In the classification of building stones, what name is given to rocks having alumina (clay) as the major constituent? Select one correct category used in civil engineering.

Introduction / Context:
Civil engineers classify stones by dominant chemistry and mineralogy to predict durability and behavior. This question asks for the term applied when alumina/clay predominates.

Given Data / Assumptions:

• Major constituent is alumina-bearing clay minerals.
• We need the standard petrographic category used in construction texts.

Concept / Approach:

Argillaceous rocks contain significant clay (aluminosilicate) content. This affects strength, water absorption, and weathering. Siliceous rocks are silica-rich (e.g., quartz), calcareous are calcium carbonate-rich (e.g., limestone). “Sedimentary” and “igneous” are process-based, not chemistry-specific labels.

Step-by-Step Solution:

Verification / Alternative check:

Examples include shales and mudstones that soften on wetting, confirming clay dominance.

Why Other Options Are Wrong:

Siliceous: silica-rich, not clay-rich. Calcareous: calcium carbonate dominant. Sedimentary/igneous: genetic classes, not specific to alumina content.

Common Pitfalls:

Equating “sedimentary” with “argillaceous”—many argillaceous rocks are sedimentary, but not all sedimentary rocks are clay-rich.

Final Answer:

argillaceous rocks