ABSTRACT
In recent years, we have come to appreciate the astounding intricacies associated with the formation of minerals from ions in aqueous solutions. In this context, a number of studies have revealed that the nucleation of calcium sulfate systems occurs nonclassically, involving the aggregation and reorganization of nanosized prenucleation species. In recent work, we have shown that this particle-mediated nucleation pathway is actually imprinted in the resultant micrometer-sized CaSO4 crystals. This property of CaSO4 minerals provides us with the unique opportunity to search for evidence of nonclassical nucleation pathways in geological environments. In particular, we focused on large anhydrite crystals extracted from the Naica Mine in Mexico. We were able to shed light on this mineral's growth history by mapping defects at different length scales. Based on this, we argue that the nanoscale misalignment of the structural subunits, observed in the initial calcium sulfate crystal seeds, propagates through different length scales both in morphological, as well as in strictly crystallographic aspects, eventually causing the formation of large mesostructured single crystals of anhydrite. Hence, the nonclassical nucleation mechanism introduces a "seed of imperfection," which leads to a macroscopic "single" crystal whose fragments do not fit together at different length scales in a self-similar manner. Consequently, anisotropic voids of various sizes are formed with very well-defined walls/edges. However, at the same time, the material retains in part its single crystal nature.
ABSTRACT
Feldspar-rich pseudotachylytes from the island of Moskenesøya, Lofoten, formed in dry granulites under lower crustal conditions during the Caledonian orogeny. The central parts of the pseudotachylytes, where the cooling rates were slowest, are characterized by microlites and spherulites of plagioclase and K-feldspar. K-feldspar surrounding plagioclase is consistent with crystallization from a melt during cooling instead of devitrification as the origin of the spherulites. Very thin (a few micrometres wide) injection veins, which experienced very rapid quenching, contain amorphous or cryptocrystalline material. The preservation of this material and of the fine-grained microstructures shows that, under fluid-absent conditions, recrystallization and reactions are slow and the original microstructures of the pseudotachylytes can be preserved. This article is part of a discussion meeting issue 'Understanding earthquakes using the geological record'.
