Richterite-magnesioarfvedsonite-magnesioriebeckite isomorphic series: Evidence from carbonatites of Eastern Siberia

Rock-forming amphiboles of alkaline rocks and carbonatites - richterite., magnesioarfvedsonite, and magnesioriebeckite belong to the isomorphic series with gradual change of the L/M ratio, where L = 3-5 is mainly Mg2+ and M = 0-2 is mainly Fe3+. The studied representatives of this series from carbonatites of Eastern Siberia are characterized by L/M = 1.4-35 with the variations of L from 4.86 (with M = 0.14) down to 2.94 (with M = 2.06). Richterite-magnesioarfvedsonite and magnesioarfvedsonite-magnesioriebeckite parts of the considered isomorphic series are characterized by different mechanisms of isomorphic replacement. The main isomorphic effect for the richterite-magnesioarfvedsonite part may be represented by the scheme: Ca2+ + L2+ <-- Na-B(1+) + M3+ with the possibility of total replacement of Ca by Na in the B-group. The main isomorphic effect for magnesioarfvedsonite-magnesioriebeckite part may be represented by the scheme: A(1+) + L2+ <--M3+ with the variation of the value of the A-group from 0-1. The clear correlation between the value of the M formula subgroup and optical properties (refractive indices n(l) and n(x), birefringence, pleochroism, optical elongation) characterize the amphiboles of richterite-magnesioarfvedsonite-magnesioriebeckite series.