The System K2CO3–CaCO3–MgCO3 at 3 GPa: Implications for Carbonatite Melt Compositions in the Shallow Continental Lithosphere

Potassic dolomitic melts are believed to be responsible for the metasomatic alteration of the shallow continental lithosphere. However, the temperature stability and range of compositions of these melts are poorly understood. In this regard, we performed experiments on phase relationships in the system K2CO3–CaCO3–MgCO3 at 3 GPa and at 750–1100 °C. At 750 and 800 °C, the system has five intermediate compounds: Dolomite, Ca0.8Mg0.2CO3 Ca-dolomite, K2(Ca≥0.84Mg≤0.16)2(CO3)3, K2(Ca≥0.70Mg≤0.30)(CO3)2 bütschliite, and K2(Mg≥0.78Ca≤0.22)(CO3)2. At 850 °C, an additional intermediate compound, K2(Ca≥0.96Mg≤0.04)3CO3)4, appears. The K2Mg(CO3)2 compound disappears near 900 °C via incongruent melting, to produce magnesite and a liquid. K2Ca(CO3)2 bütschliite melts incongruently at 1000 °C to produce K2Ca2(CO3)3 and a liquid. K2Ca2(CO3)3 and K2Ca3(CO3)4 remain stable in the whole studied temperature range. The liquidus projection of the studied ternary system is divided into nine regions representing equilibrium between the liquid and one of the primary solid phases, including magnesite, dolomite, Ca-dolomite, calcite-dolomite solid solutions, K2Ca3(CO3)4, K2Ca2(CO3)3, K2Ca(CO3)2 bütschliite, K2Mg(CO3)2, and K2CO3 solid solutions containing up to 24 mol % CaCO3 and less than 2 mol % MgCO3. The system has six ternary peritectic reaction points and one minimum on the liquidus at 825 ± 25 °C and 53K2CO3∙47Ca0.4Mg0.6CO3. The minimum point resembles a eutectic controlled by a four-phase reaction, by which, on cooling, the liquid transforms into three solid phases: K2(Mg0.78Ca0.22)(CO3)2, K2(Ca0.70Mg0.30)(CO3)2 bütschliite, and a K1.70Ca0.23Mg0.07CO3 solid solution. Since, at 3 GPa, the system has a single eutectic, there is no thermal barrier for liquid fractionation from alkali-poor toward K-rich dolomitic compositions, more alkaline than bütschliite. Based on the present results we suggest that the K–Ca–Mg carbonate melt containing ~45 mol % K2CO3 with a ratio Ca/(Ca + Mg) = 0.3–0.4 is thermodynamically stable at thermal conditions of the continental lithosphere (~850 °C), and at a depth of 100 km.

Diffusion Paths for the Formation of Half-Heusler Type Thermoelectric Compound TiNiSn

Half-Heusler compound TiNiSn is one of the most promising candidates of thermoelectric materials which can be used to directly convert the waste heat to clean electric energy at high temperatures (around 1000 K). Thermoelectric power generation is an appealing approach for conserving energy and preserving the global environment. Half-Heusler compounds have the cubic C1b type ordered structure and show semiconducting behavior when their valence electron count (VEC) is around 18. TiNiSn is the most attractive one not only because it has excellent thermoelectric properties but also it consists of eco-friendly elements which are neither toxic nor costly. However, TiNiSn has a bothersome problem that fabrication of single phase TiNiSn alloy is quite difficult. We have found that TiNiSn phase forms by the ternary peritectic reaction. Thereby, inevitable non-equilibrium solidification results in the formation of impurity coexisting phases which tend to decrease thermoelectric properties. In the present work, to establish the basis of new fabrication processes for TiNiSn alloys, we have started from the investigation on the diffusion paths which are closely related to the formation of TiNiSn phase. The diffusion behavior was evaluated using solid/liquid diffusion couples composed of the binary Ti-Ni intermetallic compounds and Sn liquid phase, where we have selected TiNi, TiNi3 and Ti2Ni as solid phases for instance. The most interesting result is that the single-phase TiNiSn phase layer forms at the TiNi/Sn(L) interface during annealing at 1073 K for only 1 h. Moreover, faceted grains of TiNiSn single-crystal grow at the interface toward the liquid Sn phase. We have confirmed two interesting microstructural features using EBSD analyses. One is that most of these TiNiSn single-crystals have the same crystallographic orientation, and the other is that TiNiSn phase layer formed on the TiNi side of the interface consists of very fine sub-microns grains. While TiNiSn solely forms at the TiNi interface, Heusler TiNi2Sn also forms with TiNiSn at the TiNi3 interface and Ti6Sn5 tends to coexist at the Ti2Ni interface.