Why does quartz lack cleavage planes?

Question

BlackTom AI · Accepted Answer

Because quartz is a framework silicate in which each SiO4 tetrahedron shares all four oxygens with neighboring tetrahedra, strong covalent Si–O bonds form a continuous three‑dimensional network. This network produces no preferred planes of weak bonding, so quartz does not split along smooth cleavage surfaces.

Relevant concepts:
- Cleavage is the tendency of a mineral to break along planes of weak atomic bonding in the crystal structure. Perfect cleavage indicates clear, flat planes where bonds are weaker in a specific direction.
- Fracture describes how a mineral breaks when it has no cleavage; conchoidal fracture (smooth, curved surfaces) is typical for minerals with strong, isotropic bonding like quartz.
- Quartz structure: SiO2 consists of SiO4 tetrahedra linked at all four corners, producing strong covalent bonds in all directions (a 3D framework).

Step-by-step reasoning:
1. Identify what would produce cleavage: planar weakness in bonding (e.g., layered or sheet-like structures have weak interlayer bonds).
2. Examine quartz bonding: each silicon is covalently bonded to four oxygens, and each oxygen bridges between two silicon atoms. No layers or easy slip planes exist.
3. Conclude that because there are no planes of weaker bonding, quartz cannot preferentially break along flat cleavage planes; instead it exhibits conchoidal fracture.

Why the correct option is correct:
- The given answer (“because it is a framework silicate and has strong, covalent bonds in all directions”) directly states the structural reason: an interconnected 3D Si–O network prevents planar weak bonds and thus prevents cleavage.

Why the other options are incorrect:
- “Very soft mineral” is false: quartz is relatively hard (Mohs hardness ≈7), not soft.
- “Unnatural mineral grown in labs” is false: quartz is a common natural mineral.
- “Plate-like mineral that forms thin sheets” describes sheet silicates (e.g., micas) which have perfect cleavage; quartz is not plate-like.

Conclusion: The correct answer is “because it is a framework silicate and has strong, covalent bonds in all directions,” which matches the provided answer.

Why does quartz lack cleavage planes?单项选择题

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