2M1 Polytype Research Articles

Reconstruction of Magma Plumbing System and Regional Magmatic Processes via Chemical and Structural Zoning of Biotite in Rhyolite from Long Valley, CA

AbstractMinerals with compositional zoning in volcanic products are widely used to decipher the history of magmatic evolution. However, structural information, which reflects physical conditions and crystallization equilibrium, has often been overlooked. This study presents the first report on the structural zoning of deep‐derived biotite phenocrysts through investigations of metaluminous rhyolite from Long Valley, CA. Biotite is enriched in Si, Mg, and K and depleted in Fe3+, Ti, and AlIV in core zones compared with rims. In situ structural analyses, including micro X‐ray diffraction, Raman spectroscopy, and transmission electron microscopy, were conducted to identify cores with perfect 2M1 polytype and disordered rims of biotite. The results demonstrate the effectiveness of these methods in revealing various (micro)polytypes of a single species, which occur at different crystallization temperatures, pressures, supersaturation levels, and oxygen fugacities. The concept of structural zoning is introduced here to describe the different structural features distributed systematically in various parts of minerals. By combining structural and chemical zoning, we illustrate a two‐step growth for samples: equilibrium crystallization of the highly ordered cores in a deep magma reservoir with high temperature and pressure, followed by rapid growth of disordered rims during magma mixing in a crystal mush. We further discuss the implications of these findings for reflecting the plumbing system structure and eruption history of rhyolitic magma over extended periods. Our study underscores the remarkable sensitivity of structural zoning in delineating the crystallization conditions of minerals and documenting the environmental changes within magma.

2M1 phlogopite–muscovite series minerals at increasing pressure to 9 GPa. I Atomic volumes and compressibilities

Muscovite (Ms) and phlogopite (Phl) series mineral is studied in the 2M1 polytype and modeled by the substitution of three Mg2+ cations in the three octahedral sites of Phl [KMg3(Si3Al)O10(OH)2] by two Al3+ and one vacancy, increasing the substitution up to reach the Ms [KAl2□(Si3Al)O10(OH)2]. The series was computationally examined at DFT using Quantum ESPRESSO, as a function of pressure from − 3 to 9 GPa. Crystal structure is calculated, and cell parameters, and geometry of atomic groups agree with experimental values. OH in the Mg2+ octahedrons are approximately perpendicular to the (001) plane, meanwhile when they are in Al3+, octahedral groups are approximately parallel to this plane. From Quantum Theory of Atoms in Molecules, the atomic basins are calculated as a function of the pressure, K+ and basal O show the largest volumes. The bulk excess volume (Vxs) and the excess atomic volumes are analyzed as a function of the composition and the pressure. K+, basal and apical O Vxs show a behavior similar to the bulk Vxs as a function of the composition, keeping qualitatively this behavior as a function of pressure; substituent atoms do not show a Vxs behavior similar to the bulk and their effect consequently is mostly translated to atoms in the interlayer space. Atomic compressibilities are also calculated. Atomic compressibilities are separated in the different sheets of the crystal cell. Atomic moduli of K and basal O are the lowest and the ones behaving as the bulk modulus of the series. The atomic bulk modulus of the H’s is different depending of their position with respect to the (001) plane.

Mineralogical and geochemical characteristics of the hydrothermal illite from Hoam granite, South Korea: Implications for episodic fluid injections in the hydrothermal alteration system

Two stages of illite mineralization are recognized in the hydrothermal alteration zone of the Hoam granite. These illites are formed as a result of pervasive alteration by re-equilibration with high water/rock in a brittle environment below <2 km; the mineralization timing is middle Oligocene (26–27 Ma), coinciding with the timing of crustal deformation related to the opening of the East Sea (Sea of Japan). The mineralogical and geochemical characteristics of the clearly distinguished illites at each site indicate that they were mineralized from different fluid injections in distinct geological environments. Illites at the site-1 alteration zone are characterized by high-K content [K0.84 per O10(OH)2], 2M1 polytype of 99 %, hexagonal plate shape, and coexistence with pyrite. These observations indicate that the illites were formed in a slow cooling system (>250 °C), high fS2, and a relatively acidic environment. The pseudomorphic replacement combined with matrix-filling texture indicates that the illites at the site-1 alteration zone recorded the changes in fluid conditions from low to high water-rock ratio. In contrast, the illites at the site-2 alteration zone show the coexistence of polytypes (2M1, 1M, and 1Md), high-K illites [(K0.83 per O10(OH)2]/low-K illites [K0.63 per O10(OH)2], platy/hairy shapes, and presence of magnetite. Furthermore, this alteration zone no longer exhibits primary textures because of pervasive alteration induced by the dissolution-precipitation process. These results indicate that they were formed in a rapid cooling system and were continuously under conditions of high water-rock ratio, as well as in a less acidic and fS2 environment than that observed at the site-1 alteration zone. The behavior of trace elements for each illite primarily depends on the constituents of the hydrothermal fluid, which reflect different degrees of fluid evolution. The enrichment of high field strength elements (Nb and Ta), large ion lithophile elements (B, Be, and Cs), rare earth elements, and actinide elements (U and Th) in illite at the site-2 alteration zone shows that these elements formed by a more evolved fluid than that of the illite at the site-1 alteration zone. In addition, negative Ce anomalies at the site-2 alteration zone indicate that these crystallized in a reducing environment. Considering the mineralogical and geochemical properties of illites at the site-1 and site-2 alteration zones, the illite mineralization in the Hoam granite was likely generated by at least two episodes of hydrothermal illite mineralization, which originated from episodic injections of fluids, rather than continuously evolved fluids.

Extraction of 40Ar-39Ar Ages from a Multicomponent Mixture: A Case Study From the Tatra Mountains, Poland

AbstractExtraction of meaningful information on the timing of fault activity from clay gouges using radiometric dating methods, such as those based on the K-Ar system, can be challenging. One of the factors complicating interpretation of the radiometric dating results is the presence of multiple K-bearing components in the gouge material. In the current study, an attempt was made to develop a new interpretative method for K-Ar and 40Ar-39Ar dating, capable of handling a three-component mixture. In addition, the mineral composition of clay gouges from the Tatra Mountains (Poland), which has not been investigated before, is reported. The mineral compositions of the bulk clay gouge material and separated size fractions were determined by X-ray diffractometry and Fourier-transform infrared spectroscopy. The gouge samples were composed of quartz, dioctahedral mica (as a discrete phase and as a component of mixed-layered illite-smectite), and chlorite, commonly with plagioclase and more rarely with K-feldspar, dioctahedral smectite, calcite, anatase, or trace kaolinite. One feldspar-free sample containing three mica polytypes (1Md, 1M, and 2M1) was chosen for dating with the 40Ar-39Ar method. The results of 40Ar-39Ar dating were interpreted using three concepts: Illite Age Analysis (IAA), a method based on the MODELAGE software, and a newly developed three-component concept. The age values obtained with IAA were −14 Ma ± 31 Ma and 180 ± 91 Ma for authigenic (1Md) and inherited (1M + 2M1) components, respectively. The MODELAGE-based approach returned –4 ± 40 Ma and 165 ± 62 Ma. The three-component approach returned age values of polytypes as follows: 1Md, 15 ± 37 Ma; 1M, 135 ± 57 Ma; 2M1, 121 ± 56 Ma based on the medians and the interquartile ranges of non-normal distributions of Monte Carlo-simulated age values. The results obtained indicated that the 1Md polytype was probably formed during the most recent stage of fault activity, while 1M and 2M1 polytypes are of equal age, roughly.

Stable isotope (δD, δ18O) data on the structural water of two sericite samples from the Cu-Au high-sulphidation deposit Chelopech, Bulgaria – a tentative study

The epithermal high-sulphidation Cu-Au Chelopech deposit is characterized by a well-developed and well-traceable hydrothermal footprint manifested in the volcanic host rocks. The economic ore mineralization is embedded in the strong silicification, included among the advanced argillic zone of alteration, smoothly transitioning to quartz-sericite alteration that evolves into widespread propylitics. The quartz-sericite alteration zone is accessible for exploration only in underground mining galleries and exploration drillings. The main mineral assemblage in this zone is quartz, sericite, pyrite, minor rutile/anatase and relics of apatite and feldspar. According to XRD data from the studied samples, sericite was defined as illite and muscovite/sericite 2M1 polytype. The abundance of heavy stable isotopes (D, 18O) in the structural water of two sericite samples is the object of this study. A special attention was paid to the separation of extraneous waters from the structural one by thermal fractionation. The extracted structural water was converted to hydrogen and carbon dioxide before the isotopic measurements. The obtained results, put into a δD vs. δ18O plot, indicate that sericite structural water is “heavier” than meteoric water, within uncertainty limits.

Polytypism of Cronstedtite from Ouedi Beht, El Hammam, Morocco

AbstractThe present study deals with accurate identification of polytypes, twins, and allotwins – oriented crystal associations of more than one polytype. The trioctahedral 1:1 layered silicate cronstedtite was studied using single-crystal X-ray diffraction data collected with a four-circle diffractometer equipped with a CCD detector. The sample from the skarn occurrence, Ouedi Beht, El Hammam, Morocco, was explored. It contains cronstedtite in fibrous massive aggregates in the central part, and euhedral crystals in surrounding veinlets and druses. The reciprocal space (RS) sections created by the diffractometer software and presented here were used to determine the OD (ordered-disordered) subfamilies (Bailey’s group A, B, C, D) and to identify polytypes. The chemical compositions of some crystals were determined thereafter by electron probe microanalysis (EPMA-WDS). Some crystals studied are more or less complicated allotwins. Polytypes were thus separated by cleaving crystals into smaller parts in many cases. All polytypes found belong to subfamilies A or D. The following polytypes of the subfamily A were identified: 2M1 (a = 5.49, b = 9.51, c = 14.40 Å, β = 97.30°, space group Cc), 1M (a = 5.51, b = 9.54, c = 7.33 Å, β = 104.5°, Cm), 3T (a = 5.51, c = 21.32 Å, P31), 6T2 (a = 5.50, c = 42.60 Å, P31). 2M1 and 3T were present as isolated crystals or separated by cleaving, otherwise all these polytypes are parts of allotwins. The 2M1 polytype is sometimes twinned by reticular pseudo merohedry with twin index n = 3 and 120° rotation about the chex axis as the twin operation. Allotwins of 1M + twinned 2M1 polytypes are also present. Another kind of twinning, with rotation by (2n+1)×60° about chex is rare. The subfamily D is represented mostly by 2H1 and 2H2 polytypes, a = 5.50, c = 14.25 Å, space groups P63cm (2H1), P63 (2H2). In addition, several six-layer (a = 5.49, c = 42.80 Å), mostly non-MDO polytypes were separated from allotwins by cleaving. In order to identify them, 24 possible stacking sequences were modeled, diffraction patterns calculated, graphical identification diagrams constructed, and comparisons made with actual RS sections. This simulation revealed that five pairs of sequences provided identical diffraction patterns. Polytypes actually found correspond to the following sequences: 1 (6T1), 5 (proposed Ramsdell’s symbol 6T3), 8+10 (6T5), 11+12 (6T4), 24 (6T6, trigonal polytypes, space group type P3), 22 (6R1), and 23 (6R2, rhombohedral polytypes, space group type R3c and R3, respectively). The RS section corresponding to the hexagonal polytype 6H2 (sequence 14) was also found. However, diffraction patterns geometrically indistinguishable can be produced by the twin with rotation by 180° about chex as a twin operation of the rhombohedral polytype 6R2 (sequence 23). Several aggregates with fiber texture of polytype 2H2 were separated from the central part. Use of EPMA-WDS revealed Fe and Si along with significant amounts of Mn and Mg. Crystals from veins were more Mn- and Mg-rich than these from the central part. Traces of Cl, S, and Al are present also.

Clay mineralogy and geochemistry of pelitic rocks in the middle Permian Fanjiatun formation, East Changchun, China: implications for metamorphism and provenance

Metamorphic pressure–temperature conditions and sedimentary provenance were determined for the middle Permian Fanjiatun Formation in east Changchun, China, based on clay mineralogy, vitrinite reflectance and geochemical data. The pelitic rocks in the Fanjiatun Formation contain illite + kaolinite + chlorite ± mixed-layer chlorite/smectite and detrital quartz + plagioclase. Illite crystallinity in the formation varies from 0.38 to 0.49 (average = 0.44), and consists dominantly of the 2M1 and mixed 2M1 and 1Md polytypes. The illite b0 dimension is 8.9860–9.0130 A (average = 9.0010 A), indicating the Fanjiatun Formation experienced very-low-grade metamorphism at <1.5 kbar. The metamorphic temperatures obtained based on vitrinite reflectance are 191–308 °C, indicating the formation experienced anchizone-zone facies metamorphism. Al2O3/TiO2 ratios vary from 23.93 to 24.22 (average = 24.11), and the chemical index of alteration is 64–69 (average = 67). Rare earth element contents are high and exhibit enrichment in light rare earth elements and negative Eu anomalies. Trace element characteristics indicate that the source of the pelitic rocks was moderately weathered silicic rocks. The sediments were deposited in a reducing continental setting.

Clay mineral formation in Permian rocks of a geothermal borehole at Northern Upper Rhine Graben, Germany

Hydrothermally altered rhyolite rocks in the Permian Donnersberg Formation of a geothermal borehole in the Northern Upper Rhine Graben (Germany) were investigated to find out answers for the low hydraulic conductivity of the rocks. The composition of clay minerals and the temperature of smectite–illite transformation were carried out using X-ray diffraction, X-ray fluorescence, transmission electron microscopy, Fourier transform infrared spectroscopy, and polarized-light microscopy analyses. Clay mineral (CM) composition includes illite/muscovite (1M and 2M1 polytypes), illite–smectite interstratifications (IS-ml), smectite, and chlorite; and non-clay minerals such as quartz, feldspars, epidote, calcite, dolomite, and hematite were detected. The 2M1-polytype mica might be the only primary sheet silicates from the parent rocks, while the others occur as authigenic neo-formed CMs under heat flow and geothermal gradient. The development of CMs indicates different mechanisms of illitization and smectitization. Based on the texture, morphology, structure/polytype, and chemistry of rocks and minerals, in particular CMs, the study grouped the CM formation into three transformation processes: smectitization during magma cooling and possible contact metamorphisms with decreasing and low temperature, smectite illitization controlled by burial diagenesis and hydrothermal alteration, and illite smectitization followed exhumation and Cenozoic subsidence with decreasing temperature. The rhyolites were altered to all of the orders IS-R0, IS-R1, and IS-R3 by the dissolution-precipitation and layer-to-layer mechanisms. The first one supported small xenomorphic plates and flakes of 1Md, elongated particles of 1M, and pseudo-hexagonal forms of 2M1. The second one could lead to the platy particles of 1Md and 2M1 polytypes. The dominant temperature range for the transformation in the area has been 140–170 °C– ~ 230 °C.

Origin of clay minerals on section of Luochuan loesspalaeosol in Shaanxi Province, northwest China

Crystallinity, polytype, and morphology of clay minerals in the Luochuan loess-palaeosols in Shaanxi Province, northwest China were studied in order to have an insight into their origin using X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) methods. The SEM observations showed that the morphology of some illites seems to be lath-shaped crystals. An analysis of illite crystallinity (IC) on the Luochuan section indicated that the origin of illite was related to the rocks of an anchizone. Most illite in the Luochuan loess-palaeosol section were of 2M1 polytype, but some were of 1M polytype formed by weathering of feldspar in the process of pedogenesis. Illite in the Luochuan section has undergone both physical and chemical weathering. These results revealed that most illite were of detrital origin related to the source area of an anchizone, but parts of the illite were of an authigenic origin formed during pedogenesis after sedimentation. Chlorites in the samples of the Luochuan section were mainly composed of irregular flaky grains and their crystallinity was good. These showed that chlorite had the detrital origin formed by physical weathering. Kaolinite crystallinity was relatively good. The value of CIA on the Luochuan section ranges from 61.9 to 69, and therefore kaolinite could not be formed during weathering and pedogenesis. These results indicated that the kaolinite had a detrital origin. Morphology of smectite seems to be capillaceous. The XRD patterns of all samples contained diffraction peaks at 1.5218 A (nontronite) and 1.5052 A (montmorillonite), thus indicating an intermediate composition between trioctahedral and dioctahedral smectite. The smectite crystallinity was very poor. These results revealed that smectite in the study area was authigenic rather than detrital origin.

Compressibility of 2M1 muscovite-phlogopite series minerals.

Muscovite (Ms) and phlogopite (Phl) belong to the 2:1 dioctahedral and trioctahedral layer silicates, respectively, and are the end members of Ms-Phl series minerals. This series was studied in the 2M1 polytype and modeled by the substitution of three Mg2+ cations in the Phl octahedral sites by two Al3+ and one vacancy, increasing the substitution up to reach the Ms. The series was computationally examined at DFT level as a function of pressure to 9GPa. Cell parameters as a function of pressure and composition, and bulk moduli as a function of the composition agrees with the existing experimental results. The mixing Gibbs free energy was calculated as a function of composition. From these data, approximated solvi were calculated at increasing pressure. A gap of solubility is found, decreasing the gap of solubility at high pressure.

Diagenesis and Very Low‐grade Metamorphism of the Upper Permian Yangjiagou Formation in Eastern Changchun, China: Evidence from Clay Mineral Geothermobarometers

The metamorphic conditions of the Upper Permian Yangjiagou Formation in eastern Changchun, China, were evaluated based on the mineral assemblage, illite crystallinity, illite polytypism, the b dimension of illite, and the chemical composition of chlorite. The pelitic rocks in the Yangjiagou Formation are characterized by illite + kaolinite + chlorite ± mixed‐layer chlorite/smectite and detrital quartz + plagioclase. Illite in the formation has a crystallinity of 0.38–0.55 and comprises mixed 2M1 and 1Md polytypes, indicating a metamorphic temperature of &gt;200°C. Based on the chemical composition of chlorite and the chlorite geothermometer, we estimated diagenetic to very low‐grade metamorphic conditions with temperatures of 185°C–204°C. The b dimension of illite varies from 8.992Å to 9.005Å. We used a mathematical algorithm to extend Guidotti and Sassi's (1986) diagram relating illite b dimension with temperature and pressure, and used this diagram, together with illite crystallinity and chlorite chemical composition, to semi‐quantitatively estimate the formation pressure at&lt;1.2 kbar. These reveal that the Yangjiagou Formation has experienced very low‐grade metamorphism.

In situ high temperature X-ray diffraction study of illite

Despite extensive researches, many aspects of the structural transformation that occur in illite on heating remain unclear. This study concentrates on samples from a mine located in Wenzhou, Zhejiang Province, China to supplement data obtained by previous investigators. In situ high temperature X-ray diffraction (HTXRD), infrared spectroscopy (IR), field emission scanning electron microscope (FESEM) and electron microprobe (EMP) analysis were used to record the characterization of the raw material. XRD pattern showed the illite sample was primarily in 2M1 polytype. The phase transition from illite into dehydroxylated phase started at about 525°C in static air and at about 550°C in vacuum, and from dehydroxylated phase into mullite occurred at above 1100°C. These lattice parameters of illite (RT – 1100°C) and mullite (1125–1150°C) both in static air and in vacuum were refined. The variations of illite lattice parameters with temperature (°C) were therefore well described. Thermal expansion coefficients of illite (RT – 1100°C) were determined. In situ high temperature X-ray diffraction study revealed four stages during heating: expansion, dehydroxylation, post-dehydroxylation and mullite stages.

Structural and chemical variations in phlogopite from lamproitic rocks of the Central Mediterranean region

Micas from mafic ultrapotassic rocks with lamproitic affinity from several localities of the Central Mediterranean region were studied through single-crystal X-ray diffraction (SC-XRD), electron microprobe analysis (EMPA) and Secondary Ion Mass Spectrometry (SIMS); Mössbauer Spectroscopy (MöS), when feasible, was also applied to minimise the number of unknown variables and uncertainties. Analysed lamproitic samples cover the most important Central Mediterranean type localities, from Plan d'Albard (Western Alps) to Sisco (Corsica), Montecatini Val di Cecina and Orciatico (Tuscany, Italy) and Torre Alfina (Northern Latium, Italy). The studied crystals show distinctive chemical and structural features; all of them belong to the phlogopite-annite join and crystallise in the 1M polytype, except for micas from Torre Alfina, where both 1M and 2M1 polytypes were found. Studied micas have variable but generally high F and Ti contents, with Mg/(Mg+Fe) ranging from ~0.5 to ~0.9; 2M1 crystals from Torre Alfina radically differ in chemical composition, showing high contents of Ti and Fe as well as of Al in both tetrahedra and octahedra, leading to distinctive structural distortions, especially in tetrahedral sites.SIMS data indicate that studied micas are generally dehydrogenated with OH contents ranging from ~0.2 apfu (atoms per formula unit) for Orciatico and Torre Alfina to ~1.4 for Plan d'Albard crystals; this feature is also testified by the length of the c parameter, which decreases with the loss of hydrogen and/or the increase of the F→OH substitution. Chemical and structural data suggest that the entry of high charge octahedral cations is mainly balanced by an oxy mechanism and, to a lesser extent, by a M3+,4+-Tschermak substitution. Our data confirm that Ti preferentially partitions into the M2 site and that different Ti and F contents, as well as different K/Al values, are both dependant upon fH2O and the composition of magma rather than controlled by P and T crystallisation conditions.The obtained data help to discriminate among lamproite-like rocks formed within a complex geodynamic framework but still related to a destructive tectonic margin and evidence different trends for micas from the youngest Torre Alfina (Northern Latium) lamproites, referred to the Apennine orogeny and those of the older lamproites from Orciatico, Montecatini Val di Cecina (Tuscany), Western Alps, and Corsica, the latter referred to the Alpine orogeny. Phlogopite crystals from the older lamproites fall within the compositional and structural field of worldwide phlogopites from both within-plate and subduction-related settings. Phlogopite from the Plio-Pleistocene lamproite-like occurrence in Tuscany and Northern Latium, despite crystals with low Mg# of the Torre Alfina rock plot well within the general field of the other crystals in less evolved samples, follows a different evolution trend similar to that of shoshonites from Tuscany and Northern Latium. On this basis, we argue that the observed differences are inherited by slight differences in the magma compositions that are related to different genetic and evolution pathways.

Very Low‐Grade Metamorphism of Clastic Rocks from the Meso‐Neoproterozoic and the Paleozoic along the Profile Yueyang‐Linxiang in Northeastern Hunan Province and Its Geological Implications

AbstractThis study uses illite crystallinity, chlorite crystallinity, illite polytypes, the b0 cell‐dimension of K‐white mica, clay mineral assemblages and mineral geothermo‐geobarometers to investigate the overprint of diagenesis and metamorphism on the Meso‐Neoproterozoic and the Lower Paleozoic along the profile Yueyang‐Linxiang in northeastern Hunan Province, China. Illite crystallinity Kübler index (KI) of the &lt;2µm fractions ranges from 0.225 to 0.485°Δ2θ while chlorite crystallinity Árkai index (AI) ranges from 0.244 to 1.500°Δ2θ. This indicates that the Meso‐Neoproterozoic and the Lower Paleozoic along the profile Yueyang‐Linxiang were overprinted with diagenesis and anchi‐ to epimetamorphism. Peak metamorphic temperature is estimated with the IV site chlorite geothermometer roughly at 360°C. The b0 cell dimension values of illites (K‐mica) range from 0.9002 to 0.9054 nm and, on average, at 0.9030 nm for the Meso‐Neoproterozoic. Based on cumulative frequency curves of illite (K‐mica) b0 cell dimension, the peak metamorphic pressure of the Meso‐Neoproterozoic along the profile Yueyang‐Linxiang is derived of an intermediate pressure type. Most illites occur in the 2M1 polytype and some of them in a mixture of 2M1+1M types especially those in the Paleozoic. This result partly agrees with the conclusion of the lower greenschist and greenschist facies of the Lengjiaxi and Banxi Groups. However, it is not agreed with the sedimentary cover from the Sinian to the Lower Paleozoic or from the Banxi Group to the Lower Paleozoic. Crustal thickening due to “collision” between the Yangtze and Cathaysia blocks led to an increase in the thickness of the Meso‐Neoproterozoic to ca. 14 km and resulted in a temperature increase in those rocks due to burial. The very low grade to low grade metamorphism overprinting the Meso‐Neoproterozoic implies that the so called “Chiangnania or Jiangnan orogen” was no relative with the “Grenvillian orogeney; instead, it might be a continuous amalgamation product between the Yangtze and Cathaysia blocks.

Illitization of Late Devonian-Early Carboniferous K-bentonites from Western Pontides, NW Turkey: Implications for their origin and age

K-bentonite (tephra) layers are exposed as thin beds within Late Devonian-Early Carboniferous carbonates of the Yilanlı formation at four different locations in northwestern, Turkey. Clays separated from K-bentonites in the Gavurpınarı, Yılanlı Burnu (Bartın) and Çimşir Çukurları (Şapça) quarries and the Güdüllü-Gökgöl highway tunnel section near Zonguldak were analyzed by X-ray diffraction, optical, scanning and high-resolution transmission electron microscopy and inductively coupled plasma-mass spectrometry. The clay mineralogy is dominated by illite and mixed-layer illite-smectite (I-Sm) along with subordinate amounts of kaolinite, dolomite, calcite, quartz, feldspar, and gypsum. Morphologically, platy shaped illite is the major clay mineral in the Gavurpınarı and Yılanlı Burnu sites, while sponge-like to platy shaped mixed-layer illite-smectites occur in the Şapca Çimşir Çukurları and Gökgöl sites. Illite Kübler index (KI, Δ°2θ) and polytype data indicate high-grade diagenesis for illite-bearing site, and low-grade diagenesis for I-Sm-bearing sites. Lattice d060 values (Ǻ) of illite and I-Sm reflect a dioctahedral composition, with relatively larger d060 values in the Yılanlı Burnu site, which is related to Mg incorporation into the octahedral layer from dolomitic limestone host-rocksIllites have relatively lower tetrahedral Al substitution and higher octahedral Fe and Mg substitutions compared to those of I-Sm. Illites with phengitic composition occur in the Gavurpınarı and Yılanlı Burnu sites, whereas muscovitic composition at Şapca Çimşir Çukurları site. Chondrite-normalized rare earth element patterns for the Gavurpınarı and Yılanlı Burnu sites exhibit similar trends, with relatively higher values when compared to trends for the Şapça Çimşir Çukurları sites. Oxygen (δ18O) isotope values of illites and I-Sm range from 17.7 to 21.9‰ (V-SMOW), whereas hydrogen (δD) isotope values range widely from −10.1 to −69.9‰ (V-SMOW). The depleted values of δ18O for the Gavurpınarı site imply geologically sudden crystallization under higher temperature conditions. K/Ar ages of different size illite fractions indicate the presence of older detrital illites (2μm) together with younger diagenetic fractions (0.5μm) that correspond with an increase of 2M1 polytype in the coarser fraction. Illitization ages of K-bentonites in the Bartın area indicate an Early Permian event corresponding to the Variscan orogeny, whereas the illitization of K-bentonites in the Zonguldak area is Early Jurassic in age, related to the Cimmerian deformation in the region.

Timing and conditions of brittle faulting on the Silltal-Brenner Fault Zone, Eastern Alps (Austria)

The Silltal Fault is the northern brittle continuation of the Brenner Fault Zone, marked by a narrow zone of cataclasis and, in three locations, clay-rich fault gouges. The clay mineral composition of these gouges is dominated by higher temperature 2M1 polytype illite/muscovite, with no 1M/1Md illite or mixed layer illite/smectite detected. Smectite is limited to the northern samples from the Stephansbrucke location, whereas chlorite is present in all samples. The K–Ar ages from the different sample size fractions (<0.1, <0.4, <2, 2–6, 6–10 μm, “whole rock gouge”) show a wide spread, from ca. 115 to 12 Ma, with ages consistently decreasing with grain size. Although the ranges overlap, ages from the northern Stephansbrucke samples are generally older (115–36 Ma) than those from the south near Matrei (55–12 Ma), possibly reflecting increasing regional metamorphic temperatures to the south. The well-defined linear relationship between apparent age and hydrogen stable isotope (δD) values establishes a direct correlation between rejuvenation of the K–Ar system and increased interaction with meteoric water introduced and focussed within the fault zone. The dependence of both apparent age and δD on grain size also indicates that radiogenic and stable isotope exchange was controlled by grain size, reflecting new 2M1 illite growth, mechanical grinding of protolith muscovite during cataclastic faulting, or both. The results demonstrate the advantages of combining radiogenic and stable isotope analysis in interpretation of K–Ar ages from clay fault gouges. The combined approach was necessary to establish the crucial influence on apparent K–Ar ages of meteoric water influx focussed on the fault zone and its interaction with clay-size-fraction grains.

Complex hydrothermal alteration and illite K–Ar ages in Upper Visean molasse sediments and magmatic rocks of the Variscan Badenweiler-Lenzkirch suture zone, Black Forest, Germany

Post-collisional Upper Visean molasse sediments and magmatic rocks of the Badenweiler-Lenzkirch Zone reveal by microscopy of thin sections different degrees of hydrothermal illitization of feldspar and mica particles, and XRD, IR and XRF data of the <2 µm fractions show illitic material as the dominant clay mineral consisting of a mixture of 1M and 2M1 polytypes. Moreover, small amounts of illite/smectite mixed-layer minerals of R1-ordering are proved in the granites. In the separates, two illite mixing lines with different Fe + Mg contents are verified between authigenic illite from feldspar alteration and detrital illite in the molasse sediments, as well as between authigenic illite from feldspar alteration and altered mica flakes in the granites. Fe-rich detrital chlorite is present within the molasse sediments, while mixtures of high aluminous Fe-poor dioctahedral/di-trioctahedral chlorite with randomly interstratified chlorite/smectite mixed-layer minerals are formed from feldspar alteration in the granites. Illite K–Ar dating of the <2 and <0.63 µm fractions yields hydrothermal illitization of feldspar and partial resetting of the K–Ar system of detrital illite and mica flakes in the molasse sediments at ≥200 °C during Upper Permian to Middle Triassic times, while the granites in the eastern part of the study area were not altered contemporaneously. In contrast, hydrothermal activity at ≤200 °C during Upper Jurassic to Lower Cretaceous times occurred in the granites, whereas these temperatures were too low for resetting the older ‘Permo–Triassic’ illite K–Ar ages in the molasse rocks. Within both K–Ar age clusters, the data are seen to decrease with grain size and portion of illite 2M1 polytype. The alteration phenomena indicate multiple hydrothermal episodes in the study area, and they match those from the Central and Western European crust as fluid supply was controlled geodynamically by episodic break up of Pangea.

Ab initio investigations of dioctahedral interlayer-deficient mica: Modeling particles of illite found within gas shale

The focus of the study presented here is the illite component of the clay mineral components found within gas shale, specifically the creation of robust, atomistic models of illite particles bearing characteristics of the 2M1 polytype. The template for the illite models is derived from crystallographic data, which is used together with the general formula, K1.67(Al3.29Fe0.38Mg0.32)[(Si6.68Al1.32)O20(OH)4] to create different particles of illite, each containing potassium plus either ammonium (NH4+) or hydronium (H3O+) ions, or water molecules in two different proportions. These atomistic models are optimized from first principles using plane waves and pseudo-potentials within the formalism of density functional theory (DFT). The resulting lattice lengths, lattice dynamics in the form of infrared frequencies, positions of interlayer molecules and simulated X‑ray powder diffraction (XRPD) patterns are analyzed with reference to the available experimental data. We conclude that all four illite models display properties that fall within the range of available experimental data, and that therefore these are state-of-the-art atomistic illites, ready for use in further studies. Furthermore, the analytical data from these models will enable the characterization of physical samples of illite with varying interlayer constituents.

Crystallography of Vanadium-bearing Micas (Bol'shoi Karatau Range, Kazakhstan)

The mineralogical study of vanadium and vanadium-bearing micas from Cambrian carbonaceous-cherty formation of North-West of Bol'shoi Karatau range has allowed to establish four basic groups: 1. chernykhite (V2O3+V2O4 up to 23%); 2. Ba-roscoelite (V2O3 up to 18%) [1]; 3. vanadium-bearing muscovite and phengite (V2O3 up to 5%) and 4. secondary mica – V-Ba-phengite (V2O3+V2O4 up to 6-8%) [2]. Physical, optical properties as well as crystal structural parameters depend on vanadium content. The crystal structure of micas was determined by X-ray and electron diffraction techniques. The polytypes and unit cell parameter b (Å) are the main structural characteristics [3]. 2M1 polytype is spread among vanadium micas. Polytypes 1M and (1M+2M1) are only in vanadium-bearing micas – muscovite and phengite. The minimum b 9.03-9.04 Å is typical for this group. For secondary mica - V-Ba-phengite the parameter b varies from 9.6 to 9.09 Å. On the figure the secondary mica (1) is associated with mica without vanadium (2) and carbon-clay-chert (3) into polished section, where epoxy resin (4). For Ba-roscoelite b is equal 9.07-9.15 Å; for chernykhite - b 9.18 Å.

Geochemical monitoring of clays for diagenetic evolution of the Paleozoic–Lower Mesozoic sequence in the northern Arabian plate: Hazro and Amanos regions, Southeastern Turkey

Clay minerals in the diagenetic/very low-grade metamorphic–sedimentary series from southeastern Anatolia in Turkey were analyzed to determine their mineralogical and chemical compositions. In the Amanos region, the lowermost unit is composed of metaclastics with primary clastic textures, as well as slaty cleavages and chlorite-mica stacks including volcanic rock intercalations. The Lower Cambrian is composed of mainly very low-grade metamorphic clastic rocks, while the Ordovician units have siliciclastic and carbonate rocks. In the Hazro region, the Late Silurian–Lower Triassic units are represented by highly diagenetic carbonate and clastic rocks. All of the rock units include illite. In addition, chlorite, mixed-layered illite–chlorite and chlorite–vermiculite are present in the Amanos region, while calcite, dolomite, kaolinite, mixed-layered illite–smectite (I–S) and glauconite occur in the Hazro region. The illites are characterized by the dominance of 2M1 polytype in the Amanos samples; and 1Md+2M1 in the Hazro samples. The I–S, glauconite and kaolin have R1 and R3, 1M and kaolinite polytypes, respectively. The illites have greater tetrahedral and lower octahedral substitutions than the I–S. Total trace element contents, elemental substitutions and chondrite-normalized trace element and REE values decrease toward illite–I–S–kaolinite. There are obvious fractionations for some major – trace and rare earth elements with respect to each other and clear enrichment with respect to the chondrite, with strong anomalies of positive for Gd and negative for P, K and Eu in the clay minerals. The textural, morphological and geochemical data indicate that kaolinite and I–S in the Hazro area occur in supergene conditions with due to a full neoformation mechanism, whereas illites in the Amanos region represent the hypogene origin. In brief, the K2O contents, ratios of Eu/Eu* and LaN/LuN and δ18O and δD values of I–S and illite exhibit notable relationships with increasing diagenetic/metamorphic grade.