Evidence for a non-classical dissolution-reprecipitation reaction path in natural high pressure-low temperature rocks

Dissolution and precipitation of minerals in the presence of a hydrous fluid is commonly assumed to occur predominantly by hydrolysis of the outermost monolayers of the reacting crystal and subsequent incorporation of atoms from aqueous solutions into favorable sites at the product surface. However, experiments have shown that different parts of this classical reaction pathway are facilitated and enhanced by the presence of nanoparticles and/or amorphous phases.   

We show in natural high-pressure/low temperature rocks, which experienced metamorphic conditions in the order of 500°C at 1 GPa, that an alkali-Al-Si-rich amorphous material can be found in nm-sized metamorphic porosity [1]. The gel-like water-rich material contains about 15wt% total dissolved solids (TDS), which is more than ten times higher than expected for aqueous solutions at these metamorphic conditions. High-resolution TEM observations show that the amorphous material forms directly by depolymerization of the crystal lattice of amphiboles and clinopyroxenes. Depolymerization of the minerals starts along grain or phase boundaries, dislocation cores within the crystal lattice or lattice defects in general that serve as effective element exchange pathways and are sites of porosity formation. High hydrogen concentrations in such dislocation cores within nominally anhydrous clinopyroxene has been confirmed by Nano-SIMS measurements and indicates hydrolysis and recrystallization within mineral grains.

The resulting amorphous material occupies large volumes in an interconnected porosity network which has been documented in 3D imaging. Precipitation of product minerals occurs directly by repolymerization of the amorphous material at the product surface.

Reference: [1] Konrad-Schmolke et al. 2018, Nature Communications 9: 1637.