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

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.

Multiple Growth Mechanisms of 2:1 Type Layered Aluminosilicates during Mineral Transformation

“…Besides the layer growth, 2:1 type LAs particles could grow through the oriented attachment of small nanoparticles. 20,22 Nanoparticles of 2:1 type LAs could occasionally attach along the c-axis but not at one identical crystal orientation (Figure 6b), as shown by the rotation of nanoparticles in Figure 5b. Additionally, the attachment of 2:1 type LAs nanoparticles could happen in the a−b plane (Figure 6c), which caused the misalignment of ∼0.45 nm lattice fringes (Figure 5c).…”

“…45 Compared with the basal surfaces of layered silicates, the layer edges contain generally abundant broken bonds, which makes the edges atomically rough and therefore possess high attachment energy. 46 This induces the attachment of growth units including atoms 46 or nanoparticle monomers (Figure 4d) 20,21 happen at the edges of layered silicates. Eventually, the incorporations of attached atoms or nanoparticle monomers further promote the growth of layered silicate particles.…”

“…17,19 Oriented attachment depicts the formation of mineral crystals through the attachment and coalescence of primary nanocrystals. 17,20,22 Lateral extension is achieved by building blocks attaching at the layer edges, which happens in an environment of increasing temperature. 21 In natural and experimental conditions, most 2:1 type LAs are formed through the alteration of other minerals containing Al or/and Si.…”

“…Interpretation of the growth mechanisms of minerals is of great importance for revealing their formation processes in various geological environments and better understanding the formation of crystal defects within minerals, which is crucial for the application of minerals. To date, several classic and nonclassic growth mechanisms including layer growth, − spiral growth, ,− Ostwald ripening, , oriented attachment, , lateral extension, as well as their coupled growth mechanisms have been proposed for explaining the crystal growth processes of layered silicates. These growth mechanisms well revealed the formation of polytypisms, crystal defects, and micromorphologies of layered silicates. , Generally, layer growth takes place under the conditions of a high oversaturation degree and temperature, , and it causes a significantly increased thickness of layered silicates .…”

“…These growth mechanisms well revealed the formation of polytypisms, crystal defects, and micromorphologies of layered silicates. , Generally, layer growth takes place under the conditions of a high oversaturation degree and temperature, , and it causes a significantly increased thickness of layered silicates . By contrast, spiral growth appears in the relatively low oversaturation degree and temperature environments such as the conditions of the late stage of mineralization, , and it generally retains growth spirals on the basal surfaces. ,, Ostwald ripening mainly involves the dissolution and crystallization of small and large crystal particles, respectively, and it generally happens in environments with active water–rock interactions. , Oriented attachment depicts the formation of mineral crystals through the attachment and coalescence of primary nanocrystals. ,, Lateral extension is achieved by building blocks attaching at the layer edges, which happens in an environment of increasing temperature …”