"The pyroxene-rich rocks which result from the contact metamorphism of impure limestones are described as pyroxene hornfelses (calc-silicate hornfelses)."

The above statement sounds inaccurate, since pyroxenes in hornfels are the byproduct of metabasite and metapelite metamorphism. If a "limestone" was that siliceous with a large mafic component, it would no longer be a limestone but a shale.