Answer the following questions in about 150 words each: (a) Describe the symmetry elements shown by the normal class of the…

(a) Orthorhombic normal class. The normal class of the orthorhombic system is 2/m 2/m 2/m (or mmm). It is the holosymmetric class, so all crystal faces occur in symmetrically equivalent pairs. It is the most symmetric orthorhombic class. It has three mutually perpendicular twofold axes (3L2), three mirror planes (3P), and a centre of symmetry (C). The axes a, b and c are unequal and mutually perpendicular. The characteristic forms are pinacoid {h00}, prism {0kl}, dome {h0l}, and pyramid {hkl}. In a sketch stereogram of the general form {hkl}, the class mmm gives eight distinct poles: four in the upper hemisphere and four in the lower hemisphere. The sketch would show them as eight symmetrically placed points. The lower-hemisphere poles do not coincide with the upper-hemisphere poles; they are related by the centre of symmetry and by the three mirror planes. The poles are arranged symmetrically about the three twofold axes. Minerals that crystallize in this system include olivine, andalusite, barite, and topaz.

(b) Optical properties of hornblende and augite. Under the petrological microscope, hornblende and augite are distinguished by relief, cleavage, extinction and colour. Hornblende is an amphibole; it shows moderate relief, two cleavages, and inclined extinction of about 10-25° to the cleavage traces. It is strongly pleochroic, commonly passing from green to brown. Augite is a clinopyroxene; it shows high relief, two cleavages, and symmetric extinction. It has little or no pleochroism, and its cleavage/optic orientation is commonly described by a 45° angle, Z∧c ≈ 45°. In cross-polarised light, hornblende’s green-brown pleochroism is readily visible, whereas augite remains nearly colourless. Both minerals show two cleavage directions, but the extinction angle and pleochroism are the most useful diagnostic features. Augite’s higher relief makes its grain boundaries sharper, while hornblende’s inclined extinction is a key distinction in thin section for identification. These properties help identify them in igneous and metamorphic rocks.

(c) Perthite texture and formation. Perthite is an intergrowth in which alkali feldspar is the host and plagioclase occurs as exsolved lamellae; antiperthite is the reverse, with plagioclase host and alkali feldspar lamellae. A neat labelled diagram would show a host alkali feldspar grain containing thin, regularly spaced plagioclase lamellae, with labels for host, exsolution lamellae, and, if present, mesoperthite-scale lamellae. The formation is explained by the Ab-Or phase diagram. At high temperature, albite and orthoclase form a solid solution. On cooling, the composition enters the solvus, the miscibility gap, below about 700 °C. The homogeneous feldspar becomes unstable and separates into two feldspar phases. Slow cooling gives coarser mesoperthite lamellae, while rapid cooling gives finer cryptoperthite. The phase diagram would show the single-phase feldspar field above the solvus and the two-phase feldspar field below it, with a cooling path crossing the solvus. This exsolution records the cooling history of granitic rocks.

(d) Diagenetic textures of carbonate rocks. A labelled diagram of carbonate diagenesis would show grains and cements modified after deposition. Micrite envelopes are thin carbonate mud coatings around grains; micritization is the replacement or infilling of grains by micrite. Hardgrounds are cemented, erosion-resistant surfaces. Intraclasts are fragments of earlier carbonate sediment, and peloids are mud clots. Ooids may show diagenetic overgrowths. Cementation includes syntaxial overgrowths, where new calcite grows with the same crystal orientation as the host grain, and epitaxial cements, where new crystals grow with a different orientation. Pressure solution produces stylolites and sutured grain contacts, where soluble material is removed along grain boundaries. The diagram would label an ooid with micrite envelope, an intraclast, a peloid, syntaxial cement, and a stylolite. These features help distinguish marine, shallow-water, and burial diagenetic environments. They also indicate compaction, cementation, and fluid flow during burial. Such textures are common in limestones and dolostones.

(e) Ultrahigh-pressure and ultrahigh-temperature metamorphism. Ultrahigh-pressure metamorphism occurs at pressures above 2.5-4 GPa and temperatures of 600-800 °C, within the blueschist to eclogite facies. Its diagnostic assemblages include coesite and diamond, minerals stable only at such high pressures. These rocks record rapid subduction of crustal material to mantle depths and later exhumation. Ultrahigh-temperature metamorphism occurs at temperatures above 900 °C, up to 1050 °C, at pressures of 0.5-1.2 GPa, in the granulite facies. Characteristic assemblages include osumilite, sapphirine, and spinel plus quartz. UHT rocks record intense heating of the lower crust, often associated with mantle-derived magmatism or deep crustal extension. Both UHP and UHT metamorphism indicate extreme tectonic processes, but UHP reflects very high pressure with moderate temperature, while UHT reflects very high temperature at lower pressure. Such assemblages are used to reconstruct P-T paths and tectonic settings. They are important for understanding plate convergence, crustal recycling, and thermal structure. Their preservation requires fast exhumation and limited retrogression.