Richard A. Eggleton

New data and a revised structural model for ferrihydrite

Synthetic 2-line and 6-line ferrihydrite samples prepared from ferric nitrate solutions have the bulk compositions Fe4(O,OH,H2O)12 and Fe4,6(O,OH,H2O)12, respectively. The composition depends on crystal size, which averages 20 Å for 2-line and 35 Å for 6-line ferrihydrite. X-ray absorption edge spectra indicate the presence of tetrahedral Fe3+, a conclusion supported by heating experiments which show the development of maghemite after heating to 300°C in the presence of N2, followed by the formation of hematite at higher temperatures. These two reactions are recorded on differential thermal analysis traces by exotherms at 350° and 450°C. Transmission electron microscopy shows that 2-line ferrihydrite has no Z-axis regularity but does show hexagonal 2.54-Å lattice fringes. Six-line ferrihydrite forms faceted crystals having 9.4-Å c-parameter only detectable in dark field. In bright field, 2.54-Å lattice fringes indicate greater atomic regularity than in 2-line ferrihydrite. Analysis of the X-ray powder diffraction pattern of 6-line ferrihydrite suggests a structure based on double-hexagonal close-packed oxygens, containing 36% Fe in tetrahedral sites. Selective chemical dissolution, surface area measurements, and magnetic susceptibility are consistent with the recorded properties of ferrihydrite.

Cation exchange capacity of kaolinite

Experimental cation exchange capacities (CEC) of kaolinites were determined and compared to theoretical calculations of CEC. The comparison reveals that the exchangeable cations occur mostly on the edges and on the basal (OH) surfaces of the mineral. It also shows that permanent negative charge from isomorphous substitution of Al3+ for Si4+ is insignificant. The CEC of kaolinite strongly depends on the particle size (both thickness and diameter in the (00l plane) and pH value. Particle size is more important than crystallinity in affecting kaolinite CEC. This study shows that the hydroxyls on the exposed basal surfaces may be ionizable in aqueous solutions. The amount of negative charge on the edges and the exposed basal hydroxyls depends on pH and other ion concentrations. A higher pH value gives rise to more negative charges, which lead to a higher CEC value. This study indicates that charge from broken edges and exposed OH planes rather than charge from Al/Si substitution determines the kaolinite CEC, even at zero point charge. A high CEC in some kaolinites is found to be due to smectite layers on the surface of the kaolinite crystals.

Apatite replacement and rare earth mobilization, fractionation, and fixation during weathering

During an electron microscope study of the weathering of granite from southern New South Wales, Australia, an assemblage of minerals including florencite and rhabdophane was discovered replacing apatite. Light rare earth elements released from allanite early in weathering apparently combined with P released by leaching of primary apatite to form secondary phases exhibiting a range of morphologies and compositions. Chondrite normalized fractionation patterns La > Nd > Sm > Ce; La > Ce > Nd ≥ Sm and La = Sm = Nd > Ce were identified. The rare earth elements were present in very small crystals and aggregates of secondary minerals (< 10 μm) and coexisted with clays and secondary Fe-Ti oxides. The weathered granite was enriched about 6 to 10 times in all rare earth elements except Ce relative to fresh granite if the abundances were corrected for apparent enrichment due to reduction in density. The rare earth elements were probably derived from higher in the weathering profile, possibly by destruction of florencite and rhabdophane in very intensively weathered rock. Ce remained relatively immobile during weathering, probably due to its oxidation to Ce4+.

WEATHERING OF BASALT: CHANGES IN ROCK CHEMISTRY AND MINERALOGY

The weathering of eastern Australian basalts, sampled from the rounded, hard, core-stone to the rind of softer weathered material, has been examined by bulk chemical analyses, thin section petrography, electron microprobe, and X-ray powder diffraction analyses. Using density as a measure of weathering intensity, data from four core-stones show that at a stage of weathering in which the total loss due to dissolution is – (i.e., at the core-stone rim), the percentages lost of the following major elements are: Ca, 85; Mg, 80; Na, 70; K, 50–80; P, 55; Si, 45; Mn, 40; Al, 5; Fe, 0; and Ti, 0. With more intense weathering, deposition of some elements, particularly rare earths and Ba, and mobilization and deposition of Al and Fe make quantification impossible. The rate of weathering of individual minerals is consistent with the well-known susceptibility series: glass ∼ olivine > plagioclase > pyroxene > opaque minerals. Clay minerals in the core-stones are dominated by smectites, whereas those in the surrounding more intensely weathered rinds are dominated by halloysite and goethite.

Transmission electron microscope study of biotite weathering

Transmission electron microscopy suggests that biotite transforms to vermiculite primarily by direct structural modification, involving the replacement of K+ by hydrated interlayer cations, and only minor reorganization of the 2:1 layer. A second relatively uncommon mechanism appears to involve redistribution of components from two biotite sheets to form a single vermiculite layer. Distortion of the surrounding structure initially inhibits growth of vermiculite in the surrounding biotite, and promotes the propagation of vermiculite layers in opposite directions. This phenomenon may contribute to the development of relatively regular, widely spaced interstratifications of biotite and vermiculite. Additional components and space are provided by the dissolution of biotite where access of solutions is greater. During weathering, biotite and vermiculite become increasingly replaced by kaolinite, which crystallizes epitactically onto existing layers, and goethite, which develops from a poorly crystalline iron oxyhydroxide precursor to form oriented laths. In areas parts of strongly weathered samples kaolinite and goethite appear to develop in proportions consistent with a reaction that conserves both Al and Fe.

Analytical transmission electron microscope studies of plagioclase, muscovite, and K-feldspar weathering.

Analytical and high-resolution transmission electron microscopy of weathered plagioclase and K-feldspar provided microtextural and chemical data that suggest a sequential formation of weathering products. An alteration layer < 1 µm thick on feldspar surfaces had short-range order and was termed protocrystalline. Relative to the parent feldspars the protocrystalline layer was depleted in Ca, Na, K, and Si and significantly enriched in Fe. On plagioclase, the protoerystalline material was replaced by Ca-Fe-K-smectite, another protocrystalline material, and spherical halloysite. Abundant tubular halloysite on the corroded surface apparently formed by reprecipitation of components released by plagioclase dissolution. The K-feldspar was markedly more resistant to weathering than the plagioclase.Recrystallization of the patchily developed protocrystalline rind produced Fe-bearing, aluminous smecrite, which was ultimately replaced by spherical halloysite and laths of kaolinite. Muscovite laths within plagioclase crystals were converted initially to illite by loss of K, then to randomly interstratified illite/smectite, and then to smectite that contained Mg, little K and Fe, and was more aluminous and contained less Ca than the smectite that originally replaced the plagioclase. Smectite was replaced epitactically by kaolinite. Kaolinite was the stable weathering product of the feldspars and muscovite in the profiles. It probably formed in equilibrium with a solution whose composition was no longer controlled by the microenvironment within the feldspar, but approached that of meteoric water.

Weathering of granitic muscovite to kaolinite and halloysite and of plagioclase-derived kaolinite to halloysite

Weathered perthite and mixed muscovite-kaolinite from a kaolinitic granite at Trial Hill in east Queensland and kaolinized sericitic alteration from a granite from the Ardlethan Tin Mine of New South Wales were examined by optical, scanning electron (SEM), and transmission electron microscopy (TEM) to determine the alteration process of muscovite to kaolinite and kaolinite to halloysite (7Å). Muscovite was found intimately interleaved with kaolinite in a variety of proportions on a sub-micrometer scale. The contact was generally parallel to the (001) layers of both minerals, and the thickness of the contact layer alternated between 10 and 7 Å over short distances. Where the kaolinite to muscovite contact was at an acute angle to the muscovite layers, a small angle existed between the layering of the two phases, consistent with a topotactic alteration of muscovite to kaolinite. One tetrahedral sheet in the muscovite appeared to have been removed over 50–100 Å, converting a 10-Å layer to a 7-Å layer. The mica near the contact with kaolinite was easily damaged in the electron beam and showed Al loss during analytical transmission electron microscopy; thus, H3O+ probably substituted for K+ in this transitional phase.An SEM examination of completely weathered plagioclase showed kaolinite plates having attached, parallel, polygonal rods of halloysite (7Å), which had planar sides and a central void, partly fused with the surfaces of the kaolinite crystals. TEM study showed that the kaolinite altered to halloysite, and that, where the kaolinite was partly altered to halloysite, a series of sharp kinks were present in the kaolinite plate in which alteration had occurred. These kinks were interspersed with linear kaolinite relics, 0.1–0.2 μm long, which appear to have provided local rigidity to the clay packet. Apparently, the altered clay first curled into loosely wound spirals, which ranged in cross-section from triangles to irregular octagons, with pentagons and hexagons being most common. The tendency to pentagons and hexagons compares well with a statistical study of the angles, which were most commonly grouped around 120°. As alteration of the kaolinite relics progressed, the linear parts of the spiral lost their rigidity and became circular or oval shaped. The long axis of the halloysite spirals was parallel to the X axis of the kaolinite. Halloysite spirals formed most readily if they had space to curl; if space was not available, the halloysite formed sheaves. Rare, thin layers of muscovite were present projecting through kaolinite into halloysite. Where muscovite relics reached open spaces, the 10-Å structure expanded to 14 Å.

Formation of iddingsite rims of olivine; a transmission electron microscope study

Iddingsite rimming olivine in a basanite from the Limberg, Germany, is composed of saponite and goethite. Transmission electron microscopy of ion-thinned, oriented crystals suggests a two-stage alteration process. At first, the olivine breaks into a mosaic of 50-Å diameter {110} bounded needle-shaped domains which change to a metastable hexagonal phase having a = 3.1 Å and c = 4.6 Å, probably of close-packed, metal-oxygen octahedra. This reaction opens solution channels in the olivine which are detectable from about 20-Å diameter and are parallel to the olivine y-axis. Laths of smectite, one or two layers thick, 20 Å wide, and as much as 100 Å long parallel to their y-axis nucleate from the metastable phase and begin to fill in the solution channels. The laths orient with smectite (001) parallel to olivine (100). As the channels widen, prismatic {110} goethite crystals form directly from the metastable hexagonal phase. This first stage thus provides heterogeneous nuclei of smectite and goethite, formed epitactically and perhaps topotactically from a metastable intermediary.In a second stage,these nuclei enlarge by deposition from solution as water migrates readily through the solution channels. A reduction in total volume allows smectite veins to form, misoriented with respect to the olivine. The reaction conserves iron, requires the addition of aluminum and water, and releases magnesium and silicon. Electron microprobe analyses of the iddingsite indicate that the smectite is saponite.РезюмеИддингсит окружающий оливин в басаните из Лимберг в Германии, состоит из сапонита и гетита. Трансмисионная электронная микроскопия направленных кристаллов указывает на двустепенный процесс изменения. Во-первых, оливин распадается на мозаику (110) иголчатой структуры диаметром 50 Å, которая изменяется в метастабильную гексагональную фазу c a = 3,1 Å и c = 4,6 Å, вероятно, октаэдр метал-кислород с плотной упаковкой. Эта реакция открывает каналы для растворителя в оливине, которые обнаруживаются от диаметра около 20 Å и которые являются параллельными к оси у. Пластинки смектита, толщиной один или два слоя, широкие на 20 Å и длиной достигающей 100 Å, параллельные к своей оси у, образуются из метастабильной фазы и начинают наполнять каналы. Пластинки ориентируются по смектиту (001), параллельно к оливину (100). Когда каналы растираются, кристаллы призматического {110} гетита образуются прямо из метастабильной гексагональной фазы. Тогда эта начальная стадия создает неоднородные ядра смектита и гетита, которые формируются путем эпитаксии или топотаксии нз метастабильного посредника.Во второй стадии эти ядра увеличиваются путем осаждения из раствора, в то время как вода двигается легко через каналы. Уменьшение полного объема позволяет формировать вены смектита, неориентированные по отношению к оливину. Реакция сохраняет железо, нуждаятся в добавлении алюминия и воды, и выделяет магний и кремний. Микроэлектронный анализ иддингсита указывает на то, что смехтит является в виде сапонита. [E.G.]ResümeeIddingsitränder von Olivin in einem Basanit von Limberg, Deutschland, bestehen aus Saponit und Goethit. Transmissionselektronenmikroskopische Untersuchungen von “ion-thinned” orientierten Kristallen deuten auf einen Umwandlungsprozeß in zwei Abschnitten hin. Zuerst zerfällt der Olivin in ein Mosaik aus etwa 50 Å großen, {110} begrenzten, nadelförmigen Domänen, die sich in eine metastabile hexagonale Phase mit a = 3,1 Å und c = 4,6 Å umwandeln, die wahrscheinlich aus dicht gepackten Metall-Sauerstoff-Oktaedern besteht. Diese Reaktion öffnet Lösungskanäle im Olivin, die ab einem Durchmesser von 20 Å erkennbar sind und parallel zur y-Achse des Olivins verlaufen. Smektitleisten, die eine oder zwei Lagen dick sind, 20 Å breit und parallel zu ihrer y-Achse bis zu 100 Å lang sind, bilden sich aus der metastabilen Phase und beginnen die Lösungskanäle zu füllen. Die Leisten sind mit der (001) Ebene des Smektits parallel zu (100) Ebene des Olivins orientiert. Wenn die Kanäle weiter werden, bilden sich prismatische {110} Goethitkristalle direkt aus der metastabilen hexagonalen Phase. Dieses erste Stadium liefert daher heterogene Keime von Smektit und Goethit, die epitaktisch und vielleicht topo-taktisch aus einer metastabilen übergangsphase entstehen.In einem zweiten Stadium vergrößern sich diese Keime durch Ablagerung aus Lösung, da Wasser sehr leicht durch die Lösungskanäle wandern kann. Eine Verminderung des Gesamtvolumens ermöglicht es, daß sich Smektitadern bilden, die im Hinblick auf den Olivin keine bestimmte Orientierung aufweisen. Die Reaktion hält das Eisen fest und fordert eine Zufuhr von Aluminium und Wasser, während sie Magnesium und Silizium freigibt. Mikrosondenanalysen des Iddingsit deuten darauf hin, daß der Smektit ein Saponit ist. [U.W.]RésuméDe l’iddingsite entourant de l’olivine dans une basanite de Limberg, Allemagne, est composée de saponite et de goethite. La microscopie électronique à transmission de cristaux orientés réduits à l’épaisseur d’ions, suggère un procédé d’altération à deux étapes. Tout d’abord, l’olivine se brise en une mosaïque de domaines en forme d’aiguilles, de 50 Å de diamètre limités par {110}, qui se transforment en une phase hexagonale métastable ayant a = 3,1 Å, et c = 4,6 Å, probablement d’octaèdres d’oxygène-métal très proches les uns des autres. Cette réaction ouvre des canaux de solution dans l’olivine, que l’on peut détecter d’à peu prés à partir de 20 Å de diamètre, et sont parallèles à l’axe-y de l’olivine. Des lattes de smectite, d’une ou deux couches d’épaisseur, de 20 Å de large, et jusqu’à 100 Å de longueur parallèles à leur axe-y sont formées à partir de la phase métastable et commencent à remplir les canaux de solution. Les lattes s’orientent avec la smectite (001) parallèle à l’olivine (100). En même temps que les canaux s’élargissent, des cristaux de goethite prismatiques {110} se forment directement à partir de la phase hexagonale métastable. Cette première étape fournit ainsi des noyaux héterogènes de smectite et de goethite, formés épitactiquement, et peut-être topotactiquement à partir d’un intermédiaire métastable.Pendant une deuxième étape, ces noyaux s’aggrandissent par déposition de solution puisque l’eau émigre facilement à travers ces canaux de solution. Une réduction de volume total permet la formation de veines de smectite, mal orientées respectivement à l’olivine. La réaction conserve le fer, exige l’addition d’aluminium et d’eau, et relâche du magnésium et de la silice. Des analyses microprobes d’électrons de l’iddingsite indiquent que la smectite est de la saponite. [D.J.]

The orthoclase-microcline inversion: A high-resolution transmission electron microscope study and strain analysis

High-resolution electron microscopy of an intermediate microcline (Or93) from a granodiorite in southeastern Australia reveals anen echelon arrangement of triclinic lens-shaped domains, twinned on the albite law. The domains are tabular on (010), are only a few unit cells wide, but extend 20 or 30 unit cells alongx, until they merge into a zone of monoclinic cells roughly aligned in the rhombic section. The domains are longer and less clearly terminated alongz. Strain calculations show that the energy released by Al/Si ordering, producing the orthoclase-microcline inversion, is equal to the strain energy developed when triclinic domains are forced to retain the original monoclinic crystal shape. This balance of strain energies thus explains the metastable persistence of intermediate microcline into the region of maximum microcline stability. Shearing along faults during deformation of the granodiorite released the strain in some of these feldspars, allowing maximum microcline to develop, and so giving rise to a bimodal distribution of triclinicities throughout the pluton. The value ofγ measured for the intermediate microcline is the average of a range of values throughout each domain, and may be considerably closer to 90° thanγ from an unstrained crystal with the same degree of Al/Si order.

Weathering of basalt; formation of iddingsite

The formation of iddingsite by the oxidative weathering of Fo80 olivine begins by solution of Mg from planar fissures, 20 Å wide and spaced 200 Å apart, parallel to (001). Oxidation of Fe within the remaining olivine provides nuclei for the topotactic growth of goethite. Cleavage cracks < 50 Å in diameter allow Na, Al, and Ca from adjacent minerals, particularly plagioclase, to enter the altering olivine while Mg and Si diffuse away. In the early stages of weathering, strips of Fe-rich smectite (saponite), 20–50 Å wide and 1–7 layers thick, form bridges 50–100 Å long across the planar fissures. Dioctahedral smectite crystallizes on the margins of wider cleavage-controlled fissures; with further weathering halloysite is formed away from the fissure walls. In the ultimate stages of alteration, the saponite and dioctahedral smectite are lost, leaving a porous, oriented aggregate of goethite crystals each measuring about 50 × 100 × 200 Å (X, Y, Z, respectively), with sporadic veins of halloysite crossing the pseudomorph.