Na-poor, Fe-bearing high sanidine from a lamproite near Cancarix (Spain) has 2Vα‖(010) = 37–43°, and C2/m, a = 8.598(15), b = 13.050(26), c = 7.209(17) Å, β = 116.00(18)°, V = 727(2) ų. Rims of sanidine crystals against vugs contain up to 60 mole % KFeSi₃O₈ and up to 10 at.% Si and 6 at.% K above the stoichiometric requirement; otherwise, they have up to 4 mole % □Si₄O₈ and 3 mole % K₂O.Si₄O₈ in solid solution. Their MgO content may reach 0.46 wt.%. The skeletons of mm sized blocky crystals (Baveno habit) indicate formation under moderate undercooling at temperatures not much above 725°C. Feldspar formation was facilitated by a high diffusion rate due to low viscosity in a highly perpotassic melt, supersaturated by pressure release and diopside fractionation, upon extrusion of a huge volume of lava in a crater. After titanian potassium-richterite largely filled the interstices in the sanidine fabric, crystals of dalyite (K₂ZrSi₆O₁₅) and Fe-rich rims of sanidine and amphibole crystals were formed from an increasingly hydrous, silicic, ferric, and peralkaline residual melt. High rate nonequilibrium crystallisation caused the incorporation of excess SiO₂ and K₂O in the defect structure of the sanidine. Retrograde boiling initiated the escape of volatiles, causing the quenching, by which the disordered structural state and the nonstoichiometric composition of the sanidine were preserved.