Admixture feats grant new ways to utilize the Admixture talent (from the Destruction sphere), adding abilities from other spheres to your destructive blast. All admixture feats replace the second blast talent you would normally apply, with the resulting destructive blast dealing normal blast damage in addition to the effect outlined in the feat. Any additional costs incurred by the additional effect must be paid as normal. If your caster level is different for the two spheres, the destructive blast is governed by your caster level for the relevant blast type and the additional effect is governed by your caster level for the appropriate ability.
Author's Note: When using an (admixture) feat, the resulting sphere effect is not one simultaneous effect, but rather a destructive blast with another sphere effect as a new, independent rider. The new sphere effect is still subject to spell resistance if the destructive blast would not be, and the target makes a new, separate saving throw against the new effect.