Another mechanism that could explain helical inclusions is screw dislocations. These crystal defects appear when crystal growth is disrupted by local deformation, creating a spiral structure that guides crystallization.
In this scenario, helical inclusions would not only be the result of Liesegang diffusion and precipitation, but also of a spiral movement of crystallizing atoms around a dislocation. This process could act in conjunction with the Liesegang phenomenon, producing a complex structuring of inclusions at both the macroscopic and microscopic scales.
Diagrams and photo : Diagrams of a screw dislocation and photo of a screw dislocation around a fluorite twin from Rogerley, United Kingdom © Rémi Bornet
Beryls and spodumenes are particularly susceptible to helical fluid inclusions due to the specific geological conditions in which they form. These minerals typically crystallize in pneumatolytic or hydrothermal environments, located at the interface between the final phases of magmatic differentiation and hot fluid circulation. These environments are rich in volatile elements (fluorine, boron, water, etc.) and light elements such as lithium and beryllium, which favors both mineral growth and fluid incorporation.
These conditions are often unstable, with variations in temperature, pressure, and chemical composition that can lead to saturation and precipitation cycles. This creates a favorable environment for periodic fluid trapping, similar to the Liesegang phenomenon.
Furthermore, beryls and spodumenes exhibit marked directional growth, particularly along their crystallographic c axis. When a fluid inclusion pattern forms regularly, this axial growth can lead to a progressive coiling of the pattern, transforming a linear distribution into a helical structure.
The presence of mobile elements such as lithium, beryllium, or iron can also promote complex diffusion processes, sometimes accelerated by the formation of intermediate complexes in the fluid, contributing to a modified periodic precipitation pattern. Finally, the presence of screw dislocations, common in large crystals, can reinforce or induce this spiral structuring by serving as a helical growth channel.
Note that helical inclusions are also known in other mineral species such as topaz, tourmaline, and natrolite.
The hypothesis that the helical inclusions observed in certain beryls and spodumenes result from the Liesegang phenomenon is a plausible explanation. By combining chemical diffusion, periodic precipitation, and anisotropic growth, this model allows us to interpret these fascinating patterns from a chemical and crystallographic perspective.
Furthermore, the presence of screw dislocations could act at a finer scale, influencing the formation of helical structures. This coupled phenomenon could open new avenues of research into the origin of these inclusions.
Future studies could explore this hypothesis by simulating growth conditions and analyzing the chemical composition of the inclusions to confirm their Liesegangian nature.
References :
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LIESEGANG, R.E. (1896). Ueber einige Eigenschaften von Gallerten. Naturwissenschaftliche Wochenschrift, 11, 353–362.
MÜLLER S. C., KAI S., ROSS J. (1982). Curiosities in Periodic Precipitation Patterns. Science, 216 (4546), 635-637
MÜLLER S. C., KAI S., ROSS J. (1982). Periodic Precipitation Patterns in the Presence of Concentration Gradients. 1. Dependence on Ion Product and Concentration Difference. Journal of Physical Chemistry, 86 (6), 903-909.
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THOMAS S., LAGZI I., MOLNÁR F. Jr., RÁCZ Z. (2013). Helices in the wake of precipitation fronts. Physical Review E, 88(2), 022141