Evidence for a two-stage particle attachment mechanism for phyllosilicate crystallization in geological processes

Abstract

Abstract The understanding of crystal nucleation and growth has evolved over the past two decades from the conventional atom-by-atom model to a non-classical approach, involving particle aggregation and amorphous transformation pathways. Whereas aggregation of particles instead of individual atoms/ions/molecules has been recognized as a common crystallization pathway at the Earth’s surface conditions, few cases are known for high-temperature (e.g., melt) mineralization, which is of great importance for understanding geological processes. Here, we present texture data for natural (e.g., igneous and metamorphic biotite and muscovite) and synthetic (e.g., fluorophlogopite) phyllosilicates suggesting that a particle attachment formation should be considered, although other crystal growth models cannot be excluded. A nonclassical crystallization model is proposed for phyllosilicates forming at elevated temperatures in magmatic and metamorphic environments whereby oriented attachment of building blocks occurs along the (001) plane or the [001] direction, or both simultaneously. In this model, the crystallization of phyllosilicates occurs in steps, with multi-ion complexes forming nanoparticles, and nanoparticles coalescing (self-assembly) to form nano-flakes that become domains in larger crystallites by oriented attachment. Adjacent domains can share a common crystallographic orientation or may be rotated at various angles relative to each other. Nanoparticles may be associated with distorted bonds or may be space separated. Thus, the phyllosilicate grows into a mosaic crystal. Mosaic crystals can also form following classical crystallization models, but the process differs in that the mosaic character involves the intergrowths of nucleation sites (classical crystal-growth process) instead of the coalescence of nanoparticles building blocks (crystallization by particle attachment). These processes may be discerned by the textural differences that result. Oriented particle attachment of building blocks in phyllosilicates is recognized by a loss of closest packing by bond distortion or by space separation at domain boundaries. Crystallization by atom attachment occurs with closest packing within layers, and particles grow independently. The two processes may occur within a single environment and are not mutually exclusive. However, defects generated, for example, by chemical inhomogeneity, mechanical deformation, or sample preparation, cannot be completely excluded, although the use of synthetic, end-member material (e.g., fluorophlogopite) generated from a melt reduces these possibilities. Nonetheless, a particle attachment model is a viable alternative to classical crystal growth processes for high-temperature phyllosilicates with the presented supporting data, although still not yet proven.