Water desorption from microcline (001): insights into the first water layer

Area of Science:

• Geochemistry
• Surface Science
• Mineral Physics

Background:

• Feldspar minerals are abundant tectosilicates vital for Earth's geochemical cycles.
• Water-feldspar surface interactions are critical for weathering and cloud formation but poorly understood.
• Experimental data on the first water layer's binding and desorption on feldspars are scarce.

Purpose of the Study:

• To investigate the binding and desorption of the first water layer on microcline (001), the most stable cleavage plane of potassium-rich feldspar.
• To elucidate the molecular-scale mechanisms governing water adsorption on feldspar surfaces.
• To provide experimental data complementing theoretical insights into water-feldspar interactions.

Main Methods:

• Temperature-programmed desorption (TPD) experiments were conducted on water desorbing from microcline (001).
• Density-functional theory (DFT) calculations were employed to simulate water-feldspar interactions.
• TPD data were analyzed in conjunction with DFT results to understand adsorption energetics.

Main Results:

• TPD spectra showed a coverage-dependent shift in desorption peak temperature from 235 K to 180 K as coverage increased.
• A saturation point was observed at four water molecules per primitive unit cell, indicating the completion of the first layer.
• Adsorption energy decreased with increasing water coverage, consistent with DFT simulations.

Conclusions:

• The study provides molecular-scale insights into the binding of the first water layer on microcline (001).
• Experimental and theoretical findings confirm and enhance understanding of water adsorption on feldspar surfaces.
• This research contributes to understanding feldspar's role in geochemical processes and cloud physics.