Some minerals melt with dissociation or incongruently. The most important examples are clinoenstatite dissociating at 1,557° into forsterite and silica, and orthoclase with a dissociation tern perature of 1,17o° yielding leucite and silica. The incongruent melting of these two minerals has important petrogenetic impli cations which will be noticed hereafter.
Binary Systems.—The manner of crystallization of binary systems is dependent on the mutual relations of the constituents.
In the simple case of two independent components each lowering the melting point of the other and not forming solid solutions the completion of crystallization takes place at a definite tempera ture (the eutectic point) at which both solids separate simulta neously from a solution of fixed composition (the eutectic mix ture). Many examples of eutectic crystallization are known among silicates. It frequently gives rise to a graphic texture in which there is an intimate intergrowth of the two phases. The intergrowths of quartz and orthoclase in graphic pegmatites and micro-pegmatite are believed to be eutectic aggregates. This is revealed not only in their simultaneous crystallization but also in their constant relative proportions (quartz 26%, orthoclase 74%). Not all graphic textures can, however, be regarded as evi dence of eutectic crystallization, nor do all eutectic mixtures form graphic intergrowths.
The conception of eutectics has played an important part in petrogenetic theory, and—as already noted—Vogt has attempted to recognize in eutectics the basis of a genetic classification of rocks. The eutectic relation is, however, only one of a number of possible relations existing between two or more components of a system. Where two components exhibit perfect isomorphous relations, eutectic crystallization is eliminated and the two sub stances crystallize as a single phase—an homogeneous solid solu tion. Of rock-forming minerals exhibiting this relation the pla gioclase felspars are the most important. The equilibrium rela tions in such a system may be illustrated diagrammatically as in fig. 1. The horizontal line represents the composition, the vertical the temperature. The pure components are albite and anorthite.
Anorthite (An.) melts at 1,550° C. The melting point of albite (Ab.) is fixed close to I,Ioo° ; its melts are highly viscous and crystallize only with great reluc tance. The upper curve of the
diagram is the freezing point curve of intermediate mixtures of the two components. It is known as the liquidus. The lower curve is the melting point curve, or solidus. Above the liquidus all mixtures are liquid, below the solidus all are crystallized. Mix tures of albite and anorthite do not melt at definite temperatures but have a melting or crystalliza tion interval, the lower limit fixed by the solidus, the upper by the liquidus.
Let us consider the phenomena attending crystallization of a mixture of given composition. A liquid of composition as the temperature falls begins to crystallize at A. The crystals in equilibrium with this liquid have the composition B (Ab1An4), the liquid being always richer in albite than the crystals with which it is in equilibrium. As the temperature slowly falls the crystals continue to form, at the same time changing in composi tion along the solidus curve by continuous reaction with the liquid. Throughout the course of crystallization, this process of reaction proceeds until finally the crystals have the composition D, that of the original liquid, the last drop of liquid having the composition C. If, however, equilibrium between the solid and liquid phases is not continuously maintained, the course of crystal lization is extended. At a moderately rapid rate of cooling the first formed crystals of composition B are not adjusted to the liquid, and there results a deposition of new layers upon the first formed crystals, each of composition corresponding to the tern perature at which they are formed. When crystallization ceases, as the bulk composition of the crystals is that of the original liquid and the cores of the crystals are richer in anorthite, the outer layers must be enriched in albite. The composition of the outer layer is represented by a point beyond D and that of the final liquid by a point beyond C. The process fir. t described is exemplified in the zonary banding or zoning of the common plagioclase felspars in igneous rocks. Either very slow cooling or very rapid cooling gives crystals free from zoning; at inter mediate rates of cooling, crystallization is accompanied by zoning, and at some definite rate of cooling it might be expected that a maximum zoning would result in which case the final drop of liquid would have the composition of pure albite.