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

Abstract

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).

Molecular Simulations of Feldspar Surfaces Interacting with Aqueous Inorganic Solutions: Interfacial Water/Ion Structure and Implications for Ice Nucleation

“…Therefore, we conclude that the thickness of the mineral lattice structure has no significant influence on the surface water orientation. There is some experimental evidence suggesting that the (100) surface contributes to the IN ability of feldspar, , but the present and earlier simulations give no indication that the pristine (100) surface can template any plane of ice.…”

The sensitivity of the ice-nucleating ability of minerals to heat and the implications for the heat test for biological ice nucleators

“…However, several exceptions to this rule have been reported (Harrison et al, 2016) and the higher INA of many microclines is related to the fact that they are more likely than sanidines to contain the exsolution microtexture thought to be to source of the highly active sites (Whale et al, 2017;Kiselev et al, 2021).…”

“…2a) in order to represent the diversity of this group of minerals. These included: BCS-376 Microcline, which was studied previously (Atkinson et al, 2013) and is considered generally representative of the INA of standard K-feldspars (Harrison et al, 2016); Amazonite and TUD#3 Microcline, which are samples of microcline that show exceptionally high INA compared to typical variants of microcline and the other Kfeldspar polymorphs for reasons that are still unclear (Harrison et al, 2016;Welti et al, 2019;Peckhaus et al, 2016); Eifel Sanidine, which exhibits much lower INA compared to the other samples due to a lack of cracks related to exsolution microtexture (Kiselev et al 2021;Whale 2017} and, in addition, we included a duplicate of the BCS-376 Microcline sample that was re-ground in a ball mill in order mitigate for possible 'ageing' of the sample and to investigate the effect of removing the largest particles in the sample.…”

“…Ice nucleation on mineral surfaces such as feldspars has been shown to occur at specific sites that become active at a specific temperature (Holden et al, 2019;Holden et al, 2021). Topographical features associated with exsolution microtexture (Whale et al, 2017;Kiselev et al, 2021) have been proposed as the locations of the highly active sites on K-feldspar. Moreover, Kiselev et al (2017) observed that ice crystals growing from the vapour phase on the surface of microcline originated on steps and cracks and were preferentially orientated between the basal face of ice and the (100) cleavage plane.…”

An evaluation of the heat test for the ice-nucleating ability of minerals and biological material

“…2a) in order to represent the diversity of this group of minerals. These included BCS-376 microcline, which was studied previously (Atkinson et al, 2013) and is considered generally representative of the INA of standard K-feldspars (Harrison et al, 2016); amazonite and TUD#3 microcline, which are samples of microcline that show exceptionally high INA compared to typical variants of microcline and the other K-feldspar polymorphs for reasons that are still unclear (Harrison et al, 2016;Welti et al, 2019;Peckhaus et al, 2016); and Eifel sanidine, which exhibits much lower INA compared to the other samples due to a lack of features related to exsolution microtexture (Kiselev et al, 2021;Whale et al, 2017).…”