EXPERIMENTAL INVESTIGATION OF AN-DI PHASE EQUILIBRIA AT 1-ATM, A COLLABORATIVE COURSE PROJECT

We performed a series of 18 1-atm phase equilibria experiments in the An-Di (CMAS) system as a class project. Four groups of 2-3 students each ran experiments on An₃₀, An₄₀, An_41.5, An₅₀, An₆₀ bulk compositions constructed from high purity oxide and carbonate powders. Starting powders were run in crimped Pt capsules in 1-atm vertical quench furnace at temperatures ranging from 1250ºC to 1400ºC for 24 to 119 hours. Run products were lightly crushed, mounted in epoxy, and polished for analysis by SEM and electron microprobe (EPMA) at the University of Oregon and results were compared to those of Bowen (1915). Backscattered electron (BSE) images were collected to assess general phase assemblage and texture. The images reveal that experimental results of each bulk composition adhered to the expected phase boundaries and presence, or absence, of mineral grains within the glass. Glass proportions are greater in higher-temperature runs and the most well developed (euhedral) crystals occurred in the longest duration experiments. Two-stage experiments (initially super-liquidus, then lowered to target temperature) showed the best-equilibrated textures. Capsules of An₃₀ and An₅₀ were run simultaneously in the furnace at the same conditions (2h at 1350ºC, 117h at 1250ºC). The An₃₀ results showed large subhedral Di and An crystals, whereas a single stage 30h run showed smaller euhedral Di and An crystals. The two-stage An₅₀ experiment showed subequal proportions of large (up to 200 µm), well-developed euhedral An and Di crystals, while the single stage run showed much smaller (<10µm) sub- to euhedral crystals. EPMA analyses of crystalline phases and glass (quenched melt) were collected. Analyses of apparent overlapping phases were discarded. Oxide data were re-cast into mineral formulas and glass compositions were converted into An-Di phase diagram plotting coordinates. Calculated mineral formulas are 98-99% pure, consistent with analytical uncertainty and expected pure end-member composition. Small discrepancies were likely due to incomplete reaction of starting materials or errors in creating original mixes. Overall, phases plotted consistently with expectations.