Complex sentence: As a result of weathering, they have a thin patina or surface layer in which some of the feldspar has been turned into clay, resulting in a softer material, rated 3 to 4 on the Mohs scale of mineral hardness, which can be scratched.
Split into simpler sentences: As a result of weathering, they have a thin patina. It consists of a weathering rind in which weathering has turned some of the feldspar into clay, resulting in a softer material, rated 3 to 4 on the Mohs scale of mineral hardness, which can be scratched.