In situ observation of the subsolidus reactions between petalite and spodumene + quartz in a hydrothermal diamond-anvil cell

Abstract Spodumene and petalite are the primary minerals mined from pegmatites, which are the most important sources of hard-rock lithium resources. In Li-mineralized pegmatites, the subsolidus reaction of petalite → spodumene + 2 quartz (1) and its reverse reaction: spodumene + 2 quartz → petalite (2) produce secondary spodumene, usually in the form of spodumene and quartz intergrowths (SQI), and secondary petalite, respectively. The pressure-temperature (P-T) conditions under which these reactions occur have been investigated using quenching-type methods (e.g., with cold-seal pressure vessels). However, the kinetic parameters of the reactions are difficult to measure in these experiments and are thus frequently overlooked, even though their determination can significantly improve our understanding of the formation of spodumene and petalite in pegmatites. In this study, we employed a hydrothermal diamond-anvil cell to monitor the reaction process and crystallization dynamics in situ. The reaction rates (Vr) of reaction 1 in sample systems with 5–95 wt% H2O ranged from 10−6 to 10−11 cm3/day at 355–753 MPa and 500–600 °C. Vr is driven by the Gibbs free energy change (ΔGr) of the reaction, which is dependent on pressure and temperature. As ΔGr (<0) decreases, Vr rises. Vr also correlates positively with the growth rate (Vg) of secondary spodumene, though Vg is lower than that of primary spodumene formed in pegmatite-forming melts. Hence, by extrapolating experimental ΔGr–Vr data to pressure-temperature (P-T) conditions of pegmatite formation, we can estimate Vr and thus assess Vg for secondary spodumene in natural SQI. For example, the formation of secondary spodumene in the Highbury pegmatite (South Africa) is attributed to reaction 1, which occurred at a Vr of 10−8 to 10−9 cm3/day under conditions of 370 MPa and 400–500 °C. This Vr could correspond to a high Vg, leading to secondary spodumene crystals reaching up to 0.5 cm in size. In contrast, the fine-grained secondary spodumene (e.g., <0.2 mm) in the Tanco pegmatite (Canada) is possibly attributed to a lower Vg under P-T conditions of ∼290–260 MPa and below 400 °C. Reaction 2 at 244 MPa and 600 °C yielded Vr of ∼10−7 cm3/day, approaching the growth rate of petalite crystallized from pegmatite-forming melts. Thus, secondary petalite is easy to form via reaction 2 during pressure drops in pegmatite formation.