The ex-situ incubation method coupled with the atomic force microscopy in contacting imaging and phase imaging modes was used to explore how the surface of biotite, a phyllosilicate K-bearing mineral, is eroded or dissolved and its microstructure is altered by citric acid (CA) in simulation of what happens in rhizosphere environment. CA, a kind of low molecular weight organic acid is one of the most important components of root exudate and, therefore, ubiquitous in rhizosphere environment. Results show that in weakly acidic aqueous solution, 4.0 in pH, 24 hours of incubation left some tiny etch pits, about 0.1~0.9 nm in depth, on the surface of biotite (001), making the surface rough. After 96 h of incubation, pits became more apparent, averaging 0.957 nm in depth, and more in number, but only covering 4.8% of the surface. After 140 h, an unstable coating precipitated on the surface, hindering the process of dissolution. However, in citric acid solution (CA) 4.0 in pH, after 24 h of incubation, large numbers of etch pits appeared on the surface of biotite (001) and dissolution of the surface layer became apparent. After 48 hr, the surface terrace dissolution rate of the biotite increased significantly, with dissolved area reaching 48.7% of the surface. And 140 hr later, circular spallings were observed on the (001) surface, leading to breakage of the layer (1~2 nm in thickness) into fragments and promoting further rise in dissolution rate. In CA solution containing Na⁺, rising Na concentration increased dissolution rate of the surface and was a secondary coating was formed, too. Meanwhile, K -Na ion exchange on the solid-fluid interface was enhanced and spalling in the (001) surface structure, about 2~10 nm in depth became more and more apparent. With the incubation going on (140 hr), biotite expanded in depth (~20 nm), causing cracks in the perimeters (0.1~1.9 nm in depth) and eventually zonal hydration of the surface microstructure, and formation of hydrous mica (illite) on the surface of the biotite.