The invention relates to surface-modified non-halogenated mineral filler compositions comprising a particulate mineral filler core comprising hydroxide groups bound to divalent or trivalent metal ions, said particulate mineral filler core having on its surface (i) one or more short-chain organosilano groups each having one to three silicon-bound short-chain hydrocarbon groups having, independently, one to three carbon atoms, and (ii) one or more long-chain organosilano groups having one to three silicon-bound straight-chained or branched, saturated or unsaturated, long-chain hydrocarbon groups containing, independently, eight to twelve carbon atoms, wherein the long-chain organosilano groups are present in an amount of up to about fifty-five mole percent of combined molar amount of long-chain and short-chain organosilano groups.