Cleavage is the tendency of a crystal to split along flat planes that follow rows of relatively weak bonds, because breaking those bonds costs far less energy than cutting across the stronger bonds elsewhere in the structure. This model represents a crystal as a cubic lattice of atoms joined by directional bonds whose strength encodes real bonding chemistry, and resolves an applied tensile stress onto each bond:
F(bond) = σ · |cos θ|
θ = angle between the pull axis and the bond direction
bond breaks when F(bond) > bond strength
Only bonds aligned with the load actually feel force (Mode-I, opening-mode loading) — exactly why a mineral cleaves cleanly on planes perpendicular to how it's pulled or struck, not randomly.
- Halite (NaCl) — ionic bonds are equally strong along all three cube edges, so it cleaves perfectly on any of three mutually perpendicular {100} planes: pick any axis and it splits into clean cubic blocks. This is the textbook example of "cubic cleavage."
- Mica — strong covalent Si–O bonds hold each silicate sheet together, but only weak van der Waals/ionic bonds link sheet to sheet. Pull along Z (interlayer) and the lattice peels into paper-thin flakes at very low stress; pull along X or Y and almost nothing happens — "perfect basal cleavage in one direction only."
- Quartz — a fully cross-linked 3D network of covalent Si–O bonds with no systematically weak direction (small random jitter stands in for the network's structural disorder). It resists far higher stress before failing, and the break front is ragged rather than flat: real quartz has no cleavage and instead fractures conchoidally (smooth curved shell-like surfaces, like broken glass).
Stress slider / Strike loads the whole lattice; bonds that exceed threshold break permanently. Heatmap colors intact bonds from weak (red) to strong (cyan) so you can see the anisotropy before anything breaks. Reset rebuilds an intact lattice for the current mineral.