Chlorite Layers Research Articles

Evolution of reservoir quality related to clay coating overgrowths in tight sandstone in the fluvial facies of the southeastern Sichuan Basin

As a result of silicate overgrowth, clay coatings influence the evolution of reservoir quality during diagenesis. Most studies indicate coatings can resist compaction and inhibit cementation, thus preserving primary porosity and serving as one of the primary controlling factors for improving reservoir quality. However, some research suggests clay coatings have insufficient hardness to resist compaction and could potentially block pore throats and prevent fluid flow within the sandstone, negatively affects reservoir quality. Therefore, to address whether coatings are positive to reservoir evolution, this study employs polarized light microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and porosity-permeability testing to investigate the characteristics of clay coatings within the fluvial facies sandstone of the Xiashaximiao Formation. The microscopy reveals abundant clay coatings were found, mainly in lithic-arenite with 30%–45% feldspar and rock fragments. The underlying Lianggaoshan Formation locally exhibits interbedded siltstones with volcanic rock fragments, providing material sources for the clay coatings formation. SEM and EDX analyses indicate two predominant forms of clay coatings: single-layer (primarily chlorite) and double-layer (inner smectite/illite, outer ferroan chlorite). Single-layer overgrowths primarily consist of thin layers of chlorite (5–15 μm), whereas double-layer overgrowths consist of a thin inner zone (1–5 μm) covering the grain surface and a thicker outer zone (10–20 μm) covering the inner zone, demonstrating characteristics indicative of multi-stage growth. Porosity-permeability testing results the sandstone samples have <10% porosity, which are typically tight sandstones, but those with clay coatings have higher porosity. Nevertheless, sandstones with double-layer overgrowths exhibit lower permeability relative to those with single-layer overgrowths or without overgrowths. Therefore, based on the integration of morphological and physical properties of silicate overgrowths, it is proposed the efficiency of clay coatings in resisting compaction, inhibiting cementation, and preserving primary porosity increases with the number of growth stages, layers, and thickness, which in turn depends on the abundance of source materials. Single-layer coatings facilitate fluid migration, whereas thicker double-layer overgrowths are prone to occluding narrow pore throats, thereby restricting fluid mobility. Consequently, this study establishes a model for the multi-stage evolution of clay coatings overgrowths, elucidating the varying fluid migration behaviors across different stages and providing a more detailed investigation of the implication of clay coatings on reservoir quality, thus offering new theoretical insights into diagenesis and the evolution of tight sandstone reservoirs.

Clay minerals of Shiranish Formation in, Erbil, Duhok northern Iraq

In the Upper Cretaceous of northern Iraq (upper Campanian-Maastrichtian), the Shiranish Formation is examined for clay minerals using X-ray diffraction in the Duhok (Sandor section) and Erbil cities (Lailuk section). In the marl section of the Shiranish Formation, the layers of chlorite, illite, kaolinite, palygorskite, and a mixed layer of montmorillonite-chlorite strata were discovered to have formed antigenically. The variation in minerals indicates the Cretaceous’ changing climate. Palygorskite is a result of leaching processes that are more successful due to variations in the parent rocks. The transition of palygorskite from montmorillonite and illite can occasionally be impacted by post-depositional diagenetic processes. Because of the warm conditions that supply the eroded clastic materials from earlier successions of Cretaceous in the area, chlorite, mixed-layers, and illite may have predominated over kaolinite. The variation in source rocks and drainage conditions is reflected in the variation of clay minerals in the investigated sequence, which could improve leaching rate processes.

Akın ve Taşkonak Köyleri (Van) Çevresinde Yüzeyleyen Orta-Geç Miyosen Yaşlı Killi Kayaçların Mineralojik Özellikleri ve Paleoiklimsel İpuçları

Bu çalışmada Orta-Geç Miyosen yaşlı denizel ortamda oluşmuş olan Van Formasyonu killi kayaçlarının mineralojik özelliklerini belirlemek ve bu kayaçların oluştuğu Orta-Geç Miyosen boyunca çalışma alanındaki eski iklim koşullarını yorumlamak amaçlanmıştır. Çalışma alanı Van Gölü’nün güney batısındadır. Orta-Üst Miyosen kayaçlarından dört stratigrafi kesiti ölçülmüş ve yetmiş üç kilce zengin kayaç numunesi derlenmiştir. Numunelerin tümünün X-ışını difraksiyonu analizleri yapılmış, tüm kayaç bileşimlerinde kuvars, mika, feldispat, kalsit, amfibol, dolomit, kristobalit ve kil mineralleri; kil fraksiyonlarında ise illit, klorit, smektit, serpantin, karışık tabakalı klorit-smektit ve illit-smektit belirlenmiştir. Seçilmiş üç örnek üzerinde yapılan SEM ve EDX çalışmalarında serpantin, klorit ve smektit gözlenmiştir. İyi kristalli illit, smektit, klorit ve feldispat minerallerinin bir arada bulunmalarına dayanılarak, kırıntılı kayaçların oluştuğu süre boyunca bölgede soğuk ve kurak iklim koşullarının hakim olduğu yorumu yapılmıştır.

Rietveld refinement of chromian clinochlore from the chromitites of Golyamo Kamenyane serpentinite (Eastern Rhodopes)

The structure of a disordered IIb chromian clinochlore from the chromitites of Golyamo Kamenyane serpentinite (Eastern Rhodopes) was studied using Rietveld refinement techniques and powder X-ray diffraction (CuKα, 3–100° 2Θ in 0.013° 2Θ increments). The refinement in space group C–l yielded the following lattice parameters: a = 5.299(2) Å, b = 9.2625(5) Å, c = 14.4726(6) Å, α = 90.103(7)°, β = 97.191(13)°, γ = 90.320(11)°, and cell volume of 704.69(21) Å3. The structure of the studied mineral is characterized by a semi-random stacking of adjacent chlorite layers creating a situation in which not all atoms positions could be precisely located. Despite this limitation, the crystal chemical plausibility of the final structural model could be achieved due to the fact that most of the atomic positions (all octahedral cations and anions) repeat at intervals of ±b/3 and could be uniquely determined. The type of structural information that can be derived from the test compound and similar structurally related materials by applying the Rietveld method is discussed and ways to achieve this are considered.

Siliciclastic-hosted zinc mineralization in the Proterozoic Vazante – Paracatu District, Brazil: Implications for metallogeny and sources of metals in sediment-hosted base metal systems

The Mesoproterozoic Upper Vazante Sequence is a mixed carbonate – siliciclastic marine succession, deposited between 1300 and 1100 Ma, which contains a variety of carbonate-hosted Zn silicate and Zn – Pb sulfide deposits in central Brazil. Carbonate-hosted Zn ± Pb mineralization is interpreted to have formed in two stages: one prior to the major deformation event and a second stage during the Brasiliano Orogeny (850 to 550 Ma). All known carbonate-hosted Zn-deposits and occurrences in the 250 km – long Vazante – Paracatu District are underlain by the Serra do Garrote Formation, a 500 + m-thick package made up mostly of phyllites and carbonaceous phyllites (>1% Total organic carbon) with minor interlayered meta-arenitic rocks. Our study revealed that siliciclastic rocks from the Serra do Garrote Formation contain horizons enriched in Zn (up to 0.46%).Siliciclastic-hosted zinc mineralization in the Serra do Garrote Formation is associated with pre-orogenic hydrothermal sulfide – quartz – chlorite layers and veinlets, which preferentially formed in carbonaceous and pyritic phyllites belonging to two subunits. The siliciclastic-hosted mineralization in these layers and veinlets is folded and crenulated (pre-orogenic). The geochemical signature of zinc-enrichment includes Cd, Cu, Hg, In, V ± Sb, Se, Mo, Re. Based on textural relationships and chemical U – Th – Total Pb dates of pre-orogenic and syn-orogenic monazite, sulfide mineralization in the Serra do Garrote Formation formed between 1200 and 650 Ma. Textural features indicate that base metals were locally remobilized within the Serra do Garrote Formation during the Brasiliano Orogeny, precipitating new generations of syn-orogenic sphalerite, chalcopyrite and pyrite. Mineralized zones in the Serra do Garrote Formation are distributed throughout the basin and have a similar geochemical signature as the overlying syn-orogenic carbonate-hosted base metal mineralization. However, there is no compelling evidence of depletion in the Serra do Garrote Formation, at the scale of this study, to suggest the ore-related metals have been leached from these siliciclastic rocks to be concentrated in the carbonate-hosted base metal deposits during the Brasiliano Orogeny. The findings of this study suggest that the Serra do Garrote Formation can be a potential target for siliciclastic-hosted base metal mineralization, similar to other Proterozoic sediment-hosted massive sulfide deposits.

Estimation of clay minerals from an empirical model for Cation Exchange Capacity: An example in Namorado oilfield, Campos Basin, Brazil

Abstract Clay minerals are important components of shales and occur predominantly in clastic sedimentary basins. Although the classification of such minerals arouses the interest of oil exploration practitioners, this is a hard task to accomplish. In order to typify the main clay minerals of turbidite sandstones in Campos Basin, this study comprises an empirical model to estimate Cation Exchange Capacity. The basic parameter for applying the empirical model is shale volume, whose values are calculated from natural gamma ray, neutron and bulk density measurements. Two-dimensional maps of Cation Exchange Capacity in specific depths are also provided in order to extend the analysis to the whole Namorado oilfield, Campos Basin, in southeast Brazil. The application of this methodology in two wells drilled in Namorado oilfield reveals, despite significant differences in shale content estimation, the predominance of kaolinite in the selected sedimentary interval, while layers of illite and chlorite are noticed at the top of the sediments. There is a total absence of smectite in all studied area. Two-dimensional maps for specific depths of Namorado reservoir enhance discrepancies associated with limitations of both methods to obtain shale content.

Specific Features of Profile Distribution and Crystallochemistry of Phyllosilicates in Soils of the Cisbaikal Forest-Steppe

The mineralogical composition of agrogray, dark gray, and agro-dark gray soils (Luvic Greyzemic Retic Phaeozems); agro-dark gray residual-calcareous soils (Calcaric Cambic Phaeozems); clay-illuvial agrochernozems (Luvic Chernic Phaeozems); and agrochernozems with migrational–mycelial carbonates (Haplic Chernozems) developed in the forest-steppe of Central Siberia within the Irkutsk Depression has been studied. The clay (<1 μm) fraction separated from these soils consists of mixed-layer minerals with alternating layers of hydromica, smectite, vermiculite, and chlorite; the proportions between them change within the soil profiles. The clay fraction also contains hydromicas, kaolinite, chlorite, and some admixture of the fine-dispersed quartz. Each type of the soils is characterized by its own distribution pattern of clay material with specific alternation of layers in the mixed-layer formations. Mixed-layer minerals of the chlorite–vermiculite type predominate in the upper horizons of texture-differentiated soils. Down the soil profile, the content of mixed-layer mica–smectitic minerals increases. In the clay fraction of arable dark gray-humus soils with residual carbonates, the distribution of the clay fraction and major mineral phases in the soil profile is relatively even. An increased content of well-crystallized kaolinite is typical of these soils. The parent material of agrochernozems has a layered character: the upper horizons are generally depleted of clay, and the middle-profile and lower horizons are characterized by the considerable kaolinite content. In general, the clay material of soils of the Tulun–Irkutsk forest-steppe differs considerably from the clay material of foreststeppe soils developed from loesslike and mantle loams in the European part of Russia. In particular, this difference is seen in the proportions between major mineral phases and between biotitic and muscovitic components, as well as in the degree of crystallinity and behavior of kaolinite and chlorite.

CLAY MINERALOGY OF THE SEDIMENTARY IRON-NICKEL ORE OF AGIOS IOANNIS, NE BOEOTIA: NEW DATA AND IMPLICATION FOR DIAGENETIC MODIFICATIONS Christidis G.

The clay mineralogy of the sedimentary Fe-Ni deposit of Agios Ioannis and a Ni lateritic profile from Pavlos, both in Lokris area, Greece, is investigated. The clay fraction of samples from Agios Ioannis consists mainly of Fe-rich chlorite and Fe-smectite, with minor serpentine, hematite and goethite. Locally, disordered talc (kerolite and/or pimelite), illite, R1 mixed layer illite/smectite and gibbsite are identified. The lateritic profile has different composition and contains mainly Fe-rich smectite (nontronite and/or ferruginous montmorillonite) with minor serpentine, goethite, magnetite, and talc, whereas chlorite is absent. It is suggested that in the Agios Ioannis deposit chlorite has formed from smectite via R0 mixed layer chlorite/smectite with &gt;80% chlorite layers. The source of Al is probably the diagenetic transformation of pedogenic goethite to hematite during burial. The lack of chlorite in the lateritic profile of Pavlos is attributed to the limited presence of goethite. Smectite is believed to be a major mineralogical constituent of the Fe-Ni ores of the broader Lokris area.

Clay Minerals Variations in Quaternary Sediments of Basrah City-Iraq

Mineralogical and chemical analysis of two well cutting in quaternary sediments at Basrah city. This study have been focused on the quaternary clay minerals variations which give a clear indication to marine transgression and or / regression in Hammar formation. Kaolinite, Illite, Palygoreskite, Illite-Palygorestike mixed layer, Chlorite, Montmorillonite, Vermiculte, and mixed layers of Illite- Smectite are the most quantitatively important phyllosilicates in soil studies in both boreholes. The mineralogical composition of cutting sediments shows significant variability in the different size fractions and depth distribution. Chlorite and kaolinite minerals were increase in the fine grain size especially in the clayey silt, whereas montmoriolllinite and vermiculite minerals were increase in coarse grains especially in silty sand texture. Kaolinite decrease in abundance with depth, which give indicative of a transition from non-marine in quaternary recent sediment ( fresh or brackish water silt) to marine facies in quaternary Hammar formation ( marine shelly silt) during last transgression of quaternary. Palygoreskite mineral percentages increasing at depths 27 and 28 meter , after decreased at 30, 32, 35 , and 40 m respectively , which give data indication that early quaternary marine transgression happened in 32m , and center of quaternary marine transgression detected in 27 and 28 meters , while late marine transgression detected in 17 m , and all of these represent by Hammar formation. Soil ageing also caused to downward increase of montmorillonite suggest to climate became more arid during the early quaternary. Variations ratio of montmorillonite versus kaolinite can be indicated to climatic fluctuation. Illite-Semectite featured indicates changes in both sediment source and paleoclimate. Illite transformation for Illite-Palygoreskite mixed layers with ageing. Vermiculite has strong susceptibility to increase with age. SiO₂, Al₂O₃, CaO, MgO, K₂O, and Fe₂O₃ were the major oxides that identified in present study and used as indicated of clay minerals in studied sediments.

Complexity of clay mineral formation during 120,000 years of soil development along the Franz Josef chronosequence, New Zealand

ABSTRACTWeathering of primary silicates to secondary clay minerals over time affects multiple soil functions such as the accumulation of organic matter and nutrient cations. However, the extent of clay mineral (trans)formation as a function of soil development is poorly understood. In this study, the degree of weathering of sediments along a 120 kyr soil formation gradient was investigated using X‐ray diffraction, Fourier transform infrared spectroscopy and X‐ray fluorescence spectroscopy. Irrespective of site age, mica and chlorite were the dominant clay minerals. During weathering, a remarkable suite of transitional phases such as vermiculite and several interstratifications with vermiculitic, smectitic, chloritic and micaceous layers developed. The degree of weathering was correlated with soil pH and depletion of K, Ca, Na, Fe and Al, regarding both soil depth and site age. Kaolinite occurred especially at the 120 kyr site, indicating slow formation via transitional phases. The findings of this study revealed that long-term soil development caused complex clay mineral assemblages, both temporally and spatially, and linking this variability to soil functioning warrants further research.

Tube fossils from gossanites of the Urals VHMS deposits, Russia: Authigenic mineral assemblages and trace element distributions

The occurrence, types, morphology, and mineralogical characteristics of tube microfossils were studied in gossanites from twelve VHMS deposits of the Urals. Several types of tube microfossils were recognized, including siboglinids, polychaetes and calcerous serpulids, replaced by a variety of minerals (e.g. hematite–quartz, hematite–chlorite, carbonate–hematite) depending on the nature of the substrate prior to the formation of the gossanites. Colonial hematite tube microfossils (~150μm across,1–2mm long) are composed of hematitic outer and inner walls, and may exhibit a cellular structure within their cavities. Spherical forms are saturated with Fe-oxidizing bacteria inside the tubes – probably analogues of trophosomes. Colloform stromatolitic outer wall surfaces are characterized by the presence of numerous interlaced filaments of hematite (2–3μm diameter, up to 1–2mm long). Between tube microfossils, the hematitized cement contains bundles of hematitized filaments with structures similar to the hyphae of fungi. Hematite–chlorite tube microfossils are scattered in gossanites, mostly as biological debris. They are typically 30 to 300μm in diameter and 1 to 5mm long. The layered structure of their tube walls is characterized by hematite–quartz and chlorite layers. Abundant filamentous bacteria coated by glycocalix and chlorite stromatolite are associated with hematite–chlorite tubes. The carbonate–hematite tube microfossils (up to 300μm across, 2–3mm long) occur in carbonate-rich gossanites. The tubes are characterized by fine (~10μm thick) walls of hematite and cavities dominated by relatively dark carbonate or hematite. Carbonates may be present both in walls and cavities. Stromatolite-like leucoxene or hematite–carbonate aggregates were also found in association with tubes. Randomly oriented filaments are composed of ankerite. Single filaments are composed of individual cells, typically smaller than 100nm across, similar to that of magnetotactic bacteria.Three dimensional tomographic images of all types of tube microfossils demonstrate a clear wavy microlayering from outer and inner walls, which may reflect segmentation of the tube worms. The traces of burrowing or fragments of glycocalix with relict spheres are typical of tube microfossils from gossanites.The carbon isotopic composition of carbonates associated with tube microfossils from hematite–quartz, hematite–carbonate, and hematite–chlorite gossanites average−7.2, − 6.8, –22.8‰, PDB, respectively. These values are indicative of a biogenic origin for the carbonates. The oxygen isotopic composition of these carbonates is similar in all three gossanite types averaging +13.5, +14.2, +13.0‰ (relative to SMOW), and indicative of active sulfate reduction during the diagenetic (and anadiagenetic) stages of the sediments evolution. The trace element characteristics of hematite from tube microfossils are characterized by high contents of following trace elements (average, ppm): Mn (1529), As (714), V (540), W (537), Mo (35), and U (5). Such high contents are most likely the result of metal and metalloid sorption by fine particles of precursor iron hydroxides during the oxidation of sulfides and decomposition of hyaloclasts via microbially-mediated reactions.

Changes in soil mineralogy due to nitrogen fertilization in an agroecosystem

Additions of nitrogen (N) fertilizer to surface soil can trigger a wide array of complex effects, including changes in soil chemical, physical, and microbial properties which impact both the nitrogen and carbon cycles. While abundant literature exists regarding the influence of N amendments on processes and properties influencing the soil carbon cycle, there is little recognition given to potential changes in soil mineralogy. We collected surface soil (Maury silt loam, Typic Paleudalf) from a long-term agroecosystem under no-tillage management which has received annual inorganic N additions (0, 168, 336kgNha−1) for 34years and characterized the soil mineralogy and other relevant properties. With recent liming, N fertilizer addition was associated with only slight changes in soil pH and the suite of exchangeable cations. Silt mineralogy was not influenced by N, but x-ray diffraction found changes in the clay fraction. The clay mineral assemblage in the control (0kgNha−1) is complex, comprised of layers of pedogenic chlorite, vermiculite, hydroxy-interlayered vermiculite, mixed-layer chlorite–vermiculite, illite, kaolinite, and quartz. Fertilizer N additions resulted in a 3- and 3.3-fold decrease in the 1.4nm/1.0nm peak area ratios for Mg-clay slides in the 168 and 336kgNha−1 treatments. This decrease is likely due in part to fixation of NH4+ in vermiculite layers, corroborated by increases in the intensity of the infrared (IR) band at 1430cm−1 (assigned to the NH bending mode of NH4+) with added N. The role of biota emerged as shown by a negative relationship between water-soluble oxalate concentrations and the 1.4nm/1.0nm peak area ratios (r=0.56, P=0.06). Past acidification due to nitrification of added NH4+ which occurred prior to liming might have played a role in weathering of pH-sensitive minerals such as chlorite. Our findings raise questions about the capacity of reactive 2:1 mineral layers to fix NH4+ and how this is moderated by biota in no-till agroecosystems.

On the mechanism of exfoliation of ‘Vermiculite’

AbstractSix samples of ‘Vermiculite’ have been studied to investigate the mechanism of its well known but poorly understood property to exfoliate. The samples were analysed quantitatively by XRD to determine their precise mineralogical composition. Electron microprobe methods, including elemental mapping of native potassium and of caesium (introduced by cation exchange) were used to examine variation in the chemical composition of the particles. Most of the samples examined show heterogeneous mineralogical compositions which occur as distinct zones within the volume of individual particles, presenting a mosaic texture. Exfoliation is related to this mosaic distribution of the different mineral phases within the particles. Lateral phase boundaries between vermiculite and mica layers, or vermiculite and chlorite layers are postulated to prevent or impede the escape of gas from a particle, resulting in exfoliation when the pressure exceeds the interlayer bonding forces that hold the layers together. This mechanism provides a common explanation for the exfoliation of ‘Vermiculite’ by thermal methods or by treatment with H2O2. Paradoxically, one sample which consists of pure vermiculite, in the mineralogical sense of the term, demonstrates that pure vermiculite does not and should not exhibit the property of exfoliation. Our explanation of the mechanism of exfoliation explains the commonly observed particle size dependence of exfoliation and the tendency for obviously poly-phase ‘Vermiculite’ samples to show the largest coefficients of expansion.

Petrographic and microprobe study of nephrites from Lower Silesia (SW Poland)

Lower Silesia hosts important European nephrite deposits of Jordanów and less-known of Złoty Stok. Nephrite artifacts were discovered in archaeological sites dated back to the Neolithic period, across Eurasia. Especially artifacts found in Poland, Italy and Bulgaria may originate from Polish nephrites. Nowadays, only one artifact is precisely linked to Jordanów. Petrographic study of nephrites and chemical analyses of constituents by means of EMPA allow accurate identification of the nephrites. The characteristic phases of Jordanów tremolite nephrite are rotated and cataclased diopside porphyroblasts with pressure shadows, chlorite layers and nests with interlocking non-pseudomorphic texture and prehnite veins. The presence of hydrogrossular, grossular, titanite, apatite with monazite inclusions, and zircon with pleochroic haloe is typical. Chlorites are usually represented by penninite, and minor clinochlore and diabantite. The characteristic features of Złoty Stok actinolite nephrite are löllingite and diopside crystals usually visible by the naked eye, with the presence of quartz and carbonates. Löllingite is chemically inhomogeneous and gold bearing. Most of the mineralogical-petrological features can be obtained using non-destructive methods

PORE‐LINING CHLORITE CEMENTS IN LACUSTRINE‐DELTAIC SANDSTONES FROM THE UPPER TRIASSIC YANCHANG FORMATION, ORDOS BASIN, CHINA

Non‐marine sandstones in the upper Triassic Yanchang Formation constitute an important reservoir rock in the Zhenjing area of SW Ordos Basin, Central China. A study of the diagenetic history of these sandstones, including the formation of pore‐lining chlorite cements, was undertaken to investigate controls on reservoir quality. Data came from the petrographic analysis of cores from five wells. The sandstones were deposited in a lacustrine‐deltaic setting and consist of fine‐to medium‐grained feldspathic litharenites and lithic arkoses, and in general have low porosity and permeability. The principal diagenetic processes observed were compaction, cementation, dissolution and replacement. Mechanical compaction and calcite cementation caused a reduction in reservoir quality. The precipitation of pore‐lining chlorite inhibited quartz cementation and preserved porosities of up to 16%. The chlorite occurs in a double layer which lines pore spaces, particularly in distributary channel sandstones. Both chlorite layers grew by precipitation from pore waters containing the products of dissolution of volcanic rock fragments, biotites and feldspars. An increase of temperature during diagenesis was probably the cause of the two generations of chlorite cement, and also of the observed variations in elemental concentrations between the earlier‐ and later‐formed chlorite. The chlorite was precipitated after early compaction but before other diagenetic phases.

Early Proterozoic weathering processes under low O2 conditions reconstructed from a 2.45 Ga paleosol in Pronto, Canada

We have examined a paleosol in Pronto, Canada, which formed at ~2.45 Ga and under very low O 2 conditions, to reconstruct ancient weathering processes. Chlorite, which replaced biotite in parent granite, consists of almost complete chlorite layers as revealed by high-resolution transmission microcopy. Chlorite grains were fragmented gradually with increased intensity of weathering. The fragmented chlorite grains were partly or largely replaced by sericite formed from K-metasomatism, which strongly suggests that almost complete chloritization of biotite preceded sericitization. In the upper part of the paleosol, chlorite-biotite mixed layers formed but the formation of these layers cannot be explained by mere chloritization and sericitization, which are usually considered to occur concurrently during post-weathering events. Our observations and analyses suggest that the chlorite-biotite mixed layers were formed as follows: chlorite, with no or little biotite, was formed from biotite in parent granite prior to weathering. Chlorite was then weathered by dissolution and fragmentation at ~2.45 Ga, and chlorite-vermiculite (or smectite) mixed layers were formed by layer-by-layer transformation in the weathering profile (note that vermiculite is not actually observed in the Pronto paleosol). Following burial of the weathering profile, the vermiculite layers in the chlorite-vermiculite mixed layers were converted to biotite layers, by means of K-metasomatism, to form the chlorite-biotite mixed layers. The chlorite-biotite mixed layers in the Pronto paleosol were therefore chlorite-vermiculite mixed layers at the time of weathering.

Mineralogy and chemistry of finely dispersed bottom sediments in the Amur River

The mineralogy and chemistry of the fine fractions separated from the bottom sediments (aquasols) in the Amur River were studied in the subfractions of the fractional peptization differing in the binding strength between the water-peptized and aggregated components. Detritus of quartz, field spars, and strongly degraded layered silicates mainly passes into the water-peptized state. Interstratified layered silicates with different combinations of chlorite and mica layers prevail in the aggregate subfraction. The distribution of the microelements in the bottom sediments, the water-peptized and aggregated clays, and the residues after their separation was determined. The content of iron, manganese, zinc, and copper in the clay subfractions was higher than in the bottom sediments and in the residue after the separation of the clay subfractions by 2–4 times, which confirmed the functional significance of the clay minerals. The presence of diatomic algae in the components of the bottom sediments was revealed by scanning electron microscopy. The contribution of the diatoms to the accumulation of iron, manganese, zinc, and copper was shown.

Metamorfismo de facies prehnita-pumpellyita en la franja oriental de la Formacion Abanico, Cordillera de los Andes de Chile central (33°50'S): controles qumicos y de escala sobre las asociaciones minerales, el progreso de la reaccion y el estado de equili

In the El Volcan and Rodeo de los Bueyes areas, Andean Principal Cordillera (east of Santiago; 33o50'S), an Upper Oligocene-Lower Miocene volcanic series belonging to the Abanico Formation (Late Eocene-Early Miocene) is exposed. The rock successions outcropping in both areas, ca. 3,300 m total thickness, have been affected by very low-grade, non-deformative metamorphism in the prehnite-pumpellyite facies. This is represented by the widespread development of secondary mineral assemblages composed of epidote, mixed-layer chlorite-smectite, albite, quartz, white mica, and titanite. These mineral assemblages also contain pumpellyite, prehnite or prehnite+actinolite in a few samples. Chemical characteristics, such as low compositional variability of mixed-layer chlorite-smectite and actino- lite independent from the metadomain where these phases are hosted, along with a high proportion of chlorite layers in the former, suggest that these phases closely represent the whole rock effective bulk composition. On the contrary, pumpellyite compositions show a higher variability and a strong metadomain control evidencing its lower equilibration kinetics and leaving only those that grow in the rock matrix as the more closely representative of a whole rock effective bulk compositional control. Mineral assemblages from both areas show evidence of having been formed during the same metamorphic event where reactions have ocurred equivalently between them. However, local differences in chemical variables controlling this process are recognizable. A higher CO 2 concentration in the fluids during the metamorphism in the El Volcan area than in the Rodeo de los Bueyes area is suggested by the scarce development of prehnite and the ubiquitous presence of calcite in the metamorphic assemblages of the former. Pressure and temperature of this meta- morphic event have been roughly estimated to be lower than 2-3 kbar and 320oC, respectively.

Clay minerals from Weichselian glaciolimnic sediments of the Sępopolska Plain (NE Poland)

Clay minerals from Weichselian glaciolimnic sediments of the Sępopolska Plain (NE Poland)Glaciolimnic deposits sampled from three sedimentation reservoirs located on the Sępopolska Plain (northeastern Poland) were investigated. The material under study was deposited in the recession phase of the Pomeranian phase of Vistula (Weichselian) glaciation. The clay fraction was separated by centrifugation after preparation according to Jackson. Mineralogical investigations were done by X-ray diffraction. The analysed deposits had a similar complex composition of clay minerals. The main components were illites mixed with the illite/smectite mineral with percentages &lt; 10 % S. Another part were minerals of the illite/smectite type which had differentiated content of smectite layers (mainly 80-90 % S). There were also chlorite minerals, probably as mixed layer minerals of the chlorite/vermiculite type or HIV with a negligible amount of chlorite layers. The results indicated that all the deposits were of the same age as well as similar deposited material and the samples are different from typical Pomeranian till and typical limnic material of the same age. Small differences observed among the deposits of specific sedimentation reservoirs were probably the result of later processes.

Diagenetic smectite-to-illite transition in clay-rich sediments: A reappraisal of X-ray diffraction results using the multi-specimen method

Smectite illitization is a common mineralogical reaction occurring during the burial diagenesis of clay-rich sediments and shales, and has thus attracted sustained interest over the last fifty years. Prior studies have concluded that smectite illitization proceeds through a steady set of homogeneous reactions involving intermediate mixed layers of varying compositions. In these intermediate structures, illite and smectite or, more generally, expandable layers (I and Exp layers, respectively) coexist among the same crystallites giving rise to non-periodic structures (I-Exp) characterized by specific diffraction effects. Consistent with this model, reaction progress was characterized by the simultaneous increase in the illite content in I-Exp and in their stacking order leading to the following mineralogical sequence: smectite → randomly interstratified I-Exp with high smectite contents (\> 50% Exp layers) → ordered I-Exp with high illite contents (\> 50% I layers) → illite. Although reaction mechanisms have been extensively debated, this structural characterization has not been challenged, possibly due to a methodological bias. In the present study, X-ray diffraction patterns typical of the diagenetic illitization of smectite are interpreted using modern approaches involving profile fitting (multi-specimen method). Novel insights into the structure of intermediate reaction products are thus obtained. In particular, original clay parageneses are described that include the systematic presence of illite, kaolinite, chlorite and a mixed layer containing kaolinite and expandable layers (K-Exp). In contrast to previous descriptions, the early stages of smectite illitization are characterized by the coexistence of discrete smectite and of a randomly interstratified I-Exp with a high content of illite layers (\>50% I layers). Both the smectite and the I-Exp are authigenic and form under shallow burial, that is at low temperature conditions. With increasing burial depth, the relative proportion of I-Exp increases, essentially at the expense of discrete smectite, and the composition of I-Exp becomes slightly more illitic. In the second stage of smectite illitization, two illite-containing mixed layers are observed. They result from two parallel reaction mechanisms affecting the randomly interstratified I-Exp present in the shallow section of the series. The first reaction implies the dissolution of this randomly interstratified I-Exp and leads to the crystallization of an ordered I-Exp without significant illitization, possibly because of the low K-availability. The second reaction affecting the randomly interstratified I-Exp implies the growth of trioctahedral (Mg, Al) hydroxide sheets in Exp interlayers, thus developing di-trioctahedral chlorite layers (Ch layers) in the initial I-Exp to form an I-Exp-Ch. A layer-by-layer mechanism is hypothesized for this reaction. In this scheme, Mg cations released by the dissolution-recrystallization reaction of I-Exp likely represent the source of Mg for the formation of brucite-like sheets in expandable interlayers, and thus of the I-Exp-Ch. The reported structural characterization of smectite illitization intermediate products contradicts the conventional wisdom of a homogeneous reaction through a series of pure mixed layers of variable composition. In contrast, the coexistence of different phases implies a heterogeneous reaction via a sequence of intermediate phases and requires reassessing the reaction mechanisms proposed in the literature. The compositional range (relative proportion of the different layer types) of these phases is limited and smectite illitization proceeds essentially as relative proportions of the different phases vary. In addition, reaction kinetics and stability of the different intermediate products also need to be reconsidered.