Although the transition of the blueschist-greenschist facies has been proposed to be modeled by the reaction glaucophane (Gin) + paragonite (Pg) + water (V) = clinochlore (Chl) + albite (Ab) + quartz (Qtz) (1), we have found in the system Na₂O-MgO-Al₂O₃-SiO ₂-H₂O that vermiculite (Ver) forms repeatedly and persistently by way of the reaction Gln + Chl = Ver (2). We have, therefore, investigated the reaction Gln + Pg + Qtz ± V = Ver + Ab (3), as a possible alternative reaction.

Reaction (3) was studied at P-T ranges of 600°C-800°C and 14-20 kbars in a ½-inch diameter piston-cylinder press. For reversal experiments water content was kept in the range of 0-4 wt%. Experiments included (a) a reversal mixture for reaction (1), (b) a mixture of synthetic Gln and Chl for reaction (2), (c) a mixture of natural Gln ((Na_1.93Ca _0.07)(Mg_1.85Fe_1.14Fe_0.43Al_1.65 )(Al_0.08Si_7.92)O₂₂(OH)₂) and natural Chl ((Mg_4.36Fe_0.48Al_1.15)(Al _1.17Si_2.85)O₁₀(OH)₉) for reaction (2), (d) a reversal mixture for reaction (3), and (e) treatments of Ver (Na _0.5(Mg_1.50Al_1.00)(Al_0.5Si _3.50)O₁₀(OH)2•nH₂O) as a single phase at various P-T conditions.

The location of reaction (1) could not be determined because of the consistent loss of Chl and the appearance of Ver at low pressures. Reaction (3) was found to be at 600°C/16±0.5 kbars; 700°C/17±0.5 kbars; and 800°C/19±0.5kbars (at 0 wt% V content), but with partial melting at 800°C/20 kbars (with V≥3 wt%). A strong reaction occurs between synthetic Gln and Chl and natural Gln and Chl yielding Ver. Powder XRD and heating stage analyses were done to detect changes in the interlayer cation composition (Mg or Na) by looking for changes in the d001 reflection at 200°C (0% relative humidity). A decrease in Na cation concentration from 0.57 to 0.37 occurs with increasing temperature. The single Ver phase is Mg-rich with Pg present at T<800°C. At T=800°C the single Ver phase becomes a Mg-rich amphibole at P>18 kbars as is confirmed by powder XRD pattern analysis, and paragonite breaks down to corundum + albite + H₂O. The amphibole Gln yields low cation totals at T=800°C and P=20 kbars, possibly, due to an intimate mixture of the amphibole grains with quartz-albite melt.

Vermiculite appears to be a stable, high-temperature metamorphic phase, as suggested earlier by Velde (1978) and as evidenced by the stable growth and breakdown via reaction (3). It appears the Ver forms especially at the expense Gln and Chl.

Keywords: Glaucophane, chlorite, vermiculite, high-temperature, interlayer, cations