Chlorite Grains Research Articles

“Retrograde Diagenesis” of Clay Minerals in the Precambrian Freda Sandstone, Wisconsin

AbstractSiltstones from the Precambrian Freda Formation in Wisconsin have been studied by scanning, scanning-transmission and analytical electron microscopy (SEM, STEM and AEM), and X-ray diffraction (XRD). XRD data for drill core samples show a change from smectite-rich Illite-Smectite (I-S) in shallow samples to illite in deeper samples, implying a transition during burial diagenesis.Transmission electron microscopy (TEM) observations of shallow samples reveal the presence of three clearly distinguishable kinds of dioctahedral clay minerals: (1) detrital grains of micrometer-sized, mature muscovite; (2) small packets in the matrix consisting of dominant (Reichweite) R1 I-S or (3) small packets of illite. The illite and I-S stacks occur separately and have similar textures, with packet thicknesses averaging ∼400 Å, within the range of anchizonal illite. Illite and detrital muscovite commonly display strain features typical of the effects of tectonic stress. Void space within I-S or illite packets is inferred to be a strain feature, and to have served as pathways for fluids. Detrital muscovite shows abundant alteration features including (001) boundaries which are continuous with parallel packets of I-S; individual layers commonly show along-layer transitions of muscovite to smectite or I-S. Trioctahedral clays consist primarily of detrital chlorite which commonly shows direct alteration to R1 I-S and smectite, as with detrital muscovite.Deep samples contain only unaltered, coarse detrital muscovite, and thin packets of illite forming stacks and comprising most of the matrix. The texture of the illite appears to be identical to that of shallow samples, with characteristics such as packet size typical of anchizonal illite. Trioctahedral clays consist almost entirely of detrital grains of chlorite and corrensite. They occur as separate grains with rather constant composition, without signs of alteration.The data imply that all of the studied rocks have been subjected to a uniform anchizonal grade of metamorphism in which detrital grains were largely unchanged but matrix clays were transformed to packets of illite. The unusually abrupt transition with depth from highly expandable I-S to illite is inferred to actually be the result of subsequent alteration of authigenic illite and detrital chlorite and muscovite to R1 I-S and smectite in shallow rocks. This late overprinting of the anchimetamorphic clay mineral assemblage is inferred to have been locally caused by fluids with temperatures less than those of peak metamorphism. This process, called “retrograde diagenesis”, gave rise to a sequence of dioctahedral I-S and illite which mimics classic prograde sequences. Interpretations of such sequences as being prograde, especially in cases of ancient rocks, should be interpreted with caution when high-resolution images of textures are not available.

Abstract: Origin of Chlorite Grain Coats in Sandstones of the Lower Vicksburg Formation, South Texas 

Abstract: Origin of Chlorite Grain Coats in Sandstones of the Lower Vicksburg Formation, South Texas 

Structure and formation mechanism of low-angle grain boundaries in chlorite

The structure and formation mechanisms of low-angle grain boundaries (LAGB) in chlorite have been investigated using HRTEM to resolve each cation sheet in chlorite. The LAGB are abundant in two hydrothermally formed ferromagnesian chlorite specimens. Abundant LAGB, which are almost parallel to (001), divide chlorite grains into packets many tens of nanometers thick. They are initiated or terminated in a grain, indicating they are not formed as conjunctions of two grains growing independently. The boundaries consist of the (001) surface of a chlorite layer at one side and terminating chlorite layers at the other side, so that the grain boundary involves a series of misorientation of several degrees. Periodic rows of triangular regions surrounded by two (001) chlorite surfaces and one edge of the terminating layer along the boundary are formed along the LAGB. Distinct crystalline structure inside these triangles is not observed and brucite-like interlayers are absent at the (001) surfaces between triangular-shaped regions, indicating local compositional change along the boundaries. To explain the origin of this microstructure, a formation mechanism of these LAGB is proposed involving termination of the growth of one brucitelike interlayer on a (001) TOT layer surface during layer-by-layer growth.

Post-entrainment mineral–melt reactions in spinel peridotite xenoliths from Inver, Donegal, Ireland

Spinel lherzolite and harzburgite xenoliths hosted in an alkali basalt dyke near Inver, Donegal, Ireland show abundant evidence of interaction between xenolith minerals and the host melt. Of particular interest are primary Cr-diopside and spinel with sieve-textured coronas. Coronas on primary Cr-diopside are up to 3 mm wide and are associated with veinlets of devitrified glass. The coronas comprise secondary Cr-diopside with vermicular, interstitial alkali feldspar and chlorite grains up to 100 µm in size. The inclusion-free Cr-diopside cores are Al- and Na-rich whereas the coronas are Al- and Na-depleted and Ti-enriched. Sieve-textured spinels have similar texture to the clinopyroxene grains and are also associated with veinlets of infiltrated glass. However, the interstitial inclusions in the sieve-textured region are chlorite and nepheline. Inclusion-free spinel is part of a chromite–spinel solid solution and is Ti-poor. Spinel in the coronas has a greater chromite and ulvospinel component and falls close to a mixing line with spinel in the host alkali basalt. In addition to the sieve-textured grains, primary olivine in contact with infiltrated glass has Fe-rich rims, and orthopyroxene has broken down to form rims of olivine, clinopyroxene and a K-rich phase similar in composition to alkali-feldspar. Comparison of the compositions of the inclusion-free cores and sieve-textured rims shows that the rims have chemical signatures consistent with partial melting, that is, Al and Na depletion for clinopyroxene and Cr-enrichment for spinel. The textures of the coronas, particularly those around spinel and the reaction margins on orthopyroxene are identical to those produced during dissolution experiments.We suggest that silicate liquid from the host magma infiltrated the xenoliths during their ascent and since it was not in equilibrium with the xenolith minerals caused reaction. The occurrence of K-bearing interstitial minerals in the sieve-textured grains and reacted orthopyroxenes indicate that the coronas did not form by simple melting since none of the minerals that underwent breakdown are K-bearing. We suggest that the sieve-textured grains formed initially by partial melting and reaction associated with decompression and infiltration of liquid from the host magma. The melts included in the reacted phases were enriched in K by diffusion from the Si-poor infiltrated melt into the more Si-rich melt inclusions in the coronas.

Subglacial comminution in till — evidence from microfabric studies and grain-size distributions

AbstractQuartz, feldspar and chlorite, the principal minerals in a basal till from the Halasi River catchment in the Altay Mountains, northwestern China, are present in approximately equal concentrations in the coarse fraction of the till, 1.0-0.125 mm. Quartz concentrations are significantly higher than those of the other two minerals in the 0.125-0.016 mm size range. Feldspar and chlorite concentrations are higher than those of quartz in the finest fraction. Quartz has a strong preferred mode at 0.063-0.032 mm. Feldspar and chlorite have two weak modes in the silt-size range, one between 0.063 and 0.032 mm and the other between 0.016 and 0.004 mm.Thin sections of oriented impregnated samples were used to study crushing and abrasion. Over 2700 daughter particles were identified as products of comminution of 925 parent grains. Quartz and feldspar are most likely to be broken into two particles of roughly equal size, as are fine chlorite grains. However, owing to their weakness and cleavage, larger grains of chlorite tend to he split into more than two daughter particles.Sizes of the daughter grains were measured and sizes of the original parent grains were estimated. Mean parent grain-sizes for quartz, feldspar and chlorite are 0.129, 0.078 and 0.059 mm, respectively, whereas mean daughter grain-sizes are 0.068,0.041 and 0.024 mm, respectively. The greater percentage reduction in the size of chlorite reflects its tendency to break into more than two daughter particles.Most grains tend to be crushed. Only a few large particles seem to have suffered from abrasion.

Subglacial comminution in till — evidence from microfabric studies and grain-size distributions

Abstract Quartz, feldspar and chlorite, the principal minerals in a basal till from the Halasi River catchment in the Altay Mountains, northwestern China, are present in approximately equal concentrations in the coarse fraction of the till, 1.0-0.125 mm. Quartz concentrations are significantly higher than those of the other two minerals in the 0.125-0.016 mm size range. Feldspar and chlorite concentrations are higher than those of quartz in the finest fraction. Quartz has a strong preferred mode at 0.063-0.032 mm. Feldspar and chlorite have two weak modes in the silt-size range, one between 0.063 and 0.032 mm and the other between 0.016 and 0.004 mm. Thin sections of oriented impregnated samples were used to study crushing and abrasion. Over 2700 daughter particles were identified as products of comminution of 925 parent grains. Quartz and feldspar are most likely to be broken into two particles of roughly equal size, as are fine chlorite grains. However, owing to their weakness and cleavage, larger grains of chlorite tend to he split into more than two daughter particles. Sizes of the daughter grains were measured and sizes of the original parent grains were estimated. Mean parent grain-sizes for quartz, feldspar and chlorite are 0.129, 0.078 and 0.059 mm, respectively, whereas mean daughter grain-sizes are 0.068,0.041 and 0.024 mm, respectively. The greater percentage reduction in the size of chlorite reflects its tendency to break into more than two daughter particles. Most grains tend to be crushed. Only a few large particles seem to have suffered from abrasion.

Geology of the zoned gold skarn system at Junction Reefs, New South Wales

The Sheahan-Grants, Frenchmans, and Cornishmens orebodies at Junction Reefs are typical gold skarns. Gold ore is restricted to a pyrrhotite-pyrite-arsenopyrite-bearing distal skarn zone in which prograde pyroxene has been altered to a retrograde chlorite-quartz-carbonate assemblage. The skarns are associated with a Late Ordovician (440 Ma) shoshonitic intrusive complex which comprises a large, central monzodiorite stock and numerous small, peripheral stocks. The skarn orebodies are located within a mineralogically zoned metasomatic alteration halo centered on one of the satellite stocks, the Junction Reefs Monzodiorite. The ore is within skarn formed from a 39-m-thick sequence of Early Ordovician limestone, siltstone, and chert, known locally, as the ore bed sequence. The ore beds were deposited between thick accumulations of massive and graded tuffaceous sandstone and siltstone in the hanging wall, and laminated chert, shale, siltstone, and sandstone in the footwall. The ore beds are thought to have been tilted, faulted, and intruded by at least one suite of Early to Middle Ordovician (465-480 Ma) latite dikes and sills prior to gold mineralization in the Late Ordovician. The orebodies are variously bounded by sharp skarn or marble fronts, preore dikes, and gradational mining cutoffs. Gold occurs mainly as 2- to 10-mu m particles on sulfide, calcite, and chlorite grain boundaries. The skarn is sequentially zoned from a grandite garnet-rich inner assemblage, near the Junction Reefs Monzodiorite stock, through clinopyroxene-dominated and amphibole-bearing assemblages, to an outer calcite-chlorite-quartz retrograde assemblage that includes the orebodies. Proximity to the skarn front has provided a major control on ore localization and gold grades reach a peak in a zone about 20 m behind the front. Preore dikes also appear to have controlled the distribution of ore in places by acting as barriers to...

Strain analysis of the Ninemile fault zone, western Montana: insights into multiply deformed regions

By means of detailed field mapping and a systematic strain analysis we investigated the development of the Ninemile fault in western Montana. The fault has been considered to be either a Mesozoic or mid-Tertiary strike-slip fault, or a Late Eocene-Early Oligocene normal fault. The rock of the study area experienced a Mesozoic to early Tertiary NE-SW shortening expressed by thrusts, folds, and pervasive cleavage. We estimated total strain by measuring preferred orientation of chlorite grains in samples from within and near the Ninemile fault zone and by calculating the strain according to the March theory. Our goal was to explain how strain accumulated by successive superposition of strains during the two deformation events onto an early compaction strain. In all samples the minimum-elongation direction (= maximum shortening) is nearly vertical. The maximum-elongation directions of all the samples outside the Ninemile shear zone are subperpendicular to the strike of the shear zone, whereas in the shear zone they are either parallel to the strike of the shear zone or form an angle of about 45°. The systematic variation in the principal strain directions is the result of two deformational events expressed by local structures created by early NE-SW shortening and later NE-SW extension. The observed strain requires that the Ninemile fault, a normal fault during the Tertiary, had earlier been either a thrust or a zone of contractional shear deformation, along which there was fault-perpendicular shortening and fault-parallel extension. Although the average value of maximum elongation is greater within the Ninemile shear zone than outside, it varies significantly inside the fault zone, suggesting a highly inhomogeneous strain distribution created by a complex deformation history.

Strain partitioning and crack-seal growth of chlorite-muscovite aggregates during progressive noncoaxial strain: an example from the slate belt of Taiwan

Chlorite—muscovite aggregates from the slate belt of southeastern Taiwan have an ellipsoidal geometry (~ k = 1 on a Flinn diagram) with an aspect ratio a:c ranging from about 7:1 to 12:1 and a crystallographic preferred orientation with the basal planes of the chlorite and muscovite at a high angle to the slaty fabric in the rock. The principal axes of the aggregates are approximately coaxial with the principal finite strain directions determined from fibrous pressure shadows. In cleavage parallel sections, straight fibers parallel to the downdip mineral lineation record plane strain. In cleavage perpendicular sections, however, pressure shadows have curved fibers that are consistent with W-directed thrusting. We propose a model in which the chlorite-mica aggregates grow syntectonically by cracking of detrital chlorite grains parallel to the basal planes and precipitation of secondary chlorite and mica in the open cracks. This model involves (1) rotation of preexiting detrital chlorite grains during progressive noncoaxial deformation, (2) cracking of these grains parallel to the basal plane when the (001) planes rotate into the incremental shortening field, with nucleation and growth of secondary chlorite and muscovite in the open microcracks. As the aspect ratio increases, the grains stabilize in orientation, with the long axes (normal to the basal planes) at a small angle to the shear direction. Extension normal to (001) is also accompanied by some shortening parallel to (001), but with an overall increase in size of the aggregates. The final grain size is controlled by the cumulative displacement associated with crack-seal events. Microprobe analyses of both detrital and secondary chlorite grains show distinct compositional differences within individual grains perpendicular to (001), particularly in the detrital grains, which we interpret as primary and secondary chlorite, consistent with a primary-and-secondary origin for the aggregates.

A Stratigraphic and Sedimentologic Analysis of a Lower Atoka Sandstone, Frontal Ouachita Thrustbelt, Western Arkansas: ABSTRACT

Abstract Prolific lower Atoka (Pennsylvanian) gas reservoirs have been exploited in the frontal Ouachita thrustbelt of eastern Oklahoma. Although the thrustbelt extends into western Arkansas, Pennsylvanian reservoirs have not been recognized. A lower Atoka sandstone unit, exposed behind the Ti Valley thrust fault, exhibits characteristics that indicate favorable reservoir quality. The outcrop is a succession of very fine to medium grained litharenite. Three zones are distinguished by grain size, mineral composition and sedimentary structures. The stratigraphically lowest zone is very fine grained and is composed mostly of monocrystalline quartz with a clay matrix. Sedimentary structures include scours, horizontal burrows and horizontal laminations. Porosity and permeability are nonexistent in this zone. Alternating sets of horizontally laminated fine to medium grained sandstone and bioclastic sandstone characterize the second zone. Secondary porosity has developed due to dissolution of skeletal grains. The stratigraphically highest zone exhibits porosity and permeability comparable to reservoirs within the Arkoma basin. Porosity values range from 6% to 24% with permeability as high as 248 millidarcies. The zone is a massively bedded, fine to medium grained sandstone. Monocrystalline and polycrystalline quartz are the dominant mineral components along with minor amounts of skeletal fragments . Porosity is secondary and has developed where pervasive chlorite grain coatings preclude silica cementation. The primary sedimentary structure is hummocky cross stratification implying deposition on a storm dominated shelf. The outcrop is significant in that it suggests: (1) the Arkoma shelf extended south of the present Ouachita thrustbelt trend and (2) reservoir quality rocks of Atokan age may be present in the subsurface behind the major thrust faults of the trend in western Arkansas.

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.

Kinematics and strain distribution of a thrust-related fold system in the Lewis thrust plate, northwestern Montana (U.S.A.)

In order to understand interactions between motion along thrusts and the associated style of deformation and strain distribution in their hangingwalls, geologic mapping and strain measurements were conducted in an excellently exposed thrust-related fold system in the Lewis thrust plate, northwestern Montana. This system consists of: (1) an E-directed basal thrust (the Gunsight thrust) that has a flat-ramp geometry and a slip of about 3.6 km; (2) an E-verging asymmetric anticline with its nearly vertical forelimb truncated by the basal thrust from below; (3) a 4-km wide fold belt, the frontal fold complex, that lies directly in front of the E-verging anticline; (4) a W-directed bedding-parallel fault (the Mount Thompson fault) that bounds the top of the frontal fold belt and separates it from the undeformed to broadly folded strata above; and (5) regionally developed, W-dipping spaced cleavage. Although the overall geometry of the thrust-related fold system differs from any previously documented fault-related folds, the E-verging anticline itself resembles geometrically a Rich-type fault-bend fold. The observed initial cut-off and fold interlimb angles of this anticline, however, cannot be explained by cross-section balancing models for the development of either a fault-bend fold or a fault propagation fold. Possible origins for the E-verging anticline include (1) the fold initiated as an open fault-bend fold and tightened only later during its emplacement along the basal thrust and (2) the fold started as either a fault-bend or a fault-propagation fold, but simultaneous or subsequent volume change incompatible with any balanced cross-section models altered its shape. Strain in the thrust-related fold system was determined by the preferred orientation of mica and chlorite grains. The direction and magnitude of the post-compaction strain varies from place to place. Strains in the foreclimb of the hangingwall anticline imply bedding-parallel thinning at some localities and thickening at others. This inhomogeneity may be caused by the development of thrusts and folds. Strain in the backlimb of the hangingwall anticline implies bedding-parallel stretching in the thrust transport direction. This could be the effect of bending as the E-verging anticline was tightened and transported across the basal thrust ramp. Strain measured next to the Gunsight thrust again indicates locally varying shortening and extension in the transport direction, perhaps in response to inhomogeneous friction on the fault or else to a history of alternating strain hardening and softening in the basal thrust zone.

Paragenesis and Reservoir Quality Within a Shallow Combination Trap: Central West Virginia

Sandstone character and reservoir quality of the Lower Mississippian Pocono Big Injun sandstone were examined in Granny Creek-Stockly field, Clay County, West Virginia. Sixty-three samples from 6 wells were analyzed using transmitted light, x-ray diffraction, and scanning electron microscopy techniques. The Pocono Big Injun formation is divided into four (Injun 1 through 4) based on composition and hydrocarbon productivity. The Injun 1 sand is a fine-grained, carbonate-cemented litharenite below the oil-producing zone. The oil-productive Injun 2 and 3 sands are well sorted, fine-grained litharenites which contain more authigenic and allogenic clay minerals than adjacent sands. These sands have produced more than 3.4 million bbl of oil from the Granny Creek part of the field since 1925. The Injun 4 sand is generally a coarse-grained sublitharenite with marginal gas production limited to the uppermost section of the sand. The paragenetic sequence consists of (1) minor quartz overgrowths, (2) illite and chlorite grain coatings, (3) quartz overgrowths, (4) early carbonate, (5) kaolinite, (6) calcite, (7) dolomite, and (8) pyrite. Porosity and permeability were not preserved once paragenesis progressed past the kaolinite stage. Porosity and permeability are variably preserved when steps in the paragenetic sequence are absent within the Pocono Group. Where any porosity is identified within the Pocono sandstones, primary porosity is dominant. However, secondary porosity and microporosity in clay-rich intervals are also important. Secondary porosity was c eated by the dissolution of cements and detrital grains. Authigenic chlorite grain coatings may not be responsible for preserving primary porosity in the Big Injun sands.

Smectite-Illite Transition in Barbados Accretionary Wedge Sediments: Tem and Aem Evidence for Dissolution/Crystallization at Low Temperature

AbstractSediments from depths to 670 m in the Barbados accretionary complex and transecting the décollement zone have been studied by transmission and analytical electron microscopy (TEM/AEM). The sediments consist of claystone and mudstone intercalated with layers of volcanic ash. Smectite comprises the bulk of the noncalcareous sediments and forms a continuous matrix enveloping sparse, irregular, large grains of illite, chlorite, kaolinite and mixed-layer illite/chlorite of detrital origin at all depths. The detrital origin of illite is implied by illite-smectite textural relations, well-ordered 2M polytypism, and a muscovite-like composition. K is the dominant interlayer cation in smectite at all depths, in contrast to the Na and Ca that are normally present in similar rocks.Deeper samples associated with the décollement zone contain small (up to 100 Å thick) illite packets included within still-dominant subparallel layers of contiguous smectite. AEM analyses of these packets imply illite-like compositions. Selected area electron diffraction (SAED) patterns show that this illite is the 1Md polytype. Packets display step-like terminations like those seen in illite of hydrothermal origin. The data collectively demonstrate that smectite transforms progressively to illite via a dissolution-recrystallization process within a depleting matrix of smectite, and not by a mechanism of layer replacement. This illite seems to form at depths as shallow as 500 m and temperatures of 20°-30°C, which is in marked contrast to the much higher temperature conditions normally assumed for this transformation. This implies that the high water/rock ratios associated with the décollement zone are significant in promoting reaction.

Fresh Water and Acid Sensitivity of Authigenic Clays in Sandstone Reservoirs

Authigenic illite-smectite and chlorite in reservoir sandstones from several Pacific rim sedimentary basins in Australia and New Zealand have been examined using an Electroscan Environmental Scanning Electron Microscope (ESEM) before, during, and after treatment with fresh water and HCl, respectively. These dynamic experiments are possible in the ESEM because, unlike conventional SEMs that require a high vacuum in the sample chamber (10-6 torr), the ESEM will operate at high pressures up to 20 torr. This means that materials and processes can be examined at high magnifications in their natural states, wet or dry, and over a range of temperatures (-20 to 1000 degrees C) and pressures. Sandstones containing the illite-smectite (60-70% illite interlayers) were flushed with fresh water for periods of up to 12 hours. Close examination of the same illite-smectite lines or filled pores, both before and after freshwater treatments, showed that the morphology of the illite-smectite was not changed by prolonged freshwater treatment. Chlorite-bearing sandstones (Fe-rich chlorite) were reacted with 1M to 10M HCl at temperatures of up to 80 degrees C and for periods of up to 48 hours. Before treatment the chlorites showed typically platy morphologies. After HCl treatment the chlorite grains were coated with an amorphous gel composed of Ca, Cl, and possibly amorphous Si, as determined by EDS analyses on the freshly treated rock surface. Brief washing in water removed this surface coating and revealed apparently unchanged chlorite showing no signs of dissolution or acid attack. However, although the chlorite showed no morphological changes, elemental analysis only detected silicon and oxygen.

Compositional homogeneity in low-temperature chlorites

Electron microprobe analyses of small chlorite grains (10–20 μm width) in diagenetically altered rocks from three deep wells and a series of outcrop samples ranging in maximum age from 4 to 1300 Ma show an increasingly smaller range in grain-to-grain composition as a function of the average age of the specimens in a series. The scatter of composition in a sample in each series changes little with depth despite present day, or estimated maximum temperature differences of 70° C to 100° C from top to bottom. Comparison of these chlorite compositions with those published for geothermal and metamorphic rocks formed under conditions of 300–450° C indicates that the homogeneity of chlorite composition is a function of both the temperature of equilibration and its duration. Apparently total homogenization of Fe/(Fe+Mg), Al/(Al+Mg+Fe) content will occur after 1010 years at low temperatures. Simple calculations indicate that a slow process such as solid state diffusion could be responsible for the eventual homogenization of mineral grain composition to reach chemical equilibrium after phase equilibrium has been reached.

Uranium Redistribution and Fixation During Chlorite Weathering at Koongarra, Australia

ABSTRACTThe chemical forms of iron minerals and uranyl species have been examined non-destructively by visible microspectroscopy and electron microprobe analysis in order to obtain a better understanding of uranium redistribution and fixation mechanisms. The uranyl species are associated with sub-micron sized iron minerals, such as hematite (Fe2O3), goethite (FeOOH), and ferrihydrite (Fe4-5(OH,O)12). Additionally, some uranium is fixed to goethite in weathered chlorite grains. Sub-micron sized saleeite, Mg(UO2)2(PO4)2. 1OH2O, is the most probable uranyl phase associated with the iron minerals. This suggests that the uranium fixation in the vicinity of the Koongarra uranium ore deposits is the result of the coprecipitation and sorption of microcrystals of saleeite on to the iron minerals released during weathering of chlorite.

Magnetic fabric implications for the relationships between deformation mode and grain growth in slates from the Borrowdale Volcanic Group in the English Lake District

In the less chlorite-rich slates from the Borrowdale Volcanic Group, chlorite magnetic fabric almost corresponds to fabric resulting from deformation following the March-Fernandez model. But chlorite grain rotation could be related to the first stages of the tectonic deformation in the other slates, the chlorite grains probably impinging after these stages upon one another and a simple grain rotation mechanism being therefore no longer possible.

Structural Variations in Chlorite and Illite in a Diagenetic Sequence from the Imperial Valley, California

AbstractSamples of cuttings from the Borchard A-2 well, Imperial Valley, California, were collected over a measured borehole temperature interval 135° to 275°C. The <0.5-µm (e.s.d.) fraction was separated using high-gradient magnetic separation (HGMS) to create a nonmagnetic fraction rich in illite and a magnetic fraction rich in chlorite. Chlorite was less easily separated from illite in lower temperature samples (<200°C), presumably due to the presence of polymineralic grains of chlorite and illite. Grains in higher temperature samples were more nearly monomineralic and more easily separated.The chlorite is the IIb polytype. The thickness of coherent scattering domains of chlorite increased until 220°C and then remained constant. The amount of 7-Å interstratified material increased downhole until 195°C and then decreased. Over the same temperature interval, the illite polytypes varied systematically from 1Md (135° to 175°C) to 1M + 2M1 (230° to 275°C) and coherent scattering domains in the mineral became thicker to about 200°C and then remained constant in thickness. The percentage of illite in mixed-layer illite/smectite (I/S) increased from 40% at 135°C to 100% at temperature >205°C; ordering in the I/S changes from R0 to R1 between 135° and 155°C, and from R1 to R ≥ 3 at temperatures >155°C.The concurrent structural changes in chlorite and illite indicate a general improvement in the overall structural order of the clay minerals with increasing temperature. Differences between chlorite and illite suggest that the minerals may have reacted differently to changing conditions or that they may have formed by different mechanisms. The exclusive occurrence of IIb chlorite at temperatures as low as 135°C extends the limit of IIb chlorite stability to temperatures lower than previous estimates.

Effects of Chlorite Alteration on Uranium Redistribution in Koongarra, Australia

ABSTRACTThe relevance of alteration of chlorite, one of the major constituent minerals of the host rock to uranium ore at Koongarra, to the redistribution of uranium in the vicinity of the ore deposit has been examined. The chlorite alteration is produced by weathering; chlorite is transformed to vermiculite, through regularly interstratified chlorite/vermiculite. The Fe released from chlorite reprecipitates to form iron minerals, possibly ferrihydrite. Vermiculite is then replaced by kaolinite (and possibly smectite), further releasing Fe. On the millimeter scale, an alpha autoradiography study has shown that uranium concentrations are qualitatively proportional to the extent of the alteration; altered chlorite grains having higher uranium concentration. On the meter scale, the X-ray diffraction study has revealed that the abundances of chlorite, vermiculite, and kaolinite correspond well to the low, intermediate, and high uranium concentration zones, respectively. These suggest that the interaction of the uraniferous solution with chlorite causes the alteration of chlorite and the precipitation and sorption of uranium in the alteration products from the solution, and thus, the uranium migration is retarded at Koongarra.