Adrien Herbillon

Characterization of a Weathering Sequence of Soils Derived From Volcanic Ash in Cameroon - Taxonomic, Mineralogical and Agronomic Implications

The characteristics of eight pedons developed in Quaternary basaltic pyroclasts in Western Cameroon under humid tropical conditions are reported. The soil weathering sequence consists of an Andept-Tropept-Udalf-Udult association. Andic properties, physico-chemical characteristics and weathering stage clearly distinguish the taxonomic units. The total reserve in bases (TRB) and the clay content measured after adequate H2O dispersion with Na+-resins both appear to be suitable weathering indices. Clay mineralogy of non andic soils shows direct relationships between interstratified halloysite-smectite content, CEC of the clay fraction and weathering stage. The weathering stage of the investigated soils is reflected not only in their components and classification, but also in their nutrient status and banana crop performance.

Characteristics of non-allophanic Andisols with hydric properties from the Ecuadorian páramos.-

So far, the occurrence of hydric properties (i.e. water retention values at 1500 kPa matrix potential larger than 1000 g kg(-1)) has generally been reported in Andisols characterised by advanced stages of weathering. Moreover, as these properties are due to the presence of short-range-ordered (SRO) minerals, they are frequently better expressed in the subsurface than in the surface horizons of these Andisols. The present paper describes the characteristics of three Hydric Melanudands whose properties distinctly depart from those briefly summarised above. These soils have been found in some of high-altitude (3200-4000 m a.s.l.) paramos of Ecuador where they are likely to cover several thousands of km(2). They either derived from or have been rejuvenated by volcanic ash deposited more recently than 3000 years BP and they develop under climatic and vegetation conditions favouring exceptionally large accumulations of organic matter. As a consequence, the deep (50-80 cm) epipedons of these Melanudands exhibit an unusual combination of properties. Though rich in primary weatherable minerals and virtually devoid of allophane, these epipedons have nevertheless 1500 kPa water contents either close to or even much larger than 1000 g kg(-1). In these Hydric Melanudands, organic colloids are the constituents responsible for their large porosity. Hydric Andisols are known to experience important irreversible physical changes on drying and this behaviour, in turn, makes them very fragile. As the hydric properties occur here in surface horizons, these paramos soils are thus expected to be especially susceptible to any land use change

Mineralogical, chemical and charge properties of Geric Ferralsols from New Caledonia

The mineralogical, chemical and surface charge properties of Geric Ferralsols of New Caledonia were examined. These soils, which corresponded to two soil mantles formed either on ultramafic rocks or volcanic ejecta and ashes, were respectively dominated by iron and aluminium oxides. The electric charge characteristics were studied by measuring retention of Ca (2+) and Cl- different pH values ranging from 3 to 7. The cation exchange capacity (CEC,) increased with soil organic carbon and pH and varied from 0 to 35 cmol(c) kg(-1) soil. The anion exchange capacity (AEC) reached 4.25 cmol(c) kg(-1) soil in Bo horizons at pH 4. The magnitude of the CECT and AEC variations was modelled according to CECT = 10(a1pH) * 10(B1) and AEC = - 10(a2pH) * 10(B2). Parameters a(1) and a(2) were low for samples with high organic carbon (from 0.10 to 0.19 and from - 0.44 to - 0.66, respectively) and could increase to 1.25 and - 0.39, respectively, when organic carbon content decreased. The parameters a(1) and a(2) could be also useful to regroup soil types according to the nature of the properties that can be manipulated for their management

Mössbauerite, Fe6 3+ O4 (OH) 8 [CO3]·3H2O, the fully oxidized green rust mineral from Mont Saint-Michel Bay, France

Abstract The new mineral mössbauerite (IMA2012 -049), Fe3+6O4(OH)8[CO3]·3H2O, is a member of the fougèrite group of the hydrotalcite supergroup. Thus, it has a layered double hydroxide-type structure, in which brucite-like layers [Fe3+6+O4(OH)8]2+ are intercalated with CO2-3 anions and water molecules. Mössbauerite is the fully oxidized analogue of fougèrite and trébeurdenite, related to them chemically by the exchange of (Fe3+O2-) with (Fe2+OH-). Mössbauerite, intimately intergrown with trébeurdenite, was discovered in intertidal gleys from Mont Saint-Michel Bay, France, along with quartz, feldspars and clay minerals. Mössbauerite is formed by the oxidation of the other members of the fougèrite group. Like them, it occurs as μm-scale platelets in gleys with restricted access to atmospheric O and decomposes rapidly when exposed to air. Identification and characterization of these minerals has relied on an electrochemical study of synthetic analogues and Mössbauer spectroscopy, which inspired the name of the new mineral. Unlike fougèrite and trébeurdenite, which are blue-green, pure synthetic mössbauerite is orange in colour. Detailed optical and other physical properties could not be determined because of the small platelet size and instability. The hardness is probably 2-3, by analogy with other members of the supergroup and the density, calculated from unit-cell parameters, is 2.950 g/cm3. Synchrotron X-ray data indicate that the natural material is a nanoscale intergrowth of 2T and 3T polytypes; the latter probably has the 3T7 stacking sequence. The corresponding maximum possible space group symmetries are P 3̅m1 and P3m1. Unit-cell parameters for the 3T cell are a = 3.032(7) Å, c = 22.258(4) = 3 x 7.420 Å and Z = ½. Mössbauer spectroscopy at 78 K indicates that two distinct Fe3+ environments exist in a 2:1 ratio. These are interpreted to be ordered within each layer, but without the development of a three- dimensional superlattice. Mössbauerite undergoes gradual magnetic ordering at 70-80 K to a ferromagnetic state, below which it splits into three sextets S1m, S2m and S3m, as measured at 15 K, and shows the same intensity ratio ½:⅙:⅓ as the three doublets for fougèrite D1f, D2f, D3f in the paramagnetic state at 78 K. This suggests that there is also short-range coupling of interlayer carbonate anions with respect to the octahedral layers and that the 2D long-range order of carbonates in interlayers remains unchanged.

Rubéfaction post-wurmienne en climat montagnard humide jurassien. Rôle du pédoclimat et actualité du processus

Abstract Post-Wurmian soils from a proglacial region (Nozeroy, Jura) have been studied. They have two B horizons which have developed under the same general climatic conditions, a reddish horizon overlying a yellowish horizon. Analysis of the mineral phases leads to the conclusions that (1) the soils have the same aeolian Wurmian origin from a nearby source; (2) the recent pedogenetic development has involved the formation of hematite essentially from in situ modification of geothite. Hematite was identified in the argillaceous granulometric fraction by three methods: colour, X-ray diffraction, and Mossbauer spectroscopy. This pedogenetic change of geothite into hematite, which may be quite recent, is apparently favoured by the extremely fissured nature of the underlying rock and the stony nature of the soil. The siting and organization of the soil-rock interface lead to repeated cycles of wetting and drying, and thus a pedoclimate with periods of desiccation sufficient to induce the formation of hematite. The severe limitations associated with the mountain climate (precipitation 1700 mm, average temperature 6°C) are in this case apparently less important than the interactions of climate and rocks. The excellent drainage due to the highly fissured rocks results apparently in a pedological environment favourable to rubification in the horizons most subject to cycles of wetting and drying.