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
(101‾0) prism planes and the (100) planes of alkali feldspar. Here
the effect of generating feldspar surfaces with close to (100) orientation
by means of chemically induced fracturing on the IN activity of alkali
feldspar was investigated experimentally. To this end, 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. By this procedure,
a system of parallel cracks with an orientation close to the (100) plane of
the feldspar was induced. Droplet-freezing assay experiments performed on
grain mounts of the cation-exchanged alkali feldspars revealed an increase
in the overall density of ice-nucleating active site (INAS) density with respect to
the untreated feldspar. In addition, annealing at 550 ∘C
subsequent to primary cation exchange further enhanced the INAS density and
led to IN activity at exceptionally high temperatures. Although very
efficient in experiment, fracturing by cation exchange with an alkali halide
salt 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 tentatively ascribe this phenomenon to the preferential exposure of
feldspar crystal surfaces oriented sub-parallel to (100).
Effect of chemically induced fracturing on the ice nucleation activity of alkali feldspar
