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. Synthetic opals, designed for photonic applications, are also sold as gem materials (fig. 3). Material applications are by no means restricted to com- mon mineral species. Recently, there has been considerable interest in synthetic triphylite (a.k.a. lithium iron phosphate, lifepo4, LFP, and olivine), an uncommon phosphate min- eral found in highly evolved granitic pegmatites, as a storage cathode for rechargeable lithium batteries (Anderson et al. 2000; Chung, Blocking, and Chiang 2002; Yang et al. 2002). Keys to the use of triphylite in batteries are its electrical and ion (Li) conductivities. Triphylite, as well as lithiophilite, is Figure 2. Top: Schematic of the Cool-System KEG GmbH an electrical insulator, which is the main impediment to its “CoolKeg®.” Bottom: One of the many alumino-silicate minerals of the zeolite family, natrolite, from Dayton area quarry, near use in batteries. Chung, Blocking, and Chiang (2002), how- Dayton, Washington. Jeffrey M. Schwartz specimen and photo. ever, have shown that controlled cation chemistry can in- The specimen measures 1.5 × 1.5 × 1 inches. crease the electrical conductivity of triphylite by as much as 108 times, well above that of Li storage cathodes currently In some cases, it is the presence of structural defects, such used in commercially available batteries. They postulated as interstitial atoms, vacancies, substituent atoms, and dislo- that in a conventional cell design, triphylite may yield the cations, that give a mineral its desired properties. An exam- highest power density yet developed in rechargeable Li bat- ple of a defect that leads to desirable property change is the teries (fig. 4). Furthermore, it has been speculated that the doping of diamond with impurities of boron. When boron same doping mechanism for increasing electrical conductiv- enters the diamond structure, it alters the electronic proper- ity in triphylite will apply to other olivine-structure phases, ties such that the doped diamond becomes a semiconductor, such as lithiophilite (Losey et al. 2004). Triphylite or LFP in contrast to its boron-absent nature as an electrical insula- is currently one of the cathodic materials used in lithium tor. The change in the electronic properties of diamond with ion batteries, and if things develop in a positive direction, the incorporation of boron can also impart a blue color and triphylite-lithiophilite may become much more common result in photoluminescence and phosphorescence. This is (through synthesis) than Earth’s pegmatites would lead us to the origin of the intense color and fluorescence of the Hope believe. Another mineral currently used as cathodic material Diamond (Eaton-Magaña et al. 2008). High-temperature in lithium ion batteries is the manganese-oxide birnessite. photosensors are being manufactured from blue, boron- Ramsdellite, also a manganese oxide, is used as a cathode in doped diamond because of its semiconducting properties Zn-alkaline batteries. and thermal stability (e.g., Apollo Diamond Inc.). Of course, The analytical skills that mineralogists learn (e.g., X-ray diamond is used extensively for two other superlative prop- diffraction, electron diffraction and imaging, and spectros- erties: hardness and thermal conductivity. The former is well copies of many types) and their ability to deal with com- known because diamond tools, including saw blades and plex structures and chemistries (such as those of zeolites and

354 ROCKS & MINERALS Examples of materials mineralogy, past, present, and potential future. Mineral, mineral-group, or structure-type Chemistry Uses Properties utilized

Apatite (and apatite- (Ca,Mn,REE)5 Phosphors, lasers Optical emission group minerals) (PO4,SiO4)3F

Ca5(PO4)3OH Prosthetic coating and bone Similarity to biological hard tissues (bones, replacement media teeth)

Ca5(PO4)3(OH,F) Environmental remediation agent Variable chemistry, stability, annealing tempurature

Na5Bi5(PO4)6(F,OH)2 Nanoparticulate drug delivery agent Size, morphology, structure 4+ Birnessite (and (Na0.3Ca0.1K0.1)(Mn , Cathodes in Li-ion batteries Electrical and ion conductivity 3+ other manganese Mn )2O4 · 1.5 H2O oxides)

Calcite CaCO3 Optical gunsight Birefringence (double refraction) and interference Lenses Refractive index Nomarski prism for DIC microscopy Birefringence (double refraction)

3+ Corundum Al2O3 with trace Cr Lasers (the first solid-state optical laser) Optical emission Diamond C, with and without added Abrasives, cutting tools, and others Hardness trace B impurity Diamond anvil cells Compressional strength plus diaphaneity Heat transport agent (heat pipes, heat Thermal conductivity spreaders) for computer chips, lasers, power supplies, and others Windows in lasers, and other high- Thermal conductivity temperature high-power devises plus optical and IR transparency Optical sensor in high-temperature Semiconduction when doped and thermal devices stability

Garnet (Y,REE)3Al5O12 Phosphors, lasers Optical emission Graphite C Electrodes (especially in Al production Electrical conductivity and thermal stability and electric steel making) Permanent inks Opacity and ability to be ground fine Carbon fibers for light-weight structural Tensile and strength, weight, thermal and materials, electrodes, and others electrical conductivity Neutron regulator in nuclear reactors Good neutron moderator with low neutron absorption cross section Anodes in lithium ion batteries Electrical conductivity and intercalation capability

Mica (K,Na,Ca)2(Al,Fe,Mg)4–6(Si, Dielectric in capacitors Electric properties Al) O (OH,F) 8 20 4 Substrate for scanning probe microscopy Cleavage

Monazite (REE)PO4 Phosphors and lasers Optical emission Anti UV materials Opal silicate and other materials Photonic band-gap materials, photonic Optical diffraction (i.e., diamond) transistors Perovskite Y-REE-Ba-Cu-oxides High-temperature superconductors Electrical properties e.g., BaTiO and Pb[Zr Ti ]O 3 x 1-x 3 Capacitors with tunable capacitance Ferroelectric

Pyrochlore A2B2O7 (A = REE, actinides; Solid nuclear-waste form Thermal and radiation stability, chemistry B = Ti, Zr, Sn, Hf)

Quartz SiO2 Frequency standard for timing devices, Piezoelectricity radio transmitters and receivers

Ramsdellite MnO2 Alkaline batteries Reduction capacity and atomic structure  Tourmaline (Ca,K,Na, )(Al,Fe,Li,Mg,Mn)3 Frequency standard for timing devices, Piezoelectricity (Al,Cr, Fe,V)6(BO3)3(Si,Al,B)6 radio transmitters and receivers O18(OH,F)4

Triphylite LiFePO4 Cathodes in Li-ion batteries Electrical and ion conductivity Zeolites Silicates, phosphates, Detergents, water softeners, metal and Ion exchange capacity arsenates, and others organic waste sequestration agent, and others Hydrocarbon cracking Catalytic capacity Molecular sieve Porosity and channel size

Volume 85, July/August 2010 355 societal importance. To quote one of the most cited earth scientists of the twentieth century (Hawthorne 1993): I have heard from Earth Scientists depicting Mineralogy as a ‘sunset’ science. These sentiments are due completely to ig- norance and lack of scientific insight. This is a tremendously exciting time to be doing Mineralogy, with the explosion of experimental and theoretical techniques, and the funda-

Figure 3. Top: Schematic of the “channeling” of specific wavelengths of light (by diffraction) in a photonic band-gap material. Bottom: A synthetic photonic material (opal) devel- oped by Dr. Alexander Bulatov, Russian Acadamy of Sciences, Chernogolovka. amphiboles) put them in an excellent position to make im- portant contributions to materials science. For example, the development of high-temperature superconductors on the basis of perovskite structures was the result of a close inter- action between mineralogists and physicists (Hazen 1988). One of the inventors of this new kind of material, 1987 phys- ics Nobel Prize winner J. Georg Bednorz, has a degree in mineralogy and crystallography. Because mineralogists are well prepared for the challenges faced in materials science, there are numerous companies that employ them. As an ex- ample, Minerals Technologies Inc. specializes in materials mineralogy. An interesting video about the company and its Figure 4. Top: Schematic of a lithium-ion battery with a products can be found on their website: http://www.minerals synthetic triphylite cathode (a.k.a. lithium iron phosphate or tech.com/about-mti/mti-video/. olivine, because it is isostructural with olivine) and a synthetic graphite anode that is intercalated with lithium ions. Bottom: I hope that this column has made it clear that materials Triphylite in a matrix of microcline, quartz, and muscovite, mineralogy, along with environmental mineralogy, biomin- Chandlers Mills, New Hampshire. Ken Larsen photo, courtesy of eralogy, and even good old geology, has great and enduring Smithsonian Institution (NMNH specimen #R9228).

356 ROCKS & MINERALS mental nature of the many complex problems that need to be solved. Minerals are the basic stuff of the Earth and their study will always remain at the core of the Earth Sciences. Indeed, the future of materials mineralogy is certain to lead to technological advances not yet fathomable by today’s hi-tech society, and mineralogists will continue to be at the forefront of these new discoveries.

Acknowledgments I thank Kendall Hauer, John Hughes, John Jaszczak, and Brian Phillips for their reviews of this manuscript. I am also grateful to An- drew Phelps for his review and for many interesting and informative discussions about mineral properties and materials mineralogy.

References Anderson, A. S., J. O. Thomas, B. Kalska, and L. Haggstrom. 2000.

Thermal stability of LiFePO4-based cathodes. Electrochemical and Solid-State Letters 3:66–68. Beck, J. V., and A. M. Osman. 1993. Maximum temperatures in dia- mond heat spreaders using the surface element method. Journal of Heat Transfer 115:51–57. Callister, W. D. 2007. Materials science and engineering: An introduc- tion. Hoboken, NJ: John Wiley & Sons. Chung, S. Y., J. T. Blocking, and Y. T. Chiang. 2002. Electrically con- ductive phospho-olivines as lithium storage electrodes. Nature Materials 1:123–28. De La Rue, R. 2003. Photonic crystals—Microassembly in 3D. Na- ture Materials 2:74–76. Eaton-Magaña, S., J. E. Post, P. J. Heaney, J. Freitas, P. Klein, R. Wal- ters, and J. E. Butler. 2008. Using phosphorescence as a fingerprint for the Hope and other blue diamonds. Geology 36:83–86. Evans, A. M. 1993. Ore geology and industrial minerals: An introduc- tion. Hoboken, NJ: Wiley-Blackwell. Galassi, C., M. Dinescu, K. Uchino, and M. Sayer, eds. 2000. Piezo- electric materials: Advances in science, technology and applications. NATO Science Partnership sub-series 3. Vol. 76. Berlin: Springer. Gunter, M. E. 2003. A lucky break for polarization: The optical properties of calcite. In ExtraLapis English, Calcite, 40–45. East Hampton, CT: Lithographie. Harlow, G., ed. 1997. The nature of diamonds. New York: Cambridge University Press. Hawthorne, F. 1993. Minerals, mineralogy and mineralogists: Past, present and future. Canadian Mineralogist 31:253–96. Hazen, R. M. 1988. The breakthrough: The race for the superconduc- tor. New York: Simon & Schuster. Krivovichev, S. V., ed. 2008. Minerals as advanced materials I. Berlin: Springer. Larsson, L. 2000. The passage of axes: Fire transformation of flint objects in the Neolithic of southern Sweden. Antiquity 74:602–10. Livshits, T., and S. Yudintsev. 2008. Natural and synthetic miner- als—Matrices (forms) for actinide waste immobilization. In Minerals as advanced materials I, ed. S. V. Krivovichev, 192–207. Berlin: Springer. Losey, A., J. Rakovan, J. M. Hughes, C. A. Francis, and M. D. Dyer. 2004. Structural variation in the lithiophilite-triphylite series and other olivine-group structures. Canadian Mineralogist 42:1105–15. Luo, Y., J. Rakovan, J. M. Hughes, and Y. Pan. 2009. Site preference of U and Th in Cl, F, Sr apatites. American Mineralogist 94:345–51. Rakovan, J. 2008. Word to the wise: Environmental mineralogy. Rocks & Minerals 83:172–75. Stapert, D., and L. Johansen. 1999. Flint and pyrite: Making fire in the stone age. Antiquity 73:765–77. Yang, S., Y. Song, P. Y. Zavalij, and M. S. Whittingham. 2002. Re- activity, stability, and electrochemical behavior of lithium iron phosphates. Electrochemistry Communications 4:239–44. Wood, E. A. 1977. Crystals and light—An introduction to optical crystallography. New York: Dover. ❑

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