Reevaluation of the Åkermanite-gehlenite Binary System

Introduction: Melilite, a solid solution of åker-manite (Ca 2 MgSi 2 O 7) and gehlenite (Ca 2 Al 2 SiO 7), is one of the major constituents of the Ca-Al-rich inclusions (CAIs), representing the oldest material of the solar system. Therefore, knowledge of the phase and compositional relationships in the åkermanite-gehlenite binary system is very important for understanding the formation and evolution of CAIs. The temperature-composition relationship in the åkermanite-gehlenite binary system was first determined by Ferguson and Buddington in 1920 [1]. They showed that åkermanite and gehlenite form a complete solid solution series with a minimum melting point of 1388ºC (Åk 73). The solidus and liquidus curves were determined by identifying quenched products by optical microscopy. In 1941, Osborn and Schairer [2] used a fast quenching technique to confirm the liquidus position , but the solidus was shifted significantly to lower temperatures (e.g., from 1443ºC to 1413ºC for starting composition corresponding to Åk 40). Such a large shift in the solidus curve results in very large differences in composition of melilite crystallizing from the melt. For example, first melilite crystallizing from the melt with composition corresponding to Åk 60 would be ~Åk 45 according to [1] and only ~Åk 30 according to [2]. The large discrepancy in the solidus position may be due to difficulties in quenching some glasses as was pointed out in [2], but also by lack of equilibrium between liquid and solid phases at low temperatures and slow crystallization kinetics of meli-lite from the melt. The later seems to explain the difference in solidus position obtained from mostly 15 minute runs in [1] and several hours long runs in [2]. The purpose of this study was reevaluate the temperature composition relationship in the åkermanite-gehlenite binary system by significantly increasing the run duration of the experiments and using modern techniques to analyze the quench products. Experimental: The starting materials (Åk 20 and Åk 50) were prepared by mixing (for ~1 hour) appropriate amounts of MgO, SiO 2 , Al 2 O 3 and CaCO 3 followed by melting at 1600ºC for at least 3 hours in air. Quenched glasses were ground under ethanol in an agate mortar for one hour and 20-30 mg of the resulting powder mixed with polyvinyl alcohol were loaded into 2.5 mm Pt wire loops. The samples were placed into a Deltech vertical furnace, sintered at 1000ºC in air, heated for 3–4 hours …