Initial Stage Of Weathering Research Articles

Evaluation of the initial weathering rate of Istria stone exposed to rain action, in Venice, with X-ray photoelectron spectroscopy

Samples of Istria stone were exposed in rain-washed conditions for 6 and 18 months, in the industrial and marine environment of Venice. To assess the initial stages of weathering, the exposed samples were analyzed by using X-ray photoelectron spectroscopy (XPS) and other analytical techniques, such as Fourier-transform infrared spectroscopy (FTIR) and scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM–EDS). Within a 6-month period, XPS revealed the deposition of sulfates, calcium silicates, carbonaceous particles, nitrogen compounds and organic compounds of lead, fluorine and chlorine. After an 18-month exposure period, the exposed surface exhibited pollutant compounds of sulfates, fluorine and nitrogen along with new-formed pollutant compounds of phosphorus and zinc. The appearance of silicon, aluminum and iron compounds indicates either dissolution of the argillaceous inclusions of the carbonate matrix or deposition of air-borne particles. The other applied analytical techniques evidence alteration features related more to morphological modification and less to chemical changes. Therefore, XPS revealing deposition of typical components of atmospheric pollution, as well as either deposition or dissolution of soil-derived and intrinsic elements evidences the initial weathering rate of exposed Istria stone to rain action.

Effect of microstructure and weathering on the strength anisotropy of porous rhyolite

To study the effect of microstructure and weathering on the strength anisotropy of rock, unconfined compressive strength (UCS) tests were carried out on three porous rhyolites having the same original lithology, but different weathering periods of 2600, 20,000 and 40,000 years. The rock is mainly composed of glassy groundmass, with flow structure. UCS tests were undertaken on a series of samples at 15° intervals, from right angles to the flow structure ( β=90°) to parallel ( β=0°), where β is the angle between the direction of the applied load and the direction of the flow structure. The test results show that UCS-values are maximum when β=0–30° and minimum when β=60–90°. This differs from previous reports for layered anisotropic rocks such as sandstone, sandy shale, schist etc., for which, UCS-values are maximum at β=0 or 90°. It is also found that UCS-values for β=60–90° reduce rapidly in the initial stage of weathering (the first 20,000 years), while for β=0–30°, the reduction rate increases after 20,000 years. This shows that the effect of weathering on strength anisotropy is not uniform, but depends on the weathering processes of the microstructures of the rock.

Vermiculitization of Phlogopite in Metagabbro, Central Turkey

AbstractDioctahedral vermiculite occurs in an isolated metagabbro klippe (Kurançalı Metagabbro) that belongs to the Central Anatolian Ophiolites from central Turkey. Both the metagabbro and the structurally underlying high-grade metamorphic rocks are intruded by granitic rocks. The Kurançalı Metagabbro is characterized by its well-developed compositional layering, and the presence of vermiculitized phlogopiterich layers. Petrographic and mineralogie studies show that the primary mineral phases in the host rock are diopside, tschermakitic hornblende, Fe-rich phlogopite, and plagioclase. Secondary minerals are hornblende, actinolitic hornblende, Fe-rich phlogopite, and vermiculite. A two-phase history of alteration involving acidic weathering and alkaline metasomatism is suggested for the dioctahedral vermiculite and secondary Fe-rich phlogopite, respectively. The alteration of phlogopite to dioctahedral vermiculite proceeded both along cleavage planes and at crystal edges. The vermiculite is colorless to pale yellow with weak pleochroism and shows optical continuity with the parent mineral. Vermiculite flakes, analyzed semi-quantitatively by scanning electron microscope-energy dispersive analysis (SEM-EDS) and electron microprobe (EMP), are characterized by partially expanded interlayers, K depletion, and Mg and/or A1 enrichment. X-ray diffraction (XRD) and differential thermal analysis-thermal gravimetric (DTA-TG) analyses indicate that phlogopite is not a pure phase, although it is the dominant one. The XRD patterns show the presence of both dioctahedral vermiculite having dehydrated interlayers and hydroxy-Al interlayers, and interstratified phlogopite-vermiculite. The transformation of phlogopite to vermiculite is thought to represent an initial stage of weathering in an acidic environment.

Microbial controls on phosphate and lanthanide distributions during granite weathering and soil formation

Both microbial and geochemical factors control the form, distribution, and abundance of lanthanides during weathering and soil formation in a profile in the Bemboka Granodiorite from southern New South Wales, Australia. During the initial stages of weathering, hydrous Ce-bearing lanthanide and lanthanide-aluminum phosphates (including rhabdophane and florencite) crystallize on etched apatite surfaces throughout the profile. However, their fate depends greatly on their location within the profile. In rocks weathered 5–6 m below the soil zone (the lower profile), secondary lanthanide phosphates persist long after all apatite has been dissolved. However, in rocks weathered within 2 m of the soil zone (the upper profile), secondary phosphates are dissolved and lanthanides other than Ce are removed. Bacteria and fungal hyphae are localized on secondary phosphate surfaces, suggesting that rhabdophane and florencite are solubilized in the upper profile due to organic complexation of dissolved lanthanides and/or uptake of phosphate by cells. In the soils (defined by loss of granitic texture), secondary phosphates are replaced by Ce-oxides. Lanthanides (other than Ce) removed in solution from the upper profile precipitate as Ce-poor phosphates in the lower profile when the solubility product is exceeded due to high dissolved phosphate concentrations in proximity to apatite. Thus, the upper profile is the source of lanthanides added to the lower profile. In both the lower and upper weathered granite, the degree to which weight-based Ce abundances increase with increasing weathering is consistent with Ce immobility on the centimeter scale. We attribute the very high weight-based Ce abundances (up to 12× concentrations in fresh rock) to extensive leaching and compaction during transformation of weathered rock to soil.

Over-estimation of efficiency of weathering in tropical “Red Soils”: its importance for geoecological problems

Weathering and soil formation rates are regarded as the main criteria of a tolerable soil loss. The efficiency of weathering in the seasonal semiarid tropics has often been greatly over-estimated especially in the geomorphologic literature in which weathering is assumed to be as fast or even faster than surface erosion. Six selected “Red Soils” in two intramontane basins of hyperthermic SW Nepal near the border with India, with 1500–1750 mm annual rainfall (5 humid months), and a “Black Soil” near Baroda, Gujarat, India (3–4 humid months) were studied mineralogically. Two of the “Red Soils” have TL ages between 10 and 30 ka, the “Black Soil” has one of about 10 ka. The yellowish silty parent material of the “Red Soils” is a preweathered soil sediment; it contains only small amounts of easily weatherable primary minerals: around 5% feldspars and 10–15% phyllosilicates, dominantly muscovites. Surprisingly, little pedogenic clay mineral formation could be identified. The illites and kaolinites are predominantly of detrital origin. The few non-regular mixed-layer minerals in the fine clay fraction (<0.2 μm) can be interpreted as resulting from the initial stage of silicate weathering. The hematites, however, are mostly of pedogenic origin. Therefore the rubefication is a recent autochthonous process, and by itself is not a reliable indicator of strong pedogenic weathering. In the dated “Black Soil”, only a small increase in 2:1 minerals, mainly smectites, could be found, although the content of weatherable minerals is high. These results support earlier conclusions from South India, where above a threshold of 2000 mm annual rainfall (6 humid months) deep weathering is a recent process leading to the formation of kaolinites over a long time interval; with 10 or more humid months per year, it leads to the formation of gibbsite. These soils are regarded as Vetusols. In the Typic and Aridic Rhodustalfs, earlier soil forming processes such as deep weathering and strong kaolinite formation have now almost ceased because of the semi-arid conditions; instead secondary carbonate is accumulating in the saprolite (Cr) and lower part of the Bt horizons. We conclude that, as well as Ultisols, most Alfisols or Lixisols in now semiarid India are relict soils or non-buried paleosols formed in an earlier period of much moister climate. Because the rate of compensatory regeneration of soil is now in effect, almost zero, the soil erosion there is a permanent loss of the country's most important natural resource, but it has not been recognized as such because the soils were not identified as paleosols.

Physical weathering of marbles caused by anisotropic thermal expansion

Marbles as building stones as well as in their natural environments show complex weathering phenomena. The most important damage scenario is based on the highly anisotropic thermal expansion coefficient α of calcite, i.e. extreme expansion parallel and contraction normal to the crystallographic c-axis. Therefore, the rock fabric and especially the lattice-preferred orientation (texture) of calcite and/or dolomite as the predominant mineral phases in marbles have a significant influence on the mechanical weathering. The textures of marbles from five different locations vary from a more or less perfect prolate to moderate oblate shape of the [006] pole figure tensor. Accordingly, the texture-derived bulk thermal dilatation anisotropy covers a broad range from –0.048 to 0.680. The modelled thermal dilatations correlate with those obtained from experimental measurements. The difference in magnitude is basically explained by the microcrack fabrics which was not considered in the computations. All samples show a deterioration due to thermal treatment regardless of the strength of texture. The directional dependence of (a) the total magnitude of the thermal dilatation coefficient and (b) of the residual strain is highest in marbles with a strong texture, whereas the Carrara marble with a weak texture exhibits a uniform crack formation. The progressive loss of cohesion along grain boundaries due to dilatancy may serve as an example for the initial stage of physical weathering.

Field weathering rates of Mt. St. Helens tephra

The initial stages of chemical weathering in tephra were examined under field conditions in a cool and humid forest ecosystem in the Cascade Mountains of Washington. Unleached tephra from the 1980 eruption of Mt. St. Helens was applied in 5 cm and 15 cm depths to simulate natural tephra deposition. Leachate solutions from the tephra were then collected and analyzed over a 4 year period. Concentrations of dissolved elements were combined with the water fluxes to determine elemental fluxes from tephra and to estimate chemical weathering rates. Solutions leached from the tephra layer indicate incongruent dissolution resulting in formation of a cation-depleted, silica-rich leached layer on glass and mineral surfaces. Measured weathering rates were 1–3 orders of magnitude less than comparable rates reported in the literature for laboratory dissolution studies, but considerably greater than those measured for entire watersheds in field studies. Dissociation of carbonic acid, originating primarily from upward transport of carbon dioxide from the buried soil, was the dominant source of protons for weathering reactions. Weathering rates in the 5 cm treatment were approximately twice those of the 15 cm treatment. A greater flux of CO 2 per unit volume of tephra in the 5 cm treatment is believed to be responsible for the differential weathering rates.

Weathering detected by Raman spectroscopy using Al-ordering in albite

Albite samples from Ontario, Canada, laboratory weathered in HCl solutions at pH 1 have been analysed and compared to unreacted mineral samples with the same crystal orientation using Raman microspectrometry. The study was focused on Raman bands reflecting Al-ordering because destabilisation of Al–O bonds with subsequent release of Al is well recognised to be a major factor in low pH dissolution of albite. Analyses of unweathered samples reveal that most bands in the albite spectra show variations in intensities for different crystal orientations in the wavenumber range 100 to 1200 cm −1. A well-defined crystal orientation is therefore essential to a successful investigation of weathered samples. Unweathered albite has two bands; 455 and 976 cm −1 in the spectrum normal to the (001) face, which have been related to Al-ordering in the structure. In spectra of weathered albite crystals, with the same orientation, the band most sensitive to alteration is 976 cm −1. This band is very strong for unreacted albite, but diminishes in intensity relative to the other bands at the initial stage of weathering. When dissolution continued, the structural information in the range 700 to 1200 cm −1 was completely lost, shown as a bandbroadening of a less organised structure, while the characteristic albite bands still could be observed in the range 100 to 600 cm −1.

The role of intragranular microtextures and microstructures in chemical and mechanical weathering: direct comparisons of experimentally and naturally weathered alkali feldspars

Electron microscopic observations of alkali feldspars from soils show that intragranular microtextures, such as exsolution lamellae, and microstructures, primarily dislocations, are both highly significant determinants of the weathering behaviour of these minerals. In particular, strained structure around intersecting edge dislocations in the plane of exsolution lamellae, ∼( 6 01), dissolves at a rate which is orders of magnitude greater than unstrained feldspar, producing a mesh of intersecting etch tubes extending >5 × 10 −3 cm into the crystal. As a result, dissolution at dislocations is the major source of solutes during initial stages of chemical weathering in the field. With progressive chemical weathering, the most highly reactive feldspar is consumed by growth and coalescence of etch tubes, but outer parts of the grain are physically weakened, leading to mechanical flaking that increases available surface area and exposes further reactive sites. In contrast, previous dissolution experiments, and microscopy of reacted surfaces, have shown little or no correlation between dissolution rate and dislocation density and few visible signs of dissolution at particularly reactive sites. To resolve the apparent discrepancy between field and laboratory behaviour we have carried out flow-through dissolution experiments using pH 2 HCl at 25°C on three alkali feldspars with carefully characterized intragranular microtextures and microstructures. These alkali feldspars were: (1) Eifel sanidine, an alkali feldspar that has no microtextures at the TEM scale and a low dislocation density (<10 6 cm −2), (2) unweathered alkali feldspars from the Shep Granite, which have a mainly coarse exsolution microtextures and higher dislocation density (>2–3 × 10 8 cm −2), and (3) naturally weathered alkali feldspars, also from the Shap Granite, which have the same microtextures as unweathered Shap Granite alkali feldspars but, because they have been weathered, have a lower density of dissolution reactive dislocations exposed on grain surfaces (<2–3 × 10 8 cm −2). Results from the experiments are ambiguous. If the rate data are normalised to the powder’s initial BET surface area, dissolution rates increase with dislocation density. Normalisation to the powder’s BET surface area as it is inferred to have changed during the experiments, yields no correlation with dislocation density. SEM and AFM images of reacted grain surfaces show that dislocation outcrops and albite exsolution lamellae have both etched more rapidly than tweed orthoclase, but dissolution at these sites makes a quantitatively insignificant contribution to the overall rate of laboratory dissolution of the feldspar powders. Major differences in the importance of dislocations to rates of early chemical weathering in field and laboratory contexts probably result from corresponding contrasts in the saturation state of ambient solutions. Observations of naturally weathered alkali feldspars show that microtextures and microstructures have the greatest impact on mineral weathering rates during advanced stages of dissolution when grain surfaces start to disintegrate. Hundreds of years of dissolution under the laboratory conditions used here would be required to reach this stage.

Weathering of Mt. St. Helens Tephra under a Cryic‐Udic Climatic Regime

AbstractThe initial stages of chemical weathering in tephra were examined during a 10‐yr period under field conditions in a cryic‐udic climatic regime. Unleached tephra from the 1980 eruption of Mt. St. Helens was applied to the surface of soils in 5‐ and 15‐cm depths to simulate natural tephra deposition on subalpine forest ecosystems. Solid‐phase tephra samples were collected 10 yr after addition and compared chemically and mineralogically to the original, unweathered tephra. The solid‐phase pH of the tephra decreased by approximately one unit, with the 5‐cm treatment being ≈0.2 units lower than the 15‐cm treatment. The accumulation of Al and Fe as alteration products in the weathered tephra was detected by selective dissolution analyses. Aluminum accumulated as Al‐humus complexes, hydroxy‐Al polymers in the interlayer of 2:1 layer silicates, and possibly as hydroxy‐Al precipitates external to the interlayer space; Fe accumulated primarily as a noncrystalline oxyhydroxide (e.g., ferrihydrite) and Fe‐humus complexes. The increase in the active Fe and Al fraction (i.e., oxalate extractable Fe and Al) contributes to a sevenfold increase in phosphate retention. Hydroxy‐Al interlayering of detrital 2:1 layer silicates present in the tephra was suggested from x‐ray diffraction and selective dissolution analyses. There was no evidence of opaline silica, allophane, or imogolite forming in the tephra. Preferential incorporation of Al into Al‐humus complexes and hydroxy‐Al interlayers of 2:1 layer silicates appears to inhibit formation of allophane and imogolite during the initial stages of weathering.

Mineralogical Investigation of Soils Formed in Calcareous Gravelly Alluvium, Eastern Crete, Greece

AbstractThe mineralogical composition of soils along a toposequence formed on Quaternary‐aged alluvium from the northeastern portion of the island of Crete, Greece, and the examination of their mineral weathering characteristics should help provide an insight into the stage of weathering and relative age of the soils. Four soils were described and sampled from three landscape positions along the toposequence and the clay and silt fractions from all horizons of each soil were examined using a combination of x‐ray diffraction and electron microprobe techniques. Coarse and fine silt fractions from each soil contained dolomite, calcite, quartz, feldspars, mica, and chlorite. Feldspar content generally decreased with depth, whereas calcite increased with depth. Minerals identified in the silt fractions resulted from a combination of physical and chemical weathering and possible eolian additions. Clay fractions were dominated by trioctahedral ferrous chlorite and dioctahedral mica (muscovite), with minor amounts of kaolinite, quartz, dolomite, and calcite. Goethite was also identified in the clay fraction of some soil horizons. The source of the phyllosilicates was inheritance from phyllite gravels in the alluvium and chemical dissolution of the dolostone releasing entrained clay minerals. Some portion of the kaolinite in surface horizons was probably the result of eolian additions. Goethite formation resulted from release of Fe from the chlorite. The extent of clay mineral inheritance, limited expansion of the chlorites, and lack of a chlorite or mica weathering product (vermiculite), as well as the presence of carbonates in the clay fractions, all suggest that these soils are still in the initial stages of weathering.

Allophane, imogolite, and gibbsite in coatings in a Costa Rican Andisol

Neoformation of allophane, imogolite, and gibbsite has been studied extensively in soils in humid climates, but little is known about their micro distribution patterns. such information may provide insight about the conditions at the micro-site scale, responsible for their formation and disappearance. Our study describes amorphous and crystalline coatings in a Melanudand on a 18,000 yr old andesitic lava in Costa Rica. Fine-textured, white to yellow coatings could be observed in the B, C, and R horizon in the field. In thin sections the coatings are translucent and isotropic in plane polarized light (PPL), indicating that they consist of amorphous material. In the B horizon, under crossed polarized light, isotropic coatings show a gradual transition towards crystalline outer margins and spots, while in PPL such transition cannot be observed, suggesting a genetic relationship. Submicroscopical analyses reveal that the amorphous coatings in the R and C horizon consist of allophanic material with a molar Al/Si ratio of 0.9, those in the B horizon consist of allophanic material and imogolite with a molar Al/Si ratio of 1.4. The amorphous coatings resulted from precipitation of Al and Si liberated upon weathering of primary minerals in an initial stage of lava weathering. The crystalline coating parts in the B horizon consist of gibbsite and represent the ultimate stage of mineral neoformation. The different coating composition in the soil horizons is the result of different leaching conditions at a macro and micro scale. Care should be taken to compare results obtained at different sampling scales with different analytical methods.

Lead isotope systematics of granitoid weathering

The isotopic composition of lead released by chemical weathering of granitoids was investigated in order to evaluate the sensitivity of this natural tracer as a tool for monitoring mineral weathering and soil development. The isotopic composition of lead was found to change systematically with the relative degree (or maturity) of weathering in both Cretaceous granitoids from the Sierra Nevada batholith (SN) and Precambrian granitoids from the Wind River Range, Wyoming (WRR). In the SN, lead released from crushed bedrock by dilute acid leaching (to simulate the initial stages of weathering) has a composition more radiogenic than observed in soils (average 206Pb 207Pb = 1.335 ). Lead released by leaching from soils developed on a relatively young terrain (~10 kyr old) is less radiogenic (average 206Pb 207Pb = 1.270 ) than crushed bedrock and lead released by leaching from soil developed on older surfaces (~ 100 kyr) is even less radiogenic (average 206Pb 207Pb = 1.255 ). Lead released from the total digestion of the rock has the least radiogenic composition (average 206Pb 207Pb = 1.240 ). In the WRR, systematic changes in the isotopic composition of lead released by leaching were observed in soils developed on glacial moraines of variable ages. Lead released from soil developed on a 21 kyr old moraine was the most radiogenic ( 208Pb 207Pb = 3.257 ). Lead released from soil developed on a 130 kyr old moraine was less radiogenic ( 208Pb 207Pb = 2.780 ) and lead released from soil developed on ≥350 kyr old moraine was the least radiogenic ( 208Pb 207Pb = 2.532 ). The observed systematic changes in the isotopic composition of lead with time in both field areas demonstrates the potential of lead isotopes to ascertain the degree (or maturity) of chemical weathering of granitoids and may have applications in determinations of the relative ages of some glacial deposits.

The weathering of basaltic rocks in Burundi and Vietnam

Various stages of tropical weathering are described using a number of examples of red soil formation derived from rift basalts in Burundi and laterite bauxites from plateau basalts of South Vietnam. The phenomena of decomposition of primary minerals are comparable in the two regions. Fine-grained montmorillonites, kaolinites, halloysites and hematite were formed during an initial stage of weathering. The two generations of kaolinites, goethites and gibbsites are the main products of a more intense stage of weathering. The gradients of chemical activities in the micro-environment (surfaces of grains, pores, microfissures) seem to be the controlling factors of weathering.

Distribution of Sc, Cr, Co, Hf, Ta and Th in a weathering profile of gabbroic rocks (Serpa, Portugal)

Trace element data (Sc, Cr, Co, Hf, Ta and Th) are reported for a weathering profile of gabbroic rocks located near Serpa, Portugal. The results obtained for the whole rock samples showed that: (i) Co and Sc contents decrease upwards the profile. Co appears to be mobilized in the soil probably as soluble cation; (ii) Cr tends to remain in the profile; and (iii) Hf and Th concentrate in the upper horizons. Trace element distribution in different size fractions of the upper horizons revealed: (i) Sc, Cr and Co have a more uniform distribution than Hf, Ta and Th; (ii) Hf and Ta are concentrated in the intermediate fractions; and (iii) Th is concentrated in the finer fractions, probably due to incorporation in weathering resistant minerals, specially in the initial stages of weathering.

Surface reactions during the early stages of weathering of albite

The aim of this study is to gain a better understanding of the nature and distribution of surface species occurring during the initial stage of weathering of albite. Instead of the classical acidbase titration experiments used extensively in previous work, the surface of freshly ground mineral was titrated by adding increasing amounts of solid to pure water. The aqueous phase was analyzed for all the elements present after a well-defined short reaction time. Compared to the classical titration experiment, this approach has the advantage of defining the entire composition of the system and limits the interferences of the dissolution process with surface reactions. The results confirm the rapid exchange of Na + by H + at the surface of albite, leading to the formation of an H-feldspar and a deprotonation producing negatively charged surface species. Addition of a large excess of Na + to the aqueous solution demonstrates the reversibility of the exchange reaction. The exchange reaction and the deprotonation can be depicted by apparent equilibrium constants which describe the density of the negatively charged species as a function of pH. These results strongly suggest that the negatively charged surface species are closely related to Na sites at the feldspar surface where three species can be identified: NaAlSi 3O 8, HAlSi 3O 8, and AlSi 3O 8 −. The fractional dependence of pH obtained (0.35) for the density of the surface charge is close to the fractional order of the dissolution rate in the same pH region (pH > 5.5). Additional experiments show that the fresh albite surface is able to react with gaseous CO 2 which results in a more disturbed surface layer.

Laboratory Weathering of Combusted Oil Shale

AbstractProper management of large volumes of spent oil shale requires an understanding of disposal environment chemistry and mineralogy. Laboratory weathering is one method to rapidly and inexpensively assess the long‐term potential for spent oil shales to degrade the environment. The objective of this study was to examine the mineralogy and leachate chemistry of three combusted oil shales (two Green River Formation and one New Albany) in a laboratory weathering environment using the humidity cell technique. The mineralogy of the combusted western oil shales (Green River Formation) is process dependent. In general, processing resulted in the formation of anhydrite, lime, periclase, and hematite. During the initial stages of weathering, lime, periclase, and anhydrite dissolve and ettringite precipitates. The initial leachates are highly alkaline (pH &gt; 10.5), saline, and dominated by Na, hydroxide, and SO4. As weathering continues, ettringite dissolves, gypsum and calcite precipitate, and the leachates are dominated by Mg, SO4, and CO3. Leachate pH is rapidly reduced to between 8.5 and 9 with leaching. The combusted eastern oil shale (New Albany) is composed of quartz, illite, hematite, and orthoclase. Weathering results in the precipitation of gypsum. The combusted eastern oil shale did not display a potential to produce acid drainage. Leachate chemistry was dominated by Ca and SO4. Element concentrations continually decreased with weathering. In a western disposal environment receiving minimal atmospheric precipitation, spent oil shale will remain in the initial stages of weathering, and highly alkaline and saline conditions will dominate leachate chemistry. In an eastern disposal environment, soluble salts will be rapidly removed from the spent oil shale to potentially affect the surrounding environment.

Formation of Trioctahedral Illite from Biotite in a Soil Profile over Granite Gneiss

AbstractClay fractions separated from the A2, B, and C horizons of a soil formed on granite gneiss showed X-ray powder diffraction (XRD) spacings characteristic of trioctahedral illite. The trioctahedral illite was derived from biotite, and its development through various stages of weathering was followed by optical and electron microscopy combined with electron microanalysis. In the initial stages of weathering, Fe2+ within biotite was oxidized, without the loss of much K. During this process, biotite flakes became slightly buckled and fractured. Solutions moved into the damaged flakes leading to chemical weathering and exfoliation along cleavages and angular fractures. Major exfoliation broke up the flakes into segments, which themselves contained minor exfoliations and alterations along cleavage planes. The extent of exfoliation and alteration continued until thinner and shorter segments consisted almost wholly of thin (&lt; 0.25 µm), parallel wafers separated by less compact layers of particles and microaggregates. The segments finally lost their shape and divided into clay-size particles. Parts of the thin wafers had the same chemical composition (and structure) as the original, intact flakes of oxidized biotite. The same parts of wafers retained some of the optical properties of the original biotite, and, when broken down to clay, they produced the XRD spacings of 10 and 1.54 Å, typical of fine-grained, trioctahedral mica (illite).

Studies on the weathering process in poly(vinyl chloride) by laser induced fluorescence spectroscopy

Excimer laser induced fluorescence spectra of plasticised commercial poly(vinyl chloride) show two distinct regions of emission bands. The one in the ultraviolet region around 360 nm and the other in the visible around 450 nm are attributed to the emission from the plasticiser (dioctyl phthalate) and the carbonyl impurities, respectively. Artificial weathering is found to quench the fluorescence from the phthalates indicating a depletion of these chromophores through photo-oxidative reactions. A corresponding increase in the visible fluorescence suggests an increase in the concentration of carbonyl groups during the initial stages of weathering (up to 200 h of exposure in a Xenotest Weather-Ometer). Prolonged weathering (>200 h) depletes the primary chromophores and the fluorescence is then governed by the conjugated polyene generated through progressive photo-reactions. The phthalates and the carbonyl units constitute natural probes for the study of the photo-oxidative processes in commercial PVC.

Geology of precambrian paleosols at the base of the huronian supergroup, elliot lake, Ontario, Canada

Detailed examination of two new profiles through the Denison Series paleosol, and one new profile through a Pronto Series paleosol in the Elliot Lake area, lend fresh insight into atmospheric and hydrogeologic environmental conditions of Precambrian Earth. These are reassessed, with reference to the results of previous studies, as true indigenous paleosols of polygenetic character. Virtually all of the criteria (Grandstaff et al., 1986) for identification of paleosols are met. The paleosols bear strong mineralogical and chemical similarities despite derivation from different parent materials, and sampling at locations distant from each other. There has evidently been a common reaction to weathering processes peculiar to the Early Proterozoic. Diagenetic and metamorphic events mask, to a considerable extent, the role played by environmental and hydrologic conditions in the development of the paleosols. In the Denison greenstone-derived paleosol, clay formed from amphibole, feldspar and chlorite is altered to sericite. Increased quartz content is due to residual concentration and to secondary development. In granodiorite-derived paleosol, quartz and sericite are generally more abundant; quartz is relatively concentrated toward the top of the profile. The paleosols are thus primarily sericite, quartz and chlorite assemblages. Rutile and leucoxene are accessory minerals. Profiles of elemental chemistry show the most drastic changes in concentration at the base, in transition to bedrock. Magnesia and lime are leached from all profiles, as in modern soils, although in common with most Precambrian paleosols, potash accumulated. Total Fe is strongly depleted from the profiles except just above the bedrock contact with granodiorite, a condition attributed to mobilization of Fe from the paleosol to a position directly over relatively impermeable parent rock. Use of Holland's method of establishing oxygenation from paleosols indicates that in terms of Fe content, parent material of granodiorite-derived paleosol should produce a reduced Fe-paleosol. The oxygen demand of this rock was greater than granite, owing to an abundant ferromagnesian content. Two fossil eluvial horizons, marked by high chlorite and a dramatic decrease in sericite content, are recognized in the Denison paleosol. Their existence shows that although initial stages of weathering may have occurred under water-saturated conditions, conditions were probably more arid than supposed by previous workers, particularly during the later stages of development of the paleosol. Although Fe-enriched, the eluvial horizons are drastically depleted in potash, Al and Th. Removal of Al 2O 3 negates normalization of chemical analyses of paleosols to Al 2O 3, for purposes of constant volume calculations. Th, associated with sericite in the profiles, increases at the contact with the overlying Matinenda Formation, in association with rutile and leucoxene, owing to interaction with diagenetic U-bearing fluids from overlying sediments. Rutile and leucoxene originate from parent rocks, and are a late-stage form of mineralization. These titaniferous phases indirectly contributed to the local dominance of brannerite-type orc through diagenetic processes of mineralization involving the Pronto Reaction. Paleotopographic greenstone highs are related to local, anomalously high concentrations of heavy minerals in the brannerite-rich orc. This involves hydraulic deposition of heavy minerals in the lee of crescent-shaped greenstone highs, with subsequent diagenetic upgrading. Granodiorite-derived paleosol contains 5–10 times more Au than the Denison paleosol, and is modestly enriched above background. A slight decrease in Au content upward, evident in all profiles, is an artefact of erosion of the top of the paleosols.