Synthesis and crystal chemistry of Li-hydro-pyroxenoids

Abstract

The phases present within the system Li₂SiO₃-Mn₂SiO₄-SiO₂+H₂O under total P_H2O=2kb at 740°C are Li₂SiO₃, MnSiO₃ (rhodonite), Mn₂SiO₄ (tephroite), LiMn₄Si₅O₁₄OH (Li-hydrorhodonite) and SiO₂ (quartz). Under 3kb at 500°C, these are Li₂SiO₃, Li₂Si₂O₅, MnSiO₃ (pyroxmangite), Mn₂SiO₄ (tephroite), Li Mn₄Si₅O₁₄OH (Li-hydrorhodonite), LiMn₂Si₃O₈(OH) (Li-serandite)? and SiO₂ (quartz). The pressure-temperature stability field of the end member Li Mn₄Si₅O₁₄OH is wide, ranging from 400°C to 800°C under 1.5kb water pressure. A reversible dehydration of synthetic Li-hydrorhodonite with the above composition vs. anhydrous rhodonite+quartz has been established at 850°C under 1.5kb, P_H2O.
A Li-serandite-like phase with the probable composition, LiMn₂Si₃O₈OH, is stable from 300°C to 600°C under 1.5kb water pressure. This compound decomposes to Li-hydrorhodonite+Li₂SiO₃ with rising temperature (6000-800°C), and finally to anhydrous rhodonite+liquid at over 800°C under the same water pressure (1.5kb).
Solid solution series of Li-hydrorhodonite are formed by the replacement of four Mn²⁺ in one unit formula by Mg²⁺, Fe²⁺ or Ca²⁺. These substitutions reach a maximum when Mg=2.5; Fe=2.0 and Ca=0.5. The unit cell dimensions of the synthetic LiMn₄Si₅O₁₄OH are a=7.549 (2), b=11.778 (3), c=6.726 (2), α=93°11′(1′), β=95°20′(1′), γ=106°19′(1′) and V=569.4 (1) with triclinic space group P^-1. Indexed powder diffraction data are also given for LiMn₄ Si₅O₁₄OH.
The idealized composition of the new mineral nambulite, LiNaMn₈Si₁₀O₂₈(OH)₂ failed to crystallize as a single phase under the diverse conditions here studied.