FERRIAN HIGH SANIDINE IN A LAMPROITE FROM CANCARIX, SPAIN

Na-poor, Fe-bearing high sanidine from a lamproite near Cancarix (Spai n) has 2Valpha parallel-to (010) = 37-43-degrees, and C2/m, a = 8.598( 15), b = 13.050(26), c = 7.209(17) angstrom, beta = 116.00(18)-degrees , V = 727(2) angstrom3. 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 % square 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 hab it) indicate formation under moderate undercooling at temperatures not much above 725-degrees-C. Feldspar formation was facilitated by a hig h diffusion rate due to low viscosity in a highly perpotassic melt, su persaturated by pressure release and diopside fractionation, upon extr usion of a huge volume of lava in a crater. After titanian potassium-r ichterite largely filled the interstices in the sanidine fabric, cryst als of dalyite (K2ZrSi6O15) and Fe-rich rims of sanidine and amphibole crystals were formed from an increasingly hydrous, silicic, ferric, a nd peralkaline residual melt. High rate nonequilibrium crystallisation caused the incorporation of excess SiO2 and K2O in the defect structu re of the sanidine. Retrograde boiling initiated the escape of volatil es, causing the quenching, by which the disordered structural state an d the nonstoichiometric composition of the sanidine were preserved.