Synthesis, Stability Relations, and Occurrence of Riebeckite and Riebeckite-Arfvedsonite Solid Solutions

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Abstract
Riebeckite, oNa2 Si8O22(OH)2, and members of a continuous splid-solution series between riebeckite and arfvedsonite extending as far as the approximate composition (Na2.4 ) ( ) Si7.7 O22(OH)2 were synthesized employing the bulk composition Na2O · 5FeOx·8SiO2 + excess H2O. Conventional hydrothermal techniques were used. Physical parameters governing the investigated phase equilibria include fugacity (partial pressure) of oxygen, temperature, and total pressure. Oxygen fugacity was controlled by copper-, iron-(± silica), and nickel-oxide buffers. The stability field of iron-rich alkali amphibole is bounded by surfaces within an volume. The five diagrams presented, each appropriate for a particular oxygen fugacity range, represent projections along the axis. Riebeckite decomposes above 469° at 250 bars at a high oxidation state where oxygen fugacity is defined by the hematite-magnetite buffer, and above 515° C. at 2,000 bars . At constant bulk composition except for oxygen content, the amphibole becomes increasingly arfvedsonitic as relative oxidation state declines, and its high-temperature stability limit is elevated; riebeckite-arfvedsonite is sTable up to 655° C. at 250 bars , and coexists with melt at 712° and 2,000 bars with low defined by the wüstite-iron buffer. In riebeckite-arfvedsonite solid solutions, sodium occupies the A, or vacant, position of the riebeckite structure in increasing amounts as oxygen fugacity declines; this site is completely filled near the low limit of the magnetite stability field with corresponding adjustment in ferrous/ferric and sodium/iron ratios in six-, eight-, and tenfold sites to maintain electrostatic neutrality. With further diminution of oxygen fugacity, iron and sodium enrichment of the amphibole takes place through limited substitution of ferric iron for silicon in tetrahedral co-ordination coupled with minor increment of sodium and possible slight oxidation of iron in six-and eightfold positions. Unit cell dimensions remain almost constant at oxygen fugaci-ties above the magnetite-wüstite boundary; at lower , cell volumes increase abruptly, reflecting the presence of Fe+++ in tetrahedral sites. Refractive indexes of riebeckite-arfvedsonite solid solutions decrease slightly and color changes from blue to green as the amphibole becomes more arfvedsonitic. No indication of exsolution between riebeckite and arfvedsonite was observed at temperatures as low as about 420° C. The experimental data seem to be consistent with field occurrences of iron-rich alkali amphiboles. Hy-droxyl-bearing pure riebeckites are sTable at low temperatures and high oxidation states in authigenic, low-grade metamorphic, and pegmatitic environments; in contrast, arfvedsonitic amphiboles appear to have crystallized at lower oxidation states and higher temperatures as primary constituents of alkalic igneous rocks.