The “chessboard” classification scheme of mineral deposits: Mineralogy and geology from aluminum to zirconium

The ancient rock record for Mars has long been at odds with climate modelling. The presence of valley networks, dendritic channels and deltas on ancient terrains points towards running water and fluvial erosion on early Mars
 1
 , but climate modelling indicates that long-term warm conditions were not sustainable
 2
 . Widespread phyllosilicates and other aqueous minerals on the Martian surface3–6 provide additional evidence that an early wet Martian climate resulted in surface weathering. Some of these phyllosilicates formed in subsurface crustal environments
 5
 , with no association with the Martian climate, while other phyllosilicate-rich outcrops exhibit layered morphologies and broad stratigraphies
 7
 consistent with surface formation. Here, we develop a new geochemical model for early Mars to explain the formation of these clay-bearing rocks in warm and wet surface locations. We propose that sporadic, short-term warm and wet environments during a generally cold early Mars enabled phyllosilicate formation without requiring long-term warm and wet conditions. We conclude that Mg-rich clay-bearing rocks with lateral variations in mixed Fe/Mg smectite, chlorite, talc, serpentine and zeolite occurrences formed in subsurface hydrothermal environments, whereas dioctahedral (Al/Fe3+-rich) smectite and widespread vertical horizonation of Fe/Mg smectites, clay assemblages and sulphates formed in variable aqueous environments on the surface of Mars. Our model for aluminosilicate formation on Mars is consistent with the observed geological features, diversity of aqueous mineralogies in ancient surface rocks and state-of-the-art palaeoclimate scenarios. A geochemical model for the state of early Mars suggests that short-term warm events grafted onto a generally cold climate could form Al-rich phyllosilicates in short-lived surface water bodies and Mg-rich ones in hydrothermal subsurface environments.

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Surface clay formation during short-term warmer and wetter conditions on a largely cold ancient Mars

Kaolin: Soil, rock and ore. From the mineral to the magmatic, sedimentary and metamorphic environments

A specific methodology was developed to refine the complex clay mineralogy commonly encountered in soil environments. The soil examined was a Cambisol developed into a ferralitic paleosol. The sample was split into four sub-fractions of different particle sizes (<0.05, 0.05-0.1, 0.1-0.2, and 0.2-2 mu m), and their respective mass contributions to the overall <2 mu m clay fraction were determined. For each sub-fraction, X-ray diffraction (XRD) patterns were modeled using a trial-and-error approach based on the direct comparison of experimental and calculated profiles. Quantitative information derived from the fitting procedure for the different sub-fractions allowed for the determination of the complex mineralogy of the <2 mu m clay fraction through the identification and quantification of eight clay phases. The results show that the finest and most reactive clay fraction (<0.05 pm) was totally hidden in the XRD pattern of the <2 mu m fraction, the fraction commonly considered in soil mineralogical analyses. Similarly, this procedure revealed the presence of illite-smectite-chlorite and kaolinite-illite mixed-layer minerals seldom described in soil literature using classical methods. The use of this methodology improved our understanding of the pedogenesis of this soil through the identification and quantification of clay phases structural properties. The analysis of the evolution of structural parameters with particle size allowed for the detection of local modifications in the interlayer composition of expandable and hydroxy-interlayered vermiculite layers. Following this approach, key information can be derived to determine subtle changes in clay mineralogical composition that are related to microorganism and/or plant activity

Unraveling complex <2 μm clay mineralogy from soils using X-ray diffraction profile modeling on particle-size sub-fractions: Implications for soil pedogenesis and reactivity

Wet-chemical extraction of soil is a standard procedure to characterise pedogenic aluminium (Al) and iron (Fe) species, especially oxides, allophanic minerals and metal–organic associations. This article critically reviews the suitability of commonly used extractants (e.g. dithionite, oxalate and pyrophosphate) and the potentials and restrictions in their use for species identification and in soil classification. None of the commonly used extractants is completely selective and quantitative. The degree of completeness differs between the extractants and depends on soil composition. Dithionite-based methods provide a ‘pseudo-total’ content of pedogenic Fe oxides, as they are not always completely dissolved. Oxalate may attack further non-target species, releasing additional Al and Fe. Therefore, the extraction of Al and Fe exclusively from poorly crystalline species is not always guaranteed. As a consequence of dispersion of aggregates, pyrophosphate solubilises both mineral particles and metals from organic associations. Thus, quantification of species based on these extractions and their implementation in pedogenic thresholds may be questionable. Alternative extractants such as citrate–ascorbate and dithionite–citrate–oxalate could be used in addition, as applicable and reliable wet-chemical extractions will be still demanded for research and practical applications. The examination of the effectiveness and selectivity of wet-chemical extraction methods by spectroscopic techniques is recommended.

Wet-chemical extractions to characterise pedogenic Al and Fe species – a critical review

In Ecuador, DINAGE (known today as the Servicio Geologico Nacional) and the German Federal Institute for Geosciences and Natural Resources have discovered a huge allophane deposit covering an area of >4000 km2 This study presents the results from an investigation of a 16-m thick vertical sequence from this deposit, supposedly the weathering product of two different volcanic ash deposits In particular, the distribution of alkali metals within the uppermost layer indicates that the weathering process is still ongoing According to the mineralogical composition, an allophane-rich layer (allophane facies) could be distinguished from the underlying halloysite-rich layer (halloysite facies) A 2-m thick transition zone is characterized by the presence of gibbsite and intermediate specific surface area values Only a few imogolite fibers could be identified (by scanning electron microscopy), indicating the dominance of allophane over imogolite in the allophane facies Single allophane particles were investigated by atomic force microscopy, though this method was less accurate than transmission electron microscopy with respect to the determination of the primary particle diameter Carbon isotope analysis (14C) suggested an age of ∼20,000 y for the allophane layer Within the allophane facies, a 4-m thick layer occurs containing 70–80 wt% allophane with an N2-BET specific surface area of >300 m2/g Based on infrared and energy-dispersive X-ray diffraction measurements, an Al/Si ratio of 13–14 was established for this allophane, which is between Al-rich and Si-rich allophane The allophane layer may be of economic value due to the large allophane content, the small amount of organic matter, and the significant thickness of the deposit

A new massive deposit of allophane raw material in Ecuador

Allophane is a very fine-grained clay mineral which is especially common in Andosols. Its importance in soils derives from its large reactive surface area. Owing to its short-range order, allophane cannot be quantified by powder X-ray diffraction (XRD) directly. It is commonly dissolved from the soil by applying extraction methods. In the present study the standard extraction method (oxalate) was judged to be unsuitable for the quantification of allophane in a soil/clay deposit from Ecuador, probably because of the large allophane content (>60 wt.%). This standard extraction method systematically underestimated the allophane content but the weakness was less pronounced in samples with small allophane contents. In the case of allophane-rich materials, the Rietveld XRD technique, using an internal standard to determine the sum of X-ray amorphous phases, is recommended if appropriate structural models are available for the other phases present in the sample. The allophane (+imogolite) content is measured by subtracting the amount of oxalate-soluble phases ( e.g. ferrihydrite). No correction would be required if oxalate-soluble Fe were incorporated in the allophane structure. The present study, however, provides no evidence for this hypothesis. Mossbauer and scanning electron microscopy investigations indicate that goethite and poorly ordered hematite are the dominant Fe minerals and occur as very fine grains (or coatings) being dispersed in the cloud-like allophane aggregates. Allophane is known to adsorb appreciable amounts of water, depending on ambient conditions. The mass fraction of the sample attributed to this mineral thus changes accordingly; the choice of a reference hydration state is, therefore, a fundamental factor in the quantification of allophane in a sample. Results from the present study revealed that (1) drying at 105°C produced a suitable reference state, and (2) water adsorption has no effect on quantification by XRD analysis.

Quantification of allophane from Ecuador

Clays consist of fine particles (clay minerals) which under certain conditions can be dispersed in water, forming a fairly stable dispersion. The stability of the clay mineral dispersions depends on the pH, solid/liquid ratio, and most of all on the type and concentration of dissolved ions. The difference between the dispersibility of different clays is not yet understood. In the present study allophane and smectite (main constituent of bentonite) are compared. Allophane surfaces are dominated by aluminol groups. Therefore, they can be used to investigate the properties of the edges of platy dioctahedral clay minerals. Dispersion and coagulation of allophane was understood based on the charge of the surface functional groups. Dispersion of allophane was only possible at pH < 5 where the aluminol groups became positive and at low ionic strength (critical coagulation concentrations (CCC) < 10 mmol/L NaCl). Dispersion at high pH may also be possible but this was not tested in the present study. In contrast to allophane, bentonite dispersions coagulated when the edges of the smectites became positive. This is probably caused by heterocoagulation leading to edge (+)/face (−) aggregation. CCC values of different bentonite dispersions ranged from 10 to 750 mmol/L. Most of the Wyoming bentonite dispersions were more stable and even above the CCC, sedimented much slower compared to other bentonites. The reason for this phenomenon could not be identified and needs further research.

https://www.mdpi.com/2504-5377/2/1/12/pdf

Annette Kaufhold

Papers

A new massive deposit of allophane raw material in Ecuador

Quantification of allophane from Ecuador

Comparison of the Critical Coagulation Concentrations of Allophane and Smectites

Complete, yet limited: The guarantee of independence for National Regulatory Authorities in the energy sector: Commission v. Germany

Large Scale to Small Scale after Mechanical Testing 2