Optical properties and cell parameters in the glaucophane-riebeckite series

A complete set of new optical and x-ray data is given for eleven analyzed alkali amphiboles [Na2(Mg, Fe″)3(Al, Fe‴)2Si8O22(OH)2]. Nine new wet chemical analyses are reported. Using additional selected data from the literature, variation in refractive indices, extinction angles (γ-α), optic angles, density, lattice constants and cell volume are expressed graphically as a function of composition in the glaucophane-riebeckite and magnesiorie-beckite-ferroglaucophane series. Four orientations (G, C, O, and R) of the optical indicatrix within the structure are described and shown to be characteristic of the chemical species glaucophane (G), crossite (C), magnesioriebeckite (O), riebeckite (O), and riebeckite-arfvedsonite (R and O). Optical properties of the pure end members by extrapolation are: X-ray parameters of the end members reffered to the C 2/m space group are: These show very good agreement with comparable measurements on synthetic counterparts. There is some indication that the two proposed synthetic polymorphs of glaucophane (Ernst, 1963) are both more disordered than the natural end member.