Brucite-like Interlayer Research Articles

Chlorite weathering linked to magnetic enhancement in Red Clay on the Chinese Loess Plateau

Magnetic parameters are widely used in the loess-palaeosol sequence tracing Quaternary climate change on the Chinese Loess Plateau (CLP). However, the climatic implication and magnetic enhancement mechanism in aeolian Red Clay on the CLP is still debated. Chlorite is a common Fe-bearing silicate in Red Clay and it is easily to be weathered. The transformation of weakly magnetic ferrihydrite, a product in chlorite weathering process, into strongly magnetic iron oxides has been linked to magnetic enhancement in many soils. Here, we present a complete chlorite weathering record within the Red Clay sequence through a detailed mineralogical investigation, from a 125-m thick eolian Red Clay deposit in the Chaona section (~8.1 to 2.6 Ma) of the central CLP to explore the link between changes in chlorite geochemical composition and magnetic susceptibility enhancement and regional climate. The results suggest a concomitant evolution of chlorite Fe content and magnetic susceptibility, indicating that Fe released from chlorite brucite-like interlayers, which induced ferrihydrite formation, is responsible for the enhanced magnetic susceptibility in the CLP Red Clay deposits. Moreover, the results also show that the chlorite Fe content exhibited a stepwise decrease, with two accelerated stages at ~6.2 Ma and ~3.6 Ma. The post-6.2 Ma evolution of chlorite Fe content is related to increased precipitation caused by phased enhancement of the East Asian summer monsoon (EASM). Our study also suggests that changes in chlorite chemical composition and crystallinity linked to soil weathering are useful proxies in tracing past weathering intensity and climate change within arid-less humid regime.

Sorption of Ni by “lithiophorite–asbolane” intermediates in Moa Bay lateritic deposits, eastern Cuba

The Moa Bay Ni–Co laterite deposits, placed in the so-called Mayari–Baracoa ophiolitic belt (eastern Cuba), are oxide type. Despite its geological relevance and economical impact no detailed studies exist with regards to cristallochemical characterization of Ni incorporated in (or attached to) the main Ni-containing minerals forming the lateritic profile. A sample corresponding to the ore limonite horizon has been studied by microfocus Raman, micro X-ray diffraction (µXRD), electron probe micro analysis (EPMA) and synchrotron radiation microfocus X-ray absorption spectroscopy (XAS) to gain structural and chemical information on Ni. The data obtained has revealed that Ni is preferably accumulated in quantities up to 21 wt.% in “lithiophorite–asbolane” intermediates. The local environment of Ni shows Ni–Mn distances ∼ 3.5 Å suggesting that Ni is sorbed mostly in inner-sphere complexes sitting on Mn vacancies and at the edge of the Mn layers. However it is shown that in the presence of Al the Ni is incorporated within the “lithiophorite–asbolane” intermediate by developing brucite-like interlayers. The understanding of Ni sorption mechanisms within the limonite horizon suggests that combined physicochemical factors such as soil porosity and pH regime have important implications for Ni mobility across the profile.

XRD and HRTEM analyses of stacking structures in sudoite, di-trioctahedral chlorite

The stacking structures of sudoite, di-trioctahedral chlorite from two different localities are investigated by powder X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). XRD analyses using a Gandolfi camera revealed that the structures of both specimens are similar, corresponding to a one-layer II bb -4 (1 A ) polytype. HRTEM observations indicate that the stacking sequence is characterized by a largely uniform intralayer shift of a /3 in the −X 1 direction (X i represent the directions along the pseudohexagonal axes) and by an interlayer displacement of similar magnitude in either the −X 2 or −X 3 direction. Stacking disorder is primarily caused by the mixing of interlayer displacements in the two directions. This disorder is more common in the lath-shaped crystalline specimen from Berezovsk, Russia than in the fine platy crystalline specimen from Ottre, Belgium. DIFFaX simulations of the powder XRD patterns for this stacking model reproduced the observed features well for both specimens. The stacking configuration of these sudoite specimens is considered to be controlled by the corrugation of basal oxygen planes in the 2:1 layer and the corresponding deformation in the brucite-like interlayer sheet.

Polytypic Constraints for Solid-State Layer Silicate Transformation Mechanisms Via Atomic-Resolution Transmission Electron Microscopy

Abstract Layer silicates are important and abundant minerals. Polytypic details can be determined directly from atomic-resolution images recorded down [010] of layer silicates such as chlorite (Fig. 1), serpentine, and micas using a transmission electron microscope with a point resolution of ∼ 0.2 nm. For chlorite, this involves determination of whether octahedral slants in the brucite-like interlayers and octahedral sheet of the tetrahedral - octahedral-tetrahedral (TOT) layers are the same or opposed, the (3 angle, and how the octahedral cations in the brucite-like interlayer are positioned relative to the tetrahedral cations in the TOT layer (Fig. 1). Images must be recorded from the thinnest areas in order to distinguish slant of the brucite-like interlayer (composition dependent, but always < 0.7 nm sample thickness; 1). Polytype analysis is aided significantly by rotational filtering of images from thin areas to remove the amorphous contribution (2). All six of the common, semi-randomly stacked chlorite polytypes can be distinguished (1).

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.

Atomic-resolution transmission electron microscope evidence for the mechanism by which chlorite weathers to 1:1 semi-regular chlorite-vermiculite

Atomic-resolution transmission electron microscope (TEM) images reveal that IIbb (beta = 97 degrees ) Mg,Al,Fe-chlorite from Koongarra, Australia, transforms to vermiculite via a range of intermediate chemical and structural states. Semi-quantitative analysis of contrast in atomic-resolution images of layer silicates with approximately 1.4 nm basal spacings indicate that the interlayers range from brucite-like to having approximately 0.3-0.6 interlayer cations per formula unit. Octahedral cations (predominantly Mg and Fe) tend to be removed from every second interlayer, leading to semi-regular 1:1 interstratifications of chlorite-vermiculite. Further loss of interlayer cations is accompanied by partial to complete interlayer collapse in the vacuum of the TEM. Resulting intergrowths of chlorite and semi-regular 1:1 chlorite-vermiculite retain the primary chlorite orientation, morphology, and sense of octahedral tilt in 2:1 layers. Although vermiculitization is a continuous process that occurs by a solid-state mechanism, the reaction involves important structural modifications. Atomic-resolution [010] images indicate initial loss of interlayer cations is accompanied by approximately a/3 shifts of 2:1 layers and cations in brucite-like interlayers. Displacements of interlayer cations change the interlayer stacking from IIbb to Iab and shift of the following 2:1 layer converts it from Iab to the Iaa. Displacements are driven by the lower energy of a-type interactions when vacancies occur in sites above tetrahedral cations. Shift of a 2:1 layer alters the subsequent interlayer from IIbb to IIab. Stabilization of every slightly altered second interlayer by introduction of a-type stacking explains development of semi-regular 1:1 chlorite-vermiculite interstratifications. Displacements occur before significant modification of interlayer electron density can be detected in high-resolution images. This observation is consistent with previously reported inhibition of layer shifts by low interlayer charge. Layer displacement may occur by an elastic process (no rupture of bonds within the 2:1 layer) at the tip of the growing vermiculite portion of the intergrowth. Removal of cations from the chlorite-vermiculite junction may be facilitated by rapid diffusion along the vacancy-rich interlayer. Mg is removed in solution, Fe is precipitated locally in aggregates of nanocrystalline Al-, Si-, and P-bearing goethite.

The Role of Randomly Mixed-Layered Chlorite/Smectite in the Transformation of Smectite to Chlorite

AbstractVesicular and groundmass phyllosilicates in a hydrothermally altered basalt from the Point Sal ophiolite, California, have been studied using transmission electron microscopy (TEM). Pore-filling phyllosilicates are texturally characterized as having coherent, relatively thick and defect-free crystals of chlorite (14 Å) with occasional 24-Å periodicities. Groundmass phyllosilicates are texturally characterized as 1) randomly oriented crystals up to 200 Å in width and 2) larger, more coherent crystals up to 1000 Å in width. Small crystallites contain predominantly 14-Å layers with some 24-Å units. Large crystals show randomly interlayered chlorite/smectite (C/S), with approximately 50% chlorite on average. Adjacent smectite-like layers are not uncommon in the groundmass phyllosilicates. Electron microprobe analyses show that Fe/Mg ratios of both groundmass and vesicular phyllosilicates are fairly constant.Termination of brucite-like interlayers has been identified in some of the TEM images. The transformation mechanisms represented by these layer terminations are 1) growth of a brucite-like interlayer within smectite interlayer regions and 2) the dissolution and reprecipitation of elements to form chlorite layers. Both mechanisms require an increase in volume as smectite transforms to chlorite.The data, combined with that from previously published reports, suggest that randomly interlayered C/S is a metastable phase formed in microenvironments with low water/rock ratios. Chlorite forms in microenvironments in the same sample dominated by higher water/rock ratios. The relatively constant number of Mg's in the structure (Mg#) of both structures indicates that in both microenvironments the bulk rock composition has influence over the composition of phyllosilicates.

Formation of regularly interstratified serpentine-chlorite minerals by tetrahedral inversion in long-period serpentine polytypes

Serpentinite from Lancaster County, Pennsylvania, consists of a variety of fme-grained serpentine minerals, chlorite, randomly interstratified serpentine-chlorite, and a series of phases based on regular interstratification of serpentine and chlorite (SxCy, where x and y are integers). Within the resolution of the AEM technique, all layer silicates have the same Mg-rich, AI-rich, Cr-rich, and Fe-poor compositions. Regularly interstratified serpentinechlorite minerals are frequently intimately intergrown with serpentines that have repeat distances identical to those of the regular interstratifications. Thus, dozyite (SICI, (3= 90°) is intimately associated with serpentine with three-layer octahedral order (1,1,11).Longer period polysomes (S2C SIC2, S2C2, S,C3, S3C2, and S,C4, all with (3 = 90°) are each accompanied by serpentines with equivalent c-axis periodicities. SxCy phases apparently form by selective growth ofIbb chlorite units from 1,11octahedral sequences in long-period serpentines. All microscopic structural evidence is consistent with the formation of regular interstratifications by tetrahedral inversion within existing serpentine. Atomic resolution images reveal that the tetrahedral sheet is displaced by a13.where it inverts to form the 2:1 layer. A :ta13 shift is required for hydrogen bonding between OH of the newly formed brucite-like interlayer and 0 atoms of the 2: 1 layer. The sense of the shift is determined by the strong interactions between the octahedral cations in the brucite-like interlayer and the Si in the 2: 1 layer (direct superimposition, previously described as a type-a interaction, is strongly unfavorable). Distortion at the inversion point probably lengthens Si-O bonds in the next tetrahedra, facilitating relocation of Si on the other side of the basal 0 plane. Reversal of the octahedral slant in the 2:1 layer occurs because the +a13 tetrahedral shift necessitates repositioning of 0 and OH coordinating octahedral cations, requiring movement of octahedral cations from type-II to type-I positions. Except in the 2: 1 layers, the stacking and octahedral slants are inherited. The result is a series of regular interstratifications characterized by a single octahedral slant (specifically, Ibbb,l) and bl3 stacking disorder. This analysis reveals the importance of cation-cation interactions in determining the relative stability of pairs of 1:1 layers and in controlling the detailed structures oflayer silicates formed in solid-state serpentine-to-chlorite reactions. Because similar constraints apply to formation of serpentine from chlorite by direct structural modification, the common 1T lizardite polytype may be produced from both IIbb and Ibb chlorites.

A Study of Weathering in a Soil Derived from a Biotite-Hornblende Rock

AbstractThe weathering of biotite in a sedentary soil profile developed on biotitehornblende- rock near Rehiran, Inverness-shire has been investigated by optical, X-ray, chemical, infrared and differential thermal methods. In the C horizon a direct transformation to hydrobiotite was observed. This involves oxidation of all the ferrous iron present and subsequent movement of ferric ions from the octahedral sheet. In the B horizon weathering proceeds to a fully expanded 14A phase and this was identified as interstratified vermiculite-chlorite in a 1:1 ratio. Chemical analysis indicates that this stage is accompanied by a substantial addition of magnesium, much of which goes to form the brucite-like interlayers. In the more acid surface horizon these interlayers tend to break down thus yielding a more vermiculitic product. The possible general implications of this type of biotite weathering are briefly discussed.

Effects of pretreatment on a 14 Å swelling mineral from Gartly, Aberdeenshire

A 14 Å swelling mineral, interlayered with what is probably an organometallic complex, occurs in the surface horizon of a gleyed soil near Gartly, Aberdeenshire. The interlayer material is thermally stable at 300°C and is easily removed by oxidation with H2O2 and NaOCl. Treatment with HCl and Na2EDTA destroys the ability of the mineral to expand with glycerol, although not if the clay is H2O2 treated beforehand. The Gartly clay mineral is also unusual in that Mg-saturation, using magnesium acetate at pH 7, leads to the rapid formation of brucite-like interlayers. It is concluded that in the investigation of soil clays it is best to avoid all pretreatments.