HYDROTHERMAL SYNTHESIS OF A CS FERRUGINOUS TRIOCTAHEDRAL MICA

Hydrothermal experiments aimed at the synthesis of trioctahedral ferru ginous Cs micas were carried out at 100 MPa and a constant temperature in the range 500-710 degrees C or in a temperature gradient. Starting mixes consisted of SiO2, Fe2O3, Al2O3, Cs2CO3; there was excess water in all runs, and in some runs also an excess of CsOH or Cs2CO3 in sol ution. Two micas were grown with good yields, the Cs analog of annite and the Cs analog of tetra-ferri-annite. The former is usually accompa nied by variable quantities of pollucite (+/- magnetite) or fayalite. An almost Al-free ''Cs-tetra-ferri-annite'' was synthesized from an al uminous mix in thermal gradient runs, and a pure ''Cs-tetra-ferri-anni te'' was synthesized from an Al-free oxide mix. Rietveld refinement of the structure of ''Cs-annite'' shows it to be similar to the structur es of annite, tetra-ferri-annite, and ''Cs-tetra-ferri-annite'', which has the largest unit-cell of any mica synthesized. to date.