(a) Describe the classification of crystals into crystal systems, and add a note on the types of symmetry shown by crystals. (10…

(a) Crystal systems and symmetry (10 marks)

Crystals are grouped into seven systems by their axial lengths (a, b, c) and interaxial angles (α, β, γ), which follow from their symmetry.

• Cubic: a = b = c; all angles 90°; four 3-fold axes.
• Tetragonal: a = b ≠ c; all 90°; one 4-fold axis.
• Hexagonal: a = b ≠ c; α = β = 90°, γ = 120°; one 6-fold axis (trigonal has a 3-fold axis).
• Orthorhombic: a ≠ b ≠ c; all 90°; three 2-fold axes.
• Monoclinic: a ≠ b ≠ c; α = γ = 90° ≠ β; one 2-fold axis.
• Triclinic: a ≠ b ≠ c; no angle 90°; at most a centre.

Symmetry elements. These are the plane of symmetry, the axis of symmetry (2, 3, 4 and 6-fold) and the centre of symmetry. Together they give 32 crystal classes.

(b) Structural classification of silicates (10 marks)

Silicates are classified by how the SiO₄ tetrahedra are linked, that is, their degree of polymerisation.

• Nesosilicates (isolated tetrahedra, Si:O = 1:4): olivine (Mg,Fe)₂SiO₄, garnet.
• Sorosilicates (paired, Si₂O₇, 2:7): epidote.
• Cyclosilicates (rings, Si₆O₁₈, 1:3): beryl Be₃Al₂Si₆O₁₈, tourmaline.
• Inosilicates: single chains (SiO₃, 1:3), pyroxenes such as augite; double chains (Si₄O₁₁, 4:11), amphiboles such as hornblende.
• Phyllosilicates (sheets, Si₂O₅, 2:5): micas (muscovite), clay minerals, chlorite.
• Tectosilicates (framework, SiO₂, 1:2): quartz, feldspars, feldspathoids; Al replaces some Si in feldspars.

The more shared oxygens, the higher the Si:O ratio and the greater the stability and hardness.

(c) Granitization (10 marks)

Granitization is the process by which solid rocks are transformed into granite without melting, by the introduction and removal of chemical constituents, so it is a form of metasomatism.

Mechanism. Ions of Na, K, Si and Al diffuse through intergranular fluids and along grain boundaries, migrating in fronts ahead of the change. Fe, Mg and Ca are removed as a "basic front", while Si, Na and K arrive as a "granite front". Porphyroblasts of feldspar grow in the old rock.

Evidence. Relict structures such as bedding and foliation continue into granite, and there are gradational contacts, no chilled margins, and the lack of room for a large magma body.

The magmatists, led by N. L. Bowen, argue from experiments that granite crystallises from melt. Most granites are magmatic, but granitisation is accepted for migmatites and some granite gneisses in deep crust.

(d) Granulites: texture and Indian terrains (10 marks)

Identification. Granulites are high-grade rocks of the granulite facies (T above about 700°C). They are medium to coarse, dry rocks with a granoblastic texture: equigranular grains with straight, polygonal boundaries meeting at about 120°. Feldspar and quartz are often elongated in flattened grains (granulitic gneissic banding). Orthopyroxene (hypersthene) with clinopyroxene, plagioclase and garnet is the diagnostic mineral assemblage. Hydrous minerals such as biotite and hornblende are scarce.

Indian terrains.

• Southern Granulite Terrane: Nilgiri, Madras (Pallavaram), Palni, Shevaroy and Biligirirangan hills, and the Kerala Khondalite Belt.
• Eastern Ghats Belt (khondalites, charnockites).
• Central Indian Tectonic Zone (Sausar belt).
• Parts of the Chotanagpur Gneissic Complex.

(e) Petrographic features of sedimentary rocks (10 marks)

Sedimentary rocks are identified and classified in thin section by texture and composition.

• Texture: grain size, sorting, roundness and sphericity, grain contacts and packing (grain-supported or matrix-supported), and fabric.
• Composition: framework grains (quartz, feldspar, lithics), matrix (grains below 30 μm) and cement (calcite, silica, iron oxide).
• Classification: sandstones by Folk or Dott/Pettijohn schemes in a QFL diagram (quartz arenite, arkose, lithic arenite, greywacke); carbonates by Folk (allochems and micrite or sparite) and Dunham (mudstone to grainstone).
• Diagenetic features: compaction, cementation, replacement, dissolution and porosity type (intergranular, moldic, vuggy).

Together these show source, transport, environment and burial.