The (Mg,Fe2+) substitution in ferri-clinoholmquistite, squareLi2(Mg,Fe2+)3Fe3+2Si8O22(OH)2
Résumé
Amphiboles have been synthesized by hydrothermal techniques at 500–600 °C, 1 Kbar P(H2O) and low oxygen fugacity (close to NNO) at nominal compositions along the join ferri-clinoholmquistite [squareLi2Mg3Fe3+2Si8O22(OH)2] – ferri-clino-ferroholmquistite [squareLi2Fe2+3Fe3+2Si8O22(OH)2]. SEM-EDS and X-ray powder diffraction show that at 500 °C, amphibole is the dominant phase in the run assemblage, together with minor clinopyroxene + quartz, in the Fe-rich side of the join, up to sim50 mol. % ferri-holmquistite component in the system. For higher Mg contents, an assemblage consisting of predominant clinopyroxene + quartz is obtained, and amphibole is only a minor phase. At 600 °C, the situation is reversed: amphibole is the predominant phase in the Mg-rich side of the join (sim60–100 mol. % ferri-holmquistite component in the system). X-ray powder-diffraction patterns can be indexed in space group C2/m and cell parameters show well-defined linear trends as a function of nominal amphibole composition. Mössbauer spectra show that Fe3+ is strongly ordered at the M2 site, whereas Fe2+ is disordered over the B and C sites. The infrared OH-stretching spectra of intermediate compositions show fine structure caused by ordering of Mg and Fe2+ over the M1 and M3 sites in the amphibole; this fine structure consists of four main bands assigned to the four local combinations of (Mg,Fe2+) at M(1,3). The relative intensities of these bands show that there is almost complete Mg-Fe2+ substitution at M(1,3) in ferri-clinoholmquistite and that there is no significant short-range ordering (cluster and/or anti-clustering) between these two cations at the M(1,3) sites. The IR spectra show, in addition, four minor components that are assigned to (Fe2+,Mg) at M(4), indicating that the amphibole composition departs from the nominal one following the vector M4Fe2+ M2Mg M2Fe3+-1M4Li-1.