Garnet is a group of silicate minerals that form under geological conditions involving high pressure and temperature, resulting in distinct geometric shapes. The question of how many sides a garnet crystal has is complex because the word “side” is geometrically ambiguous, and the mineral naturally forms in two primary shapes. To provide a precise answer, one must consider the specific crystal form and the accurate terminology used to describe the external appearance of the mineral.
Clarifying Crystal Terminology
What most people refer to as a “side” of a crystal is accurately known as a face—a flat, smooth surface that develops during the crystal’s growth. Two faces meet to form an edge, which is a straight line segment on the crystal’s exterior. Finally, a vertex is the point where three or more edges intersect. For garnet, the number of flat faces is the most relevant measurement, as this determines the crystal’s overall shape, or habit. A well-formed garnet crystal can have either 12 or 24 of these faces.
The Dodecahedral Form
The most commonly encountered shape for a garnet crystal is the rhombic dodecahedron, defined by having 12 flat faces. Each of the 12 faces is a rhombus, or diamond shape, and all are identical in size and shape. This structure gives the crystal a somewhat rounded, symmetrical appearance. The dodecahedron is considered the simpler of the two common garnet habits, and garnets rich in the calcium-aluminum species, such as grossular, often favor this 12-sided form.
The Trapezohedral Form
The second major crystal habit that garnet forms is the trapezohedron. This shape is significantly more complex than the dodecahedron, featuring 24 faces. Each of the 24 faces is an irregular kite-shaped polygon, hence the name trapezohedron. This 24-sided form is common, particularly in garnets rich in iron, manganese, or magnesium, such as almandine or pyrope. The trapezohedron can be visually understood as a more complicated version of the dodecahedron, where each of the original 12 faces has been replaced by two smaller surfaces. External growth conditions often lead to crystals that exhibit a mix of both the 12-faced and 24-faced forms.
The Underlying Crystal System
The reason garnet crystals form these specific shapes stems from their fixed internal atomic structure, which belongs to the isometric, or cubic, crystal system. This system is characterized by three axes of equal length that are all mutually perpendicular. The atoms of silicon, oxygen, and various metals are arranged in a highly ordered, repeating three-dimensional lattice, and Garnet’s chemical formula, X3Y2(SiO4)3, describes a framework of silicate tetrahedrons and octahedrons. This internal arrangement possesses a very high degree of symmetry, which dictates that the external faces of the growing crystal must reflect this highly ordered structure. Only geometric shapes that conform to the isometric system’s symmetry, such as those with 12 or 24 faces, can form as the crystal grows outwards. The specific combination of temperature, pressure, and chemical composition during the mineral’s formation influences which one of these two common habits, the 12-faced dodecahedron or the 24-faced trapezohedron, will dominate the crystal’s final external appearance.