Geology 2021 Paper II 50 marks 150 words Compulsory Discuss

Q1

Answer the following questions in about 150 words each: (a) How does one designate mirror plane, rotational axes of symmetry and centre of symmetry in Hermann-Mauguin notation? A crystal has one 4-fold axis, four 2-fold axes, five mirror planes and centre of symmetry. What will be its Hermann-Mauguin notation? Show on a stereographic projection diagram the symmetry elements of the above crystal and plot the faces of the form (hkl). (b) Discuss with the help of a suitable diagram the crystallization of a melt having composition Diopside-70 and Anorthite-30 under 1 atmospheric pressure. After complete crystallization, what would be the texture of the rock? (c) Describe 'continuous' and 'discontinuous' reaction series of minerals proposed by Bowen. Why is one series called as 'continuous' and the other as 'discontinuous'? (d) Define 'prograde' and 'retrograde' metamorphism with suitable examples. Discuss the process of metasomatism. (e) Give a brief account of Folk's classification of limestones.

हिंदी में प्रश्न पढ़ें

निम्नलिखित प्रश्नों में से प्रत्येक का लगभग 150 शब्दों में उत्तर दीजिए : (a) हर्मन-मौगुइन नोटेशन में मिरर प्लेन, सममिति के घूर्णी अक्ष और सममिति के केन्द्र को कैसे नामित किया जाता है? एक क्रिस्टल में एक 4-गुणा अक्ष, चार 2-गुणा अक्ष, पाँच मिरर प्लेन और सममिति का केन्द्र होता है। इसका हर्मन-मौगुइन नोटेशन क्या होगा? एक त्रिविम प्रक्षेप (स्टीरियोग्राफिक प्रोजेक्शन) आरेख पर उपर्युक्त क्रिस्टल के सममिति तत्वों को दिखाइए और (hkl) स्वरूप के फलकों को आलेखित कीजिए। (b) समुचित आरेख की सहायता से 1 वायुमण्डलीय दबाव में डायोपसाइड-70 और एनोर्थाइट-30 के गठन वाले गलित (मेल्ट) के क्रिस्टलीभवन पर चर्चा कीजिए। पूर्ण क्रिस्टलीभवन के बाद शैल का गठन क्या होगा? (c) बावेन द्वारा प्रस्तावित खनिजों की 'सतत' और 'असतत' प्रतिक्रिया श्रृंखला का वर्णन कीजिए। एक श्रृंखला को 'सतत' और दूसरे को 'असतत' क्यों कहा जाता है? (d) उपयुक्त उदाहरणों द्वारा 'अग्रगतिक (प्रोग्रेड)' व 'पश्चगतिक (रिट्रोग्रेड)' कायान्तरण को परिभाषित कीजिए। तत्वान्तरण (मेटासोमेटिज्म) की प्रक्रिया पर चर्चा कीजिए। (e) फोक के चुनाश्म के वर्गीकरण का संक्षिप्त वर्णन कीजिए।

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Directive word: Discuss

This question asks you to discuss. 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 multi-part question requires approximately 150 words per sub-part (10 marks each). For (a), begin with Hermann-Mauguin notation rules, derive 4/mmm as the symbol, and sketch stereographic projection with symmetry elements and {hkl} faces. For (b), draw the Diopside-Anorthite binary phase diagram, trace crystallization path, and identify ophitic/subophitic texture. For (c), contrast Bowen's reaction series with mineral examples. For (d), define terms with Indian examples (Tamil Nadu charnockites for prograde; retrograde chloritization) and explain metasomatism. For (e), outline Folk's allochem/matrix classification with limestone types. Allocate ~3 minutes per part with 30-40 seconds for diagrams.

Key points expected

  • (a) Hermann-Mauguin notation: rotation axes as digit (1-6), mirror planes as 'm', centre of symmetry as /m; derived symbol 4/m 2/m 2/m or 4/mmm; stereographic projection showing 4-fold axis vertical, 2-fold axes horizontal/diagonal, mirror planes as great circles, centre as small circle, and {hkl} form as 8 faces in general position
  • (b) Binary phase diagram of Diopside-Anorthite system with eutectic at ~1270°C, 40% An; crystallization path for Di70An30 melt showing initial diopside crystallization at liquidus, composition moving to eutectic, final solid as mixture of diopside + anorthite; texture: ophitic or subophitic with large diopside enclosing anorthite laths
  • (c) Continuous reaction series: plagioclase feldspar (Ca-rich to Na-rich) with gradual compositional change; Discontinuous series: olivine → pyroxene → amphibole → biotite with distinct mineral changes at specific temperatures; named for gradual vs. abrupt phase transitions
  • (d) Prograde metamorphism: increasing P-T conditions (e.g., Tamil Nadu granulites); Retrograde: decreasing P-T with rehydration/reaction (e.g., garnet to chlorite); Metasomatism: chemical alteration by fluid infiltration (e.g., fenitization at Sivamalai, Tamil Nadu)
  • (e) Folk's classification based on allochems (intraclasts, oolites, fossils, peloids) and matrix (micrite vs. sparite); categories include biosparite, micrite, intramicrite, oosparite; significance for porosity/permeability in carbonate reservoirs

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5Precise Hermann-Mauguin symbol derivation (4/mmm), accurate phase diagram interpretation for Di-An system, correct distinction between continuous (plagioclase) and discontinuous (ferromagnesian) series, and accurate application of Folk's allochem terminologyCorrect basic concepts but minor errors in symbol notation, vague description of crystallization sequence, or incomplete distinction between reaction series typesFundamental errors such as wrong Hermann-Mauguin symbol, confused binary system behavior, or mixing up continuous/discontinuous series definitions
Diagram / cross-section25%12.5Clear stereographic projection with all symmetry elements (4-fold axis, 2-fold axes, mirror planes, centre) and {hkl} faces plotted; accurate Di-An binary phase diagram with liquidus/solidus, eutectic point, and tie-lines; Bowen reaction series diagramRoughly correct diagrams missing some elements (e.g., missing 2-fold axes in stereonet, or unlabeled axes in phase diagram) but showing essential featuresMissing diagrams, incorrect projections (e.g., confusing stereographic with orthographic), or diagrams that contradict text description
Field evidence15%7.5Specific Indian examples: Sivamalai fenitization for metasomatism, Tamil Nadu charnockite belt for prograde metamorphism, or mention of Deccan Trap ophitic textures; links texture to observable field relationshipsGeneric examples without Indian specificity, or mention of field terms without clear connection to the theoretical conceptsNo field examples, or incorrect application (e.g., confusing contact metamorphism with regional prograde metamorphism)
Quantitative reasoning20%10Correct calculation/derivation of Hermann-Mauguin symbol from symmetry elements; precise use of Di70An30 composition on phase diagram to predict crystallization sequence and final modal proportions; temperature estimates for reaction seriesQualitative description of quantitative relationships without explicit numbers, or correct but incomplete quantitative treatmentNo quantitative treatment where required, or serious errors (e.g., wrong eutectic composition, incorrect symbol derivation)
Indian / economic relevance15%7.5Mention of economic significance: Folk's classification applied to Kutch or Rajasthan limestone reservoirs; metasomatism related to Indian rare-earth element deposits; or dimension stone quality linked to crystallographic symmetryBrief mention of economic importance without specific Indian context, or generic statement about limestone as cement sourceNo economic or Indian relevance discussed, or irrelevant economic information

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