Properties of synthetic triclinic KFeSi3O8, iron-microcline, with some observations on the iron-microcline⇌iron-sanidine transition

Triclinic KFeSi ₃ O ₈ , iron-microcline, has been synthesized from oxide mixes and by complete conversion of monoclinic KFeSi ₃ O ₈ , iron-sanidine. Iron-microcline is triclinic, C ī, a =8·68±0·01 Å, b =13·10±0·01, c =7·34±0·01, α =90° 45′±10′, β =116° 03′±10′, γ=86°14′±10′. The optical properties (Na light) are: α =1·585±0·002, β =1·596±0·002, γ=1·605±0·002, 2 V _α =85° (calc.), X≈ b , Z ∧ c =20°±5°. A reversible phase transition between triclinic and monoclinic KFeSi ₃ O ₈ occurs at 704°±6° C at 2000 bars total pressure. Iron-microcline is the low-temperature polymorph; no intermediate polymorphs were observed in either hydrothermal or dry heating experiments.