Basics

Crystallography is a discipline of mineralogy that studies crystalline forms. These crystalline forms are related to the arrangement of the atoms of a crystal which form what is called an elementary lattice. For example, for diamond, the smallest basic shape is a cube shape made up of 8 carbon atoms which are the corners of the cube. When you stack these little cubes, you get a crystal made up of billions of carbon atoms. Keep in mind that whatever the shape of the crystal, it is made up of microscopic cubes.

Each crystal system has an elementary cell. The characteristics of these meshes can be found in the raw crystals, in particular the angles which exist between the different axes.

There are 7 crystal systems on Earth:

• the cubic system
• the quadratic system
• the rhombohedral system
• the hexagonal system
• the orthorhombic system
• the monoclinic system
• the triclinic system

Each mineral species must belong to one of these systems. We add, although they are not crystalline systems, amorphous materials (such as glass) and microcrystals. Crystal systems can be divided into groups having similar optical characteristics. This makes it possible, during the gemological analysis, to be able to rule out certain possibilities. For example, anisotropic stones, that is to say those which will have a pleochroism with the dicroscope, which turn on and off with the polariscope, which often have two lines with the refractometer, etc. cannot be of the cubic system. So if, when testing a gem, you see two distinct colors with a dichroscope, then that stone cannot be a spinel which is of the cubic system and therefore anisotropic.

Before going any further, I invite you to download paper models of crystals to build yourself. You can find it all over the web, but here is a link that offers excellent quality. This makes it possible to perfectly illustrate the characteristics of each crystal system.

In crystallography, to describe a crystalline system, we use the notions of axis of symmetry, center of symmetry and plane of symmetry. Depending on the number of axes, centers and planes of symmetry, we know in the presence of which crystalline form we are. The shape of a crystal, regardless of its crystal system is called a habitus. The image below represents a crystal with a cubic habit. There are natural crystals with this habit (fluorite, pyrite, etc.)

Let’s take a cube (see image below). By inserting a rod in the middle of a face and which comes out in the middle of the opposite face, we represent an axis. Let us now hold this axis vertically, one side of the cube facing you. When we slowly rotate the cube around the axis, we see that a complete turn presents us with 4 identical faces. We then say that this axis has an order of 4 (denoted 1A4). By observing the cube, we can see that there are two other positions of axes which have an order 4. We will then say that the cube has 3 axes of order 4 (denoted 3A4). In the present case, 3A4 is diagnostic of the cubic system. No need for additional testing!

The image below shows a crystal of the cubic system with an octahedral habitus. Despite the different habit of the cube, we find our three axes of order 4. Fluorite and diamond often have this shape.

Another habit of the cubic system: the dodecahedron. We find the 3A4. Rough garnets often have this shape.

We will see in detail in a series of articles the different crystal systems.

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