Experimental study of the compositions and stabilities of synthetic nyböite and nyböite-glaucophane amphiboles

Abstract

Synthesis of end-member glaucophane (Na 2 Mg 3 Al 2 Si 8 O 22 (OH) 2 ) has0 not been achieved in any previous experimental studies. Instead, compositions are displaced from the end-member, often along the exchange vector NaAl□ -1 Si -1 , i.e. towards nyboite (Na 3 Mg 3 Al 3 Si 7 O 22 (OH) 2 ). In this study nyboite synthesis was attempted in a piston-cylinder apparatus at 15-32 kbar / 600-900°C from various bulk compositions : Na 3 Mg 3 Al 3 Si 7 O 23 + H 2 O (OH-NY experiments), Na 3 Mg 3 Al 3 Si 7 O 23 .7SiO 2 + H 2 O (OH-Q experiments) and compositions on the nyboite - glaucophane joint (G-N experiments). Run products were characterised by optical microscopy, powder X-ray diffraction and electron microprobe analysis. Amphiboles obtained in OH-NY experiments are quaternary solid solutions of nyboite, glaucophane, Mg-substituted-katophorite and eckermannite (Ny-Gl-Kt-Ek), with maximum Ny contents of ∼0.7 mole fraction, whose compositions migrate along MgMgSi -1 with increasing pressure, and along AlAlMg -1 Si -1 with increasing temperature. They coexist with Jd or Ab +Ne, plus a ternary sodium phlogopite - talc - preiswerkite (Sp-Tc-Pw) sheet silicate ; are stable to the highest pressure investigated (32 kbar) ; and break down below about 15 kbar through either the metastable reaction Ny-Gl-Kt-Ek ss +Sp-Tc-Pw ss = Ab + Ne + Sp, or the stable reaction Ny-Gl-Kt-Ek ss + Sp-Tc-Pw ss = Jd +Sp. Amphiboles synthetised in OH-Q experiments coexist with Qz/Co + Ab/Jd ± Tc. They are ternary Ny-Gl-Kt solid solutions, with low Ny contents which decrease slightly with increasing pressure and temperature through the reaction MgMgNa -1 Al -1 . Their P, T stability field is reduced with respect to that of OH-NY amphiboles : the low-pressure limiting reaction Am + Qz = Ab +Tc has been bracketed at 14.8-18.5 kbar/700°C - 18.2-19.5 kbar/800°C. Experiments using bulk compositions between Ny and Gl also yield amphiboles that are either quaternary Ny-Gl-Kt-Ek, or ternary Ny-Gl-Kt solid solutions, the latter obtained only at a high pressure (22 kbar) in an experiment on the glaucophane bulk composition. The presence of Qz in any experiment inhibits the growth of Sp, with the result that Tc is the only stable sheet silicate, and the amphibole composition is buffered onto the Ny-Gl-Kt triangle. If Qz is not present, a ternary sheet silicate can grow, and hence a quaternary amphibole. These observations are supported by results of recent studies, as well as by natural data, and reveal the true nature of Ernst's (1963) polymorphic reaction between low- and high-pressure forms of glaucophane : it is the reaction Am + Qz = Ab + Tc located in this study, with the low-pressure polymorph Gl I a quaternary amphibole, and the high-pressure polymorph Gl II a ternary amphibole