Phyllosilicate Grains Research Articles

Re-equilibration of detrital muscovite and the formation of interleaved phyllosilicate grains in low temperature metamorphism, northern Apennines, Italy

Low temperature metamorphosed clastic rocks of northern Apennines (Verrucano) contain detrital and metamorphic muscovite along with aggregates of interleaved phyllosilicate grains. The extent of celadonite substitution in the metamorphic muscovite progressively increases from Al-rich pyrophyllite-bearing samples to Al-poor K-feldspar-bearing samples and appears to be compatible with the phase relation in the AKF and AKNa diagrams. Some detrital muscovite grains, not equilibrated with the present metamorphic mineral assemblages, seem to be unreacted grains retaining their premetamorphic composition. The chemistry of the re-equilibrated “detrital muscovite” is mainly controlled by the activity of Al2O3 that is implied by mineral assemblages in the host rocks and is independent of the original composition of detrital mica. Four types of phyllosilicate associations in the interleaved phyllosilicate grains were recognized: 1) muscovite- pyrophyllite-sudoite; 2) muscovite-pyrophyllite-chlorite; 3) muscovite-paragonite-chlorite; 4) muscovite-chlorite. A microstructural and petrological model is proposed for the origin of interleaved phyllosilicate grains in the Verrucano rocks. The model supports the idea that the interleaved phyllosilicate grains are the result of the trend towards equilibrium between detrital muscovite, metamorphic mineral assemblages and the fluid phase.

Mineralogy of the rhizosphere in forest soils of the eastern United States

Chemical and mineralogical studies of forest soils from six sites in the northeastern and southeastern United States indicate that soil in the immediate vicinity of roots and fine root masses may show marked differences in physical characteristics, mineralogy and weathering compared to the bulk of the forest soil. Examination of rhizosphere and rhizoplane soils revealed that mineral grains within these zones are affected mechanically, chemically and mineralogically by the invading root bodies. In SEM/EDS analyses, phyllosilicate grains adjacent to roots commonly aligned with their long axis tangential to the root surface. Numerous mineral grains were also observed for which the edge abutting a root surface was significantly more fractured than the rest of the grain. Both the alignment and fracturing of mineral grains by growing roots may influence pedogenic processes within the rhizosphere by exposing more mineral surface to weathering in the root-zone microenvironment. Chemical interactions between roots and rhizosphere minerals included precipitation of amorphous aluminium oxides, opaline and amorphous silica, and calcium oxalate within the cells of mature roots and possible preferential dissolution of mineral grains adjacent to root bodies. Mineralogical analyses using X-ray diffraction (XRD) techniques indicated that kaolin minerals in some rhizosphere samples had a higher thermal stability than kaolin in the surrounding bulk forest soil. In addition, XRD analyses of clay minerals from one of the southeastern sites showed abundant muscovite in rhizoplane soil adhering to root surfaces whereas both muscovite and degraded mica were present in the immediately surrounding rhizosphere soil. This difference in mineral assemblages may be due to either K-enrichment in rhizoplane soil solutions or the preferential dissolution of biotite at the root-soil interface

Pedon Distribution of Minerals in Coastal Plain Paleudults

AbstractMineral distributions in three Florida Paleudults were investigated with respect to particle size, horizon, depth, and location within the soil matrix. Mineralogy of sand, silt, and clay was determined using optical microscopy, x‐ray diffraction, and thermal analysis. Also, mineralogy of sand‐grain coatings was compared with intergrain matrix material. Elemental‐oxide composition of siltsized phyllosilicate grains was determined by electron microprobe analysis. Sand fractions consisted mainly of quartz, with lesser amounts of feldspar and mica, which were more prevalent in lower horizons. Coarse‐ and medium‐silt fractions were also dominated by quartz, but contained appreciable feldspar and mica in B horizons. Kaolinite and hydroxy‐interlayered vermiculite (HIV) were most prevalent in fine‐silt and clay fractions. Abrupt changes in silt and clay mineralogy across the E‐ and B‐horizon boundary (E‐B) included a decrease in quartz and HIV and an increase in kaolinite. Relative amounts of HIV did not markedly change with depth in arenic epipedons, but decreased steadily below the E‐B boundary for all pedons. Sand‐grain coatings from Bt horizons contained more mica than did the intergrain matrix, whereas coatings in A and E horizons contained appreciably more HIV. Distribution of phyllosilicates in these pedons could be explained by a hypothetical sequence of processes which includes (i) illuviation of kaolinite formed from weathering or dispersed from clay lenses in the sediments, (ii) comminution of sand‐ and silt‐sized mica, (iii) incorporation of finegrained mica in sand‐grain coatings, and (iv) transformation of finegrained mica to HIV in highly permeable eluvial horizons.

Porosity and Cementation in Upper Cretaceous Mooreville and Demopolis Chalks, Central Alabama: ABSTRACT

Cyclically arranged chalky marl, marl, limestone, and sand facies comprise the Upper Cretaceous Mooreville and Demopolis Chalks, the lower two formation in the Selma Group, inner Coastal Plain of Alabama. In the central Alabama study area (Dallas, Lowndes, and Montgomery Counties), the Mooreville-Demopolis section is 305 m thick and the two main facies are chalky marl and marl. Chalky marl consists of 50-70% carbonate (nannofossil component plus isopachous cement) and is relatively impermeable (average permeability is 0.9 md). The marl has 30-50% carbonate (mainly nannofossil component) and has an average permeability of 0.13 md. Helium-calibrated porosity values range from 31 to 35% in chalky marl and 36 to 41% in the marl. Scanning-electron microscopy (SEM) of chalky marl and marl shows a relatively poor alignment of phyllosilicate grains in both facies. Under SEM, the calcareous nannofossil component, mainly coccoliths and rhabdoliths, shows pristine to cement-coated exterior surfaces. The cement coating is most common in the chalky marl. Sampling throughout the Mooreville-Demopolis section shows no apparent vertical (stratigraphic) trends in facies-specific petrologic characteristics such as permeability, helium-calibrated porosity, phyllosilicate grain alignment, nannofossil content, and extent of cementation. The most parsimonious explanation of the development and evolution of porosity and cementation inmore » the study area is as follows. First, simple mechanical compaction from burial under a few hundred meters of sediment can readily explain the reduction in porosity to less than 40% in most samples from the Mooreville-Demopolis section. Secondary (facies-selective) cementation is a further cause for porosity reduction in chalky marl. Cementation is likely the result of calcareous nannofossil dissolution during compaction and carbonate solution-transfer via groundwater movement.« less

Nonexchangeable Potassium Associated with Hydroxy‐Interlayered Vermiculite from Coastal Plain Soils

AbstractHydroxy‐interlayered vermiculite (HIV) from a Quartzipsamment, a Haplaquod, and a Paleudult from Florida was studied to determine levels of nonexchangeable K associated with this mineral. Mineralogy was characterized by x‐ray diffractometry (XRD) and thermal analysis. A scanning electron microscope equipped with an energy‐dispersive x‐ray system was used to examine morphology of silt‐sized phyllosilicate grains and to qualitatively determine elements present. An electron microprobe equipped with a wavelength‐dispersive x‐ray system was used to obtain K x‐ray images and determine elemental oxide compositions of 82 randomly selected grains. Total K was determined for bulk samples using Na2CO3 fusion. Extractable K by double acid, HNO3, and H2SO4 was determined for clay fractions. Results confirmed that most phyllosilicate grains from quartz‐ and HIV‐dominated silt fractions of all samples contained sufficient K to be detected in K x‐ray dot images, though little or no mica was detectable by XRD. Mean K2O contents for mediumsilt grains from the Quartzipsamment, Haplaquod, and Paleudult were 2.3, 4.1, and 5.6%, respectively. Fine‐silt grains averaged 1.3, 1.3, and 2.3%, respectively, despite lack of infinite thickness. High K2O values, mica‐like grain morphology, and relatively large size of these grains suggest that they are transformation products of mica. Acid extracts from the clay also contained detectable levels of K. Nonexchangeable K may reside within grains as small occluded mica zones with insufficient periodicity for XRD detection, or could be retained in the 1.4‐nm structure. Results suggest a more direct transformation of mica to HIV rather than one involving a significant vermiculite intermediate.

Superposed deformations in the Eastern Alps: strain analysis and microfabrics

Superposition, mainly of two Eoalpine (Cretaceous) deformation events, can be observed in vertical sections and horizontal traverses throughout a decollement zone in the Eastern Alps consisting of crystalline basement and autochthonous and allochthonous cover stacked in several nappes. The first event is an episode with a non-coaxial strain history caused by W-directed thrusting, the second a N-directed shortening by folding and fold imbrication. Strain analysis based both on deformed pebbles and on the preferred orientation of phyllosilicate grains (March theory), microfabric analysis, and observations of the relative timing of deformation and metamorphism, together indicate that the superposition of structures caused by the two events amounts to one continuous, progressive act of deformation. We attribute gradual and consistent variations in intensity and axial ratio of the tectonic strain to a history influenced by: (a) the position of samples in the pile; (b) the relative importance of the strain increments caused by the two events; and (c) rock ductility. We interpret variations of the c-axis fabrics of quartz in the same way, and draw tentative kinematic conclusions for the Eoalpine orogeny in the Eastern Alps.

Cleavage development in dolomite of the Elbrook Formation, southwest Virginia

Well-developed cleavage in argillaceous Cambrian Elbrook Formation dolomite has been analyzed petrographically and chemically to evaluate the deformation mechanisms responsible for cleavage formation and the lithologic controls on these mechanisms. Two main types of cleavage occur: moderately spaced cleavage defined by coarse, continuous, wavy domains, and narrowly to closely spaced cleavage defined by fine, discontinuous, anastomosing domains. The two most important deformation mechanisms during cleavage formation were pressure solution and mechanical rotation. Evidence for pressure solution includes off-set sedimentary structures, grain dissolution, syn-cleavage fibrous veins, and accumulations of quartz, phyllosilicates, and pyrite in cleavage domains. Pressure solution was enhanced by the presence of clay minerals. A model for cleavage development in dolomite is presented in which cleavage initiates by the dissolution of dolomite grains at dolomite-phyllosilicate grain boundaries. Rotation of phyllosilicate grains concomitant with dolomite removal resulted in a strong preferred orientation of layer silicates within cleavage domains. Finer cleavage domains coalesced to form coarser domains as the intervening lithon dolomite was dissolved. This model is comparable to those previously proposed for cleavage development in argillaceous limestones and siltstones.

Evidence for syntectonic crystallization for the mudstone to slate transition at Lehigh gap, Pennsylvania, U.S.A.

Data primarily for phyllosilicates have been obtained for the continuous transitional sequence from mudstone to slate with well-developed slaty cleavage at Lehigh Gap, Pennsylvania, and for slates from quarries in the same area. Samples were studied by optical microscopy, powder X-ray diffraction and electron microprobe analysis, with emphasis on transmission and analytical electron microscopy. Mineral grains are virtually free of deformation-induced strain. Concomitant with the gradual development of cleavage normal to bedding the following changes are observed or confirmed: (1) the orientation of phyllosilicate grains changes discontinuously from being preferentially parallel to bedding to being parallel to cleavage; (2) crystal imperfections as expressed in layer terminations, low angle grain boundary-like features and other defects decrease in density; (3) complex mixed layering is replaced by homogeneous packets of layers of single phases and (4) illite transforms to muscovite, with increase in K + Al and change from a 1M d to 2M polytype. Slaty cleavage apparently develops due in part to pressure solution of phyllosilicates oriented parallel to bedding, mass transport of components, and crystallization to form new grains parallel to cleavage. It reflects transitions from imperfect, metastable phases toward ordered stable phases in a low temperature (∼225°C) metamorphic environment.

The relationship of strain and preferred orientation of phyllosilicate grains in rocks—a review

The calculation of strain from the preferred orientation of platy phyllosilicate grains, taking strain to be homogeneous, is surprisingly often successful from a pragmatic point of view, although numerous theoretical objections to this procedure can be and have been raised. For the calculation to succeed, the original distribution of mica or clay mineral grains must be known or assumed. The usual assumption that the starting distribution was random and therefore nearly uniform implies that the strain history of pelitic sediments is taken to start when the watery sediment was last stirred by waves or bioturbation, or if it was laid down in still, anoxic waters, from the moment of first deposition. It therefore includes compaction as part of the strain history, which is usually neglected by structural geologists. Originally random distribution of clay flake orientations has been observed in recently deposited sediments in many instances and has been theoretically explained in various ways. Exceptions probably exist, but their frequency has not been estimated. Biologically disturbed sediments may be brought nearer to a state of random orientation of clay flakes than they were at deposition. With the exception of whole beds or concretions in which pores are early filled with cement, mudrocks are almost always compacted by the weight of later deposits. The strain so induced is axially symmetric about the vertical and thus about the pole of the bedding planes. Compaction acts on soft, unconsolidated rocks but may continue with increasing overburden on rocks strengthened by cementation and by increasing interlocking of grains. Tectonic deformation may start early, before the rock is fully compacted, or it may begin only much later and the mechanisms of deformation increase in complexity with the increasing temperature and hydrostatic stress of deeper burial. Up to a certain level of mild metamorphism, however, phyllosilicate grains seem to be rather permanent, if not as mineral species then at least by preservation of their constituent sheets of silica tetrahedra. Given this permanence, the mechanisms of rock deformation, whether intergranular slip, cataclasis, or dissolution and reprecipitation and various intragranular processes in the non-phyllosilicate matrix hardly matter as far as the recording of strain is concerned. Even a certain amount of stress solution at especially heavily loaded points of the phyllosilicate grains themselves does not destroy their value as strain markers, as long as the dissolved material is disposed of mainly by crystallographically continuous overgrowth on existing grains. The same holds for local dissolution of phyllosilicates in portions of old grains that have become thermodynamically unstable by bending. Widespread irregular or random nucleation and growth of newly nucleated phyllosilicate grains at intermediate or high grades of metamorphism can reduce the reliability of the strain record, and so can domainal structure of the deformed rock if both the volume fraction of phyllosilicate grains and the intensity of total strain differ sharply from one domain to another, if distinct domains are not equant and have themselves a strong preferred orientation of shape. Nevertheless, rocks containing an anastomosing network of zones rich in phyllosilicate grains, as slates and other deformed mudrocks commonly do, have in many cases been found to have average preferred orientations of mica or chlorite which, if used to estimate the strain, give results similar to those obtained from independent strain markers in the same rocks. Thus the most significant verification of the usefulness of strain estimates by measurement of preferred orientation is the mutual confirmation with independently determined strain. The limit to such verification is the rarity of strain markers, so that the coexistence of two independent ones is exceptionally rare. For this reason, however, it is important to make full use of the orientation of phyllosilicate grains where they are available. The alternative, to have no strain measurements at all, causes structural geologists to miss potential information.

Preferred orientation of phyllosilicate [001] in matrix of Murchison meteorite and possible mechanisms of generating the oriented texture in chondrites

Preferred orientation of the dominant phyllosilicate phase (serpentine) in the matrix of the Murchison meteorite (C2) was determined by X-ray pole figure goniometry. The basal plane (001) of the phyllosilicate shows a clear preferred orientation of an axial concentration type. The preferred orientation is very weak in comparison with the orientations of known terrestrial tectonites, and it resembles the weakest case observed in the loosely consolidated clayey sediments from the deep ocean bottom. However, gentle sedimentation of platy phyllosilicate grains on a flat surface under a weak gravity field does not generate a sufficient preferred orientation. It is suggested that the preferred orientation in the matrix of Murchison was generated by uniaxial compaction, and the magnitude of strain to give rise to the observed degree of preferred orientation is evaluated as −9%. There are two possible explanations of the deformation: dynamic compression caused by impact, and static compaction in a parent body. Though the latter case appears to be appropriate in the present case, it is not straightforward, however, to conclude that the suggested deformation is caused by burial compaction along gravity under lithostatic stress in a small primitive parent body.

Aspects of strain history in folded sediments from the Schefferville Mining District, Labrador Trough, Canada

Preferred orientations of phyllosilicate grains in the shaley rocks of the Labrador Trough were studied using a pole-figure goniometer. Transmitted X-rays permit determination of the basal planes of chlorite and muscovite. From their preferred orientation, strain is calculated according to March's theory. By choosing samples in successively more strongly deformed domains, a strain history can be recognized which started with compaction by loss of pore volume under an overburden and was later followed by an early homogeneous tectonic shortening parallel to bedding and at a right angle to the fold axis. Buckling occurred next, with strain in the limbs becoming different from that in the hinges; the limbs were lengthened parallel to bedding and nearly normal to the fold axis. Continuing appression, finally, led to pervasive shortening normal to the axial plane with concurrent stretching both updip along the axial plane and along the fold axis. This latter stretching may have occurred when variable-plunge basin and dome structures of the Labrador Trough were formed.

Rotation and Dewatering during Slaty Cleavage Formation: Some New Evidence and Interpretations

Microstructures in the Martinsburg Formation in east-central Pennsylvania provide strong evidence supporting Maxwell's hypothesis (1962) that the slaty cleavage formed during deformation of unconsolidated sediments by rotation of phyllosilicate grains in a loosely compacted sedimentary succession held open by abnormally high pore-fluid pressures. The structures fall into two classes: those which indicate the unconsolidated condition of the strata at the time of slaty cleavage development, and those which indicate that pore-fluid pressures were high prior to dewatering. The absence of dewatering phenomena across the strata where slaty cleavage parallels bedding provides a clue to the process of cleavage development. Because sedimentary intrusions and other dewatering structures occur only where phyllosilicate particles are at some angle to bedding, the inference is that dewatering across bedding occurred when clays rotated away from parallelism with the strata, providing avenues of escape for pore water. It is concluded, therefore, that rotation and alignment of phyllosilicates caused the dewatering rather than the converse, as is commonly thought.