Ice crystals growing on K-feldspar (microcline) have preferential orientation dictated by feldspar lattice structure

Abstract:

Recently, we have conducted experiments on deposition nucleation and growth of ice on freshly cleaved natural K-feldspar (microcline) crystals exposed to water vapor in the Environmental Scanning Electron Microscope (ESEM, FEI Quanta 650 FEG). Independently adjusting the partial water vapor pressure in the sample chamber and the temperature of the substrate mounted on top of the double-stage Peltier element, deposition ice nucleation, growth, and sublimation can be studied within the temperature range from -5°C to -60°C. By using small crystal size and tilted geometry we have been able to record the video sequences of ice nucleation taking place on both 001 and 010 crystallographic planes simultaneously.

Here, we report the following general features of ice nucleation and growth observed in these experiments:

1. Nucleation of ice always starts before the water saturation is reached.
2. Ice was preferentially nucleating on surface defects (steps, cracks, and pits) or on the debris particles scattered over the surface of feldspar crystal.
3. Ice crystals grown via deposition at temperatures above -30°C on any of the feldspar crystal faces have shown the same directional and rotational orientation, with c-axis of ice aligned with the c-axis of microcline unit cell. Below -35°C no preferential orientation has been observed whatsoever.
4. The majority of observed ice crystals exhibit the evaporation groove at the waist of hexagonal prism, indicting the presence of lattice dislocations in the crystal nucleation plane.

We discuss a possible mechanism of crystal lattice alignment by considering layer of ordered water on the surface of feldspar crystal forming prior to ice nucleation. Using density functional theory we show how the mineral surface interacts with water, particularly addressing the interaction of surface cations and hydroxyl groups with a water overlayer. We argue that the misalignment of the 001 lattice planes for microcline and ice (inherently following from the alignment of their c-axis) should favor the stacking disorder of ice lattice in the nucleation plane, the feature we observe as an “evaporation groove” in the majority of the aligned ice crystals. Finally, we discuss the implications of the observed phenomenon for the understanding of ice nucleation mechanism on the atmospheric mineral dust particles.