Geology 2022 Paper II 50 marks 150 words Compulsory Calculate

Q1

Answer the following in about 150 words each: (a) How are Miller Indices of a crystal face calculated ? Calculate Miller Indices of following two crystal faces : (i) A face intersects all three crystallographic axes at 3-unit distance. (ii) A face intersects a-axis at 4-unit distance and is parallel to b and c axes. (10 marks) (b) Explain the phenomena of solid solution and exsolution in minerals. (10 marks) (c) Describe with suitable sketches 'intergranular' and 'sub-ophitic' textures. How do you explain presence of both these textures in a mafic rock ? (10 marks) (d) How do increasing pressure and temperature either singularly or jointly, metamorphose a rock ? (10 marks) (e) Describe the classification of sandstones on the basis of their composition and matrix. (10 marks)

हिंदी में प्रश्न पढ़ें

निम्नलिखित में से प्रत्येक का उत्तर लगभग 150 शब्दों में दीजिए : (a) क्रिस्टल फलक के मिलर सूचकांक की गणना किस प्रकार की जाती है ? निम्न दो क्रिस्टल फलकों के मिलर सूचकांक की गणना कीजिए : (i) एक फलक जो तीनों क्रिस्टलोग्राफिक अक्षों को 3 इकाई दूरी पर काटता है । (ii) एक फलक जो a-अक्ष को 4 इकाई दूरी पर काटे तथा b और c अक्षों के समानांतर हो । (10 अंक) (b) खनिजों के ठोस विलयन तथा अपविलयन की क्रिया को समझाएं । (10 अंक) (c) उचित चित्रों की सहायता से 'अंतरकणिक' एवं 'अधःओफाइटी' गठनों का विवरण दीजिए । इन दोनों गठनों की मैफिक शैलों में उपस्थिति को किस प्रकार समझाएंगे । (10 अंक) (d) बढ़ते दाब एवं तापमान एकल या संयुक्त रूप से शैलों को किस प्रकार कायांतरित करते हैं ? (10 अंक) (e) बलुआ पत्थर के वर्गीकरण को उसके संघटन तथा आधात्री के आधार पर वर्णन कीजिए । (10 अंक)

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Directive word: Calculate

This question asks you to calculate. 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 demands precise, concise responses (~150 words each) across five sub-topics. For (a), state the Miller Index calculation procedure (reciprocals → clear fractions → integers), then solve both faces showing working: (i) yields (111), (ii) yields (100). For (b), define solid solution (ionic substitution, e.g., plagioclase series) and exsolution (unmixing, e.g., perthite), contrasting their P-T conditions. For (c), sketch intergranular (plagioclase laths with pyroxene in interstices) and sub-ophitic (smaller pyroxene partly enclosing plagioclase) textures, explaining their coexistence through differential cooling rates in mafic dolerites. For (d), explain metamorphic facies concept: contact (T-dominant), burial (P-dominant), and regional (P-T combined) metamorphism with mineralogical changes. For (e), classify sandstones using Pettijohn's compositional categories (quartz, feldspathic, lithic) and matrix content (arenite vs. wacke). Allocate ~20% time per part, prioritizing accuracy over elaboration.

Key points expected

  • (a) Miller Indices: correct procedure stated (reciprocals of intercepts, clear fractions, reduce to smallest integers); calculation (i) intercepts 3,3,3 → 1/3,1/3,1/3 → (111); calculation (ii) intercepts 4,∞,∞ → 1/4,0,0 → (100)
  • (b) Solid solution: definition, substitutional types (complete vs. limited), example plagioclase albite-anorthite series; Exsolution: definition, unmixing on cooling, example perthite (orthoclase with albite lamellae), contrast temperature requirements
  • (c) Intergranular texture: sketch showing plagioclase laths with pyroxene/olivine in angular interstices; Sub-ophitic texture: sketch showing pyroxene crystals partially enclosing smaller plagioclase laths (contrast with ophitic); Explanation: both textures in same mafic rock due to variable cooling rates—intergranular in rapidly cooled margins, sub-ophitic in slower-cooled interiors of dolerite sills/dykes
  • (d) Contact metamorphism: T increase, low P, hornfels facies, aureoles around intrusions; Burial metamorphism: P increase, low T, zeolite facies; Regional metamorphism: combined P-T increase, Barrovian/isograd sequences, index minerals; P-T-t paths and metamorphic facies series
  • (e) Compositional classification: quartz arenite (>90% quartz), feldspathic arenite (>25% feldspar), lithic arenite (>25% rock fragments); Matrix-based: arenite (<15% matrix) vs. wacke (>15% matrix); mention Dott's classification or Pettijohn's scheme; Indian examples: Vindhyan quartz arenites, Siwalik feldspathic sandstones

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5All five sub-parts demonstrate crystallographically and petrologically precise definitions: Miller Index calculation steps explicitly stated with correct final indices; solid solution vs. exsolution mechanisms clearly distinguished with correct mineral examples; intergranular vs. sub-ophitic textures accurately differentiated with correct genetic explanation; metamorphic processes correctly linked to specific P-T regimes and facies; sandstone classification correctly applies both compositional and matrix criteriaMost concepts broadly correct but with minor errors: Miller Indices calculated correctly but procedure vaguely stated; solid solution and exsolution defined but examples generic or confused; textures described but genetic explanation weak; metamorphism explanation conflates P-T effects; sandstone classification partially correct with mixing of criteriaFundamental conceptual errors: incorrect Miller Index calculations; confusion between solid solution and exsolution; failure to distinguish intergranular from ophitic/sub-ophitic; metamorphic processes described as merely 'heat and pressure' without facies linkage; incorrect or missing sandstone classification
Diagram / cross-section20%10Clear, labeled sketches for part (c): intergranular texture showing plagioclase laths with pyroxene in interstices, and sub-ophitic showing pyroxene partially enclosing plagioclase; crystal shapes and spatial relationships accurately depicted; may include supplementary P-T diagram for (d) showing metamorphic facies fieldsSketches attempted for textures but poorly labeled or confusing intergranular with intersertal; crystal relationships not clearly shown; diagrams present but lack essential labels (minerals, texture names)No diagrams despite explicit requirement; or diagrams completely unrecognizable with wrong minerals and relationships; text description substitutes for required sketches
Field evidence15%7.5Specific field contexts cited: Deccan Trap dolerite sills for intergranular/sub-ophitic texture zonation; Aravalli or Dharwar sequences for metamorphic grades; Vindhyan Supergroup quartz arenites and Siwalik feldspathic sandstones as Indian examples; mentions thin section/petrographic identification criteriaGeneric field references without Indian specificity; mentions 'mafic rocks' or 'sandstones' without formation names; some awareness of where textures occur but lacks concrete examplesNo field evidence cited; entirely theoretical treatment; incorrect associations (e.g., textures in granites instead of gabbros/dolerites)
Quantitative reasoning20%10Precise calculations in (a): explicit reciprocals (1/3,1/3,1/3), clearing fractions, final (111) and (100) with clear reasoning for infinite intercepts; quantitative compositional thresholds stated in (e): >90% quartz, >25% feldspar/lithics, <15% vs. >15% matrix boundaries; P-T values or ranges mentioned for metamorphic faciesCalculations present but steps unclear or partially shown; final indices correct but working implicit; compositional percentages mentioned but thresholds fuzzy; qualitative rather than quantitative treatment of P-T effectsNo calculations shown despite 'calculate' directive; incorrect arithmetic; purely descriptive treatment where quantitative treatment required; missing numerical thresholds for classification
Indian / economic relevance20%10Indian geological significance emphasized: Vindhyan quartz arenites as building stone and reservoir (Bhander/Sirbu formations); Siwalik feldspathic sandstones in hydrocarbon exploration; Deccan Trap dolerite sills/dykes for texture studies and groundwater/aquifer significance; metamorphic belts of India (Eastern Ghats, Aravallis) cited; economic importance of sandstone dimension stone and aggregate mentionedBrief mention of Indian examples without elaboration of economic significance; standard textbook examples without Indian context; or Indian examples cited but relevance not explainedNo Indian examples; entirely foreign/locality-unspecific treatment; misses economic dimensions of sandstone as construction material and reservoir rock

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