Distribution Of Secondary Minerals Research Articles

Analysis of Distribution of Secondary Minerals and their Associations on The Surface of Diamonds and in Derrivative Products of Metasomatically Altered Kimberlites

The theoretical and experimental studies resulted in establishing regularities in the distribution of secondary minerals and theirassociations in metasomatically altered diamond-bearing kimberlites and products of their processing.Based on integrated mineralogical research, it was found out that the composition of the altered kimberlites and the fine-dispersedclayey slurries formed during their processing constituted the basis of hydrophilic formations on the surface of diamond crystals notrecovered by the methods of grease and froth separation. Particles of these minerals concentrate in fine-dispersed slurry productsof kimberlite ore processing, interact with the crystal surface, reduce their hydrophobic properties and, accordingly, recovery inprocesses of grease and froth separations into concentrates.

Geochemical characteristics and behaviour of elements during weathering and pedogenesis over karst area in Yunnan–Guizhou Plateau, southwestern China

A study of two typical red soil profiles, Gumu (GM-limestone) and Sanjiacun (SJC-dolomite) was conducted at Yunnan–Guizhou Plateau, China. The study employed mineralogy, geochemical and elemental ratios to assess the mobilisation and distribution of elements during weathering and pedogenesis. The mineralogy results revealed similarity in distribution of secondary minerals in both profiles, with exception at the ferricrete layer. The results revealed inheritance relationship between red soils and their underlying bedrocks indicating that clay minerals were likely derived from in situ weathering products or metasomatic process rather than detrital process. During weathering and pedogenesis process, alkaline elements such as K and Rb behave similarly and were closely related. All lanthanide elements except for Ce shared similar transport trend with Y and Be; however, Be exhibited significant enrichment at ferricrete layer of the dolomite profile. Transitional elements such as Co and Mn behave similarly; while, the High Field Strength Elements (HFSE) were stable, except for Th and Sc which indicate depletion in upper soil layer of the dolomite profile. Our findings further revealed that both profiles have experienced intense chemical weathering with high Chemical Index of Alteration values (CIA: 83–93) and a constant Ba/Sr ratio > 2. Overall, both profiles experienced similar elemental behaviour with the accumulation of major elements, the leaching of alkaline earth elements and the fluctuation of trace elements including Rare earth elements (REEs). The elemental behaviour was primarily controlled by redox reaction, pH, water availability, ionic radii and the stability of secondary oxides/hydroxides. Our study further highlights the use of newly proposed geochemical proxy Fe/Na ratio, which revealed high correlation (R2 ≈ 0.90) with U, Ba, Zn, Cu, Mo, Ni, Sb and REEs of power functions. These findings indicate that Fe/Na ratio may be used to investigate the behaviour of trace elements including REEs in highly weathered soils.

Mineralogical Study and Genetic Model of Efflorescent Salts and Crusts from Two Abandoned Tailings in the Taxco Mining District, Guerrero (Mexico)

The mineralogy of surface crusts and efflorescent salts of two old abandoned tailings (Xochula and Remedios) of the mining district of Taxco, Guerrero, was studied by short-wave infrared spectroscopy (SWIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The chemistry of the selected samples was studied with induced coupled plasma mass spectrometry (ICP–MS). Principal mineralogy is composed of hydrated sulfates of iron (di-trivalent), other less-soluble sulfates such as gypsum and jarosite, and oxides–hydroxides of iron, all of which are the result of the weathering of sulfides (mainly pyrite) accumulated in mining waste. Using quantitative X-ray diffraction (Rietveld method) and the spatial distribution of secondary minerals, two main zones (lateral and central) have been established in both tailings. The genetic model for their formation shows that the secondary iron minerals (sulfates, oxides, and hydroxides) in these deposits have been formed at least in three different stages, mainly by oxidation, dissolution, and precipitation processes in combination with dehydration and primary sulfides neutralization reactions. The chemical analyses of some salts confirm the presence of potentially toxic elements (PTE) in their structure and therefore indicate that the dissolution of the efflorescences in these deposits is a potential source of contamination during the wet season.

Effects of climate on distribution of soil secondary minerals in volcanic regions of Tanzania

In the volcanic regions of Tanzania, including Mt. Kilimanjaro, Mt. Meru, and Mt. Kieyo, soils are formed on relatively recent tephra deposits (<0.45 Ma). Because of the diversity of climate caused by the volcanic terrain in these regions and different parent material compositions, various soil secondary minerals were expected to form. The objectives of this study were to determine the distribution of secondary minerals in soils formed on tephras in the volcanic regions of Tanzania and to estimate the effects of climate and parent material composition on the distribution of secondary minerals. A total of 30 soil samples from the B-horizon from different parent materials and under different climatic conditions (540–2800 m altitude) were collected. The soils were classified into five groups depending on location and considering the total elemental contents. Selective extraction methods, X-ray diffraction, and differential thermal analyses were applied to determine the composition of the secondary minerals. The composition of the soil water extract was determined to assess the stability of the minerals. Excess precipitation (EP), as an index to evaluate the soil water regime, was calculated by subtracting potential evapotranspiration from precipitation. The soils in Mt. Kilimanjaro and Mt. Meru were rich in Fe or Mg, whereas soils in Mt. Kieyo were rich in Si, indicating mafic and felsic parent materials, respectively. At the low moisture sites (EP < 500 mm), Alo + 1/2Feo (the sum of acid oxalate soluble Al and Fe) were low and increased with EP. Meanwhile, the values of Alo + 1/2Feo negatively correlated with mean average temperature (MAT) for all sites. At low moisture sites, low EP would be favorable for the crystallization of nanocrystalline Al and Fe. At the sites with sufficient moisture, the temperature was the primary factor controlling the distribution of nanocrystalline minerals. Furthermore, gibbsite was found at the sites with high EP (EP > 500 mm) with low H4SiO40 activities. More kaolinite was detected at the low elevation sites with high H4SiO40 activities in soil solutions. Soil moisture and temperature are critical factors influencing the distribution of secondary minerals, whereas parent material compositions have less effect on soils in the volcanic regions of Tanzania.

Parent Materials and Climate Control Secondary Mineral Distributions in Soils of Kalimantan, Indonesia

Core Ideas A unified interpretation of clay mineral distributions in Kalimantan was achieved. The conditions under which soil clay minerals formed in Kalimantan were clarified. Systematic sampling, considering parent material and climate, allowed clear analysis. Thermodynamic analysis of clay mineral stability was performed. The clarified mineralogy will allow better ecological and agricultural understanding. It is important to clarify which factors control the distribution of secondary minerals because secondary minerals affect soil physicochemical properties. This study investigated the effects of parent materials and climate at different elevations on secondary mineral distributions in soils under a humid tropical climate with a hilly landscape and without recent volcanic activity. Soil material from B horizons was sampled at 60 sites on gentle slopes at different elevations (20–1700 m altitude). Each major parent material (sedimentary, felsic, and intermediate to mafic) was represented at different elevations. The soil samples were classified based on their total elemental compositions using cluster analysis. Secondary minerals were measured by X‐ray diffraction and selective extractions. The samples were divided into ferric (high Fe contents), K&amp;Mg (high K, Mg, and Si), and silicic (high Si) groups. The ferric soils were derived from mafic parent materials and characterized by high Fe oxides and gibbsite contents. The K&amp;Mg and silicic soils were derived from felsic or sedimentary parent materials and had similar secondary mineral (kaolinite and vermiculite) contents, although more mica was found in the former group. Climate affected secondary mineral distributions only in the silicic group soils; higher temperatures at lower elevations may enhance weathering and increase kaolinite and crystalline Al and Fe contents. The hilly landscape of this study is different from those of shields in tropical humid regions in Africa or South America. Here, secondary mineral distributions were primarily determined by parent material (mafic vs. felsic or sedimentary) and secondarily by climate, which varied with elevation.

Distribution of secondary minerals in crusts developed on sandstone exposures

ABSTRACTPrevious research on rock weathering crusts has revealed their large variability depending on the type of host rocks and development of weathering processes. The composition of crusts developed on natural sandstone exposures is less documented in the literature in comparison to those developed on architectonic stones. In both cases, previous research has focused mainly on the progress of salt weathering. This study considers the surfaces of sandstone tors in the Polish Outer Carpathians. The exposed parts of the rocks in this area are often covered by crust, which is up to several centimetres thick, and differs from the internal part in colour and composition. The crusts were characterized using light and electron microscopy, X‐ray diffractometry, thermal analyses, Mössbauer spectroscopy, bulk chemical analyses and sequential chemical extractions. Porosity was estimated by digital image processing. The following two hardened zones were observed: (1) thin (up to 30 µm), black, external layer, rich in carbon and composed of opal‐type silica, covered in places by sulphate incrustations and numerous spherical particles of anthropogenic origin; (2) thicker (up to several millimetres), internal part composed of a set of laminae of variable colouration, enriched in iron (oxyhydr)oxides (goethite and hematite) in comparison to the rock interior. Development of the crust results from silicon and iron redistribution during the sandstone alteration. The chief source of silica is hydrolysis of aluminosilicates, whilst that of iron is decomposition of aluminosilicates, carbonates and sulphides. Hematite is probably a result of goethite transformation. However, air pollutants may play an important role in the formation of sulphates. Silica and iron compounds affect the properties of the rock, hardening the surface and lowering porosity by formation of secondary cement. Crystallization of sulphate salts, in turn, may contribute to mechanical disintegration of the rock. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Martian subsurface fluid pathways and 3D mineralogy of the Nakhla meteorite

The three dimensional structure of the Nakhla meteorite has been investigated in order to provide a detailed picture of the fluid pathways in the volcanic subsurface of Mars. A combination of computed tomography and electron microscopy have been used to identify, characterise and interpret the distribution, size, interconnectivity and secondary mineralisation of fractures through which water flowed. Secondary minerals which formed during aqueous alteration are found as fracture-filling silicates in olivine grains and a range of carbonates, sulfates and halite are found throughout olivine, pyroxene and mesostasis. The fractures which acted as fluid pathways are highly interconnected, branching and reconnecting at multiple locations, leading to a pervasive and homogeneous distribution of secondary minerals throughout the sample. Miniature topographic basins have been identified and provide a unique method of identifying the true orientation of the meteorite while on its host planetary surface. Halite and sulfate are found to be related to one another, likely formed in the same episode of fluid flow, and are found to postdate both carbonate and olivine-hosted silicate-alteration products. Later episodes of fluid flow may have interacted with, and potentially eroded, earlier generations of secondary minerals. Nakhla, and by implication the other nakhlite meteorites, therefore preserves a complex record of multiple episodes of fluid flow on Mars in the past billion years.

THE REDISTRIBUTION OF RARE-EARTH ELEMENTS IN SECONDARY MINERALS OF HYDROTHERMAL VEINS, SCHWARZWALD, SOUTHWESTERN GERMANY

Minerals of the rare-earth elements (REE) occur as supergene phases in the Schwarzwald ore district, southwestern Germany. They form by alteration of hydrothermal fluorite – barite – quartz – carbonate veins with various associations, including Cu– Pb, Pb–Zn and Co–Bi–Ag–U assemblages in sandstones, gneisses and granites. The REE minerals, including mixite-group minerals ([REE,Bi,Ca,Pb]Cu 6 (AsO 4 ,AsO 3 OH) 3 OH 6 •3H 2 O), rhabdophane and churchite (REEPO 4 •H 2 O and REEPO 4 •2H 2 O), chukhrovite (Ca 3 REEAl 2 SO 4 F 13 •10H 2 O) and bastnasite (REECO 3 F), were analyzed by electron microprobe and LA–ICP–MS. In addition, REE concentrations in secondary fluorite, calcite and Mn oxides cogenetic with the REE minerals were determined by LA–ICP–MS. Results of analyses of 74 mixite-group samples from 20 localities in the Schwarzwald ore district show that continuous miscibility is possible between REE and Ca and between Bi and Ca end-members. In contrast, no miscibility seems to exist between the Bi and REE end-members, and only Ca-rich members can accommodate small amounts of both Bi and REE. The REE phosphates churchite and rhabdophane do not occur at the same locality in the Schwarzwald, which is probably dependent on which REE (light or heavy) predominate at a certain locality. Whereas churchite incorporates heavy REE (HREE), rhabdophane prefers light REE (LREE). Dependent on the source of the REE, either HREE or LREE dominate in an alteration fluid and, consequently, only one type of REE phosphate forms. The HREE dominate in such a fluid if REE originate from dissolved “pitchblende”, whereas the LREE dominate if the altered host-rock or dissolved fluorite are the source of the REE. The REE contents of Mn oxides and calcite cogenetic with REE-bearing mixite-group minerals show that the REE distribution of secondary minerals can be influenced by each other. Whereas the Mn oxides incorporate Ce 4+ resulting in a positive Ce anomaly, a cogenetic mixite-group mineral develops a negative anomaly, which shows that Ce anomalies are coupled. Calcite intergrown with a mixite-group mineral can only incorporate those REE that are not consumed by the latter. These processes involve the distribution of REE between minerals on the micrometer scale and do not take place until minerals precipitate. However, sorption and complexation processes also take place during transport on a larger scale and produce decoupled Ce anomalies. This interplay between large- and small-scale processes results in a complex redistribution of REE during remobilization.

Near Infrared Analysis of Fossil Bone from the Western Cape of South Africa

The burial environment of a fossil bone may be poorly suited to long-term preservation of important chemical constituents. Studies of fossil bone chemistry consequently benefit from knowledge of burial environmental change (burial history). Several techniques have been developed to describe alteration of fossil bone, including measurements of crystallinity using infrared spectroscopy. The utility of near infrared (NIR) spectroscopy was investigated for describing the burial history of fossil bone. Spectra from large sample areas (bulk NIR) identified secondary calcite and clay within fossil bone, and were useful for distinguishing fossils from different localities. Spectra from small sample areas (hyperspectral NIR) were used to map constituents within fossil bones. Calcite and clay pervaded the internal cavities of fossil horn cores, indicating saturation with groundwater. The identification and distribution of secondary minerals is relevant for studies of taphonomic process, hence NIR is a useful supplement to existing techniques for assessing the burial history of fossil bone.

Numerical modeling of injection and mineral trapping of CO 2 with H 2S and SO 2 in a sandstone formation

Carbon dioxide (CO 2) injection into deep geologic formations could decrease the atmospheric accumulation of this gas from anthropogenic sources. Furthermore, by co-injecting H 2S or SO 2, the products respectively of coal gasification or combustion, with captured CO 2, problems associated with surface disposal would be mitigated. We developed models that simulate the co-injection of H 2S or SO 2 with CO 2 into an arkose formation at a depth of about 2 km and 75 °C. The hydrogeology and mineralogy of the injected formation are typical of those encountered in Gulf Coast aquifers of the United States. Six numerical simulations of a simplified 1-D radial region surrounding the injection well were performed. The injection of CO 2 alone or co-injection with SO 2 or H 2S results in a concentrically zoned distribution of secondary minerals surrounding a leached and acidified region adjacent to the injection well. Co-injection of SO 2 with CO 2 results in a larger and more strongly acidified zone, and alteration differs substantially from that caused by the co-injection of H 2S or injection of CO 2 alone. Precipitation of carbonates occurs within a higher pH (pH > 5) peripheral zone. Significant quantities of CO 2 are sequestered by ankerite, dawsonite, and lesser siderite. The CO 2 mineral-trapping capacity of the formation can attain 40–50 kg/m 3 medium for the selected arkose. In contrast, secondary sulfates precipitate at lower pH (pH < 5) within the acidified zone. Most of the injected SO 2 is transformed and immobilized through alunite precipitation with lesser amounts of anhydrite and minor quantities of pyrite. The dissolved CO 2 increases with time (enhanced solubility trapping). The mineral alteration induced by injection of CO 2 with either SO 2 or H 2S leads to corresponding changes in porosity. Significant increases in porosity occur in the acidified zones where mineral dissolution dominates. With co-injection of SO 2, the porosity increases from an initial 0.3 to 0.43 after 100 years. However, within the CO 2 mineral-trapping zone, the porosity decreases to about 0.28 for both cases, because of the addition of CO 2 mass as secondary carbonates to the rock matrix. Precipitation of sulfates at the acidification front causes porosity to decrease to 0.23. The limited information currently available on the mineralogy of naturally occurring high-pressure CO 2 reservoirs is generally consistent with our simulations.

Controls on the Karaha–Telaga Bodas geothermal reservoir, Indonesia

Karaha–Telaga Bodas is a partially vapor-dominated, fracture-controlled geothermal system located adjacent to Galunggung Volcano in western Java, Indonesia. The geothermal system consists of: (1) a caprock, ranging from several hundred to 1600 m in thickness, and characterized by a steep, conductive temperature gradient and low permeability; (2) an underlying vapor-dominated zone that extends below sea level; and (3) a deep liquid-dominated zone with measured temperatures up to 353 °C. Heat is provided by a tabular granodiorite stock encountered at about 3 km depth. A structural analysis of the geothermal system shows that the effective base of the reservoir is controlled either by the boundary between brittle and ductile deformational regimes or by the closure and collapse of fractures within volcanic rocks located above the brittle/ductile transition. The base of the caprock is determined by the distribution of initially low-permeability lithologies above the reservoir; the extent of pervasive clay alteration that has significantly reduced primary rock permeabilities; the distribution of secondary minerals deposited by descending waters; and, locally, by a downward change from a strike-slip to an extensional stress regime. Fluid-producing zones are controlled by both matrix and fracture permeabilities. High matrix permeabilities are associated with lacustrine, pyroclastic, and epiclastic deposits. Productive fractures are those showing the greatest tendency to slip and dilate under the present-day stress conditions. Although the reservoir appears to be in pressure communication across its length, fluid, and gas chemistries vary laterally, suggesting the presence of isolated convection cells.

George Patrick Leonard Walker

George Patrick Leonard Walker

Origin, timing, and temperature of secondary calcite–silica mineral formation at Yucca Mountain, Nevada

The origin of secondary calcite–silica minerals in primary and secondary porosity of the host Miocene tuffs at Yucca Mountain has been hotly debated during the last decade. Proponents of a high-level nuclear waste repository beneath Yucca Mountain have interpreted the secondary minerals to have formed from cool, descending meteoric fluids in the vadose zone; critics, citing the presence of two-phase fluid inclusions, argued that the minerals could only have formed in the phreatic zone from ascending hydrothermal fluids. Understanding the origin, temperature, and timing of these minerals is critical in characterizing geologically recent fluid flux at the site, and has significant implications to whether waste should be stored at Yucca Mountain. Petrographic and paragenetic studies of 155 samples collected from the Exploratory Studies Facility (ESF) and repository block cross drift (ECRB) tunnels indicate that heterogeneously distributed calcite with lesser chalcedony, quartz, opal, and fluorite comprise the oldest secondary minerals. These are typically overgrown by intermediate-aged calcite, often exhibiting distinctive bladed habits. The youngest event recorded across the site is the deposition of Mg-enriched (up to ∼1 wt%) and depleted, growth-zoned calcite intergrown with U-enriched opal. The cyclical variation in Mg enrichment and depletion is probably related to climate changes that have occurred during the last few million years. The distribution of secondary minerals is consistent with precipitation in the vadose zone. Fluid inclusion petrography of sections from the 155 samples determined that 96% of the fluid inclusion assemblages (FIAs) contained liquid-only inclusions that formed at ambient temperatures (<35°C). However, 50% of the samples (n = 78) contained relatively rare FIA that contain both liquid-only and liquid plus vapor inclusions (herein termed two-phase FIAs) that formed at temperatures above 35°C. Virtually all of these two-phase FIAs occur in paragenetically old calcite; rare two-phase inclusion assemblages were also observed in early fluorite and quartz, and early–intermediate calcite. Homogenization temperatures (≡ trapping temperatures) across Yucca Mountain are generally 45 to 60°C, but higher temperatures reaching 83°C were recorded in calcite from the north portal and ramp of the ESF. Cooler temperatures of ∼35 to 45°C were recorded in the intensely fractured zone. Multiple populations of two-phase FIAs from lithophysal cavities in the ESF and ECRB cross drift indicate early fluid cooling with time from temperatures >45°C in early calcite, to <35 to 45°C in paragenetically younger calcite. Freezing point depressions range from −0.2 to −1.6°C, indicating trapping of a low salinity fluid. The majority of intermediate calcite and all outermost Mg-enriched calcite contains rare all-liquid inclusions and formed from ambient temperature (<35°C) fluids. Carbon and oxygen isotope data reveal a consistent trend of decreasing δ 13C (from 9.5 to −8.5‰) and increasing δ 18O (from 5.2 to 22.1‰) values from paragenetically early calcite to Mg-enriched growth-zoned calcite. Depleted δD values (−131 to −90‰) of inclusion fluids from intermediate and the youngest Mg-enriched calcite indicate derivation from surface meteoric fluids. Recalculation of δ 18O H 2 O values of −12 to −10‰ is consistent with derivation from paleometeoric fluids. Results of integrated U-Pb dating (opal and chalcedony) and fluid inclusion microthermometry indicate that two-phase FIAs that trapped fluids of >50°C are older than 6.29 ± 0.30 Ma. Two-phase FIAs in paragenetically later calcite, which formed from fluids of 35 to 45°C, are older than 5.32 ± 0.02 Ma. There is no evidence for trapping of fluids with elevated temperatures during the past 5.32 my. The youngest Mg-enriched calcite intergrown with opal began to precipitate between about 1.9 to 2.9 Ma and has continued to precipitate within the past half million years. The presence of liquid-only inclusions and the consistent occurrence of Mg-enriched calcite and opal as the youngest event indicate a minor, but chemically distinct, ambient temperature (<35°C) fluid flux during the past 2 to 3 my.

Mapping secondary mineral formation in porous media using heavy metal tracers

A new technique is presented which allows easy identification of secondary minerals, formed during experimental diagenesis. Reacting fluids are doped with Ba and Sr; heavy metals which substitute into the alkali‐bearing authigenic minerals that precipitate because of alteration of primary labradorite and quartz. The main secondary phases observed are a Ba‐rich zeolite mineral, strontianite, and a Ba‐rich calcite. When polished sections of postexperiment specimens are viewed using scanning electron microscope backscatter imaging, secondary phases appear as bright patches due to their higher density and average atomic number. The more difficult method of X‐ray elemental mapping confirms that these zones represent secondary minerals rich in Ba and Sr. This technique allows the spatial distribution of secondary minerals to be mapped out at the specimen scale in a short time frame. Image analysis shows that observed reductions in permeability are caused primarily by mineral precipitation in narrow pore throats and in areas containing an abundance of fine‐grained material. Secondary mineral formation is concentrated near the top of the sample and is interpreted to migrate through the sample as a reaction front. Mineral coverage is then quantified and used to explain the observed permeability evolution.

Mineral micro- and nano-variability revealed by combined micromorphology and in situ submicroscopy

A combination of micromorphology and different in situ sub microscopical techniques, performed on thin sections of soil, saprolite, and rock samples, was used to assess possible relationships between micro- and nano-variability and macro- and micro-environmental conditions of mineral transformation during weathering. Four examples, presented in this study illustrate variability in morphology, chemistry and mineralogy of secondary minerals present in pseudomorphs after primary silicate minerals in volcanic soils present in the humid tropics. The study was based on micromorphological characterization of secondary (clay) minerals, with complementary submicroscopical analysis such as scanning electron microscopy with energy dispersive X-ray analysis (SEM–EDXRA), performed on uncovered thin sections, step scan X-ray diffraction (SSXRD) and transmission electron diffraction (TEM–EDS) carried out either on disturbed powder samples or undisturbed microsamples, obtained by micro-drilling in thin sections. These techniques highlight a very complex mineralogy in pseudomorphs after primary minerals, including secondary minerals that generally are assessed to be formed under contrasting physicochemical conditions (gibbsite and 2:1 phyllosilicates in a pseudomorph after plagioclase; halloysite, kaolinite, smectite and aluminum and iron oxi-hydroxides in pseudomorphs after pyroxenes). The study by TEM of undisturbed microfabrics of secondary minerals in a clay pseudomorph after pyroxene revealed typical distribution patterns, with mineralogical compositions varying over some microns. The combination of techniques gave an improved insight on the complexity and variability of the nature composition and distribution of secondary minerals formed during weathering. It indicates the influence of sequential weathering processes on mineral micro- and nano-variability.

Petrological differentiation patterns and geomorphic distribution of ferricretes in Central Africa

Geomorphic distribution and petrological differentiation patterns of ferricretes widespread on landsurfaces were studied in the Dembia-Zemio area, southeastern Central African Republic. Four types of ferricretes are distributed on high plateaux, hillslopes and low plateaux. The main contrast corresponds to the differentiation between ferricretes of high plateaux rich in poorly hydrated minerals, hematite and kaolinite, and those covering hillslopes and low plateaux richer in hydrated minerals, goethite and gibbsite. A Principal Component Analysis of geochemical and mineralogical data was used to characterize the main differentiation patterns of ferricretes in relation with their petrographic facies and the geomorphic features of the landscape. The distribution of secondary minerals and their relationships to trace elements within and between the ferricrete types reflects differences in weathering and erosion processes. Our results document that the petrological patterns and the distribution of ferricretes are related to geomorphic features depending on hydroclimatic variations that govern the landscape evolution of many tropical shields.

Depth trends in magnetic properties in an area of prolonged cold seawater drawdown in uppermost Troodos-type oceanic crust

Study of the oxide minerals and measurements of the magnetic properties of about 100 samples distributed over a 475 m continuous drill-core section from the uppermost part of an extrusive sequence of the Troodos, Cyprus, ophiolite close to 35°02′N, 33°11′E, in the vicinity of Malounda, yields the following principal results: (1) Low-temperature alteration is intense to a depth of 260 m, below which it is relatively much subdued. (2) Saturation magnetization, natural remanence intensity, and initial susceptibility all increase continuously with depth throughout the section. (3) Curie temperature decreases with depth to 260 m, below which a change in the style of the thermal decay of magnetization behaviour takes place. (4) Pillowed and massive flows differ in their magnetic properties.The results are consistent with the downwards decrease in low-temperature alteration also noted by K. Gillis from the distribution of secondary minerals. The downwards variations in magnetic properties closely follow in both sense and magnitude the trends observed in DSDP hole 417A, where marked downwards decrease in low-temperature alteration also occurs. This comparison and the discontinuous nature of the lower units of the Troodos sedimentary cover in the vicinity of hole CY-1 support the interpretation of the upper, reddened facies of the Troodos extrusive sequence as being due to prolonged drawdown of cold seawater and suggest that the magnetization of at least the upper part of the Troodos ophiolite is a reliable guide to ocean crust magnetization.

Feldspar Weathering in Lateritic Saprolite

AbstractFeldspars in granitic saprolite in southwestern Australia have altered to halloysite, kaolinite, and gibbsite with no evidence of noncrystalline material. The secondary minerals are commonly present as intimate mixtures within altered feldspar grains, but discrete zones of gibbsite or halloysite-kaolinite also are present. Variations in the chemical microenvironment within micrometer-size zones in grains apparently controlled the type and distribution of secondary minerals. The alteration of both plagioclase and alkali feldspars involved congruent dissolution by soil solution and crystallization of halloysite, kaolinite, and gibbsite from this solution. Highly altered feldspar grains consist of etched feldspar fragments embedded within a highly porous framework of subhedral to euhedral platy crystals of kaolinite and gibbsite, or of spherical and felted aggregates of halloysite.

Weathering and morphogenesis in a tropical plateau

Weathering phenomena on the Campos do Jorda˜o Plateau have been analysed in order to show their relationship to geomorphic compartition and morphogenetic dynamics. Correlations between weathering characteristics of surficial materials and geomorphic compartments are more conspicuous in saprolite than in soils. On the plateau highest areas gibbsite is the most important secondary mineral; at lower altitudes kaolinite prevails. Thus the spatial distribution of secondary minerals seems to out-line two weathering processes—alitization and monosialitization—defined and interpreted as two different stages within a general alitic weathering trend. On hillslopes, bedrock weathering degree reflects the intensity of morphogeneticactions. Hillslope deposit profiles frequently show degrees and sequences of weathering related to downslope mouvement processes rather than to incipient post-depositional pedogenetic activity. Red ferralitic materials overlaid by superficial podzolized ones point to the super-position of two pedogenetic trends linked to variations in environmental conditions. Ferralitic materials would be remnants of weathering processes active during the Tertiary at lower altitudes, prior to the plateau uplift.

Weathering and morphogenesis in a tropical plateau

Weathering phenomena on the Campos do Jordão Plateau have been analysed in order to show their relationship to geomorphic compartition and morphogenetic dynamics. Correlations between weathering characteristics of surficial materials and geomorphic compartments are more conspicuous in saprolite than in soils. On the plateau highest areas gibbsite is the most important secondary mineral; at lower altitudes kaolinite prevails. Thus the spatial distribution of secondary minerals seems to outline two weathering processes — alitization and monosialitization — defined and interpreted as two different stages within a general alitic weathering trend. On hillslopes, bedrock weathering degree reflects the intensity of morphogenetic actions. Hillslope deposit profiles frequently show degrees and sequences of weathering related to downslope mouvement processes rather than to incipient post-depositional pedogenetic activity. Red ferralitic materials overlaid by superficial podzolized ones point to the superposition of two pedogenetic trends linked to variations in environmental conditions. Ferralitic materials would be remnants of weathering processes active during the Tertiary at lower altitudes, prior to the plateau uplift.