Geology 2025 Paper II 50 marks 150 words Compulsory Describe

Q1

Answer the following questions in about 150 words each: (a) Describe the symmetry elements shown by the normal class of the orthorhombic system. What are the different forms for this class? Draw the sketch stereogram of the form {h k l}. Give examples of two minerals that crystallize in this system. (10 marks) (b) Discuss in brief the characteristic optical properties of hornblende and augite under petrological microscope. (10 marks) (c) Draw a neat labelled diagram for perthite texture. Explain the formation of perthite with the help of a suitable phase diagram. (10 marks) (d) With the help of diagrams, describe the various diagenetic textures of carbonate rocks. (10 marks) (e) Describe ultrahigh-pressure and ultrahigh-temperature metamorphism. (10 marks)

हिंदी में प्रश्न पढ़ें

निम्नलिखित में से प्रत्येक प्रश्न का उत्तर लगभग 150 शब्दों में दीजिए : (a) विषमलम्बाक्ष समुदाय के सामान्य वर्ग में विद्यमान सममिति तत्त्वों का वर्णन कीजिए। इस वर्ग में विभिन्न रूप क्या हैं? एक विविम प्रक्षेप (स्टीरियोग्राफिक प्रोजेक्शन) आरेख पर {h k l} स्वरूप को आलेखित कीजिए। दो खनिजों के उदाहरण दीजिए जिनका क्रिस्टलीकरण इस समुदाय में होता है। (10 अंक) (b) हॉर्नब्लेंड और अगाइट के शैलविज्ञान सूक्ष्मदर्शी प्रकाशीय लक्षणात्मक गुणों की संक्षेप में विवेचना कीजिए। (10 अंक) (c) परथिटिक गठन का स्वच्छ नामांकित चित्र अंकित कीजिए। समुचित फेज आरेख की सहायता से परथाइट बनने की प्रक्रिया समझाइए। (10 अंक) (d) कार्बोनेट शैलों के विभिन्न प्रसंयाती गठनों का सचित्र वर्णन कीजिए। (10 अंक) (e) अतिउच्च-दाबीय एवं अतिउच्च-तापीय कायांतरण का वर्णन कीजिए। (10 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

The directive 'describe' demands systematic, detailed exposition of each sub-part with appropriate visual support. Allocate approximately 30 words each to parts (a), (c), and (d) which require diagrams; 25 words each to (b) and (e). Structure: begin each sub-part with a precise definition, follow with characteristic features, and conclude with specific examples or geological significance.

Key points expected

  • (a) Orthorhombic normal class (mmm): three mutually perpendicular 2-fold axes, three mirror planes, center of symmetry; forms include pinacoid, prism, dome, pyramid; stereogram shows {hkl} in general position; examples: olivine, andalusite, barite, topaz
  • (b) Hornblende: inclined extinction (10-25°), moderate relief, pleochroism from green to brown; augite: 45° cleavage angle, high relief, no pleochroism, symmetric extinction; both show two cleavages
  • (c) Perthite: intergrowth of alkali feldspar (host) with plagioclase (exsolved lamellae); antiperthite reverse; phase diagram from Ab-Or system showing solvus and exsolution below ~700°C; mesoperthite intermediate
  • (d) Diagenetic carbonate textures: micrite envelope, micritization, hardgrounds, intraclasts, peloids, ooids with diagenetic overgrowths; syntaxial and epitaxial cementation; pressure solution features like stylolites
  • (e) UHP: coesite and diamond stability, P>2.5-4 GPa, T=600-800°C, blueschist to eclogite facies; UHT: osumilite, sapphirine, spinel+quartz assemblages, T>900°C, P=0.5-1.2 GPa, granulite facies; both indicate extreme tectonic processes

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5Precisely identifies orthorhombic symmetry elements (3L²3PC), correctly distinguishes hornblende's oblique from augite's symmetric extinction, accurately explains perthite exsolution mechanism via solvus, and correctly defines UHP/UHT P-T thresholds with diagnostic mineralsIdentifies basic symmetry elements but confuses crystal class notation; mixes some optical properties of hornblende and augite; describes perthite texture but misrepresents phase diagram; gives approximate P-T ranges without specific mineral indicatorsConfuses orthorhombic with other systems; fails to distinguish key optical properties; describes perthite as simple intergrowth without exsolution; conflates UHP and UHT conditions or gives incorrect P-T values
Diagram / cross-section25%12.5Neat stereogram for {hkl} in orthorhombic system with proper pole projection; clear perthite texture diagram showing host-exsolved relationship; accurate Ab-Or phase diagram with solvus curve; well-labelled diagenetic texture sketchesDraws basic stereogram but misses some symmetry elements; perthite diagram shows texture but lacks labels; phase diagram present but missing critical temperatures; diagenetic textures described with minimal sketchingNo diagrams or unrecognizable sketches; confuses stereogram with crystal drawing; perthite diagram shows wrong mineral relationship; omits required phase diagram entirely
Field evidence15%7.5Cites specific Indian/locality examples: olivine from Deccan Traps or Salem; UHP rocks from Tso Morari or Kaghan Valley (Pakistan Himalaya analog); UHT granulites from Eastern Ghats or Southern Granulite Terrain; mentions specific thin section observationsMentions general field occurrences without specific Indian localities; gives textbook examples (e.g., olivine from Hawaii) without Indian context; describes field relationships in generic termsNo field examples cited; purely theoretical treatment; incorrect or fabricated locality information
Quantitative reasoning20%10Provides exact angles: hornblende extinction 10-25°, augite 45° cleavage; precise P-T values for UHP (>2.5 GPa, ~700°C) and UHT (>900°C); correct Miller indices notation {hkl}; quantitative phase diagram interpretationApproximate angles and ranges given; correct order of magnitude for P-T conditions; understands {hkl} notation but may confuse with (hkl); qualitative description of phase relationshipsNo quantitative data; incorrect angles or P-T values; confuses different notations; purely descriptive without numerical support
Indian / economic relevance15%7.5Links orthorhombic minerals to Indian resources: barite from Mangampeta (Andhra Pradesh), andalusite from Lapsa Buru (Jharkhand); notes UHT Eastern Ghats granulites for crustal evolution studies; mentions economic importance of carbonate diagenesis in oil/gas reservoirs of Mumbai High or Cambay BasinMentions Indian localities without economic context; notes general importance of metamorphic studies for mineral exploration; describes carbonate reservoirs without specific Indian examplesNo Indian examples; no economic significance discussed; purely academic treatment ignoring applied aspects

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