Structural change of alkali feldspar by ball milling

Abstract

The structural changes and amorphization of alkali feldspar by ball milling were investigated using X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, Raman spectroscopy, scanning electron microscopy (SEM), thermal analysis, particle size analysis, and specific surface area measurements. The Or₇₇Ab₂₃ alkali feldspar crystal has an Ab-rich lamella texture in Or-rich host crystals. The mean particle size decreased from ∼ 67 μm to almost 3 μm after 50 h of ball milling. After 600 h of ball milling, the apparent mean particle size calculated from the observed particle size distribution increased prominently because of the agglomeration of small particles. It reached ∼ 38 μm after 1200 h of ball milling. While the density variation was not consistent with that of the particle size, it was consistent with that of the crystallinity estimated from X-ray diffraction data. The FTIR and Raman spectra indicated breaking of the Si-O-Si or Al-O-Si bonds and the formation of defects and distortion of the TO₄ (T = Si or Al) tetrahedra as a result of the milling. An increase in the intensity of luminescence with milling supported these results. XRD analysis revealed the sample that was ball milled for 1200-h was amorphous. However, the structure and properties of this amorphous phase were different from those of fused feldspar glass and had weak crystalline features. These amorphous materials were similar to those in the fault zone that were formed naturally and experimentally without melting. The Si-OH (or Al-OH) shoulders ν = 890 cm^-1) in the FTIR spectra were formed by the chemical reaction between the broken Si-O-Si (or Al-O-Si) bonds and atmospheric water surrounding the 300-h milled samples. Because of this reaction, the surface and interior structure of the milled particles depended on the agglomeration and formation of defects formation resulting from the long milling time. XRD results of the milled samples indicated that grains with a larger amount of Ab-rich lamella structures were downsized more rapidly than the Or-rich host.