The (Mg,Fe (super 2+) ) substitution in ferri-clinoholmquistite, []Li (sub 2) (Mg,Fe (super 2+) ) (sub 3) Fe (super 3+) (sub 2) Si (sub 8) O (sub 22) (OH) (sub 2)

Amphiboles have been synthesized hydrothermally at 500-600 degrees C, 1 kbar P(H (sub 2) O) and low fO (sub 2) along the join ferri-holmquistite-ferri-clino-ferroholmquistite. At 500 degrees C, on the Fe-rich side of the join, up to approximately 50 mol.% ferri-holmquistite occurs. For higher Mg contents, an assemblage consisting predominantly of clinopyroxene + quartz is obtained, and amphibole is only a minor phase. At 600 degrees C, the situation is reversed, with amphibole the dominant phase in the Mg-rich side of the join( approximately 60-100 mol % ferrri-holmquistite); XRD patterns can be indexed in space group C2/m and cell parameters show well-defined linear trends. Moessbauer spectra show that Fe (super 3+ ) is ordered strongly at the M2 site, whereas Fe (super 2+) is disordered over the B and C sites. The IR spectra show fine structure caused by ordering of Mg and Fe (super 2+) over the M1 and M3 sites; this fine structure consists of four main bands assigned to the four local combinations of (Mg,Fe (super 2+) ) at M(1,3). There is almost complete Mg-Fe (super 2+) substitution at M(1,3) in ferri-clinoholmquistite. The IR spectra also show, in addition, four minor components that are assigned to (Fe (super 2+) ,Mg) at M4, indicating that the amphibole composition departs from the nominal one following the vector (super M) (super 4) Fe (super 2+) (super M) (super 2) Mg (super M) (super 2) Fe (super 3+) (sub -1) (super M) (super 4) Li (sub -1) .