Microscopic properties and glasses
Published:January 01, 2001
Iron makes up around 5 wt% percent of the Earth’s crust and is second in abundance to aluminium among the metals and fourth in abundance behind oxygen, silicon, and aluminium among the elements. Both in its ferrous (Fe2+) and ferric (Fe3+) forms, iron can be involved in direct or coupled substitutions with a variety of divalent, trivalent, and tetravalent cations in 4-, 6-, and 8-coordinated environments. It is consequently a major component in natural oxide and silicate solid solutions. Iron has a partially-filled 3d shell and occurs principally in a high-spin state, giving it a permanent magnetic moment. The presence of a magnetic species in a solid solution not only changes its macroscopic properties in fundamental ways but also provides a sensitive probe of the local chemical environment (Harrison & Putnis, 1999a).
Despite the abundance of Fe in the crust, only a small number of naturally occurring oxide, hydroxide, and sulphide minerals display magnetic ordering phenomena at room temperature (Dunlop & Özdemir, 1997). The most important of these are the titanomagnetite and titanohematite solid solutions. These oxides are the dominant carriers of natural remanent magnetisation (NRM) in rocks, and display diverse and complex behaviour resulting from the interaction between chemical and magnetic ordering (Burton, 1991; Harrison, 2000). No silicates display ordering at room temperature, but several (e.g. pyroxenes, amphiboles, micas, olivines, garnets) do show strong antiferromagnetic ordering at temperatures below 100 Κ (Coey & Ghose, 1987). The presence of magnetic ordering at such low temperatures can still have a significant impact on the thermodynamic properties of the solid solution at high temperatures.
Figures & Tables
Solid Solutions in Silicate and Oxide Systems
The EMU book series or notes, as they are called, were introduced to provide university teachers with up-to-date reviews in important, rapidly evolving areas of mineralogy, petrology and geochemistry. They are also meant to introduce scientists into special and often interdisciplinary fields of research. In this regard, a volume on solid solutions is current and sorely needed. The solid Earth, as well as many meteorites and the other solid planets, consists for the most part of mineral solid solutions. Research on solid solutions is extremely broad encompassing work in physics and chemistry, metallurgy, materials science and, last but not least, mineralogy and petrology. Hence, because the theme is so strongly interdisciplinary in nature, the workshop was organised to include solid state physicists, physical chemists, crystallographers, mineralogists and petrologists. The various chapters reflect some of this diversity and show what mineralogy has become. Experimental investigations in mineralogy now routinely include different types of spectroscopies along with more traditional phase equilibrium, X-ray diffraction, calorimetry, and TEM methods. There have also been new and impressive developments in theory and computation. Many computational approaches relating to the study of solid solutions, for example, the Cluster Variation Method or Monte Carlo simulations, have been brought in from materials science, chemistry and physics. It can be concluded that the traditional or historical, and perhaps artificial, boundaries between the various disciplines are disappearing. Many current research efforts in mineralogy are similar to those in chemistry, materials science and physics.