New Mexico Mineral Symposium — Abstracts

Most minerals are chemically complex and can exhibit large variations in chemistry. We often see tables of analyses of one mineral or another where the chemistry of the mineral reflects the differences in geochemistry from one locale to another. Therefore most of us are aware of such changes. However, many of us are only vaguely aware that if the chemical and physical conditions change while any one crystal of such a mineral is growing, this individual crystal will show chemical variations within it. This phenomenon is called zoning and, while we have all seen this, few of us have thought about what causes it or what it is telling us. In this talk, I plan to remedy that by examining some of the properties of zoning as well as discussing some of the good (potentials) and bad (problems) features of this phenomenon.

In discussing properties I plan to mention phenomena, such as diadochy, that allow zoning to occur as well as discuss the various properties that zoning exhibits. These properties include the fact that some elements can change concentration while others don't. Zoning occurs in various styles or types (normal, reverse, sector, and oscillatory), several of which can be exhibited by any one crystal at the same time. Zoning can be manifested in many different ways (color, birefringence, microprobe analyses, etc.). It is important in discussing zoning to remember that essentially all minerals are zoned, some obviously, some not so obviously.

I will also discuss some of the potentials (benefits) of mineral zoning. For example, zoning helps to simplify the mineral kingdom by making minerals chemically "flexible". Without this flexibility, ordinary rocks might have become amazingly complex--a different mineral for every element in the sample. Zoned minerals aid in our understanding of mineral chemistry and mineral relationships. This understanding has allowed the creation of materials with tailored properties. To the academic geologist, a major benefit of zoning is the easily interpreted record of geochemical history preserved in these minerals. To the hobbyist, a major benefit is the beauty which zoning can create.

I also need to discuss some of the problems that zoning creates. Few people are aware of how difficult it can be to obtain, or manufacture, some materials in pure and/or homogeneous forms. For example, the separation of pure praseodymium from neodymium wasn't possible until the late 1920's when ion exchange techniques were perfected. If one wants to make a silicon/germanium alloy, one has to expect significant inhomogeneity. Some elements are hard to obtain because zoning has dispersed them in small amounts in some minerals, such as rubidium in feldspar, or has zoned them with elements from which they are difficult to extract, such as tantalum in niobates. Of immediate concern to mineral collectors is how the very wonderful chemical flexibility of zoning can make the naming of a particular specimen frustrating. If the central half of a specimen is nickel-rich skutterudite and the out half is cobalt-rich nickel-skutterudite--what do I call it?

Examples of zoning in a wide variety of minerals and using a number of techniques will be used to illustrate the talk. Because the electron microprobe was used in many of these examples, a short description of this important instrument and the data it produces will be included.

*A portion of this work was performed at Sandia National Laboratories and was supported by the U.S. Department of Energy under contract no. DE-ACO4-76DP00789.