Order-disorder in sapphirine

Abstract

Yellow sapphirine with a doubled b-axis occurs in enstatite-hornblende-sapphirine rocks at the contact between dolomite marble and yoderite-quartz-talc schist on Mautia Hill, Tanganyika. It has refractive indices α 1·725± 0·002, γ 1·732±0·002, 2V_γ66°, pleochroism a pale yellow, β pale lime green, γ pale pinkish orange, and a composition corresponding to Mg_3·67Mn_0·04Fe²⁺_0·17Ti_0·01Fe³⁺_0·33Al_s·07Si_1·75O2_0·00. Its unit-cell dimensions are approximately a 9·85 Å, b 28·6Å, c 9·96 Å, β 110° 30′, and its diffraction pattern exhibits diffuse streaks parallel to b*, on which lie diffuse intensity maxima representing the relics of certain types of sharp reflexions of normal sapphirine ; this is interpreted as indicative of partial MgAl ordering. The associated enstatite and hornblende have compositions Mg_7·58Mn_0·02Fe²⁺_0·02Ti_0·01Fe³⁺_0·13Al^[6]_0·25Al_0·41^[4]Si_7·59O_24·00 and K_0·04Na_0·16Ca_1·95Mg_4·37Mn_0·01Fe_0·05²⁺Ti_0·04Fe_0·25³⁺Al_0·42^[6]Al_0·91^[4]Si_7·09O2_2·24(OH)_1·76respectively and are accompanied by accessory hematite and pseudobrookite. Preliminary heating experiments have demonstrated the transformation of single crystals of partially ordered Mautia sapphirine to single crystals of a defect spinel phase after 48 hours at 1287° C in air; normal disordered Panrimali sapphirine remains unchanged by similar heat treatment.