Which of the following rock types is a hydrous rock formed by chemical processes ?

Question

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (b) 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.  (c) 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.  (d) 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.

Concept: 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.

State PCS 2025 Q24 — Geography PYQ with Answer

State PCS 2025 Q24 — Geography PYQ with Answer