Geology 2024 Paper II 50 marks Describe

Q2

(a) What are the symmetry elements present in the normal class of an isometric system ? Write the Hermann-Mauguin notation of the normal class of isometric system. Plot the face (hkl) and deduce the form generated by operation of symmetry elements from the face (hkl) on a stereogram of the normal class of isometric system. (15 marks) (b) Draw and describe the structure of mica group of minerals. Describe the chemical composition and optical properties of minerals of mica group. (15 marks) (c) Define polymorphism and discuss different types of polymorphic transitions. What are the different types of polymorphs of SiO₂ and Al₂SiO₅ ? (20 marks)

हिंदी में प्रश्न पढ़ें

(a) घनीय समुदाय के सामान्य वर्ग में विद्यमान सममिति तत्व क्या होते हैं ? घनीय समुदाय के सामान्य वर्ग के हरमन-मौगुइन संकेतन को लिखिए । घनीय समुदाय के सामान्य वर्ग के फलक (hkl) को त्रिविम चित्र में दर्शाइए तथा सममिति तत्वों से बनने वाली आकृति की व्युत्पत्ति कीजिए । (15 अंक) (b) अभ्रक समुदाय के खनिजों की संरचना का सचित्र वर्णन कीजिए । अभ्रक समुदाय के खनिजों का रासायनिक संयटन और प्रकाशिक गुणधर्मों का वर्णन कीजिए । (15 अंक) (c) बहुरूपता को परिभाषित कीजिए और विभिन्न प्रकार के बहुरूपीय संक्रमणों की चर्चा कीजिए । SiO₂ और Al₂SiO₅ के विभिन्न प्रकार के बहुरूप क्या हैं ? (20 अंक)

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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

This question demands descriptive-cum-analytical treatment across three distinct domains: crystallographic symmetry, mineral structure, and phase transitions. Allocate approximately 35% time/words to part (a) given its stereographic projection complexity, 30% to part (b) for structural diagrams, and 35% to part (c) as the highest-mark section requiring systematic polymorphism analysis. Structure as: concise definitions → systematic elaboration with diagrams → integrated conclusion linking crystal chemistry to natural occurrences.

Key points expected

  • Part (a): Identify all 13 symmetry elements of isometric normal class (3A⁴, 4A³, 6A², 9P, C); state Hermann-Mauguin notation 4/m 3̄ 2/m; construct stereogram showing {hkl} form development into hexoctahedron or related form with proper great circle traces
  • Part (b): Depict T-O-T layer structure of mica with interlayer K⁺; show tetrahedral-octahedral sheet linkage; distinguish dioctahedral (muscovite, Al-rich) vs trioctahedral (biotite, phlogopite, Mg-Fe-rich) chemistry; specify optical properties (biaxial negative, 2V, pleochroism, perfect {001} cleavage)
  • Part (c): Define polymorphism as same composition, different structure; classify transitions as reconstructive (quartz-tridymite-cristobalite, high Ea) vs displacive (α-β quartz, low Ea) vs order-disorder; list SiO₂ polymorphs (quartz, tridymite, cristobalite, coesite, stishovite) and Al₂SiO₅ polymorphs (kyanite, andalusite, sillimanite) with P-T stability fields
  • Part (c): Explain Al₂SiO₅ triple point significance for metamorphic facies series (Barrovian vs Buchan) and index mineral usage in Indian Precambrian terrains
  • Integrated application: Cite Indian occurrences—mica from Koderma (Jharkhand), Nellore (Andhra); sillimanite from Sonapahar (Meghalaya), Pipra (Madhya Pradesh); stishovite as impact indicator

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5Precisely enumerates all 13 symmetry elements of 4/m 3̄ 2/m; correctly distinguishes reconstructive vs displacive transitions with activation energy reasoning; accurately describes T-O-T layer topology and mica polytypism; no confusion between crystal system and crystal classLists major symmetry elements but misses some (e.g., diagonal A² or S₆); basic polymorphism definition correct but transition types poorly differentiated; mica structure described without clear sheet terminology; minor errors in Hermann-Mauguin notationConfuses isometric with other systems; fundamental errors in symmetry element identification; describes polymorphism as 'same mineral different colour'; no understanding of layer silicate structure; incorrect notation
Diagram / cross-section25%12.5Clean stereogram with primitive, great circles for (hkl) operation showing 48-fold form development; accurate mica structure diagram with labelled tetrahedral, octahedral sheets and interlayer cations; P-T diagram for Al₂SiO₅ triple point with stability fields; all diagrams properly titled and referenced in textAttempted stereogram but incorrect projection or missing symmetry operation traces; mica diagram shows layers but lacks detail on cation coordination; basic Al₂SiO₅ diagram without triple point precision; diagrams present but not integratedNo stereogram or completely wrong projection; mica diagram confused with chlorite/talc; no phase diagram; diagrams unlabelled or irrelevant to question
Field evidence15%7.5Cites specific Indian localities: Koderma-Nellore mica belt with production statistics; Sonapahar sillimanite deposits; stishovite from Lonar crater or Ramgarh structure; links Al₂SiO₅ polymorphs to Barrovian metamorphism in Eastern Ghats or Aravalli-Delhi beltMentions India generally without specific localities; basic awareness of mica mining states; knows kyanite-sillimanite are index minerals but no Indian examples; vague reference to metamorphic beltsNo Indian examples; completely generic treatment; confuses field occurrence with laboratory synthesis
Quantitative reasoning20%10States correct number of faces in {hkl} form (48); gives approximate triple point conditions for Al₂SiO₅ (~500°C, 0.4 GPa); cites lattice parameters for mica polytypes; mentions coordination number changes in SiO₂ polymorphs (4, 6 for stishovite); uses Miller indices correctly throughoutGeneral awareness that forms have multiple faces but no specific numbers; knows P-T increases affect polymorph stability but no quantitative values; basic Miller index notation correctNo numerical data; incorrect face counts; confuses pressure and temperature axes; no understanding of coordination geometry changes
Indian / economic relevance15%7.5Discusses strategic importance of Indian mica (Koderma once world's largest producer, now illegal mining issues); sillimanite for refractory industry in Bhilai/Rourkela steel plants; significance of Al₂SiO₅ as geobarometer-geothermometer for Indian Precambrian crustal evolution; stishovite as impact metamorphism proxyMentions mica as economically important without specifics; aware of refractory minerals but no industrial linkage; basic statement that polymorphs indicate P-T conditionsNo economic or strategic context; misses relevance entirely; irrelevant discussion of gemstone value

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