Word to the Wise JOHN RAKOVAN Department of Geology Miami University Oxford, Ohio 45056 [email protected] Materials Mineralogy Synthetic analogues of long-known minerals, and in some cases, the natural materials themselves, are quickly establishing a place of importance in today’s high-tech world. n what contexts is the study of minerals noteworthy? represent the first example of materials science and certainly Many people are aware that minerals are the constitu- predate the formal study of mineralogy. As a case in point, Ients of most rocks and thus are the building blocks of the use of flint to spark fires is a technological application our Earth as well as other solid celestial bodies. As such, min- of a mineral whose origin is lost in antiquity (Stapert and eralogy has historically been a study of Earth materials, and Johansen 1999; Larsson 2000). Moreover, native metals and traditionally it has been framed by the context of the Earth’s their uses have defined major technological milestones in hu- composition and dynamics. This is of great and enduring man civilization (Hawthorne 1993), and the use of minerals significance, but the other contexts in which mineralogy is of in technological applications grew exponentially throughout relevance are stunningly broad. Human health, water quality, the industrial revolution and during the twentieth century. life on Mars, art history and media, the origin of life, quality Almost all mineral collectors know of at least some mate- of the environment, and technological materials are but the rial applications of minerals. A familiar example is the use tip of the mineralogical iceberg. Of course, icebergs are made of quartz for its piezoelectric properties in timing devices of the mineral ice, making this metaphor more of a pun than such as watches and clocks. Piezoelectric materials are those a simile. In a previous Word to the Wise column, “Envi- that generate an electrical voltage when physically deformed ronmental Mineralogy” (Rakovan 2008), we looked at the (e.g., squeezed). Conversely, an applied voltage or current relevance of minerals and mineralogy in sustaining the envi- will change the shape of the material, causing it to oscillate at ronment in which we live. In this column, we focus on another a regular frequency to which timing devices can be synchro- area where minerals profoundly affect our everyday lives. nized (Galassi et al. 2000). Originally, natural quartz crystals For lack of a better term, I have chosen “Materials Miner- were used; however, twinning, which is common in quartz, alogy” as the title for this column. Materials mineralogy is not interferes with the performance, and most quartz oscillators a commonly used term (this column may possibly change today are manufactured from untwinned synthetic crystals. that), and the subject is most often found in the literature as A more recent application of this same property is in sensors the mineralogy of materials. The concept behind materials of some automobile airbags. The stress applied to a piezo- mineralogy is the use of minerals in technological applica- electric sensor during an impact sends an electrical signal tions based on their physical and chemical properties, rather to the airbag discharge device causing it to deploy. Synthetic than as a source of their constituent elements, as in the case piezoelectric materials have many other uses, including of ore minerals (Evans 1993). I chose materials mineralogy spark igniters on grills, furnaces, and stoves. It is interesting because it relates to the discipline of materials science: an to note that flint, mentioned above as a neolithic tool for interdisciplinary field involving the properties of matter and starting fires, is a variety of microcrystalline quartz. their applications to various areas of science and engineer- Examples of high-tech applications of minerals abound, ing (Callister 2007). The science of materials mineralogy fo- and many pages could be filled describing them (e.g., cuses on the relationship between the atomic structure and Krivovichev 2008). Previous Word to the Wise columns, in- chemistry of minerals (and their synthetic analogues) and their resulting macroscopic properties, whereas engineering pursues the application of these properties. Dr. John Rakovan, an executive editor of Rocks & Minerals, is Mineralogy has always been intimately tied to materials a professor of mineralogy and geochemistry at Miami Univer- science. In fact, informal mineral “studies” can be said to sity in Oxford, Ohio. 352 ROCKS & MINERALS though they may have different compositions. Two exam- ples of this are the use of olivine to denote synthetic triphy- lite and perovskite to denote the Y-Ba-Cu oxide family of superconductors. An interesting historical application is that of calcite in optical ring sights (Wood 1977; Gunter 2003). With the ad- vances in airplane speeds and technologies leading up to World War II, and the attacks on Pearl Harbor, it became ap- parent to the U.S. military that it was sorely lacking an ade- quate sighting device for antiaircraft guns. What was needed was a gunsight that did not suffer from parallax, as did the simple two-element sighting system that was in use. Capital- izing on some of the optical properties of calcite, E. H. Land (founder of the Polaroid Corporation) invented the optical ring sight (fig. 1). Utilizing optical interference effects, a se- ries of concentric rings are created in the ring site. To ac- curately fire on a target, all that is needed is sighting within these rings. In this application natural calcite was used, mak- ing sources of optical-grade calcite important strategic min- eral deposits during the war. Minerals are also being used in many applications for environmental remediation and sustainability (Rakovan 2008). Likewise, advanced materials, including minerals, have numerous applications in medicine. Thus, the subdis- ciplines of materials mineralogy, environmental mineralogy, and medical mineralogy have many aspects in common. A striking example of such wide-ranging links is illustrated by two very different applications of synthetic apatite (sensu lato). This mineral is being investigated as a potential solid nuclear-waste form because it has many properties desirable for long-term, stable containment of radioactive waste (i.e., low solubility, low annealing temperature, and a high affinity for many radionuclides [Livshits and Yudintsev 2008; Luo et al. 2009]). Synthetic apatite is also used for coatings on bone and tooth prosthetics that promote better acceptance by the Figure 1. Top: Optical ring sight, Smithsonian Institution, body and coupling to tissues such as cartilage and muscle. Washington, D.C. Photo courtesy of Jeff Post. Bottom: Cleav- One of my favorite developments in materials mineralogy age rhombohedron of calcite showing double refraction in the involves the use of zeolites in a novel cooling system. In this splitting of a single line on paper into two lines when viewed through the crystal. device, a metal container is jacketed with a closed two-layer chamber. The inner chamber contains water-saturated fleece under a vacuum and is connected to the outer chamber by cluding “Zeolites,” “Epitaxy,” and “A-mica,” highlight many an externally operated valve. The outer chamber, similarly uses of minerals as advanced materials. The remainder of under a vacuum, is filled with zeolite crystals. When the this column provides a few additional, interesting examples. connecting valve is opened, the difference in pressure be- A more extensive, but far from comprehensive, list is found tween the inner and outer chambers allows water vapor to in the table. In most cases, naturally occurring minerals are flow from the fleece to the zeolite, where it is absorbed. The too impure or defective for technological applications, and evaporative enthalpy extracted from the fleece turns the re- their carefully grown synthetic analogues are employed. maining water to ice, thus cooling the contents of the central Henceforth, I freely use mineral names when describing metal container. The device was developed by Cool-System synthetics. In some cases alternative names are used in the KEG GmbH to cool beer. Imagine an ice-cold beer, even in materials literature; these are given in parentheses, such as the hottest desert or jungle, without the need for carrying triphylite (a.k.a. lithium iron phosphate, lifepo4, LFP, and ice or a refrigeration device. The Cool-System “CoolKeg®” olivine). Also, synthetic materials are sometimes referred to (fig. 2) is currently used by brewers, such as Tucher Brau by the name of a mineral isomorph (same structure) even of Germany. Volume 85, July/August 2010 353 drill bits, are found in just about any hardware store. The latter is seeing increased usage in advanced electronics. As semiconducting chips are getting smaller, heat buildup is becoming a formidable problem. The need to quickly and efficiently cool these devices requires materials with high thermal conductivity, and, as with hardness, diamond is sec- ond to none in its ability to conduct heat (Beck and Osman 1993). This is also the reason that diamonds feel cold to the touch and one of the reasons for the colloquial use of ice as an epithet for this mineral (Harlow 1997). An exciting new class of substances currently under in- tense investigation, photonic band-gap materials, possesses structures similar to those found in opal (De La Rue 2003). One application of these materials is to use photons (parti- cles of light) as carriers of information, similar to the way in which electrons are used today. Light has several significant advantages over electrons for this purpose, including higher speed and lower power loss. Photonic band-gap materials have the ability to steer light (the mechanism behind the play of colors in precious opal) in the same way that electrons are manipulated in semiconductor chips. Thus, there is the po- tential that future computers and other devices may operate on light rather than electricity.