Structural morphology of alkali feldspars and zircon and oriented intergrowths causing asterism in gemstones

Crystalline material grown in aqueous solutions or from vapour phase is often bounded by plane crystal faces. The surface configuration of such faces, that is controlled by the internal crystal structure, can be derived by the Hartman-Perdok theory with atomic precision. The crystal faces can be classified into three different categories. The most important faces with the lowest growth rates are the F faces, which are parallel to at least two non-collinear Periodic Bond Chains (PBCs). A PBC is an uninterrupted chain of strong bonds formed during the crystallization. As the strong bond is a relatively short bond within the first coordination sphere the number of PBCs and F faces is limited. The other two categories are the S and K faces, which should not be present on the growth form. Conclusion The influence of the crystal structure on crystal growth processes is clearly demonstrated by the investigations on zircon and potassium feldspars. The crystal structure determines the surface structure of the crystalline interface of which the atomic configuration controls the growth and the incorporation of impurities. Internal interfaces caused by the oriented intergrowth of host and guest phases not only reflect the (re)crystallization processes, but can also produce physical phenomena such as chatoyance and asterism.