Winchite and the actinolite–glaucophane miscibility gap
Leake et al.1 have recently emphasized the apparent scarcity of winchite s.l., NaCa(Mg, Fe2+)4(Al, Fe3+) [Si8O22/(OH)2] (ref. 2), an amphibole intermediate in composition between the glaucophane–riebeckite series of high-pressure blueschists and the tremolite–actinolite series of lower-pressure greenschists. The rarity of winchite is considered consistent with the existence of a miscibility gap3,4 between these two amphibole series at the metamorphic P–T conditions normally attained in nature. Here we present optical and X-ray properties of Mg- and Al-rich winchite from Venezuela and draw attention to several other recent winchite analyses. In fact, as Schliestedt's5 new data on co-existing sodic and calcic amphiboles in blueschists show, only the present Mg–Al-rich winchite definitely lies within the above miscibility gap.
Optical properties and cell parameters in the glaucophane-riebeckite series
AbstractA 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:αβγ(γ−α)c^nϱGlaucophane1.5941.6121.6180.025b=βc^γ=6°3.03Riebeckite1.7021.7121.7190.015b=γC^α=6°3.40Magnesioriebeckite1.6551.6711.6720.02b=γC^γ=6°3.15
X-ray parameters of the end members reffered to the C 2/m space group are:a0(Å)b0(Å)C0(Å)β(°)V(Å3)Glaucophane9.5017.675.29103.7864Reibeckite9.7818.085.34103.5918Magnesioriebeckite9.7617.975.31103.9904
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.
The determination of the unit-cell dimensions of non-cubic substances
A graphical extrapolation method for the accurate determination of the unit-cell dimensions of cubic substances from high-angle reflexions on a powder photograph was described by Bradley & Jay (1932).