Mica cleaves into thin, flexible sheets, while quartz fractures conchoidally (in smooth, curved surfaces) and cannot be split into sheets. What accounts for this difference in physical behavior?
AMica is softer than quartz, so it breaks more easily along any surface
BMica has weak ionic bonds between its silicate sheet layers but strong covalent bonds within each layer; quartz has uniformly strong covalent Si-O bonds throughout its framework
CMica is an amorphous solid while quartz has a crystal structure, giving quartz no preferred cleavage planes
DMica cleaves because it contains metalite elements, while quartz is a pure silicate
The physical behavior of a mineral is a direct expression of its crystal structure and bonding. Mica's sheet silicate structure has strong covalent bonds within each silicate layer but only weak van der Waals or ionic bonds between layers — so the crystal naturally separates along those weak interlayer planes. Quartz has a three-dimensional framework of Si-O covalent bonds with similar strength in all directions, so there is no preferred weak plane; instead, it fractures in smooth curves (conchoidal fracture) wherever stress is applied. Option A confuses hardness with cleavage — mica is indeed softer (Mohs ~2–3 vs. quartz at 7), but hardness measures scratch resistance, not cleavage tendency.
Question 2 Multiple Choice
Diamond is the hardest known mineral (Mohs hardness 10). What does this tell us about whether diamond can be cleaved?
ADiamond cannot be cleaved — its extreme hardness means no force can separate its atoms along any plane
BDiamond can be cleaved along its octahedral crystal planes, despite its hardness, because hardness and cleavage measure different properties
CDiamond's hardness implies it would fracture conchoidally rather than cleave, since all bonds are equally strong
DDiamond can only be cleaved by other diamonds, since only a harder material can break its bonds
Hardness (resistance to scratching) and cleavage (tendency to split along crystallographic planes) are distinct properties that measure different aspects of a crystal's bonding. Diamond is the hardest mineral because every carbon atom is covalently bonded to four neighbors with strong, uniform bonds — a scratch requires breaking many bonds simultaneously. But diamond also has perfect octahedral cleavage: the (111) planes are less densely bonded per unit area than other orientations, so a sharp impact along the correct angle causes the crystal to split cleanly. Gem cutters exploit this exact property to shape diamonds. A common misconception conflates 'hard = unbreakable' with 'hard = no preferred fracture plane.'
Question 3 True / False
Hardness and resistance to cleavage measure the same underlying property of a mineral's crystal structure.
TTrue
FFalse
Answer: False
False — this is a persistent misconception. Hardness (as defined by the Mohs scale) measures resistance to being scratched, which depends on the overall bond strength and density throughout the crystal. Cleavage measures the tendency to split along specific crystallographic planes where bonding is relatively weaker. Diamond is the hardest mineral (Mohs 10) but has perfect octahedral cleavage. Conversely, a mineral can be relatively soft but have no well-defined cleavage planes (like quartz at Mohs 7, which fractures conchoidally rather than cleaving). The two properties often correlate loosely but are conceptually and physically distinct.
Question 4 True / False
Rocks are aggregates of one or more minerals, while a single mineral has a definite chemical composition and an ordered internal crystal structure.
TTrue
FFalse
Answer: True
True. The rock/mineral distinction is fundamental in geology. Granite is a rock composed of multiple minerals — quartz, feldspar, and mica — interlocked in irregular patches. Quartz alone is a mineral: it has a fixed composition (SiO₂) and a specific crystalline lattice structure regardless of where it forms. A rock has neither a fixed composition nor a single crystal structure. Glass and obsidian fail the mineral test not on composition but on structure — they lack the ordered internal lattice required for a material to be classified as a mineral.
Question 5 Short Answer
How does the type of chemical bonding in a mineral's crystal lattice determine its physical properties, specifically hardness and cleavage? Use at least one specific mineral example.
Think about your answer, then reveal below.
Model answer: The bonds within a crystal lattice control both how hard the mineral is and whether it preferentially breaks along specific planes. Strong, directional covalent bonds resist scratching (high hardness) but may be anisotropic — if bond density varies by direction, the mineral cleaves along weaker planes. Diamond is the hardest mineral because every carbon atom is bonded to four neighbors with strong, uniform covalent bonds, making scratching in any direction difficult; yet it cleaves along octahedral planes where fewer bonds cross the fracture surface. Halite (NaCl) has ionic bonds that are moderately strong but break cleanly along cubic lattice planes where the repulsive like-charge ions align, producing perfect cubic cleavage at moderate hardness (Mohs 2.5). Mica has mixed bonding: strong Si-O covalent bonds within each tetrahedral sheet but weak forces between sheets, producing perfect basal cleavage into thin sheets despite moderate hardness.
The key insight is that physical properties are not arbitrary — they are the macroscopic expression of atomic-scale bonding geometry. Reading a mineral's behavior (hardness, cleavage, fracture) gives direct information about its internal structure.