Classification of rocks by composition: Rocks in which argil (clay) predominates are termed as which class of rocks?

Introduction / Context:
In building materials, rock classification by dominant constituents influences strength, durability, and weathering behavior. Civil engineers often classify sedimentary rocks into calcareous, siliceous, and argillaceous groups based on predominant minerals.

Given Data / Assumptions:

• “Argil” is clay-rich constituent (alumino-silicates).
• We must pick the class where clay dominates.

Concept / Approach:
Argillaceous rocks are clay-rich (e.g., shale, slate as a metamorphosed derivative). Siliceous rocks are silica-rich (quartzites, sandstones), calcareous rocks are rich in calcium carbonate (limestones, marbles), and igneous is a genetic, not compositional, term in this context. The name “argillaceous” derives from “argilla,” Latin for clay.

Step-by-Step Solution:

Verification / Alternative check:
Standard materials texts list shale and claystones as argillaceous; their engineering behavior (low permeability, potential for swelling if montmorillonite is present) follows clay mineralogy.

Why Other Options Are Wrong:

• Siliceous: dominated by silica; not clay.
• Calcareous: dominated by calcium carbonate; not clay.
• Igneous: origin-based term (formed from magma), not a clay-dominance descriptor here.

Common Pitfalls:

• Confusing genetic (igneous, sedimentary, metamorphic) with compositional (siliceous, calcareous, argillaceous) classifications.

Final Answer:
argillaceous rocks