The hydroxyl orientations in the 2H2 polytype of amesite, Mg2Al(SiAl)O5(OH)4, have been determined by minimizing the electrostatic potential energy as a function of OH orientation. The angles, ρ, between the hydroxyls and (001) vary between 81.5 and 88.8°. All surface hydroxyls form hydrogen bonds with oxygens of the adjacent layer. The OHs tend to tilt away from the higher charged A1VI ions, and the spread in ρ values is due to variations in the positions of the receptor oxygens. The inner hydroxyls noticeably weaken the interlayer bonding. The substitution of Al in both T and M sites creates a dipolar layer charge and the resulting attraction between layers forms an important part of the interlayer bonding. The 2H2 polytype of amesite has substantial interlayer bonding even with all surface hydroxyls replaced by fluorine. This is not true for a composition of Mg3Si2O5(OH)4 where a similar F for OH substitution destroys all interlayer bonding as in kaolinite.

The thermal transformation of a Brazilian antigorite heated dry from 600° to 1300°C, was studied by high resolution electron microscopy and selected area electron diffraction (SAD). Below 600°C, the antigorite particles did not change morphologically or crystallographically, as evidence by the presence of the superlattice in the SAD patterns and lattice images. At 650°C, traces of antigorite remained, although the superlattice structure had disappeared and the first indications of forsterite formation were observed in the same SAD. Diffraction patterns of the transition phases showed various topotactical relationships between antigorite and forsterite. At 800°C, the crystallization to forsterite was much more pronounced. At 900°C, the individual particles retained their original shapes, but they were actually pseudomorphs, for their SAD patterns showed only forsterite. Between 1000° and 1300°C various topotactical relationships were observed. Although the overall transformation to forsterite appears to be similar for both chrysotile and antigorite, the transformation of antigorite took place at a slightly higher temperature (~50°C) with a larger number of topotactical relationships than for chrysotile. Enstatite was formed at 1300°C, but it was impossible to determine the topotactic relations between enstatite and forsterite, which was possible with chrysotile.

Quaternary debris flows derived from ophiolite melange in northern Southland, New Zealand, are locally cemented and veined by cross-fiber and fine-grained chrysotile. Chrysotile was identified by optical, X-ray powder diffraction, electron microprobe, differential thermal (DTA), and infrared analysis methods. Microprobe and DTA also suggested the presence of as much as 6% of sub-microscopic, noncrystalline alumina, probably in the form of pseudoboehmite. Cavity fillings and irregular veins containing chrysotile are as thick as 40 cm and commonly contain lizardite clasts. Calcite commonly accompanies chrysotile as a vein mineral. Magnetite and pyrite are found near and adjacent to chrysotile veins. These accessory minerals imply that the chrysotile formed under alkaline, reducing conditions. Surface water seepages have pH = 9. Remnant debris-flow topography, Holocene radiocarbon dates, and the absence of hot spring activity confirms that chrysotile can form under near-surface, low-temperature conditions. Growth of chrysotile fibers on a lizardite substrate suggests that the chrysotile formed by solution of detrital lizardite and subsequent precipitation in cavities within the debris flow.

The crystal structure of a magnesian cronstedtite-2H2 from Pribram, Czechoslovakia, was refined in space group C1 to a residual of 5.4% with 1832 independent reflections. Tetrahedral ordering between Fe3+ and Si is judged to be complete on the basis of electron density maps, the first confirmation of such ordering in a layer silicate. Octahedral cations are disordered on the M sites. Mean T-O bond lengths do not confirm the tetrahedral ordering, perhaps due to tetrahedral distortion in order to relieve the extreme corrugation of the sheet that would arise from ordering of such different size cations.

In the initial stages of refinement the structure of the crystal under study could not be described adequately on the basis of a single 2H2 polytype. The structure contains two kinds of 2H2 domains that appear shifted by b/3 relative to one another due to a mistake in the interlayer stacking sequence. This mistake of zero shift (the normal 2H2 polytype consists of alternating —b/3 and +b/3 shifts) affects only the tetrahedral sheets. It creates adjacent enantiomorphic domains of 2H2 packets such that domains with small Si tetrahedra sit above larger Fe3+-rich tetrahedra and vice versa. This mechanism to relieve strain may indeed be the cause of the stacking mistakes. A third kind of domain contains regions in which the sense of tetrahedral rotation is reversed, giving rise to split basal oxygens on electron density maps. This multiple domain model best explains extra atomic positions observed on Fourier maps, lack of streaking of k ≠ 3n reflections, and possible nonintegral “satellitic” reflections observed on Weissenberg films.

Six standard polytypes of micas exist that have c-periodicities between 1 and 6 layers; these can be subdivided into groups A and B according to whether the octahedral cations are in the same set of positions in every layer or alternate regularly between I and II possible sets of positions (alternate slant directions), respectively. Correspondingly, 12 standard polytypes exist for 1:1 layers of the serpentine-type structures, which are divisible into four groups A-D. Groups A and B differ as in the micas, as do groups C and D. Groups A and B have ±a/3 interlayer shifts, whereas groups C and D have zero or ±b/3 interlayer shifts. Six structural types of semi-randomly or regularly stacked 1-layer chlorites exist that differ in the slant of the interlayer sheet (I or II) and its position (a or b) relative to 6-member rings in adjacent 2:1 layers.

Criteria have been established that permit the X-ray diffraction identification of the above-mentioned groups as well as the individual polytypes within each group. These criteria involve inspection of the strong X-ray diffraction reflections of index k = 3n for identification of the groups and of the generally weaker k ≠ 3n reflections for identification of the polytypes (assuming indexing on orthohexagonal axes throughout for convenience). In all hydrous phyllosilicates the octahedral cations and anions repeat at intervals of b/3 and thus contribute strongly to X-ray diffraction reflections of index k = 3n. The intensities of these reflections identify the two mica groups, the four serpentine-type groups, and the six chlorite groups. The periodicities of these reflections along Z* is that between identical octahedral sheets. The basal oxygen atoms do not repeat at intervals of b/3 and are a primary contributor to the intensities of k ≠ 3n reflections. The period along Z* for these reflections is that between identical basal oxygen planes, and the periodicity plus the symmetry identify the individual trioctahedral polytypes. For dioctahedral polytypes of the kaolin-group minerals and chlorites, the position of the vacant octahedral site must be considered also.

These general principles can be illustrated especially well by single-crystal precession photographs and extrapolated to powder photographs.

Secondary minerals formed during simulated weathering of nuclear waste glasses have been identified by analytical electron microscopy. A complete description of the reacted glass, from the outermost surface in direct contact with the leachant solution to the reacting front that migrates into the bulk glass, was obtained. Manganese and iron oxyhydroxide phases and saponite were found to have precipitated onto the residual glass surface from the leachant solution. Iron-bearing smectite, serpentine, and manganese and uranium-titanium oxyhydroxides formed in situ in the glass in several distinct bands at different depths beneath the original surface. This sequential development of secondary phases displays a clear trend toward more order and crystallinity in the phases farthest from the reaction front and indicates that complete restructuring of the glass into crystalline phases did not occur at the interface with fresh glass. Additionally, the formation of a discrete uranium-bearing phase, as opposed to uranium uptake by precipitated phases, suggests that stable actinide phase formation rather than ion exchange may be a possible mechanism for retarding radionuclide release to the environment.

The formation of iron-rich phyllosilicates can occur at different natural or engineered settings. In this study, the influence of pH in the hydrothermal synthesis of iron-rich phyllosilicates was investigated in the pH range 8.50–12.10 after the ageing of the precursor. The synthesized samples were characterized by powder X-ray diffraction, Raman and Mössbauer spectroscopies and transmission electron microscopy. Three domains of pH were identified, and these correlated with silica availability and its speciation in the solution. The formation of 1:1-type FeIII/FeII phyllosilicate was observed between pH 9.67 and 10.75. Above pH 10.75, two types of phyllosilicate-like mineral phases were observed. In addition to 1:1-type FeIII/FeII phyllosilicate, 2:1-type FeIII/FeII phyllosilicate was observed. Below pH 9.67, mainly amorphous silica and iron oxides were observed. The findings show that pH governed the crystallinity and nature of the obtained phyllosilicate-like phases.

Alteration of the crystal structure of Mulhacén antigorite caused by dry, vibration grinding was investigated by X-ray diffraction (XRD), infrared spectroscopy (IR), thermal analyses (TG), grain-size distribution, and transmission and analytical electron microscopy (TEM, AEM). Grinding for 1 min reduces particles to a size ideal for IR and TG. With prolonged grinding, XRD and electron diffraction patterns showed that the crystal structure was affected mainly along the c axis, causing a partial loss of crystallinity. TG analyses revealed that vibration grinding modified mineral dehydration, accelerating the dehydroxylation process and transforming the structural OH to adsorbed water in the resulting matrix. IR spectra and AEM showed that grinding affected the tetrahedral sheet to a lesser extent than the octahedral sheet. Partial release of Mg by preferential destruction of the octahedral sheet after 10 min grinding produced an increase in the Si/Mg ratio in semi-crystalline particles, whereas the amorphous material product after 120 min showed the same composition as the initial antigorite. TEM and grain-size distribution results revealed that grinding led to a general decrease in particle size at the beginning of the experiment followed by the agglomeration of ultrafine particles as grinding proceeded.

In a hitherto unpublished manuscript dated 1794–95, H.-B. de Saussure described three types of rudimentary laboratory experiments on weathering, each one lasting about 4 hours, undertaken on samples of jade, serpentine, feldspar, and mica schist. He submitted these samples to dry heating, heating with continuous addition of water, and heating with periodic addition of water. Experimentation was performed at temperatures of the order of 200–250°C, and Saussure seems to have realized that a reasonable increase of temperature was necessary for laboratory simulation of long-lasting geological processes. No changes were observed during dry heating whereas increasing alteration was noticed in the presence of water, particularly when heating was combined with periodic wetting. He concluded that these tests demonstrated his preconceived idea that weathering of minerals (mostly silicates) resulted from alternating periods of drying and wetting. This concept is basically very close, if not identical, to that underlying modern ideas on seasonal weathering.

The objective of this paper is to describe the new serpentine group mineral, guidottiite, which is analogous to cronstedtite. Guidottiite has an ideal chemical composition of (Mn2Fe3+)(SiFe3+)O5(OH)4. The sample is from the N’chwaning 2 mine, Kalahari manganese field, Republic of South Africa, and apparently forms from hydrothermal solutions. Grains are optically near opaque [average index of refraction 1.765, with variable extinction on the (001)], vitreous, and black, with perfect {001} platy cleavage. A non-separable fibrous substructure exists perpendicular to cleavage that results in a silky luster under optical examination. The average chemical analysis determined from electron microprobe based on four grains with ten analyses each resulted in a structural formula of (Mn1.86Fe0.613+Mg0.54)Σ=3.01(Si1.36Fe0.643+)Σ=2.00O5(OH)4, with calculated density of 3.236 g/cm3. Analysis from another area of the sample showed a slightly different chemical composition and resulted in a formula of (Mn1.70Fe0.963+Mg0.24Σ=2.89(Si1.26Fe0.743+)Σ=2.00O5(OH)4, with calculated density of 3.291 g/cm3. The measured density on a bulk sample (with impurities) was 3.33 g/cm3. Thermal analysis suggested a dehydroxylation temperature of 535°C, a decomposition/recrystallization temperature of 722°C, and weight loss (= H2O loss) of 9.4%. The derived Mohs hardness from nano-indentation is H = 4.25.

The sample is mostly the 2H1 polytype with minor amounts of the 2H2 polytype. Using a predominantly 2H2 crystal, which has better crystallinity, the strongest observed X-ray peaks are: 7.21 Å (Io/Io = 100%), 3.543 (50), 2.568 (39), 1.982 (26), and 2.381 (25). All Gandolfi simulations, even with three crystal remountings, showed preferred orientation effects. Transmission electron microscope (TEM) analysis showed stacking disorder within Group D serpentine polytypes. Thus, a regular alternation of the occupancy of octahedral sets within each layer along the stacking exists, but disorder of the layer displacements of 0 and ±b/3 (b defined here as the orthohexagonal cell) exists. Ordered 2H1 (no layer displacement) and 2H2 (alternating + and —b/3 displacement) domains were also frequently observed. X-ray diffraction analysis showed that even apparent single crystals contain impurity phases, presumably Mn-rich and Ca phases that were detected in the microprobe study. The single-crystal structure refinement used a well (stacking) ordered apparent 2H2 crystal with little to no streaking in the diffraction pattern. Results showed that the crystal has a random interstratification of 2H2 and 2H1. The 2H2 polytype is hexagonal, space group P63, with a = 5.5472(3), c = 14.293(2) Å, and Z = 2, and was refined to R1 = 0.072 and wR = 0.108 from 656 unique reflections. Because the two polytypes in the composite have only small differences in the lower 1:1 layer, a large displacement parameter for the basal oxygen atom results, which was constrained to B = 1.5 Å2 (Ueq = 0.0190) in the refinement. Half of the tetrahedral sites in the 2H1 upper layer superpose over half of the tetrahedral sites in the 2H2 upper layer (T1 sites only) per unit cell. This superposition produces an apparent excess of electron densities of the T1 site relative to the T2 site (T1 = 21.9 electrons, T2 = 15.8). Comparison with the microprobe data indicates that observed tetrahedral bond lengths are generally not affected by this intergrowth. Tetrahedral bond lengths indicated that the tetrahedral sites contain T1 = Si0.678 Fe0.3223+ and T2 = Si0.631Fe0.3693+. This excess of electron densities and other refinement problems associated with the guidottiite single-crystal refinement closely parallel all single-crystal cronstedtite-2H2 refinements to date, suggesting that these refinements also involve random interstratifications of 2H2 and 2H1 polytypes.

When present at elevated levels in drinking water, arsenic is toxic, and magnesian clays are gaining recognition as a source of elevated arsenic in groundwater. In the crust and upper mantle of Earth, arsenic incorporation into clay minerals is influenced by geochemical conditions associated with hydrothermal fluids and metamorphic processes (e.g. serpentinization), meaning that As is a useful tracer of fluid-flow in the deep Earth. To improve understanding of arsenic speciation in groundwater, sediments, soils, and hydrothermal-metamorphic systems, the present study examined arsenic incorporation into magnesian clays by synthesis of serpentine minerals (200oC, 10 d) with varied concentrations of Si, Al, As5+, and As3+. The synthesis experiments produced two distinct crystal types, tubular and platy serpentines, each with 10–15% randomly interstratified talc layers. X-ray absorption spectroscopy indicated that As5+ and As3+ occurred in the tetrahedral sheet. Single-crystal analysis revealed that tubular crystals contained up to 1 wt.% arsenic [Mg2.8(Si1.8As0.2)O5(OH)4] (mean 0.2 wt.% As). The mean composition of platy, high-Al crystals is (Mg1.8Al0.7)(Si2.0)O5(OH)4, and that of platy, medium-Al crystals with As3+ is (Mg2.07Al0.52) (Si1.97As3+0.03)O5(OH)4. Charge, geometry, and radius of tetrahedral AsO43– oxyanions are similar to tetrahedral SiO44–, and this facilitates fixation of As5+ into the tetrahedral sheet of clay minerals. The geometry and size of the larger As3+ in tetrahedral sites (as a pyramidal AsO33– oxyanion) may limit incorporation relative to As5+. Arsenic-bearing Mg clays crystallize in alkaline environments where AsO43– or AsO33– are the dominant As species and where high pH accompanies crystallization of serpentine, talc, chlorite, or Mg-smectite. The presence of tetrahedral As in these clays raises the possibility of tetrahedral As in other Mg clays (e.g. sepiolite or kerolite) as well.

The formation of 2:1 and 1:1 phyllosilicates in Fe–Si–O–H systems occurs in various geological and engineering settings; however, the identification and characterization of these minerals is very challenging due to the limited amount that is accessible, the very small particle size, and often the large degree of heterogeneity of these samples. To overcome these drawbacks, the synthesis of iron-rich phyllosilicates was attempted in this study with an initial Fe/Si molar ratio ranging from 0.50 to 2.33. The synthesis was performed at 150°C under hydrothermal conditions over a period of 7 days. Synthesis products were characterized by X-ray diffraction and fluorescence, by infrared, Raman, and Mössbauer spectroscopies, and by transmission electron microscopy. Results revealed that the stability field of the 2:1 clay mineral was wider than that of the 1:1 clay mineral. The 2:1 clay mineral was less sensitive to redox conditions compared to the 1:1 clay mineral. In addition, a heterogeneity of phases formed (iron oxides, nontronite, cronstedtite, or greenalite) was identified.

Serpentinites, which contain high concentrations of Cr and Ni, weather easily into layer silicates and are therefore a possible source of metal contamination in soils. In the present study three soil profiles formed on serpentinites in a paddy field in eastern Taiwan were investigated to understand pedogenic clay-mineral transformations and to determine the relationship between the mineralogical characteristics and labile Cr and Ni in the soil. To this end, physicochemical analyses, micromorphology, X-ray diffraction, and Fourier transform infrared spectroscopy were employed. Serpentine and chlorite were the dominant minerals in the soil parent material, with smaller amounts of pyroxene, amphibole, and talc. Progressive weathering and the release of cations from the parent material resulted in the pedogenic formation of smectite, vermiculite, and interstratified chlorite-vermiculite, demonstrated by their presence in all Ap and AC horizons but their absence from the C horizons. Serpentine, pyroxene, amphibole, and talc are proposed to be transformed to low-charge smectite, while chlorite transformed to vermiculite through an interstratified chlorite-vermiculite phase. The surface soils were enriched in oxalate-extractable Fe relative to the subsoils, which was probably generated by the artificial flooding and draining of the paddy soils. The artificial flooding, which typically releases Fe, may also drive the observed partial hydroxyl interlayering of smectite and incomplete interlayer OH sheets of chlorite. Labile Cr and Ni (extracted with 0.1 N HCl) ranging from 4.7 to 26.8 mg kg−1 and from 56 to 365 mg kg−1, respectively, increased significantly toward the surface soil, consistent with weathering. The heavy metals released may pose a threat to the environment as well as to human health by entering the food chain.

The enhanced spatial and spectral resolution provided by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on the Mars Reconnaissance Orbiter (MRO) has led to the discovery of numerous hydrated silicate minerals on Mars, particularly in the ancient, cratered crust comprising the southern highlands. Phases recently identified using visible/near-infrared spectra include: smectite, chlorite, prehnite, high-charge phyllosilicates (illite or muscovite), the zeolite analcime, opaline silica, and serpentine. Some mineral assemblages represent the products of aqueous alteration at elevated temperatures. Geologic occurrences of these mineral assemblages are described using examples from west of the Isidis basin near the Nili Fossae and with reference to differences in implied temperature, fluid composition, and starting materials during alteration. The alteration minerals are not distributed homogeneously. Rather, certain craters host distinctive alteration assemblages: (1) prehnite-chloritesilica, (2) analcime-silica-Fe,Mg-smectite-chlorite, (3) chlorite-illite (muscovite), and (4) serpentine, which furthermore has been found in bedrock units. These assemblages contrast with the prevalence of solely Fe,Mg-smectites in most phyllosilicate-bearing terrains on Mars, and they represent materials altered at depth then exposed by cratering. Of the minerals found to date, prehnite provides the clearest evidence for subsurface, hydrothermal/metamorphic alteration, as it forms only under highly restricted conditions (T = 200–400°C). Multiple mechanisms exist for forming the other individual minerals; however, the most likely formation mechanisms for the characteristic mineralogic assemblages observed are, for (1) and (2), low-grade metamorphism or hydrothermal (<400°C) circulation of fluids in basalt; for (3), transformation of trioctahedral smectites to chlorite and dioctahedral smectites to illite during diagenesis; and for (4), low-grade metamorphism or hydrothermal (<400°C) circulation of fluids in ultramafic rocks. Evidence for high-grade metamorphism at elevated pressures or temperatures >400°C has not been found.

A number of chemical reactions (mass-action expressions) have been proposed to describe serpentinization of olivine in terrestrial serpentinites. Some have been applied to interpreting the formation of serpentine-group minerals in CM carbonaceous chondrites. The widely used olivine + pyroxene → serpentine (ol + px → srp) reaction is quantitatively inconsistent with observed ratios of olivine to pyroxene and olivine to serpentine in CM carbonaceous chondrites. The reaction of 5ol → 2srp is most consistent with constraints from textural observations at the scale of individual partial and complete pseudomorphs of serpentine after coarse olivine in CM chondrites, observed co-variations in modal abundances of reactant and product minerals in CM chondrites, and experimental geochemical kinetics. Furthermore, the 5ol → 2srp reaction provides insight into the mobility of solute species in the aqueous alteration environment of CM carbonaceous chondrites.

Outer main-belt asteroids are predominantly of the C-type (carbonaceous), suggesting that they are likely parent bodies of carbonaceous chondrites. Abundant phyllosilicates in some classes of carbonaceous chondrites have chemical compositions, mineral associations, and textures that preserve direct evidence of the processes by which carbonaceous chondrites and their parent asteroids originated and evolved to their present state. Serpentine is the dominant hydroxyl-bearing mineral in the most abundant (CM) group of carbonaceous chondrites. Serpentine may have formed as a direct nebular condensate during cooling of the solar nebula, or by aqueous alteration of anhydrous Mg,Fe-silicate precursors. Such alteration of anhydrous precursors may have occurred in the solar nebula prior to assembly of the meteorites’ parent bodies or on the parent bodies. The relative proportions of Fe and Mg in fine-grained CM2 serpentines have been used to compare the degree of aqueous alteration of different CM2 chondrites with one another. The Mg content of serpentine increases with increasing overall degree of aqueous alteration, so CM2 chondrites with Mg-rich serpentines experienced a more advanced degree of aqueous alteration than CM2 chondrites with Fe-rich serpentines. Attempts to quantify aqueous alteration of CM chondrites by interpreting electron microprobe analyses in terms of charge-balance and site-occupancy constraints from serpentine stoichiometry have met with mixed success. Despite its imperfections, one widely used alteration index based on serpentine stoichiometry is strongly correlated with the elapsed time since the fall and recovery of witnessed CM chondrite falls. Additionally, volatile organic contaminants introduced during sample processing in the laboratory are associated with serpentine and other matrix phyllosilicates. Together, these post-recovery changes in scientifically important sample attributes imply that oxidation-reduction and other types of weathering and contamination affect these meteorites even during curatorial storage and laboratory processing. The same phyllosilicates that make their carbonaceous-chondritic host rocks scientifically important research targets also render those same rocks extraordinarily vulnerable to terrestrial contamination of some of their most scientifically important attributes. This has possible implications for reconstructing pre-terrestrial (parent body) aqueous alteration phenomena from carbonaceous chondritic meteorites and eventually from samples returned by future missions to asteroids with spectral reflectance properties similar to carbonaceous chondrites.

There has been limited research on clay mineral transformation in serpentinitic soils under humid tropical conditions. In this study, four soil pedons were selected along a toposequence from the summit (Entisol), shoulder (Vertisol), backslope (Alfisol) to footslope (Ultisol) positions to explore the contributions and the significance of landscape and weathering status of serpentinitic rock with regard to clay mineral transformations in eastern Taiwan. Experimental results indicated that the large amount of dithionite-citrate-bicarbonate-extractable Fe (Fed) and clay in the subsurface horizon were mainly caused by the strong leaching potential from intensive rainfall and weathering of the fine-grained parent rocks. The clay mineralogy reflected the clear weathering trend of the soils along the toposequence: (1) the soils on the summit and shoulder contained smectite and serpentine, which are predominant in the young soils derived from serpentinitic rocks; and (2) vermiculite gradually increased in the relatively old soils on backslope and footslope. The mineralogical transformations observed along the toposequence indicated that chlorite and serpentine, initially present in the Entisol on the summit, weather into smectite and interstratified chlorite-vermiculite in the intermediate soil on the shoulder under strong leaching and oxidizing conditions. Furthermore, vermiculite formed as the major weathering product of chlorite and smectite in the soil developed on the backslope. In addition to vermiculite, kaolinite and quartz formed in the soils on the footslope with the greatest concentration of Fed along the toposequence.

The influence of dissolved species and particle morphology on the electrokinetic behavior and the initial yield stress values of Cuban lateritic aqueous suspensions was studied. The lateritic samples were mixtures of serpentine and goethite in different relative proportions. The addition of silicate and Mg ionic species, which are normally found in natural waters used in industrial processes, affected the electrokinetic and flow behavior of the lateritic suspensions. Specific adsorption of these species on particle surfaces was shown by a shift of the isoelectric point and the maximum of the initial yield stress to more acidic pH (Si ionic species adsorption) and more basic pH (Mg ionic species adsorption), when compared to suspensions containing only non-adsorbing electrolytes. The initial yield-stress values determined in samples consisting entirely of goethite varied from sample to sample. A detailed crystallochemical characterization revealed that these changes were associated with the axial ratio (i.e. ratio of particle length to width) of the mineral particles. Goethite samples with larger particle size (smaller number of particles for a given solid concentration) and greater axial ratios presented initial yield-stress values greater than those goethites with smaller particle size and lower axial ratio.

The first occurrence of Ni-rich stevensite found in the ophiolite complex of Othrys, Central Greece is described. The stevensite, which develops in cracks in a host serpentinite, formed at the expense of serpentine. Two varieties of stevensite have been described: a Mg-rich, Ni-poorer variety with 0.4–1.2 octahedral Ni atoms per half formula unit (p.h.f.u.) and a Ni-rich variety with >2 Ni atoms p.h.f.u. The layer charge in both varieties is −0.24 p.h.f.u.. Stevensite layers are completely separated when dispersed in dilute polyvinylpyrrolidone (PVP) solutions and begin to convert to talc after heating at 250°C for 90 min. Total conversion to talc is observed at 550°C. Formation of Ni-rich stevensite took place at ambient temperature during supergene processes. The scarcity of Ni-rich stevensite occurrences in nature is attributed to the metastability of smectite and to the analytical procedures used in previous studies. Stevensite is considered a phase containing domains with variable numbers of octahedral vacancies. A new experimental protocol is proposed for the determination of Ni-rich stevensite, based on a combination of XRD after solvation with various organic liquids and subsequent heating at 750°C.