Anorthite melting temperature

At the top of the diagram, the section is drawn at the melting temperature of anorthite crystals. Therefore, the liquid and solid curves join at the An axis, because Going toward component Ab, the liquid curve... 

Let us assume that, at high temperature and ambient pressure, the binary system albite-anorthite (ab-an) is ideal. The temperature T and enthalpy AH( of melting of each component is... 

Goldsmith J. R. (1980). The melting and breakdown reactions of anorthite at high pressures and temperatures. Amer Mineral, 66 1183-1188. 

Figure 4-34 is a phase diagram for the system titanite-anorthite. Suppose a crystal of titanite is initially in contact with a crystal of anorthite. The two are heated to 1350°C. Either phase by itself would not melt. But because the temperature is higher than the eutectic point of the two phases, at the interface there is melting. As melting proceeds, a thin melt layer would form between the two crystals. The melting of the two phases continues and the rate may be controlled by different factors. The rate would depend on the controls, as outlined below. 

Olivine occurs mainly as inclusions in enstatite and is also found in albite, spinel, and troihte (Brearley, 1989). At least three kinds of poly-crystalline clusters appear to have formed separately (i) enstatite-rich units with forsterite inclusions, (ii) coarser versions with albite and olivine inclusions, and (iii) forsterite-anorthite units with no enstatite. Enstatite crystals are intergrowths of ortho- and clinopyroxene and their microstructures indicate cooling from >1,000 °C at -1,000 °Ch Brearley (1989) suggests that the Kakangari matrix formed from amorphous or partly crystalline particles <10 p,m in size that were annealed at 1,100-1,200 °C or possibly higher, and then rapidly cooled in an hour. The chondrules in Kakangari could have formed from similar material that was heated to higher temperatures, partly melted, and quenched at comparable rates, provided that the chondrules acquired lower concentrations when molten. 

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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