What is a radical ? Geology question?

I presume you are talking chemistry, when a radical is an atomic or molecular form that will react readily because of the existence of an unpaired electron in its outer shell. Electrons are less stable when unpaired than when paired. I do not know any other way that the term "a radical" is used in geology. Radical as an adjective is used, in its usual sense, but the noun form tends to apply to chemistry.