Determination of Mn speciation in aragonitic freshwater bivalve shells using synchrotron x-ray absorption and emission techniques

SOLDATI, ANALIA L.; GECK, JOCHEN; JACOB, DORRIT E.; GLATZEL, PIETER; SWARBRICK, JANINE C.

Budapest, Hungry

Congreso; IMA 2010 20th General Meeting of the International Mineralogical Association; 2010

International Mineralogical Association

We investigated shells from seven different freshwater bivalvespecies by means of spin-resolved x-ray absorption near edgespectroscopy (XANES) and x-ray emission spectroscopy(XES). The shells are composed of aragonite (CaCO3) and lessthan 5 wt% of organic material. The aim of the experiments wasto clarify whether Mn replaces the Ca2+ ion in the carbonatestructure or if it is associated with the organic molecules in theshells (inter- and intra-crystalline).Bivalve shells used here comprise specimens Hiriopsis cumingii, Margaritifera margaritifera, Margaritifera falcata, Anodonta cygnea, Anodonta Anatina, Unio tumidus and Diplodon chilensis from two different patagonian lakes. XANES spectra of the different bivalve shells (independent ofthe bivalve type, provenance, or if modern or archaeological)show the same absorption features with identical positions ofthe Mn K-edge (6.548 keV), the white line (6.552 keV), as wellas of the position (6.540 keV) and intensity of the pre-edge peak[2]. The Mn pre-edge feature is a result of quadrupole 1s 3dtransitions and is present at the same energy in all shells as wellas in Mn-bearing calcite (CaCO3) and in synthetic and naturalrhombohedral MnCO3. In addition, the spin-resolved XANESshows that the electronic configuration of Mn in these materialsis largely 3d5 corresponding to Mn2+.Mn-K! emission spectra provide detailed informationregarding the chemical environment (i.e. the next neighbours).All shells show the same Kb´ and Kb1,3main, and Kb2,5 satellitelines. Most importantly, all spectra show the same features asthe emission spectra of a Mn-bearing calcite crystal and ofMnCO3, while they clearly differ from Mn-bearing aragonitesand reference compounds containing other Mn coordination.These data therefore reveal that: (i) Mn in the aragonitic shellsresides largely in the mineral part and not in the organicmaterial. (ii) Mn in the orthorhombic shell aragonite is presentas Mn2+ in a calcite microenvironment with rhombohedralcoordination.