Accumulation of organo‐metallic complexes in laterites and the formation of Aluandic Andosols in the Nilgiri Hills (southern India): similarities and differences with Umbric Podzols

We have investigated the speciation and distribution of iron (Fe) and aluminium (Al) between minerals and organic species in A and B horizons of two Aluandic Andosols with X‐ray diffraction, thermal analyses, visible diffuse reflectance and Fourier transform infrared (FTIR) spectroscopies, together with selective and total chemical extractions. The two Aluandic Andosols of the Nilgiri Highlands (south India) have formed at the expense of intensively weathered lateritic formations of the Eocene. Data revealed that Al and Fe were predominantly stored in end‐weathering products of laterites, mainly as gibbsite and Fe (hydr)oxides in B horizons of the Aluandic Andosols. These secondary minerals are gradually replaced by organo‐metallic complexes in the topsoil A horizons exhibiting andic properties. We then indicate that formation of the organo‐metallic complexes results from weathering of the dominant crystalline Al‐ and Fe‐(hydr)oxides mediated by the organic ligands and complexation of the polyvalent metals following the accumulation of organic matter. Such weathering and complexation mechanisms are therefore similar to those recently ascribed to deferralitization and the formation of freely drained Umbric Podzols (Humus‐Podzols) in the upper Amazon Basin. In the present case, large supplies of both Al‐ and Fe‐bearing minerals provide large metal:carbon ratios that prevent the mobility of the organo‐metallic complexes and induce the formation of Aluandic Andosols rather than Podzols.     

中国福建省土壤的主要粘土矿物

The clay minerals of more than 200 soil samples collected from various sites of Fujian Province were studied by the X-ray diffraction method and transmission electron microscopy to study their distribution and evolution.Montmorillonite was found in coastal solonchak,paddy soils derived from marine deposit,lacustrine deposit and river deposit,and some lateritic red soil,red soil and yellow soil with a low weathering degree.Chlorite existed mainly in coastal solonchak and paddy soil developed from marine deposit.1.4nm intergradient mineral appeared frequently in yellow soil,red soil and lateritic red soil.The content of 1.4nm intergradient mineral increased with the decrease of weathering degree from lateritic red soil to red soil to yellow soil.Hydrous micas were more in coastal solonchak,paddy soils derived from marine deposit,lacustrine deposit and river deposit.and puple soil from purple shale than in other soils.Kaolinte was the most important clay mineral in the soils iun this province.The higher the soil weathering degree,the more the kaolinite existed.From yellow soil to red soil to lateritic red soil,kaolinite increased gradually,Kaolinite was the predominant clay mineral accompanied by few other minerals in typical lateritic red soil. Tubular halloysite was a widespread clay mineral in soils of Fujian Province with varying quantities.The soil derived from the paent rocks rich in feldspar contained more tubular halloysite.Spheroidal halloysite was found in a red soil and a paddy soil developed from olivine basalt gibbsite in the soils in this district was largely“primary gibbsite” which formed in the early weathering stage.Gibbsite decreased with the increase of weathering degree from yellow soil to red soil to lateritic red soil.Goethite also decreased in the same sequence while hematite increased.     

An Indigenous Soil Bacterium Facilitates the Mitigation of Rocky Desertification in Carbonate Mining Areas

Carbonate minerals are extensively distributed across China, and their special rock structures make them vulnerable to land damage through mining, leading to rocky desertification. Soil microorganisms play an important role in mineral weathering. However, little is known about the utilization of the mineral‐weathering microorganisms to alleviate the problem of rocky desertification in mines. In the present study, the mineral‐solubilizing bacterium NL‐11 was isolated from soil around weathered dolostones and identified as Bacillus thuringiensis based on the Biolog identification system and 16S rDNA sequence analysis. The mineral dissolution experiments revealed that inoculation with the live bacterium significantly increased the mineral sample dissolution via significantly enhancing Ca and Mg release, with increase values of 303·27 and 50·55 mg L⁻¹ respectively, compared with that with the inactivated bacterium. Moreover, the acetic acid secreted by strain NL‐11 markedly decreased the size of particle diameter (quantified with a laser diffraction particle size analyser) through reducing pH value. The eroded traces were observed by scanning electron microscopy analysis, and the results further verified the erosional effects of this strain. In addition, this bacterium contributed to the establishment and proliferation of plants by providing nutrient elements, such as phosphorus (P) and potassium (K). Our study not only provided an efficient bacterial strain NL‐11 but also enriched the technologies to mitigate problems associated with ecological restorations of carbonate mines. Copyright © 2017 John Wiley & Sons, Ltd.     

On the mechanism of specific phosphate adsorption by hydroxylated mineral surfaces: A review

Abstract

The mechanism of specific phosphate adsorption by hydroxy‐lated mineral surfaces comprises two aspects: the phosphate‐hydroxyl surface reaction and the configuration of the adsorbed phosphate ion. Evidence pointing to ligand exchange as the mechanism of the phosphate‐surface hydroxyl reaction include kinetics of adsorption and desorption; hydroxyl ion release; infrared spectroscopy, and stereochemical calculations. Data pertaining to the coordination of adsorbed phosphate on hydroxy‐lated mineral surfaces have not been conclusive overall. Isotopic exchange experiments and studies of desorption kinetics do not provide definitive information on surface coordination. Measurements of hydroxyl ion release and crystallographic calculations provide support for the existence of both monodentate and bidentate surface complexes of phosphate ions. Infrared spectroscopic investigations suggest a binuclear complex on dried, phosphated goethite. However, these studies cannot be extrapolated automatically to soil minerals, since the addition of water favors formation of a monodentate surface complex. Further research is needed to establish the configuration of adsorbed phosphate ions.     

Minerals controlling arsenic distribution in floodplain soils

As a consequence of intensive mining of the western Erzgebirge since medieval times, floodplain soils of the Mulde river contain large concentrations of arsenic (As) (>50 mg kg⁻¹). Arsenic in soil is often bound to poorly crystalline Fe and Mn (hydr)oxides, which may dissolve under reducing conditions. Part of the As may also exist in primary minerals, predominately sulphides, or in secondary minerals formed upon weathering. In order to better understand the impact of seasonal flooding, we surveyed As‐bearing mineral phases, especially of iron (Fe) (hydr)oxides. Because Fe (hydr)oxides are clay‐sized, soil samples were fractionated into six particle‐size fractions. The fractions were digested with aqua regia for determination of total element concentrations, extracted with hydroxylammonium chloride (NH₃OHCl; selective for Mn (hydr)oxides and NH₄ oxalate), and analysed by X‐ray diffraction and scanning electron microscopy. The largely similar distribution of As and lead (Pb) suggested the potential co‐existence of the two elements in primary or secondary mineral phases. However, neither As–Pb minerals nor any other As mineral were detected. Association with Mn oxides was negligible. The predominant As‐bearing phases were poorly crystalline Fe (hydr)oxides, which also incorporated large amounts of Pb and were affected by redox dynamics.     

Red soil chemistry and mineralogy reflect uniform weathering environments in fluvial sediments, Taiwan

Purpose

Information on the physicochemical properties, mineral species and micromorphology of lateritic soils and gravel soil layers in paleo-environmental soil profile is severely lacking. Red soil profile of the Taoyuan terrace was employed to demonstrate its different extents of lateritic weathering. The objectives of this study were to compare the physicochemical properties of lateritic soils and gravel soil layers and identify using conventional and synchrotron X-ray diffraction (XRD) analyses mineral species in nanoparticles separated by automated ultrafiltration device (AUD) apparatus.

Materials and methods

Soil samples were collected from paleo-environmental lateritic soils. Soil samples were examined using elemental analysis, conventional and synchrotron XRD analyses, high gradient magnetic separation, separation and collection of nanoparticles by AUD apparatus, and transmission electron microscopy (TEM).

Results and discussion

The soil pH, redness index, quantities of free Al- and Fe-oxides (Al_d and Fe_d), and clay content of lateritic soils are higher than those of gravel soil layers. Illite, kaolinite, gibbsite, quartz, goethite, and hematite were identified in clay fractions and nanoparticles by conventional and synchrotron XRD analyses. TEM images show presence of hematite nanoparticles on the surface coating of kaolinite nanoparticles and aggregated hematite nanoparticles overlapping the edge of a kaolinite flake in a size range of 4?C7?nm. Synchrotron XRD techniques are more straightforward and powerful than conventional XRD with random powder methods for identifying nanoparticles in red soils, particularly for illite, kaolinite, goethite, and hematite nanoparticles. According to chemical compositions of clay fractions and red soil features in the Taoyuan terrace, these red soils can be taken as lateritic red earths or red earths.

Conclusions

This work suggests that physicochemical properties, mineral species, and micromorphology of red soil at all depths can shed light on the extent of paleo-environmental lateritic weathering.     

Intensive kaolinization during a lateritic weathering event in South-West Spain: Mineralogical and geochemical inferences from a relict paleosol

This study documents the mineralogical and geochemical record of a lateritic weathering event during the Pliocene in South-west Spain. The paleoweathering profile derived from arkosic sands and comprises a white sandy clayey saprolite, a red mottled clay zone overlain by a thick soft layer, and a ferruginous pisolitic hardcap partially dismantled by erosive processes. Kaolinite, quartz and degraded K-feldspars are the major minerals forming the saprolite, along with Al-goethite (mottled zone) as well as gibbsite and boehmite (soft layer), while the pisolitic duricrust is essentially composed of quartz grains embedded in a matrix of Fe oxy-hydroxides (hematite, goethite, and/or maghemite). Besides quartz, the phases most resistant to alteration were a variety of heavy minerals (mostly titanium oxides) that are present as minor impurities in the residual kaolin deposit.     

土壤风化速率研究及其应用

土壤矿物风化是土壤、也是整个生态系统中无机矿质养分的最重要来源,不仅为植物长期提供养分和保持土壤的化学平衡稳定性,并缓冲土壤和地表水的酸化,还影响全球气候变化。风化速率在全球碳循环、酸临界负荷和土壤侵蚀等研究中都是非常重要的参数。土壤化学风化是一个不断进行的自然释放过程,气候是最主要的驱动力,而矿物本身的稳定性也影响了风化速率的快慢。由于人为影响下的大气酸沉降和农业活动已经非常普遍,目前的土壤风化速率也因此而改变。本文从风化速率的研究方法、风化速率的影响因素以及风化速率在全球变化中的应用3个方面介绍了近年来在风化速率方面的研究进展,并探讨了相关研究未来的发展趋势。     

Identification of nutritional status,emphasizing minerals,in northwestern zaire

Abstract

A study was conducted in northwestern Zaire during the beginning of the dry season (December 1994) at a specific ranch to determine the crude protein (CP), in vitro organic matter digestibility (IVOMD), and mineral content of the principal pasture forages. This region is located in Zaire at 4°36'N latitude and 20°28'E longitude. Average rainfall is 1695 mm a year and mean monthly temperature is 25.5°C. Soils are red lateritic clays. Thirty forage samples were collected at 2‐, 3‐, and 4‐week regrowths of Imperata cylindrica and Savanna Stylosanthes (excluding 2‐week age) from low and high elevation grazing areas. Eighteen soil samples and a mineral supplement fed to cattle (primarily N'dama breeding) were collected for analysis. There were decreases (PO.05) in IVOMD as plants matured and deficiencies of protein, sodium (Na), phosphorus (P), magnesium (Mg), and copper (Cu) at 4 weeks of regrowth. The mineral mixture fed to cattle provided less than 1% of the requirements for the deficient minerals. This study suggests the need for provision of a suitable free‐choice mineral supplement.     

Contribution of SO3 to the acid neutralizing capacity of Andosols exposed to strong volcanogenic acid and SO2 deposition

Soil response to acid and sulphur inputs is influenced largely by the soil's physico‐chemical properties. We studied the effects of such depositions in two types of Andosols exposed to volcanogenic emission (Masaya, Nicaragua), namely Eutric Andosols rich in allophanic constituents, and Vitric Andosols rich in volcanic glass. Small mineral reserves and large contents of secondary short‐range ordered minerals indicate a more advanced weathering of the Eutric than the Vitric Andosols. Strong correlations between soil specific surface and oxalate‐extractable Al, Si and Fe contents highlight the predominant contribution of short‐range ordered minerals to surface area. Both types of Andosols showed a decrease in pH upon acid input. Sulphur deposition increased the soil's S content to 5470 mg S kg^?1. However, the acid neutralizing capacity of the soil solid phase (ANC_s) was not significantly affected by the acid and S inputs. Non‐exchangeable (mineral reserve) and exchangeable cations and total contents of sulphur and phosphorus dictate most of the ANC_s variation. In the Vitric Andosols, mineral reserves contributed up to 97% to these four additive pools, whereas the exchangeable cations accounted for 1–4%. In the Eutric Andosols, the contribution of mineral reserves was less (71–92%), but the exchangeable cation content was greater (1–20%), whereas the contribution of sulphur and phosphorus was significant at 1–15% and 2–7%, respectively. The main process involved in H⁺ consumption is mineral weathering in Vitric Andosols and ion exchange in Eutric Andosols.     

Exchange properties and mineralogy of some soils derived from lavas of lower old red sandstone (devonian) age. II. mineralogy

The mineralogy of some soils developed on tills derived from basalt and andesite of Lower Old Red Sandstone age has been investigated by X-ray diffraction and optical and scanning electron microscopy. All particle-size fractions as well as some unweathered Lower Old Red Sandstone lavas are rich in layer silicate minerals. The lavas contain abundant saponite of hydrothermal or late-stage deuteric origin. This mineral occurs in rõck fragments which are found in the coarse sand and as aggregated grains in the fine sand and silt and is responsible for the high cation-exchange capacity of these fractions. As well as saponite, the clays contain kaolinite, illite and a variety of dioctahedral interstratified minerals. Scanning electron microscope observations show that plagioclase felspar grains in the fine sand fraction commonly bear regularly shaped etch pits which seem to indicate weathering by solution. The exchangeable calcium and magnesium in the soils derive mainly from decomposition of plagioclase felspar and saponite, respectively, the increasing magnesium content down the profile reflecting the higher content of saponite in the lower horizons. In all probability the exchangeable potassium which is concentrated in the clay fraction is preferentially sorbed at exchange sites on particle edges and is not associated with any particular clay mineral. Clay-mineral aggregates in lava-derived soils may be common, but they can originate in three different ways, viz,, pedogenic cementation, weathering of primary minerals, and inheritance from parent rock.     

Tracing Sediment Sources Using Mid‐infrared Spectroscopy in Arvorezinha Catchment,Southern Brazil

The widespread adoption of the sediment fingerprinting approach to guide catchment management has been limited by the cost and the difficulty to prepare and process samples for geochemical and radionuclide analyses. Spectral properties have recently been shown to provide a rapid and cost‐efficient alternative for this purpose. The current research objective was (i) to quantify the sediment source contributions in a 1∙19‐km² rural catchment of Southern Brazil by using mid‐infrared (MIR) spectroscopy and (ii) to compare these results with those obtained with geochemical approach and near‐infrared and ultraviolet–visible spectroscopy methods. The sediment sources to discriminate were cropland surface (n  = 20), unpaved roads (n  = 10) and stream channel banks (n  = 10). Twenty‐nine suspended sediment samples were collected at the catchment outlet during nine significant flood events. The sources could be distinguished by MIR spectroscopy. Cropland and channel bank sources mainly differed in their clay mineral contents, but their similar organic matter content complicated the MIR‐model predictions. Unpaved road contributions were discriminated from the other sources by their lower organic carbon content. When the results of the current research based on MIR spectroscopy are compared with those obtained using other sediment fingerprinting approaches, based on geochemistry and near‐infrared and ultraviolet–visible spectroscopy, an overestimation of channel banks contribution and an underestimation of cropland and unpaved road contributions is found. These results suggest that MIR spectroscopy can provide a useful tool that is non‐destructive, rapid and cheap for tracing sediment sources in rural catchments and for guiding the implementation of soil and water conservation measures. Copyright © 2016 John Wiley & Sons, Ltd.     

Assessing parent material uniformity of a red and black soil complex in the landscapes

The distribution of red and black soil (Xeralfs–Xerolls) associations in the Monarto area (South Australia) is complex and their genesis either being derived from a uniform parent material or a lithologic discontinuity is not known. The objectives of this study were (i) to assess Zr- and Ti-bearing grains as minerals resistant to chemical weathering prior to employing Zr and Ti in determining parent material uniformity, and (ii) to confirm whether pedological processes or a lithologic discontinuity may be responsible for the textural contrast within the red and black soil profiles. Scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX) were used to study chemical weathering and elemental composition of surfaces of Zr- and Ti-bearing grains. X-ray fluorescence (XRF) analysis was used to determine elemental concentrations in various soil fractions. Results showed that Zr-bearing grains consisted of only zircon having smooth clean surfaces, which are characteristic of a mineral resistant to chemical weathering. EDAX spectra confirmed that Zr was a specific element to represent zircon, suggesting its reliability to be used in assessing parent material uniformity. On the other hand, Ti derived not only from minerals (rutile/anatase) resistant but also from minerals (ilmenite/pseudorutile, biotite) susceptible to chemical weathering. This limited the usefulness of Ti as an index mineral for soil development studies. Except for Zr:Ti ratio, all other indicators of parent material uniformity, i.e., depth distribution curves of Zr and Y in various fractions and Y:Zr ratio showed no considerable inflection and variation with depth in a Xeralf (red soil). This indicated the Xeralf derived from a uniform parent material (mica schists), thereby texture contrast within a profile is due to pedological processes. On the other hand, depth distribution curves showed clear inflection and variation in Xerolls (black soils), indicating soils developed from lithologic discontinuities, so different trends in particle size fractions between 0–48 cm for MA5 and 0–59 cm for MA3 profiles compared to the underlying layers are due to parent material differences and not pedogenesis. The presence of complex red and black soil association in the landscape is attributed to the difference in parent materials, where the red soil developed from mica schist and the black soils from calcareous deposits.     

玄武岩发育的几种红壤的矿物特征

本文主要研究不同生物气候条件下由玄武岩母质发育的红壤、赤红壤和砖红壤的理化性质及矿物组成。粘粒中氧化铁的含量都很高,为16.98±0.83%。砖红壤与赤红壤粘粒中高岭石与非晶物质的含量相近,它们之间的差异是赤红壤中没有三水铝石,而含有7—11%蒙皂石。红壤粘粒中高岭石和非晶物质的含量都低于砖红壤和赤红壤,而水云母和蒙皂石的含量较高,也没有三水铝石。粘粒含量、阳离子交换量、硅铝率、硅铁铝率、铁的游离度和风化淋溶系数等都反映了土壤风化程度上的差异,它与水热条件特别是年均温和积温有关,进一步说明生物气候因素引起的土壤性质及矿物组成变化比其它因素强烈。砖红壤与砖红壤性水稻土的差异是在氧化铁形态上;红壤与红壤性水稻土相比,后者粘粒中蒙皂石含量略高,水云母含量略低。     

Clay Mineralogy of the Wadi M’Goun Sediments,Their Source,and Distribution on the Southern Flank of the Central High Atlas Mountains (Morocco)

Eurasian Soil Science - Clay minerals are regarded as the most critical chemical and weathering components in ground. To establish the origin and distribution of the clay mineral associations of...     

Cation exchange resin and test vermiculite to study soil processes in situ in a toposequence of Luvisol and Cambisol on loess

Both the ion accumulation on cation exchange resin and the transformation of test vermiculite in situ have been used to identify current processes in acid forest soils. We used such test materials to study weathering in a toposequence Dystric Luvisol–Spodo‐Dystric Cambisol on loess under deciduous forest in Belgium. The resin and a trioctahedral vermiculite were inserted for 2 years in the major horizons, down to a depth of 60 cm. The cation accumulation on the resin revealed that four main acid‐consuming systems are currently active in the toposequence. With decreasing acid neutralizing capacity, these systems are in the Luvisols: (i) the pool of exchangeable bases, (ii) the Al‐bearing minerals controlling the Al concentration in the liquid phase; and in the podzolized Cambisols: (iii) the less weatherable K‐bearing minerals, (iv) the Mg‐bearing phyllosilicates made free of Al interlayers in complexing conditions. The loss of cation exchange capacity in the test vermiculite is related to Al interlayering. However, this process masks a significant interlayer accumulation of magnesium, which is generated by the weathering of the test mineral itself. The largest interlayer accumulation of Mg occurs in the podzolized Cambisol, suggesting more intense weathering of the test vermiculite in this soil.     

Predicting carbon content in illitic clay fractions from surface area, cation exchange capacity and dithionite-extractable iron

We used the specific surface area (SSA), the cation exchange capacity (CEC) and the content of dithionite‐extractable iron (Fe_d) to predict the content of organic carbon in illitic clay fractions of topsoils from loess. We determined SSA (BET‐N₂ method) and CEC of clay fractions after removing organic C or reducing oxides or both. The CEC and the SSA of the carbon‐ and oxide‐free clay fraction explained 56% and 54% of the variation in C content, respectively. The Fe_d content of the clay fractions was strongly and negatively related to the C content, and with the SSA of the carbon‐free clay fraction it predicted C content almost completely (R² = 0.96). The results indicate that the amount of cations adhering to the silicate clay minerals and the size of the silicate mineral surface area are important properties of the mineral phase for the storage potential of C. The reason for the negative relation between iron oxides and C content remains unclear.     

Genesis of a Xeralf on feldspathic sandstone, South Australia

A Xeralf on feldspathic sandstone was sampled from a hillslope in the Mount Lofty Ranges in order to study its genesis and, in particular, the origin of the strong texture contrast. Micromorphological study demonstrated that the clay present in void argillans and papules in the B2 horizon accounted for only a small amount of the clay present. Elemental analysis of the whole soil (< 2 mm) and sand, silt and clay fractions showed that there had been considerable weathering of both quartz and microcline, which were the dominant minerals present. Kaolinite is the dominant clay mineral weathering product. Illite appears to be forming from vermiculite in the A horizon. Using zircon as an internal standard, it was shown that elemental losses of SiO₂ and reductions in weight and volume were similar in A and B horizons. Losses of aluminium and potassium were greatest in the A horizon, least in the B3. There has been an absolute increase in the amount of iron. A possible source is iron from heavy mineral bands upslope. It was concluded from the similarity of the quartz particle-size distributions of the A2, B2 and B3 horizons that the intensity of weathering of quartz was the same in A and B horizons. In the case of feldspar (mostly microcline), there is a greater proportion of feldspar in the fine sand and silt fractions of the A2 horizon than in the B horizon. Weathering of feldspar is greatest in the A horizon. The texture profile is principally a function of greater lateral loss of clay from the A horizons compared to the B horizons.     

Organic matter in particle-size fractions from A and B horizons of a Haplic Alisol

The organic matter in soils may be stabilized by its interactions with minerals. We have studied such interactions in a Haplic Alisol under forest in which clay and organic matter have migrated from an eluvial A horizon to accumulate in an illuvial B horizon. We have tried to trace the fate of organic matter in these horizons (Ah and Bvt) by determining clay mineralogy, carbon and nitrogen content, hydrolysable amino acids, lignin signature by alkaline CuO oxidation and carbon species by ¹³C CPMAS NMR of bulk soils and particle‐size fractions. In both horizons, most of the organic matter was present in O–alkyl and methylene structures, each contributing one‐third to the bulk organic matter. In the Ah horizon the ratios of carbon‐to‐nitrogen, and yields for lignin and hydrolysable amino acids decreased as the particle‐size class decreased, but side‐chain oxidation of lignin compounds increased with decreasing particle size. In contrast to previous observations, the proportions of O–alkyl carbon increased as particle size decreased, constituting a major proportion of the organic carbon in the clay‐size fractions from both the Ah and Bvt horizons (≥ 38%), while proportions of methylene carbon decreased. Illite was the dominant mineral in the fraction ≤ 6 μm, whereas the mobile fine clay fraction (<0.2 μm) was rich in smectites – minerals with large surface areas. Our results support the hypothesis that potentially labile organic matter, such as O–alkyl carbon typically present in polysaccharides, may be stabilized against further degradation in organomineral complexes.     

Effect of termites on clay minerals in tropical soils: fungus-growing termites as weathering agents

Termites of the subfamily Macrotermitinae play an important role in tropical ecosystems: they modify the soil's physical properties and thereby make food available for other organisms. Clay is important in the architecture of Macrotermitinae termite nests, and it has been postulated that termites could modify the mineralogical properties of some clays. We have tested this hypothesis of clay transformation by termites in the laboratory under controlled conditions, using Odontotermes nr. pauperans termite species, one of the main fungus‐growing species at Lamto Research Station (Côte d'Ivoire). Soil handled by termites in nest building was saturated with SrCl₂, glycol or KCl and afterwards heated at 250°C for X‐ray diffraction analyses. Termite handling led to an increase in the expandable layers of the component clay minerals. Heating and saturation by potassium of modified clays did not close the newly formed expandable clay layers. However, differences occurred between parts of the constructions built by termites, and the clays can be ranked according to their degree of alteration in the following order: unhandled soils < galleries < chamber walls. Consequently, termites can be seen as weathering agents of clay minerals, as previously shown for micro‐organisms and plants.