Why Do Some Crystals Split Cleanly? Weak Planes Inside the Structure

A crystal is a regular three-dimensional arrangement of atoms held by bonds, and those bonds are not equally strong in every direction. Where a structure contains layers held to each other by weaker forces than those operating within each layer, the crystal breaks preferentially between the layers, producing a flat surface. Mica is the extreme case, with sheets of strongly bonded atoms held together weakly, so it splits into flexible transparent sheets with almost no effort. Where the bonding is comparably strong in every direction, as in quartz, there is no preferred plane and the mineral breaks along curved irregular surfaces instead, which is called fracture rather than cleavage and is equally diagnostic.

Cleavage is recorded by how many directions exist, at what angles and how well it works:

• •One direction, producing sheets, as in mica
• •Two directions at ninety degrees, producing rectangular blocks, as in feldspar
• •Two directions not at ninety degrees, as in the amphibole group
• •Three directions at ninety degrees, producing cubes, as in halite
• •Three directions not at ninety degrees, producing rhombs, as in calcite
• •Quality graded from perfect through good to poor, describing how flat and how easily produced

The combination of number, angle and quality is characteristic and frequently decisive. Two minerals of similar colour and hardness are distinguished immediately if one cleaves in two directions at ninety degrees and the other at about sixty and one hundred and twenty, which is exactly the standard test separating the pyroxene and amphibole groups and which a geologist performs by looking at the ends of grains in a rock. Calcite breaking into rhombs is unmistakable. The absence of cleavage is equally informative, since a mineral with none must be one of a limited set. The test needs only a hand lens and a hammer, and it works on small grains in a rock as well as on individual crystals, which is why it appears in every identification key.

Cleavage is one of a handful of simple tests that together identify most common minerals in the field. Hardness is tested by what scratches what, using a scale of ten reference minerals and, in practice, a fingernail, a coin, a knife blade and a piece of glass, which between them bracket most specimens. Streak, meaning the colour of the powder produced by rubbing the mineral on unglazed porcelain, is far more reliable than the colour of the specimen itself, since surface colour varies with impurities and streak generally does not. Lustre describes how the surface reflects light. Specific gravity distinguishes heavy minerals by feel. Reaction with dilute acid identifies carbonates immediately. Crystal form helps where crystals grew freely, which is uncommon.

The property has consequences well outside the laboratory. Gem cutting depends on it entirely, since a stone must be cut in orientations that avoid cleavage planes and a blow in the wrong direction destroys it, which is why diamond, despite being the hardest natural material, can be split with a well-placed tap and why cutters spend a long time deciding orientation before starting. Rock strength depends on the cleavage of its constituent minerals, since a rock full of micas splits along them. Asbestos minerals are hazardous because their structure produces fibres rather than blocks when broken, and the fibre shape is what makes them dangerous in the lung. And slate roofing exists because the mineral alignment produced during metamorphism gives the rock a cleavage of its own.

Bonds within a crystal differ in strength by direction, so breaking follows the weakest planes and produces flat surfaces, while uniform bonding gives irregular fracture instead. The number of directions, the angles between them and the quality are characteristic enough to identify minerals with a hand lens. Diamond is the hardest natural material and splits with a well-placed tap.