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

Spodumene and petalite are the primary minerals mined from pegmatites, the most important sources of hard-rock Li resources. In Li-mineralized pegmatites, the subsolidus reaction of petalite → spodumene + 2quartz (1) and its reverse reaction: spodumene + 2quartz → 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 equipment (e.g., 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 they 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 observe the reaction process and crystallization dynamics in situ. The reaction rates (V_r) 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. V_r is driven by the Gibbs free energy change (ΔG_r) of the reaction, which is pressure- and temperature-dependent. As ΔG_r (< 0) decrease, V_r rises. V_r also correlates positively with the growth rate (V_g) of secondary spodumene, though V_g is lower than that of primary spodumene formed in pegmatite-forming melts. Hence, by extrapolating experimental ΔG_r–V_r data to pressure-temperature (P–T) conditions of pegmatite formation, we can estimate V_r and thus assess V_g for secondary spodumene in natural SQI. For example, the formation of secondary spodumene in the Highbury pegmatite (South Africa) is ributed to Reaction (1), which occurred at a V_r of 10⁻⁸–10⁻⁹ 34 cm³/day under conditions of 370 MPa and 400–500 °C. This V_r could correspond to a high V_g, 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 V_g under P–T conditions of ∼290–260 MPa and below 400 °C. Reaction (2) at 244 MPa and 600 °C yielded V_r 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.