Oriented transformation of grunerite to clinoferrosilite at 775° C and 500 bars Argon pressure

A member of the grunerite series (Fe_6.685Mn_0.142Ca_0.110Mg_0.096) (Si_7.968Al_0.016)O₂₂(OH)₂, has been transformed to clinoferrosilite, amorphous silica and water at 775° C and 500 bars Argon pressure. Single crystal photographs of an oriented intergrowth of the amphibole and pyroxene show that the clinoferrosilite retains the cristallographic a, b and c axes of the original grunerite in the I2/m orientation and has the cell dimensions: a=9.77, b=9.08, c=5.30 Å, =109.5°; space group P2₁/c. Mössbauer resonance spectroscopy confirmed the identity of the olinoferrosilite.A transformation mechanism requires a minimum of ionic movements in an acceptor region (7/8 of the total crystal), where pyroxene is formed by expulsion of protons and acceptance of Fe²⁺ ions; simultaneous destruction of donor regions provide the Fe²⁺ ions, the residue being silica and water. The formation of metastable clinoferrosilite rather than fayalite and quartz indicates the strong structural control imposed by the host grunerite structure on the nature of the transformation products.