The Problem of Expressing the Specific Surface Areas of Clay Fractions
Estimations of the external specific surface areas (S.S.A.) by the N2-BET method of clay separates that were further fractionated by the high gradient magnetic separation technique revealed that the magnetic fractions had consistently lower S.S.A. compared to non-magnetic fractions. This phenomenon has been attributed, in the past, to the intimate association of Fe-oxides with silicate clays. It is the contention of this study that this reasoning is insufficient due to the following reasons. X-ray diffractograms (XRD) confirmed that heavy minerals were abundant in the magnetic fractions of these clays. Total chemical analyses and energy dispersive X-ray analyses showed that these heavy minerals contained Fe and Ti, which were not completely extracted by the dithionite-citrate bicarbonate (DCB) treatments. Crystallinity and quantity of these oxides in the different fractions did not show any relationships with the S.S.A. Lower S.S.A. were found in the magnetic fractions of both coarse and fine clays in the untreated as well as DCB-treated samples. The average particle density of the magnetic fractions was found to be higher than the non-magnetic fractions, resulting in an underestimation of the S.S.A. This underestimation was further proven when clay-sized illmenite (density = 4.79 Mg m−3) was found to have lower S.S.A. than quartz (density = 2.65 Mg m−3) and well-crystallized Georgia kaolinite (density = 2.61 Mg m−3), even though the illmenite particles were smaller in size compared to the kaolinite particles and similar in size compared to the quartz particles. It is, therefore, proposed that the specific surface areas should be expressed either on a volumetric basis or corrected for differences in density to avoid underestimations when heavy minerals are present in the samples.
Key WordsDensity Fe and Ti oxides Magnetic and non-magnetic fractions Specific surface area
Unable to display preview. Download preview PDF.
- Buol, S. W. 1985. Mineralogy classes in soil families with Low Activity Clays. In Mineral Classification of Soils. J. A. Kittrick, ed. Soil Sci. Soc. Am. Special Pub. No. 16. Soil Sci. Soc. Am. Inc. Madison, Wisconsin: American Soc. of Agron. Inc., 169–178.Google Scholar
- Deer, W. A., R. A. Howie, and J. Zussman. 1967. An introduction to the rock-forming minerals. London: Longmans, Green and Co. Ltd. 528 pp.Google Scholar
- Hillel, D. 1982. Introduction to soil physics. New York: Academic Press Inc. 364 pp.Google Scholar
- Jackson, M. L. 1969. Soil chemical analysis—Advanced course. 2nd. Edition, 8th. Printing, 1973. Published by M. L. Jackson. Dept of Soil Science, Univ. of Wisconsin, Madison, Wis.Google Scholar
- McKeague, J. A. Ed. 1978. Manual on sampling and methods of analysis. 2nd. Edition. Can. Soc. of Soil Sci., Ottawa. Canada.Google Scholar
- Rieke, R. D., T. S. Vinson, and D. W. Mageau. 1983. The role of specific surface area and related index properties in the frost heave susceptibility of soils. Permafrost: Fourth Int. Conf., Proc. (National Academy of Sciences), Washington, D.C.: National Academy Press, 1066–1071.Google Scholar
- Sposito, G. 1984. The surface chemistry of soils. N.Y.: Oxford Univ. Press., Oxford: Clarendon Press. 234 pp.Google Scholar
- Van Olphen, H., and J. J. Fripiat Ed. 1979. Data handbook for clay materials and other non-metallic minerals. Oxford, New York, Toronto, Sydney, Paris, Frankfurt: Pergamon Press Inc., 346 pp.Google Scholar
- Wann, S. S., and T. C. Juang. 1985. Grouping soils for management practice. FFTC Book series No. 29: 41–54.Google Scholar