GOFF AND LACKNER: CARBON DIOXIDE SEQUESTERING 89

Carbon Dioxide Sequestering Using

Ultramafic Rocks
FRASER GOFF* and K. S. LACKNER‡
* EES-1, MS D462, Los Alamos National Laboratory, Los Alamos, NM 87545
‡ T-3, MS B216, Los Alamos National Laboratory, Los Alamos, NM 87545

ABSTRACT •
Fossil fuels continue to provide major sources of energy to the
of magnitude greater than is the annual production of CO2
by volcanoes and metamorphic processes and an order of
modern world even though global emissions of CO2 are pres- magnitude greater than is the consumption rate of CO2 by
ently at levels of $19 gigatons/yr. Future antipollution measures natural geologic processes (Kerrick et al., 1995). During the
may include sequestering of waste CO2 as magnesite (MgCO3) last two centuries, CO2 levels in the atmosphere have in-
by processing ultramafic rocks. Common ultramafic rocks react creased z30% (Ramanathan, 1988) raising legitimate con-
easily with HCl to form MgCl2 which is hydrolyzed to form cerns about global warming and the terrestrial carbon cycle
Mg(OH)2. CO2 would be transported by pipeline from a fossil (Sabine et al., 1997; Weart, 1997).
fuel power plant to a sequestering site and then reacted with Various schemes have been devised to reduce CO2 emis-
Mg(OH)2 to produce thermodynamically stable magnesite. sions while allowing continued consumption of fossil fuels
Huge ultramafic deposits consisting of relatively pure Mg-rich (Blok et al., 1992). In one of these schemes, Lackner et al.
silicates exist throughout much of the world in ophiolites and, to (1995) described two chemical processes to sequester CO2 by
a lesser extent, in layered intrusions. Peridotites and associated formation of carbonate minerals. Sequestering CO2 involves
serpentinite are found in discontinuous ophiolite belts along reaction with divalent cations (principally Mg and/or Ca) de-
both continental margins of North America. Serpentinites and rived from natural mineral deposits by either direct carbon-
dunites comprise the best ores because they contain the most Mg ation at high temperature or reaction in aqueous solution. Both
by weight (35 to 49 wt-% MgO) and are relatively reactive to sequestering processes are thermodynamically favorable, but
hot acids such as HCl. Small ultramafic bodies (z1 km3) can details of actual implementation require much further investi-
potentially sequester z1 gigatons of CO2 or z20% of annual gation (Lackner et al., 1995, 1997; Butt et al., 1996).
U.S. emissions. A single large deposit of dunite (z30 km3) Mg and Ca comprise z2.0 and 2.1 mol-% of the earth’s
could dispose of nearly 20 years of current U.S. CO2 emissions. crust, respectively, primarily bound in silicate minerals
The sequestering process could provide Mg, Si, Fe, Cr, Ni, and (Brownlow, 1979). Although molar abundances are similar,
Mn as by products for other industrial and strategic uses. Be- Mg silicates contain more reactive material per ton of rock
cause “white” asbestos (chyrsotile) is a serpentine mineral, CO2 due to the lower molecular weight of Mg. Two types of Mg
sequestering could dispose of some waste asbestos. The cost and silicate minerals occur in relatively pure deposits and have
environmental impact of exploiting ultramafic deposits must be thermodynamic and chemical properties desirable for CO2
weighed against the increased costs of energy and benefits to the waste processing. These minerals, forsterite and serpentine,
atmosphere and climate. occur as peridotites and serpentinites in ultramafic rocks.
The purpose of this article is to describe briefly CO2 seques-
Key Words: carbon dioxide, environmental geology, geochemis- tering by magnesite formation, discuss the distribution and
try, global warming, mining, ultramafic rocks, waste disposal. geochemistry of ultramafic deposits, estimate the CO2 se-
questering potential of some U.S. resources, and consider
INTRODUCTION •
Numerous resource evaluations show that worldwide re-
some industrial and environmental impacts that would re-
sult from large-scale ultramafic mining.
serves of fossil fuels can provide mankind’s energy needs
for many centuries (e.g., United Nations, 1995). The major CO2 SEQUESTERING IN SOLID FORM
obstacle to consuming these resources is growing emissions In the presence of CO2, serpentine and forsterite are both
of CO2 to the atmosphere. The United States produces z5 thermodynamically unstable. Given time, they are trans-
gigatons of CO2 annually, which is z25% of the present formed into magnesite plus additional materials that contain
global output of $19 gigatons. These quantities are an order the silica and water that made up the stoichiometric balance
of the original minerals. These carbonation reactions are
© 1998, Environmental Geosciences, 1075-9565/98/$10.50/0
Environmental Geosciences, Volume 5, Number 3, 1998 89–101 exothermic and lower the free energy of the system. The en-
Volume , Number , 199
Copyright © 2013 AAPG Division of Environmental Geosciences
 90 ENVIRONMENTAL GEOSCIENCES

ergy release is 64 and 95 kJ per mol of CO2 for serpentine 2508C to form Mg(OH)Cl and HCl (Kelley, 1945). The
and forsterite, respectively (Robie et al., 1979). This should Mg(OH)Cl disassociates into Mg(OH)2 and MgCl2 in aque-
be compared with the heat of combustion of 394 kJ that is ous solution. In this cycle, it is possible to recover virtually
released in the formation of 1 mol of CO2. Because they are all of the HCl. The most serious loss mechanism is the for-
thermodynamically favored, magnesite and silica are com- mation of alkali chlorides. However, the alkali content of
mon in serpentinized ultramafic rocks (Barnes et al., 1973; peridotite and serpentinite rocks is very small, usually #0.5
O’Hanley, 1996). The formation of these silica-carbonate wt-% (see below). A major design concern is to accomplish
rocks is promoted by natural CO2-rich fluids permeating the these processing steps by using only energy that is available
mineral deposits. The resulting magnesite is stable and is from heat sources within the processing scheme.
not likely to release the bound CO2 again. Fortunately, the carbonation reaction is quite exothermic
One approach to CO2 disposal is to accelerate this natural and, in principle, can provide all the energy necessary to re-
process and form magnesite on a rapid scale. The resulting cover the HCl. To harness this energy, the carbonation is
mineral is environmentally safe and provides a permanent performed in a gas-solid reaction between Mg(OH)2 and
storage for the large volumes of CO2 resulting from power CO2. In the aqueous alternative, the reaction rates may be
generation. The cost of such a process can be held low be- faster, but because of the high degree of dilution, the heat of
cause the reaction is exothermic and, if properly designed, reaction would be lost for practical purposes.
does not require the additional input of energy. The CO2 would be pipelined to the disposal site from a
The carbonation of serpentinite is broken up into several power plant. Separating and shipping of CO2 has similari-
steps (Figure 1). First, the mineral ore is mined and ground ties with all other CO2 disposal methods. It is not discussed
to a powder. To improve the carbonation reaction kinetics, further here because its implementation will differ greatly
the magnesium is extracted from the ore and put in the form between various plant designs. The emphasis of this present
of magnesium hydroxide. The extraction is accomplished work has been on the downstream disposal process. In par-
with HCl, which dissolves the mineral forming MgCl2, and ticular, it has been demonstrated experimentally that the
silica, which is readily precipitated along with iron oxide gas-solid carbonation reaction goes virtually to completion
that was present in the original minerals. This process was in ,30 min at pressures of 50 bar and temperatures of z500
described in the 1940s and 1950s when Mg shortages were to 6008C. Such a process would already be economically
driving research into alternative extraction technologies feasible if one takes advantage of the fact that the pipelined
(Houston, 1945; Barnes et al., 1950). The resulting MgCl2, CO2 is already pressurized. However, based on experience
which is always hydrated, can be hydrolyzed at 200 to in the analogous sulfation chemistry of calcium oxide, it is

FIGURE 1: Processing stream for CO2 dis-

posal (Lackner et al., 1997); the upper
branch shows the carbon flow from energy
source (coal mine) through the power plant
and the sequestration unit. The large amount
of earth moved reflects the overburden of a
typical surface mine. It is assumed that the
CO2 gas is delivered to the disposal site
through a pipeline. The bottom half of the di-
agram represents the disposal process (see
text). Kt, Kilotons.
 GOFF AND LACKNER: CARBON DIOXIDE SEQUESTERING 91
expected that a more careful investigation of the reaction
mechanisms will lead to a significantly improved imple-
mentation of the reaction (Lackner et al., 1997).
The cost of mining and grinding the ore can be roughly
estimated from the analogous processing steps in copper
mining. It would amount to z$8/ton of CO2. Very simple
estimates suggest that an additional $8/ton of CO2 would be
a reasonable goal to aim for in the cost of downstream pro-
cessing. To achieve such favorable costs would require that
the overall process does not demand significant amounts of
external energy in the form of heat or recompression of
gases. The $8/ton of CO2 would pay for a $300 million pro-
cessing plant, plus operating and personnel costs, plus
makeup of lost HCl. To set the scale, for a power plant with
44% conversion efficiency, $16/ton of CO2 amounts to
$0.012 per kW hr (Lackner et al., 1995).

RESOURCES
The magnesium-rich, ultramafic rocks (primarily perido-
tites and serpentinites) that are candidate ores in the seques-
tering process are distributed throughout the world. There are
at least nine types of ultramafic rock associations, but they
occur in magmatic-tectonic settings too varied to document
here (Coleman, 1977). The most voluminous and widespread
ultramafic rocks are the alpine (“metamorphic”) peridotites
that form the basal sequence of ophiolites, slabs of oceanic
crust uplifted and eroded along subduction zones and plate
boundaries. The basal peridotites represent detached slices of
the Earth’s upper mantle exposed by these tectonic processes FIGURE 2: (A) Polar projection of the world, showing generalized locations of
(Dickinson et al., 1996). Because they occur mostly along the ophiolite belts (from Coleman, 1977). (B) Locations of ophiolite belts and major lay-
ered intrusions of North America. Example ophiolite bodies mentioned in text are B,
upper plate of present and past subduction zones, ophiolites Baltimore Complex; BM, Belvidere Mountain; CM, Canyon Mountain; G, Green
Mountain; TS, Twin Sisters (from Coleman, 1977 and references in text).
are found as belts throughout most of the world, having dis-
continuously exposed dimensions of as much as 1000 3 100
km. Within North America, ophiolite belts are found along
the Appalachian mountain chain stretching from the south- (usually #8 km). The larger bodies tend to be Precambrian
east United States into Quebec and Newfoundland and along age ($650 Ma (million years ago)). Because of their great
the Cordilleran mountain chain stretching from Alaska initial volumes, these magmas cooled slowly within the
through British Columbia to California (Figure 2). Smaller crust and the first formed minerals of crystallization (prima-
belts are found in Guatemala and in the Caribbean. rily Mg-rich silicates) settled by gravity toward the bottom
When examined more closely, the basal ultramafic rocks of the intrusions. As a result, layers of peridotite as thick as
in ophiolite belts are found to be elongate ribbons and frag- a few hundred meters can be found in exposures as long as
ments that parallel regional geologic structures (Figure 3). several tens of kilometers. The largest such body is the fa-
The tectonic processes that create ophiolites and expose mous Bushveld Complex in South Africa but other well-
elongate fragments of the upper mantle are complex and known bodies occur at the Stillwater, Sudbury, and Skaer-
usually take several million years to complete (Coleman, gaard locations in North America (Figure 2). Of these three,
1977; Harper, 1984). Individual exposures of ultramafic the late Archean (2.7 Ga (billion years ago)) Stillwater
rock may occupy hundreds of square kilometers or may be Complex in Montana has the largest exposures of gravity-
as small as hand samples incorporated into fault zones. settled peridotite in North America (z48 km long; Czaman-
The second most voluminous class of ultramafic rocks ske and Zientek, 1985).
occurs in large, layered intrusions at many localities world-
wide (Hess, 1960; Cawthorn, 1997). These magma bodies CALIFORNIA STUDY REGION
generally had initial compositions similar to mantle basalt This present investigation focuses on California ultramafic
and were intruded into shallow levels of the Earth’s crust bodies (Figure 3), because their basic geology is well known,
 92 ENVIRONMENTAL GEOSCIENCES

FIGURE 3: Map of California (modified

from Jennings, 1977), showing locations of
ultramafic provinces and some example sites
described in text. The Vulcan Peak peridotite
in extreme southern Oregon is actually part
of the greater Josephine ophiolite, the largest
in North America. The ophiolites have been
deformed and exposed during prolonged
subduction of the Pacific Plate (west) be-
neath the North American Plate (east) during
late Paleozoic to early Tertiary time. Since
z30 Ma, the rocks have experienced consid-
erable right-lateral shear along the present
San Andreas transform zone.

their distribution and volume are significant, their proximity (magnesite), Mn (pyrolusite, MnO2), cinnabar (HgS), and
to population and power manufacturing centers is favorable, chromite (Hawkes et al., 1942; Bodenlos, 1950; Maddock,
and their previous exploitation is well established (Cole- 1964). The body is part of an elongate slab of ophiolite whose
man, 1996). ultramafic part (up to 300 m thick and z40 km2) is variably
California ultramafic bodies occur in four locales: the serpentinized (Evarts and Schiffman, 1982).
Coast Ranges, Big Sur, Sierra Nevada foothills belts, and The contact between ultramafic rock and underlying ma-
the Klammoth-Trinity region (Saleeby, 1982; Harper, 1984; rine deposits of the Franciscan Complex (Jurassic to
Dickinson et al., 1996). It is well beyond the scope of this Eocene) is relatively flat and sharp (Figure 4A). The ex-
article to review the age and tectonic history of each prov- treme base of the ultramafic body consists of strongly foli-
ince in detail, but each was formed during subduction- ated antigorite schist grading upward into sheared, serpenti-
related events that occurred from z300 to 50 Ma (Paleozoic nized harzburgite and local zones of dunite. The western
to Early Tertiary). Two ultramafic bodies in the Coast crest of the deposit consists of material that is only 5 to 40%
Ranges belt were chosen for preliminary study: the Del Pu- serpentinized and contains $45 mol-% MgO (Table 1). The
erto body because it contains a large mass of relatively un- east portion of the deposit is more highly serpentinized
serpentinized ultramafic rock, and the Wilbur Springs body (Maddock, 1964; Himmelberg and Coleman, 1968).
because it is mostly serpentinized. Within the deposit, cross-cutting faults are pervasive. A set
of high-angle, northwest-trending faults and fractures host
Del Puerto Ultramafic Body magnesite veins, pods, and eroded spring deposits (Bodenlos,
The Del Puerto ultramafic body lies 60 km due east of San 1950). Considerable geochemical research has shown that the
Jose in the California Coast Ranges (Himmelberg and Cole- magnesite forms from groundwater alteration of the ultrama-
man, 1968; Evarts, 1977). The ultramafic and surrounding fic rock and transport of Mg to favorable sites for precipita-
rocks are well explored as they have been mined for Mg tion (Barnes et al., 1967, 1973). Obviously, the host rocks are
 GOFF AND LACKNER: CARBON DIOXIDE SEQUESTERING 93

FIGURE 4: (A) Photograph looking north

of the west side of the Del Puerto ultramafic
body (UM) overlying marine rocks of the
Franciscan complex (F). Note sharp vegeta-
tion contrast between units. This boundary is
a thrust fault developed during late Creta-
ceous–early Tertiary subduction. Ultramafic
rocks change from highly foliated antigorite
schist at the base to relatively massive
harzburgite and dunite near the top. Fractur-
ing is pervasive and serpentinization varies
from 5 to 60% within this zone. The ultrama-
fic mass thickens to the east (right). (B) Pho-
tograph looking north of the Wilbur Springs
serpentinite along Kilpepper Creek, 1 km
west of Complexion Spring. The serpen-
tinite, which is over 100 m thick at this loca-
tion, is sheared and fractured into blocky
rubble. Only z5% of the original peridotite
minerals remain unaltered.

completely compatible with the magnesite waste that would small subdivisions and “ranchettes” of single-family homes
be generated by CO2 sequestering. have been built in San Antonio Valley on the west margin
Although the magnesite has been largely mined out, the of the ophiolite. The impact of renewed mining for carbon-
haul roads, shafts, pits, and dumps are still visible. Since ate waste disposal would be examined carefully by the local
World War II, the region has been used mostly for cattle public. Historic (and dilapidated) mining infrastructure is
ranching and hunting clubs. The ultramafic rocks are cov- visible over all of the Del Puerto body.
ered primarily with brush of manzanita and live oak with
scattered pines. The area may be reached by paved roads Wilbur Springs Serpentinite
from the west, east, and north. A small county park is lo- The Wilbur Springs serpentinite mass is located $200 km
cated on the northeastern margin of the area. More recently, NNE of San Francisco on the eastern side of the Coast
 94 ENVIRONMENTAL GEOSCIENCES

TABLE 1. Chemical compositions (major elements in wt-%) of ultramafic rocks from the Del Puerto and Wilbur Springs ultramafic
masses, California.

Del Puerto Ultramatic Body

Wilbur Springs Serpentinite
DPS-AVEd UM96-26a
Sample No. UM96-19a UMDP-AVEb 66R22c 66R20c Serpentinized Serpentinized UM96-3a UM96-13a UMWS-AVEe
Type Dunite Peridotite Dunite Harzburgite Peridotite Lherzolite Serpentinite Serpentinite Serpentinite
SiO2 38.22 40.50 6 1.89 39.0 44.9 38.8 40.8 39.75 41.73 40.6 6 1.2
TiO2 0.002 0.000 6 0.001 0.02 0.02 0.013 0.011 0.080 0.065 0.040 6 0.02
Al2O3 0.71 0.40 6 0.33 0.04 0.91 0.47 0.032 2.28 2.59 1.99 6 0.40
Fe2O3 4.54 4.10 6 1.32 2.8 0.80 3.23 5.49 6.80 4.58 5.06 6 1.3
FeO 4.68 4.25 6 1.15 5.0 7.0 4.74 1.88 1.38 3.53 2.94 6 1.0
MnO 0.159 0.134 6 0.11 0.11 0.12 0.13 0.14 0.147 0.128 0.138 6 0.15
MgO 44.24 42.92 6 1.18 46.1 43.0 42.5 34.7 34.61 36.85 36.2 6 1.1
CaO 0.00 0.00 0.00 1.50 0.53 5.76 0.17 1.87 0.42 6 0.1
Na2O 0.00 0.00 0.00 0.02 0.013 0.00 0.00 0.00 0.00
K2O 0.00 0.00 0.23 0.08 0.15 0.00 0.00 0.00 0.00
P2O5 ,0.005 ,0.005 0.03 0.03 0.051 ,0.005 ,0.005 ,0.005 ,0.005
NiO 0.396 0.34 6 0.03 0.35 0.32 0.27 0.116 0.34 0.30 0.32
Cr2O3 1.18 0.53 6 0.25 0.44 0.47 0.60 0.413 0.45 0.36 0.40
CO2 —f — 0.21 ,0.05 0.18 — — — —
H2O(1) 5.02 6.23 6 2.42 5.6 1.0 8.43 11.3 13.76 8.50 12.6 6 1.1
H2O(2) — — 0.50 0.09 0.45 — — — —
TOTAL 99.15 99.40 100.4 100.3 100.6 100.9 99.7 100.5 100.7

MgO/SiO2 1.16 1.06 1.18 0.96 1.09 0.850 0.871 0.883 0.892
Mol-% MgO 52.5 48.2 51.1 52.7 45.6 36.7 35.9 37.8 37.6
r(g/cm3) 3.25 2.95 2.83 3.22 2.85 2.69 2.55 2.85 2.65
% Serpentinite 10.0 45 6 30 58.5 10.0 56.1 90 100 60 95 6 5
aAnalysis from Goff et al. (1997); LOI (loss on ignition) listed as H 2O(1).
bAverage of nine dunite and harzburgite samples with SD (ls) from Goff et al. (1997); LOI listed as H 2O(1).
cAnalysis from Himmelberg and Coleman (1968).
dAverage of eight peridotite samples from Himmelberg and Coleman (1968).
eAverage of 15 samples with SD (1s) from Goff et al. (1997); LOI listed as H2O(1).
f (—), not analyzed.

Ranges (McLaughlin et al., 1989). The serpentinite is the #1408C at .2000 m). The presence of low-temperature
preserved base of an extensive, north-trending sheet of mineral springs, which occur sporadically throughout most
ophiolite that is z50 km long and averages 2 to 6 km wide. of the serpentinite, indicates that modern day serpentiniza-
The thickness of the deposit varies from a few tens of tion is happening by reactions with groundwaters. Com-
meters on the west to several hundred meters on the east plexion Spring (#208C), near the heart of the study area,
(Figure 4B). precipitates brucite (Mg(OH)2) and has a pH #12 (Barnes
In contrast to the Del Puerto deposit, our examinations et al., 1972).
show that most outcrops have very little preserved perido- The southern margin of the serpentinite also hosts several
tite minerals (#5% overall), although original textures are small cinnabar mines, last worked in the early 1950s, and
occasionally well preserved. The rocks are serpentinized one small gold mining district that has not been worked
harzburgites that are pervasively faulted and sheared since World War I (Peters, 1991; Goff and Janik, 1993).
throughout much of the deposit. Typical samples from 15 Homestake Mining Company re-explored the deposit in the
widespread locations in the mass contain z36 mol-% MgO 1980s but decided to drop their lease due to low tonnage of
(Table 1). The serpentinites overlie deformed Franciscan gold-bearing rock. Wilbur Springs proper is a small but
Complex rocks on the west and south and are overlain by thriving hot spring resort first developed before the turn of
Great Valley sequence marine sediments (Jurassic–Creta- the century (Goff and Janik, 1993). The resort now caters to
ceous) on the east. people who seek quiet, natural surroundings.
A WNW-trending group of hot springs in an 8-km-long Most of the land occupied by the serpentinite belongs to
zone occurs at the extreme southern edge of the serpentinite the U.S. Bureau of Land Management or to a few cattle
body and is the surface expression of a small geothermal ranches. Indian Valley on the west side of the mass contains
reservoir (Goff and Janik, 1993). Drilling for geothermal re- a reservoir that is used for recreational purposes, when there
sources in the 1960s failed to find sufficiently high tempera- is water. The serpentinite hosts scrubby vegetation consist-
tures for electrical generation (reservoir temperature is ing of manzanita, buckthorn, live oak, scattered pines, and
 GOFF AND LACKNER: CARBON DIOXIDE SEQUESTERING 95
rare cypress trees which can be nearly impenetrable to hu- nor presence of plagioclase. Contents of Cr, Ni, and Mn are
mans on foot. The area may be reached by dirt road from the roughly equivalent to alpine peridotites.
south, east, and northwest. Most peridotites are partly to completely reconstituted into
hydrated Mg-rich silicates (serpentine and related minerals).
The three serpentine minerals (lizardite, chrysotile, and antig-
GEOCHEMISTRY OF orite) are isochemical with very similar sheet-like structures.
ULTRAMAFIC ROCKS The resulting serpentinites may contain some relict olivine
Mineral contents and chemical analyses of “fresh” peri- and pyroxene, but more often they contain only serpentine
dotite bodies ($50 modal-% unaltered rock) are listed in minerals (Mg3(Si2O5) (OH)4), magnetite (Fe-rich spinel), and
numerous reports (e.g., Goff et al., 1997). Typical perido- residual chromite, plus brucite (Mg(OH)2), carbonates (usu-
tites are harzburgites containing #90 modal-% of forsteritic ally magnesite, MgCO3), and free silica (SiO2). Serpentinites
olivine and $10 modal-% of orthopyroxene with accessory may contain as much as 14 wt-% water. Textural evidence in
chrome spinel 6 chrome diopside. Most peridotites contain rocks shows that the hydration of forsterite to form serpentine
39 to 44 wt-% SiO2, 42 to 50 wt-% MgO, 7 to 9 wt-% FeO (and brucite) is accompanied by a volume increase of as
(as total Fe), #4000 ppm Cr, #3000 ppm Ni, and #1200 much as 53%. Thus, the serpentinites are low-density rocks
ppm Mn (Tables 1 and 2). CaO and Al2O3 contents are gen- (z2.5 g/cm3) relative to the original peridotites (z3.3 g/cm3).
erally #1.5 wt-% and #5 wt-%, respectively. Total Ti, Na, Chemical analyses of serpentinites are widely available in
K, and P contents are usually #0.3 wt-%. The Fe is rela- the literature (Tables 1 and 2). Because of their lower densi-
tively reduced (Fe31 /Fe21 < 0.3). Peridotites in layered in- ties and high water contents, serpentinites contain substan-
trusions generally contain less magnesia and more of the tially less magnesia than do peridotites, usually between 32
other oxides due to higher proportions of pyroxenes and mi- and 36 wt-% MgO. Formation of magnetite during serpenti-

TABLE 2. Chemical compositions (major elements in wt-%) of ultramafic rocks from selected locations in the United States.

Baltimore
Twin Sisters, Canyon Complex, San Mateo, Stillwater,
WA Vulcan Peak, OR Belvidere Mtn., VT Mtn., OR MD CA MT
Sample No. STD a UNIMb VP-AVEc 19VP68 1VP68 BM-AVd BMS-AVEe CM-AVEf BC-AVEg FG96-312h ST-AVEi
Type Dunite Dunite Peridotite Dunite Harzburgite Dunite Serpentinite Harzburgite Serpentinite Serpentinite Peridotite
SiO2 40.41 42.52 41.2 39.6 43.4 39.7 33.0 42.1 41.2 41.08 47.7
TiO2 0.005 —j 0.035 0.03 0.02 0.0 0.0 0.04 0.05 0.054 0.12
Al2O3 0.19 0.19 0.38 0.07 0.25 0.4 0.6 1.70 1.33 1.76 4.82
Fe2O3 1.03 7.68 1.52 1.1 0.52 — — 2.49 6.17 8.92 2.94
FeO 6.97 — 6.93 9.8 7.8 8.9 14.1 4.79 2.43 — 6.54
MnO 0.12 — 0.12 0.14 0.14 0.23 0.2 0.13 0.12 0.133 0.17
MgO 49.59 48.01 45.5 47.4 45.3 48.3 38.0 35.6 35.09 33.06 29.0
CaO 0.17 0.02 0.70 0.30 0.91 — — 5.58 1.67 1.76 2.44
Na2O 0.015 0.02 0.006 0.02 0.00 — — 0.55 0.01 0.00 0.19
K2O 0.001 0.01 0.068 0.06 0.07 — — 0.03 0.06 0.00 0.02
P2O5 0.002 — 0.032 0.04 0.04 — — 0.02 0.008 0.011 0.01
NiO 0.30 0.37 0.26 0.27 0.25 — — — 0.18 0.33 —
Cr2O3 0.58 0.13? 0.25 0.30 0.18 1.6 0.7 — 0.16 0.37 0.48
CO2 0.08 — ,0.06 0.05 ,0.05 — — 0.12 0.03 — 0.11
H2O(1) 0.44 1.05 2.97 1.1 1.3 1.23 12.5 6.94 11.06 12.46 4.91
H2O(2) 0.06 — 0.18 0.10 0.05 — — — 0.31 — 0.49
TOTAL 99.96 100.0 100.2 100.4 100.2 100.4 99.1 100.1 99.88 99.99 99.95

MGO/SiO2 1.23 1.13 1.10 1.20 1.04 1.22 1.15 0.845 0.852 0.804 0.608
MOL-% MgO 59.9 57.1 53.2 57.0 54.6 57.9 39.3 40.4 37.7 35.5 35.5
r(g/cm3) 3.32 3.3 3.18 3.28 3.27 3.27 2.63 2.95 2.71 2.63 3.07
% Serpentinite 4 4 21 9 10 10 100 50 80 100 35
a Rock standard DTS-1 (Govindaraju, 1994).
b Analysis provided by Unimin Corp. (UNIM); Fe as Fe 2O3 and LOI (loss on ignition) listed as H 2O(1).
c Average of 13; all Vulcan Peak analyses from Himmelberg and Loney (1973).
d Average of three (Labotka and Albee, 1979); Fe as FeO and LOI listed as H O(1).
2
e Average of three (Labotka and Albee, 1979); Fe as FeO and LOI listed as H O(1).
2
f Average of 11 (Thayer, 1977); LOI listed as H O(1).
2
g Average of four (Morgan, 1977).
h Average of eight homogenized pieces (total weight 5 2 kg) from single outcrop (Goff et al., 1997); Fe as Fe O and LOI listed as H O(1).
2 3 2
i Average of 26 (Hess, 1960); no analyses for Ni reported.
j (—), not analyzed.
 96 ENVIRONMENTAL GEOSCIENCES

nization causes the average oxidation state of iron to rise than is the three-reagent mixture ($35 wt-% versus #15
(Fe31/Fe21 < 2). Contents of other transition metals are wt-% Mg). Residual products (z45 to 60 wt-%) from
slightly less than in the original peridotites. HCl dissolution include silica gel, spinels, and pyroxenes
and additional silicates such as talc, amphiboles, chlorite,
and sericite. The three-acid mixture apparently precipi-
Acid Dissolution Experiments
tates MgF compounds while dissolving the rock.
Acid dissolution experiments were conducted on various
• Hot HCl is slightly better at dissolving Mg from serpen-
peridotite and serpentinite samples to determine the relative
tinite than from peridotite, including dunite, because
merits of HCl as opposed to a more complex rock reagent
serpentinites contain less nonreactive silicates such as
such as HCl-HNO3-HF (Table 3). The results of these ex-
pyroxene. Much of the Fe in serpentinites occurs as mi-
periments show that:
crocrystalline magnetite that is relatively easy to dis-
• Hot HCl is better at dissolving Mg from ultramafic rocks solve in HCl.

TABLE 3. Results of dissolution and analysis using 1 g of ultramafic rock sample mixed in hot 1:1 HCl or
in HCl-HNO3-HF. The results are compared with X-ray fluorescence (XRF) analyses. The residues are
mixtures of silica gel, spinels, and pyroxenes. No single reagent can effectively dissolve all components
from ultramafic rocks, but HCl is more effective at dissolving Mg than is the mixed reagent and works
better on serpentinites than peridotites.

Residue MgO Mn Ni Cr
Sample Types (wt-%) (wt-%) (ppm) (ppm) (ppm)
Peridotites
DTS-1 dunite (standard value, Table 1) —a 49.59 929 2360 3990
DTS-1 (powder, 3 acids) — 8.04 916 2454 157
DTS-1 (powder, hot HCl) 59.6 53.05? 951 2530 20
Three Sisters dunite (Unimin, #4% serpentinite) — 48.01 — 2907 886
Three Sisters dunite (XRF) — 47.93 930 2830 3820
Three Sisters dunite (ore, hot HCl) 56.5 42.47 747 2524 21
Three Sisters dunite (crushed ore, hot HCl) 63.1 40.38 761 2430 27
Three Sisters dunite (powder, hot HCl) 53.0 46.89 830 2728 127
Green Mtn. peridotite (Unimin, 10% serpentinite) — 47.65 — 3500 1710?
Green Mtn. peridotite (XRF) — 46.68 1005 140 3832
Green Mtn. peridotite (powder, hot HCl) 48.1 41.28 738 2479 657
PCC-1 peridotite (standard value) — 43.43 930 2380 2730
PCC-1 (powder, 3 acids) — 8.04 899 2484 804
JP-1 peridotite (standard value) — 44.72 930 2460 2970
JP-1 (powder, 3 acids) — 7.73 914 2445 709
Del Puerto dunite (CRF, 40% serpentinite) — 44.31 951 2754 3160
Del Puerto dunite (powder, hot HCl) 43.4 41.82 828 2398 25
Del Puerto dunite (powder, 3 acids) — 11.46 822 2653 33
Del Puerto harzburgite (XRF, 50% serpentinite) — 41.63 1050 2560 2794
Del Puerto harzburgite (powder, hot HCl) 46.7 34.64 784 2141 185
Del Puerto harzburgite (powder, 3 acids) — 12.79 908 2285 435
Serpentinites
UB-N serpentine (standard value) — 35.21 929 2000 2300
UB-N (powder, 3 acids, 5/96) — — 981 2064 2145
UB-N (powder, 3 acids, 2/97) — 13.71 985 2102 2200
San Mateo serpentinite (XRF, Table 1) — 33.06 1045 2514 2688
San Mateo serpentinite (powder, 3 acids) — — 1014 2312 1998
San Mateo serpentinite (powder, 608C HCl) 48.8 30.92 765 2385 1440
San Mateo serpentinite (powder, #1288C HCl) 46.6 32.75 865 2120 1595
San Mateo serpentinite (powder, #2088C HCl) 47.5 32.66 840 2270 2135
San Mateo serpentinite (608C residue, 3 acids) — — 103 57 1481
San Mateo serpentinite (608C residue, 1 leachate) — — 868 2442 2921
Wilbur Springs serpentinite (XRF, 85% serpentinite) — 35.41 1155 2253 3054
Wilbur Springs serpentinite (powder, hot HCl) 42.2 34.04 985 1960 1992
Wilbur Springs serpentinite (powder, 3 acids) — 15.19 1039 2137 2849
a (—), not analyzed.
 GOFF AND LACKNER: CARBON DIOXIDE SEQUESTERING 97
• Hot HCl is less effective at dissolving many trace met- Partially serpentinized peridotite and dunite in large masses
als from ultramafic rocks than is the three-acid mixture. (20 to 80% serpentine) are more common. Examples include
This is especially true for high field-strength elements the Belvidere Mountain prospect in Vermont (Labotka and
like Cr and slightly true for Mn. On the other hand, Co Albee, 1979), the Canyon Mountain and Vulcan Peak depos-
(not listed in Table 2) and Ni mainly reside in olivine; its in Oregon (Himmelberg and Loney, 1973; Thayer, 1977),
thus, HCl dissolution works well for these elements. the Del Puerto and Burro Mountain bodies of California
• HCl dissolution at 608C and atmospheric pressure is (Page, 1967; Himmelberg and Coleman, 1968; Goff et al.,
nearly as effective as HCl at 2008C and 15 bars for dis- 1997), and most peridotite in the Stillwater Complex (Tables
solving Mg from serpentinite (see results for San Mateo 1 and 2). The Belvidere, Vulcan Peak, Del Puerto, and Still-
serpentinite, Table 2). However, the gain in trace metal water bodies contain small zones (#4 km2) of relatively unal-
dissolution is too small to make this procedure worth- tered dunite.
while. Completely serpentinized peridotite is exceptionally com-
mon in certain areas of eastern and western North America.
Best Ultramafic Ores for Carbonate Disposal Ultramafic rocks in the Baltimore Complex contain $80%
Because olivine and serpentine are the most reactive Mg- serpentinite and nearly all California deposits consist of
rich minerals in the HCl dissolution process, rocks comprised $95% sepentinite (Rice, 1957; Morgan, 1977; Goff et al.,
solely of these minerals would make the best ores. In contrast, 1997). Perhaps the largest body of continuous serpentinite
spinels and pyroxenes are not as reactive; thus, these residual outcrop in the United States occurs at the Josephine Ophio-
minerals (and silica) must be separated from primary reactants lite of northwest California and Oregon, which extends over
during processing. Although Cr-spinel (and other potentially $800 km2 (Harper, 1984). These deposits, although some
valuable metals) may contribute greatly to the economics of are huge, are mostly serpentinized harzburgite and are not
the process, the pyroxenes contribute virtually nothing of as desirable as the unaltered dunites.
value. Thus, peridotites and serpentinites with little pyroxene
comprise the most desirable ores (Figure 5). SEQUESTERING POTENTIAL OF
Fresh dunite (or any unserpentinized peridotite for that mat- TYPICAL ULTRAMAFIC BODIES
ter) is relatively uncommon in large quantities. The largest The CO2-sequestering potential of some U.S. ultramafic
such body in the United States is the Twin Sisters Dunite bodies is compared in Table 4. Volume, bulk density, and
(Ragan, 1963), which occurs in the Cascade Range of north- Mg content for each body were estimated using published
western Washington (Table 2). This dunite body covers z90 geologic maps, reports, and chemical analyses. As men-
km2 and is presently mined by open-pit methods for refractory tioned above, fresh peridotites, especially dunites, contain
(foundry) sand. An examination of a 20-kg sample provided the most Mg per unit mass of rock. Stillwater peridotite con-
by the operator showed that it contains #4% serpentine and tains relatively more pyroxene plus some plagioclase but
other secondary reaction products. The relatively large, unser- less olivine and is the least attractive ultramafic body evalu-
pentinized Green Mountain Peridotite ($5 km2 ) occurs in the ated in terms of Mg content. All economic trade-offs for
Appalachians of North Carolina and is also mined primarily mining and processing various ultramafic rocks have not
for foundry sand. An examination of this material showed that been evaluated, but some are discussed below.
it contains $5% orthopyroxene and z10% of high-grade The sequestering potential of small ultramafic bodies is
metamorphic products including talc and Mg-rich amphibole considerable. For example, the Belvidere Mountain ultra-
(minerals not soluble in HCl). This material is less desirable mafic body, having an estimated volume of z2 km3, is ca-
than is dunite as a CO2-sequestering ore even though it con- pable of handling the equivalent of 0.5 year of present total
tains nearly comparable amounts of Mg (Goff et al., 1997). U.S. CO2 emissions (z5 gigatons/yr; this value includes

FIGURE 5: Diagram showing lower right

corner of the Opx-Cpx-Ol ternary and lower
half of the Opx-Ol-Serp ternary (modal-%).
Because pyroxenes (and accessory chromite)
are not soluble in HCl, dunite and thoroughly
serpentinized dunite or harzburgite comprise
the best ores for CO2 sequestering. Numbers
around perimeter of diagram are percent
Opx.
 98 ENVIRONMENTAL GEOSCIENCES

TABLE 4. Physical-chemical and CO2-sequestering properties of example peridotite/serpentinite bodies (data from Czamanske and
Zientek, 1985, Goff et al., 1997, Hess, 1960, Himmelberg and Coleman, 1968, Himmelberg and Loney, 1973, Labotka and Albee, 1979,
Morgan, 1977, and Ragan, 1963).

Twin Vulcan Del Belvidere Wilbur Baltimore San

Sisters, Peak, Puerto, Mtn., Springs, Complex, Mateo, Stillwater,
WA OR CA VT CA MD CA MT
Volume-Density
Area (km2) 91 16 40 2.3 200 100 4? 26
Depth (km) 0.6 0.5 0.3 #1 $0.2 0.3 0.25 0.5
Est. Vol. (km3) 54 8 12 2 40 30 1 13
Density (g/cm3) 3.3 3.2 2.8 2.9 2.65 2.7 2.6 3.1
Wt-% Mg
Peridotite 29.9 27.4 27.2 29.1 — — — 17.5
Serpentinite —a — 20.9 23.1 21.8 21.2 19.9 —
Combined 29 27 23 26 21 21 20 17
Sequestering Properties
R(CO2)b 1.91 2.05 2.40 2.13 2.63 2.61 2.76 3.25
Mg (109 tons) 52.0 6.91 7.73 1.5 22.3 17.0 0.5 6.85
CO2 (109 tons) 94.1 12.5 14.0 2.7 40.3 30.8 0.9 12.4
USA (yr)c 18.8 2.50 2.80 0.55 8.05 6.15 0.18 2.48
World (yr)c 4.95 0.66 0.74 0.14 2.12 1.62 0.048 0.65
a (—), not significant.
b R(CO
2) is the calculated mass ratio of rock processed to CO 2 disposed.
cAssumes annual U.S. and world CO emission rates of z5 3 109 and z19 3 109 tons/yr, respectively.
2

auto emissions). The large dunite at Twin Sisters, Washing- ADDITIONAL CONSIDERATIONS
ton could dispose of nearly 19 years of U.S. CO2 emissions Strategic Minerals
and z5 years of global emissions. From another perspec- Ultramafic rocks contain many mineral resources. Chrome,
tive, the abundance of Mg in the Earth’s crust (2.0 mol-%) platinum group metals, nickel, cobalt, and diamonds come
is nearly 60 times greater than is the abundance of C (0.035 from various ultramafic rocks and their eroded products,
mol-%; Brownlow, 1979); thus, it is not surprising that whereas manganese, copper, mercury, and other metals are
there should be more than enough Mg in ultramafic deposits sometimes obtained from within the bodies or from enclosing
to sequester global CO2 emissions. rocks (Maddock, 1964). Metals such as these are of strategic
At a deposit such as the Del Puerto ultramafic body, Cali- importance during desperate political periods but may be of
fornia (Evarts and Schiffman, 1982), a ton of sequestered environmental concern in large-scale mining operations. The
CO2 would require on average mining, crushing, and dis- metal-bearing residues could be backfilled into excavations or
solving 2.4 tons of ultramafic rock (Table 4, where R 5 could be stockpiled for future use. Some metals such as Cr are
mass ratio of rock processed to CO2 sequestered). Every ton bound primarily in chromite and minor clinopyroxene which
of CO2 would precipitate 1.8 tons of magnesite that would are relatively benign in the sequestering process. Most of the
be backfilled into the existing mine. An additional 1.2 tons Ni and Mn is hosted in the olivine or products formed during
of silica and residual minerals such as pyroxenes and serpentinization and are released by acid dissolution.
chromite would be backfilled with the magnesite, although Following the example used above, complete utilization of
some could be stockpiled and sold for other industrial uses. the Del Puerto ultramafic body would yield at least 80 mega-
The process would generate z0.15 tons of dissolved Fe per tons of Cr, another 80 megatons of Ni, and perhaps 30 mega-
ton of fixed CO2 or a total of 2 gigatons of Fe for the entire tons of Mn. These quantities could impact dramatically the
deposit which would be precipitated as the oxide for feed- economics of existing metals industries but would be a bene-
stock in other industries. If completely mined, total Mg in ficial byproduct of CO2 sequestering.
the Del Puerto deposit could fix 14 gigatons of CO2.
Compared with a typical coal mine, the Del Puerto de-
posit is quite large. By itself, the Del Puerto deposit could Chrysotile Asbestos
dispose of all of the CO2 emissions associated with a popu- Serpentinites host commercial deposits of chrysotile or
lation of 10 million people for z70 years. Thus, it would “white” asbestos (Coleman, 1996; O’Hanley, 1996). Because
suffice for the local region. However, to absorb the CO2 most serpentinites contain appreciable chrysotile (usually in
output of the United States, many more such mines would noncommercial form), special environmental precautions
be needed. Del Puerto could only handle 2.8 years of total may be required during mining. Chrysotile accounts for
U.S. output. z90% of the asbestos that has been used historically in the
 GOFF AND LACKNER: CARBON DIOXIDE SEQUESTERING 99
United States (Ross, 1981), and it is the primary type of as- mafic rocks and other formations. Because serpentinite is the
bestos used in insulation and many other construction materi- “state rock” of California, special care is generally taken to
als. Chrysotile forms the bulk of asbestos-contaminated waste restore or reseed areas where serpentinites are disturbed in
resulting from removal activities. this state (i.e., Dellinger, 1997).
The health risks associated with chrysotile have been the
focus of extensive scientific and public debate (Alleman and
Mossman, 1997). Many of the arguments deliberate whether
CONCLUSIONS •
Steady increases in global emissions will require new
chrysotile can cause mesothelioma (a rare type of cancer) in technologies to capture and immobilize waste CO2. Conver-
humans. Although this issue is largely unresolved, the risk ap- sion of CO2 into thermodynamically stable magnesite is one
pears to be much less than that posed by amphibole asbestos of many technologies under current examination. Abundant
(Mossman et al., 1990; Hume and Rimstidt, 1992). Chrysotile resources of Mg-rich peridotite and serpentinite exist within
asbestos continues to be removed from buildings, albeit at a the United States and many other countries. Peridotite and
lower level than in years past. This asbestos-containing ma- serpentinite are relatively soluble in HCl; thus, huge quanti-
terial must be disposed of in landfills with special precau- ties of Mg can be dissolved easily for further chemical uses.
tions prescribed by the U.S. Environmental Protection Engineering and technology advances could lead to con-
Agency (e.g., the use of a .15-cm cover of asbestos-free struction of coal- or gas-fired power plants in which waste
material). Waste chrysotile asbestos could be used as feed- CO2 is fed to a sequestering plant adjacent to an open-pit ul-
stock in the sequestering of CO2 if it was relatively uncon- tramafic mine. Peridotite, serpentinite, and waste asbestos
taminated by other materials. would be consumed, whereas magnesite and silica residues
would be backfilled into the pit. Byproducts would include
Large-scale Open-Pit Mining Fe, Cr, Ni, Mn, and possibly other metals.
Open-pit mining on the scale envisoned here would have Retrofitting all existing fossil fuel plants for benign CO2 dis-
profound economic and environmental impact. Projected posal is surely an impractical task, but the global community
costs would probably be similar to open-pit copper mining, must deal eventually with the CO2 dilemma. CO2 sequestering
as mentioned above. Precedents for open-pit mining of ser- in magnesite is one of many technologies that may eventually
pentinite presently exist at New Idria and at other mines in reduce or stabilize emissions. The environmental impact of
the California Coast Ranges (Coleman, 1996). large-scale ultramafic mining and CO2 sequestering, with asso-
The McLaughlin gold mine (Homestake Mining Com- ciated increases in energy costs, would have to be weighed
pany) was constructed in a similar geologic and physio- against the counterimpact of continued CO2 emissions to the
graphic environment to the Wilbur Springs serpentinite in the atmosphere and the risk of accelerated climate change.
late 1970s. The McLaughlin mine is located z30 to 40 km
south of the Wilbur Springs region and was over 100 m deep
and 1000 m long during maximum exploitation (Sherlock et
ACKNOWLEDGMENTS •
We thank the following people: G. Guthrie (EES-1) for in-
al., 1995). The gold has been mined out now, and the site is put on asbestos; D. Counce, E. Kluk, and M. Snow (Los Ala-
being reclaimed. Processing of stockpiled ore will yield 19.2 mos National Laboratory [LANL]) for various analyses; D.
tons of gold before Homestake abandons the site in the year Bergfeld (LANL) for graphics; A. Adams (LANL) for thin
2003 (Field, 1996). A large open-pit mine designed for car- sections; P. Canelli (UNIMIN Corp.) for peridotite samples
bonate waste disposal would probably have considerable from two active mines; California Division of Mines and Ge-
community support in this economically depressed area. ology for maps and information; and R. G. Coleman (Stanford
However, the environmental impact would have to be evalu- University) for good advice. Initial reviews were provided by
ated fully before any development proceeded. M. J. Aldrich and B. Carey (LANL). Final reviews were ob-
tained from R. G. Coleman and two anonymous individuals.
Ultramafic Rocks and Ecology This research was funded by Program Directorate Energy
Because ultramafic rocks are exceptionally low in K and P Technologies (PDET) (E. Joyce; LANL).
and rich in Mg and Fe, they sustain a unique flora and fauna
that are noticeable to even the most casual of observers
(O’Hanley, 1996). Most grasses grow with difficulty on ser-
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