Abstract: A layer of so-called Mung Bean Rock was recently discovered at Hexidong in Daijiagou, Chongqing. The formation is about 1.5 m thick, in which many hard grains with diameter ranging from 2-5 mm can be easily seen. Compositional analyses show that the rock consists of illitic claystone, with an illite
content of 95 -97%. X-ray refraction and SEM examination reveal that the main constituents of the grains are quartz (80%) and calcite (20%). This discovery provides not only a new site to investigate Mung Bean Rock, but also a potential industrial resource of illite.

Key words: Mung Bean Rock, illite, quartz, calcite, Chongqing