The igneous rocks are formed from the cooling and solidification of the Magma/Lava. The minerals and the chemical makeup of igneous rocks are important in order to understand their formation. Experimental studies of the crystallization sequence of minerals from a melt have helped greatly in understanding the origin of igneous rocks. The temperature-composition diagrams involving a liquid phase are called the “liquidus diagram”.
Most of the igneous rocks are multicomponent. Generally, they consist of two, three, four, and five principal mineral constituents. To study the solidification of magma, they are taken in the order of increasing complexity.
01. Crystallization of Binary Magmas:
The crystallization of magma consisting of two non-mixing constituents may be illustrated by the use of the temperature composition diagram. The two end members of the bicomponent system are A and B, which lack a solid solution between them. The melting temperatures of pure A and B are Ta and Tb respectively. The addition of some amount of B to a melt of A lowers the melting temperature of the liquid that can coexist with A along the curve between Ta and E. Here E is the “eulectic point”. Similarly, the melting temperatures of the liquid that can coexist with B is lowered by the addition of some amount of substance A as shown by the curve between Tb and E. The lowest temperature at which the crystal and the melts are in equilibrium is Te.
Let’s study the crystallization sequence of a melt of composition M in the following figure
- The melt M cools and its temperature drops from Tb without any crystallization.
- At temperature T1 Substance B begins to crystallize. The crystallization of B from the melt continues along the curve from Tb to E. As a result of this, the content of A in the melt increases continuously.
- At the eutectic point, E both A and B crystallize simultaneously at constant temperature TE until the melt is exhausted. At this point, no further change in the composition of the melt occurs. “Eutectic” is the constant proportion in which the two constituents crystallize simultaneously.
Mixed Crystals: When the two components are isomorphous and miscible in all proportions in solid-state, they form “mixed crystals”. The plagioclases are good examples of these types of crystals. The crystallization of mixed crystals from a melt can be represented by a temperature composition diagram shown in the above figure. A complete solid solution exists between the two end-members A and B. The melting point of pure A is Ta and that of pure B is Tb. A mixed-crystal does not melt at a definite temperature, but melting is spread over an interval other lower limit of which is fixed by the “solidus” and the upper limit by the “liquidus”. In the above figure, the upper curve is “liquidus”. It represents the locus of melt compositions in equilibrium with crystals. The lower curve is known as “solidus”. It is the locus for the composition of the crystalline phases in equilibrium with the melt. Let us study the crystallization of a melt of composition M and temperature Tb.
- The crystallization of the melt M starts at temperature T1 and crystals of composition D(A-14%, B-86%) begin to form. As these crystals contain greater amount of B, the melt becomes enriched in A and its composition moves along the liquidus curve towards A.
- Due to continual lowering of temperature, the crystals of original composition D react wth the melt and their composition changes in the direction of the arrow on the solidus curve. As a result content of A will increase in both the crystalline product and the melt with falling temperature.
- The crystallization stops at temperature T2 when the crystals have the same composition as that of the original melt M. If the temperature is lowered further the composition of the crystals will remain constant at composition M.
02. Crystallization of Ternary Magmas:
The crystallization of three-component magma can be represented by the triangular diagram. An example of such a diagram is shown in the following figure in terms of three components: (i) albite, (ii) anorthite, and (iii) diopside. The contours in this diagram represent melting temperatures and are known as contours in this diagram represent melting temperature and are known as “isotherms”.
The surface defined by these isotherms is called liquids surface. Let us study the crystallization of a magma of composition F. The composition F reprents 50% diopside, 25% albite, and 25% anorthite. In the above figure there is one boundary curve, which separates the field of plagioclase from the field of diopside. The point F is in the diopside field and represents a temperature 1275 degree Celcius.
- As the temperature falls, diopside begins to crystallize. With the separation of deopside the liquids are enriched in albite and anorthite, and the point F moves downwards in the direction of the arrow. Ultimately it reaches the boundary of the plagioclase field at G(temperature 1235 degree C)
- Point G represents 17% diopside crystals and 83% liquid. Here anorthite-rich plagioclase (Ab1.An4) starts crystallizing. With the separation of crystals of both diopside and An-rich-plagioclase, the composition of the melt moves along the boundary curve in the direction of the arrow. It means that the melt progressively becomes rich in albite.
- With falling temperature, the earlier formed crystals of plagioclase react with the melt and their composition changes continuously. For example at K plagioclase of composition L(Ab1.An2) separates out and all earlier crystals of plagioclase will also change over to this composition.
- All the liquid disappears at M and the plagioclase arrives at the composition Ab1.An1. The composition of the diopside, however, remains unchanged.
In the light of the above examples and discussion, a perfect equilibrium is assumed throughout the crystallization. If however the equilibrium between crystals and liquid is not maintained due to rapid cooling, new plagioclase will coat the early ones thereby forming zoned crystals. Magmas that give rise to igneous rocks, are made up of several components and their crystallization is rather a complex process.