Authigenic crystallization of hausmannite and romanechite induced by limestone–clay mineral Interactions in the Apulian karst system (Southern Italy)

Nodules and encrustations within a small paleo-doline in the Apulian karst on Mesozoic carbonate successions, contain Mn and/or Mn-Ba phases, which have been investigated with a multidisciplinary approach, combining scanning electron microscopy with energy-dispersive spectrometer, Micro-Raman spectroscopy, wavelength dispersive X-ray fluorescence spectroscopy, and powder X-ray diffraction. This approach revealed the presence of authigenic hausmannite and romanechite with recrystallized calcite and clay minerals. Hausmannite forms within fractures and pockets with euhedral habitus. Romanechite is more widespread, and typically surrounds the hausmannite indicating later-stage crystallization. The textural characteristics of hausmannite and romanechite attest to their relationship with mud deposits. The presence of kaolinite, detrital quartz grains, plagioclase and K-feldspars suggests that these phases are fundamental constituents of the mud. Clay minerals play a crucial role in the formation of Mn-oxides and Mn-Ba hydroxides. They capture divalent cations such as Fe2+, Mg2+, Mn2+, and Ba2+ stabilizing the octahedral layers and promoting the growth of Mn- and Mn-Ba phases on their surface. The genetic process responsible for the growth of hausmannite and romanechite may be described as follows: i) the limestone substrate interacts with stormwater runoff enriched in mud and CO2, ii) these muddy waters settle on the carbonate surface, leading to the dissolution of CaCO3; iii) under these conditions, Mn2+ and Ba2+ derived from the mud and/or limestone dissolution penetrate the fractures of the karstified limestone; iv) the subsequent evaporation of water leads to oxidizing conditions with release of CO2, calcite re-precipitation and authigenic formation of hausmannite and romanechite.