rhomboédriques

Rhomboédriques refers to a crystal system characterized by a unit cell that is a rhombohedron. A rhombohedron is a three-dimensional figure formed by six congruent rhombi. In this crystal system, all three crystallographic axes are of equal length, and the angles between them are equal but not necessarily 90 degrees. This means that while the edges are of equal length, the unit cell is generally skewed.

The rhombohedral crystal system is often considered a subdivision of the hexagonal crystal system. This is because a rhombohedron can be described using a hexagonal lattice as well, although with a different set of axes and origin. The characteristic symmetry of the rhombohedral system includes a three-fold rotation axis.

Minerals that crystallize in the rhombohedral system include calcite, dolomite, and quartz (though quartz is more commonly described in the hexagonal system). Understanding the rhombohedral system is important in mineralogy and crystallography for identifying and classifying crystalline structures. The geometric properties of rhombohedral crystals influence their physical properties, such as cleavage and optical behavior.