Synthesis and stability relations of Li2Mg2[Si4O11]
Vitek, Eva; Maresch, Walter V.
European Journal of Mineralogy Volume 5 Number 6 (1993), p. 1121 - 1132
published: Dec 1, 1993
manuscript accepted: Jun 23, 1993
manuscript received: Nov 24, 1992
ArtNo. ESP147050506007, Price: 29.00 €
Abstract The recently discovered phase Li2Mg2[Si4O11] represents a new silicate structure type. It is a "loop-branched dreier single-chain silicate", in the classification of Liebau, and plays an important role in the phase relations of the system Li2O-MgO-SiO2-(H2O). New synthesis studies show that the composition is narrowly defined, with no evidence of a solid solution field in the stated system. A maximum substitution of 15 mol% of Mg by Fe2+ has been achieved in the present study. The stability relations of Li2Mg2[Si4O11] have been investigated at P(H2O) = P(total). The phase possesses a cusp-shaped stability field bounded to lower temperatures by reaction to amphibole + Li2Si2O5 + Li2SiO3 (reaction boundary passes through approximately 750°C/0.5 kbar and 795°C/1 kbar). A curve of incongruent melting passing through the coordinates 920°C/1 bar, 875°C/0.5 kbar and 855°C/1 kbar limits the stability field to higher temperatures. Detailed phase relations near the projected intersection of the two terminal reactions at approximately 1.8 kbar are still undetermined. At pressures higher than 1 bar, melts can not be quenched to glass at the cooling rates attainable in the routine high-pressure apparatus employed. The metastable assemblage quartz + Li2SiO3 + orthoenstatite always results. The present study indicates that the phase Li2Mg2[Si4O11] must possess a liquidus field at 1 bar and that a redetermination of the classical Li2O-MgO-SiO2 liquidus phase diagram will be necessary. Two brackets at 700 and 900°C indicate that the phase Li2Si2O5 breaks down to the higher-pressure/higher-temperature assemblage Li2SiO3 + quartz at 8.75 ± 0.25 kbar and 3.25 ± 0.25 kbar, respectively.