Q2
(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)
Directive word: Describe
This question asks you to describe. The directive word signals the depth of analysis expected, the structure of your answer, and the weight of evidence you must bring.
See our UPSC directive words guide for a full breakdown of how to respond to each command word.
How this answer will be evaluated
Approach
The directive 'describe' demands comprehensive, systematic exposition with visual support. Allocate approximately 30% time/words to part (a) on crystallographic systems, 30% to part (b) on pyroxene structure and orthopyroxene properties, and 40% to part (c) on silicate classification given its higher mark weightage. Structure with a brief integrative introduction on crystal chemistry fundamentals, then address each sub-part sequentially with labeled diagrams, and conclude with the significance of silicate structural diversity in crustal petrogenesis.
Key points expected
- Part (a): All seven crystallographic systems defined by their characteristic symmetry elements (rotation axes, mirror planes, inversion centers) with correct Hermann-Mauguin symbols; distinction between holohedral and hemihedral forms
- Part (a): Tabular or diagrammatic presentation of axial relationships (a, b, c) and interaxial angles (α, β, γ) for each system from cubic to triclinic
- Part (b): Single chain silicate structure of pyroxenes showing SiO₃ tetrahedral chains; cleavage at ~87° and ~93° reflecting chain periodicity; comparison with amphibole double chains
- Part (b): Orthopyroxene chemistry in the MgSiO₃ (enstatite)-FeSiO₃ (ferrosilite) solid solution series; orthorhombic system, 2V angle, pleochroism, and birefringence characteristics
- Part (c): Six structural types of silicates with their Si:O ratios: nesosilicates (1:4), sorosilicates (2:7), cyclosilicates (1:3), inosilicates (single chain 1:3, double chain 4:11), phyllosilicates (2:5), tectosilicates (1:2)
- Part (c): Specific Indian examples for each type: zircon/olivine (nesosilicate), epidote (sorosilicate), beryl (cyclosilicate), diopside/enstatite (inosilicate), muscovite/biotite (phyllosilicate), quartz/feldspar (tectosilicate)
Evaluation rubric
| Dimension | Weight | Max marks | Excellent | Average | Poor |
|---|---|---|---|---|---|
| Concept correctness | 25% | 12.5 | Precise definitions of all 7 crystal systems with correct symmetry elements; accurate description of pyroxene chain structure and cleavage angles; flawless Si:O ratios and coordination numbers for all six silicate classes with correct examples | Correct identification of most crystal systems but confused symmetry elements; basic pyroxene chain description without cleavage explanation; mostly correct silicate types but errors in Si:O ratios or mismatched examples | Fundamental errors in crystal system definitions; confused pyroxene with amphibole structure; major errors in silicate classification or missing structural types entirely |
| Diagram / cross-section | 25% | 12.5 | Neat, labeled diagrams for: (a) at least 3 crystal system unit cells showing symmetry elements; (b) pyroxene single chain structure with cleavage angles marked and optical orientation diagram; (c) all six silicate structural types with correct tetrahedral linkages and dimensional sketches | Basic diagrams for most parts but missing labels or symmetry notations; pyroxene chain shown but cleavage angles omitted; 4-5 silicate types sketched with some structural inaccuracies | Few or no diagrams; poorly drawn structures without labels; diagrams contradict textual description; missing critical diagrams for highest-mark section (c) |
| Field evidence | 15% | 7.5 | Specific Indian occurrences: orthopyroxene from charnockites of Eastern Ghats (Tamil Nadu/Odisha) or Karnataka; pyroxene from Deccan trap basalts; sillimanite-kyanite index minerals from Kerala khondalites linked to metamorphic grade; field criteria for distinguishing orthopyroxene from clinopyroxene in hand specimen | Generic mention of Indian localities without specificity; basic field distinction between ortho- and clinopyroxene; general reference to metamorphic belts without mineral-paragenesis links | No Indian field examples; confused field occurrences; irrelevant international localities cited without Indian context |
| Quantitative reasoning | 20% | 10 | Precise Si:O ratios calculated for all silicate types; correct 2V angles for orthopyroxene (~50-90°); refractive indices (nα, nβ, nγ) with birefringence; cleavage angles 87°/93° specified; axial ratios for crystal systems where relevant | Most Si:O ratios correct but some calculation errors; approximate 2V mentioned without values; cleavage angles rounded or approximate; missing refractive index data | No quantitative data; incorrect Si:O ratios; confused optical properties between ortho- and clinopyroxene; missing cleavage angle specifications |
| Indian / economic relevance | 15% | 7.5 | Economic significance: pyroxene-rich rocks in Dharwar craton for iron ore exploration (magnetite-orthopyroxene associations); sillimanite from Meghalaya for refractory industry; beryl from Rajasthan for beryllium; feldspar-quartz from pegmatites in Andhra Pradesh for ceramics; linkage of silicate structures to industrial applications based on cleavage and stability | Some Indian economic examples cited but superficially; generic mention of refractory or ceramic uses without specific deposits; missing structural-property-application linkages | No Indian economic relevance; irrelevant international examples; no connection between crystal chemistry and practical applications |
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