Preprints
https://doi.org/10.5194/acp-2021-18
https://doi.org/10.5194/acp-2021-18

  12 Feb 2021

12 Feb 2021

Review status: this preprint is currently under review for the journal ACP.

Effect of chemically induced fracturing on the ice nucleation activity of alkali feldspar

Alexei Kiselev1, Alice Keinert1, Tilia Gaedecke1, Thomas Leisner1,2, Christoph Sutter3, Elena Petrishcheva3, and Rainer Abart3 Alexei Kiselev et al.
  • 1Karlsruhe Institute of Technology, Institute of Meteorology and Climate Research, Karlsruhe, Germany
  • 2Institut für Umweltphysik, Universität Heidelberg, Heidelberg, Germany
  • 3University of Vienna, Department of Lithospheric Research, Althanstrasse 14, A-1090 Vienna, Austria

Abstract. Feldspar is an important constituent of airborne mineral dust. Some alkali feldspars exhibit particularly high ice nucleation (IN) activity. This has been related to structural similarities of the ice (10-10) prism planes and the (100) planes of alkali feldspar. Here the effect of generating surfaces with close to (100) orientation by means of chemically induced fracturing on the IN activity of alkali feldspar was investigated experimentally. Gem quality K-rich alkali feldspar was shifted towards more Na-rich compositions by cation exchange with an NaCl-KCl salt melt at 850 °C, and a system of parallel cracks with an orientation close to (100) was induced. Droplet freezing assay experiments performed on grain mounts of the cation exchanged alkali feldspars revealed an increase of the overall density of ice nucleating active sites (INAS) with respect to the untreated feldspar. In addition, annealing at 550 °C subsequent to primary cation exchange further enhanced the INAS density and lead to IN activity at exceptionally high temperatures. Although very efficient in experiment, fracturing by cation exchange is unlikely to be of relevance in the conditioning of alkali feldspars in nature. However, parting planes with similar orientation as the chemically induced cracks may be generated in lamellar microstructures resulting from the exsolution of initially homogeneous alkali feldspar, a widespread phenomenon in natural alkali feldspar known as perthite formation. Perthitic alkali feldspars indeed show the highest IN activity. We ascribe this phenomenon to the preferential exposure of crystal surfaces oriented sub-parallel to (100).

Alexei Kiselev et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Review of Kiselev et al.', Thomas F. Whale, 11 Mar 2021
  • RC2: 'Comment on acp-2021-18', Anonymous Referee #2, 12 Mar 2021

Alexei Kiselev et al.

Alexei Kiselev et al.

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Short summary
Alkali feldspar is the most abundant mineral in the Earth's crust and is often present in the mineral dust aerosols, that are responsible for the formation of rain and snow in clouds. However, the cloud droplets containing pure potassium-rich feldspar would not freeze unless cooled down to a very low temperature. Here we show that partly replacing potassium for sodium would induce fracturing of feldspar, exposing a crystalline surface that could initiate freezing at higher temperature.
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