Experimental investigation of trioctahedral micas in the Na2O-FeO-Fe2O3-Al2O3-SiO2-H2O-HF system

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.