Does Chlorine Gas react with K-Feldspar? If so, what is the reaction like (reaction conditions, rates, safety), and what are the products? Can it form useful potassium compounds (eg K-fertilisers)?
In short, no. The potassium can be selectively extracted with calcium chloride solutions, but the mineral is essentially unreactive towards elemental chlorine gas. It is quite well oxidized already and very stable.
Yes. The "general" way is by the electrochemical potential. That is, whether the energy exchange/release from chlorine reduction and the oxidation of the other element is positive or not. That would only happen with a metal ion that is capable of further oxidation (for example, Fe+2 to Fe+3). It would NOT affect the silicate ion. Chloride ions are a different matter, as then there can be displacement reactions, as in my original reply. A cursory search for reactions of chlorine gas with silicates does not indicate any reactions.