Minerals can be formed directly from lava or volcanic action. They can also be formed through weathering, evaporation as well as the settling of hydrothermal solutions. Changes in temperature and pressure can trigger alteration as well. Despite the differences in mineral formation environments, every mineral not only has its own particular range of chemical composition it is also arranged in a specific geometric way inside. We call this the "Crystal Structure".
Naturally occurring minerals may vary in their atomic or ionic structure, or have different crystal structures, but where the geological conditions are right the homogenous solids formed by these atoms or ions all have regular and symmetrical crystal structures. These along with the smooth facets that form on the surface allow all kinds of different "Mineral Crystals" to form.
People began analyzing the internal structure of mineral crystals in the early 20 th Century using X-rays. Only then were they able to work out the true arrangement of the crystal structure and particles (atoms, ions or molecules). The studies revealed that regardless of what the mineral or crystal looked like externally, the internal particles all followed a set arrangement that in turn determines the shape unique to that crystal.
Native copper for example is made up of copper atoms stacked tightly in a cubic manner. The atoms are located at the corners and the center of each face of the cubic cell. Another example is rock salt. The chemical composition is sodium chloride and in Image 2, the large green ball and the small red ball represent the Chlorine (Cl-) and Sodium (Na+) irons. These particles always appear in a three-dimensional space at set intervals. If we use a solid circle and a hollow circle to represent the center of the chlorine and sodium ions then link them together with straight lines, we end up with a cubic lattice. This is the reason why rock salt crystals usually appear as cubes.
Depending on the composition of each mineral and the way their atoms or ions are stacked, the result is a complex world of minerals filled with many different varieties and compositions.