ELEMENTS, VOL. 9, PP. 415–419 DECEMBER 2013415

1811-5209/13/0009-415$2.50 DOI: 10.2113/gselements.9.6.415

Garnet: Common Mineral, Uncommonly Useful

GARNET IS EVERYWHEREThe dark red crystals that frequently adorn common mica schists are garnet (FIG. 1A). The purple-red hue that sometimes decorates the crests and troughs of wave ripples at the beach or concentrates in deep red bands and rivulets after a winter storm is the result of millions of garnet grains (FIG. 1B). A dazzling green gemstone that might be mistaken for an emerald is really a garnet (FIG. 1D). That red woodworking sandpaper on the workbench and the red side of a common “emery” board are covered with garnet grains (FIG. 1F). Garnet has even been documented in meteorites (e.g. Krot et al. 1998) and in association with microbial life (Ménez et al. 2012). Indeed, garnet is one of the best-known minerals in the Earth and is particularly notable for its commonality in a wide range of environ-ments, from igneous and metamorphic to sedimentary, from the mantle to the crust, and from nature to industry. Most of Earth’s garnet occurs as a primary ingredient of the upper mantle. However, with the exception of xenoliths and scarce, exhumed sections of mantle lithosphere (e.g. Van Roermund and Drury 1998; Keshav et al. 2007; FIG. 1C), garnet is rarely observed in this context. In the crust, garnet is a common constituent of metamorphic rocks derived from almost any protolith, from lower greenschist facies rocks to ultrahigh-temperature (UHT) granulites and ultrahigh-pressure (UHP) eclogites. Garnet can crystallize in igneous rocks, such as peraluminous granites. Due to its density

and relative resistance to surface weathering processes, garnet is also a common detrital phase in the heavy-mineral fraction of sediments and sedimentary rocks. Finally, garnet is a useful mineral in geoscientifi c inquiry and for its role in industrial, technological, and societal contexts.

In this issue of Elements, we portray some of the richness and variety of garnet, focusing on its widespread geological occurrence (i.e. it is a common mineral) and its remarkably broad applications (i.e. it is uncommonly useful). The articles in this issue provide an appreciation of the role of garnet

from its place in the deep Earth, up through the crust, and to its applications in society. Wood, Kiseeva, and Matzen begin with a discussion of the largest reservoir of garnet in the planet—the mantle—where the mineral has profound infl uence over geodynamic and geochem-ical processes. Caddick and Kohn outline the role of garnet in the metamorphic rocks of the crust, including its use as a monitor of evolving metamorphic conditions and underlying tectonic processes. Baxter and Schererdiscuss the growing fi eld of garnet geochronology, whose temporal resolution permits us to know more than just “when” garnet grows (and the timing of processes that may be linked to it), but also “how fast” and “for how long.” Ague and Carlson showcase the use of garnet crystals to constrain the kinetics of metamorphic processes, such as mineral nucleation, the approach to equilibrium, and thermal evolution. Geiger reviews how garnet crystal chemistry and structure give rise to macroscopic properties, including those that have driven technological applica-tions of synthetic garnets. Last, Galoisy writes about the cultural and historical relevance of garnet, while describing different gem varieties and the underlying crystal chemistry that creates a rainbow of colors.

WHAT IS GARNET?According to the updated garnet nomenclature published by Grew et al. (2013), “the garnet supergroup includes all minerals isostructural with garnet regardless of what elements occupy the four atomic sites.” However, in common natural occurrences, garnet is a silicate mineral belonging to the nesosilicate group (i.e. it is constructed of isolated silicon tetrahedra [SiO4

4–] bound together by other cations). Its general formula is X3Y2Si3O12, where X is an eightfold-coordinated site most commonly fi lled by a solid solution of divalent Fe, Mg, Ca, and Mn, and Y is a sixfold-coordinated site typically fi lled by trivalent Al (i.e. the aluminosilicate

Garnet is a widespread mineral in crustal metamorphic rocks, a primary constituent of the mantle, a detrital mineral in clastic sediments, and an occasional guest in igneous rocks. Garnet occurs in ultramafi c to

felsic bulk-rock compositions, and its growth and stability span from <300 to 2000 ºC and from atmospheric pressure to 25 GPa. More than merely a constituent of these rocks, garnet possesses chemical and physical attributes allowing it to record, and infl uence, a diverse suite of tectonic, metamorphic, and mantle processes, making it uncommonly useful in geoscientifi c inquiry. Because of its myriad colors, garnet has been used through the ages in jewelry. More recently, nonsilicate crystals with the garnet structure have been fabri-cated for sophisticated laser, magnetic, and ion-conducting technologies.

KEYWORDS: garnet, mantle, crust, metamorphism, geothermobarometry, geochronology, technology

Ethan F. Baxter1, Mark J. Caddick2, and Jay J. Ague3,4

1 Department of Earth & Environment, Boston University 675 Commonwealth Avenue, Boston, MA 02215, USAE-mail: efb@bu.edu

2 Department of Geosciences, Virginia Tech 4044 Derring Hall, Blacksburg, VA 24061, USA

3 Department of Geology and Geophysics, Yale University P.O. Box 208109, New Haven, CT 06520-8109, USA

4 Peabody Museum of Natural History, Yale University New Haven, CT 06511, USA

Garnet crystals are not only beautiful, but they can contain a vast storehouse of informa-

tion about the evolving Earth. PHOTO COURTESY

OF GEORGE ROSSMAN AND MARK GARCIA

ELEMENTS DECEMBER 2013416

garnets) or sometimes by Fe3+ or Cr. The formulas and names of some common species are given in TABLE 1. Many additional end-member species (32 in total) and elemental substitutions exist in natural garnets; these are reviewed in Grew et al. (2013) and several are discussed in Geiger (2013 this issue) and Wood et al. (2013 this issue). Synthetic crystals with the garnet structure (e.g. YIG and YAG; FIG. 1E, TABLE 1) have also been fabricated for industrial use. While such synthetic compositions do not occur naturally (at least not as suffi ciently pure end-members), these crystalline oxide materials are garnet in the structural sense and thus share certain key properties with common silicate garnets.

Garnet’s wide-ranging chemical composition and its atom-scale structure manifest themselves in important and/or desirable physical and optical properties, such as isometric crystal structure, high bulk modulus, high density (up to 4.5 g/cm3 for almandine), hardness (7.5), magnetism, and a diverse range of vibrant colors. Garnet’s large edge-sharing sites can incorporate signifi cant amounts of heavy rare earth elements, allowing for the identifi cation of a “garnet signature” in the source of mantle melting (e.g. Wood et al. 2013) and for suffi cient enrichment of radioactive lutetium (over daughter hafnium) and samarium (over daughter neodymium) to make garnet useful for geochronology (e.g. Baxter and Scherer 2013 this issue). Synthetic oxide garnets may possess properties making them unique and useful in several applications; such properties include magnetism (for use in electronics), lasing ability (for use in lasers), and ion conduction (for use in batteries) (see Geiger 2013).

UNCOMMONLY USEFULJust how useful is garnet in comparison to other minerals? Usefulness is of course largely subjective, though modern search engines provide a means (albeit imperfect) of quanti-fying the scientifi c usefulness, or frequency of application.

At the time of writing, the Web of Science indicated over 26,000 published papers (since 1965, when the Web of Science database begins) that include the “topic” of garnet. This places garnet (as a “topic”) behind only fi ve other minerals or broad mineral groups that were searched for (clay, graphite, quartz, diamond, zeolite) and ahead of important and/or common minerals like feldspar, calcite, zircon, and olivine. It is noteworthy that these highest-scoring topic minerals include those with important industrial or technological applications. Garnet is thus unusual in providing both geoscientifi c value and indus-trial, technological, and cultural value.

In what ways has garnet been used or applied? A Web of Science search for papers that include the topic of garnet plus one other term yields the greatest number for “garnet” plus “metamorphism” and “garnet” plus “mantle/magma/melt,” driving home the importance of garnet in the evolving crust and mantle and in metamorphic and igneous rocks. In terms of uses, the list is topped by established industrial applications, including “garnet” plus “laser” and “garnet” plus “magnetism.” A Web of Science topic search for “YAG” (yttrium aluminum garnet, important in laser technology) alone yields over 40,000 papers! Emerging technologies such as Li-stuffed garnets and their use in rechargeable battery technology have begun to attract signifi cant atten-tion in recent years (see Geiger 2013 for a discussion of this and other technological applications). These applications are followed in number by geoscientifi c uses, including closely related “partitioning” and “geo/thermo/barometry.” While the former includes the role of garnet in controlling magma compositions, geothermobarometric applications of garnet, mainly involving the calibrated exchange of Fe and Mg between garnet and other minerals in the mantle and crust, comprise the top three most cited papers on the

FIGURE 1 Garnet in its many settings, both natural (A–C) and societal (D–F). (A) A euhedral, ~3 cm garnet crystal in

a metamorphic schist from Wrangell, Alaska. (B) Garnet beach sand near Nome, Alaska. (C) Garnet harzburgite from the Boshoff Road Dumps, Kimberley, South Africa. The garnet crystals are up to 3 mm in diameter. (D) Demantoid garnet gemstones. (E) Neodymium-YAG rods for use in laser technology. (F) Garnet as an abrasive in common sandpaper. PHOTOS COURTESY OF GEORGE ROSSMAN (A), EVELYN MERVINE (B), GRAHAM PEARSON (C), WIMON MANOROTKUL/PALAGEMS.COM (D), AND SCIENTIFIC MATERIALS CORP. (E)

A

D

B

E

C

F

TABLE 1 Some important garnet end-member compositional names and abbreviations

Almandine Fe3Al2Si3O12 Andradite Ca3Fe2Si3O12

Grossular Ca3Al2Si3O12 Majorite Mg3(MgSi)Si3O12

Pyrope Mg3Al2Si3O12 Spessartine Mn3Al2Si3O12

Uvarovite Ca3Cr2Si3O12

YAGSynthetic yttrium aluminum garnet

Y3Al2Al3O12 YIGSynthetic yttrium iron garnet

Y3Fe2Fe3O12

ELEMENTS DECEMBER 2013417

topic of “garnet”: Ellis and Green (1979), Ferry and Spear (1978), and Brey and Kohler (1990). The only other paper registering over 1000 citations with “garnet” as a topic is Christensen and Mooney (1995), which illuminates the role of garnet in the physicochemical properties of the deep continental root as manifested in seismic velocity data. “Garnet” plus “spectroscopy,” “geochronology,” and “diffu-sion,” which are key topics covered in this issue by Geiger (2013), Galoisy (2013), Baxter and Scherer (2013), and Ague and Carlson (2013), round out the most frequently published application areas of garnet.

Predating Web of Science citation metrics, since the fi rst commercial development of garnet quarries at Gore Mountain in New York State in 1878 (FIG. 2), the primary industrial application of natural garnet has been as an abrasive. Uses have included abrasive powders, water-jet cutting, abrasive blasting (garnet replaced quartz in the late 1980s as a sandblasting medium due to health concerns over airborne crystalline silica), and garnet sandpaper (Olson 2006). Finally, garnet has been a popular gemstone for thousands of years due to its many colors, its hardness, its commonality, and its luster (Galoisy 2013).

GARNET IN EARTH’S DEEP INTERIORGarnet is one of the primary constituents of the deep Earth, occurring in garnet granulites and pyroxenites at the base of the crust and throughout the upper mantle, where it is the primary storehouse of aluminum. Garnet transforms to perovskite and disappears below the 670 km seismic discon-tinuity. In this context, garnet is stable at temperatures up to almost 2000 °C and pressures as high as ~25 GPa. In the mantle, garnet’s structure can accommodate diverse

elemental substitutions, including Si in the Y site and Na in the X site, leading to Si-enriched, Al-poor compositions collectively known as majorite (see Wood et al. 2013). The incorporation of Fe3+ in garnet has been used to monitor the oxygen fugacity ( fO2) of the mantle (see later discussion and Berry et al. 2013). Garnet’s physical properties are also signifi cant (e.g. Hacker et al. 2003). For example, the high density of garnet-rich eclogites creates the primary “slab-pull” driving force for plate motion as subducted oceanic crust transforms to eclogite and descends into the mantle. Dense garnet pyroxenites in the roots of continents may similarly lead to delamination or “drips” of dense mafi c material from the base of the continental crust, contrib-uting to the long-term stability and bulk chemistry of the continents (e.g. Ducea 2011). The high density and bulk modulus of garnet can be signifi cant in modifying the seismic wave velocities that are useful in the imaging of Earth’s layered interior (e.g. Wood et al. 2013).

GARNET IN THE CRUST—A TECTONIC “TAPE RECORDER”Most of the garnet we see at the surface derives from metamorphic rocks. Garnet may form in rocks that are suffi ciently rich in Al (or Fe3+ or Cr) and in many metamor-phic contexts (i.e. contact, regional, and subduction-related metamorphism). It usually forms at temperatures above ~400 °C and pressures above ~0.4 GPa (e.g. Spear 1993; Caddick and Kohn 2013 this issue), though lower- temperature Mn- and Ca-rich garnets have been reported in nature (e.g. at ≤300 °C and 0.1–0.2 GPa; Coombs et al. 1977; Theye et al. 1996; Ménez et al. 2012), and spessar-tine garnet has been crystallized experimentally from melt at atmospheric pressure (e.g. Van Haren and Woensdregt 2001). Garnet can persist up to UHT and UHP conditions within the hottest orogens (e.g. >1000 °C; Harley 1998) or the deepest subducted materials (well into the diamond stability fi eld at more than ~4 GPa; e.g. Schertl and O’Brien 2013). Garnet may also form as a consequence of anatexis (i.e. partial melting at high metamorphic temperatures) and occurs as an igneous phase in some S-type and peraluminous granites, resulting from the melting of Al-rich sedimentary rocks (e.g. Clemens and Wall 1981). In addition, calcic garnets (grossular and andradite) may form in calcsilicate rocks, including skarn-type contact metamorphic rocks (e.g. D’Errico et al. 2012) and in hydro-thermal systems (e.g. Ménez et al. 2012). Garnet crystals frequently grow in a simple concentric pattern, not unlike tree rings, such that the chemical, isotopic, textural, and inclusion records of these zoned crystals can yield invalu-able information about the evolution of Earth’s crust (e.g. FIGS. 3, 4), sometimes spanning millions or even tens of millions of years (e.g. Skora et al. 2009; Pollington and Baxter 2010; FIG. 4D). Fantastic spiral or “snowball” garnet (FIG. 3) has been interpreted to refl ect rotation of growing garnet crystals during tectonic deformation (for a discus-sion, see Johnson 1993). In many cases, garnet zonation can be disturbed by cracking, retrogression, fl uid processes, or thermally activated diffusion, but it still retains records of those specifi c processes (e.g. Angiboust et al. 2012; Ague and Carlson 2013; FIG. 4B). Garnet growth and intracrystal-line zonation are increasingly used to constrain the timing, duration, and kinetics of tectonometamorphic processes (e.g. Ague and Carlson 2013; Baxter and Scherer 2013; FIG. 4C, D).

GARNET AT EARTH’S SURFACEWhile garnet is not known to be an authigenic phase, it may be found in the heavy-mineral fraction in clastic sediments and sedimentary rocks. Garnet in some beach

FIGURE 2 Barton Mine at Gore Mountain, New York State, USA, where garnet has been mined since 1878. Deep red

garnet crystals are suspended in black amphibolite. Crystals nearly 1 meter in diameter have been reported here. Coauthor Caddick for scale.

ELEMENTS DECEMBER 2013418

sands and alluvial deposits may be suffi ciently concen-trated to be mined as an abrasive (e.g. Olson 2006). Given its large compositional range, detrital garnet has been used by sedimentologists as a powerful provenance tracer (e.g. Morton 1985), including use as an indicator mineral in diamond exploration (Dawson and Stephens 1975). A recent report on deep-sea serpentinites within shallow oceanic crust reveals the presence of past microbial

communities within cavities in low-temperature hydro-andradite (Ménez et al. 2012). Garnet in this context appears well suited for colonization by microbial life and may have been an important player in early hydrothermal, prebiotic environments.

GARNET AND THE EARTH’S VOLATILE BUDGET Garnet is a nominally anhydrous mineral and water (hydroxyl) does not appear in garnet’s ideal formula. However, garnet plays a major role in monitoring and infl uencing the Earth’s water cycle, as well as the cycling of other important volatiles, like oxygen. In the mantle, garnet can be a storehouse of a signifi cant amount of water as a trace constituent, with concentrations up to 0.1 wt% (e.g. Bell and Rossman 1992; Mookherjee and Karato 2010). In the crust, garnet occasionally incorporates signifi cant hydroxyl into its tetrahedral structure (e.g. “hydrogarnet”; Rossman and Aines 1991; Grew et al. 2013). In this case, hydroxyl can reduce the symmetry of the usually isometric garnet, which changes its crystallographic properties from isotropic (black in transmitted light under crossed polar-izers; e.g. FIG. 3) to anisotropic, imparting a subtle play of dark to light gray colors under crossed polarizers. The growth of garnet during metamorphism also typically heralds the major dehydration of hydrous minerals such as chlorite, mica, amphibole, and lawsonite, which are reactants in many garnet-forming reactions (e.g. Spear 1993; Baxter and Caddick 2013). Garnet growth can record

0.05

0.10

0.15Fe3+Laserbulk

Stable fluidcomposition

Meteoricflooding Rebound

200 600 1000

3.5

-4.5-2.5-0.51.5

18O(Grt)

X X’

1.00.5 2.0 2.51.520

24

26

28

Fe

Ca

20 1030

F

E

DC

A B

F

E

DC

A B

FIGURE 4 Garnet zonation records changing conditions.

(A) Oscillatory zoning of aluminum in a hydrothermal Ca–Cr–Fe3+ garnet, refl ecting subtle changes in hydrothermal fl uid composition. MAP COURTESY OF CHARLES GEIGER. (B) Magnesium zonation in an eclog-itic garnet showing healed fractures (light blue-green, cutting across darker blue) related to subduction zone seismicity (from Angiboust et al. 2012). (C) Complex major element zonation revealing a growth morphology indicative of garnet growth far from chemical equilibrium (Wilbur and Ague 2006). (D) Age-zoned garnet revealed by geochronology of color-contoured compositional growth zones showing a 7.5 My duration and pulses (gray bands) of accelerated garnet growth (Pollington and Baxter 2010). (E) Oxygen isotope zonation (δ18O) in skarn garnet refl ecting infi ltration of meteoric fl uids during hydro-thermal mineralization (D’Errico et al. 2012). (F) Fe3+/ΣFe zonation measured by microXANES, refl ecting changing oxygen fugacity due to mantle metasomatism (Berry et al. 2013).

FIGURE 3 Rotated spiral garnet (~1 cm across) in thin section. The photo was taken in transmitted light under

crossed polarizers such that the garnet appears black (isotropic). The sample is from the garnet zone below the Main Central Thrust, Nepal Himalaya. IMAGE COURTESY OF SCOTT JOHNSON

A

D

B

E

C

F

ELEMENTS DECEMBER 2013419

the infi ltration of external fl uid in metamorphic or hydro-thermal systems, for example, in its major element zoning (FIG. 4A) or in its oxygen isotope composition (e.g. Kohn et al. 1993; D’Errico et al. 2012; FIG. 4E), which can now be measured at high spatial resolution using a secondary ion microprobe (SIMS) (e.g. Page et al. 2010). Ongoing debate about the fO2 of the mantle has been aided by efforts to link the measurement of the Fe3+/Fe2+ ratio of garnet in mantle xenoliths to fO2 (see Wood et al. 2013). Recent work has illuminated possible mantle fO2 variations based on microXANES mapping of Fe3+/Fe2+ in mantle garnet (Berry et al. 2013; FIG. 4F).

ACKNOWLEDGMENTSWe thank Barbara Dutrow and Lawford Anderson for providing thoughtful reviews of this article. We also thank everyone who contributed to this issue of Elements, including George Rossman and Ed Grew who offered generous support, discussions, and fi gure material, all of the authors and reviewers, and especially Georges Calas and Pierrette Tremblay of the Elements editorial team, without whom this issue would not have been possible. EFB, MJC, and JJA acknowledge support from NSF Grants EAR-1250497, EAR-1250470, and EAR-12 50269, respectively.

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