The theoretical investigations of Roozeboom have shown that five types of crystallization of isomorphous substances may occur; in three the mutual solubility of the two components is unlimited, in two it is limited, so that only mixed crystals of certain types may occur. Some of these have been identified in rock-forming minerals, others are suspected though not yet proven.
Ternary Systems.—Crystallization is more complicated in ternary systems than in binary. Roozeboom and Schreinemakers worked out the theory of these systems. To represent their be haviour resort must now be made to a solid figure. All possible mixtures of three components can be represented by points within an equilateral triangle, the apices of which represent the three components. Binary mixtures appear as points on the sides of the triangle. If lines be drawn through any point within the triangle and parallel to its sides, they will cut the sides at distances which represent the relative proportions of the components represented by the point. To represent temperature, vertical ordinates are erected upon the equilateral triangle. The solid figure thus obtained is a triangular prism bounded above by freezing point surfaces meeting in boundary lines. These can be conveniently represented in plane projection. In the simplest ternary systems each pair of the components form a binary eutectic and the three together a ternary eutectic, which represents the composition in the system possessing the lowest fusibility.
The crystallization course of such a simple system may be illus trated in the diagram (fig. 2). A, B and C represent the three pure components, AB, BC and AC represent binary mixtures, and and the respective binary eutectics. The ternary eutectic is represented by the point within the figure. A liquid of composition a when cooled to the freezing point surface sepa rates crystals of the composition A, the composition of the liquid moving away from A towards b. At b, the liquid becomes saturated for the component C which will begin to crystallize, both A and C crystal lizing while the composition of the liquid changes along the boundary curve bEabe. Finally B also begins to crystallize and the ternary eutectic point is reached at which the three components crystallize simul taneously in definite proportions (at Eabe ) until the whole is com pletely solidified. This simple ternary system though realized among metallic alloys—for example in the system lead-tin bismuth, where the ternary eutectic melts at 96°—has not been yet discovered among rock-forming silicates. There the relations are much more complex.
Occurrence of inversion points in one or more of the compo nents; the formation of binary or ternary compounds with con gruent and incongruent melting points; the formation of isomor phous mixtures, limited or unlimited, among the phases, are some of the many complications in silicate systems already investigated.
As an example of a very complicated system, the ternary system Ca0 — may be briefly noted.
In addition to the three components, there are nine binary compounds, some with enantiotropic inversions ; and three ternary compounds, only two of which are stable. The fields meet three together in a large number of quintuple points eight of which are ternary eutectics. Investiga tion of this system has involved the undertaking of over 7,000 heat treatments and microscopi cal preparations.
Among the ternary silicate sys tems that have been studied three in particular are of great importance in the interpretation of the crystallization-history of rock magmas. They include those mixtures from which the phases, plagioclase, pyroxene, olivine, spinel and silica have separated. An important system is that of diopside — albite —anorthite. The equilibrium diagram is shown in fig. 3. The crystallizing phases are diopside and plagioclase. These mixtures are theref ore comparable with simple basaltic or gab broic magmas on the one hand and simple dioritic (augite-diorite) magmas on the other according to the nature of the plagioclase.
As before, consider the crystallization of a liquid whose compo sition is represented by the point F. (5o% diopside, 5o% plagio Crystallization begins with ,a separation of diopside (supposed to be a simple mineral and not an isomor phous mixture, as it would usually be in rocks) at about 1,275°. At 1,235° the excess of diopside (G) has separated out, and felspar begins to crystallize. It has about 8o% anorthite (H). Thereafter diopside and felspar both crystallize, but as the temperature travels along the line EGD from G to M the compo sition of the felspar changes from H to Z (if we suppose that all the early felspar which is unduly rich in anorthite is stage by stage resorbed). The resulting rock has the mineral composition above stated ; but if resorption of felspar is incomplete the last f ormed felspar is richer in albite and has a composition T. The felspar crystals in that case are zonal with basic centres. If at any time crystallization is suddenly brought to an end a glassy ground-mass will be formed, which is richer in soda and silica than the original magma and contains zoned felspar crystals. This is exceedingly like what takes place in many basaltic lavas. Again, if the original mixture had been richer in felspar, so that the composition point lay below the line DE, felspar would have crys tallized out first. This seems to be in keeping with the structure of many dolerites, which contain felspar partly enclosed in augite crystals of later formation (ophitic structure), while others show that the augite appears in porphyritic crystals and began crystallizing before felspar.