Engineering geology—rock classification: Quartzite belongs to which category based on its origin and transformation history?

Introduction / Context:

Classifying rocks by origin—igneous, sedimentary, and metamorphic—helps civil engineers anticipate strength, durability, weathering behavior, and suitability for aggregates. Quartzite is a common construction stone and aggregate source; recognizing its genesis informs its performance in structures and pavements.

Given Data / Assumptions:

• Quartzite is derived from silica-rich parent material.
• Field identification may rely on texture (granoblastic), hardness, and interlocking grains.
• Engineering interest: crushing strength, abrasion resistance, and durability.

Concept / Approach:

Quartzite forms when sandstone (a sedimentary, siliceous rock) is subjected to metamorphism—heat and pressure—leading to recrystallization of quartz grains and a strong, interlocking mosaic texture. Hence, quartzite is unequivocally a metamorphic rock. Although it is also ‘‘siliceous’’ by composition, the question seeks classification by genesis; therefore, the metamorphic category is the correct answer.

Step-by-Step Solution:

Verification / Alternative check:

Petrology texts describe quartzite's conchoidal fractures and extreme hardness (scratches steel), confirming its metamorphic nature and suitability as high-quality aggregate.

Why Other Options Are Wrong:

• Argillaceous: clay-rich rocks (e.g., shale), not quartzite.
• Calcareous: calcium carbonate-based (e.g., limestone), not silica-dominant.
• Silicious rock: compositionally true but not a genetic class; the question focuses on classification by origin.
• Sedimentary: quartzite's parent is sedimentary, but quartzite itself is metamorphic.

Common Pitfalls:

• Confusing quartzite with quartz-rich sandstone due to similar color; quartzite is harder and has fused grains.

Final Answer:

metamorphic rock