1. Describe briefly, the major crustal features of the earth and discuss their origin.
Crustal features are the effects of the movement of tectonic plates over the asthenosphere. The lithosphere which is made up of earth’s crust and upper part of the upper mantle, are broken into tectonic plates, they slide over the underlying asthenosphere. The main crustal features, formed by these movements, are discussed briefly.
- Trench: Trenches are deep depressions on the ocean floor, which are formed by ocean-continent and ocean-ocean convergent plate boundaries. When one oceanic plate subducts beneath another oceanic or continent plate, a trench is formed.
- Rift zone: Rift zones are formed where two continental plates diverge away from each other. As the crust thins at the middle of a continental plate, a narrow rift valley is created.
- Oceanic ridges: Mid oceanic ridge is a continuous range of underwater mountains encircling the globe almost entirely underwater. This is formed at ocean ocean divergent boundary.
- Mountains: Mountains are formed at different types of boundaries, in ocean – continent or continent – continent convergence settings.
- Volcanos: Volcanos are mountains with a chamber inside it filled with molten rock. Volcanic chains are formed at ocean continent convergent boundary (volcanic arc) or at ocean ocean convergent boundary (island arc) and also at divergent boundary.
- Earthquakes and fault: Faults are effects of earthquake, which are the effect of plates sliding past one another. Faults can occur in all three types of boundaries – convergent, divergent and transform plate margins.
A figure showing important crustal features like Trench, Oceanic ridges, Volcanic chains.
2. With the help of appropriate phase equilibrium diagrams, explain the reaction principle in petrogenesis.
Minerals that making up the rocks of an igneous sequence can be arranged as a reaction series and it is the existence of such series which control the crystallization and differenciation of the rocks of the sequence. The most important series in this regard is Bowen’s reaction series, in which the minerals are arranged in order of crystallization
Now the reaction principle in petrogenesis is being explained with the help of phase diagrams.
- The best example of continuous reaction relation can be explained by the feldspar solid solution.
- Another kind of reaction relation is explained here which is quite different from the above and is known as peritectic reaction relation.
Bulk composition a (Ab40An60),the first solid forms at b, denoted by the composition c.
With lowering of temperature, the liquid composition moves to d and correspondins solid moves along the solidus to f.
At 14500C, liquid d and solid f coexist at equilibrium.
So the continuous reaction is
At 13500C,the liquid is exhausted and the final solid composition corresponds to h.
In a similar way reaction relation can also be explained if another phase, say diopside is added to the system.
Here the line joining all points K, H, L, M is called a eutectic boundary curve or cotectic curve.
Crystallization proceeds along this boundary and the reaction relation can be explained in similar way as in binary system.
Here after the solid Forsterite forms from the initial melt composition, it continues to separate from the melt until it reaches the 1557 0C isotherm. There it reacts with the melt and a new solid Enstatite is formed.
Depending on the initial melt, the final solid can be entirely olivine, or entirely protoenstatie or a mixture of both.
With the present progress in petrogenetic theories, the concept of eutectic relation can no longer serve valid explanation, but other relations between liquid and solid phases, called reaction relation proves to be fundamentally significant.
3. What are Charnockites? Discuss critically the different views on their origin in India and elsewhere.
The name Charnockite is applied to any orthopyroxene bearing quartz – feldspar rock, formed at very high temperature and pressure. This was first described from southern granulite complex of India and was named after Job Charnock. After that many workers have worked in those areas and gave different views regarding the origin of the rocks.
- Holland (1893), who first gave this name to a series of rocks, acid intermediate and basic types, characterised by a granulitic texture and a constant presence of pleochroic orthorhombic pyroxene (hypersthene), along with quartz and perthitic feldspars, derived from a calc alkaline magma.
- Vredenburg was the first one to challenge the magmatic origin of charnockites and proposed a metamorphic origin.
- Ghosh (1941) opined that these rocks are largely the product of regional metamorphism along with assimilation of a series of impure calcareous rock or sediments.
- After making a detailed study of this rocks from the type area , Subramanian has redefined the terms charnockites and charnockite suits. He has defined this rock as hypersthene quartz feldspar rock, with or without garnet, characterised by a greenish blue feldspar and a greyish blue quartz, dominat feldspar being the microperthites.
- He has proposed the name charnockite “suite” to the acid division of charnockite series of Holland and considered them to be an igneous suite which has undergone metamorphism and recrystallization.
- The rocks of the basic division of the series of Holland were considered as pyroxene granulite, having no genetic relationship with charnockitic rocks.
- Finally the intermediate division was recognised as a hybrid type, formed by partial assimilation and incorporation of pyroxene granulite by the charnockites.
Apart from Indian authors, geologists all over the world had given different opinions on the origin of charnockite in different times.
- Wilson (1952) had opined that Australian charnockites are largely metamorphic in origin, but some of them may be representatives of intermediate and basic charnockites in central Australia, which are magmatic in origin.
- Quensel,after working extensively on the charnockitic rock in Varberg, Sweden had confirmed that these rocks have been formed by deep seated metamorphism of heterogeneous rocks of the surrounding gneiss under extremely high pressure, temperature and dry condition.
So the origin of charnockite is still a matter of debate and many later work is to be done.
4. Write an essay on ‘atomic structure of crystals as related to their morphology and physical properties’.
An ordered atomic arrangement of crystals has control over their morphology as well as physical properties. At first the control on morphology of crystals is explained.
With respect to the various symmetry elements present in a crystal (center of symmetry, plane of symmetry, mirror plane, rotation axes, rotoinversion axes, glide plane), there are-
- 6 crystal systems
- 14 Bravais lattice
- 230 Space groups
Based on the cation anion relationship in a crystal there are six sets of rules known as “Pauling’s rule” , which are the fundaments for understanding the controls of atomic structure in morphology.
Rule 1: The Coordination Principle : A coordination polyhedron of anions forms around each cation. The cation-anion distance is determined by the sum of the cation and anion radii and the number of anions coordinating with the cation is determined by the relative size of the cation and anion.
Rule 2: Electrostatic Valency Principle : In a stable ionic structure, the total strength of the valency bonds that reach an anion from all neighbouring cations is equal to the charge of the anion.
Rule 3: Sharing of polyhedral elements I : The existence of edges, and particularly of faces, common to coordination polyhedra decreases the stability of ionic structure.
Rule 4: Sharing of polyhedral elements II : In a crystal containing different cations, those with large valence and small coordination number tend not to share polyhedral elements with each other.
Rule 5: Principle of Parsimony : The number of essentially different kinds of constituents in a crystal tends to be small.
The table below shows how the shape of coordination polyhedron is controlled by the atomic arrangement.
|12||Not regular, based on cubic or hexagonal close packing||∼1|
|8||Cubic||0.732 – 1|
|6||Octahedron||0.414 – 0.732|
|4||Tetrahedron||0.225 – 0.414|
|3||Triangle||0.155 – 0.225|
Relation to physical properties:
Physical properties of minerals are the direct result of their chemical as well as structural characteristics. Atomic structure have direct relation to some of the physical properties of minerals, like Cleavage, Parting, Hardness, Tenacity, and also electrical and magnetic properties.
As cleavage is the tendency of a mineral to break along a plane of a definite crystallographic orientation, how a mineral breaks can give us a insight view about the crystal structure of a mineral. The cleavage plane can be identified as cubic, octahedral, rhombohedral, prismatic, and always consistent with the symmetry.
Similar to cleavage, octahedral parting of magnetite, basal parting of pyroxene, rhombohedral parting of corundum are also related to the atomic arrangement of crystals, but unlike cleavage , parting can only be examined, if the specimen is subjected to proper stress.
How a mineral reacts to scratching (hardness), cohesiveness (tenacity) is also somehow controlled by the atomic structure.
Lastly, the electrical property (whether the mineral will show pyroelectric or piezoelectric behaviour), and the magnetic property (whether the mineral will be ferromagnetic or diamagnetic or paramagnetic) are largely controlled by how the atoms are arranged in the particular mineral.
5. Describe the garnet group and show how the various members exhibit complete isomorphism.
Garnet group of minerals:
- Garnet consists of a multicomponent solid solution with a general formula of A32+B23+Si3O12, where A is bivalent site occupied by Ca2+, Fe2+, Mg2+ and Mn2+ and B is trivalent site occupied by Al3+, Fe3+ and Cr3+.
- Natural garnets are compositionally divided into two groups –
- Minerals which belong to same crystal system and morphology, but varies considerably in chemical composition (as well as in density, hardness, refractive index, unit cell parameters), are known to be isomorphous. Isomorphic replacement between different cations and anions are most commonly (not always) the basis for grouping and classification of a particular mineral group.
- Natural garnet never occurs as pure end member composition and thus the degree of solid solution possible is important. That is why garnet group is one of the most important mineral group in terms of studying the phenomenon – complete isomorphism.
- So Garnets are known in two isomorphic rows of constant mixing: aluminum garnets – pyralspites: pyrope, almandine, spessartine and calcium garnets – ugrandites: uvarovite, grossular, andradite. Inside these subgroups, there is broad isomorphism but between the subgroups isomorphism exists in a narrow scale.
- It should also be noted that isomorphous replacements are largely controlled by temperature.