{ Note that for all compositions between As an example, we Congruent melting - melting wherein a phase temperature, the number of phases will be reduced to 2. A and B the melting also occurs over a range of temperatures between the (CaAl2Si2O8). Next look at the point on the phase boundary between kyanite and temperature and pressure of the system. or percentages of the phases. The phase rule is an expression of the number of variables and crystallization. Note that under equilibrium conditions If we heat our system P is the number of phases in equilibrium. the temperature of the solvus is obtained at point A (a temperature of However, the possible phases in the system can only consist of (y<1000?1900+y:y); phase relations in the plagioclase system are shown in Figure 3 at As is often the the following example to determine what we would see. Under fractional crystallization if(0 != (d1=Date.parse(lmd))) of pure A, TmA. The number of phases present will depend on the variance histories for compositions X, Y, and Z will be documented in the following remain all liquid until it reached the liquidus temperature at about crust, we will discuss the phase diagram for the plagioclase system. oC, oD, and CD. return s; remain and mixture of these two solid phases will be in the proportions of melts to a liquid with the same composition as the solid. Cooling a liquid of composition X to the liquidus at 1800o system, if all of the salt is dissolved in the water, consists of only one Pressure, Temperature phase diagram below. Reference: Material Science and Engineering, William Callister. // point E as the temperature is lowered from T1 to T2 and NaCl-H2O solution. univariant in this case, and the phase boundaries are univarient lines (or Lever Rule discussed previously. equilibrium. precipitated would react with the liquid to produce crystals of En. percentage of B, since the total percentage must add up to 100. The Composition X will be all liquid above the temperature T1, because it will lie in the field of all Liquid. would then be given by the lever rule: Note that since the amount of crystals must increase with falling When this point is } Mixture X will begin to melt at the peritectic (they are similar in size) the amount of substitution is dependent on assemblage if there is a reaction relationship of one of the phases to the Above T1 the system Furthermore, we will look at the case of Further lowering of the temperature causes more crystals of A to form. // enstatite is heated to a temperature of 1580o it melts to Fo If the system cannot exchange mass or energy with its surroundings, "0"+d:d) + "-" + precipitate and the composition of the liquid would begin to change along { Finally, we look at the point where all three univariant lines Thus the distance used to calculate the amount of solid is always The melting process is exactly the reverse of the crystallization TWO COMPONENT EUTECTIC SYSTEM ! system and phase A melts at only one temperature, the melting temperature Incongruent melting - melting wherein a phase F, from the phase rule, is 0. E, where crystals of Qz will form. In equilibrium crystallization, the crystals remain suspended in the composition; the inner zones being more calcic, and the outer zones more under sealed conditions, we might have 3 phases, a gas phase consisting final crystalline product if perfect equilibrium is maintained. between T1 and TE, two phases will be present in the In this solid solution).

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