Reports: UR250759-UR2: Experimental Synthesis of Authigenic Clay Minerals: Implications for Lacustrine Deposition and Diagenesis

Daniel Deocampo, Ph.D., P.G., Georgia State University

Project Objectives

        This project is using lab synthesis experiments to test several hypotheses based on detailed studies of clay minerals in saline, alkaline lakes in East Africa and other regions of the world (Deocampo, 2004).  In these settings, magnesium enrichment observed in ancient lake sediments has been used as an indicator of aridity in ancient times.  The recent development of high quality geochronologically controlled stratigraphic sections allows these geochemical indicators to be used in developing high quality paleoenvironmental and paleoclimate change reconstructions (Deocampo et al., 2009).
        Project hypotheses address 1) the role of detrital aluminum-rich substrates on clay authigenesis; 2) aqueous silica saturation state effects on authigenesis; 3) carbonate brines, cation exchange (Ca desorption), and interstratification; 4) biogenic CO2-induced clay dissolution; and 5) the importance of alkali concentrations on low-temperature illitization of Fe-reduced smectite.

        To test hypothesis #1, the hydrothermal sepiolite experiments of Wollast et al. (1968) and Mizutani et al. (1991) are being modified by addition of an aluminum-rich detrital component (Clay Minerals Society standard clay SWy-2).  To test hypothesis #2, synthetic brines are being formulated with varying silica concentrations, and in contact with buffering diatomite.  To test hypothesis #3, simulated brines have been formulated (Jones et al., 1977) to test for cation exchange from calcium-saturated standard clays.  To test hypothesis #4, clay suspensions are placed in CO2-enriched waters under ambient and high-pressure conditions.
        Experimental fluids are being monitored by AA and ICP-AES.  Mineralogy is being monitored with a new NSF-supported Panalytical X-ray diffractometer recently acquired by the project PI.  Geochemistry of 500mg separates is being determined by Rigaku wavelength dispersive X-ray fluorescence spectroscopy, and SEM and TEM are being used to characterize experimental materials.

Student Participation
        In Year 2, three undergraduates and one M.S. student were supported by the project.    During Year 2, six presentations were made at national or regional meetings of the Geological Society of America by either currently-supported students, or those supported in Year 1.  These include a presentation by a student who helped develop the brine synthesis methods for this project's experiments (Yurman and Deocampo, 2012); a student who helped develop the high-temperature Parr bomb experimental methods (Raines and Deocampo, 2012); and preliminary tests of hypothesis #1 (Pickering et al., 2013a), hypothesis #3 (Taylor et al., 2013), and hypothesis #5 (Razumov et al., 2013).

            Taylor et al. (2013) showed the predicted increase in cation exchange associated with carbonate brines, a key initial step assumed to be necessary in the development of Mg-rich clay interstratifications (Deocampo et al., 2009).  Razumov et al. (2013) demonstrated how brines increase the amount of K taken up to Fe-rich clays after artificial Fe-reduction, implying more effective illitization under such conditions.  Pickering et al. (2013a; 2013b) showed the development of trioctahedral (Mg-rich) domains during the clay synthesis experiments (Figure 1), an important indicator of active diagenetic mineral crystallization. 

Figure 1.  X-ray diffractograms of clay synthesis experimental materials (Pickering and Deocampo, 2013), powdered and randomly oriented, and limited to the range of 2-theta values where 060 peaks occur attributable to the octahedral layer in sheet silicates.  SWy-2 (Clay Minerals Society standard source clay) and Mg-Silicate gel patterns are raw materials, the green pattern shows the mixture of the two following thermal treatment.  Subsequent analysis suggests the peak in the synthesized materials represents trioctahedral (Mg-rich) domains formed in increasingly crystalline sepiolite.

The students in the lab are now following up on these initial results, in particular exploring X-ray diffraction data indicating increases in crystallinity of synthetic products.  One of the undergraduates from Year 2 of the project (Lucy Taylor) has now graduated and entered the MS program, and she is continuing to work with the project as a senior mentor for the new team of undergraduates.  The group is planning to submit their results at the upcoming southeast section meeting of the Geological Society of America in Virginia. 

References (*student author*)

Deocampo, D.M., 2004.  Authigenic clays in East Africa: Regional trends and paleolimnology at the Plio-Pleistocene boundary, Olduvai Gorge, Tanzania.  Journal of Paleolimnology, vol. 31, p. 1-9.

Deocampo, D.M., Cuadros, J., Wing-Dudek, T., Olives, J., and Amouric, M., 2009.  Saline lake diagenesis as revealed by coupled mineralogy and geochemistry of multiple ultrafine clay phases: Pliocene Olduvai Gorge, Tanzania.  American Journal of Science, vol. 309, p. 834-868.

*Huckins, S.*, and Deocampo, D., 2012.  Carbon sequestration in northeast Georgia: potential clay dissolution in the Cambrian Weisner Quartzite (Sandstone).  Geological Society of America Abstracts with Programs, vol. 44, p. 62.

Jones, B.F., Eugster, H.P., and Rettig, S.L., 1977.  Hydrochemistry of the Lake Magadi basin, Kenya.  Geochimica et Cosmochimica Acta, vol. 41, p. 53-72.

*Raines, J.E.*, and Deocampo, D.M., 2012.  Caprock interactions with supercritical CO2 and brine: a laboratory study of the effects of simulated geological CO2 sequestration on shale's from the Black Warrior River Basin, Alabama.  Geological Society of America Abstracts with Programs, vol. 44, p. 395.

Mizutani, T., Fukushima, Y., Okada, A., and Kamigaito, O., 1991.  Hydrothermal synthesis of sepiolite.  Clay Minerals, vol. 26, p. 441-445.

*Pickering, R.A., Taylor, L.C., Razumov, P.*, and Deocampo, D.M., 2013.  Role of Al-rich detritus in Mg-silicate supersaturated solutions.  Geological Society of America Abstracts with Programs, vol. 45, n. 2, p. 60.

*Pickering, R.A.*, and Deocampo, D.M., 2013.  Tri-Octahedral domains in synthetic clays: implications for lacustrine paleoenvironmental reconstruction.  Geological Society of America Abstracts with Programs, vol. 45, no. 7, p. 781.

*Razumov, P., Taylor, L.C., Pickering, R.A.*, and Deocampo, D.M., 2013.  Clay synthesis: Modeling the uptake of K+ during low-temperature illitization of smectite.  Geological Society of America Abstracts with Programs, vol. 45, n. 2, p. 12.

*Taylor, L.C., Razumov, P., Pickering, R.A.*, and Deocampo, D.M., 2013.  Effects of sodium carbonate brines on cation exchange and clay interstratification.  Geological Society of America Abstracts with Programs, vol. 45, n. 2, p. 12.

*Yurman, S.*, and Deocampo, D.M., 2012.  Carbon sequestration potential in simulated saline lake waters.  Geological Society of America Abstracts with Programs, vol. 44, p. 395.

Wollast, R., Mackenzie, F.T., and Bricker, O.P., 1968.  Experimental precipitation and genesis of sepiolite at earth-surface conditions.  American Mineralogist, vol. 53, p. 1645-1662.