Answer the following questions in about 150 words each: (a) How does one designate mirror plane, rotational axes of symmetry and…

(a) Hermann–Mauguin notation. In Hermann–Mauguin notation, rotational axes are written as digits 1, 2, 3, 4 and 6; mirror planes as m; and a centre of symmetry as 1 with a bar, or implied by n/m when a mirror is perpendicular to an axis. The positions denote principal, perpendicular and diagonal axes. The crystal has one 4-fold axis, four 2-fold axes, five mirror planes and a centre. The principal axis is therefore 4/m; the perpendicular 2-fold axes and diagonal 2-fold axes are each 2/m. The symbol is 4/m 2/m 2/m, abbreviated 4/mmm. On a stereographic projection, the 4-fold axis is at the centre; the four 2-fold axes lie horizontally as opposite pairs along crystallographic and diagonal directions; the five mirror planes are great circles, one horizontal and four vertical; the centre is a small circle. The general form {hkl} plots as eight face poles symmetrically related by these elements.

(b) Crystallization of Di70–An30 melt. The Diopside–Anorthite binary system is eutectic. At 1 atm the eutectic is near 1270–1275°C at about 40–42% anorthite. A melt of 70% diopside and 30% anorthite lies on the diopside side of the eutectic. On cooling, its liquidus is about 1300–1310°C, where diopside begins to crystallize. As diopside is removed, the residual liquid becomes richer in anorthite and follows the liquidus toward the eutectic composition. At the eutectic temperature the remaining liquid solidifies simultaneously as diopside and anorthite. The final rock is therefore a mixture of diopside and anorthite. The early diopside crystals grow as large phenocrysts, while anorthite crystallizes later as laths in the interstices and in the eutectic intergrowth. This produces an ophitic or subophitic texture, in which large diopside grains enclose smaller anorthite laths. The diagram shows the liquidus, eutectic horizontal, and path moving rightward to the eutectic.

(c) Bowen's reaction series. Bowen's reaction series describes the order in which minerals crystallize from a cooling basaltic magma. The continuous series is the plagioclase feldspar series. As temperature falls, plagioclase changes composition gradually from calcium-rich anorthite toward sodium-rich albite. Because the same mineral species persists and only its composition changes smoothly, the series is called continuous. The discontinuous series involves separate mineral species that appear in succession: olivine crystallizes first, then reacts with the melt to form pyroxene, followed by amphibole and finally biotite. Each step replaces the earlier mineral by a new, chemically distinct phase, so the series is discontinuous. The continuous series is thus a solid-solution trend, while the discontinuous series records abrupt phase changes as the melt evolves. The two series operate together during fractional crystallization, with continuous plagioclase crystallization accompanying the stepwise changes in the mafic minerals. This explains igneous mineral assemblages and mafic-to-felsic trends.

(d) Prograde and retrograde metamorphism. Prograde metamorphism occurs when rocks are subjected to increasing pressure and temperature, usually during burial, heating or tectonic thickening. Mineral assemblages react progressively toward higher-grade conditions. A classic example is the Barrovian sequence in pelites, where chlorite, biotite, garnet, staurolite and finally sillimanite appear with rising temperature. In India, the high-grade granulite–charnockite terrains of Tamil Nadu record prograde heating to granulite facies, with orthopyroxene and other high-temperature minerals. Retrograde metamorphism occurs during cooling, uplift or fluid ingress, when high-grade minerals break down to lower-grade assemblages. Examples include garnet breaking down to chlorite, quartz and biotite, orthopyroxene changing to amphibole, and retrograde chloritization of pyroxene. Metasomatism is chemical alteration caused by infiltration of fluids that add or remove components. It produces new minerals and compositions, as in skarns, greisen and fenitization around alkaline intrusions, e.g. Sivamalai, Tamil Nadu.

(e) Folk's classification of limestones. Folk's classification of limestones is based on the type of grains, called allochems, and the nature of the fine-grained matrix or cement. Allochems include intraclasts, ooids, fossils and peloids. If the grains are set in micrite, the rock is a micrite; if they are cemented by sparite, it is a sparite. Thus fossil grains in micrite give biomicrite, while the same grains in sparite give biosparite. Ooids give oomicrite or oosparite, intraclasts give intramicrite or intrasparite, and peloids give pelmicrite or pelsparite. A limestone with no significant grains is simply micrite. The classification is useful because grain type and matrix control porosity and permeability. Sparitic limestones with well-cemented grains may have interparticle porosity, while micritic limestones often have lower permeability unless micrite is dissolved or fractured. Folk's scheme therefore links carbonate texture to depositional environment and reservoir quality. It also helps distinguish grain-supported from matrix-supported limestones in reservoir studies.