Epitaxy

Many mineral specimens—and by definition almost all rocks—are intergrowths of crystals. Crystals in an intergrowth are usually randomly oriented, but those exceptions, where we find nonrandom or crystallographically controlled intergrowths, are fascinating and often quite beautiful in their symmetry. One of several types of nonrandom (or regular) crystal intergrowths is epitaxy. Generally speaking, epitaxy is the oriented overgrowth of one mineral or crystalline material on another. In this case, specific crystallographic directions of the substrate and overgrowth are aligned. This occurs when certain structural planes in the overgrowth and the substrate have similar spacings between atoms. In such instances the directions of similar atomic spacing in the two crystals prefer to be aligned rather than randomly oriented. There are many instances of epitaxy in the mineral kingdom. A few common examples are pyrite on marcasite, rutile on hematite (figs. 1, 2), and chalcopyrite on sphalerite (figs. 4, 5). It is not difficult to imagine why two isomorphic minerals (same structure but different chemistry) will have similar atomic spacings and will form epitaxic relationships. Examples of this include albite epitaxic on microcline and pyromorphite epitaxic on apatite. In some cases the two minerals involved in an epitaxic relationship are polymorphs (i.e., they have the same composition but different atomic arrangements, such as pyrite and marcasite or sphalerite and wurtzite). Yet, in many cases both the chemistry and structure of the two minerals are different; however, along a certain few directions in the two crystals there is similar spacing between their constituent atoms. Recognition of epitaxy between two crystals can be easy if the substrate and overgrowth have well-developed morphologies from which the orientation of the crystallographic axes are readily apparent. Thus, if you can see that the axes of one crystal are parallel to those of the other, you know that an oriented relationship exists between the two. In many mineral samples, you find numerous small crystals of the overgrowth on a single larger substrate crystal (figs. 1, 4). Often the crystals in the overgrowth are physically separated from one another. If you rotate the sample to a position where light reflects directly off a face on one of the overgrowth crystals and into your eye (specular reflection), you will notice that many or all of the other crystals are giving specular reflection at the same time (fig. 4). For this to occur, all of the overgrowth crystals must have a similar orientation. If they are physically separated from one another on the substrate surface, their mutual orientation is often, but not always, due to an epitaxic relationship between the overgrown crystals and the substrate. Epitaxy, in the strictest sense, refers to the oriented relationship between two different minerals or crystalline mateDr. John Rakovan, an executive editor of Rocks & Minerals, is a professor of mineralogy and geochemistry at Miami University in Oxford, Ohio. Epitaxy