Transformation Of Mica Research Articles

Mineralogical characterization of clay fraction of Chalan Beel soils of Bangladesh

Top soils of four different series viz Jaonia (Typic Haplaquept), Halti (Typic Haplaquept), Taras (Aeric Haplaquept) and Digli (Typic Haplaquept) developed on Atrai river alluvium were studied to investigate the clay mineralogical composition. X-ray diffraction technique was employed to identify and quantify the minerals. The XRD patterns were obtained with Ni-filter Cu kα - radiation at a scanning speed of 2º2θ min and the amount of minerals was estimated by XRD peak intensities at parallel powder mount. Mica was the dominant clay mineral (33 - 51%) in all soils, followed by quartz (13 -19%). The kaolinite contents were small (8 - 10%) over the soils. The occurrence of vermiculite in the Digli and Taras soils (9 and 7% respectively) indicates a considerable transformation of mica. There is very little or no smectite minerals in the soils which might be due to poor parent material and disappearance of smectite from the top soils. Appreciable amount of primary minerals like quartz and feldspars have also been observed in the clay fraction of the soils. Keywords: Clay minerals; Chalan beel; X-ray diffraction DOI: 10.3329/jbau.v6i2.4819 J. Bangladesh Agril. Univ. 6(2): 253-260, 2008

Note sur la relation entre la vermiculitisation des micas et l'orientation des dipoles OH

The characteristics of constitutional mica hydroxyls and, especially, their distribution between directions close to the c* axis or to the ab plane seem to be important for the transformation of micas into vermiculite. It is suggested that studies of this transformation should take account of the water and fluorine contents and of the detailed hydroxyl structure. Moreover some of these data are relevant of the composition and the organisation of octahedral cations.

Clay Mineral Weathering in Southern Wisconsin Soils Developed in Loess and in Shale-Derived Till

Abstract Loess is the dominant soil parent material covering the western and central portions of southern Wisconsin. Glacial till derived mainly from Devonian shales and Silurian dolomite is the most extensive parent material in extreme southeastern Wisconsin. The soils developed in loess are coarser in texture, leached of carbonates to greater depths and more acid and contain a higher percentage of zirconium in their coarse silt than those developed in the till. The clay of the soils developed in loess contains a higher proportion of minerals of more advanced weathering indices (montmorillonite, “pedogenic” chlorite and kaolinite) formed by weathering. The clay mineralogy was fairly uniform in the four soils developed from loess, but the two Gray-Brown Podzolic soils were found to contain only about one-half as high a percentage of total clay and one-fourth as high a percentage of medium and fine clay in their A horizons (which were also 7 in. less in thickness) than the A horizons of their prairie soil analogues. The Varna soil developed in the till contains clay with a higher proportion of minerals of less-advanced weathering indices (mafic chlorite and dioctahedral mica), largely inherited from the parent material. There has been some transformation of mica and mafic chlorite to expansible layer silicates and amorphous material in the Varna solum and the ratio of ferrous to total iron decreased both with decreasing particle size (from fine silt through fine clay) and with approach to the soil surface (in the whole clay).

Discussion

DR. E. A. VINCENT asked whether the author would elaborate on his suggestion that the finer grain of the rocks late in his time-sequence might be due to the concentration of volatiles causing an attenuation of the crystallization range of the minerals. Could the author explain how this was brought about ? Dr. S. I. T omkeieff said that he would like to suggest a different hypothesis in explanation of kaolinite genesis in the St. Austell granite, namely : a biphyletic hypothesis, according to which an exogenetic phase was superimposed on the endogenetic phase. Many years ago, G. Hickling in his study of the Cornish china-clay deposits (1908, Trans. Inst. M. E. 36 , 23) suggested that muscovite was an intermediate product of the transformation of feldspar into kaolinite. The transformation of mica into kaolinite was a well-known sedimentary process. Numerous German kaolinite deposits justified the “moor water” theory, according to which kaolinite and other clay minerals were formed through the action of ground-waters charged with humic substances. The widespread kaolinite deposits of the western Ukraine occurred in granites and other feldspar-bearing rocks of the Pre-Cambrian foundation overlain by Tertiary deposits often containing seams of brown coal. According to I. I. Ginzburg, who studied these deposits (1912, Ann. St. Petersburg Polytechn. Inst 17 , and 1915, ibid 22 ), they were of the nature of residual formations and, according to the evidence, were formed through leaching of underlying rocks by ground-waters in the lower zone of weathering. Dr. Tomkeieff added that he