SIZE-DEPENDENT PHASE DIAGRAMS FOR MINERAL NANO-CRYSTALS

Mineral nano-crystals are common in the earth environments. The stability and reactivity of nano-crystals are different from their macroscopic crystals. The size-dependent phase diagrams for mineral nano-crystals are important for us to better understand geochemical processes in the earth system. Integrated experimental study and computer modeling will help us to construct nano-crystals’ stability fields and their formation processes. Both shape and size of nano-crystals will affect their stability and reactivity. In the zirconia system, thermodynamically unstable tetragonal phase becomes stable when the crystal size decreases (< 13 nm). The sized-dependent phase transition between the tetragonal and monoclinic (baddeleyite) structure also depends on crystal shapes and solution acidity in aqueous environment. In aqueous solution, high acidity will expand baddeleyite stability field. Similar phenomena occur in anatase-rutile system. Solution acidity increases rutile stability due to the formation of stable surface complex on rutile nano-crystals. In nano-gold system, a field of quasi-molten that is between melt and solid phase increases as the crystals size decrease. The transition temperature between icosahedral gold and decahedral gold increases as the crystal size decreases. However, the transition temperature between FCC gold and decahedral gold increases as the crystal size decreases. The phenomena of size-dependent crystal stabilities also occur in mineral nano-precipitates. For instance, nano-lamella of ultra-high pressure titania phase (with alpha-PbO2 type structure) will be stable at low pressure in host crystal of rutile; nano-precipitates of protoenstatite (a high-temperature polymorph of enstatite) will be stable at low temperature in host crystal of plagioclase.