The scientific debate about the mineralogical form of manganese in anoxic sediments is centred around the
question of which distinct phases may be formed. Pore-water studies allow some conclusions about the possible
controlling mineral phases. However often such solubility calculations may not be conclusive, since analytical errors, uncertainties in the thermodynamic database, kinetic effects such as Ostwald ripening and non-steady -state conditions near the sediment surface all may jeopardise the reliability of the solubility approach. It is important to combine both, pore water studies and direct structural determinations of the dominant mineral phases.
In this work we apply extended x-ray absorption fine structure (EXAFS) spectroscopy in order to obtain information on the mineralogical transformations of manganese at the oxic/anoxic boundary of a productive lake. This sensitive spectroscopic method yields structural information on a molecular scale (× 10 Å) in "x-ray amorphous" minerals. Rather low concentrations (down to 0.05 wt %) can be analysed in dried sediment material. We combine this method with electron spin resonance (ESR) spectroscopy, scattering electron microscopy (SEM), sequential extraction and pore water analysis. The study site was located in an eutrophic hardwater lake, for which the intensity of the benthic manganese cycle has already been established (Wehrli et al., 1995). Microprobe analyses show that manganese is mainly associated with authigenic (Ca,Mn)CO3 particles. A minor fraction is found in phosphate particles such as (Fe,Mn)3(PO4)2*8H2O. The average concentration of manganese amounts to 23 mol% in carbonate particles and 16 mol% in phosphate. EXAFS spectra show that Mn(II) is incorporated in carbonates. Some features of the spectra can explained by the occurrence of (Ca,Mn)CO3 solid solution. ESR spectra point in the same direction: The coexistence of MnCO3 with a dilute Mn-phase like (Ca,Mn)CO3. Sequential extraction indicate that up to 50% of the Mn is present as weakly surface bound manganese.
This multiple method approach enables us to identify the Mn-phases in the sediment. Which one of the solid phases, MnCO3, (Ca,Mn)CO3 or (Fe,Mn)3(PO4)2*8H2O, controls Mn concentration in the porewater is difficult to prove. The
saturation state of the pore water was calculated in respect to MnCO3 and the solid solutions (Ca,Mn)CO3 and (Fe,Mn)3(PO4)2*8H2O. From spring to autumn, the pore water remains over saturated in respect to all three phases . This indicates that a different phase, for instance adsorbed Mn as determined by sequential extraction, might be important.
Wehrli B., Friedl G. & Manceau A., In Aquatic Chemistry: Principles and Applications of Interfacial and Inter-Species Interactions in Aquatic Systems (ed. C. P. Huang et al.), Vol. 244, pp. 111-133, ACS Advances in Chemistry Series (1995).