Synthesis and stability relations of Li (sub 2) Mg (sub 2) [Si (sub 4) O (sub 11) ]

This phase represents a new type of silicate structure: a 'loop- branched dreier single-chain silicate' in the classification of Liebau. In the system Li (sub 2) O-MgO-SiO (sub 2) -(H (sub 2) O), its composition is narrowly defined, with no evidence of solid solution; a maximum substitution of 15 mol.% of Mg by Fe (super 2+) has been achieved. The phase has a cusp-shaped stability field bounded at lower T by reaction to amphibole + Li (sub 2) Si (sub 2) O (sub 5) + Li (sub 2) SiO (sub 3) (reaction boundary passes through approx 750 degrees C/0.5 kbar and 795 degrees C/1 kbar). A curve of incongruent melting passing through 920 degrees C/1 bar, 875 degrees C/0.5 kbar and 855 degrees C/1 kbar limits the stability field at higher T (detailed phase relations near the projected intersections of the two terminal reactions at approx 1.8 kbar have yet to be determined). This study indicates that the phase Li (sub 2) Mg (sub 2) [Si (sub 4) O (sub 11) ] must possess a liquidus field at 1 bar and that a redetermination of the classical Li (sub 2) O-MgO-SiO (sub 2) liquidus phase diagram is necessary.