MPSC 2025 Prelims Paper-I · Question 24 of 100

Correct — option (1), Gypsum. Gypsum is a chemically precipitated (evaporite) sedimentary rock made of hydrous calcium sulphate — its mineral formula, CaSO4·2H2O, literally carries water molecules bound into its crystal structure, which is exactly what 'hydrous' means in rock classification. It forms when calcium-sulphate-rich water, typically in shallow seas, lagoons or salt lakes in arid climates, evaporates and leaves the dissolved mineral behind as a chemical precipitate — a process of chemical deposition, not the mechanical settling of eroded rock fragments that builds a rock like shale. That combination — chemically precipitated AND water-bearing in its very molecular formula — is what the question is isolating, and among the four options only gypsum satisfies both conditions at once.

• (2)Shale — Shale is sedimentary, but it is a clastic (mechanically formed) rock, not a chemical one. It forms from the compaction and lithification of fine clay and silt particles that were eroded elsewhere, transported by water, and deposited in thin, fissile layers — a physical, particle-accumulation process, not a chemical precipitation from solution. Shale is not classified as a hydrous mineral rock the way gypsum is; its defining trait is grain size and layering, not bound water in its chemical formula.
• (3)Granite — Granite is an igneous rock, formed by the slow cooling and crystallisation of molten magma deep underground — a thermal process entirely unrelated to water chemistry or sedimentary deposition. It is composed chiefly of quartz, feldspar and mica, none of which carry water in their crystal structure the way gypsum's calcium sulphate does, so it fails both the 'chemical/sedimentary process' and the 'hydrous' halves of the question.
• (4)Pumice — Pumice is an igneous rock too, formed when gas-rich, silica-rich lava is ejected explosively during a volcanic eruption and cools so rapidly that trapped gas bubbles freeze into the rock as a light, highly vesicular (full of holes) texture — this is why pumice is famously light enough to float on water. Its formation is a rapid-cooling volcanic process, not a chemical precipitation from water, and it carries no bound water in its mineral structure.

Rocks are classified by origin into three great families — igneous (from cooled molten rock), sedimentary (from compacted or precipitated surface material), and metamorphic (from existing rock altered by heat and pressure) — and sedimentary rocks are further split into clastic (mechanically deposited fragments, like shale or sandstone), organic (built from living material, like coal or some limestones), and chemical/evaporite rocks (precipitated directly out of solution as water evaporates, like gypsum and rock salt). A 'hydrous' rock or mineral is one whose chemical formula includes water molecules as an intrinsic structural component, not merely dampness on its surface — gypsum's CaSO4·2H2O is a textbook example, in contrast to its close chemical cousin anhydrite (CaSO4), which is the same mineral without the bound water.

This question tests whether a candidate can classify a rock along two axes at once — process of formation (mechanical versus chemical) and chemical composition (hydrous versus anhydrous) — rather than recalling a single fact in isolation. MPSC and UPSC geography sections regularly probe this kind of two-part classification with rock and mineral examples, because it distinguishes rote memorisation of 'gypsum is sedimentary' from a genuine understanding of why it is sedimentary and what makes its chemistry distinctive among evaporites.

• Gypsum is a chemically precipitated (evaporite) sedimentary rock, composed of hydrous calcium sulphate, CaSO4·2H2O.
• Evaporite rocks like gypsum and rock salt form when mineral-rich water evaporates in arid, shallow-basin settings, leaving dissolved salts behind as a solid deposit.
• Shale is a clastic sedimentary rock, formed mechanically from compacted clay and silt, not by chemical precipitation.
• Granite is an igneous rock formed by the slow underground cooling of magma; pumice is an igneous rock formed by the rapid cooling of gas-rich lava during explosive volcanic eruption.
• Anhydrite (CaSO4) is gypsum's water-free chemical cousin — the comparison is a useful way to remember what 'hydrous' specifically means for this mineral.

• Treating 'sedimentary' as a single category and assuming any sedimentary rock qualifies as 'chemically formed' — clastic sedimentary rocks like shale are mechanical, not chemical, in origin
• Confusing 'hydrous' (water bound into the chemical formula) with simply being found near water or feeling damp
• Mixing up igneous rock textures — assuming any light-coloured or light-weight rock must be sedimentary, when pumice is a classic igneous example

MPSC's geology-adjacent items typically ask for a rock or mineral matching two or three criteria simultaneously — origin process, chemical composition, and sometimes texture or use — rather than a single-fact identification. Building a mental table of common rocks (granite, basalt, pumice, sandstone, shale, limestone, gypsum, marble, slate) against their family (igneous/sedimentary/metamorphic), sub-type (clastic/chemical/organic, or intrusive/extrusive) and any distinctive chemical trait is the most transferable way to prepare for this recurring question style.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which of the following is the water-free chemical equivalent of gypsum, formed from the same calcium sulphate but without water bound into its crystal structure ?

  • (a)Anhydrite
  • (b)Halite
  • (c)Dolomite
  • (d)Limestone

  Answer(a) Anhydrite — chemically CaSO4, it is gypsum's dehydrated counterpart; gypsum is CaSO4·2H2O, with two water molecules bound into its crystal structure, which anhydrite lacks.

• practice — not a real PYQ

  Evaporite rocks such as gypsum and rock salt typically form in which kind of environment ?

  • (a)Deep ocean trenches with constant high pressure
  • (b)Shallow, arid-climate basins, lagoons or salt lakes where mineral-rich water evaporates
  • (c)Volcanic craters during active eruptions
  • (d)Deep underground magma chambers

  Answer(b) Shallow, arid-climate basins, lagoons or salt lakes where mineral-rich water evaporates — as the water evaporates faster than it is replenished, dissolved salts become concentrated and eventually precipitate out as solid evaporite deposits like gypsum and halite (rock salt).