Available online at www.sciencedirect.com
Geochimica et Cosmochimica Acta 87 (2012) 69–80
www.elsevier.com/locate/gca
Signatures in magnetites formed by (Ca,Mg,Fe)CO3
thermal decomposition: Terrestrial and extraterrestrial
implications
Concepcion Jimenez-Lopez a,⇑, Carlos Rodriguez-Navarro b,⇑,
Alejandro Rodriguez-Navarro b, Teresa Perez-Gonzalez a, Dennis A. Bazylinski c,
Howard V. Lauer Jr. d, Christopher S. Romanek e
b
a
Departamento de Microbiologı́a, Facultad de Ciencias, Universidad de Granada, Campus de Fuentenueva s/n, 18071 Granada, Spain
Departamento de Mineralogı́a y Petrologı́a, Facultad de Ciencias, Universidad de Granada, Campus de Fuentenueva s/n, 18071 Granada, Spain
c
School of Life Sciences, University of Nevada at Las Vegas, Las Vegas, NV, USA
d
ESCG/Barrios Technologies, Houston, TX 77258, USA
e
NASA Astrobiology Institute and Department of Earth and Environmental Sciences, University of Kentucky, Lexington, KY, USA
Received 29 June 2011; accepted in revised form 19 March 2012; available online 28 March 2012
Abstract
It has never been demonstrated whether magnetite synthesized through the heat-dependent decomposition of carbonate
precursors retains the chemical and structural features of the carbonates. In this study, synthetic (Ca,Mg,Fe)CO3 was
thermally decomposed by heating from 25 to 700 °C under 1 atm CO2, and by in situ exposure under vacuum to the electron
beam of a transmission electron microscope. In both cases, the decomposition of the carbonate was topotactic and resulted in
porous pseudomorphs composed of oriented aggregates of magnetite nanocrystals. Both calcium and magnesium were incorporated into nanophase magnetite, forming (Ca,Mg)-magnetites and (Ca,Mg)-ferrites when these elements were present in the
parent material, thus preserving the chemical signature of the precursor. These results show that magnetites synthesized in this
way acquire a chemical and structural inheritance from their carbonate precursor that indicates how they were produced.
These results are not only important in the determination of the origin of chemically-impure, oriented nanophase magnetite
crystals in general, but they also provide important insights into the origin of the large, euhedral, chemically-pure, [111]elongated magnetites found within Ca-, Mg- and Fe-rich carbonates of the Martian meteorite ALH84001. Based on our
experimental results, the chemically-pure magnetites within ALH84001 cannot be genetically related to the Ca-, Mg- and
Fe-rich carbonate matrix within which they are embedded, and an alternative explanation for their occurrence is warranted.
Ó 2012 Elsevier Ltd. All rights reserved.
1. INTRODUCTION
Nanometer-sized magnetite (Fe3O4) crystals having a
variety of morphologies, size distributions and chemical
⇑ Corresponding authors. Tel.: +34 958 24100020426; fax: +34
958 249486 (C. Jimenez-Lopez), tel.: +34 958 246616; fax: +34 958
243368 (C. Rodriguez-Navarro).
E-mail addresses: [email protected] (C. Jimenez-Lopez), carlosrn@ugr.
es (C. Rodriguez-Navarro).
0016-7037/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.gca.2012.03.028
compositions have been recovered from modern and ancient environments (Thomas-Keprta et al., 2000). Magnetite is found on Earth in igneous, metamorphic and
sedimentary rocks as well as in extraterrestrial materials
including meteorites and interplanetary dust particles.
Whereas magnetites formed at high temperature are clearly
abiotic, those formed in low-temperature environments e.g.,
Precambian stromatolites, deep-sea and lake sediments and
soils, may have an abiogenic or biogenic origin (ThomasKeprta et al., 2000). Biogenic magnetite is known to form
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C. Jimenez-Lopez et al. / Geochimica et Cosmochimica Acta 87 (2012) 69–80
through biologically-induced (BIM; Frankel and Bazylinski, 2003) or biologically-controlled mineralization (BCM;
Bazylinski and Frankel, 2003). The origin of many lowtemperature terrestrial magnetites is unknown, and there
is still great controversy over how to distinguish inorganically- and biologically-produced sedimentary magnetites
(Jimenez-Lopez et al., 2010). BCM magnetites have been
proposed as magnetofossils by many authors because they
simultaneously meet all the criteria summarized in the
MAB package defined by Thomas-Keprta et al. (2000).
BIM magnetites have not been generally accepted up to
now as magnetofossils, with the exception of the extraordinary tabular single-domain magnetite crystals induced by
Geobacter metallireducens GS-15 (Vali et al., 2004). The criteria contained in the MAB package, especially morphology, crystal-size and size distribution (i.e., Arató et al.,
2005) have been widely used for many years to recognize
biotic origin of natural terrestrial magnetites. However,
many discussions have risen when trying to apply those criteria to extraterrestrial magnetites, and the debate still goes
on (Thomas-Keprta et al., 2009). These problems must be
resolved to recognize mineral biomarkers in terrestrial
and extraterrestrial materials (Jimenez-Lopez et al., 2010).
In the laboratory, magnetite can be synthesized abiotically as a primary or secondary mineral phase. Secondary
magnetite forms through the transformation of Fe-bearing
minerals at low or high temperature. At low temperature
(<100 °C), magnetite can be formed in solution by recrystallization of ferrihydrite, green rust and FeOOH under anoxic conditions (Ishikawa et al., 1998; Zachara et al., 2002).
Alternatively, a solid-state transformation mechanism is
proposed for the thermal decomposition of siderite
(FeCO3), or other Fe-rich carbonates, at higher temperatures in the absence of oxygen. Such a mechanism may be
responsible for magnetite formation in natural environments resulting from the contact of a carbonate precursor
with a magmatic fluid or from a meteoritic impact on
sedimentary carbonates (Golden et al., 2001, 2004;
Thomas-Keprta et al., 2009; Bell, 2007). The decomposition
mechanism has been explored in some detail with distinct
precursor minerals and under different environmental conditions (Golden et al., 2001, 2004; Thomas-Keprta et al.,
2009), including thermal shock under conditions mimicking
meteoritic impact (Bell, 2007). The thermal mechanism of
magnetite formation has not been extensively explored with
regard to the origin of terrestrial magnetites. Nevertheless,
it has largely been debated in the context of the subset of
magnetites with unusual chemical and physical properties
that are intimately associated with carbonate disks in the
ALH84001 Martian meteorite, related by some to a bacterial origin (McKay et al., 1996) and by others to an exclusively inorganic origin through the thermal decomposition
of the carbonate matrix in which they are embedded (Golden et al., 2000, 2001, 2004; Brearley, 2003; Treiman, 2003;
Bell, 2007). Golden et al. (2001) reported that magnetite
crystals produced by the thermal decomposition of a mixture of pyrite, siderite, magnesite and ankerite were chemically-pure, single-domain and defect-free, similar to the
[111]-elongated magnetite crystals observed in ALH84001.
To bolster their results, Golden et al. (2006) thermally
decomposed Copper Lake siderite in vacuo (closed system)
whereas Bell (2007) decomposed this Copper Lake siderite
by shocking samples to pressures up to 49 GPa. They obtained magnetites with compositions ranging from pure
magnetite [when pyrite was present, (Golden et al., 2006)]
to Mg-bearing magnetite [610 wt %, (Golden et al., 2006)
and 620 wt %, (Bell, 2007)] and claimed that these magnetites displayed the same range of sizes, compositions and
morphologies as inorganic magnetites synthesized by Golden et al. (2004). Although a chemically diverse suite of
magnetites were produced in these studies, no reaction
products contained Ca. On the one hand, this is not surprising as Golden et al. (2006) did not report the presence of Ca
in their starting material. On the other hand, Bell (2007) reported 11–14 wt % CaCO3 in their siderite target, yet no Ca
was observed in the solid phase product of the shock reaction. It is unclear why Ca was detected in the same starting
material in one study but not the other but it is clear that a
complex interaction of materials and processes influences
the nature of solids produced by thermal decomposition.
Thomas-Keprta et al. (2009) challenged these results by
thermally decomposing Roxbury siderite, which is considered to be a terrestrial analoge of the Fe-rich component
of the carbonate disks in ALH84001 meteorite. They observed the formation of Mg- and Mn-magnetites and proposed that a CaCO3 component separated from the
(Ca,Mg,Fe)CO3 solid solution prior to the formation of
the magnetite. However, this phase, or CaO, was not observed in their experimental products.
To resolve discrepancies observed in these studies
involving Ca incorporation in magnetites, and to shed light
on signatures that may help recognize magnetite formation
by the thermal decomposition of a carbonate matrix in natural environments, we studied the chemical composition
and microtextural features of magnetites resulting from
the decomposition (both thermally from 25 to 700 °C,
and in situ, under the electron beam of a transmission electron microscope) of a synthetic (Ca,Mg,Fe)CO3 (characterized in Romanek et al. (2009), Fig. 1). In particular, the fate
of calcium in the resulting magnetite was monitored, and
the structural relationship(s) between the precursor and
product phases were determined.
2. METHODS
Synthetic mixed carbonate samples were provided by
Romanek et al. (2009). They were precipitated in laboratory experiments conducted at 25 and 70 °C in free-drift
experiments from solutions containing NaHCO3,
Fe(ClO4)2, Mg(ClO4)2 and Ca(ClO4)2. The resulting solids
were identified by XRD as Mg and/or Ca-substituted siderites (samples FD15 and FD16) and Fe and/or Ca-substituted magnesites (samples FD20 and FD21). No
magnetite was detected in these samples by XRD and/or
TEM. The FD carbonates have a chemical composition
(determined by inductively coupled plasma optical emission
spectrometry; ICP-OES; Perkin-Elmer 4300DVS) listed in
Table 1. A comparison of the chemistry of these synthetic
carbonates with natural terrestrial and extraterrestrial
(Ca,Mg,Fe)CO3 is presented in Fig 1. Each sample was
C. Jimenez-Lopez et al. / Geochimica et Cosmochimica Acta 87 (2012) 69–80
Fig. 1. (Ca,Mg,Fe)CO3 ternary diagram representing the composition of FD samples used in this study in comparison to other
mixed cation carbonates found in terrestrial environments and
ALH84001. Light grey field contains natural occurrences of siderite
and natural magnesites, and dark grey field is siderite–magnesite
solid solution boundary at 250 °C (Romanek et al., 2009); dash-line
circumscribed field represents ALH84001 carbonates from Treiman
and Romanek (1998).
separated into two aliquots: (1) a dry, powder sample and
(2) sliced sections embedded in resin. In the latter case, samples were fixed with glutaraldehyde and post-fixed with osmium tetraoxide. Afterwards, the sample was dehydrated
with ethanol, and embedded in Embed 812. Ultrathin sections (50–70 nm) were prepared with a microtome (Reicher
Ultracut S microtome, DIATOME diamond blade),
mounted onto copper grids and carbon coated.
Both the dry powder sample and the sliced sections of
(Ca,Mg,Fe)CO3 were maintained in anaerobic conditions
inside an anaerobic chamber (Coy Laboratory Products
Inc.) and each decomposed to magnetite as described
below.
2.1. Procedure 1
Mixed cation carbonates were decomposed by heating
the dry, powder FD samples by differential scanning calorimetry (DSC). A sample was weighed in an alumina crucible to ±2 lg. The sample was then placed in the center of
the hot zone of a small volume SENSYS evo DSC (Seteram
Instrumentation; www.Setatam.com) differential scanning
calorimeter. The instrument was sealed, evacuated and then
flushed with pure CO2 carrier gas for 10 min. The calorimeter was evacuated and back flushed with CO2 two additional times to obtain a low background for O2, which is
especially critical for the DSC analysis of redox-sensitive
elements (e.g., Fe). The calorimeter was then purged with
CO2 at a rate 5 cm3 min1. After CO2 purged for 30 min,
the sample temperature was ramped up to 700 °C at a rate
of 20 °C min1. The sample was held at 700 °C for 10 min
and then ramped down to room temperature. In a duplicate
run, once the sample reached the maximum T and was then
ramped down to room temperature (after 30 min), the
71
process was repeated to ensure complete reaction and to
obtain a curve for baseline subtraction. No microstructural,
mineralogical or chemical differences were observed between magnetite crystals subjected to one or two heating
events. Results reported here correspond to DSC-magnetites heated twice.
Sufficient material was produced in the reaction to determine the mineralogy of DSC-samples using a single-crystal
X-ray diffractometer equipped with an area detector (Bruker D8 SMART APEX, Germany). A frame (or 2-dimensional (2D) diffraction pattern) was collected under the
following operating conditions: Mo Ka, 50 kV, 30 mA,
0.5 mm collimator diameter and 120 s exposure time. The
XRD equipment used for the analyses has a CCD area
detector that can register a 2D diffraction pattern containing complete Debye–Scherrer rings. These patterns were later integrated and converted into conventional 2theta scan
for mineral identification. XRD2DScan software (Rodriguez-Navarro, 2006) was used to convert these 2D diffraction patterns into regular 2h linear scans. This software
was also used for background subtraction and integration
of peaks in the 2h scans. The integration of the intensity
of rings allows better particle statistics even in very small
samples (of a few lg) and the detection of individual mineral grains of minor phases. The detection limit for minor
phases is generally established to be a few percent units
(<5%) for conventional X-ray powder diffractometers but
the detection limit of XRD equipment with area detector
is much lower (Bhuvanesh and Reibenspies, 2003; He,
2009). In order to accurately determine the d-spacings for
the main magnetite reflections, LaB6 was used as an internal standard for angular calibration. The unit cell parameter (a) was determined using a line regression model:
dhkl = a/(h2 + k2 + l2)0.5. For this calculation, four of the
strongest reflections of magnetite (e.g., 113, 004, 115 and
044) were considered.
The microstructure and composition of individual crystals was determined using a Philips model CM200 transmission electron microscope (TEM) (Philips, Eindhoven, The
Netherlands; 200 kV, 10–20 lA, windows for analyses:
100 200 nm) equipped with an EDAX solid-state energy
dispersive X-ray detector with an ultrathin window
(UTW). For these analyses, DSC-magnetites were embedded in resin and sliced using the same protocol described
above for the FD carbonate samples. At least 20 individual
crystals were analyzed for each decomposed mixed cation
carbonate sample. Quantitative analytical electron microscopy (AEM) and selected area electron diffraction (SAED)
analyses were performed on the same spots of individual
crystals on thin edges in scanning TEM (STEM) mode
(<400 counts s1) using a 4 nm diameter beam, a
100 20 nm scanning area for AEM–TEM and a ca.
500 nm diameter circular area centered in the squared area
for SAED. For AEM–TEM analyses, a low background
condenser aperture and an analytical Be sample holder were
employed to improve spectrum quality. Muscovite, albite,
biotite, spersertine, olivine, and titanite standards were used
to obtain k-factors (kx/kSi), allowing X-ray intensities to be
corrected for element analysis (Champness et al., 1982). The
determined k-factor for Ca is 1.15, 1.35 for Fe and 1.07 for
C. Jimenez-Lopez et al. / Geochimica et Cosmochimica Acta 87 (2012) 69–80
Fe(n) ± 3
56.2–74.3
48.8–65.2
17.3–50.7
12.6–39.8
Ca(n) ± 3
Ca(n) ± 3
Mg(n) ± 4
TEM-magnetites (wt.%)
DSC-magnetites (wt.%)
2.1–4.3 (8)
2.7–8.2 (10)
24–33 (8)
6.7–41.4 (9)
(8)
(10)
(8)
(9)
Mg. Based on these k-factors, average errors for analyzed
elements (two standard deviations), expressed as a percentage of the atomic proportions, are 5 (Mg), 3 (Ca), and 3
(Fe). Determination of the mineralogy of the crystals analyzed by AEM–TEM was performed by calculating d-spacings from SAED patterns, using the same analytical
window. Measured d-spacings were compared to JCPD files
from the International Center for Diffraction Data (ICDD)
for mineral identification.
n = number of crystals analyzed. nd = non detected.
a
Most of the samples (10/11) exhibited Mg wt % P 4.2%.
b
XRD pattern shifted with respect to pure end member, see Romanek et al. (2009) for details.
3–6.8 (4)
0.5–8.7 (11)a
6.8–33.4 (8)
1.4–32.9 (6)
3.7–5.6 (4)
nd (11)
0.3–13.2 (8)
nd (6)
Fe(n) ± 3
XRD
Sideriteb
Sideriteb
Magnesiteb
Magnesiteb
Ca ± 0.1
Mg ± 0.2
Fe ± 0.3
FD15
FD16
FD20
FD21
27.4
0
21.7
0
13.3
12.3
54.6
70.9
59.3
87.7
23.7
29.1
497
490
577
560
Mg(n) ± 4
54.9–57.9
57.8–63.3
21.9–54.3
26.8–68.5
(4)
(11)
(8)
(6)
7.1–11.2 (8)
nd (10)
7.5–16.4 (8)
nd (9)
2.2. Procedure 2
Decomposition
T (°C)
Precursor carbonates (mol%)
Sample
Table 1
Analytical and transmission electron microscopy (AEM–TEM) analyses of DSC-magnetites and TEM-magnetites. ± = standard deviation. (Ca,Mg,Fe)CO3 precursors data are taken from
Romanek et al. (2009) (analyses performed by inductively coupled plasma optical emission spectrometry).
72
Sliced sections of FD samples were decomposed in vacuo
under the electron beam in a Philips model CM200TEM
operating at 200 kV (Eindhoven, The Netherlands; 10–
20 lA, beam spot size: 100 nm). Selected area electron diffraction (SAED) was performed to determine the zone axis
of each crystal. Measured d-spacings were compared to
JCPD files from the ICDD for both mineral identification
and the determination of Miller Indexes of atomic planes
parallel to the electron beam. The individual mixed cation
carbonate crystal, held in the determined orientation, was
then kept under the electron beam for 5–20 min to decompose it into magnetite. Decomposition was only achieved
when using a large condenser aperture (200 lm). After
phase transformation, SAED analysis was performed to
determine the zone axis of the resulting product.
This procedure was performed on 8 crystals from each
mixed cation carbonate sample listed in Table 1 (with the
exception of FD15). The resulting set of magnetite crystals
are referred to as TEM-magnetites. The chemical composition of individual mixed cation carbonate precursors and
the resulting TEM-magnetite crystals were analyzed by
AEM–TEM, as described above for the DSC magnetites.
In the present study, low-substituted magnetites would
be called (Ca,Mg)-magnetites while high-substituted
magnetites would be addressed as (Ca,Mg)-ferrites, following the terminology in Orewczyk (1990) and ThomasKeprta et al. (2009).
3. RESULTS AND DISCUSSION
3.1. Chemical signatures in magnetites formed by the
decomposition of synthetic (Ca,Mg,Fe)CO3
3.1.1. DSC-decomposition
Each mixed cation carbonate sample that was heated by
DSC completely decomposed pseudomorphically into a
porous aggregate of magnetite nanocrystals (Fig. 2) that retained the gross morphology of the precursor (Fig. 2c and
d). None of the original carbonate matrix was detected by
XRD in any case. Magnetite formed as a consequence of
the decomposition of the precursor carbonate, in the absence of oxygen, but in the presence of CO2. Note that
the thermal decomposition of siderite in the absence of O2
results in the formation of wüstite (FeO) and CO2. CO2 acts
as an electron acceptor, thereby oxidizing the Fe2+ to Fe3+,
during the reaction CO2 ! CO + 1/2O2 (Gallagher and
Warne, 1981). As a result, part of the Fe2+ in wüstite is
C. Jimenez-Lopez et al. / Geochimica et Cosmochimica Acta 87 (2012) 69–80
oxidized to Fe3+, resulting in magnetite as the final solid
phase (Gallagher and Warne, 1981; Brearley, 2003). Because of the molar volume differences between carbonate
precursor and product oxide, a significant porosity (ca.
50%) was generated.
Non-isothermal DSC results displayed a maximum
endothermal peak (representing carbonate decomposition)
at 497 °C for DSC–FD15, 490 °C for DSC–FD16
(Fig. 2a), 577 °C for DSC–FD20 (Fig. 2b) and 560 °C for
DSC–FD21 (Table 1). Because sample size/morphology,
mass, and heating rate were similar for the different
DSC–FD samples, variations in the decomposition temperature were mainly related to the carbonate composition,
although other factors such as grain size and crystallinity
may also account for slight changes in peak characteristics.
The higher the Ca and Mg content (FD15 vs. FD20) the
higher the decomposition temperature; conversely, the
higher the Fe content (FD15 vs. FD20 and FD16 vs.
FD21) the lower the decomposition temperature (Table 1).
This is consistent with differences in decomposition temperature for the pure end-member minerals (Hurst et al., 1993;
Brearley, 2003).
The size of DSC-magnetite crystals ranged from 5 to
40 nm and their morphologies ranged from hexagonal, to
rectangular, rhombic and irregular (Fig. 2i and g). Welldeveloped chains of magnetite crystals were detected in all
samples (Fig. 2g and e) with the exception of DSC–FD20.
DSC-magnetites displayed a preferred crystallographic orientation with respect to each other, as revealed by both
brightfield and darkfield TEM images (Fig. 2g; darkfield
images not shown). Because the DSC–FD20 precursor
was composed of an aggregate of randomly oriented submicrometer- to micrometer-sized carbonate crystals forming
spherulites (Romanek et al., 2009), a preferred orientation
(see the absence of Debye rings in SAED of Fig. 2h) occurred in small magnetite aggregates (ca. 200 nm in size;
see squared section in Fig. 2h) corresponding to the individual crystals in the precursor.
Bulk X-ray diffraction (XRD) analyses of the decomposed solids revealed the presence of magnetite for samples
DSC–FD15 and DSC–FD21, and a mixture of magnetite
and a small percentage of hematite for samples DSC–
FD16 and DSC–FD20 (possibly due to partial oxidation
of Fe2+ during the analysis). No other minerals were detected by XRD (e.g., CaO, CaCO3, and/or MgO minerals).
Furthermore, systematic shifts in the d-spacings of magnetite reflections were detected, indicating that there is a modification of the unit cell dimensions due to the incorporation
of Ca and/or Mg into the magnetite structure. The presence
of Ca and Mg in magnetite was also confirmed by the detection of these cations by AEM–TEM in sliced sections of the
magnetite crystals, as discussed below. In the case of sample
DSC–FD16, XRD analyses reveal a significant contraction
of the unit cell dimensions (a = 8.253 ± 0.005 Å, compared
to a = 8.396 Å for pure magnetite reported in the ICCD
PDF 16–629). In sample DSC–FD20, there is only a slight
contraction in the dimensions of the unit cell
(a = 8.347 ± 0.005 Å). In contrast, sample DSC–FD15
shows a significant expansion of the unit cell dimensions
73
(a = 8.495 ± 0.005 Å). The contraction or expansion of
the unit cell is due, respectively, to the incorporation of
smaller Mg2+ (i.e., DSC–FD16) or larger Ca2+ (i.e.,
DSC–FD15) ions into the magnetite structure, consistent
with results of the AEM–TEM analyses. No evidence of
oxidation of magnetite to maghemite (c-Fe2O3) was detected by XRD.
Selected area electron diffraction (SAED) and analytical
and transmission electron microscopy (AEM–TEM) were
performed sequentially on the same selected areas while
illuminating one or several nanocrystals of each DSC–FD
samples. In all cases, SAED data yielded the typical dspacings for magnetite, although these d-spacings were lower than those of magnetite in DSC–FD16, DSC–FD20 and
DSC–FD21 samples and slightly larger than those of magnetite in DSC–FD15. These shifts could not be attributed to
cation incorporation due to error associated with the
SAED measurements. AEM–TEM analyses of the same selected area showed the presence of varying amounts of Ca
and Mg in the magnetite nanocrystals, forming (Ca,Mg)–
magnetites and (Ca,Mg)–ferrrites. DSC–FD15 and DSC–
FD20 crystals showed the presence of 66.8 wt % (11 wt %
in precursor) and 33 wt % (48 wt % in precursor) Mg,
respectively, and the presence of 65.6 wt % (26 wt % in precursor) and 13.2 wt % (23 wt % in precursor) Ca, respectively (Table 1). Analyses of individual crystals from
DSC–FD16 and DSC–FD21 (no Ca in precursor) also
showed the presence of 69 wt % Mg (9 wt % in precursor)
and 33 wt % Mg (64 wt % in precursor) Mg, respectively.
Variability of Mg and Ca content between single crystals
is attributed to chemical zoning in the precursor material,
since these carbonates were grown in closed, free-drift
experiments, in which the relative concentration of aqueous
iron, calcium and magnesium varied throughout the timecourse experiment (Romanek et al., 2009).
3.1.2. In situ TEM decomposition
After exposure to the electron beam for 20 min in the
TEM, each mixed carbonate crystal decomposed to a pseudomorph consisting of a porous aggregate of magnetite
nanocrystals 5–10 nm in size (Fig. 3). As for the DSCsamples, SAED and AEM–TEM analyses were performed
sequentially on a specified area of individual crystals.
SAED analyses yielded typical magnetite d-spacings. As observed in DSC magnetites, the calculated d-spacings were
shifted to lower values in TEM–FD16, TEM–FD20 and
TEM–FD21 and to higher values in TEM–FD15. AEM–
TEM analyses of TEM–FD15 and TEM–FD20 crystals
showed that they contained up to 4.3 and 33 wt % Mg,
respectively, and up to 11.2 and 16.4 wt % Ca, respectively
(Table 1), whereas TEM–FD16 and TEM–FD21 crystals
(no Ca in precursor) contained up to 8.2 and 41 wt % Mg,
respectively. As in the case of DSC magnetites, the variability of Mg and/or Ca within crystals of the same sample is
correlated to the heterogeneous composition of the carbonate precursor. Although the Mg and/or Ca concentrations
for some magnetites were close to the detection limit (±4
and 3 wt %, respectively), most of the crystals exhibited concentrations well above the detection limit.
74
C. Jimenez-Lopez et al. / Geochimica et Cosmochimica Acta 87 (2012) 69–80
Fig. 2. (a and b) DSC spectra of FD16 (a) and FD20 (b) samples. Both spectra show one endothermal peak, indicating the thermal
decomposition of the carbonate; (c and d) TEM micrograph of mixed carbonate crystals from FD16 (c) and FD20 (d). Inset: AEM–TEM
analysis showing the presence of Mg and Ca; (e and f) TEM images of product pseudomorphs showing porous aggregates of magnetite
crystals that preserve the morphology of the precursor [DSC–FD16 (e) and DSC–FD20 (f)]. (g, h, i, and j) TEM images of DSC–FD16 (g and
i) and DSC–FD20 (h and j) magnetite crystals. The arrows indicate magnetite crystals with facing corners. SAED patterns (d-spacing
corresponding to magnetite) and AEM–TEM analyses (showing Mg and Ca) are included. AEM–TEM spectra: Mg is marked with a solid
arrow and Ca is marked with a dotted arrow. Squares correspond to areas analyzed by AEM–TEM. SAED patterns correspond to a ca.
500 nm diameter circular area centered in the squared areas.
C. Jimenez-Lopez et al. / Geochimica et Cosmochimica Acta 87 (2012) 69–80
3.1.3. Chemical signatures observed in DSC- and TEMmagnetites
Considered collectively, XRD, SAED and AEM–TEM
results show that the decomposition of the mixed carbonate
results in the formation of (Ca,Mg)-substituted magnetite
to (Ca,Mg)–ferrites as a solid solution, and not a mixture
of separate phases (CaO + MgO + Fe3O4 or CaO +
(Mg,Fe)3O4). Moreover, these data support the contention
that spinodal decomposition of the substituted magnetites
into pure end-members does not occur, nor does the re-carbonation of the oxides. The formation of a Mg-substituted
magnetite (solid solution) from the thermal decomposition
of Mg-substituted siderites is consistent with results reported by other researchers (Gallagher and Warne, 1981;
Gallagher et al., 1981; Dubrawski, 1991; Gotor et al.,
2000; Cohn, 2006; Isambert et al., 2006; Bell, 2007;
Thomas-Keprta et al., 2009).
One of the most interesting results of this study is that
Ca was incorporated in magnetite as a result of the carbonate mineral decomposition. These results are intriguing because Mg and/or Mn are known to co-precipitate in
magnetite (ionic radius of 72 and 83 pm, respectively; Shannon, 1976), but the incorporation of Ca is more difficult because the ionic radius of Ca is relatively large compared to
that of Fe2+ [ionic radius of 100 pm (Shannon, 1976) and
78 pm (Bloss, 2000), respectively]. Whereas the substitution
of calcium in magnetite is not widely recognized in natural
settings, it may occur by thermal decomposition mechanism. Vidyasagar et al. (1984) demonstrated that a variety
of complex metal oxides, including calcium ferrite (CaFe2O4) and brownmillerite (Ca2Fe2O5), can be produced
by the thermal decomposition of CaCO3–FeCO3 solid solutions under air or oxygen at >400 °C. Calcium magnetite
has been synthesized at high temperature from CaO– or calcite–hematite melts (Hrynkiewicz et al., 1971; Orewczyk
and Jasienska, 1987; Orewczyk et al., 1995; Orewczyk,
2000), and low-temperature Ca–magnetite is documented
in the remediation of acid mine drainage waters (Wang
et al., 1996; Morgan et al., 2003).
In contrast, other researchers have not observed the
incorporation of Ca in magnetite during the thermal
decomposition of mixed-cation siderite (Gallagher and
Warne, 1981; Gallagher et al., 1981; Ware and French,
1984; Dubrawski, 1991; Hurst et al., 1993; Isambert and
Valet, 2003; Cohn, 2006; Isambert et al., 2006; Bell, 2007;
Thomas-Keprta et al., 2009). Thomas-Keprta et al. (2009)
suggest that the decomposition of an ankerite–dolomite solid solution would follow a sequential pathway in which the
solid solution precursor separates first into calcite and a siderite–magnesite phase prior to the nucleation and growth of
the magnetite. This proposed mechanism would yield either
CaCO3 (or CaO) in the resulting products, but these phases
were not observed in this study or those obtained by others
(Gallagher and Warne, 1981; Dubrawski, 1991; Cohn,
2006; Isambert et al., 2006; Thomas-Keprta et al., 2009).
The failure to detect Ca in magnetite is most likely due to
the low concentration of Ca in the precursor solids used
in previous decomposition studies [maximum 1.3 wt %
(Gallagher and Warne, 1981); trace amounts (Gallagher
et al., 1981; Cohn, 2006); maximum 1.8 wt % (Isambert
75
Fig. 3. Brightfield TEM images of mixed-cation carbonate crystals.
(a) carbonate crystals FD21 prior to e-beam induced decomposition; (b) detail of the crystal in the center of (a) (arrow) before
decomposition, observed along the [010]carbonate zone axis (SAED
pattern in inset); (c) carbonate crystal in (b) after exposure to the
electron beam, now transformed into a mesoporous pseudomorph
of magnetite nanocrystals (with size 65 nm) showing a preferred
crystallographic orientation as shown by the discrete SAED spots
(inset) for the 222 (d222 = 2.42 Å), 400 (d400 = 2.09 Å) and 140
(d440 = 1.48 Å) reflections, corresponding to the [110]magnetite zone
axis.
et al., 2006; shock pressures 8.4–25.9 GPa); 1.8 wt %
(Thomas-Keprta et al., 2009)] compared to the present
study (up to 25.5 wt % in FD15 and 22.8 wt % in FD20).
The effect of temperature on the chemistry of the resulting solid may also be critical. High temperatures could
facilitate the spinodal separation of CaO from a Mgmagnetite (or Mg-ferrite) over time, as reported for the
decomposition of double carbonates (Spinolo and
76
C. Jimenez-Lopez et al. / Geochimica et Cosmochimica Acta 87 (2012) 69–80
Anselmi-Tamburini, 1989). Nevertheless, the results in the
present study suggest that this process does not occur.
Firstly, no CaO, calcite or other Ca-bearing mineral phase
was detected by bulk XRD of the DSC-magnetites. Secondly, our results support the contention that the thermal
history of the samples does not affect the chemical signature
inherited from the precursor. In fact, TEM-magnetites
(and/or ferrites) and DSC-magnetites (and/or ferrites) show
the same chemical signatures, although they had different
thermal histories [TEM-magnetites, analyzed directly after
decomposition, were exposed to electron-beam heating
(Egerton and Malac, 2004) at a T corresponding to the
decomposition of siderite/impure-siderite in vacuum, i.e.,
385–450 °C (Brearley, 2003 and references therein),
whereas DSC-magnetites were heated to 700 °C (higher
temperatures and time after decomposition compared to
TEM-magnetites)]. Therefore, higher temperatures and/or
longer heating times do not cause the spinoidal separation
of the Ca and/or Mg-magnetites (and/or (Ca,Mg)–ferrites).
The main difference between these two sets of samples
(TEM- and DSC-) is the crystal size (5 nm for TEMmagnetites and 40 nm for DSC-magnetites). These differences are caused by the coarsening of the oxide nanocrystals formed after decomposition [as previously reported
for siderite (Angermann et al., 2010) and other carbonates
(Rodriguez-Navarro et al., 2009)].
The formation of metastable solid solutions (rather than
spinoidal separation), as a result of a thermal decomposition process, is also described for other double carbonates,
such as dolomite (Spinolo and Anselmi-Tamburini, 1989)
and impure siderite (Angermann et al., 2010). In fact, the
thermal decomposition of double carbonates has been proposed as an efficient method for the low-T synthesis of
(metastable) mixed metal oxides (e.g., (Mg,Ca)O (Spinolo
and Anselmi-Tamburini, 1989) and (Mn,Zn,Fe)3O4 (Angermann et al., 2010)). The resulting mixed oxides form via diffusionless, topotactic replacement of the parent carbonate
(Spinolo and Anselmi-Tamburini, 1989), thus enabling the
incorporation of metals cations of dissimilar radii in the
oxide. According to the model proposed by RodriguezNavarro et al. (2009), the topotactic transformation occurs
as a consequence of CO2 loss, accompanied by limited atom
displacement, and shrinkage along specific [hkl] directions
in the transformed carbonate. During this process, new
transient structures appear as well as empty spaces (vacancies) between cations (Rodriguez-Navarro et al., 2009). Calcium fills these spaces as the transient structure evolves in
the inverse spinel oxide. Such metastable solid solutions
persist over several hundred degrees above the decomposition temperature (Spinolo and Anselmi-Tamburini, 1989;
Angermann et al., 2010). This explains why magnetites in
the DSC and TEM experiments include Ca and/or Mg despite their different thermal history and the different relative
concentration of cations in the precursor samples.
3.2. Topotaxy between the precursor (Ca,Mg,Fe)CO3 and
the resulting magnetite
An additional experiment was performed with TEM–
FD16, TEM–FD20 and TEM–FD21 to determine whether
the transformation of the mixed carbonate to magnetite
was topotactic (signifying a three-dimensional lattice continuity) between the reactant and product phases (Bloss,
2000). This study was performed to determine whether or
not there was a genetic structural relationship between the
precursor carbonate and the resulting magnetite to explain
the pseudomorphic relationship between reactant and product observed in DSC-magnetites as well as the preferred
crystallographic orientation among DSC-magnetites. Ultimately, this study provides a mechanistic model for the
incorporation of Ca and/or Mg in magnetite.
Fig. 3a and b show mixed carbonate crystals (TEM–
FD21) prior to decomposition by the electron beam. The
SAED pattern shows that this crystal was a single phase,
not a mixture of various carbonate phases. The crystal is
oriented along the [010] zone axis (SAED in inset in
Fig. 3b). After irradiation with the electron beam for
20 min, using a large condenser-lens aperture, a porous
pseudomorph comprised of nanometer-sized magnetite
crystals was produced (Fig. 3c). The pseudomorph had a
typically mottled texture that is characteristic of beamdamaged carbonates (Wenk et al., 1983). The SAED pattern of the magnetite pseudomorph shows diffuse Debye
rings where maxima are clearly observed, indicating that
the magnetite nanocrystals display a highly oriented
arrangement, consistent with the bright field image of the
pseudomorph presented in Fig. 3c. In fact, the SAED in
Fig. 3c shows that the [110] zone axis of magnetite crystals
is parallel to the [010] zone axis of the precursor carbonate
(Fig. 3b). A second systematic orientation relationship is
the ½
441carb//[110]mag. It follows that two of the periodic
bond chains (PBC) of the carbonate precursor (i.e., those
parallel to h
441i and [010] directions) transform into the
<110> direction in the oxide product. These topotactic orientation relationships, observed in all samples studied (see
also Fig. 4), are identical to those found in other carbonates
such as calcite, whose thermal decomposition is also topotactic (Rodriguez-Navarro et al., 2009). Interestingly, other
researchers such as Thomas-Keprta et al. (2009) and Bell
(2007) have shown SAED patterns of magnetite nanocrystal produced upon thermal decomposition of Fe-rich carbonates with similar features as those presented here,
demonstrating a preferred orientation of the magnetite
crystals [see Fig. 6b in Thomas-Keprta et al. (2009) and
Fig. 9 in Bell (2007)]. Nevertheless, these authors do not
comment on the fact that their observations indicate a
topotactic decomposition of the precursor carbonate.
Rodriguez-Navarro et al. (2009) reported that oxide
nanocrystals (about 5 nm in size), formed during the early
stages of in situ carbonate (calcite) decomposition in the
TEM, aggregated in an oriented fashion following the collapse of the mesoporous structure, similar to that shown in
Fig. 3c. This transformation is associated with a release of
mechanical stress from the mismatch between the structure
of the reactant and product phase during the topotactic
decomposition. After this diffusionless coarsening process,
sintering associated with increased temperature and heating
time contributes to the final growth of the crystals to produce the grain sizes observed in DSC experiments when
heating continues above the decomposition temperature
C. Jimenez-Lopez et al. / Geochimica et Cosmochimica Acta 87 (2012) 69–80
77
Fig. 4. TEM of carbonate crystals FD16 (a) and a FD20 (e) before electron beam decomposition with their corresponding AEM–TEM
analysis (inset) and SAED patterns [(b) and (f) for FD16 and FD20, respectively]. TEM of the carbonate (pseudomorphs) transformed into
magnetite nanocrystals after electron-beam heating: TEM–FD16 (c) and TEM–FD20 (g). AEM–TEM analyses (inset) and SAED patterns
[(d) and (h) for TEM–FD16 and TEM–FD20, respectively]. Squared areas correspond to those sequentially analyzed by SAED and AEM–
TEM. SAED patterns correspond to a ca. 500 nm diameter circular area (centered in the squared areas) and show that the magnetite
nanocrystals display a preferred crystallographic orientation with the precursor carbonate (i.e., absence of Debye rings). Solid and dashed
arrows in AEM–TEM spectra indicate the peaks for Mg and Ca, respectively.
of carbonates (Rodriguez-Navarro et al., 2009; Angermann
et al., 2010). Most important, the coarsening process does
not modify the crystallographic relationship between parent
and product phases, nor the chemical signature (i.e., Ca
and/or Mg incorporation), thus preserving the original
topotactic relationship and the chemical fingerprint under
temperature conditions spanning from very low-T (382–
450 °C, TEM) up to high-T (700 °C, DSC). In fact, thermally-activated decomposition of carbonates such as dolomite, ankerite, siderite, magnesite, ottavite, smithsonite,
78
C. Jimenez-Lopez et al. / Geochimica et Cosmochimica Acta 87 (2012) 69–80
cerusite, and calcite (Rodriguez-Navarro et al., 2009 and
references herein) has been shown to be topotactic and
not dependent on heating rate, peak T or heating time,
and pCO2 (Rodriguez-Navarro et al., 2009). Thus, a similar
crystallographic relationship is expected between reactant/
product regardless of the thermal history of the sample.
3.3. Implications for terrestrial and extraterrestrial
magnetites
The experiments from the present study demonstrate that
magnetite produced by the thermal decomposition of
(Fe,Mg,Ca)CO3 preserves a legacy of the geochemistry
and structural attributes of the phase from which it formed.
These signatures could be important in recognizing the origin of some terrestrial and extraterrestrial nanomagnetites
formed through this inorganic pathway. In this context,
the chemical and structural signatures observed in this study
could provide insights, for example, in recognizing the origin of nanophase (Mg,Ca)-magnetites, that may result from
the thermal decomposition of (Fe,Mg,Ca)CO3 under a CO2
rich atmosphere [as likely existed on Early Earth (Ohmoto
et al., 2004) and possibly on Early Mars], following events
such as meteoritic impacts or metasomatism.
In the case of magnetites of the ALH84001 Martian
meteorite, our results suggest that aggregates of anhedral,
fine-grained magnetites observed by Barber and Scott
(2003), typically near fracture zones, are consistent with
the partial decomposition of ALH84001 carbonates, since
these authors show a crystallographic orientation with the
surrounding carbonate similar to that observed in the present study. This thermal decomposition probably occurred
at a temperature not intense enough to fully decompose
the carbonate and to coarsen the magnetites to produce larger euhedral crystals. On the other hand, the textural and
chemical analyses presented in this study show that chemically pure, euhedral magnetite crystals ranging in size between 30 and 50 nm in ALH84001 carbonate disks are
not consistent with a thermal decomposition event. It is
important to note that the chemical purity of these magnetites is still being debated. Some authors claim that they
contain up to 3% Mg (Golden et al., 2006; Bell, 2007) while
others defend the chemical purity of these magnetites
(Thomas-Keprta et al., 2000). Nevertheless, and in the
context of structural relationships, Barber and Scott
(2002) concluded that there was topotaxy between euhedral
magnetite nanocrystals and Ca,Mg,Fe-carbonates in
ALH84001. Note, however that their conclusions were
based on the orientation relationship defined by McTigue
and Wenk (1985) in calcite. The latter have been recently
reinterpreted by Rodriguez-Navarro et al. (2009) as follows: during a thermal decomposition event, and upon
the devolatilization of CO2, the CO2
3 groups in the PBC
directions of siderite (Fig. 5a) convert to O2 groups, which
subsequently rearrange along with metal cations along the
[110] direction for the product oxide (Fig. 5b). In other
words, magnetites align along the [110] directions, which
are parallel to the original PBC direction of the carbonate
precursor (Fig. 5c and d). Such an orientation relationship
between the carbonate precursor and the chemically-pure,
Fig. 5. Orientational relationships between siderite and magnetite structures. (a) structure (left) of the hexagonal unit cell of siderite, R3c
space group, showing the stacking of alternate layers of Fe2+ and CO2
3 ions along the c-axis. The projection of the structure on the (10 14)
rhombohedral cleavage plane (right) shows the different PBCs (Periodic Bond Chains) existing in this carbonate; (b) the structure of
magnetite, Fd3 m space group, showing the orientation of the [110] and [111] directions; (c) structural/orientational relationships between the
carbonate (10
14) planes, and PBC directions, and the magnetite crystals formed upon thermal decomposition of the carbonate precursor, with
<
441>carbonate // [110]magnetite and [010]carbonate // [110]magnetite; (d) example of magnetite crystals aligned // to [110]magnetite formed upon
thermal decomposition of the siderite precursor in the DSC and observed in the TEM.
C. Jimenez-Lopez et al. / Geochimica et Cosmochimica Acta 87 (2012) 69–80
euhedral [111]-elongated magnetite crystals has not been
described in ALH84001. Furthermore, the absence of
porosity and the presence of carbonates that fully surround
the euhedral, >50 nm magnetites are inconsistent with the
topotactic decomposition of the carbonate into magnetite.
In fact, whereas some degree of sintering (at temperatures
higher than those corresponding to the ferroan carbonate
decomposition temperature) is required to reach those sizes
(>50 nm) for the magnetite crystals, the surrounding
ALH84001carbonates are intact (Barber and Scott, 2002;
Thomas-Keprta et al., 2009). These critical points were
not addressed by Barber and Scott (2002). Alternatively,
the relationships determined by Barber and Scott (2002)
may indicate an epitaxial overgrowth of the ferrous carbonate around pre-existing magnetite crystals, as was suggested
by Thomas-Keprta et al. (2009), and, thus, a genetic relationship between these two phases is not guaranteed. Finally, if thermal decomposition of the Ca,Mg,Fe-ALH84001
carbonates was the origin of the euhedral, [111]-elongated,
chemically-pure subset of ALH84001 magnetites, other
chemically-pure oxides (MgO and CaO) should have been
detected, in contrast with findings of the present study
and those of Thomas-Keprta et al. (2009). Furthermore,
no CaO (or CaCO3), that could be indicative of the spinoidal separation of an intermediate mixed oxide, has been
found associated with magnetite in ALH84001. With respect to MgO, Barber and Scott (2002, 2003) describe oriented “foamy” MgO nanocrystals (ca. 3 nm in size)
associated with nanopores and in contact with partially
decomposed magnesite, but none in association with ferroan carbonates.
The chemical and structural fingerprints of magnetite
produced through thermal decomposition of a precursor
phase lead to the conclusion that large, chemically-pure,
euhedral, [111]-elongated ALH84001 magnetites are allochthonous with respect to the carbonate matrix in which
they are embedded, supporting the conclusions of
Thomas-Keprta et al. (2009). Therefore, these magnetites
could not be produced by the thermal decomposition of
the carbonate matrix in which they are embedded, and an
alternative mechanism for their formation must be sought.
ACKNOWLEDGEMENTS
Financial funding was provided by Grants CGL2007-63859,
CGL2010-18274 and MAT2009-11332 from the Spanish Ministry
of Science and Innovation (MCI) and GREIB (BIO103) from
Junta de Andalucia. This research was also funded in part by a
Grant to C.S.R. through the NASA Astrobiology Institute.
D.A.B. is supported by US National Science Foundation Grant
EAR-0920718 and A.R.N. by Grant CTM2007-65713. Thanks go
to Johnson Space Center (NASA) for the DSC analyses, and to
the CIC personal from the University of Granada for technical
assistance. We also thank A. Mucci and H. Vali for valuable comments that greatly improved this manuscript.
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Associate editor: Alfonso Mucci