Physics
Scientific paper
Mar 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995gecoa..59..927g&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 59, Issue 5, pp.927-937
Physics
10
Scientific paper
Carbonatite magmas have been suggested as important agents of mantle metasomatism and yet, the structures of this important class of melt, which would be expected to control physical and chemical behaviour, have only been poorly constrained. The infrared and Raman spectra of carbonate glasses quenched from the systems La(OH) 3 ---Ca(OH) 2 ---CaCO 3 ---CaF 2 ---BaSO 4 and MgCO 3 ---K 2 CO 3 at 1 kbar place constraints on the structures of these glasses and natural carbonatite magmas. The activity of the fundamental modes of the carbonate ion indicate that at least two structural populations of CO 3 2- exist in carbonate glass structure, one of which, by virtue of the large vibrational splitting of its 3 mode, is suggested to occupy a highly asymmetric site. The spectral activity of the O---H stretching region suggests that water exists both as molecular H 2 O and OH, interacting variably with carbonate ions and as metal complexes occupying relatively high symmetry sites in these glasses. The presence of bicarbonate groups, however, is prohibited by the absence of characteristic O---H stretching frequencies. It is suggested on the basis of the vibrational spectra that carbonate glass structures represent "flexible" frameworks constructed by the "bridging" of carbonate ions by strongly interacting metal cations and that the flexible framework is supported by framework modifying cations and molecular groups. The presence of at least two structural populations of CO 3 2- in carbonate glasses implies a level of medium range order and the existence of extended structural units in carbonate melts and it is suggested that such groups represent metal-carbonate complexes. The possible effects of such complexes on the geochemical behaviour of elements in carbonatite melts is discussed and a general model by which variations in elemental solubility could be understood is proposed.
Genge Matthew J.
Jones Adrian P.
Price Geoffrey D.
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