Chemical weathering causes iron in rock to oxidize.
When iron oxidizes it produces iron oxide, more commonly referred to as rust.
Chemical weathering of iron in rock causes rust.
Rust of iron in rocks is a direct result of 
a: solar heating
b: high speed fall from space
c: Chemical energy
d: The area was once underwater
e: permineralization
f: Temperature
g: volcanic ejection
h: chemical weathering