Garnet Crystal: Gemstone Information
The three-dimensional order of atoms in the internal geometric structure of garnets produces a repeatable lattice structure that results in isotropic crystals. Thus garnets belong to the cubic system. It should be noted, however, that distortion of the crystal lattice in ugrandite garnets results in anomalous double refraction. Predominant morphologies are the trapezohedron (icosotetrahedron) and rhombic dodecahedron: crystals with 24 trapezoidal faces or 12 rhombic faces respectively, or combinations of these and some other forms. Factors controlling the morphology have variously been attributed to the A2+/B3+ ionic radius ratio, unit cell dimension, temperature, pressure and admixture. This has resulted in the generalization that ugrandite garnets normally form as rhombic
dodecahedrons whilst pyralspite garnets normally form as trapezohedrons. However, field evidence for almandine, linking morphology to geological environment, indicates that the rhombic dodecahedral form is predominant in schists whilst trapezohedrons are predominant in pegmatites and granites. Rhombic dodecahedron and trapezohedron combination forms are also common. Hexoctahedrons are occasionally seen whilst other forms are rare.