Geology (Elective): Minerals, Rocks, Structure and Odisha's Stratigraphy
What this paper tests
CHSE Odisha Geology (Elective) examines mineral and rock identification, labelled diagrams, structural measurements, palaeontology, and the stratigraphy and economic minerals of India with a strong Odisha emphasis. Marks come from correct terminology and correct order of events, not from long description.
Mineralogy and crystallography
- Mohs order: talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8, corundum 9, diamond 10. Finger nail ≈ 2.5, copper coin ≈ 3, steel knife and glass ≈ 5.5.
- Silicate structure follows SiO₄ polymerisation: isolated tetrahedra (olivine), single chain (pyroxene), double chain (amphibole), sheets (mica, clay), framework (quartz, feldspar).
- Cleavage angle separates the ferromagnesian chains: pyroxene near 90°, amphibole near 60°/120°.
Petrology
Bowen's reaction series runs olivine → pyroxene → amphibole → biotite (discontinuous branch) beside Ca-plagioclase → Na-plagioclase (continuous branch), ending at K-feldspar, muscovite, quartz; quartz crystallises last and therefore survives weathering best. Clastic sediments follow the Wentworth scale (clay < 1/256 mm, sand 1/16–2 mm, gravel > 2 mm). Progressive metamorphism of a pelite gives slate → phyllite → schist → gneiss, marked by index minerals chlorite, biotite, garnet, staurolite, kyanite and sillimanite.
Odisha stratigraphy and economic geology
- Archaean basement: Singhbhum craton plus the Eastern Ghats Mobile Belt (khondalite, charnockite, anorthosite).
- Iron Ore Group banded iron formation hosts the Barajamda–Noamundi–Barbil horseshoe deposits; Sukinda valley ultramafics carry chromite.
- Gondwana basins: Talcher and Ib Valley coalfields, with Talchir tillite below the Barakar coal measures, then Kamthi and Maleri.
- Deccan Trap basalt, then Tertiary and Quaternary sediments around Chilika and the Mahanadi delta.
Worked example
A dark green specimen is scratched by a steel knife (H ≈ 6) but not by orthoclase; its two cleavages meet near 120° and it does not effervesce in dilute HCl. Hardness 6 excludes calcite (3) and quartz (7); 120° cleavage means amphibole, not pyroxene (≈90°); no reaction rules out a carbonate. Answer: hornblende.
Traps that cost marks
Mixing hardness with cleavage; writing "acid test positive" for silicates; naming Barakar as the basal Gondwana unit (Talchir tillite is); drawing an anticline with limbs dipping towards the axis.