Q2
(a) Give the classification scheme of silicate minerals on the basis of atomic structure, Si : O ratio and number of shared oxygen. Give suitable examples of each class. (20 marks) (b) Define 'birefringence' and 'extinction angle' in minerals. How does one proceed to measure extinction angle of a mineral under microscope? Substantiate your answer with suitable sketches. (20 marks) (c) Define 'twin plane', 'twin axis' and 'composition plane' in crystals. State different types of twinning observed in feldspars. (10 marks)
हिंदी में प्रश्न पढ़ें
(a) परमाणु संरचना, Si : O अनुपात और साझा ऑक्सीजन की संख्या के आधार पर सिलिकेट खनिजों की वर्गीकरण योजना दीजिए। प्रत्येक वर्ग का उपयुक्त उदाहरण दीजिए। (20 अंक) (b) खनिजों में 'द्विवर्तन' और 'विलुप्ति कोण' को परिभाषित कीजिए। सूक्ष्मदर्शी द्वारा किसी खनिज के विलुप्ति कोण को नापने की प्रक्रिया बताइए। उपयुक्त रेखाचित्रों से अपने उत्तर की पुष्टि कीजिए। (20 अंक) (c) क्रिस्टल में 'युग्म तल', 'युग्म अक्ष' और 'संयोजन तल' को परिभाषित कीजिए। फेल्डस्पार में पाये जाने वाले विभिन्न प्रकार के यमलन (ट्विनिंग) का वर्णन कीजिए। (10 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.
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How this answer will be evaluated
Approach
This descriptive question requires systematic exposition across three parts: spend approximately 40% of effort on part (a) given its 20 marks, covering all six silicate classes with structural formulas and examples; allocate 35% to part (b) for precise optical definitions, measurement procedure, and labeled microscope sketches; and 25% to part (c) for crystallographic twin definitions and feldspar twinning types. Begin with clear definitions, proceed with structured classification/tables, and conclude with Indian mineral occurrences where relevant.
Key points expected
- Part (a): Six silicate classes with correct 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) and shared oxygen counts (0, 1, 2, 2-3, 3, 4 respectively)
- Part (a): Representative Indian examples for each class (e.g., zircon/olivine, epidote, beryl, pyroxene/amphibole, mica, quartz/feldspar)
- Part (b): Precise definition of birefringence as difference between highest and lowest refractive indices (δ = nγ - nα or nε - nω)
- Part (b): Definition of extinction angle as angle between vibration direction of slower ray and crystal edge/trace of cleavage
- Part (b): Step-by-step measurement procedure: oriented grain selection, stage rotation to extinction positions, angle recording from both sides, calculation of acute angle
- Part (c): Clear distinction between twin plane (mirror plane), twin axis (rotation axis), and composition plane (common boundary)
- Part (c): Feldspar twin types: Carlsbad, Baveno, Manebach, albite, pericline with their crystallographic elements
Evaluation rubric
| Dimension | Weight | Max marks | Excellent | Average | Poor |
|---|---|---|---|---|---|
| Concept correctness | 25% | 12.5 | Demonstrates flawless command of silicate structural chemistry, correct Si:O ratios for all classes, precise optical crystallography definitions, and accurate twinning nomenclature; for (a) correctly identifies shared oxygen vertices; for (b) distinguishes between parallel and inclined extinction; for (c) correctly associates twin laws with specific feldspar types | Minor errors in Si:O ratios (1-2 classes confused), vague birefringence definition without quantitative expression, incomplete twin type listing with some crystallographic confusion | Fundamental errors in silicate classification (e.g., confusing chain and sheet structures), incorrect extinction angle measurement description, conflation of twin plane and composition plane |
| Diagram / cross-section | 25% | 12.5 | For (a): clear 2D/3D sketches of all six silicate structures showing Si-O tetrahedral linkage; for (b): labeled microscope field sketches showing extinction position determination with crosshairs, cleavage traces, and angle measurement; for (c): stereographic projections or block diagrams illustrating twin operations | Basic silicate structure diagrams missing vertex-sharing details, simple extinction angle sketch without measurement annotations, twin diagrams lacking crystallographic orientation | Absent or irrelevant diagrams, confused sketches showing isotropic behavior for anisotropic minerals, no visual representation of twin geometry |
| Field evidence | 15% | 7.5 | Cites specific Indian occurrences: for (a) Kodurite belt nesosilicates, Bihar mica belts, Rajasthan feldspars; for (b) mentions thin section studies from Dharwar or Singhbhum craton rocks; for (c) references twinning in plagioclase from Deccan traps or Himalayan leucogranites | Generic mention of Indian localities without specific belt names, standard textbook examples without regional context | No Indian examples, purely theoretical treatment, incorrect attribution of mineral occurrences |
| Quantitative reasoning | 20% | 10 | Precise numerical ratios for all silicate classes; for (b) includes birefringence calculation formula, mentions interference color charts (Michel-Lévy), and specifies extinction angle ranges for common minerals (e.g., orthopyroxene ~90°, clinopyroxene 35-48°); for (c) gives angular relationships in twin laws | Correct Si:O ratios without structural explanation, mentions interference colors qualitatively, gives approximate extinction angles without mineral specificity | Incorrect ratios, no quantitative optical data, purely descriptive treatment without numerical backing |
| Indian / economic relevance | 15% | 7.5 | Links silicate classes to India's mineral economy: forsterite in refractories, beryl for beryllium in Rajasthan, mica from Andhra/Jharkhand for electrical industry, feldspar from Rajasthan/Tamil Nadu for ceramics; mentions GSI/IBM exploration significance; optical mineralogy applications in Indian ore microscopy | Mentions economic importance without specific Indian production data, generic industrial uses without regional mining context | No economic significance discussed, misses India's dominance in mica and feldspar production, no connection to mineral-based industries |
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