Diopside melting temperature

The following are experimental data for diopside melting in its own melt (Kuo and Kirkpatrick, 1985). Diopside melting temperature is 1664 K. The fusion entropy is 82.76 J moC K Assume that the activation energy is 300 kj/mol. 

Figure 4-6 Interface reaction rate as a function of temperature, pressure, and composition. The vertical dashed line indicates the equilibrium condition (growth rate is zero), (a) Diopside growth and melting in its own melt as a function of temperature with the following parameters Te= 1664K at 0.1 MPa, A5m-c = 82.76J mol K , E/R —30000 K, 4 = 12.8 ms K, and AV c l. l x 10 m /mol. The dots are experimental data on diopside melting (Kuo and Kirkpatrick, 1985). (b) Diopside growth and melting in its own melt as a function of pressure at 1810 K (Tg = 1810 K at 1 GPa from the equilibrium temperature at 0.1 MPa and the Clapeyron slope for diopside). (c) Calcite growth and dissolution rate in water at 25 °C as a function of Ca " and CO concentrations.

This means that to fix all the properties of both kinds of crystals, we need only choose the temperature (pressure being already fixed at 1 bar). However, when the first drop of liquid forms, p = 3 (diopside crystals, anorthite crystals, and liquid), and / = 0. Another word for / = 0 is invariant. When p = 3 on an isobaric plane, we have no choice as to T, P, or the compositions of the phases -they are all fixed. This explains why all mixtures begin to melt at the same temperature, and why the liquid formed is always the same composition no matter what the proportions of the two kinds of crystals. No other arrangement would satisfy the phase rule. 

The binary phase diagram shown in Figure 3.3 actually relates to the system anorthite (CaAl2Si208)-diopside (CaMgSi20s), with melting points of the pure components of Ta = 1550°C and Tb = 1390 C, respectively. The eutectic temperature Te is 1270 °C, with a composition Cab of 42 mass% anorthite -1- 58 mass%... 

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