Igneous petrology – texture and origin of pegmatite Pegmatite, noted for its very coarse crystals, is best classified as which type of rock in civil/geological practice?

Introduction / Context:
Pegmatite is a distinctive rock that often contains extraordinarily large mineral crystals. In engineering geology and construction materials, correctly classifying rock types helps anticipate durability, workability, and potential for aggregate use. This question tests recognition of pegmatite’s genetic origin and where it fits within standard rock classifications used by civil engineers and geologists.

Given Data / Assumptions:

• Pegmatite exhibits very coarse grain size, sometimes crystals several centimeters to meters.
• It forms in the late stages of magma crystallization.
• We are to identify the correct rock class among intrusive/extrusive igneous, sedimentary, and metamorphic.

Concept / Approach:

Igneous rocks are classified by where the magma solidifies. Intrusive (plutonic) rocks crystallize at depth and typically have coarse grains due to slow cooling. Extrusive rocks cool quickly at or near the surface, forming fine-grained textures. Pegmatites are extreme coarse-grained bodies produced by volatile-rich residual melts that crystallize in fractures and pockets within the crust—hence intrusive. Their mineralogy commonly mirrors granitic compositions (feldspar, quartz, mica), though many variants exist.

Step-by-Step Solution:

Verification / Alternative check:

Field relationships (veins cutting earlier rocks, association with granites) and mineral textures confirm intrusive genesis. Laboratory thin-section analysis shows intergrowths typical of slow crystallization and late-stage hydrothermal activity, aligning with intrusive classification.

Why Other Options Are Wrong:

Extrusive igneous: Would be fine-grained or glassy from rapid cooling.

Sedimentary: Formed by deposition/diagenesis, not crystallization from melt.

Metamorphic: Formed by solid-state alteration; pegmatite crystallizes from melt instead.

Common Pitfalls:

Equating large crystals with metamorphism; confusing pegmatite with quartzite or gneiss due to shared minerals; ignoring the intrusive vein/dyke field context.

Final Answer:

Intrusive igneous rock