Growth mechanism of long-period biotite polytypes in the Long Valley magmatic system: A non-equilibrium crystallization model

Polytypism in minerals, particularly in phyllosilicates, holds significant interest as it reflects geochemical conditions. Short-period, such as 1 M, 2 M ₁ and 3 T polytypes are commonly found in micas across sedimentary, igneous, and metamorphic rocks. In contrast, long-period/complex polytypes predominantly occur in extrusive systems, these correlations remain subject to ongoing debate. Various mica polytypes, particularly biotite polytypes, have been frequently proposed as potential indicators of magmatic or hydrothermal crystallization environments, though this interpretation remains inconclusive. This study provides in-situ micro-nanoscale evidence to elucidate the genesis and crystallographic growth mechanisms of long-period/complex polytypes in biotite using transmission electron microscopy (TEM). Numerous biotite phenocrysts from rhyolites of the Long Valley caldera (California, USA) were examined, identifying abundant polytypes, including common (i.e., 1 M, 2 M ₁ and 3 T), long-period (4, 5, 6, and 15- layers repetition) and complex polytypes. The biotite phenocrysts display a core-rim zonation, with the core tends to develop ordered short-period polytypes, while the rim giving rise to long-period and complex polytypes. Our high-resolution TEM results suggest that long-period and complex polytypes might be fundamentally composed by more than two common polytype units, presenting a dense dislocation network along the (001) plane of biotite. In the context of equilibrium crystallization producing short-period ordered micas and non-equilibrium crystallization enabling oriented attachment (OA), a crystallization model for the formation of long-period/complex polytype biotite under non-equilibrium conditions was proposed. In this model, the crystallization of long-period/complex polytypes occurs in steps: (1) multi-ion complexes forming different nanoparticles (polytypes) who nucleate simultaneously due to a chemically and structurally fluctuation in the non-equilibrium crystallization environments; and (2) long-period polytypes are formed through the assembly between these different nano-crystals via OA and subsequent spiral growth along screw dislocation generated between the neighboring nanao-crystals. Our findings elucidate the potential origin and growth mechanism of long-period/complex biotite polytypes under non-equilibrium crystallization conditions such as magmatic systems. The proposed model offers a framework for non-equilibrium crystallization environments and insights into complex polytypes and interlayered clay minerals in diverse phyllosilicates and corresponding geological settings.