Geology 2022 Paper II 50 marks Describe

Q2

(a) Describe the crystallographic, physical, optical and chemical properties of garnet group of minerals. Give examples of rocks in which each species of garnet occurs as an essential mineral. (20 marks) (b) What are symmetry elements present in normal class of orthorhombic system ? Show the stereographic projection of a crystal face (hkl) for normal class of orthorhombic system. Write down Hermann-Mauguin notations of all classes of orthorhombic system. (15 marks) (c) Why does an anisotropic mineral, viewed under crossed polars, suffer four times of complete extinction during a 360° rotation of microscope stage ? What is pleochroism and how is it determined ? (15 marks)

हिंदी में प्रश्न पढ़ें

(a) गारनेट समूह के खनिजों की क्रिस्टलोग्राफीय प्रकाशीय, भौतिक एवं रासायनिक गुणों का वर्णन कीजिये । उन शैलों का उदाहरण दीजिये जिनमें गारनेट की विभिन्न जातियां अनिवार्य रूप से मिलती हैं । (20 अंक) (b) विषम लंबाक्ष समुदाय के सामान्य वर्ग में कौन से सममिति तत्व उपस्थित होते हैं ? इसी के क्रिस्टल फलक (hkl) का त्रिविम प्रक्षेप दिखाइए । विषम लंबाक्ष समुदाय के सभी वर्गों के हेरमन मौगुइन संकेतक लिखिये । (15 अंक) (c) क्रांसित ध्रुवीय निकाल में देखा गया विषमदैशिक खनिज सूक्ष्मदर्शी स्टेज के 360° घुमते समय चार बार विलुप्त क्यों होता है ? बहुवर्णिता क्या है और इसको किस प्रकार ज्ञात करते हैं ? (15 अंक)

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Directive word: Describe

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How this answer will be evaluated

Approach

Begin with a brief introduction defining garnet as a nesosilicate group and the orthorhombic system as one of the six crystal systems. For part (a), allocate ~40% of content covering crystallographic (isometric system, dodecahedral/trapezohedral forms), physical (hardness 6.5-7.5, no cleavage), optical (isotropic, high relief), and chemical properties (X3Y2Si3O12 formula with end-members), followed by Indian examples like pyrope in Kodurite of Eastern Ghats and almandine in Rajasthan schists. For part (b), allocate ~30% detailing the three 2-fold axes and three mirror planes of orthorhombic normal class (mmm), with accurate stereographic projection showing poles of (hkl) face and symmetry elements, plus all three Hermann-Mauguin notations (222, mm2, mmm). For part (c), allocate ~30% explaining the four extinction positions due to 90° periodicity of birefringence, with clear distinction between isotropic and anisotropic behavior, and systematic description of pleochroism determination using rotating stage and comparison with dichroism.

Key points expected

  • Part (a): Garnet crystallography—isometric system, common forms {110} dodecahedron and {211} trapezohedron; physical properties including conchoidal fracture, specific gravity 3.4-4.3 varying with composition; optical isotropy with high refractive index (1.74-1.89); chemical formula X3Y2Si3O12 with X=Ca,Mg,Fe2+,Mn and Y=Al,Fe3+,Cr,V
  • Part (a): Six principal garnet end-members (pyrope Mg3Al2Si3O12, almandine Fe3Al2Si3O12, spessartine Mn3Al2Si3O12, grossular Ca3Al2Si3O12, andradite Ca3Fe2Si3O12, uvarovite Ca3Cr2Si3O12) and their Indian occurrences—pyrope in Kodurite (Eastern Ghats), almandine in Rajasthan and Karnataka schists, grossular in Rajmahal traps contact zone
  • Part (b): Symmetry elements of orthorhombic normal class (mmm/2/m 2/m 2/m)—three mutually perpendicular 2-fold rotation axes (L2) coinciding with crystallographic axes, three mirror planes (m) perpendicular to each axis, and center of symmetry (i)
  • Part (b): Stereographic projection of (hkl) face in orthorhombic normal class showing general position pole with 8 equivalent faces generated by symmetry operations, and Hermann-Mauguin notations for all three classes: 222 (disphenoidal), mm2 (pyramidal), mmm (bipyramidal/normal)
  • Part (c): Explanation of four extinction positions—anisotropic minerals have two vibration directions (fast and slow rays) perpendicular to each other; at 45° to these directions brightness is maximum, at 0° and 90° alignment with polarizer/analyzer causes extinction, giving 4 extinctions per 360° rotation
  • Part (c): Pleochroism definition—differential absorption causing color change with vibration direction; determination method using single polar, rotating stage to find maximum/minimum absorption directions, distinguishing from dichroism (uniaxial) and trichroism (biaxial)

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5Precise definitions across all parts: correctly identifies garnet as isometric (not hexagonal), distinguishes orthorhombic 2-fold from 4-fold axes, accurately explains extinction mechanism through interference figure geometry and indicatrix orientation, uses correct Hermann-Mauguin symbols with proper orderGenerally correct mineralogy but confuses crystal systems (e.g., calls garnet hexagonal), misidentifies symmetry elements (e.g., 4-fold instead of 2-fold in orthorhombic), vague explanation of extinction as 'happens four times' without optical path geometryFundamental errors: states garnet is anisotropic, claims orthorhombic has 6-fold axis, describes pleochroism as fluorescence or phosphorescence, completely wrong Hermann-Mauguin notations
Diagram / cross-section20%10Clean stereographic projection for orthorhombic normal class showing primitive, all symmetry elements (2-fold axes as dots on primitive, mirror planes as great circles), (hkl) pole in general position with 8 equivalent points; optional but welcome: interference figure sketch for part (c), crystal drawings for garnet formsAttempted stereographic projection with correct primitive but misplaced symmetry elements or missing equivalent faces; OR describes projection verbally without drawing; garnet forms mentioned but not illustratedNo diagram for part (b) despite explicit demand; OR completely wrong projection (e.g., cubic stereogram instead of orthorhombic); illegible sketch with no labels; diagrams contradict text description
Field evidence15%7.5Specific Indian localities for garnet species: pyrope in Kodurite (Eastern Ghats, Andhra Pradesh), almandine in Rajasthan-Aravalli schists and Karnataka's Bababudan hills, grossular/andradite in Rajmahal traps contact aureole, spessartine in Nellore manganiferous rocks; mentions metamorphic grade indicators (pyrope=high P, almandine=medium grade)Generic 'garnet occurs in metamorphic rocks' without specific Indian examples; OR only mentions international localities (Bohemian garnet, Arizona peridotite); vague 'Himalayan rocks' without specificityNo field occurrences mentioned; OR completely wrong associations (e.g., garnet in basalts as primary mineral, confusing with olivine); fabricated localities
Quantitative reasoning20%10Numerical values: garnet refractive index range 1.74-1.89 with compositional variation (pyrope 1.74, almandine 1.83), hardness 6.5-7.5, specific gravity correlation with Fe/Mg ratio; extinction angle calculations showing 90° between extinction positions; correct Hermann-Mauguin symbol construction with priority rulesSome numerical values but incomplete or approximate (e.g., 'RI around 1.8'); mentions hardness but no values; understands extinction is periodic but no angular quantificationNo quantitative data; OR wrong values (e.g., garnet hardness 3, RI 1.5); confuses angles (says 45° extinction positions instead of 90°); cannot write proper Hermann-Mauguin symbols
Indian / economic relevance20%10Economic significance: garnet as industrial abrasive ( garnet sand from Tamil Nadu, Odisha beaches for water jet cutting), gemstone varieties (rhodolite from Orissa, hessonite from Rajasthan); mentions GSI/IBM exploration for garnet-bearing rocks; recent discoveries in Ladakh ultramafics; connection to India's abrasive industry and export potentialMentions garnet as gemstone or abrasive without Indian context; OR lists Indian occurrences without economic significance; generic 'used in industry' statementNo economic or Indian relevance discussed; OR confused economic use (e.g., garnet as iron ore, confusing with hematite); irrelevant discussion of non-Indian deposits

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