Summary: 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, 2Vγ66°, pleochroism a pale yellow, β pale lime green, γ pale pinkish orange, and a composition corresponding to Mg3·67Mn0·04Fe0·172+Ti0·01Fe0·333+Al8·07Si1·75O20·00. 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 Mg7·58Mn0·02Fe0·022+Ti0·01Fe0·133+Al0·25Al0·41Si7·59O24·00 and K0·04Na0·16Ca1·95Mg4·37Mn0·01Fe0·052+Ti0·04Fe0·253+Al0·42Al0·91Si7·09O22·24(OH)1·76 respectively 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.