352 ROCKS & MINERALS

Materials Mineralogy

In what contexts is the study of minerals noteworthy? Many people are aware that minerals are the constitu-ents of most rocks and thus are the building blocks of

our Earth as well as other solid celestial bodies. As such, min-eralogy has historically been a study of Earth materials, and traditionally it has been framed by the context of the Earth’s composition and dynamics. This is of great and enduring significance, but the other contexts in which mineralogy is of relevance are stunningly broad. Human health, water quality, life on Mars, art history and media, the origin of life, quality of the environment, and technological materials are but the tip of the mineralogical iceberg. Of course, icebergs are made of the mineral ice, making this metaphor more of a pun than a simile. In a previous Word to the Wise column, “Envi-ronmental Mineralogy” (Rakovan 2008), we looked at the relevance of minerals and mineralogy in sustaining the envi-ronment in which we live. In this column, we focus on another area where minerals profoundly affect our everyday lives.

For lack of a better term, I have chosen “Materials Miner-alogy” as the title for this column. Materials mineralogy is not a commonly used term (this column may possibly change that), and the subject is most often found in the literature as the mineralogy of materials. The concept behind materials mineralogy is the use of minerals in technological applica-tions based on their physical and chemical properties, rather than as a source of their constituent elements, as in the case of ore minerals (Evans 1993). I chose materials mineralogy because it relates to the discipline of materials science: an interdisciplinary field involving the properties of matter and their applications to various areas of science and engineer-ing (Callister 2007). The science of materials mineralogy fo-cuses on the relationship between the atomic structure and chemistry of minerals (and their synthetic analogues) and their resulting macroscopic properties, whereas engineering pursues the application of these properties.

Mineralogy has always been intimately tied to materials science. In fact, informal mineral “studies” can be said to

represent the first example of materials science and certainly predate the formal study of mineralogy. As a case in point, the use of flint to spark fires is a technological application of a mineral whose origin is lost in antiquity (Stapert and Johansen 1999; Larsson 2000). Moreover, native metals and their uses have defined major technological milestones in hu-man civilization (Hawthorne 1993), and the use of minerals in technological applications grew exponentially throughout the industrial revolution and during the twentieth century. Almost all mineral collectors know of at least some mate-rial applications of minerals. A familiar example is the use of quartz for its piezoelectric properties in timing devices such as watches and clocks. Piezoelectric materials are those that generate an electrical voltage when physically deformed (e.g., squeezed). Conversely, an applied voltage or current will change the shape of the material, causing it to oscillate at a regular frequency to which timing devices can be synchro-nized (Galassi et al. 2000). Originally, natural quartz crystals were used; however, twinning, which is common in quartz, interferes with the performance, and most quartz oscillators today are manufactured from untwinned synthetic crystals. A more recent application of this same property is in sensors of some automobile airbags. The stress applied to a piezo-electric sensor during an impact sends an electrical signal to the airbag discharge device causing it to deploy. Synthetic piezoelectric materials have many other uses, including spark igniters on grills, furnaces, and stoves. It is interesting to note that flint, mentioned above as a neolithic tool for starting fires, is a variety of microcrystalline quartz.

Examples of high-tech applications of minerals abound, and many pages could be filled describing them (e.g., Krivovichev 2008). Previous Word to the Wise columns, in-

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.

Word to the Wise

JOHN RAKOVANDepartment of GeologyMiami UniversityOxford, Ohio 45056rakovajf@muohio.edu

Dr. John Rakovan, an executive editor of Rocks & Minerals, is a professor of mineralogy and geochemistry at Miami Univer-sity in Oxford, Ohio.

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cluding “Zeolites,” “Epitaxy,” and “A-mica,” highlight many uses of minerals as advanced materials. The remainder of this column provides a few additional, interesting examples. A more extensive, but far from comprehensive, list is found in the table. In most cases, naturally occurring minerals are too impure or defective for technological applications, and their carefully grown synthetic analogues are employed. Henceforth, I freely use mineral names when describing synthetics. In some cases alternative names are used in the materials literature; these are given in parentheses, such as triphylite (a.k.a. lithium iron phosphate, lifepo4, LFP, and olivine). Also, synthetic materials are sometimes referred to by the name of a mineral isomorph (same structure) even

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 body and coupling to tissues such as cartilage and muscle.

One of my favorite developments in materials mineralogy involves the use of zeolites in a novel cooling system. In this 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 an externally operated valve. The outer chamber, similarly under a vacuum, is filled with zeolite crystals. When the connecting valve is opened, the difference in pressure be-tween the inner and outer chambers allows water vapor to flow from the fleece to the zeolite, where it is absorbed. The evaporative enthalpy extracted from the fleece turns the re-maining water to ice, thus cooling the contents of the central metal container. The device was developed by Cool-System KEG GmbH to cool beer. Imagine an ice-cold beer, even in the hottest desert or jungle, without the need for carrying ice or a refrigeration device. The Cool-System “CoolKeg®” (fig. 2) is currently used by brewers, such as Tucher Brau of Germany.

Figure 1. Top: Optical ring sight, Smithsonian Institution, Washington, D.C. Photo courtesy of Jeff Post. Bottom: Cleav-age rhombohedron of calcite showing double refraction in the splitting of a single line on paper into two lines when viewed through the crystal.

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In some cases, it is the presence of structural defects, such as interstitial atoms, vacancies, substituent atoms, and dislo-cations, that give a mineral its desired properties. An exam-ple of a defect that leads to desirable property change is the doping of diamond with impurities of boron. When boron enters the diamond structure, it alters the electronic proper-ties such that the doped diamond becomes a semiconductor, in contrast to its boron-absent nature as an electrical insula-tor. The change in the electronic properties of diamond with the incorporation of boron can also impart a blue color and result in photoluminescence and phosphorescence. This is the origin of the intense color and fluorescence of the Hope Diamond (Eaton-Magaña et al. 2008). High-temperature photosensors are being manufactured from blue, boron-doped diamond because of its semiconducting properties and thermal stability (e.g., Apollo Diamond Inc.). Of course, diamond is used extensively for two other superlative prop-erties: hardness and thermal conductivity. The former is well known because diamond tools, including saw blades and

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 an electrical insulator, which is the main impediment to its use in batteries. Chung, Blocking, and Chiang (2002), how-ever, have shown that controlled cation chemistry can in-crease the electrical conductivity of triphylite by as much as 108 times, well above that of Li storage cathodes currently used in commercially available batteries. They postulated that in a conventional cell design, triphylite may yield the highest power density yet developed in rechargeable Li bat-teries (fig. 4). Furthermore, it has been speculated that the same doping mechanism for increasing electrical conductiv-ity in triphylite will apply to other olivine-structure phases, such as lithiophilite (Losey et al. 2004). Triphylite or LFP is currently one of the cathodic materials used in lithium ion batteries, and if things develop in a positive direction, triphylite-lithiophilite may become much more common (through synthesis) than Earth’s pegmatites would lead us to believe. Another mineral currently used as cathodic material in lithium ion batteries is the manganese-oxide birnessite. Ramsdellite, also a manganese oxide, is used as a cathode in Zn-alkaline batteries.

The analytical skills that mineralogists learn (e.g., X-ray diffraction, electron diffraction and imaging, and spectros-copies of many types) and their ability to deal with com-plex structures and chemistries (such as those of zeolites and

Figure 2. Top: Schematic of the Cool-System KEG GmbH “CoolKeg®.” Bottom: One of the many alumino-silicate minerals of the zeolite family, natrolite, from Dayton area quarry, near Dayton, Washington. Jeffrey M. Schwartz specimen and photo. The specimen measures 1.5 × 1.5 × 1 inches.

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Examples of materials mineralogy, past, present, and potential future.

Mineral,mineral-group, orstructure-type Chemistry Uses Properties utilized

Apatite (and apatite- (Ca,Mn,REE)5 Phosphors, lasers Optical emissiongroup 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

Birnessite (and (Na0.3Ca0.1K0.1)(Mn4+, Cathodes in Li-ion batteries Electrical and ion conductivityother manganese Mn3+)2O4 · 1.5 H2Ooxides)

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

Corundum Al2O3 with trace Cr3+ 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)8O20(OH,F)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., BaTiO3 and Pb[ZrxTi1-x]O3 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

356 ROCKS & MINERALS

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 products can be found on their website: http://www.minerals tech.com/about-mti/mti-video/.

I hope that this column has made it clear that materials mineralogy, along with environmental mineralogy, biomin-eralogy, and even good old geology, has great and enduring

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.

Figure 4. Top: Schematic of a lithium-ion battery with a synthetic triphylite cathode (a.k.a. lithium iron phosphate or olivine, because it is isostructural with olivine) and a synthetic graphite anode that is intercalated with lithium ions. Bottom: Triphylite in a matrix of microcline, quartz, and muscovite, Chandlers Mills, New Hampshire. Ken Larsen photo, courtesy of Smithsonian Institution (NMNH specimen #R9228).

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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.

ACKNOWLEDGMENTSI 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.

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