Experimental investigation of trioctahedral micas in the Na2O-FeO-Fe2O3-Al2O3-SiO2-H2O-HF system
Résumé
The purpose of this work is to explore the solid solution and stability of trioctahedral micas in the system: Na2O-FeO-Fe2O3-Al2O3-SiO2-H2O-HF. Synthesis were carried out in cold-seal pressure vessels, at 600°C, 1kbar PH2O, under fO2 conditions set by NNO and MW buffers. Starting compositions belong to the annite - siderophyllite join and can be expressed as Na(Fe3-xAlx)(Si3-xAl1+x)O10(OH,F)2, where x represents the extent of the Tschermark-type substitution. In the F-free system, under the NNO buffer, the Na-annite(OH) (x = 0) composition produced a two phase assemblage albite + magnetite. Along the join, in the 0.5 x 1 compositional range, hydrated mica is found to coexist with minor amounts of hercynite and a silicate, possibly analcime. Powder XRD shows the presence of three hydration states (2W: two water layers, 1W: one-layer water and 0W: anhydrous state) whose proportions depend on the bulk aluminum content in the system. Virtually the same results are obtained using the MW buffer. The starting composition corresponding to the Na-annite(OH) produced fayalite and albite, while Al-richer compositions gave hydrated micas with 2W and 1W hydration states; anhydrous mica occurs in very weak amounts. FTIR spectroscopy in the OH-stretching region of the hydrated micas indicates a high trioctahedral character. Experiments carried out in F-bearing system gave micas with high proportions of anhydrous state (0W). At the starting composition of Na-annite (F), a mica associated with minor magnetite and quartz is produced. Na-Al annite(F) (x = 0.5) is obtained as a pure anhydrous. It’s one of the most promising compounds obtained in this study.