EXPERIMENTAL PERSPECTIVES on METAMORPHIC VERMICULITE

The interplay between experimental petrology that provides constraints on metamorphic terranes and metamorphic parageneses that provide the basis for experimental work was well demonstrated by the research of Bob Newton. In a similar manner, we present experimental evidence that vermiculite (Vm) is stable at metamorphic conditions.

Vermiculite, i.e. a tri-octahedral sheet silicate capable of reversible gain and loss of interlayer water, has been observed in nearly all studies dealing with glaucophane (Gl) in hydrous systems over most of the P-T region of 500-800°C and 2-25 kbar. Although regarded as a nuisance phase that forms metastably on quenching, we have demonstrated earlier (Basora & Jenkins, 2009, GSA Abs) that it grows and breakdowns by the reaction Gl + paragonite + quartz ± V = Vm + albite (Ab) (1) in the range of 16-19 kbar and 600-800°C. It is completely reversible like any other experimentally verifiable reaction. The central questions that remain are (i) what compositional conditions are needed to form vermiculite and (ii) what is its stability field? Vermiculite formation was ubiquitous in the numerous (>100) experiments done by the authors at or below 25 kbar if the amount of free water rose above 3-4 wt% of the total assemblage. Since interlayer water is readily driven out at relatively low temperatures (<200 °C at 1 atm), we suggest another water-dependent reaction is occurring to form vermiculite at high temperatures. By analogy with the rapid hydration that occurs in the octahedral strip of Na-Mg-chain silicates with excess OH, we propose that the formation of high-grade vermiculite may involve formation of sheet silicates with excess octahedral-layer OH, possibly including the exchange 2Mg2+ O2- = Al3+ + OH-. Assuming this exchange occurs in the Vm of reaction (1), giving the composition Na_0.2Mg_2.4Al_0.6Si_3.8O_9.7(OH)_2.5, the derived ΔH_f = -6015 kJ and S° = 252 J/K for a volume of 135 cm³/mol. Its upper-thermal stability via the reaction Vm = Ab + talc + corundum + V is calculated to be 20-50°C higher than that of paragonite.

Current experimental results support earlier field deductions that vermiculite can be part of the metamorphic paragenesis in at least medium-grade conditions. It is also suggested that vermiculite provides a sensitive indicator of the amount of free water during metamorphism.