Compositional limits and analogs of monoclinic triple-chain silicates (original) (raw)

2012, Contributions to Mineralogy and Petrology

Growing recognition of triple-chain silicates in nature has prompted experimental research into the conditions under which they can form and the extent of solid solution that is feasible for some key chemical substitutions. Experiments were done primarily in the range of 0.1-0.5 GPa and 200-850°C for durations of 18-1,034 h. A wide range of bulk compositions were explored in this study that can be classified broadly into two groups: those that are Na free and involve various possible chemical substitutions into jimthompsonite (Mg 10 Si 12 O 32 (OH) 4), and those that are Na bearing and involve chemical substitutions into the ideal end-member Na 4 Mg 8 Si 12 O 32 (OH) 4. Numerous attempts to synthesize jimthompsonite or clinojimthompsonite were unsuccessful despite the type of starting material used (reagent oxides, magnesite ? SiO 2 , talc ? enstatite, or anthophyllite). Similarly, the chemical substitutions of Ffor OH-, Mn 2? , Ca 2? , or Fe 2? for Mg 2? , and 2Li ? for Mg 2? and a vacancy were unsuccessful at nucleating triple-chain silicates. Conversely, nearly pure yields of monoclinic triple-chain silicate could be made at temperatures of 440-630°C and 0.2 GPa from the composition Na 4 Mg 8 Si 12 O 32 (OH) 4 , as found in previous studies, though its composition is most likely depleted in Na as evidenced by electron microprobe and FTIR analysis. Pure yields of triple-chain silicate were also obtained for the F-analog composition Na 4 Mg 8 Si 12 O 32 F 4 at 550-750°C and 0.2-0.5 GPa if a flux consisting of Nahalide salt and water in a 2:1 ratio by weight was used. In addition, limited chemical substitution could be documented for the substitutions of 2 Na ? for Na ? ? H ? and of Mg 2? ? vacancy for 2Na ?. For the former, the Na content appears to be limited to 2.5 cations giving the ideal composition of Na 2.5 Mg 8 Si 12 O 30.5 (OH) 5.5 , while for the latter substitution the Na content may go as low as 1.1 cations giving the composition Na 1.1 Mg 9.4 Si 12 O 31.9 (OH) 4.1 based on a fixed number of Si cations. Further investigation involving Mg for Na cation exchange may provide a pathway for the synthesis of Na-free clinojimthompsonite. Fairly extensive solid solution was also observed for triplechain silicates made along the compositional join Na 4 Mg 8 Si 12 O 32 (OH) 4-Ca 2 Mg 8 Si 12 O 32 (OH) 4 where the limit of Ca substitution at 450°C and 0.2 GPa corresponds to Na 0.7 Ca 1.8 Mg 7.8 Si 12 O 31.9 (OH) 4.1 (with the OH content adjusted to achieve charge balance). Aside from the Na content, this composition is similar to that observed as wide-chain lamellae in host actinolite. The relative ease with which Na-rich triple chains can be made experimentally suggests that these phases might exist in nature; this study provides additional insights into the range of compositions and formation conditions at which they might occur. Keywords Jimthompsonite Á Clinojimthompsonite Á Triple-chain silicates Á Chemical analogs Á Mineral synthesis Communicated by M. W. Schmidt.