ROLE OF AL-RICH DETRITUS IN MG-SILICATE SUPERSATURATED SOLUTIONS

The proportion of authigenic to detrital sediment in East African Neogene Lakes is variable, and is subject to many parameters. The importance of Al-rich detritus (e.g. montmorillonite) as a substrate on which authigenic phases (e.g. Mg-rich smectite) may precipitate has previously been hypothesized. For example, it has been hypothesized that the occurrence of sepiolite and kerolite (pure Mg silicates) in marshes of Amboseli National Park, Kenya, is due to the absence of Al-rich detritus. This question will be examined by preparing fluids supersaturated with respect to both sepiolite and kerolite and synthesizing Mg-rich silicates in experimental solutions containing A) No detrital material; B) Clay Minerals Society Al-rich smectite (SWy-2); C) Kaolinite (KGa-1); and D) Amboseli sepiolite collected by Deocampo (2004).

Preliminary results show some effects of adding Al-rich clay to Mg-silicate synthesis experiments. Wollast et al.'s (1968) sepiolite synthesis was replicated by using synthetic seawater and mixing in a buffered sodium silica solution to produce an X-ray amorphous Mg-silicate. The precipitate was examined using XRD and XRF and found to have a broad diffraction hump from ~3.0 – 4.0 Å, but no discernable 060 peak, which is commonly associated with clay octahedral layers. The experiment was then repeated with the addition of a purified sub-micron Na-montmorillonite standard (SWy-1) dispersed in the seawater. The second experiment produced a clay mixture with no discernable difference from SWy-1 oriented XRD, but a new broad 060 peak was observed at ~ 1.55-1.56 Å. This is slightly larger than the predicted sepiolite peak (1.54-1.55 Å) and similar to palygorskite (1.56 Å). Investigation is ongoing to characterize the newly synthesized phase and determine the compositional and textural relationships to the Al-rich detritus present.

Continuing experimentation will test whether the presence of aluminous detritus offers kinetically more favorable sites for the interstratification of Mg-silicate or aids in digenetic reactions involving Al-rich smectite. Fluid chemistries will be monitored over the course of the experiment, and precipitates will be characterized by XRD, XRF, SEM and FTIR.