Q1
Answer the following questions in about 150 words each: (a) How are the symmetry elements in a crystal of class 6/m 2/m 2/m oriented with respect to its crystallographic axes? (10 marks) (b) Why do some minerals show pleochroism? Explain. (10 marks) (c) Describe the compositional changes in the magnesium-rich magma due to progressive removal of olivine. (10 marks) (d) Given a mafic protolith, what would be the characteristic mineral assemblages in (i) greenschist facies, (ii) amphibolite facies and (iii) granulite facies metamorphism? (10 marks) (e) Explain the diagenetic changes in carbonate rocks. (10 marks)
हिंदी में प्रश्न पढ़ें
निम्नलिखित में से प्रत्येक प्रश्न का उत्तर लगभग 150 शब्दों में दीजिए : (a) स्फटिक कक्ष 6/m 2/m 2/m के एक स्फटिक में सममिति तत्त्व (सिमेट्री एलिमेंट) इसके स्फटिकीय अक्षों के संदर्भ में कैसे अनुयुक्त (ओरिएन्टेड) हैं? (10 अंक) (b) कुछ खनिज बहुवर्णता (प्लियोक्रोइज्म) क्यों दिखाते हैं? समझाइए। (10 अंक) (c) मैग्नीशियम-समृद्ध मैग्मा में से ओलिविन के लगातार निकलने से होने वाले संयोजनात्मक बदलाव का वर्णन कीजिए। (10 अंक) (d) मैफिक प्रोटोलिथ से (i) ग्रीन्सचिस्ट फेसीज, (ii) एम्फीबोलाइट फेसीज एवं (iii) ग्रैन्यूलाइट फेसीज में कायांतरण होने पर किस तरह के विशिष्ट खनिज समूह बनेंगे? (10 अंक) (e) कार्बोनेट चट्टानों में प्रसायाती (डायजेनेटिक) परिवर्तनों की व्याख्या कीजिए। (10 अंक)
Directive word: Explain
This question asks you to explain. 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
This multi-part question demands precise, concise explanations across crystallography, optical mineralogy, igneous petrology, metamorphic petrology and sedimentary diagenesis. Allocate approximately 30 words (20% time) per sub-part, ensuring (a) includes a clear stereographic projection sketch, (b) links crystal structure to light absorption, (c) tracks Mg/Fe enrichment trends, (d) compares three facies assemblages systematically, and (e) distinguishes early from late diagenetic processes. No introduction or conclusion is needed; dive directly into technical content for each part.
Key points expected
- (a) Hexagonal system: 6-fold axis along c-axis; mirror plane ⊥ c (6/m); three 2-fold axes ⊥ c at 120°; three mirror planes ∥ c containing a-axes; stereogram showing symmetry element orientation
- (b) Pleochroism arises from anisotropic crystal structures with differential light absorption along different crystallographic directions; requires colored minerals with distinct vibration directions; examples like biotite, tourmaline, hornblende
- (c) Progressive olivine fractionation en residual magma in MgO and FeO, driving magma toward tholeiitic vs. calc-alkaline differentiation trends; Bowen's reaction series application; eventual pyroxene saturation
- (d) Greenschist: chlorite-actinolite-albite-epidote; Amphibolite: hornblende-plagioclase (typically andesine-labradorite) ± garnet; Granulite: orthopyroxene-clinopyroxene-garnet-plagioclase; P-T conditions implied by each assemblage
- (e) Early diagenesis: micritization, cementation (marine, meteoric, burial cements), neomorphism; late diagenesis: dissolution, stylolitization, dolomitization; porosity evolution from primary to secondary
Evaluation rubric
| Dimension | Weight | Max marks | Excellent | Average | Poor |
|---|---|---|---|---|---|
| Concept correctness | 35% | 17.5 | Crystallographic symbols in (a) precisely match hexagonal holohedral class; (b) correctly identifies anisotropy and differential absorption mechanism; (c) accurately describes iron-enrichment trend and tholeiitic differentiation; (d) correctly pairs index minerals with facies; (e) distinguishes cement types and neomorphism from replacement | General descriptions of symmetry and facies without precise mineral lists; vague explanation of pleochroism as 'color change'; conflates early and late diagenetic processes; minor errors in mineral chemistry or stability fields | Fundamental errors such as confusing crystal systems, attributing pleochroism to isotropic minerals, describing olivine accumulation instead of removal, or listing minerals incompatible with stated facies conditions |
| Diagram / cross-section | 15% | 7.5 | Clear stereographic projection or block diagram for (a) showing all symmetry elements with proper angular relationships; labeled diagrams for (d) showing P-T facies series; sketches of cement types in (e) | Rough sketches without proper labeling or scale; diagrams present but missing key elements like mirror plane orientations or mineral stability boundaries | No diagrams where essential; completely wrong projections; diagrams that confuse rather than clarify, such as cubic symmetry for hexagonal class |
| Field evidence | 15% | 7.5 | Cites specific Indian occurrences: Deccan Traps for (c) olivine fractionation sequences; Eastern Ghats granulites for (d); Kutch or Himalayan carbonate sequences for (e) diagenetic studies; references thin-section observations for (b) | Generic references to 'basaltic magma' or 'regional metamorphism' without specific localities; mentions field relationships without Indian examples | No field context provided; entirely theoretical treatment where field examples would strengthen answer; invented or incorrect localities |
| Quantitative reasoning | 20% | 10 | For (a): precise angular relationships (120° between 2-fold axes); for (c): approximate Mg/Fe enrichment ratios or mentions CIPW norm calculations; for (d): approximate P-T ranges for each facies; for (e): porosity percentage changes through diagenesis | Qualitative statements about 'increasing temperature' or 'higher pressure' without numerical ranges; mentions quantitative methods without applying them | No quantitative content where expected; incorrect numerical values; confuses pressure and temperature units or ranges |
| Indian / economic relevance | 15% | 7.5 | Links Deccan Traps differentiation to Ni-Cu-PGE mineralization in (c); notes Eastern Ghats granulite-hosted graphite/charnockite resources in (d); emphasizes carbonate reservoir quality in Bombay High or Kutch petroleum systems in (e); mentions pleochroism in gem-quality Indian tourmaline for (b) | Brief mention of Indian localities without economic significance; generic statements about 'economic importance' without specifics | No Indian context; missed opportunities to connect academic content to GS Paper III economic geography or resource management dimensions |
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