Because the calcareous feldspar (An) and albite (Ab) are the two end elements of plagioclase isomorphic series, the Di-An system is similar to the Di-Ab system. An can be regarded as representing the plagioclase isomorphic mixtures containing Ab. Fig. 5-6 is a binary phase diagram of diopside (Di) and albite (Ab). It is noteworthy that the co-junction of the binary system is biased to the side of Ab, and the co-junction ratio is Di3Ab97. The composition of acidic magma generally falls to the left of the co-junction point, but is close to the co-junction point, so the dark minerals always crystallize first, forming semi-automorphic to automorphic crystals; followed by plagioclase, which is semi-automorphic crystals, forming a typical granite structure.
Binary Near-knot Systems
If there is an intermediate compound between the two terminal elements of the binary system and the intermediate compound is "inconsistent melting compound", the binary system is called binary near-junction system. The characteristic of inconsistent melting compounds is that when they are heated, they are transformed into melts with different composition and another solid phase before full melting. This melting process is also called inconsistent melting or melting. In the opposite cooling crystallization process, different melts and solid phases of the above components react to form this "inconsistent melting compound". Here are two examples
To illustrate the characteristics of this kind of binary near-knot system phase diagrams.
(1) Magnesium olivine Mg2Si04-Si02 system
The phase diagram of this system (FcrSiQ system) is shown in Fig. 5-7. Magnesium olivine and quartz do not coexist in equilibrium at one atmospheric pressure, but they can react to form a more stable intermediate compound, persistent pyroxene (Mg2Si2Q). The refractory pyroxene is the inconsistent melting compound of the phase diagram.
If the initial melt contains 50 wt% silicon dioxide, its formation is similar to basalt. It begins to cool at 1900 C (point O in Fig. 5-7) and reaches the liquidus when the temperature drops to point P. At this time, forsterite begins to crystallize and the composition of the melt changes along the liquidus. When further cooling to 1558 degree C, according to the lever principle, 60 wt% forsterite and 40 wt% melt were crystallized in the system. At 1557 degree C, the reaction of forsterite with silicon dioxide in the melt produces pyroxene.
Mg2Si04+Si02 3^Mg2Si2Oe+Latent Heat
On the contrary, the refractory pyroxene (En) can also be decomposed into forsterite (Fo) and Si02 melts at 1557 degrees C. From the point of view of crystallization, the point here is called peritectic point, and from the point of view of melting, it is also called melting point. The melt crystallization process proceeds here. The three phases of forsterite, melt and newly formed refractory pyroxene coexist at i? Point equilibrium. At this point, the phase law can be written as follows: - 3 + 1, and the degree of freedom F = 0, indicating that the temperature of the system remains unchanged during the process of three-phase coexistence, until all the melts of the refractory pyroxene formed by reaction with forsterite are exhausted. Because the crystal structure and physical properties of the two minerals formed before and after this point have undergone qualitative changes, Bowen called this reaction discontinuous.
When the content of silicon dioxide in the initial melt is high (such as 70wt%100wt%), the high temperature melt rich in silicon will decompose into two immiscible solutions during cooling process. When it is cooled along the liquidus represented by the dashed line on the right side, both sides of the liquidus will precipitate cristobalite.
When the content of silicon dioxide in the initial melt is not particularly high, the equilibrium crystallization process of the phase diagram can be shown in Fig. 5-7.
(a) To illustrate.
When the initial melt composition is point a, the melt crystallization process ends at the near node/> and when the melt is exhausted, forsterite is converted to refractory pyroxene completely, and the final crystallization product is refractory pyroxene (original refractory pyroxene is formed, then it is transformed into refractory pyroxene in the cooling process, the same below).
When the initial melt composition is / point, when the temperature drops to the liquidus, Fo crystallizes first, then P = 2, F = 1, so the system enters the solid-liquid two-phase equilibrium, and the composition moves along the liquidus. As the temperature decreases, Fo crystallizes continuously, and the composition of the calcine becomes richer and richer in Si02. When the temperature drops to the near node, Fo becomes unstable, and more Fo reacts with fewer melts to form pyroxene. The crystallization process also ended at this point, and the final product was composed of more forsterite + less pyroxene.
If the initial melt composition is at the) point, it is said that the 02 content is higher than the first two points. As the temperature drops to the liquidus, forsterite begins to crystallize, but the amount is less. The melt composition quickly reaches the near-node P along the liquidus. At this time, after a small amount of forsterite reacted with the melt and completely converted into refractory pyroxene, more silicon-rich melts remained. Because the coexistence of coexistence of coexistence of coexistence of coexistence of coexistence of coexistence of coexistence of coexistence of coexistence of coexistence of coexistence of coexistence of coexistence of coexistence of coexistence of coexistence of coexistence of coexistence pyroxene and silicon-rich melt. At the co-junction point, the coexistence of anhydrite and xenopyroxene occurs, and the F = 0 of the system. The remaining melt is transformed into a mixture of coexistence and xenopyroxene at the co-junction ratio until the melt is exhausted. Therefore, the final crystallization products of point melt are pyroxene and cristobalite.