Disorder and modulation in first aragonite precipitates from Obstanser Eishöle (Austria)

Aragonite is thermodynamically metastable at near-surface conditions, and still it is relatively widespread in marine and terrestrial sediments. In this contribution we propose the detailed chemical and crystallographic analysis of fresh aragonitic precipitates. Wet samples, collected from a dolomitic cold cave in the East Alpine range, were directly taken from underground dripping water and from the surface of aragonite speleothems. Calcium carbonate nano- and microcrystals are always found in association with magnesite and hydromagnesite and incorporate variable amounts of magnesium and possibly hydroxyl groups. The typical size of the analyzed precipitates ranges from some tens of nanometers to few microns. Advanced electron crystallographic tools were therefore necessary for a proper structural characterization. Indeed, in the last ten years electron diffraction (ED) turned into a robust protocol for phase identification and ab-initio structure determination. Such evolution was mostly propelled by the development of semi-automatic routines for 3D data collection (Mugnaioli & Gemmi, 2018). The concept at the basis of 3D ED is the same as for single-crystal X-ray diffraction, but electrons allow sampling single crystals 10 to 1000 times smaller, despite the presence of surrounding crystals of other mineralogical phases. 3D ED revealed that first calcium carbonate precipitates have a structure strictly related to conventional aragonite. Still, diffuse scattering and satellite reflections appear along aragonite {110} and point to a reduction of symmetry into the monoclinic system (Németh et al., 2018). Following the order-disorder description of aragonite proposed by Makovicky (2012), such disorder can be associated with the same mechanism responsible for the twinning in mature aragonite. The frequent (or systematic) inversion of the stacking vector can be imposed by the incorporation of magnesium in the structure, whose atomic radius and coordination significantly differ from those of calcium. In turn, the necessity to include magnesium and hydroxyl groups in the lattice may be the very factor that favors the crystallization of aragonite in respect to calcite, which should otherwise be the stable mineral phase at near-surface conditions. Such ‘monoclinic-aragonite’ seeds might therefore represent the key step for the formation of large amount of metastable aragonite sediments.