Weathering index of clay minerals

Weathering index indicates the ease of weathering of a mineral. The mineral having high weathering index is more resistant to weathering. 

The weathering index of mica is 7, whereas it is 10 for kaolinite. Therefore, kaolinite is more resistant to weathering than mica. 

Kaolinite, being the most resistant to weathering, is an indicator of highly weathered soil.

Note: The correct answer for the question no. 6 (page no. 151 in An Insight into Agronomy book) is “a” i.e. mica is the least resistant to weathering.