Properties of minerals: Crystal structure, cleavage and fracture

Minerals are solid inorganic materials found in nature. Every mineral is made up of a particular mix of chemical elements that are arranged in a specific way. Chemical composition and atomic structure are difficult to see and test directly, but they determine a mineral’s type. They also determine how a mineral looks and behaves under certain conditions — its physical properties. By carefully examining the physical properties of a mineral, we can identify it.

In most minerals, particles are arranged in an organised manner, forming a regular, repetitive pattern. We call this crystal lattice or crystal structure. Because of this, mineral crystals have: regular, often symmetrical, shapes, straight edges, flat sides, or faces, and consistent angles between them. Some minerals, like halite, also known as table salt, form crystals in the shape of cubes. Others, like quartz and topaz, have a prismatic form.

There are also some mineral-like substances, such as obsidian or opal, that do not have crystalline structures. Instead, the atoms are arranged in a random, disorganised way. Their structure is amorphous. In most cases, crystals are too small to be seen with an unaided eye. Although crystal structure is a very important property when identifying minerals, it can be difficult to observe!

Luckily, there are other properties that can give us clues about the internal atomic structure of minerals. They become apparent when a mineral is broken. In the atomic structure of most minerals, there are areas where bonds are weaker. When you split a mineral, it typically breaks along that weak plane, creating a smooth surface. This property is called cleavage.

There are several types of cleavage, based on the number of planes in a mineral along which cleavage occurs. Mica is one example of a mineral with cleavage. It seems to be made up of layers placed on top of each other, that peel off easily in very thin sheets. This type of cleavage, along only one plane, is called basal cleavage. And here is a diamond.

It has four different planes along which cleavage occurs. Diamond has octahedral cleavage. Quality of cleavage depends on how strong the atomic bonds are. Minerals with perfect cleavage have very weak bonds between planes, so they break easily, leaving a smooth surface. Bonds in minerals that have good to poor cleavage are stronger, so it is harder to obtain a smooth surface.

Some minerals don't have any clear planes of weakness in their atomic structure, so they don’t produce a cleavage. All minerals, whether they have cleavage or not, can still break irregularly. This property is called fracture. There are five main types of fractures. Quartz, for instance, fractures in a very distinct fashion — producing a curved surface that resembles the inside of a seashell — a conchoidal fracture.

Hackly, or jagged fracture is when the edges are sharp and uneven. It often occurs in metals such as copper or silver, in their pure form. Soft, loosely bound minerals such as limonite have earthy fracture, which resembles a lump of soil or dry clay. Some minerals, for example white asbestos, fracture in a way that produces long, pointy fibres — a splintery fracture. Uneven fracture is when breaking a mineral produces a rough surface with random irregularities.

To sum up: crystal structure is a physical property of minerals, which determines the external shape of a mineral crystal — its mineral form. Cleavage describes the way a mineral breaks along smooth, flat planes. Fracture describes the way a mineral breaks in an irregular pattern with no flat surfaces. Knowing how to recognize these properties can help you identify minerals.