analysed for: (1) a series of synthetic 2M(1) muscovitephengite-aluminoceladonite samples; and (2) Al-rich, K-dioctahedral 2M(2) micas with previously published refined structural data. The dependences of the unit-cell parameters on cation composition and the variations in tetrahedral and octahedral lateral dimensions and sheet thicknesses, interlayer distances and tetrahedral rotation angles were analysed and compared with those found previously for the series 1M trans-vacant (tv) illite-1M aluminoceladonite. The similarities in the variations of unit-cell parameters with cation composition observed in 2M(1) and 1M natural and synthetic K-dioctahedral micas imply that these variations are controlled by similar - albeit not identical - structural factors. A major structural factor is the readjustment of the differently sized tetrahedral and octahedral sheets, which is realized in a different manner in micas formed under different pressure and temperature conditions.

Previously published and new data on secondary transformations of the globular and platy phyllosilicates of the glauconite–illite series from the Upper Proterozoic terrigenous rocks of the Olenek and Anabar uplifts (East Siberia), Srednii Peninsula (Murmansk coast), and Vendian–Cambrian boundary rocks of the Podolian Dniester area (Ukraine) are generalized for the first time. Plastic deformation, aluminization, chloritization, berthierinization, as well as replacement of phyllosilicates of different morphology by corrensite- chlorite and pyrite at different lithogenesis stages, are considered and lithological-mineralogical characteristics of the glauconite-bearing rocks are reported. The structural, crystal-chemical, genetic, and isotopegeochronological features of di- and trioctahedral phyllosilicates are discussed.

Iron-bearing K-dioctahedral 1M and 1Md micas are abundant in diverse geological environments and vary in composition from illite to celadonite through Fe-illite, Al-glauconite, and glauconite. The chemistry and structural features of these micas are complex and heterogeneous, reliable diagnostic criteria are lacking, and the conventional mineralogical nomenclature is ambiguous, which complicate the identification of these mica varieties. The objectives of the present study were to reveal the structural and crystal-chemical variability in Fe-bearing, K-dioctahedral 1M micas and to define composition ranges and identification criteria for the mica varieties in the series. A collection of samples of various compositions was studied using 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 included four groups, namely, Fe-bearing illites, Al-glauconites, glauconites, and celadonites and each group was characterized by a specific combination of unit-cell parameters and variation ranges. The illite group contained two distinct subgroups; Fe-bearing, Mg-rich illites and Feillites; which differ in the range of cation compositions and in FTIR characteristics. The boundary between Fe-illites and Al-glauconites occurs at a unit cell b value of ~9.05 Å and at ratios of octahedral Al to total trivalent octahedral cations that range between 0.60 and 0.65. The partially overlapping cation composition and cell parameter ranges may complicate the distinction between Al-glauconites and glauconites, which can still be unambiguously differentiated using FTIR data. The dramatically different XRD and FTIR characteristics confirmed that glauconite and celadonite should be treated as separate mineral species. The distinctive features of celadonite are relatively low csinβ values and reduced |ccosβ/a| values combined with b parameters lower than glauconites, but similar to Fe-illites. Celadonites also have distinct and sharp FTIR absorption bands at specific positions in the Si-O and OH stretching regions.

The phase composition of partially dehydroxylated specimens of two Clay Mineral Society standards, named KGa-1 and KGa-2 samples, were studied by powder X-ray diffraction patterns (XRD), infrared (IR) spectroscopy, and thermogravimetric (TG) data. Each specimen was preheated for 6 h in isothermal conditions, and the heating temperatures were selected to cover the entire range of dehydroxylation for both kaolinites (380-550 degrees C for KGa-2 and 400-600 degrees C for KGa-1). Products preheated at 550 and 600 degrees C are named metakaolinite, as their XRD patterns showed no traces of kaolinite reflections. The contribution of metakaolinite to the experimental XRD pattern and IR spectrum of each preheated specimen (including 3550-3750 and 500-1300 cm(-1) ranges) was determined by the simulation of the experimental pattern with a sum of the XRD patterns or IR spectra corresponding to the untreated sample and metakaolinite. This procedure showed that the contents of metakaolinite determined independently by diffraction and spectroscopic methods for each of the preheated specimen have almost identical values. Although the modeled and experimental XRD and IR spectra generally match, they contain particular ranges where the intensity differences are particularly significant. The misfits suggest the presence of a third phase forming in partially dehydroxylated kaolinite. To reveal the diffraction and spectroscopic features of the kaolinite and possible intermediate phase (or phases), the contribution of metakaolinite was subtracted from the experimental XRD pattern and diffuse reflectance infrared Fourier transform (DRIFT) spectrum of each preheated specimen. Analysis of the metakaolinite-free XRD patterns and DRIFT spectra supports the idea that each preheated specimen, along with metakaolinite and initial kaolinite, contains a kaolinite-like component termed the intermediate phase. To determine the contents and specific diffraction features of the intermediate phases, the contribution of initial, untreated kaolinite was subtracted from the metakaolinite-free XRD patterns for each preheated specimen. In both samples, the content of the intermediate phase, C-inter, decreases with the growth of metakaolinite from 23-25% at the beginning of dehydroxylation to 2% when the XRD-determined content of metakaolinite is 98%. The analysis of the diffraction features of the intermediate phases formed during partial dehydroxylation of the KGa-1 and KGa-2 samples indicates that they represent a new type of defective kaolinite-like structure, in which, along with the one-dimensional periodicity along the c* axis and the layer displacement vectors typical for natural kaolinite samples, the octahedral sheets of the individual layers of the structure are partially dehydroxylated.

The paper presents the generalized results of structural and crystal-chemical study of globular dioctahedral 2 : 1 layer silicates of the glauconite–illite composition (45 samples) taken from the Lower Cambrian sequences of North Siberia (Olenek Uplift); Riphean, Vendian, and Lower Cambrian sequences of East Siberia; Upper Riphean sequences of the South Urals and Srednii Peninsula; and Vendian–Cambrian boundary rocks of the Podolian Dniester region (Ukraine). Monomineral fractions of grains were studied using modern chemical and physical methods: X-ray analysis, oblique-texture electron diffraction (OTED), scanning electron microscopy (SEM), classical chemical and microprobe analyses, IR and Mössbauer spectroscopy, and others. Simulation of the experimental diffraction patterns of glycolated specimens (Sakharov et al., 1999) allowed us to determine the degree of expandability (4–13%), type of expandable layers (16.85 Å, smectite and 13.2 Å, vermiculite), their proportions in twoand three-component mixed-layers, unit cell parameters b (9.02–9.11 Å) and csinβ (9.94–10 Å) of mica layers, as well as the degree of short-range order in the alternation of layers of different types (R = 0, 2, 3). The classification of low-charge (layer charge ≈ 0.6–0.85 per f.u.) dioctahedral 2: 1 layer silicates is given with account taken of the recommendations of the International Nomenclature Committee on Micas and Clay Minerals (IMA NC, AIPEA NC) (Reider et al., 1998; Guggenheim et al., 2006). Based on the crystal-chemical data on 79 samples obtained in (Ivanovskaya et al., 2012) and in the present paper, it is shown that the globular micaceous varieties are represented by a continuous isomorphic series ranging from illites through Fe-illite and Al-glauconite to glauconites (VIAl/(VIFe3+ + VIAl) = 0.81–0.91, 0.60–0.78, 0.51–0.59, and 0.11–0.50, respectively). Following (Kossovskaya and Drits, 1971; Drits et al., 2013), we distinguish Fe-illites (VIAl/(VIFe3+ + VIAl) = 0.6–0.8) and retain the term Alglauconite for mineral varieties with the Al index K Al = VIAl/(VIFe3+ + VIAl) between 0.51 and 0.59. The structural characteristics (unit cell parameters b, csinβ, and others) and IR spectroscopic data on illite, Fe-illite, Al-glauconite, and glauconite are compared. Special attention is given to samples with elevated Mg contents found in these groups of mineral varieties. Since compositional variations of the studied glauconite–illite minerals fall beyond the fields accepted for these minerals by IMA NC and AIPEA NC, the existing classifications for low-charge dioctahedral micaceous micas may need to be revised.

Al-rich K-dioctahedral 1 M and 1 Md 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 c sinβ and | c cosβ/ 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 + Fe 2+ = ~ 0.6 atoms per O 10 (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.

В статье приведены обобщенные результаты исследования структурно-кристаллохимических особенностей глобулярных диоктаэдрических 2 : 1 слоистых силикатов глауконит-иллитового состава (45 образцов) из разрезов нижнего кембрия Северной Сибири (Оленекское поднятие), рифея, венда и нижнего кембрия Восточной Сибири, верхнего рифея Южного Урала и полуострова Средний, а также из пограничных отложений венда и кембрия Подольского Приднестровья (Украина). Изучение мономинеральных фракций зерен проводилось с помощью комплекса современных химических и физических методов (рентгеновский анализ, электронография косых текстур (ЭКТ), сканирующая электронная микроскопия (СЭМ), классический химический и микрозондовый анализы, ИК и мессбауровская спектроскопия и др.). С помощью метода моделирования экспериментальных дифракционных картин от препаратов, насыщенных этиленгликолем [Sakharov et al., 1999], в изученных образцах была определена степень смешанослойности (4–13%), тип разбухающих слоев (смектит 16.85 A и 13.2 A, вермикулит), их соотношение в двух- или трехкомпонентной смешанослойной структуре, параметры элементарной ячейки b (9.02–9.11 A) и c sin (9.94–10 A) слюдистых слоев, а также степень ближнего порядка в чередовании разнотипных слоев (R = 0, 2, 3). Классификация низкозарядных (слоевой заряд 0.6–0.85 на формульную единицу (ф.е.)) диоктаэдрических 2 : 1 слоистых силикатов проведена с учетом рекомендаций Международных номенклатурных комитетов по слюдам и глинистым минералам (IMA NC, AIPEA NC) [Rieder et al., 1998; Guggenheim et al., 2006]. Опираясь на результаты исследования кристаллохимических особенностей 79 образцов, полученных ранее [Ивановская и др., 2012] и приведенных в данной статье, показано, что среди глобулярных слюдистых разновидностей наблюдается непрерывный изоморфный ряд от иллитов через Fe-иллиты и Al-глаукониты до глауконитов (VIAl/(VIFe3+ + VIAl) = 0.81–0.91, 0.60–0.78, 0.51–0.59 и 0.11–0.50 соответственно). Среди иллитов согласно работам [Коссовская, Дриц, 1971; Дриц и др., 2013], мы выделяем Fe-иллиты (VIAl/(VIFe3+ + VIAl) = 0.6–0.8) и оставляем термин “Al-глауконит” за минеральными разновидностями, у которых степень алюминиевости KAl = VIAl/(VIFe3++ VIAl) лежит в интервале от 0.51 до 0.59. Сравниваются структурные характеристики (параметры b, c sin и др.) и ИК-спектроскопические особенности иллитов, Fe-иллитов, Al-глауконитов и глауконитов; особое внимание уделяется образцам с повышенным содержанием Mg, встреченным в этих группах минеральных разновидностей. Поскольку вариации состава изученных глауконит-иллитовых минералов выходят за рамки, принятые IMA NC и AIPEA NC, авторы считают, что необходим пересмотр существующей классификации для низкозарядных диоктаэдрических слюдистых минералов.

To determine the relationships between the symmetry of the overall pyrophyllite and talc structure and the symmetry of individual layers, the geometry and symmetry of each 2:1 layer of pyrophyllite and talc were analyzed. For each, the previously published, refined unit cell may be rotated clockwise by ~60° for comparison to a layer unit cell. In pyrophyllite, the layer unit cell is ideal and shown to be orthogonal with C2/m symmetry. The agreement between the refined atomic coordinates and those calculated for the layer with C2/m symmetry confirms that the symmetry of the pyrophyllite layer is C2/m. The obliquity of the pyrophyllite refined cell results from the layer stacking and the choice of unit cell, but the interlayer stacking sequence does not disturb the layer symmetry. In contrast, talc has an oblique layer cell, without a mirror plane. For the most part, the distortion of the talc 2:1 layer is probably caused by an elongation of unshared O-O lateral edges around M1 that creates a slight corrugation of the octahedral sheet surface. Perhaps of lesser importance, the distortion of the talc layer cell may result from Coulombic interactions between cations of adjacent layers, and these cation-to-cation distances are sufficiently large (~6–7.5 Å) that the weak van der Waals forces that stabilize the stacking are not overcome. Because pyrophyllite has a vacant octahedral site, similar interactions are not present, and this results in a more idealized layer symmetry.Phyllosilicates consisting of layers with an orthogonal cell and mirror plane (pyrophyllite, kaolinite, sudoite) were shown to have similar stacking faults. In these structures, the 2:1 or 1:1 layers have uniform orientation, and stacking faults occur owing to interstratifications of two alternative interlayer displacements in the same crystal that are related by a mirror plane in the projection on the (001) plane. In talc, stacking faults are associated with layer rotations by ±120°, whereas the lateral displacement between the adjacent tetrahedral sheets across the interlayer region is relatively ordered.

В статье приведено обобщение структурно-кристаллохимических особенностей изученных ранее глобулярных диоктаэдрических 2 : 1 слоистых силикатов глауконит-иллитового состава из разрезов верхнего протерозоя Северной Сибири (Анабарское и Оленекское поднятия). Рассмотрены литолого-минералогические особенности глауконитсодержащих пород. Приведены геохронологические данные отдельных образцов. Изучение мономинеральных фракций зерен проводилось с помощью комплекса современных химических и физических методов (рентгеновская дифракция, электронография косых текстур, сканирующая электронная микроскопия, ИК- и мессбауровская спектроскопия, классический химический и микрозондовый анализы и др.). Классификация низкозарядных диоктаэдрических 2 : 1 слоистых силикатов проведена с учетом рекомендаций Международных номенклатурных комитетов по слюдам и глинистым минералам (IMA NC, AIPEA NC) [Rieder et al., 1998; Guggenheim et al., 2006]. На основе этой классификации среди изученных глобулярных Al- и Fe-содержащих разновидностей установлен непрерывный изоморфный ряд от глауконита до иллита. Слоистые силикаты промежуточного состава, не включенные в классификацию IMA NC и AIPEA NC, предложено отнести к Al-глауконитам (Fe-иллитам). К истинным иллитам предлагается добавлять термины “глобулярный” или “пластинчатый”, поскольку традиционно для глобуль зеленого цвета используется термин “глауконит”, а для тонкодисперсных Al-содержащих глинистых минералов – “иллит”.

An improved algorithm has been elaborated for computing atomic coordinates in K-dioctahedral micas-2M(1) from the experimental data on cation composition and unit-cell parameters. The structure modeling procedure is based on regression equations relating the structural features and chemical composition of micas that were obtained from the analysis of published data on 27 refined structures of dioctahedral micas of various compositions including 20 K-dioctahedral micas-2M(1), 3 paragonites-2M(1), 2 margarites-2M(1), and 2 celadonites-1M. The empirical relationships accurately describe the observed structural distortions in dioctahedral micas, such as tetrahedral tilt and rotation, tetrahedral elongation, octahedral flattening, hydroxyl depression, etc. The majority of the regressions have r(2)>0.8 and p-values <0.05, which means that the results are statistically significant. The predicted structural parameters are used to calculate the atomic coordinates for K-dioctahedral micas-2M(1) with disordered distribution of tetrahedral and octahedral cations. The estimated standard deviations (e.s.d.) for modeled atomic coordinates vary for different atomic positions and range from 0.0001 to 0.003 (fractional units); the e.s.d. values for structural characteristics obtained from the calculated atomic coordinates are 0.002-0.007 angstrom for mean and individual tetrahedral bond and edge lengths, 0.004-0.013 angstrom for mean and individual octahedral bond and edge lengths, 0.013-0.015 angstrom for K-O distances, and 0.5 degrees for the tetrahedral ditrigonal rotation angle. Computation of atomic coordinates for additional three dioctahedral mica-2M(1) structures that were not included in the derivation of the empirical structure-composition relationships used in the algorithm yielded close agreement between the modeled and observed structural characteristics.The structure modeling algorithm can be used as an inexpensive and express method for evaluation of fine structural features in large collections of K-dioctahedral mica samples of diverse compositions.