COORDINATION AND LOCAL STRUCTURE OF MAGNESIUM IN SILICATE MINERALS AND GLASSES : Mg K . EDGE XANES STUDY

We present Mg K-edge XANES spectra of selected Mg-bearing oxide and silicate minerals with different coordination states of Mg. The Mg K-edge peak shifts to higher energy with increasing coordination, from talMg in spinel, to tslMg in grandidierite, to t61Mg in diopside and many other silicates, and to I8lMg in pyrope. The correlation between the energy of the Mg K-edge and Mg-O bond distance of the model minerals is also established; it can be used to estimate the average Mg-O bond distance in disordered systems. The curve fitting of Mg K-edge XANES spectra may be used to distinguish the coordination of Mg, and to determine the relative proportion of different coordination sites, as demonstrated in yoderite, which contains both t5lMg and t6lMg. The structural role of Mg in CaMgSi2O6 @i) NaAlSi3Os (Ab) glasses was studied using Mg K-edge XANES spectra. The Mg-O bond distance in these glasses is estimated to be 2.00 + 0.04 A. Thus, Mg in these glasses may be five-coordinated with oxygen, or Mg may have multiple structural sites, t41Mg, t5lMg and t6lMg. Structutally, the Di-Ab glasses may possess a medium-range order, and have dramatically different multiple scattering (MS) paths from those of crystalline model minerals.