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Geology 2025 Paper II 50 marks Describe

Paper II — Q2

(a) Give a detailed account related to the classification of crystals into different crystallographic systems based on symmetry…

(a)

Give a detailed account related to the classification of crystals into different crystallographic systems based on symmetry elements. 15 marks

(b)

Describe the structure of the pyroxene group of minerals with suitable diagrams. Discuss the chemical compositions and optical properties of orthopyroxene. 15 marks

(c)

Describe the structural classification of silicates with neat sketches. Give one example for each type. 20 marks

हिंदी में प्रश्न पढ़ें
(a)

सममिति तत्त्वों के आधार पर विभिन्न क्रिस्टलीय प्रणालियों में क्रिस्टलों के वर्गीकरण का विस्तृत वर्णन कीजिए। (15 अंक)

(b)

पाइरोक्सीन समुदाय के खनिजों की संरचना का उपयुक्त चित्र सहित वर्णन कीजिए। ऑर्थोपाइरोक्सीन के रासायनिक संघटन और प्रकाशीय गुणों की विवेचना कीजिए। (15 अंक)

(c)

सिलिकेट के संरचनात्मक वर्गीकरण का सुस्पष्ट चित्र सहित वर्णन कीजिए। प्रत्येक प्रकार का एक उदाहरण दीजिए। (20 अंक)

Q2 of the 2025 UPSC Mains Geology Paper II, as printed
The question as printed in the 2025 Geology paper
Evaluate your answer to this question → See all 2025 Geology questions

Model answer

Drafted with AI by UPSC Answer Check against this question's marking rubric, to the expected length. UPSC does not publish answers for Mains — this is one way to score well, not an official key. How we build rubrics and answers.

(a) Crystal systems

Crystals are grouped by their essential (minimum) symmetry into seven crystallographic systems, which contain the 32 crystal classes (point groups) and 14 Bravais lattices. The seven systems are ordered from highest to lowest symmetry and are defined by their characteristic rotation axes, mirror planes and inversion centres; their Hermann-Mauguin symbols encode these elements, with m denoting a mirror plane and -1 an inversion centre. Cubic (holosymmetric class 4/m -3 2/m) has four 3-fold axes and a=b=c, α=β=γ=90°. Tetragonal (4/mmm) has one 4-fold axis, a=b≠c, all angles 90°. Hexagonal (6/mmm) has one 6-fold axis, a=b≠c, α=β=90°, γ=120°. Trigonal (holosymmetric class -3 2/m) has one 3-fold axis; in the rhombohedral setting a=b=c, α=β=γ≠90°, and it can also be referred to hexagonal axes (a=b≠c, γ=120°). Orthorhombic (mmm) has three mutually perpendicular 2-fold axes, a≠b≠c, all angles 90°. Monoclinic (2/m) has one 2-fold axis or mirror plane, a≠b≠c, α=γ=90°, β≠90°. Triclinic (1 or -1) has at most an inversion centre, a≠b≠c, α≠β≠γ≠90°. The axial relationships can be visualised as a table, with cubic at the top and triclinic at the bottom; each step reduces symmetry and increases metric freedom. Holohedral forms possess the full symmetry of the system and show all crystal faces allowed by that symmetry; hemihedral forms lack some symmetry elements, often mirrors or inversion, and may occur as enantiomorphic pairs.

(b) Pyroxene structure and orthopyroxene

Pyroxenes (general formula M2M1T2O6, e.g., XYZ2O6) are inosilicates built from single chains of corner-sharing SiO4 tetrahedra, (SiO3)2−. A side-view diagram shows each tetrahedron sharing two basal oxygens with neighbours, producing a repeating chain parallel to the c-axis; Mg, Fe, Ca and Mn occupy octahedral sites between chains.

Diagram: Pyroxene single-chain structure and I-beams

How this answer will be evaluated

What earns full marks: Complete classification with all symmetry elements, accurate pyroxene structure with optical data, all 6 silicate types with sketches and examples.

Evaluation rubric

Each sub-part is marked on its own, against the marks and word limit printed on the paper.

  1. (a) Classification of crystals into 7 systems based on symmetry elements. 15 marks

    account for— state the phenomenon → the causes in order of weight → conclusion

    Must cover

    • Seven crystallographic systems listed with their symmetry elements (cubic 4×3-fold; tetragonal 1×4-fold; hexagonal 1×6-fold; trigonal 1×3-fold; orthorhombic 3×2-fold; monoclinic 1×2-fold; triclinic ≤inversion centre)
    • Axial parameters and angle relationships for each system; ordered from highest to lowest symmetry
    • Hermann-Mauguin notation and holosymmetric classes (e.g., 4/m -3 2/m cubic, -3 2/m trigonal); 32 classes, 14 Bravais lattices
    • Holohedral vs hemihedral (enantiomorphic pairs) distinction

    Loses marks

    • Confusing systems with lattices
    • Missing symmetry element definitions

    Earns more

    • Mention 32 crystal classes
    • Reference Indian mineral examples

    Extra mark

    • Mention 14 Bravais lattices
    • Reference specific Indian crystallographic studies
  2. (b) Pyroxene structure, orthopyroxene composition and optical properties. 15 marks

    describe— define → structure or process in order → labelled diagram → significance

    Must cover

    • Pyroxenes as inosilicates with (SiO3)n single chains; corner-sharing tetrahedra parallel to c-axis; M1/M2 octahedral sites
    • Cleavage ≈87°/93° vs amphibole double chains (Si4O11)6− with 56°/124°; structure diagram provided
    • Orthopyroxene: enstatite–ferrosilite (MgSiO3–FeSiO3) solid solution with minor Ca, Mn, Al
    • Optical properties: pleochroism (hypersthene pink–green); birefringence values; optic sign/2V vary with composition (enstatite +, hypersthene −); straight extinction; Indian charnockite occurrences

    Loses marks

    • Confusing pyroxene with amphibole
    • Missing optical property details

    Earns more

    • Mention enstatite/ferrosilite series
    • Reference Indian occurrences

    Extra mark

    • Mention specific Indian pyroxene deposits
    • Reference stratigraphic age of occurrences
  3. (c) Structural classification of silicates with sketches and examples. 20 marks

    describe— define → structure or process in order → labelled diagram → significance

    Must cover

    • Structural classification by tetrahedral polymerisation with Si:O ratios (1:4; 2:7; 1:3; 1:3; 4:11; 2:5; 1:2)
    • Named examples for each class: olivine/zircon (neso), hemimorphite/melilite/epidote (soro), beryl/tourmaline (cyclo), pyroxene (single chain), hornblende (double chain), mica (phyllo), quartz/feldspar (tecto)
    • Neat sketches for each structural type provided (figure)
    • Link of structure to cleavage, density and petrogenetic behaviour (Indian examples cited)

    Loses marks

    • Missing sketches or diagrams
    • Confusing structural types

    Earns more

    • Mention specific mineral examples
    • Reference Indian occurrences

    Extra mark

    • Mention specific Indian silicate deposits
    • Reference economic importance
What "Describe" is asking you to do

Give a full, ordered account of the thing named — its parts, stages or mechanism — in the sequence in which it actually exists or occurs. Most describe questions come from the science optionals, where the marks sit in correct technical detail and, where the stem says so, a labelled diagram.

Structure that answers it

One-line identification of the subject → the parts or stages in their real order, each with its defining detail → labelled diagram where the subject is structural → closing line on function or significance

Where marks are lost

Loose general prose where the examiner is ticking named parts, correct terminology and their sequence; and in the General Studies papers, turning to evaluation before the description is finished.

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