Microscopy Studies of the Palygorskite-to-Smectite Transformation
Tóm tắt
The transformation process between palygorskite and smectite was studied by examining the morphological and structural relationships between these two minerals in an assemblage from the Meigs Member of the Hawthorne Formation, southern Georgia. Studied samples were related to an alteration horizon with a tan clay unit above and a blue clay unit below. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) were used to study the mechanism of transformation. From AFM data, both clay units contain euhedral palygorskite fibers. Many fibers are found as parallel intergrowths joined along the [010] direction to form ‘raft-like’ bundles. Degraded fibers, which are common in the tan clay, have a distinctly segmented morphology, suggesting a dissolution texture. Many of the altered palygorskite fibers in the tan clay exhibit an oriented overgrowth of another mineral phase, presumably smectite, displaying a platy morphology. This latter mineral forms along the length of the palygorskite crystals with an interface parallel to {010} of the palygorskite. The resulting grain structures have an elongate ‘wing-like’ morphology. Imaging by TEM of tan clay material shows smectite lattice-fringe lines intergrown with 2:1 layer ribbon modules (polysomes) of the palygorskite. These features indicate an epitaxial overgrowth of smectite on palygorskite and illustrate the structural relationship between platy overgrowths on fibers observed in AFM data. The epitaxial relationship is described as {010} [001] palygorskite ‖ {010} [001] smectite. Energy dispersive spectroscopy indicates that the smectite is ferrian montmorillonite. Polysomes of palygorskite fibers involved in these textures commonly vary and polysome widths are consistent with double tetrahedral chains (10.4 Å), triple tetrahedral chains (14.8 Å), quadruple tetrahedral chains (21.7 Å) and quintuple tetrahedral chains (24.5 Å). The transformation of palygorskite to smectite and the resulting intergrowths will cause variations in bulk physical properties of palygorskite-rich clays. The observation of this transformation in natural samples suggests that this transformation mechanism may be responsible for the lower abundance of palygorskite in Mesozoic and older sediments.
Tài liệu tham khảo
Bickmore, B., Bosbach, D., Hochella, M.F. Jr., Charlet, L. and Rufe, E. (2001) In situ atomic force microscopy study of hectorite and nontronite dissolution: Implications for phyllosilicate edge surface structures and dissolution mechanisms. American Mineralogist, 86, 411–423.
Galán, E. (1996) Properties and applications of palygorskite-sepiolite clays. Clay Minerals, 31, 443–453.
Golden, D.C. and Dixon, J.B. (1990) Low temperature alteration of palygorskite to smectite. Clays and Clay Minerals, 38, 401–408.
Golden, D.C., Dixon, J.B., Shadfar, H. and Kippenberger, L.A. (1985) Palygorskite and sepiolite alteration to smectite under alkaline conditions. Clays and Clay Minerals, 33, 44–50.
Güven, N. and Carney, L.L. (1979) The transformation of sepiolite to stevensite and the effect of added chloride and hydroxide. Clays and Clay Minerals, 27, 253–260.
Jones, B. and Galán, E. (1988) Sepiolite and palygorskite. Pp. 631–674 in: Hydrous Phyllosillicates (S.W. Bailey, editor). Reviews in Mineralogy, 19. Mineralogical Society of America, Washington, D.C.
Khoury, H.H., Eberl, D.D. and Jones, B.F. (1982) Origin of magnesium clays from the Amargosa desert, Nevada. Clays and Clay Minerals, 30, 327–336.
Kim, J., Peacor, D.R., Tessier, D. and Elass, F. (1995) A technique for maintaining texture and permanent expansion of smectite interlayers for TEM observations. Clays and Clay Minerals, 43, 51–57.
Krekeler, M. and Guggenheim, S. (2005) Defects in micro-structure in palygorskite-sepiolite minerals: A transmission electron microscopy (TEM) study. American Mineralogist, (in review).
Krekeler, M., Guggenheim, S. and Rakovan, J. (2004) A microtexture study of palygorskite-rich sediments from the Hawthorne Formation, southern Georgia, by transmission electron microscopy and atomic force microscopy. Clays and Clay Minerals, 52, 263–274.
Leguey, S., Martin-Rubi, J.A., Casas, J., Marta, J., Cuevas, J., Alvarez, A. and Medina, J.A. (1995) Diagenetic evolution and mineral fabric in sepiolitic materials from the Vicalvaro deposit (Madrid Basin). Pp. 383–392 in: Clays Controlling the Environment (G.J. Churchman, R.W. Fitzpatrick and R.A. Eggleton, editors). Proceedings of the 10th International Clay Conference, Adelaide, Australia, 1993. CSIRO Publishing, Melbourne, Australia
Merkl, R.S. (1989) A sedimentological, mineralogical, and geo-chemical study of the fuller’s earth deposits of the Miocene Hawthorne group of south Georgia-north Florida. PhD dissertation, Indiana University, Bloomington, Indiana 182 pp.
Nagy, K.L. and Blum, A.E., editors (1994) Scanning Probe Microscopy of Clay Minerals. Workshop Lectures, 7, The Clay Minerals Society, Boulder, Colorado, 239 pp.
Patterson, S.H. (1974) Fuller’s earth and industrial mineral resources of the Meigs-Attapulgus-Quincy district, Georgia and Florida. U.S. Geological Survey Professional Paper: Report P0828, 45 pp.
Randall, B.A.O. (1956) Stevensite from the Whin Sill in the region of the North Tyne. Mineralogical Magazine, 32, 218–229.
Weaver, C.E. (1984) Origin and geologic implications of the palygorskite deposits of the S.E. United States. Pp. 39–58 in: Palygorskite-Sepiolite: Occurrences, Genesis, and Uses (A. Singer and E. Galán, editors). Developments in Sedimentology, 37. Elsevier, New York.
Weaver, C.E. and Beck, K.C. (1977) Miocene of the S.E. United States: a model for chemical sedimentation in a perimarine environment. Developments in Sedimentology, 22, Elsevier, New York, 234 pp.