Chlorite-mica Stacks Research Articles

Mineral Chemistry of Low-Temperature Phyllosilicates in Early Paleozoic Metaclastic Rocks, Eastern Tauride Belt, Türkiye

The mineral chemistry of illite/mica and chlorites, together with the evaluation of textural data of low-temperature metaclastic rocks, plays an important role in determining their origin and metamorphic grade. This study aimed to investigate the chemical properties of phyllosilicates in early Paleozoic metaclastic rocks in the Eastern Tauride Belt, Türkiye. The textural (electron microscopy) and chemical (mineral chemistry analysis) analyses were performed on the samples representing different grades of metamorphism. The illites/micas and chlorites are observed as detrital (chlorite–mica stacks) and neoformation origin. Trioctahedral chlorites (chamosite) exhibit different chemistry for detrital and neoformed origin as well as the metamorphic grade. Tetrahedral Al and octahedral Fe + Mg increase, whereas octahedral Al decreases together with the increasing grade of metamorphism. The detrital chlorites have higher tetrahedral Al and Fe contents than their neoformed counterparts. Chlorite geothermometry data (detrital: 241–≥350 °C; neoformed: 201–268 °C) are compatible with the texture and illite Kübler index data. Illite/white-mica compositions display muscovite and Na-K mica. Tetrahedral Al and interlayer K + Na contents of illites/micas increase with metamorphic grade. Na-K mica and paragonite are observed as replacement-type developments within the detrital CMS. The obtained data indicate that phyllosilicate chemistry can be used effectively for determining the geological evolution of low-grade metamorphic sequences.

Two-step mode of clay formation in the extensional basins: Cambrian–Ordovician clastic rocks of the Antalya unit, SW Turkey

AbstractOrdovician clastic rocks of the Antalya unit in SW Turkey bear mineralogical/geochemical evidence of Triassic extensional rift timing and spatial relations. The crystal chemistry of the phyllosilicate assemblages (illite, chlorite, kaolinite, mixed-layer illite-smectite, chlorite-vermiculite and chlorite-smectite) is consistent with the rock experiencing a multi-generational burial history. The appearance of kaolinite and illite-smectite-bearing rocks in the Antalya unit is characteristic of diagenetic-anchimetamorphic conditions and is of higher grade than their anchi-epizonal equivalents in other regions of the Tauride belt. Illites and chlorites are of both detrital and authigenic origin, whereas I-S and kaolinites are authigenic. Detrital micas have been altered to chlorite and K-white mica stacks in which relicts suggest the chlorites were derived from detrital biotites. The broad X-ray diffraction illite peaks show that they are composed both of illite and illite-smectite. Na,K-mica and paragonite occur within the chlorite-mica stacks as replacements of muscovite, probably driven by Na-rich solutions. The authigenic clays were formed within the microporous matrix and the interplanar spaces of {001} planes of chlorite-mica stacks, with textures independent of the bedding and foliation planes of the rocks. The authigenic chlorites exhibit higher Si and Fe and lower Mg contents than their detrital counterparts. Authigenic chlorite thermometry indicates rift-related temperatures of 50–150°C, whereas pre-rift detrital chlorites formed at temperatures of >200°C. Authigenic illite and illite-smectite are phengitic in composition and contain more Si, Mg, Fe and Ca and less Al and K than detrital K-white micas. The textural, mineralogical and chemical characteristics support the hypothesis that the mineral assemblages were a result of a two-step mode of formation with diagenetic overprints of previously anchizonal rocks in extensional basin conditions.

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.

Clay mineral assemblages in British Lower Palaeozoic mudrocks

AbstractLower Palaeozoic rocks crop out extensively in Wales, the Lake District of northern England and the Southern Uplands of Scotland; they also form the subcrop concealed beneath the English Midlands and East Anglia. These mainly marine sedimentary rocks were deposited in basins created during plate tectonic assembly of the various terranes that amalgamated to form the British Isles, 400-600 Ma ago. Final amalgamation occurred during the late Lower Devonian Acadian Orogeny when the basins were uplifted and deformed, producing belts of cleaved, low-grade metasediments, so-called slate belts, with a predominantly Caledonian (NE-SW) trend. The clay mineralogy of mudrock lithologies - including mudstone, shale and slate - found in these belts is reviewed. Using X-ray diffraction data from the <2 μm fractions of ~4500 mudrocks samples, clay mineral assemblages are summarized and discussed in terms of diagenetic and low-grade metamorphic reactions, and the metapelitic grade indicated by the Kübler index of illite crystallinity.Two sequences of clay mineral assemblages, or regional assemblages, are recognized. Regional Assemblage A is characterized by a greater diversity of clay minerals in assemblages from all metapelitic grades. It includes K-rich, intermediate Na/K and Na-rich white micas, chlorite and minor amounts of pyrophyllite. Corrensite, rectorite and pyrophyllite are found in the clay assemblages of contact or hydrothermally altered mudstones. K-white micas are aluminous and phengite-poor, with b cell dimensions in the range 8.98-9.02 Å. Regional Assemblage B has fewer clay minerals in assemblages from a range of metapelitic grades. Phengite-rich K-mica is characteristic whereas Na- micas are rare, and absent in most assemblages; chlorite is present and minor corrensite occurs in mudrocks with mafic-rich detritus. Minor amounts of kaolinite are sporadically present, but dickite and nacrite are rare; pyrophyllite and rectorite are generally absent. The b cell dimensions of K-white mica in Regional Assemblage B are in the range 9.02-9.06 Å. The two regional assemblages are found in contrasting geotectonic settings. Regional Assemblage A is characteristic of the extensional basin settings of Wales, the northern Lake District and the Isle of Man. These basins have a history of early burial metamorphism associated with extension, and syn-burial or post-burial intrusive and extrusive volcanic activity. Intermediate Na/K mica probably developed from hydrothermal fluids generated around submarine volcanic centres. Deep diagenetic and low anchizonal clay mineral in these basins may develop a bedding-parallel microfabric. Chlorite-mica stacks also occur in the extensional basins and the stacking planes represent another type of bedding-parallel microfabric. Both types of microfabric are non-tectonic and developed by burial during the extensional phase of basin evolution. Regional Assemblage B is developed in the plate-convergent settings of the Southern Uplands and the southern Lake District. In the accretionary complex of the Southern Uplands the processes of burial diagenesis, metamorphism and tectonism were synchronous events. In both plate- convergent basins, low temperatures and tectonic fabric-formation had an important role in clay mineral reactions, whereas hydrothermal fluids played no part in clay genesis.

Very Low Grade Metamorphism of Upper Paleozoic–Lower Mesozoic Sedimentary Rocks Related to Burial and Thrusting in the Central Taurus Belt, Konya, Turkey

Metasedimentary and sedimentary rocks that represent the allochthonous Bolkardagi Unit crop out in the Central Taurus Belt. Devonian units include mainly slate, metadolomite, metadolomitic limestone, and metasandstone. Slates with slaty cleavage and chlorite-mica stacks are characterized by phyllosilicate, quartz, calcite, dolomite, feldspar, and goethite. Phyllosilicates consist of 2M1 and lesser amounts of 1M muscovite, IIb chlorite, pyrophyllite, paragonite. PM, C-V, C-S. rectorite, and dickite, and reflect conditions of the low epizone-anchizone. In the Carboniferous-Triassic units, limestone, clayey limestone, dolomitic limestone, marl, shale, and sandstone retaining primary textures are composed of calcite, dolomite, quartz,1Md illite, chlorite, and I-S, and locally smaller quantities of 2M1 muscovite, PM, paragonite, pyrophyllite, and rectorite. In contrast, the Triassic formation is made up of calcite, 1Md illite, I-S, kaolinite, smectite, chlorite, C-S, C-V, dolomite, and quartz. Textural and mineralogical data indicate that development of the diagenetic-metamorphic grade in the Central Taurus was related to sedimentary burial and thrusting; moreover, the metasedimentary rocks were metamorphosed in a typical anticlockwise P-T-t pathway in an extensional setting.

Relationships between metamorphism and structure on the northern edge of eastern Avalonia in the Manx Group, Isle of Man

Abstract The early Ordovician rocks of the Isle of Man, comprising fine-grained clastic sedimentary rocks at low regional metamorphic grade, contain a range of variably developed porphyroblasts. These are used to establish the relationship between the metamorphism and early Devonian deformation. Evidence for pre-deformation mineral growth is limited to diagenetic chlorite-mica stacks which may occur in most lithostratigraphical units. The first deformation (D 1 ) is marked by a pervasive cleavage, related to folding at various scales. Subsequent events formed crenulation cleavages in association with generally minor folding, including a gently dipping fabric developed relatively widely in response to the second phase of deformation (D 2 ). The main metamorphic peak, represented by growth of ilmenite, Mn-rich garnet, chloritoid and cordierite in muddy facies in the Manx Group, occurred after D 1 and before the onset of D 2 . Local heating contributions are also recognized in restricted metamorphic aureoles around dioritic dykes and intermediate size granitic intrusions of syn-D 1 (Dhoon Granodiorite), syn-D 2 (Black Hut Dyke) and post-D 2 (Crosby and Foxdale Granite) ages. Porphyroblast phases are best developed along the spine of the island and previous workers have suggested a metamorphic pattern related to underlying granitic intrusions. However, the timing of the growth of porphyroblasts within and outside the aureoles around exposed granites in this zone are very different, militating against any simple relationship between porphyroblast growth and igneous activity. Metamorphic mineral maps reveal a strong lithological control on the distribution of porphyroblasts. Chloritoid, in particular, is largely restricted to two prominent carbonaceous black mudstone units, the Barrule and Glen Rushen Formations, which strike northeast-southwest along the central part of the island. Cordierite is developed in the mudrocks of the Injebreck and Maughold Formations which crop out either side of the Barrule Formation. Further out across-strike, the absence of porphyroblasts (other than ilmenite) in the turbidite sandstone-rich parts of the sequence may again be, at least in part, related to composition. Here, however, low grades in the outermost tectonic tracts may be a real feature resulting from early D2 extensional reactivation and subsidence relative to the elevated central tracts.

The Formation of Sulfides During Alteration of Biotite to Chlorite-Corrensite

AbstractTransmission electron microscopy/analytical electron microscopy (TEM/AEM) were utilized to study pyrite and sphalerite inclusions in chlorite or mixed-layer chlorite-corrensite from an analcimized ash bed in the Etalian stage (Middle Triassic), South Otago, New Zealand. These sulfide inclusions occur as elongated crystals up to 1 × 15 µm in size, within lens-shaped voids between separated, deformed (001) layers of (primarily) chlorite and mixed-layer chlorite-corrensite grains of typical detrital shape or chlorite packets in chlorite-mica stacks (intergrowths of chlorite and phengite packets) up to 40 × 150 µm in size. Relict biotite layers within chlorite, mixed-layer chlorite-corrensite and berthierine have textures implying replacement of the former by the latter, whereas in other unaltered samples only fresh biotite was observed. Anatase occurs in otherwise Ti-free chlorite, whereas relict biotite contains significant Ti (0.3 moles per 22 oxygen atoms). No sulfide minerals have been found in fresh biotite and phengite.Mass balance considerations indicate that S and Zn were introduced via pore fluids and that the Fe was provided by the decomposition of biotite to secondary phyllosilicates. The alteration of biotite and the reaction of biotite to form chlorite and pyrite is controlled by aH+/aK+ as well as oxidation of reduced S species or reduction of oxidized S species from solution. Simple calculations with the observed compositions of chlorite and biotite suggest that some of the Fe in biotite was actually removed in solution rather than precipitated in pyrite and chlorite. Similar textures are abundant in ferroan phyllosilicates elsewhere, implying that the mechanism may apply widely to precipitation of sulfides in phyllosilicates during early diagenesis of sediments.

Mineralogy and microstructure of roofing slate: thermo-optical behaviour and fissility

The mineralogy and microstructure, which affect the slaty cleavage, are linked with the strong preferred orientation of phyllosilicates and this enables the rock to be split into large, thin, flat sheets. Roofing slate samples with different commercial fissilities have been analyzed by radioluminescence (RL), thermoluminescence (3DTL), by X-ray diffraction (XRD), by scanning electron microscopy (SEM) using the back-scattered mode (BSEI) and by electron microprobe (EMP). They are made up of white micas, chlorite, quartz, detrital feldspars, ilmenite, pyrite, rutile apatite and tourmaline. Texturally, all consist of silt-sized clasts of detrital quartz, feldspars, chlorite-mica stacks, muscovite and ilmenite in a recrystalline, lepidoblastic matrix of white micas and chlorite with quartz lenses, all showing a very strong preferred orientation. The luminescence emission centers are a low broad blue band around the 400 nm spectra positions linked with alkali losses and formation of [AlO 4 ]o defects; a peak at 473 nm interpreted as a the first thermal step (150-300oC) of a non-isothermal dehydroxylation of the slate phyllosilicates; and a 568 nm peak which agrees with Mn 2+ point defects in aluminosilicate lattices. The studies on the slaty cleavage could be significant because Spain is the largest producer of roofing slate tiles in the world (87% of world production).

TEM and AEM constraints on the origin and significance of chlorite-mica stacks in slates: an example from Central Wales, U.K.

Chlorite-mica stacks (grains of intergrown chlorite and white mica) in a matrix of fine-grained white mica and chlorite have been studied using XRD, SEM, EMPA, TEM and AEM methods. The stacks occur in a weakly cleaved Llandoverian mudstone, central Wales, which has a white mica (illite) crystallinity index of 0.35° Δ2φ corresponding to the lower anchizone. White mica occurs as packets (100 Å to 8 μm thick) interleaved with dominant chlorite packets in stacks, with both apparent coherent interfaces or cross-cutting small angle boundaries with chlorite layers. It is well-crystallized 2M1 polytype with phengitic composition and low paragonite component [Na/(Na + K) ≤ 0–0.07]. Chlorite in stacks is Fe-rich and relatively homogeneous. TiO2 crystals surround stacks and occur within chlorite, and white mica is Ti-rich compared to matrix white mica.Fine-grained white mica and chlorite in the matrix have two different orientations: one parallel to hedding and one parallel to cleavage, which is approximately 30–50° to bedding. Matrix white mica is predominantly a 2M1 polytype, but some cleavage-parallel white mica is 3T and some bedding-parallel white mica is 1Ma. It is Na-rich [(Na/(Na + K) ∼ 0.14–0.42] and relatively heterogeneous; some discrete paragonite and phengitic muscovite are observed to coexist in the cleavage orientation. Matrix white mica and chlorite each contain less Fe than corresponding white mica and chlorite in stacks. Both matrix and stack chlorite are one-layer polytypes.The data imply that chlorite in the stacks is largely derived from the replacement of volcanogenic biotite and other ferromagnesian minerals (probably via intermediate expandable trioctahedral phyllosilicates). Most intergrowths of chlorite and mica in stacks formed by mica replacement of chlorite and altered biotite along cleavage fissures. Subsequent deformation caused further modification of pre-existing chlorite-mica stacks whereas partial dissolution of stacks and bedding-parallel matrix phyllosilicates resulted in the formation of cleavage-parallel phyllosilicates.

Chlorite-mica stacks in low-strain rocks from central Wales

SummaryWeakly deformed, low grade, Lower Palaeozoic metasediments from central Wales contain abundant stack-like intergrowths of chlorite and white mica that closely resemble stacks described from the Devonian Hunsruckschiefer of West Germany; the Ordovician Martinsburg Slate, New Jersey, U.S.A.; and elsewhere. Several theories have been proposed to explain the origin of such stacks, including a detrital origin; strain-controlled growth of chlorite on a detrital mica nucleus; and strain-controlled intergrowth during metamorphism. None of these satisfactorily explains the central Wales stacks. A detrital origin is precluded by the presence of many stacks with shapes too delicate to have survived transportation, and a lack of hydrodynamic equivalence between the stacks and the clastic host grains. Features inconsistent with strain-controlled growth are constant alignment parallel to bedding but non-systematic orientation with respect to tectonic cleavage, their common occurrence in undeformed rocks, and petrographic evidence that they precede the tectonic cleavage. It is proposed that the stacks formed during diagenesis and low-grade metamorphism, and before the onset of deformation, through mimetic growth on a primary bedding fabric composed of clay minerals.