Download Garnet: Common Mineral, Uncommonly Useful

Garnet: Common Mineral,
Uncommonly Useful
Garnet crystals are
not only beautiful, but
they can contain a vast
storehouse of information about the evolving
Earth. PHOTO COURTESY
OF G EORGE R OSSMAN AND
MARK GARCIA
Ethan F. Baxter1, Mark J. Caddick 2, and Jay J. Ague3,4
1811-5209/13/0009-415$2.50
DOI: 10.2113/gselements.9.6.415
G
arnet 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 ultramafic 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 influence, a diverse suite of tectonic, metamorphic,
and mantle processes, making it uncommonly useful in geoscientific 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 fabricated for sophisticated laser, magnetic, and ion-conducting technologies.
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 geoscientific 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
KEYWORDS : garnet, mantle, crust, metamorphism, geothermobarometry,
(i.e. it is uncommonly useful). The
geochronology, technology
articles in this issue provide an
appreciation of the role of garnet
GARNET IS EVERYWHERE
from its place in the deep Earth, up through the crust,
The dark red crystals that frequently adorn common and to its applications in society. Wood, Kiseeva, and
Matzen begin with a discussion of the largest reservoir
mica schists are garnet (FIG. 1A). The purple-red hue that
of garnet in the planet—the mantle—where the mineral
sometimes decorates the crests and troughs of wave ripples
has profound influence over geodynamic and geochemat the beach or concentrates in deep red bands and rivulets
after a winter storm is the result of millions of garnet ical processes. Caddick and Kohn outline the role of
grains (FIG. 1B). A dazzling green gemstone that might be garnet in the metamorphic rocks of the crust, including
its use as a monitor of evolving metamorphic conditions
mistaken for an emerald is really a garnet (FIG. 1D). That
red woodworking sandpaper on the workbench and the and underlying tectonic processes. Baxter and Scherer
discuss the growing field of garnet geochronology, whose
red side of a common “emery” board are covered with
temporal resolution permits us to know more than just
garnet grains (FIG. 1F). Garnet has even been documented
“when” garnet grows (and the timing of processes that
in meteorites (e.g. Krot et al. 1998) and in association with
may be linked to it), but also “how fast” and “for how
microbial life (Ménez et al. 2012). Indeed, garnet is one of
long.” Ague and Carlson showcase the use of garnet
the best-known minerals in the Earth and is particularly
notable for its commonality in a wide range of environ- crystals to constrain the kinetics of metamorphic processes,
such as mineral nucleation, the approach to equilibrium,
ments, from igneous and metamorphic to sedimentary,
and thermal evolution. Geiger reviews how garnet crystal
from the mantle to the crust, and from nature to industry.
Most of Earth’s garnet occurs as a primary ingredient of the chemistry and structure give rise to macroscopic properties,
including those that have driven technological applicaupper mantle. However, with the exception of xenoliths
tions of synthetic garnets. Last, Galoisy writes about the
and scarce, exhumed sections of mantle lithosphere (e.g.
Van Roermund and Drury 1998; Keshav et al. 2007; FIG. 1C), cultural and historical relevance of garnet, while describing
garnet is rarely observed in this context. In the crust, garnet different gem varieties and the underlying crystal chemistry
is a common constituent of metamorphic rocks derived from that creates a rainbow of colors.
almost any protolith, from lower greenschist facies rocks
to ultrahigh-temperature (UHT) granulites and ultrahighWHAT IS GARNET?
pressure (UHP) eclogites. Garnet can crystallize in igneous According to the updated garnet nomenclature published
rocks, such as peraluminous granites. Due to its density by Grew et al. (2013), “the garnet supergroup includes all
1 Department of Earth & Environment, Boston University
675 Commonwealth Avenue, Boston, MA 02215, USA
E-mail: [email protected]
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
E LEMENTS , V OL . 9,
PP.
415–419
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 [SiO44– ] bound together by other cations). Its
general formula is X3Y2 Si3O12 , where X is an eightfoldcoordinated site most commonly filled 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
415
D ECEMBER 2013
A
B
C
D
E
F
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 G EORGE
ROSSMAN (A), EVELYN M ERVINE (B), GRAHAM PEARSON (C), WIMON
MANOROTKUL /PALAGEMS.COM (D), AND SCIENTIFIC MATERIALS CORP. (E)
FIGURE 1
TABLE 1
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 sufficiently 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 atomscale 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 significant amounts of heavy rare
earth elements, allowing for the identification of a “garnet
signature” in the source of mantle melting (e.g. Wood et al.
2013) and for sufficient 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 USEFUL
Just 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 quantifying the scientific usefulness, or frequency of application.
E LEMENTS
Some important garnet end-member compositional
names and abbreviations
Almandine
Fe3Al2Si3O12
Andradite
Grossular
Ca3Al2Si3O12
Majorite
Ca3Fe2Si3O12
Mg3 (MgSi)Si3O12
Pyrope
Mg3Al2Si3O12
Spessartine
Mn3Al2Si3O12
Uvarovite
Ca3Cr2Si3O12
YAG
Synthetic yttrium
aluminum garnet
Y3Al2 Al3O12
YIG
Synthetic yttrium
iron garnet
Y3Fe2Fe3O12
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 five 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 highestscoring topic minerals include those with important
industrial or technological applications. Garnet is thus
unusual in providing both geoscientific value and industrial, 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 significant attention in recent years (see Geiger 2013 for a discussion of this
and other technological applications). These applications
are followed in number by geoscientific 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
416
D ECEMBER 2013
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
significant (e.g. Hacker et al. 2003). For example, the high
density of garnet-rich eclogites creates the primary “slabpull” 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 mafic
material from the base of the continental crust, contributing 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 significant 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”
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.
FIGURE 2
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 “diffusion,” 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 INTERIOR
Garnet 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 discontinuity. 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
E LEMENTS
Most of the garnet we see at the surface derives from
metamorphic rocks. Garnet may form in rocks that are
sufficiently rich in Al (or Fe3+ or Cr) and in many metamorphic contexts (i.e. contact, regional, and subductionrelated 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 lowertemperature 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 spessartine 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 field 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 hydrothermal 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 invaluable 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 reflect rotation of growing
garnet crystals during tectonic deformation (for a discussion, see Johnson 1993). In many cases, garnet zonation
can be disturbed by cracking, retrogression, fluid processes,
or thermally activated diffusion, but it still retains records
of those specific processes (e.g. Angiboust et al. 2012; Ague
and Carlson 2013; FIG. 4B). Garnet growth and intracrystalline 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 SURFACE
While 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
417
D ECEMBER 2013
communities within cavities in low-temperature hydroandradite (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
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
FIGURE 3
sands and alluvial deposits may be sufficiently concentrated 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
30
A
A
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
influencing 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 significant 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 significant
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 polarizers; 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
20 PRO
10
B
B
Garnet zonation records
changing conditions.
(A) Oscillatory zoning of aluminum
in a hydrothermal Ca–Cr–Fe3+
garnet, refl ecting subtle changes in
hydrothermal fluid composition.
MAP COURTESY OF CHARLES G EIGER.
(B) Magnesium zonation in an eclogitic 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 reflecting infiltration
of meteoric fluids during hydrothermal 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 4
$O
—P
0J
—P
C
C
D
D
*DUQHWDJH0D
28
26
Fe
0J
24
PP
Laser
bulk
į18O(Grt)
3.5
1.5
-0.5
-2.5
-4.5
20
Ca
0Q
0.5
1.0
Fe3+Ȉ)H 0.15
FF
Stable fluid Meteoric
composition flooding
200
'LVWDQFH—P
600
—P
0.10
Rebound
1000
0.05
X’
X
*DUQHWUDGLXVFP
1.5
2.0
2.5
EE
E LEMENTS
418
D ECEMBER 2013
the infi ltration of external fluid in metamorphic or hydrothermal 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).
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E LEMENTS
ACKNOWLEDGMENTS
We 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 figure 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,
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