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Feldspar: The common mineral that’s always wet

Feldspar: The common mineral that’s always wet

2 February 2024

An investigation of the atomic surface of certain feldspars reveals why those minerals bond with water so easily.

A diagram and microscopy image of water bonding with feldspar, a pale rock.
Clouds form when water bonds with potassium feldspar particles in the atmosphere. Recent atomic force microscopy (seen in the detail) has shown how those bonds form so successfully. The key detail is the alternating aluminum and silicon ions at the surface. Credit: Adapted from G. Franceschi et al., J. Phys. Chem. Lett. 15, 15 (2024)

Around 60% of Earth’s crust is composed of feldspar. Fine particles of the material are also found in the atmosphere. Water molecules bond to the mineral’s surface and, in the case of a potassium-rich subset of feldspars, ice nucleation occurs much faster than it does with other minerals. Potassium feldspar is a vital part of cloud formation, yet why water molecules prefer that mineral is not fully understood.

Two studies, led respectively by Ulrike Diebold of the Technical University of Vienna and Angelika Kühnle of Bielefeld University in Germany, recently examined the atomic surface of potassium feldspar. Within a UHV, the researchers broke the sample along the natural break—the cleavage plane—to get a flat, dry surface. But when they looked at the sample, it was already wet. Water released by tiny pockets in the mineral during the breaking had already bonded with the alternating aluminum and silicon atoms on the surface of the potassium feldspar.

A visualization of the atomic structure of the surface layer of potassium feldspar with a layer of water bonds.
A visualization of the water structure on top of potassium feldspar. On this surface, water molecules coordinate to aluminum hydroxyl groups (magenta dots), silicon hydroxyl groups (yellow dots), or potassium ions (green dots). The underlying structure had previously only been modeled. Credit: Adapted from T. Dickbreder et al., Nanoscale (2024), doi: 10.1039/d3nr05585j

Yet that wasn’t an experimental failure. That the dry surface almost instantly became hydroxylated matched with what happens in nature. Potassium feldspar in the atmosphere bonds immediately with water in the air. Because of potassium feldspar’s surface atomic structure made of alternating aluminum and silicon ions, the water on the mineral surface creates a regular pattern of hydroxyl groups. That first layer of dissociated water molecules then helps additional layers of water easily bond to the surface.

Evidence shows that ice nucleation can occur around potassium feldspar at higher temperatures than it can around water, and that may be partly due to the structure of hydroxyl groups in the initial layer of water. Experimentally seeing the atomic structure of potassium feldspar is only a first step for atmospheric scientists. There is still work to be done to understand why potassium feldspars are better than other minerals at facilitating cloud formation. (G. Franceschi et al., J. Phys. Chem. Lett. 15, 15, 2024; T. Dickbreder et al., Nanoscale, 2024, doi:10.1039/d3nr05585j.)

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