B is a rock composed of a mosaic of interlocking crystals. These crystals are visible to the naked eye. What type of rock is this?

Question

B is a rock composed of a mosaic of interlocking crystals. These crystals are visible to the naked eye. What type of rock is this?

BlackTom AI · Accepted Answer

Interlocking, visible crystals indicate a phaneritic texture characteristic of intrusive igneous rocks.

1) Relevant knowledge points and concepts
- Igneous rock texture: "phaneritic" describes coarse-grained rocks whose mineral crystals are large enough to see with the naked eye. This texture forms when magma cools slowly underground, allowing crystals time to grow and interlock.
- Sedimentary rocks typically form from deposition and lithification of particles (clastic) or from precipitation/biochemical processes; they commonly show layering, clasts, or fossils rather than a mosaic of interlocking crystals.
- Metamorphic rocks often show foliation or banding (from directed pressure) or recrystallized mineral grains; some non-foliated metamorphic rocks (e.g., marble, quartzite) can have interlocking crystals, but their origin and textural clues differ from igneous phaneritic textures.

2) Step-by-step reasoning
- Observation: rock B is a mosaic of interlocking crystals visible to the naked eye.
- Interpretation of visibility: visible crystals mean coarse grain size → phaneritic texture.
- Link texture to formation: phaneritic texture forms when molten rock (magma) cools slowly beneath Earth’s surface (intrusive/plutonic environment).
- Therefore the rock is igneous, specifically an intrusive igneous rock (common example: granite).

3) Calculations
- Not applicable (classification problem).

4) Why the correct answer is correct
- The key diagnostic feature is interlocking crystals that are visible without magnification. That exact combination is the hallmark of igneous rocks that cooled slowly (phaneritic intrusive). The interlocking nature shows crystallization from a melt, and the crystal size indicates slow cooling.

5) Why other rock types are incorrect
- Sedimentary: would show clastic fragments, cement, layering, or fossils rather than a mosaic of intergrown crystals.
- Metamorphic: while some metamorphic rocks have recrystallized grains, they often exhibit foliation or banding; when they don’t (e.g., marble), context and mineralogy distinguish them. The description given best matches an igneous phaneritic texture.

6) Conclusion
- The correct classification is igneous, which matches the provided answer.

B is a rock composed of a mosaic of interlocking crystals. These crystals are visible to the naked eye. What type of rock is this?单项选择题

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