STRUCTURAL ENVIRONMENT OF ZR IN 2 INOSILICATES FROM CAMEROON - MINERALOGICAL AND GEOCHEMICAL IMPLICATIONS

The structural environments of Zr in a Na- and Fe2+-rich clinopyroxene and an arfvedsonitic amphibole (both with 1-2 wt% ZrO2) from a phonol ite of the Rumpi Hills volcanic complex, west Cameroon, were examined using X-ray absorption fine-structure (XAFS) spectroscopy at the ZrK e dge. In the clinopyroxene, tetravalent Zr is coordinated by six O atom s. Mean d[Zr-O] almost-equal-to 2.07(1) angstrom; second neighbors aro und Zr include (Si,Na) and Fe at almost-equal-to 3.16-3.18(5) angstrom . No evidence was found for Zr second neighbors. These observations ar e consistent with the presence of Zr in the M1 site. This structural a rrangement leads to local bond valence satisfaction and supports the m echanism for Zr substitution in inosilicates suggested by Jones and Pe ckett (1980). Similarities in the XANES and XAFS spectra of clinopyrox ene and arfvedsonite suggest that Zr is located in the M2 site in arfv edsonite, with mean d[Zr-O] almost-equal-to 2.04(4) angstrom. The loca l environment of Zr in these Na- and Fe2+-rich inosilicates is like th at observed in Zr-bearing garnets and Zr-bearing silicate glasses, in which [6]Zr is favored by the presence of nonbridging O atoms, to whic h Zr preferentially bonds. The similarity of the short-range structura l environment of Zr in silicate melts and in [6]Zr-, Na-, and Fe2+-bea ring inosilicates may explain why this normally incompatible trace ele ment behaved compatibly during the crystallization of the phonolites o f the Rumpi Hills volcanic complex, in which Na- and Fe2+-rich inosili cates are found. Simple bond valence arguments are used to explain why increases in the Fe3+-Fe(tot) ratio during later stages of differenti ation of the Rumpi Hills peralkaline melt should favor the incompatibl e behavior of Zr.