The magnetic properties and morphology of metallic iron produced by subsolidus reduction of synthetic Apollo 11 composition glasses

Metallic iron has been precipitated from a synthetic high-iron, high-titanium Apollo 11 composition glass powder in a furnace with the oxygen fugacity controlled by gas mixing techniques. Measurable quantities of iron, as determined with a vibrating sample magnetometer capable of detecting 0.01 wt% iron in the absence of ferromagnetic minerals, were produced in experiments at temperatures between 700°C and 1045°C, with run times between 3 hr and 95 hr, and oxygen fugacities between 1 and 2 orders of magnitude below the iron-wustite (IW) buffer curve. Such conditions of ?_O2 and T are probably not greatly different from those occurring in a large lunar ejecta blanket. The oxygen fugacity determines the amount of iron produced for a given time and temperature, with about 1% produced if log ?_O2 is 1.4 units below the IW buffer curve and about 3.5% produced if log ?_O2 is 1.7 units below at 990°C. Above 950°C essentially all the iron is multidomain (>300Å) while below 950°C as much as 15% is single domain (150Å–300Å) and an appreciable quantity remains even smaller. Compaction of the sample slows the rate of reduction but does not influence the grain size of metal. The quantities and size distribution of the reduced iron in a number of the experimental runs are strongly analogous to certain lunar soils and breccias and indicate that reduction in an ejected blanket could partly account for the excess iron of lunar soils and breccias relative to the igneous rocks.     

Magnetic signature of hydrocarbon-contaminated soils and sediments at the former oil field H?nigsen, Germany

Magnetic properties of hydrocarbon (HC) containing soils and sediments from two sites (Site A and B) of the former oil-field H?nigsen were analyzed in order to determine whether magnetic methods can be employed to delineate HC contamination of soils and sediments. Magnetic parameters such as magnetic susceptibility and induced isothermal remanent magnetizations, as well as soil and sediment properties such as pH, iron content and water content, HC content and most probable number counts of iron-metabolizing microorganisms were determined. The magnetic concentration-dependent parameters for HC contaminated samples were 25 times higher in soils from Site A than in sediment samples from Site B. However, at Site B the magnetic susceptibility was still four times higher in comparison to lithologically similar non-contaminated sediment samples from a third Site C. Newly formed magnetite containing mainly single domain particles was responsible for the magnetic enhancement, whereas superparamagnetic grains represented only a minor component. Site A had an acidic pH compared to neutral pH at Site B, and a higher crystalline and bioavailable total iron content. Nevertheless, Site B samples contained significant numbers of both iron(II)-oxidizing and iron(III)-reducing microorganisms indicating that microbial iron cycling might have taken place at this site and potentially played a role for iron mineral transformation, including magnetite (trans)formation. The content of total non-polar hydrocarbons (TNPH) at Site A was one order of magnitude higher than at Site B. Only at Site A magnetic susceptibility correlated well with TNPH. Our results demonstrate that HC contaminated samples had an enhanced magnetite content compared to non-contaminated soils and sediments. Therefore, magnetic methods may provide a quick and cost-effective way to assess HC contamination in soils and sediments. However, more field sites and laboratory investigations are needed to reveal the complex nature of the processes involved.     

Die Entstehung von Pyrit in rezenten Sedimenten des Piburger Sees

The occurrence of pyrite (FeS₂) and iron sulfide in surficial sediments of Piburger See was compared with thermodynamic calculations based on chemical analyses of iron and hydrogen sulfide in the interstitial water. The area below 17 m, where black spots were found in the sediments, showed ion products (log K_FeS=a_Fe2·a_HS/a_H+) between?3.11 and ?4.01. In areas with no visible FeS concretions logK_FeS values were in the range of ?4.74 to ?5.77, thus thermodynamic calculations seem to be in accordance with the appearance of iron sulfide. Nevertheless pyrite framboids, composed by more than 1,000 single crystals, coul be found even in shallow parts of the lake. Therefore the formation of pyrite is assumed to occur in microniches (diatom frustules, testacean shells). Inside these microcompartments high concentrations of hydrogen sulfide are reached due to the anaerobic decomposition of organic matter, whereas iron and additional sulfur are supplied by the diffusion of ferrous iron and sulfate from the anoxic environment.     

Rock magnetic properties and ore microscopy of the iron ore deposit of Las Truchas, Michoacan, Mexico

Iron ore and host rocks have been sampled (90 oriented samples from 19 sites) from the Las Truchas mine, western Mexico. A broad range of magnetic parameters have been studied to characterize the samples: saturation magnetization, Curie temperature, density, susceptibility, remanence intensity, Koenigsberger ratio, and hysteresis parameters. Magnetic properties are controlled by variations in titanomagnetite content, deuteric oxidation, and hydrothermal alteration. Las Truchas deposit formed by contact metasomatism in a Mesozoic volcano-sedimentary sequence intruded by a batholith, and titanomagnetites underwent intermediate degrees of deuteric oxidation. Post-mineralization hydrothermal alteration, evidenced by pyrite, epidote, sericite, and kaolin, seems to be the major event that affected the minerals and magnetic properties. Magnetite grain sizes in iron ores range from 5 to >200 μm, which suggest dominance of multidomain (MD) states. Curie temperatures are 580±5°C, characteristic of magnetite. Hysteresis parameters indicate that most samples have MD magnetite, some samples pseudo-single domain (PSD), and just a few single domain (SD) particles. AF demagnetization and IRM acquisition indicate that NRM and laboratory remanences are carried by MD magnetite in iron ores and PSD–SD magnetite in host rocks. The Koenigsberger ratio falls in a narrow range between 0.1 and 10, indicating the significance of MD and PSD magnetites.     

Dust production and the release of iron oxides resulting from the aeolian abrasion of natural dune sands

Global dust trajectories indicate that signi?cant quantities of aeolian‐transported iron oxides originate in contemporary dryland areas. One potential source is the iron‐rich clay coatings that characterize many sand‐sized particles in desert dune?elds. This paper uses laboratory experiments to determine the rate at which these coatings can be removed from dune sands by aeolian abrasion. The coatings impart a red colour to the grains to which previous researchers have assigned variable geomorphological signi?cance. The quantities of iron removed during a 120 hour abrasion experiment are small (99 mg kg^?1) and dif?cult to detect by eye; however, high resolution spectroscopy clearly indicates that ferric oxides are released during abrasion and the re?ectance of the particles alters. One of the products of aeolian abrasion is ?ne particles (<10 µm diameter) with the potential for long distance transport. Copyright © 2004 John Wiley & Sons, Ltd.     

The Néel transition and magnetic properties of terrestrial,synthetic, and lunar ilmenites

The magnetic susceptibility of a terrestrial, synthetic and lunar ilmenite specimen has been measured from 4 to 300 K. All specimens had a single Néel temperature transition which ranged from 56 to 57.7 K. In one case the powdered specimen was partially aligned in the magnetic field prior to the susceptibility measurements and the Néel transition was observed to shift to 60 K indicating magnetic anisotropy. A study of several magnetic parameters calculated from the experimental data showed gross impurities in the terrestrial specimen, single-domain to multi-domain metallic iron in the synthetic specimen, and a small amount of superparamagnetic metallic iron in the lunar sample. No multidomain iron was observed in the lunar ilmenite. The results of electron spin resonance measurements were also in general agreement with these findings. Because of the absence of Fe³⁺ compared to most terrestrial samples it is suggested that the anisotropic magnetic parameters be determined on lunar ilmenite when a large enough single crystal becomes available.     

Greigite (Fe3S4): magnetic properties and first domain observations

Recently the ferrimagnetic iron sulphide greigite (Fe₃S₄) has been recognized as an important carrier of magnetic remanence in young sediments. New magnetic measurements and properties of this rare mineral are presented. Magnetic domain observations were made using the Bitter method and the magneto-optical Kerr effect. In most cases the domain structures observed are very complicated and their interpretation is rather difficult. The cause of this domain structure is very likely high magnetostriction. A first attempt to establish a relationship between grain size and domain state is made.     

Magnetite dissolution,diachronous greigite formation,and secondary magnetizations from pyrite oxidation: Unravelling complex magnetizations in Neogene marine sediments from New Zealand

Detailed rock magnetic and electron microscope analyses indicate that the magnetic signature of Neogene marine sediments from the east coast of New Zealand is dominated by the authigenic iron sulphide greigite. The greigite is present as a mixed population of stable single domain and superparamagnetic grains, which is consistent with authigenic growth from solution. This growth can result from pyritization reactions soon after deposition, which also leads to dissolution of most detrital magnetite; however, where constrained by field tests, our data suggest that remanence acquisition can occur > 1 Myr after deposition, and can vary in timing at the outcrop scale. Strong viscous overprints result from oxidation of the iron sulphides, probably during percolation of oxic ground water. This process can sometimes destroy any ancient remanent magnetization. This complex magnetic behaviour, particularly the presence of late-forming magnetizations carried by greigite, means that the remanence in New Zealand Cenozoic sediments, and in similar sediments elsewhere, cannot be assumed to be primary without confirmation by field tests. The reversals test should be employed with caution in such sediments, as patchy remagnetizations can lead to false polarity stratigraphies.     

Stability and identification for rational approximation of frequency response function of unbounded soil

Exact representation of unbounded soil contains the single output–single input relationship between force and displacement in the physical or transformed space. This relationship is a global convolution integral in the time domain. Rational approximation to its frequency response function (frequency‐domain convolution kernel) in the frequency domain, which is then realized into the time domain as a lumped‐parameter model or recursive formula, is an effective method to obtain the temporally local representation of unbounded soil. Stability and identification for the rational approximation are studied in this paper. A necessary and sufficient stability condition is presented based on the stability theory of linear system. A parameter identification method is further developed by directly solving a nonlinear least‐squares fitting problem using the hybrid genetic‐simplex optimization algorithm, in which the proposed stability condition as constraint is enforced by the penalty function method. The stability is thus guaranteed a priori. The infrequent and undesirable resonance phenomenon in stable system is also discussed. The proposed stability condition and identification method are verified by several dynamic soil–structure‐interaction examples. Copyright © 2009 John Wiley & Sons, Ltd.     

Groundwater Inflow Controls Acidity Fluxes in an Iron Rich and Acidic Lake

To ascertain the influence of hydrological boundary conditions on acidity fluxes in lakes influenced by acid mine drainage, acidity budgets were developed for two sediments in areas of differential groundwater inflow (approx. 1 L m^?2 d^?1 and 10 L m^?2 d^?1). In both sediments iron was deposited as schwertmannite leading to iron(III) enriched sediments (3.9…6.2 mmol g^?1, referred to dry weight). Compared to the surface water, the inflowing groundwater had higher pH (4.5 vs. 3), ferrous iron (6…20 mmol L^?1 vs. 0.8…2.0 mmol L^?1), and sulfate (5…60 mmol L^?1 vs. 8…13 mmol L^?1) concentrations. The inflow changed the sediment pore water chemistry and triggered a further increase in pH to above 5.5. In both sediments acidity generation in the surface water (10…30 mol m^?2 a^?1) strongly prevailed over acidity consumption in the sediments (> ?0.6 mol m^?2 a^?1). With advective groundwater inflow, however, more acidity was consumed due to TRIS formation (?0.12 mol m^?2 a^?1 vs. ?0.017 mol m^?2 a^?1), iron carbonate burial (upper estimate: ?0.14 mol m^?2 a^?1 vs. ?0.022 mol m^?2 a^?1), and unspecific ferrous iron retention (?0.39 mol m^?2 a^?1 vs. ?0.08 mol m^?2 a^?1). Also, less acidity was generated due to schwertmannite transformation (?2.4 mol m^?2 a^?1 vs. ?0.11 mol m^?2 a^?1). The acidity balance of internal processes in the sediment with groundwater inflow was negative, whereas it was positive in the other sediment. The study demonstrates that in acidic and iron rich lakes the hydrological boundary conditions strongly affect geochemical processes as subsumed in acidity fluxes.     

Activity of iron and olivine solubility in basaltic liquids

Some of the factors which control the solubility of ferrous iron and olivine in basaltic liquids have been determined from experiments conducted in equilibrium with metallic iron at temperatures of 1150° to 1306°C and oxygen fugacities of log?O₂ = ?12.5to?14.5atm. In order to insure that the samples were in equilibrium with metallic iron and the gas atmosphere during the experiment, the samples were continuously mixed (60 r.p.m.) for at least 90 hours with a metallic iron mixing rod.The solubility of ferrous iron and olivine in basaltic liquid was found to decrease as the alkali and alumina content of the liquid increased. The activity coefficient of ferrous iron in the liquid was calculated from the oxygen fugacity and mole fraction of ferrous oxide in the liquid and varied from 0.79 to 2.00. The ferrous iron/magnesium ratio in the liquid relative to olivine was insensitive to the changing alkali content as measured by the constant distribution coefficient of0.30 ± 0.03 (SD). The calculated silica activity of these liquids varied from 0.11 to 0.50 and the distribution coefficient of ferrous iron-manganese between olivine and liquid was found to be1.4 ± 0.2 (SD). It was found that the major element content of olivine does not exert a strong structural control on the manganese content of olivine.The magnitude of depletion in the liquid of the metal oxides by the crystallization of olivine is in the order Ni > Mg > Co > Fe > Mn, which reflects the relative octahedral site preference of these elements. The composition of the crystallizing olivine reflects the ratio of the above elements in the liquid and is relatively insensitive to temperature, variation of the other elements in the liquid or to the changing magnesium/ferrous iron ratio in the olivine.     

Internal flow structures in columnar jointed basalt from Hrepphólar, Iceland: II. Magnetic anisotropy and rock magnetic properties

The anisotropy of magnetic susceptibility (AMS) and rock magnetic properties were measured on specimens from a basalt plate that was cut from a vertical section of a basalt column from Hrepphólar, Iceland. Macroscopic structures are clearly distinguishable in the plate, including banding inferred to represent viscous fingering parallel to the vertical axis of the column. Rock magnetic experiments indicate that the dominant ferromagnetic (sensu lato) mineral is titanomagnetite, Fe _3?x Ti _x O₄, with a Ti-composition of x?=?~0.6. Magnetic properties are related to the position within the plate and reveal a dominant volume fraction of single domain titanomagnetite in the center of the basalt column, with multidomain titanomagnetite away from the center. The AMS determined by low-field measurements shows an inconclusive relationship with the visual structures, which arises from variation of the grain size (i.e., single domain versus multidomain) across the column. In contrast, the AMS measured with a high-field torsion magnetometer avoids the complication of magnetic domain state, as is demonstrated in this contribution, and additionally allows for the separation of ferrimagnetic from paramagnetic sub-fabrics. Both sub-fabrics display a clear relationship with the macroscopic structures and support the hypothesis that vertical flow of melt took place during development of the Hrepphólar columnar basalt. Maximum susceptibility axes of the ferrimagnetic sub-fabric are grouped near the vertical axis of the column. The paramagnetic sub-fabric varies systematically across the column in coincidence with internal structure. The shape of the magnetic susceptibility ellipsoid varies across the basalt column, showing an increasingly prolate fabric toward its center.     

Heavy Metal Removal from Soils Using Magnetic Separation: 1. Laboratory Experiments

The removal of Cu, Zn, and Cd from a sandy soil was investigated using iron filings as an adsorbent, and subsequently recovering the iron filings by magnetic separation. The best treatment was obtained by using 5% iron filings and 3 h contact time between iron filings and the soil. The metal removal efficiency from soil extracts was evaluated, using MetPLATE^TM, a toxicity test that is specific for heavy metals, and the 48 h Ceriodaphnia dubia acute toxicity test. The toxicity removal was generally higher than 95% for Cu after a single treatment. With regard to Zn-spiked soil, the toxicity removal was 96.1%, 70.0%, and 49.6% after single treatment at the input concentration of 200 mg/kg, 400 mg/kg, and 800 mg Zn²⁺/kg soil, respectively. After two or three successive treatments, more than 90% of the toxicity was removed for 400 mg/kg and 800 mg/kg Zn-spiked soils. In the case of Cd-spiked soil, a single treatment removed 51.1% of the toxicity from 200 mg/kg Cd-spiked soil extracts while more than 90% of the toxicity was removed after two or three treatments. Chemical analysis and a mass balance study were also carried out to investigate the Cu distribution in the soil fractions. The results indicate that, before treatment, a large portion of Cu was immobilized in the soil matrix. Following magnetic separation, Cu was removed from both the soil matrix and extracts and was indeed adsorbed and concentrated on the iron filings. The retrieval of Cu by iron filings was further examined by energy dispersive X-ray spectroscopy (EDS).     

Unique phenomenon of the accumulation of <Emphasis Type="Italic">terrestrial</Emphasis> metal iron particles in lacustrine deposits: Zhombolok volcanic region,East Sayan

The native iron particles that were previously detected by thermomagnetic and microprobe analyses in the sediments of different age in many regions of the world are of extraterrestrial origin. The similarity in the compositions, grain shapes, and sizes observed in the extraterrestrial and terrestrial particles of native iron testifies to the common production conditions of iron particles during the formation of planets. In this paper, the single finding of terrestrial iron in the lacustrine sediments of the Zhombolok volcanic region, East Sayan, is discussed. The uniqueness of the results indicates that the spatial distribution of the particles of native iron is limited to a fairly narrow area around their source—volcanic eruption or/and the fall of a large meteorite.     

Rock-magnetic and paleomagnetic results from the Tepic-Zacoalco rift region (western Mexico)

A rock-magnetic and paleomagnetic investigation was carried out on eleven Pleistocene and Pliocene ⁴⁰Ar/³⁹Ar dated lava flows from the Tepic-Zacoalco rift region in the western sector of the Trans-Mexican Volcanic Belt (TMVB) with the aim of obtaining new paleomagnetic data from the study region and information about the Earth’s magnetic field recorded in these rocks. Rock-magnetic experiments including measurement of thermomagnetic curves, hysteresis parameters and isothermal remanence acquisition curves were carried out to find out the carriers of remanent magnetisation and to determine their domain structure. Although some samples were characterised by the presence of a single ferromagnetic phase (magnetite), in most cases more phases were observed. Analysis of hysteresis parameters showed a mixture of single domain and multidomain particles, the fraction of the latter varying between 40% and 80%. Paleomagnetic results were obtained in all sites, although in 7 sites characteristic remanence directions and remagnetisation circles had to be combined in order to calculate site means. The six Pliocene sites not showing intermediate polarity yielded a paleomagnetic pole (latitude ? = 81.1°, longitude λ = 94.3°) which roughly agrees with the expected one. Paleomagnetic directions do not indicate significant vertical-axis block rotations in the western TMVB area. Reversed polarities observed can be correlated to the Gilbert chron, normal polarities to the Gauss chron or the Brunhes chron and intermediate polarities to the Cochiti-Gilbert or the Gilbert-Gauss transition. The reversed or intermediate polarity magnetisation recorded in one of the sites (542 ± 24 ka) corresponds either to the West Eifel 4 or the West Eifel 5 excursion, while the reversed polarity observed in the other site (220 ± 36 ka) very likely provides new evidence for the Pringle Falls excursion or the event recorded in the Mamaku ignimbrite.     

Die Bestimmung des Gesamtphosphors in Abwässern mit Ammoniummolybdat und 1-Amino-2-naphthol-4-sulfonsäure («ANS-Heizmethode»)

Zusammenfassung Es wird eine Arbeitsvorschrift zur Bestimmung des Gesamtphosphors in h?uslichen bzw. kommunalen Abw?ssern, Kl?ranlagenabflüssen, auch nach P-Elimination, und in mit Abwasser verunreinigten Vorflutern angegeben. Bei industriellen Abw?ssern ist die Vorschrift, je nach der Art der darin enthaltenen Phosphorverbindungen, nicht in jedem Falle anwendbar. St?reinflüsse durch Eisen und Silikate wurden überprüft.

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Summary A method is described for the determination of total phosphorus in domestic and municipal sewage as well as in effluents of sewage treatment plants, even after treatment for the elimination of phosphorus, and in polluted waters. The method is not in any case appropriate for industrial wastes. This depends on the nature of their phosphorus compounds. Interferences by iron and silicate have been studied.

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Frau Martha Vogel-Huber sei für die gewissenhafte Ausführung eines grossen Teils der experimentellen Untersuchungen bestens gedankt.     

亚热带富铁土的磁学性质及其磁性矿物学

通过５５个亚热带富铁土的磁化率（χ）、频率磁化率(χ_ｆｄ)、非滞后剩磁（ＡＲＭ）和饱 和等温剩磁（ＳＩＲＭ）等磁性参数测定,结合氧化铁化学形态分析和矿物鉴定,初步明确了亚热 带富铁土的磁学特征及其磁性矿物．磁测数据表明富铁土中存在强磁性的矿物,其磁化率χ 与土壤游离氧化铁（Ｆｅ_ｄ）含量呈极显著指数正相关（Ｒ^２＝０．５９７１）,频率磁化率χ_ｆｄ与土壤游离氧 化铁含量呈极显著直线正相关（Ｒ^２＝０．４２８９）．富铁土的χ_ｆｄ和非滞后磁化率χ_ＡＲＭ。值表明土壤 中的磁性矿物以超顺磁性（ＳＰ）和稳定单畴（ＳＳＤ）颗粒为主,富铁上的χ和χ_ＡＲＭ呈极显著直线 正相关（Ｒ^＝ ０．９４２９）,证明富铁土的磁性是由风化成土过程产生的 ＳＰ和 ＳＳＤ磁性颗粒贡献 的．矿物磁测结合Ｘ－衍射证明富铁土中的氧化铁矿物由赤铁矿、磁赤铁矿和针铁矿组成。     

Diffraction imaging by prestack reverse‐time migration in the dip‐angle domain

Prestack image volumes may be decomposed into specular and non‐specular parts by filters defined in the dip‐angle domain. For space‐shift extended image volumes, the dip‐angle decomposition is derived via local Radon transform in depth and midpoint coordinates, followed by an averaging over space‐shifts. We propose to employ prestack space‐shift extended reverse‐time migration and dip‐angle decomposition for imaging small‐scale structural elements, considered as seismic diffractors, in models with arbitrary complexity. A suitable design of a specularity filter in the dip‐angle domain rejects the dominant reflectors and enhances diffractors and other non‐specular image content. The filter exploits a clear discrimination in dip between specular reflections and diffractions. The former are stationary at the specular dip, whereas the latter are non‐stationary without a preferred dip direction. While the filtered image volume features other than the diffractor images (for example, noise and truncation artefacts are also present), synthetic and field data examples suggest that diffractors tend to dominate and are readily recognisable. Averaging over space‐shifts in the filter construction makes the reflectors? rejection robust against migration velocity errors. Another consequence of the space‐shift extension and its angle‐domain transforms is the possibility of exploring the image in a multiple set of common‐image gathers. The filtered diffractions may be analysed simultaneously in space‐shift, scattering‐angle, and dip‐angle image gathers by means of a single migration job. The deliverables of our method obviously enrich the processed material on the interpreter's desk. We expect them to further supplement our understanding of the Earth's interior.     

Association of iron oligomeric species with natural organic matter: a combined EELS and M?ssbauer investigation

The nature of the interaction between iron oxyhydroxide compounds and natural organic matter (NOM) may take various forms and is still a matter of debate. It is an important field to understand, especially for water treatment applications and for the knowledge of iron transport in the environment. The nature of association reached between iron oligomeric species and NOM is here investigated using M?ssbauer spectroscopy and electron energy-loss spectroscopy (EELS) at the Fe-L3 edge. Raw water NOM taken from Moselle River (France), natural humic substances extracted from the riverine suspended matter, and a synthesized humic-like substance, are coagulated with iron nitrate according to a jar-test procedure. The results from M?ssbauer spectroscopy indicate that Fe is present in an octahedral coordination environment, which is consistent with prior X-ray absorption spectroscopy reported in the literature. The areas beneath the peaks (Fe L3 edge) and the peak shapes of EELS spectra differ according to the origin of the organic matter, suggesting that various types of Fe populations can be distinguished using the EELS technique. Combining the selectivity of both M?ssbauer spectroscopy for identifying trace, poorly crystalline Fe solids, oxidation state and of EELS for being able to characterize the population of Fe based on L-edge spectra, appears promising for characterizing Fe in systems containing NOM.     

Mineral accumulations induced by biological activity on granitic rocks in Qinghai Plateau,China

Fragments of weathered granitic rocks from the Kunlun Shan, Qinghai Plateau (China) were investigated to elucidate the in?uence of biotic crusts on the breakdown of granitic rocks in an alpine environment. Scanning electron microscopy with energy dispersive system and X‐ray diffractometry were used to describe the nature and properties of mineral accumulations on the rock surface. Results showed that organic salts such as calcium oxalate and calcium formate are associated with Aspicilia caesiocinera (Nyl.ex Malbr.) Arnold, Caloplaca sp., Xanthoria elegans (Link) Th.Fr., and Lecidea plana (Lahm) Nyl. Secondary accumulations of 2 : 1 clays minerals are found in A. caesiocinera while oxides of manganese are associated with X. elegans. Coatings of goethite (iron oxides) are believed to form from biological activity associated with the presence of hyphae and rodlet structures on the ?akes. Calcium oxalate crystallizes into several morphologies such as druse, hexagonal plates, and lenticular containing between 20 and 48 per cent calcium by weight. Calcium formate and iron oxide (goethite) occur together in the form of ‘red’ desert varnish. Observed ‘black’ coatings contain as much as 37 per cent manganese and 22 per cent iron. Clay accumulations have plate‐like morphology and contain c. 2 : 1 silicon to aluminium contents. We argue that organic acids from the activities of biotic crusts contribute to the breakdown of granitic rocks. Fungi accelerate the breakdown of granitic rocks through the growth of fungal hyphae along the 001 cleavage planes in primary chloritic minerals. Copyright © 2003 John Wiley & Sons, Ltd.