In engineering geology, identify the polymineralic rock (i.e., a rock composed of multiple distinct minerals) from the following list of common materials.

Introduction / Context:
In civil and geological engineering, correctly classifying earth materials is essential for assessing durability, strength, and weathering behaviour. One frequently tested distinction is between monomineralic rocks (built predominantly from a single mineral species) and polymineralic rocks (composed of several minerals). This question asks you to choose the polymineralic rock from a list that includes several monomineralic materials used in construction and industry.

Given Data / Assumptions:

• Quartz sand is essentially composed of quartz (SiO₂) grains.
• Pure gypsum is the mineral CaSO₄·2H₂O.
• Magnesite is the mineral MgCO₃.
• Granite is an igneous rock typically containing multiple minerals.

Concept / Approach:

A rock is polymineralic when it consists of more than one principal mineral in appreciable quantities. Granite is the textbook example: a coarse-grained plutonic igneous rock comprising quartz, alkali feldspar (orthoclase/microcline), and plagioclase, commonly with biotite and/or hornblende. In contrast, materials like gypsum, magnesite, and quartz sand are dominated by a single mineral species, making them monomineralic (or nearly so in practical terms).

Step-by-Step Solution:

Verification / Alternative check:

Petrology handbooks and standard classifications (e.g., QAPF diagram for granitoids) confirm granite’s multi-mineral composition. Thin-section microscopy shows interlocking grains of several distinct minerals—clear evidence of polymineralic texture.

Why Other Options Are Wrong:

Quartz sand — Predominantly quartz; not a polymineralic rock.
Pure gypsum — A single mineral species; not a rock assemblage.
Magnesite — A single mineral carbonate; not polymineralic.
None of these — Incorrect because granite is clearly polymineralic.

Common Pitfalls:

Equating an aggregate (sand) with a polymineralic rock; assuming any granular material must contain many minerals. Remember: “rock” implies a consolidated, coherent mass, and the key is the number of constituent mineral species present in significant amounts.

Final Answer:

Granite