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Donald R. Peacor - One of the best experts on this subject based on the ideXlab platform.
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Contrast in processes and products of weathering of carpholite and associated phyllosilicates A TEM study
European Journal of Mineralogy, 2000Co-Authors: Giovanna Giorgetti, Isabella Memmi, Donald R. PeacorAbstract:Weathered metapelites (from Verrucano metasediments, northern Apennines, Italy) containing coexisting, altered carpholite, muscovite, and chlorite have been studied by scanning and analytical-transmission electron microscopy in order to compare the products and mechanisms of weathering. TEM images and SAED patterns demonstrate that primary magnesiocarpholite and phyllosilicates are well-crystallized with few defects. Fractures parallel to (100) in magnesiocarpholite are the sites of the weathering products halloysite and goethite. Halloysite occurs as hollow tubes with variable orientations which are apparently unrelated to the parent magnesiocarpholite structure. In sharp contrast, muscovite crystals consist primarily of alternating packets of layers of kaolinite and muscovite. The layers of kaolinite are parallel to those of original muscovite, with along-layer transitions, as consistent with direct replacement of muscovite layers by kaolinite. Chlorite is replaced by Corrensite and goethite with along-layer transitions from chlorite to Corrensite (001). Microtextures imply that weathering of magnesiocarpholite occurs by dissolution at fracture-mineral interfaces. The reaction involves almost complete breakdown of the structure components of magnesiocarpholite, and crystallization of products with unrelated orientation and with the disordered structure of the halloysite. Muscovite and chlorite are replaced by more highly ordered kaolinite and Corrensite, respectively. The changes in structure and composition required by direct replacement of muscovite by kaolinite imply a dissolution-neocrystallization process, occurring at the muscovite-kaolinite interface. The contrasting mineralogical, orientation, and textural relations between halloysite and kaolinite thus occur because halloysite crystallized as a pore-filling mineral in fractures within the structurally dissimilar magnesiocarpholite, but kaolinite formed as a direct layer replacement due to the similarity in structures.
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The Formation of Sulfides During Alteration of Biotite to Chlorite-Corrensite
Clays and Clay Minerals, 1998Co-Authors: Donald R. Peacor, Eric J. EsseneAbstract:Transmission 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.
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Prograde Transitions of Corrensite and Chlorite in Low-Grade Pelitic Rocks from the Gaspé Peninsula, Quebec
Clays and Clay Minerals, 1994Co-Authors: Wei-teh Jiang, Donald R. PeacorAbstract:A prograde sequence of Corrensite and chlorite in pelitic rocks of the diagenetic zone, anchizone, and epizone (illite crystallinity indices = 0.17–0.58°Δ2 θ ) of the Gaspé Peninsula, Quebec, was studied by analytical and transmission electron microscopy (AEM and TEM). The data collectively suggest that diagenesis/metamorphism of chlorite and Corrensite follows a sequence of phase transitions, compositional homogenization, and recrystallization, approaching a state of equilibrium for which chlorite is the stable phase. Corrensite occurs as coalescing, wavy packets of layers intergrown with chlorite and illite in the diagenetic and low-grade anchizonal rocks. Intergrowths of discrete chlorite and Corrensite crystals, interstratified packets of chlorite and Corrensite layers, terminations of smectite-like layers by chlorite layers, and 2–3 repeats of R2- and R3-ordered chlorite-smectite mixed layers occur. These materials are alteration products of detrital biotite or other precursor phases like trioctahedral smectite. The crystal size and proportion of Corrensite decrease significantly from the diagenetic zone to the anchizone. Deformed Corrensite is crosscut by straight packets of chlorite and Corrensite in the diagenetic sample. Some chlorite occurs as discrete, euhedral to subhedral crystals intergrown with or enclosed by other phases in the absence of Corrensite. The crystal size of chlorite and definition of crystal boundaries increase whereas density of crystal imperfections and randomness in orientation decrease with increase in grade of diagenesis/metamorphism. Crystals that are kinked or bent, or display gliding along (001) form low-angle boundaries with relatively defect-free crystals, implying deformation during crystal growth. Abundant well-defined low-angle boundaries associated with dislocations are observed in the higher grade rocks, consistent with a stage of readjustment of crystal boundaries during crystal growth. The AEM analyses show that the Corrensite has lower Fe/(Mg + Fe) and Al/(Si + Al) than the coexisting chlorite in the diagenetic sample, and that the ranges of composition of chlorite of different grades overlap and become smaller with increasing grade, implying prograde homogenization. The data imply that Corrensite is a unique phase that is metastable relative to chlorite: its conversion to chlorite occurred at a grade as low as that of the high-grade diagenetic zone. The textural relations suggest that the metamorphic crystallization and recrystallization were coeval with deformation processes due to tectonism, partially modified by subsequent contact metamorphism. The data, combined with those of previous reports, suggest that the Gaspé Ordovician rocks constitute a part of a regional distribution of trioctahedral phyllosilicate-rich rocks in the northern Appalachians. The regional occurrence of abundant chloritic minerals is thus directly related to a specific tectonic regime with precursor sediments largely derived from an andesitic arc system(s).
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Formation of Corrensite, chlorite and chlorite-mica stacks by replacement of detrital biotite in low-grade pelitic rocks
Journal of Metamorphic Geology, 1994Co-Authors: Wei-teh Jiang, Donald R. PeacorAbstract:Transmission and scanning electron microscopy were utilized to investigate the nature and mechanisms of alteration of abundant detrital biotite of volcanic origin and progressive modification of phyllosilicate aggregates in a prograde sequence of pelitic rocks (illite crystallinity index = 0.19-0.58" A28) from the Gasp6 Peninsula in Quebec. Detrital biotite has been diagenetically altered to form Corrensite and chlorite through two mechanisms; (1) layer-by-layer replacement gave rise to interstratification of packets of layers and complex mixed layering via several kinds of layer transitions between biotite and chlorite, Corrensite or smectite; (2) dissolution-transport-precipitation resulted in the formation of relatively coarse-grained aggregates of randomly orientated, Corrensite-rich flakes and fine-grained Corrensite intergrown with chlorite and illite in the matrix. The data show that stacks consisting of alternating packets of trioctahedral and dioctahedral phyllosilicates originated during early diagenesis when lenticular fissures in strained altering biotite were filled by dioctahedral clays. Subsequent prograde evolution of dioctahedral clays occurred through deformation, dissolution and crystallization, and overgrowth. Illite evolved to muscovite, with K in part provided through biotite alteration, and Corrensite/chlorite to homogeneous chlorite. The alteration of detrital biotite is closely related to the formation of titanite and magnetite in diagenetic rocks, and pyrite, calcite and anatase or rutile in the higher grade rocks. The observations demonstrate that detrital biotite of volcanic origin may be the principal precursor of chlorite in chlorite-rich metapelites originating in marginal basins. The mineral parageneses suggest that the transitions from Corrensite to chlorite and illite to muscovite may be a function of local chemistry and time.
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Phyllosilicates in hydrothermally altered basalts from DSDP Hole 504B, Leg 83 — a TEM and AEM study
Contributions to Mineralogy and Petrology, 1992Co-Authors: Yen-hong Shau, Donald R. PeacorAbstract:Phyllosilicates occurring as replacements of olivine, clinopyroxene and interstitial materials and as veins or fracture-fillings in hydrothermally altered basalts from DSDP Hole 504B, Leg 83 have been studied using transmission and analytical electron microscopy. The parageneses of phyllosilicates generally change systematically with depth and with the degree of alteration, which in turn is related to permeability of basalts. Saponite and some mixed-layer chlorite/smectite are the dominant phyllosilicates at the top of the transition zone. Chlorite, Corrensite, and mixed-layer chlorite/Corrensite occur mainly in the lower transition zone and upper levels of the sheeted dike zone. Chlorite, talc, and mixed-layer talc/chlorite are the major phyllosilicates in the sheeted dike zone, although replacement of talc or ohvine by saponite is observed. The phyllosilicates consist of parallel or subparallel discrete packets of coherent layers with packet thicknesses generally ranging from< 100 A to a few hundred A. The packets of saponite layers are much smaller or less well defined than those of chlorite, Corrensite and talc, indicating poorer crystal-linity of saponite. by contrast, chlorite and talc from the lower transition zone and the sheeted dike zone occur in packets up to thousands of A thick. The Si/(Si+Al) ratio of these trioctahedral phyllosilicates increases and Fe/(Fe+Mg) decreases in the order chlorite, Corrensite, saponite, and talc. These relations reflect optimal solid solution consistent with minimum misfit of articulated octahedral and tetrahedral sheets. Variations in composition of hydrothermal fluids and precursor minerals, especially in Si/(Si+Al) and Fe/(Fe+Mg) ratios, are thus important factors in controlling the parageneses of phyllosilicates. The phyllosilicates are generally well crystallized discrete phases, rather than mixed-layered phases, where they have been affected by relatively high fluid/rock ratios as in high-permeability basalts, in veins, or areas adjacent to veins. Intense alteration in basalts with high permeability (indicating high fluid/rock ratios) is characterized by pervasive albitization and zeolitization. Minimal alteration in the basalts without significant albitization and zeolitization is characterized by the occurrence of saponite±mixed-layer chlorite/smectite in the low-temperature alteration zone, and mixed-layer chlorite/Corrensite or mixed-layer talc/chlorite in the high-temperature alteration zone. Textural non-equilibrium for phyllosilicates is represented by mixed layering and poorly defined packets of partially incoherent layers. The approach to textural equilibrium was controlled largely by the availability of fluid or permeability.
S. Hillier - One of the best experts on this subject based on the ideXlab platform.
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Berthierine/chamosite, Corrensite, and discrete chlorite from evolved verdine and evaporite-associated facies in the Jurassic Sundance Formation, Wyoming
American Mineralogist, 2002Co-Authors: Peter C. Ryan, S. HillierAbstract:Late Jurassic sandstones of the shallow-marine Sundance Formation contain authigenic chlorite minerals that occur as rosette-like pore fillings of interstratified berthierine/chamosite (B-C) and honeycomb-like pore linings of Corrensite and discrete chlorite. B-C is nearly ubiquitous in Sundance sandstones, but is absent near the top of the formation, whereas Corrensite and discrete chlorite were detected only in uppermost Sundance sandstones, within 4 m of the contact with the overlying non-marine Morrison Formation. Glauconite grains are common and occur as laminae along bedding planes and cross-beds, indicating reworking and deposition as clasts in tidally influenced regimes. The mineralogical, chemical, and morphological properties of the B-C and Corrensite indicate that they are authigenic and formed during burial diagenesis from precursor minerals, odinite in the case of B-C, and saponite in the case of Corrensite and discrete chlorite. Odinite has been recognized in numerous shallow-marine sands of the Holocene verdine facies, and the shallow-marine conditions associated with Sundance deposition would have been ideal for odinite formation. Saponite commonly forms in aeolian and evaporitic environments, implying that the saponite precursor to Corrensite and discrete chlorite formed in uppermost Sundance sands that were exposed to an influx of oxidizing groundwater following regression of the Sundance sea. X-ray diffraction (XRD) indicates that the proportion of 7 A B layers in B-C ranges from 5 to 28%, and chemical analysis by scanning electron microscope-energy dispersive X-ray spectrometry (SEM-EDS) indicates positive correlation between %B and Fe/(Fe + Mg). The polytype of the B-C is Ibb , and the Corrensite and discrete chlorite are disordered IIb . These are the expected polytypes and %B for sandstones exposed to burial diagenetic conditions of 3000–4000 m and temperatures 90–120 °C.
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berthierine chamosite Corrensite and discrete chlorite from evolved verdine and evaporite associated facies in the jurassic sundance formation wyoming
American Mineralogist, 2002Co-Authors: Peter C. Ryan, S. HillierAbstract:Late Jurassic sandstones of the shallow-marine Sundance Formation contain authigenic chlorite minerals that occur as rosette-like pore fillings of interstratified berthierine/chamosite (B-C) and honeycomb-like pore linings of Corrensite and discrete chlorite. B-C is nearly ubiquitous in Sundance sandstones, but is absent near the top of the formation, whereas Corrensite and discrete chlorite were detected only in uppermost Sundance sandstones, within 4 m of the contact with the overlying non-marine Morrison Formation. Glauconite grains are common and occur as laminae along bedding planes and cross-beds, indicating reworking and deposition as clasts in tidally influenced regimes. The mineralogical, chemical, and morphological properties of the B-C and Corrensite indicate that they are authigenic and formed during burial diagenesis from precursor minerals, odinite in the case of B-C, and saponite in the case of Corrensite and discrete chlorite. Odinite has been recognized in numerous shallow-marine sands of the Holocene verdine facies, and the shallow-marine conditions associated with Sundance deposition would have been ideal for odinite formation. Saponite commonly forms in aeolian and evaporitic environments, implying that the saponite precursor to Corrensite and discrete chlorite formed in uppermost Sundance sands that were exposed to an influx of oxidizing groundwater following regression of the Sundance sea. X-ray diffraction (XRD) indicates that the proportion of 7 A B layers in B-C ranges from 5 to 28%, and chemical analysis by scanning electron microscope-energy dispersive X-ray spectrometry (SEM-EDS) indicates positive correlation between %B and Fe/(Fe + Mg). The polytype of the B-C is Ibb , and the Corrensite and discrete chlorite are disordered IIb . These are the expected polytypes and %B for sandstones exposed to burial diagenetic conditions of 3000–4000 m and temperatures 90–120 °C.
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MAFIC PHYLLOSILICATES IN LOW-GRADE METABASITES. CHARACTERIZATION USING DECONVOLUTION ANALYSIS -- DISCUSSION
Clay Minerals, 1995Co-Authors: S. HillierAbstract:Any X-ray diffraction (XRD) pattern of mixed-layered maximum R1 ordered chlorite-smectite has an exact equivalent R0 smectite-Corrensite or R0 chlorite-Corrensite. The identification of a mineral as chlorite (0.8)-smectite R1 is, therefore, not evidence that the fundamental layers in such an interstratified structure are actually chlorite and smectite. Furthermore, there are no obvious differences between XRD patterns of R0 and R1 chlorite-smectite at high or low smectite contents. Therefore the frequently supposed identification of R0 chlorite-smectite with high smectite contents is also questionable. It follows that the identification of R1 chlorite (0.8)-smectite by Robinson & Bevins (1994) and of R0 chlorite (~ 0.2)-smectite in an earlier publication (Robinson et al., 1993) cannot be used as evidence regarding the debate over the nature of the component layers in interstratified minerals intermediate between trioctahedral smectite and chlorite. Diffraction patterns of natural mixed-layer chlorite minerals from metabasites often show a strong reflection at 31 A yet a lack of resolution of other mixed-layer peaks from those of discrete chlorite. These features cannot be matched by any single mixed-layer mineral consisting of layers of chlorite and smectite and were matched by Robinson & Bevins (1994) using a mixture of different chlorite- smectite minerals. However, they can be matched by a single chlorite-Corrensite mineral in which there is an element of segregation of layer types. These different possibilities need to be investigated further.
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Origin, Diagenesis, and Mineralogy of Chlorite Minerals in Devonian Lacustrine Mudrocks, Orcadian Basin, Scotland
Clays and Clay Minerals, 1993Co-Authors: S. HillierAbstract:Chlorite and Corrensite are common clay minerals in lacustrine mudrocks from the Devonian Orcadian Basin, Scotland. The relationship of their occurrence to vitrinite reflectance data demonstrate that they are authigenic minerals, formed during burial diagenesis/metamorphism at temperatures of ≥120°C. Whole rock mineralogical and chemical analyses show that chlorite authigenesis occurred by reactions between the detrital dioctahedral clay mineral assemblage and dolomite that was formed under early evaporitic conditions in the lacustrine environment. XRD and electron microprobe analyses indicate that phases intermediate between Corrensite and chlorite are probably mixed-layer chlorite/Corrensite with a tendency towards segregation of layer types. Chemically, the conversion of Corrensite to chlorite involves an increase in Al for Si substitution in tetrahedral sites, but there is no change in the Fe/Mg ratio of octahedral cations. There is also no relationship of mixed-layer proportions to paleotemperature; only a general paleotemperature interval of approximately 120° to 260°C in which a range of phases between Corrensite and chlorite occurs. Chlorite polytypes are exclusively IIb, indicating the formation of this polytype at diagenetic temperatures. The occurrence of Corrensite and Mg-rich chlorite in evaporite and carbonate successions is probably a reliable indicator of diagenetic alteration at temperatures of >- 100°C. Burial diagenetic reactions between dioctahedral clay minerals and Mg-rich carbonates may possibly explain many occurrences of Corrensite and Mg-rich chlorite in such rocks.
Werner Leu - One of the best experts on this subject based on the ideXlab platform.
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CLAY MINERAL DIAGENESIS AND THERMAL HISTORY OF THE THONEX WELL, WESTERN SWISS MOLASSE BASIN
2016Co-Authors: Roland Schegg, Werner LeuAbstract:Abstract--Results are presented of a diagenetic study from the 1300 m thick Oligocene Molasse deposits penetrated by the Th6nex geothermal exploration well (Geneva, Switzerland). The x-ray diffraction (XRD) studies of fine-grained rocks indicate the following diagenetic hanges: a decrease of illite/smectite (US) expandability from approximately 90 % to 30 % with depth, a decrease of the amount of US in the clay mineral fraction, and the appearance of Corrensite at depths>750 m. The transition from random US to ordered I/S occurs at the base of the Th6nex well (1200 to 1300 m) and is associated with a coal rank of about 0.7 % Rr (mean random vitrinite reflectance) corresponding to paleotemperatures of 110 to 115 ~ Corrensite appears at a vitrinite reflectance value of 0.6 % Rr and a corresponding paleotemperature of 100 ~ The amount of post-Molasse rosion is estimated to be approximately 2 km. Thermal history modeling of the Th6nex well suggests maximum paleotemperatures of 80 to 115 ~ and an average paleogeothermal gradient of 27 ~ during Late Miocene maximum burial conditions
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Clay Mineral Diagenesis and Thermal History of the Thonex Well, Western Swiss Molasse Basin
Clays and Clay Minerals, 1996Co-Authors: Roland Schegg, Werner LeuAbstract:Results are presented of a diagenetic study from the 1300 m thick Oligocene Molasse deposits penetrated by the Thonex geothermal exploration well (Geneva, Switzerland). The x-ray diffraction (XRD) studies of fine-grained rocks indicate the following diagenetic changes: a decrease of illite/smectite (US) expandability from approximately 90% to 30% with depth, a decrease of the amount of US in the clay mineral fraction, and the appearance of Corrensite at depths >750 m. The transition from random US to ordered I/S occurs at the base of the Thonex well (1200 to 1300 m) and is associated with a coal rank of about 0.7% Rr (mean random vitrinite reflectance) corresponding to paleotemperatures of 110 to 115 °C Corrensite appears at a vitrinite reflectance value of 0.6% Rr and a corresponding paleotemperature of 100 °C. The amount of post-Molasse erosion is estimated to be approximately 2 km. Thermal history modeling of the Thonex well suggests maximum paleotemperatures of 80 to 115°C and an average paleogeothermal gradient of 27 °C/km during Late Miocene maximum burial conditions.
Tarmo Kiipli - One of the best experts on this subject based on the ideXlab platform.
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Chloritization of Upper Ordovician Pirgu bentonites –
2015Co-Authors: Kalle Kirsimäe, Rutt Hints, Toivo Kallaste, Enli Kiipli, Tarmo KiipliAbstract:Mg–Fe-rich rocks/sediments begins usually with the formation of saponite-type Fe-rich smectite, which in progressive diagenesis or hydrothermal alteration transforms into Corrensite (Reynolds 1988). The next stage in Corrensite-to-chlorite conversion is the growth of chlorite layers in Corrensite to form discrete chlorite domains in a Corrensite matrix (Beaufort et al. 1997). Corrensite, however, can form directly under hydrothermal conditions at temperatures between about 100 and 200 °C (Inoue & Utada 1991). In alternating pyroclastic sedimentary sequences the formation of dioctahedral smectite and subsequent smectite-to-illite conversion is related to acidic vol-canic materials, whereas the trioctahedral smectite (saponite) and chlorite-to-smectite transformatio
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Chloritization of Upper Ordovician Pirgu bentonites - source material or diagenetic environment?
2004Co-Authors: Kalle Kirsimäe, Rutt Hints, Toivo Kallaste, Enli Kiipli, Tarmo KiipliAbstract:The sedimentary sequence of the Palaeozoic Baltic Basin contains numerous bentonite layers whose composition is dominated by the mineral assem- blage of illite-smectite-K-feldspar-kaolinite. The contents of these minerals may vary between in- dividual bentonite layers as well as laterally from almost pure illite-smectite to K-feldspar and/or kaolinite end-member compositions, but the assem- blage remains principally the same. In this respect the bentonites of the Upper Ordovician Ashgill Pirgu (Regional) Stage are exceptional and unique in the Baltic Basin. The clay mineral composition of these bentonites is characterized by the chlorite- smectite (Corrensite) and illite-smectite assemblage. The micritic-bioclastic to argillaceous limestones of Pirgu age in the northern Baltic Basin include up to three (four?) individual bentonite beds, all of which contain chlorite-smectite and/or Corrensite miner- als. In this contribution we present preliminary data on the clay mineral composition of these beds. The clay fraction (
Takashi Murakami - One of the best experts on this subject based on the ideXlab platform.
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Chlorite and biotite weathering, Fe2+-rich Corrensite formation, and Fe behavior under low PO2 conditions and their implication for Precambrian weathering
American Mineralogist, 2008Co-Authors: Hirokazu Sugimori, Teruki Iwatsuki, Takashi MurakamiAbstract:Fresh and weathered granite from drill cores in Tono, Gifu, Japan, was examined to understand weathering products and the mechanisms of chlorite and biotite weathering under low P O 2 conditions. A fresh sample from 365 m depth, a slightly weathered light-green sample from 367 m depth, and a nearly fresh sample from 369 m depth (but with brown stains on fractures), were investigated. The XRD, SEM, EMPA, and TEM analysis of green grains present within chlorite, biotite, and plagioclase grains and in veins was found to be Fe 2+ -rich Corrensite [about 40 wt% FeO with Fe/(Fe + Mg) = 0.94]. The Corrensite is interpreted to have formed from chlorite and biotite via a dissolution-precipitation mechanism. The 2+ /∑Fe value of 0.69, which, when combined with the presence of amorphous Fe 3+ (hydr)oxides confirmed by TEM, indicates that the Fe 2+ /∑Fe value of Corrensite is >0.69. These results indicate that on dissolution of chlorite and biotite, Fe 2+ was transported as Fe 2+ and precipitated as Fe 2+ -rich Corrensite and a part of the dissolved Fe 2+ was oxidized to amorphous Fe 3+ (hydr)oxides under low P O 2 conditions. The formation of Fe 2+ -rich Corrensite and that of Fe 2+ -rich smectite or vermiculite in the laboratory at 1 atm of P CO 2 and ≤3 × 10 −5 atm of P O 2 (Murakami et al. 2004) suggest that a possible Fe 2+ -bearing product during Precambrian weathering is Fe 2+ -rich sheet silicates but not siderite.
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HRTEM evidence for the process and mechanism of saponite-to-chlorite conversion through Corrensite
American Mineralogist, 1999Co-Authors: Takashi Murakami, Tsutomu Sato, Atsuyuki InoueAbstract:To elucidate the process and mechanism of the prograde conversion of saponite to chlorite through Corrensite, the microstructures of a series of chlorite-smectite (C-S) mixed-layer samples from Kamikita, northern Japan were examined by high-resolution transmission electron microscopy using both lattice and structure imaging. Corrensite grows epitaxially as domains of 5-20 nm thick mainly within homogeneous saponite domains, without forming randomly interstratified C-S. Then, chlorite domains grow outside homogeneous Corrensite domains without forming randomly interstratified C-S or chlorite-Corrensite (C-Co), and finally are predominant. An atomic resolution image suggests that Corrensite essentially consists of the 1 M stacking of alternating chloritic and smectitic layers. The structure and occurrence suggest Corrensite is mineralogically a unique species. Comparison of the stacking vectors of Corrensite along [001] to those of chlorite reveals that the stacking sequence is not inherited during the process from Corrensite to chlorite. We rarely observed layer terminations of the hydroxide sheets both in Corrensite and chlorite domains, and the layer terminations that do exist can be explained as defects rather than the formation of Corrensite or chlorite. Our data strongly suggest that the saponite-to-chlorite conversion series progresses stepwise from saponite to Corrensite and from Corrensite to chlorite, and that the dominant reaction mechanisms are dissolution and precipitation.
