Mineralogical Transformations and Soil Development in Shale Across a Latitudinal Climosequence

To investigate factors controlling soil formation, we established a climosequence as part of the Susquehanna-Shale Hills Critical Zone Observatory (SSHCZO) in central Pennsylvania, USA. Sites were located on organic matter-poor, iron-rich Silurian-aged shale in Wales, Pennsylvania, Virginia, Tennessee, Alabama, and Puerto Rico, although this last site is underlain by a younger shale. Across the climosequence, mean annual temperature (MAT) increases from 7 to 24°C and mean annual precipitation (MAP) ranges from 100 to 250 cm. Variations in soil characteristics along the climosequence, including depth, morphology, particle-size distribution, geochemistry, and bulk and clay mineralogy, were characterized to investigate the role of climate in controlling mineral transformations and soil formation. Overall, soil horizonation, depth, clay content, and chemical depletion increase with increasing temperature and precipitation, consistent with enhanced soil development and weathering processes in warmer and wetter locations. Secondary minerals are present at higher concentrations at the warmest sites of the climosequence; kaolinite increases from \u3c5% at northern sites in Wales and Pennsylvania to 30% in Puerto Rico. The deepest observed weathering reaction is plagioclase feldspar dissolution followed by the transformation of chlorite and illite to vermiculite and hydroxy-interlayered vermiculite. Plagioclase, although constituting \u3c12% of the initial shale mineralogy, may be the profile initiating reaction that begins shale bedrock transformation to weathered regolith. Weathering of the more abundant chlorite and illite minerals (∼70% of initial mineralogy), however, are more likely controlling regolith thickness. Climate appears to play a central role in driving soil formation and mineral weathering reactions across the climosequence

Considering Soil Potassium Pools with Dissimilar Plant Availability

Soil potassium (K) has traditionally been portrayed as residing in four functional pools: solution K, exchangeable K, interlayer (sometimes referred to as “fixed” or “nonexchangeable”) K, and structural K in primary minerals. However, this four-pool model and associated terminology have created confusion in understanding the dynamics of K supply to plants and the fate of K returned to the soil in fertilizers, residues, or waste products. This chapter presents an alternative framework to depict soil K pools. The framework distinguishes between micas and feldspars as K-bearing primary minerals, based on the presence of K in interlayer positions or three-dimensional framework structures, respectively; identifies a pool of K in neoformed secondary minerals that can include fertilizer reaction products; and replaces the “exchangeable” K pool with a pool defined as “surface-adsorbed” K, identifying where the K is located and the mechanism by which it is held rather than identification based on particular soil testing procedures. In this chapter, we discuss these K pools and their behavior in relation to plant K acquisition and soil K dynamics

Transformation expérimentale des micas en vermiculites ou smectites. Propriétés des smectites de transformation

The studies carried on these last years, have shown that the ability of micas to release their potassium is governed by the composition of the octahedral sheet and the hydroxylic constitution. The results here presented show the importance of the tetrahedral composition (rate of replacement of Si by Al) upon the ability of micas or micaceous clay minerals to transform into expansible minerals close to smectites : the lower the rate of substitution will be, the easier the transformation will be. According to this point of view, there is an opposition between trioctahedral micas for which the a-mount of tetrahedral aluminum is higher than 1 (for Si4O10) and in which evolution toward vermiculite prevails and dioctahedral micaceous clay minerals (illites -glauconites) in which, substitution being weak, transformation into smectite will be possible. More accurate studies on the minerals obtained during these transformations seem to demonstrate a certain «continuity» in the swelling properties of dioctahedral 2/1 clay minerals (vermiculites, beidellites, montmorillonites), especially by using ethylene-glycol and glycerol. Otherwise, these results can lead to a certain review of the tests of behaviour for the identification of these expansible 2/1 clay minerals.Les études de ces dernières années ont montré que l'aptitude des micas à libérer leur potassium est gouvernée par la composition fine de la couche octaédrique et de la constitution hydroxylique. Les résultats présentés ici montrent l'importance de la composition tétraédrique (taux de substitution de Si par Al) sur l'aptitude des micas ou phyllites micacées à se transformer en minéraux gonflants voisins des smectites : plus le taux de substitution sera faible , plus la transformation sera aisée. De ce point de vue, il existe une opposition entre les micas trioctaédriques pour lesquels la teneur en aluminium tétraédrique est supérieure à 1 (pour Si4O10) et où l'évolution en vermiculite sera la règle et les phyllites micacées dioctaédriques (illites-glauconites) où la substitution étant plus faible, la transformation en smectite sera possible. Des études plus précises sur les minéraux obtenus au cours de ces transformations semblent démontrer une certaine «continuité» dans les propriétés d'expansion des minéraux 2/1 dioctaédriques (vermiculites - beidellites - montmorillonites ) en particulier vis-à-vis de l'éthylène glycol et du glycérol. Ces résultats peuvent d'ailleurs conduire à une certaine remise en cause des tests de comportement pour la détermination de ces minéraux expansibles 2/1.Robert Michel, Barshad I. Transformation expérimentale des micas en vermiculites ou smectites. Propriétés des smectites de transformation. In: Bulletin du Groupe français des argiles. Tome 24, fascicule 2, 1972. pp. 137-151