Garnet, crystal symmetry

If the diffusion medium is isotropic in terms of diffusion, meaning that diffusion coefficient does not depend on direction in the medium, it is called diffusion in an isotropic medium. Otherwise, it is referred to as diffusion in an anisotropic medium. Isotropic diffusion medium includes gas, liquid (such as aqueous solution and silicate melts), glass, and crystalline phases with isometric symmetry (such as spinel and garnet). Anisotropic diffusion medium includes crystalline phases with lower than isometric symmetry. That is, most minerals are diffu-sionally anisotropic. An isotropic medium in terms of diffusion may not be an isotropic medium in terms of other properties. For example, cubic crystals are not isotropic in terms of elastic properties. The diffusion equations that have been presented so far (Equations 3-7 to 3-10) are all for isotropic diffusion medium. 

The determination of garnet structure was first made on natural garnet minerals nearly fifty years ago (Menzer, 1926). Structural refinements were carried out on synthetic YIG crystals (Geller and Gillco, 1957 Batt and Post, 1962 Euler and Bruce, 1965). The symmetry of the single crystal X-ray photograph is Oh - m3m. The following types of reflections are present ... 

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