Empirical reaction progress diagrams showing the trends of major element oxide concentrations (in g/cm3) as functions of bulk density for a diabase saprolite reveal discontinuities in the trends of Al2O3, MgO, H2O+, and nonextractable Fe2O3. The discontinuities coincide with discontinuities in the trends of (1) the kaolinite-smectite 001 peak-intensity ratio, (2) the smectite 002-001 peak intensity ratio, and (3) the smectite basal spacing as functions of bulk density. The discontinuities are apparently related to redox conditions in the weathering profile because they occur at a depth where siderite veins first appear in the saprolite. Oxidizing conditions in the upper part of the profile appear to have favored the formation of Ferich smectite over kaolinite, whereas reducing conditions deeper in the profile favored the formation of kaolinite over Al-rich smectite. These results indicate that where geochemical conditions favor retention of Fe over Al, smectites can form in preference to kaolinite or gibbsite even under conditions of strong leaching.

The mineralogy of the clay fraction was studied for soils and saprolite on two Blue Ridge Front mountain slopes. The clay fraction contained the weathering products of primary minerals in the mica gneiss and schist parent rocks. Gibbsite is most abundant in the saprolite and residual soil horizons, where only chemical weathering has been operable. In colluvial soil horizons, where both physical and chemical weathering have occurred, the clay fraction consists largely of comminuted primary phyllosilicates —muscovite, chlorite, and possibly biotite—and their weathering products: vermiculite, interstratified biotite/vermiculite (B/V), and kaolinite. The clay-size chlorite contains Fe2+ as indicated by Mössbauer spectroscopy, and is more resistant to weathering than biotite. The vermiculite and B/V, both weathering products of biotite, contain Fe3+. Vermiculite in colluvial soils and, especially, surface horizons is weakly hydroxy-interlayered. The kaolinite in the clay fraction resulted at least partly from the comminution of kaolinized biotite in coarser fractions.

The hematite content ranged from 0 to 8% of the clay fraction and strongly correlates (r =.95) with dry clay redness, as measured by the redness rating: RR = (10 - YR hue) × (chroma) ÷ (value). The hematite is largely a product of the weathering of almandine; thus, the soil redness is dependent on the amount of almandine in the parent materials and its degree of weathering in the soils. Goethite (13–22% of the clay fraction) imparts a yellow-brown hue to soils derived from almandine-free parent rocks. The release of Fe in relatively low concentrations during the weathering of Fe-bearing primary minerals, particularly biotite, appears to have promoted the formation of goethite.

An end-member natroalunite of composition (K0.04Na0.96)(Al2.98Fe0.02)(SO4)2(OH)6 occurs in saprolitic clays in a laterite profile developed over Deccan Trap basalts at Matanumad in the Kutch region of Gujarath state, India. Pseudocubic natroalunite crystals, ranging from 0.5 to 1 µm in size, have replaced pseudohexagonal, 2-µm-size platelets of kaolinite. The reverse reaction (kaolinization of natroalunite) has also been observed in the laterites and bauxites immediately adjacent to the saprolite. The kaolinite crystallites that replaced natroalunite occur as a mosaic of anhedral platelets that range in size from 0.1 to 0.3 µm. The formation of natroalunite at Matanumad postdates the laterization of the basalts. It is related to the post-diagenetic formation of sulfuric acid-rich solutions by the oxidation of pyrite in the Eocene-Oligocene black shales of the Matanumad basin. The kaolinitic clays at the top of the saprolite zone have been intensely altered by the acidic solutions, leading to a high concentration of natroalunite.

Gabbroic saprolite at Mont Mégantic, Quebec, was studied in detail mineralogically to gain a better understanding of the origin of ferruginous smectite reported previously from these rocks. The parent rock is a ferrogabbro composed of plagioclase, augite, calc-alkalic amphiboles, biotite, olivine, magnetite, ilmenite, and apatite. Extensive weathering has decomposed most of the mafic minerals and magnetite to goethite, lepidocrocite, and iron-rich clay minerals, which occur in numerous microcracks distributed irregularly in the outer shells of boulders and in cracks and fissures in the bedrock. Some of the felsic minerals have altered to kaolinite. The secondary minerals and the more resistant primary minerals, such as plagioclase, ilmenite, and apatite, have subsequently moved to the lower part of the saprolite. The major ferruginous clay minerals present are smectite and vermiculite, which are compositionally similar, except that the smectite is slightly richer in SiO2 and MgO and poorer in Fe2O3 than the vermiculite.

To establish a structural formula for the ferruginous smectite, the oxidation state of Fe in a sample treated with dithionite-citrate-bicarbonate was examined by Mössbauer spectroscopy. All structural iron was found to be ferric. The calculated structural formula of a Na-saturated sample is:

$$N{a_{0.61}}\left( {M{g_{1.39}} + Fe_{0.85}^{3 + }A{l_{0.17}}M{n_{0.03}}} \right)\left( {S{i_{3.49}}A{l_{0.51}}} \right){O_{10}}{\left( {OH} \right)_2},$$

The weathering products of primary biotite, chlorite, magnetite, and almandine in mica gneiss and schist in the North Carolina Blue Ridge Front were determined. Sand-size grains of biotite, the most abundant, readily weathered mineral in the parent rock, have altered to interstratified biotite/vermiculite, vermiculite, kaolinite, and gibbsite in the saprolite and soil. Fe2+-chlorite in the parent rock was relatively resistant to chemical weathering, which appears to be confined to the external surfaces of particles. Magnetite grains in the saprolite are essentially unaltered, but those in the soil contain abundant crystallographically controlled etch pits and are coated with oxidation crusts. Almandine altered to goethite, hematite, and gibbsite as the rock weathered to saprolite. Extensively weathered almandine grains were found to contain etch pits and what appeared to be oxide coatings. Apparently, a rapid release of Fe during weathering produced hematite, whereas slower release of Fe favored the formation of goethite.

The clay particles in a kaolin deposit from Brazil were investigated by X-ray diffraction (XRD), differential thermal analysis (DTA), analytical transmission electron microscopy (ATEM), and electron paramagnetic resonance (EPR) to examine the relationships between morphological and chemical properties of the crystals and to relate these properties to formation conditions. The XRD patterns show the dominant presence of kaolinite with minor amounts of gibbsite, illite, quartz, goethite, hematite, and anatase. ATEM observations show two discontinuities in the deposit as indicated by changes in morphology and size of the kaolinite crystals. At the base of the deposit, hexagonal platy and lath-shaped particles (mean area of 001 face = 0.26 μm2) maintain the original fabric of the parent rock which characterizes an in situ evolution. In the middle of the deposit a bimodal population of large (mean area of 001 face > 0.05 μm2) and small (mean area of 001 face < 0.05 μm2) sub-hexagonal platy kaolinite crystals occurs. This zone defines the boundary between the saprolitic kaolinite and the pedogenic kaolinite. Near the top of the profile, laths and irregular plates of kaolinite, together with sub-hexagonal particles, define two different depositional sources in the history of formation of the deposit. Crystal thickness as derived from the width of basal reflections and the Hinckley index are compatible with the morphological results, but show only one discontinuity. At the base of the deposit, kaolinite has a low- defect density whereas in the middle and at the top of the profile, kaolinite has a high-defect density. Likewise, EPR spectroscopy shows typical spectra of low-defect kaolinite for the bottom of the deposit and typical spectra of high-defect kaolinite for the other portions of the deposit. Despite the morphological changes observed through the profile, the elemental composition of individual kaolinite crystals did not show systematic variations. These results are consistent with the deposit consisting of a transported pedogenic kaolinite over saprolite consisting of in situ kaolinized phyllite.

Widespread lateritized ultramafic rocks in the southern part of the Muratdağı region of Turkey constitute a significant source of Ni-Cr-bearing ore with economic potential. However, no mineralogical or geochemical characterizations of these important materials have been performed previously. The aim of the present study was to describe the mineralogy, geochemistry, and genesis of Ni-Cr-bearing smectite in garnierite and ferruginous saprolite associated with the lateritized ophiolite-related ultramafic rocks. The lateritic zones are well developed over serpentinized harzburgitic mantle peridotites. The lateritized units and related bedrocks were examined using polarized-light microscopy, X-ray diffraction, scanning and transmission electron microscopies, and chemical and isotopic methods. The garnierite-containing saprolites are enriched in smectite, Fe-(oxyhydr)oxide phases, and opal-CT. Micromorphological images revealed that flaky smectite and, locally, Fe-rich particles, alunite, gypsum, gibbsite, and sulfur crystals developed along the fractures and dissolution voids. The development of saprolite demonstrates chemical weathering. The presence of silicified and Fe-(oxyhydr)oxide phases associated with gypsum, alunite, and local native sulfur in vertical and/or subvertical fractures and fault infillings are indicative of hydrothermal processes along the extensional, tectonically related fault systems. Chemical weathering and hydrothermal processes, which probably started during the Oligocene and Miocene, led to the formation of nontronite, Fe-bearing montmorillonite, and local Fe-rich kaolinite. Nickel and Cr are concentrated significantly in the saprolite zone and are positively correlated with Fe2O3 content, which is controlled by the formation of nontronite, montmorillonite, and Fe-(oxyhydr)oxide phases. Nickel-Cr-bearing nontronite and montmorillonite precipitated from alkaline water as a result of the increasing (Fe2O3+Al2O3+Cr2O5+Ni+Co)/(MgO+SiO2) ratio under the control of both chemical weathering and hydrothermal processes. The Fe and Mg (associated with Ni and Cr) required for the formation of smectite were supplied by solutions from both chemical weathering and hydrothermal alteration of Ni-Cr-bearing olivine and pyroxene in the harzburgitic bedrock; the Al was supplied by schists, granite, and volcanic units.

Secondary surface layers form by replacement of almandine garnet during chemical weathering. This study tested the hypothesis that the kinetic role of almandine’s weathering products, and the consequent relationships of primary-mineral surface texture and specific assemblages of secondary minerals, both vary with the solid-solution-controlled variations in Fe and Al contents of the specific almandine experiencing weathering.

Surface layers are protective (PSL) when the volume of the products formed by replacement is greater than or equal to the volume of the reactants replaced. Under such circumstances, reaction kinetics at the interface between the garnet and the replacing mineral are transport controlled and either transport of solvents or other reactants to, or products from, the dissolving mineral is rate limiting. Beneath PSLs, almandine garnet surfaces are smooth, rounded, and featureless. Surface layers are unprotective (USL) when the volume of the products formed by replacement is less than the volume of the reactants replaced. Under such circumstances, reaction kinetics at the interface between the garnet and the replacing mineral are interface controlled and the detachment of ions or molecules from the mineral surface is rate limiting. Almandine garnet surfaces beneath USLs exhibit crystallographically oriented etch pits. However, contrary to expectations, etch pits occur on almandine garnet grains beneath some layers consisting of mineral assemblages consistent with PSLs.

Based on the Pilling-Bedworth criterion, surface layers are more likely to be protective over a broad range of reactant-mineral compositions when they contain goethite, kaolinite, and pyrolusite. However, this combination requires specific ranges of Fe and Al content of the natural reacting almandine garnet. To form a PSL of goethite and kaolinite, an almandine garnet must have a minimum Al stoichiometric coefficient of ~3.75 a.p.f.u., and a minimum Fe stoichiometric coefficient of ~2.7 a.p.f.u.

Product minerals also influence the mobility of the least-mobile major rock-forming elements. A PSL consisting of goethite, gibbsite, and kaolinite yields excess Al for export during almandine garnet weathering. As the quantity of kaolinite present in the PSL decreases, the amounts of Al available for export increases.

The Brittany region of France is located in a low seismicity intraplate zone. Most of the instrumented earthquakes are limited to a shallow crustal depth without surface rupture. A paleoseismological analysis was performed on deposits on the Crozon Peninsula and in the Elorn estuary. We highlight hydroplastic deformations induced by liquefaction leading to clay diapirism, which were likely triggered by past earthquakes. This diapirism seems to be frequent in continental nonconsolidated sediments and to develop on the inherited tectonic structures, when a shallow water table and confining layers exist. Timing of deformation is dated using paleoenvironmental data, and electron spin resonance and infrared-stimulated luminescence dating methods. Two seismic periods were identified in western Europe during early Marine Oxygen Isotope Stage (MIS) 10 (~380 ka) and early MIS 8 (~280–265 ka). The lack of similar deformations affecting the Holocene tidal deposits in the Bay of Brest suggests that the magnitude of the triggering paleoearthquakes is probably higher (Mw ~6) than the recent events (Mw 5.4). These unusual intraplate major paleoearthquakes need specific factors affecting the far-field crustal stress loading to be triggered, such as a brief acceleration of the Africa-Eurasia lithospheric plate convergence, glacio-isostatic stress perturbations associated with the onset of major glaciations in northern Europe, or other processes induced by orbital forcing.

Widespread chemical weathering of Tertiary alkaline volcanic rocks in the vicinity of Trabzon (NE Turkey) has led to the formation of well developed reddish-brown saprolite. These saprolites are dominated by kaolin minerals (predominantly kaolinite), with minor quantities of halloysite, pyrophyllite, chlorite, smectite, hematite and illite. Other common minerals are opal-CT, quartz and occasional feldspar, clinopyroxene, hematite and Fe-Mn-Ti oxides. The significant degree of fracturing and fragmentation of primary igneous minerals within the pyroclastic units, and subsequent penetration by reactive fluids, resulted in partial chloritization of clinopyroxene, albitization of plagioclase and precipitation of Fe-Mn-Ti-oxides within fractures and dissolution voids. Kaolin minerals occur as spongy fibrous meshes surrounding partly dissolved volcanic glass and devitrified sub-rounded grains that may be relict allophane. Field observations and mineralogical-chemical characteristics indicate that the alteration of the Tertiary volcanic units of the Black Sea region was controlled by chemical weathering during circulation of oxic meteoric or mixed meteoric and marine waters. Intense fracturing and fragmentation of the pyroclastic rocks resulted in the formation of clay minerals in the upper saprolite level under acidic and largely oxidizing conditions. Incongruent dissolution was associated with the leaching and downward transport of Si, Na, K and Ca and the concentration of Al. In such an environment, early precipitation of halloysite from allophane was probable, and it gradually transformed into kaolinite during aging and progressive weathering.

This paper describes the ways in which the major rock-forming primary minerals (olivine, pyroxenes, amphiboles, feldspars, micas and chlorites) break down during weathering, the products that develop during this breakdown and the rates at which this breakdown occurs. The perspective chosen to illustrate this vast topic is that of the residual soil weathering profile. Different physical and chemical conditions characterize the various parts of such a profile. Thus, in the slightly weathered rock at the base of the profile, mineral weathering will take place in microfissures and narrow solution channels and the capillary water in such spatially restricted volumes may be expected to be close to equilibrium with the primary mineral. In these circumstances, the weathering product formed may be closely related to the primary mineral both compositionally and structurally. The saprolite higher up in the weathering profile may or may not retain the fabric and structure of the original parent rock, but in either case the close relationship observed between primary mineral and weathering product in the slightly weathered rock may be lost. This part of the profile will usually be affected by freely flowing drainage waters, the composition of which will be far from equilibrium with specific primary minerals. Weathering products which do form are likely to reflect the interaction between bulk water and bulk parent material. In the soil profile, the situation will be further complicated by organic ligands derived from decomposing organic matter or from the direct activities of soil microbes or plant roots. Thus, biological weathering will assume a much greater significance in this part of the profile compared with the mainly inorganic processes dominating in the saprolite and the slightly weathered rock. The general nature of any particular weathering profile will reflect the interactions between climate, topography, parent material, soil biota and time and superimposed upon this complexity, when considering how individual primary minerals break down in detail, will be factors related to the nature of the mineral itself. Particularly important in this respect is the inherent susceptibility of the mineral to weathering, which is related to overall chemical composition and structure, as well as the distribution and density of defects, dislocations and exsolution features, which often control the progress of the weathering reaction.

Ka Loko Dam, in Kauai, Hawaii, failed suddenly and catastrophically on March 14, 2006. The resulting breachwas marked by three topographic benches, the lowest of which exposed native volcanic deposits once resident in the dam foundation. These deposits were found to contain outcrops of a waxy, gel-like material that appeared to result from in situ weathering processes. This unusual material was found to be highly enriched in halloysite. Gravel-size pieces in the hydraulic fill of the embankment derived from these materials also exhibited significant in situ weathering and significant halloysite content. Engineers and geologists generally recognize that bedrock materials weather progressively into soil constituents over ‘geological time’, and that this process is accelerated in tropical environments. Still, the strength, stiffness and durability of bedrock, earth and embankment materials are not expected to vary significantly over the geologically short life of a dam. In the case of Ka Loko Dam, however, the volcaniclastic sediments that comprise the local bedrock experienced substantial in situ weathering over its geologically brief 115-year operational lifetime. Prolonged exposure to seepage of anoxic water weathered the sediments completely to saprolite, including weak, sensitive, fine, spherical halloysite.