Final answer:
The feldspar grains in the granite primarily undergo chemical weathering when exposed to rainy weather.
Step-by-step explanation:
The feldspar grains in the granite primarily undergo chemical weathering when exposed to rainy weather. Chemical weathering refers to the process by which rocks are broken down and altered chemically due to the interactions with substances like water and acids. In the case of feldspar, it alters to clay with chemical weathering and erosion.