Chapmanite [Fe Sb(Si O (OH)]: thermodynamic

 

 

Eur. J. Mineral., 33, 357–371, 2021
https://doi.org/10.5194/ejm-33-357-2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Chapmanite [Fe2Sb(Si2O5)O3(OH)]: thermodynamic
properties and formation in low-temperature environments

Juraj Majzlan1, Stefan Kiefer1, Kristina Lilova2, Tamilarasan Subramani2, Alexandra Navrotsky2,
Edgar Dachs3, and Artur Benisek3
1Institute of Geosciences, Friedrich Schiller University, Burgweg 11, 07749 Jena, Germany
2School of Molecular Sciences and Center for Materials of the Universe, Arizona State University,
Tempe, Arizona 85287, USA
3Department of Chemistry and Physics of Materials, University of Salzburg,
Jakob-Haringer-Str. 2a, 5020 Salzburg, Austria

Correspondence: Juraj Majzlan (juraj.majzlan@uni-jena.de)

Received: 18 March 2021 – Revised: 1 June 2021 – Accepted: 4 June 2021 – Published: 2 July 2021

Abstract. In this work, we have determined or evaluated thermodynamic properties of synthetic Sb2O5,
MgSb2O6 (analogue of the mineral byströmite), Mg[Sb(OH)6]2 · 6H2O (brandholzite), and natural chapman-
ite [(Fe1.88Al0.12)Sb(Si2O5)O3(OH)]. Enthalpies of reactions, including formation enthalpies, were evaluated
using reference compounds Sb, Sb2O3, Sb2O5, and other phases, with high-temperature oxide melt solution
calorimetry in lead borate and sodium molybdate solvents. Heat capacity and entropy were determined by relax-
ation and differential scanning calorimetry. The best set of (cid:49)f H o (kJ mol−1) and So (J mol−1 K−1) is byströmite
−1733.0±3.6, 139.3±1.0; brandholzite −5243.1±3.6, 571.0±4.0; and chapmanite −3164.9±4.7, 305.1±2.1.
The data for chapmanite give (cid:49)f Go of −2973.6 ± 4.7 kJ mol−1 and log K = −17.10 for the dissolution reac-
tion (Fe1.88Al0.12)Sb(Si2O5)O3(OH) + 6H+ → 1.88Fe3+ + 0.12Al3+ + 2SiO0
+ 2H2O. Analysis of
2
the data showed that chapmanite is finely balanced in terms of its stability with schafarzikite (FeSb2O4) and
tripuhyite (FeSbO4) under a specific, narrow range of conditions when both aqueous Fe(III) and Sb(III) are abun-
dant. In such a model, chapmanite is metastable by a narrow margin but could be stabilized by high SiO0
2(aq)
activities. Natural assemblages of chapmanite commonly contain abundant amorphous silica, suggesting that
this mechanism may be indeed responsible for the formation of chapmanite. Chapmanite probably forms dur-
ing low-temperature hydrothermal overprint of pre-existing Sb ores under moderately reducing conditions; the
slightly elevated temperatures may help to overcome the kinetic barrier for its crystallization. During weathering,
sheet silicates may adsorb Sb3+ in tridentate hexanuclear fashion, thus exposing their chapmanite-like surfaces
to the surrounding aqueous environment. Formation of chapmanite, as many other sheet silicates, under ambient
conditions, is unlikely.

+ Sb(OH)0
3

1

Introduction

Antimony is an element that enters into both quite soluble
and quite insoluble minerals as it moves through the aque-
ous environment. The solubility of such reservoirs was pre-
viously quantified by Filella and May (2003), Diemar et al.
(2009), Leverett et al. (2012), Roper et al. (2015), and oth-
ers using thermodynamic data. The discrepancies between
the observations of antimony being soluble at some sites but
insoluble at other ones were addressed and resolved by Maj-

zlan et al. (2016). The rich mineralogy of antimony was ex-
tensively summarized by Majzlan (2021), and the details will
not be repeated here.

Two insoluble minerals, considered to be the “ultimate
sinks” of antimony, are tripuhyite (FeSb5+O4) and scha-
farzikite (FeSb3+
2 O4) (Leverett et al., 2012). Tripuhyite has
been identified at a number of sites polluted by Sb (see Ma-
jzlan, 2021), but schafarzikite is rare, restricted to a few lo-
calities where it seems to be primary and not secondary (e.g.,

Published by Copernicus Publications on behalf of the European mineralogical societies DMG, SEM, SIMP & SFMC.

358

J. Majzlan et al.: Thermodynamics of chapmanite

Table 1. Chemical formulae and mineral names of phases investi-
gated in this work. With the exception of chapmanite, all samples
used in this work were synthetic. Space groups and refined lattice
parameters for the antimony phases can be found in Table 2.

2 Materials

valentinite
cervantite
–

Sb2O3
Sb2O4
Sb2O5 · nH2O
Mg[Sb(OH)6]2 · 6H2O brandholzite
MgSb2O6
Fe2Sb(Si2O5)O3(OH)
Fe2O3
MgO
SiO2

byströmite
chapmanite
hematite
periclase
quartz

Sejkora et al., 2007). Its rarity could be explained by the
scarcity of research in reduced environments because most
of the work at the polluted sites is concentrating on their ox-
idized portions. They are believed to release toxic elements,
such as antimony, into the environment. An alternative expla-
nation is that schafarzikite does not form since its nucleation
and growth is kinetically hindered. Another possibility is that
there is a competing phase or phases that scavenge antimony
under such conditions. Iron oxides, the usual scavengers of
many anions, are not good candidates, as they may undergo
reductive dissolution under such conditions. On the other
hand, it has been shown that during reduction–oxidation cy-
cles antimony adsorbed onto goethite will be locked into
tripuhyite and not into the structure of schafarzikite (Burton
et al., 2020).

The aim of this work is to evaluate the thermodynamic
stability of chapmanite, a rare mineral that could, however,
constitute an alternative sink of antimony in slightly reduc-
ing environments. To this goal, first we verified the method-
ology of high-temperature oxide melt solution calorimetry in
molten lead borate on antimony phases, doing a number of
cross-checks. Once assured that this method can yield accu-
rate and precise data, the enthalpy of formation of chapman-
ite was measured. Entropy was obtained by integration of
low-temperature heat capacity data measured by relaxation
calorimetry. Calculations of stability and solubility of chap-
manite in selected exemplary systems document its possible
role in the environment.

Throughout this paper, the phases investigated can be re-
ferred to by their mineral names. In their synthetic form, they
are equivalents of the naturally occurring minerals. The use
of these names improves the clarity of the presentation be-
cause a mineral name is linked not only to a specific chemical
composition, but also to a crystal structure. It is particularly
advantageous in systems with polymorphism, such as among
the antimony oxides. The chemical formulae and mineral
names of the phases considered in this paper are summarized
in Table 1.

Synthetic Sb2O3 (equivalent of valentinite) and Sb2O5 were
purchased from suppliers and used as received. Sb2O4
(equivalent of cervantite) was synthesized by treatment of
Sb2O3 at 700 ◦C for 1 d (Konopik and Zwiauer, 1952). Pow-
dery Sb2O3 was placed into a platinum crucible, covered by
a platinum lid and heated in air. In contrast to the results of
Konopik and Zwiauer (1952), we found that prolonged heat
treatment does not lead to better crystallinity or phase purity
but to amorphization of the sample.

Crystals of Mg[Sb(OH)6]2 · 6H2O (equivalent of brand-
holzite) were synthesized according to the procedure of
Diemar et al. (2009). Two separate solutions were prepared
initially. One of them was 1 M Sb5+ solution, prepared by
mixing deionized water and KSb(OH)6. The suspension was
heated on a heating plate at ≈ 60 ◦C until most of the solid
dissolved. The undissolved residue was separated by de-
cantation. The other solution was 0.1 M Mg2+, prepared by
mixing of deionized water and MgCl2 · 6H2O. The two so-
lutions were mixed, resulting in the immediate formation of
a white precipitate. The suspension was allowed to stand at
room temperature for 2 months and then filtered and washed
several times by deionized water. The filtrate consisted of eu-
hedral crystals of Mg[Sb(OH)6]2 · 6H2O and white, powdery
aggregates of an unknown phase, perhaps of the same com-
position. The crystals were up to 1 mm in size and were sep-
arated from the rest of the sample under a binocular micro-
scope.

sample

chapmanite,

MgSb2O6 was prepared from Mg[Sb(OH)6]2 · 6H2O by
heating at 1000 ◦C for 1 h. The crystals of Mg(Sb(OH)6)2 ·
6H2O were placed into a platinum crucible, covered by a
platinum lid and heated in air. The resulting sample was pow-
dery and grayish.
A natural

nominally
of
Fe2Sb(Si2O5)O3(OH), used in this work originated from the
Pezinok Sb deposit in Slovakia (Polák, 1983, 1988). The
sample consisted of a coating of powdery greenish-yellow
crusts of chapmanite on dark gray quartz with sparse tiny
pyrite crystals. The crusts were scraped of the specimens
and separated by a standard protocol for clay mineral
separation. Briefly, 20 g of the sample under 0.16 mm (after
grinding) was mixed with 300 mL distilled water in a beaker.
Afterwards, 3–4 mL of 0.1 M solution of sodium hexam-
etaphosphate were added, the suspension was ultrasonicated
for 5 min, the volume added up to 2 L in a cylinder. After
24 h, the water column was removed with a suction pump
and the sediment at the bottom discarded. The suspension
from the suction pump was transferred into a beaker, and
a few drops of 15 % HCl were added to coagulate the clay
particles. After coagulation, water was removed with the
suction pump and discarded. The slurry was transferred onto
a thin plastic sheet and dried at 50 ◦C. Further treatment,
owing to the analytical results, is described below.

Eur. J. Mineral., 33, 357–371, 2021

https://doi.org/10.5194/ejm-33-357-2021

J. Majzlan et al.: Thermodynamics of chapmanite

359

3 Methods

Powder X-ray diffraction (PXRD) patterns of all minerals
and reference compounds were collected using a Bruker D8
Advance DaVinci diffractometer employing Cu Kα radiation
(λ = 1.54058 Å). The patterns were collected at room tem-
perature between 5 and 90 ◦C 2θ , with a step size of 0.02◦ 2θ
and a time per step of 1 s. Lattice parameters and quantitative
fractions of the studied phases were refined by a full-profile
fit using the software suite JANA2006 (Petˇríˇcek et al., 2014).
Thermogravimetric (TG) and differential thermal analy-
sis (DTA) of chapmanite was done with a TG 92 Setaram
TG/DTA instrument. The samples were heated from room
temperature up to 900 ◦C in a flow of argon at a heating
rate of 10 ◦C min−1. Thermogravimetric (TG) analysis of
synthetic Sb2O5 was performed with a Labsys Evo instru-
ment from room temperature to 600 ◦C. Heating rate was
10 ◦C min−1, and the measurement was done in flowing air.
The quantitative chemical composition of chapmanite was
determined by electron microprobe using a JEOL JXA-8230.
The operating conditions were set to an accelerating volt-
age of 15 kV, a beam current of 5 nA, and a beam diame-
ter between 5 and 10 µm. The wavelength-dispersive X-ray
spectrometers were used to measure the elements and X-ray
emission lines of Al, Mg, Si, Ca, K, P, S, Fe, Mn (Kα), and
Sb (Lα). To improve the count-rate statistics, the counting
times were 40 s. The standard specimens used for calibration
were Al2O3 for Al, wollastonite for Si and Ca, MgO for Mg,
celestine for S, InSb for Sb, orthoclase for K, apatite for P,
hematite for Fe, and rhodonite for Mn. Peak overlap correc-
tion was used to avoid interference between the lines of Sb
and K. The detection limits, calculated from the peak and
background counts, the measurement time, the beam current,
and the standard material concentration, are 0.07 wt % for Si,
Ca, Mg, and Mn; 0.08 wt % for Fe and Al; 0.09 wt % for P
and S; 0.11 wt % for K; and 0.20 wt % for Sb.

Morphological characterization of the chapmanite sample
was performed using a Carl Zeiss ULTRA Plus FEG scan-
ning electron microscope (SEM) operating with an acceler-
ation voltage of 20 kV. Selected crystals were analyzed by
energy-dispersive X-ray (EDX) analysis.

A portion of the sample was brought into solution by total
digestion in a microwave in a mixture of HNO3, HClO4, and
HF. The concentration of selected elements was measured
by inductively coupled plasma mass spectrometry (ICP-MS).
The instrument used was Thermo Fisher Scientific.

High-temperature oxide melt solution calorimetry has
been described in detail by Navrotsky (1997, 2014). The
experiments were performed at 1073 K in both lead bo-
rate (2PbO · B2O3) and sodium molybdate (3Na2O · 4MoO3)
solvents. Oxygen gas was flushed over the solvent at
90 mL min−1 and bubbled through it at 5 mL min−1.

Low-temperature heat capacity (Cp) was measured by re-
laxation calorimetry using a commercial Physical Properties
Measurement System (PPMS, from Quantum Design, San

Diego, California) at the University of Salzburg, Austria.
With due care, accuracy can be within 1 % at 5 to 300 K and
5 % at 0.7 to 5 K (Dachs and Bertoldi, 2005; Kennedy et al.,
2007). The powdered samples were wrapped in a thin Al foil
and compressed to produce a ≈ 0.5 mm thick pellet, which
was then placed onto the sample platform of the calorimeter
for measurement. Differential scanning calorimetry (DSC)
was used to measure heat capacities near and above room
temperature using a Perkin Elmer Diamond DSC. Details of
the method are described by Benisek et al. (2012). The en-
tropy was calculated by integration of the Cp/T function in
the interval from 0 to 298.15 K.

4 Crystal structure of chapmanite

Chapmanite is a rare sheet silicate with unique structural
features. The crystal structure of this fine-grained mineral
was solved early by electron diffraction (Zhukhlistov and
Zvyagin, 1977). It is related to kaolinite with tetrahedral–
octahedral layers. The vacant sites in the dioctahedral sheets
are capped by Sb3+ ions (Fig. 1), with their lone electron
pairs pointing into the interlayer. Charge balance is achieved
by deprotonation of three OH groups. Zhukhlistov and Zvya-
gin (1977) report a limited (1 %) Si-Al substitution in the
tetrahedral sites, slight excess of Fe, and deficiency of Sb.

A later study by Ballirano et al. (1998) confirmed the
structural model of Zhukhlistov and Zvyagin (1977). Balli-
rano et al. (1998) used Rietveld refinement of powder XRD
data to improve the structural model. They also showed that
the natural samples are mixtures of chapmanite, quartz, mi-
crocline, calcite, and dolomite.

5 Results

antimony

5.1 Characterization and calorimetry of oxides of

The powder XRD patterns of all oxides of antimony show
only sharp peaks of one phase (see supporting electronic in-
formation). The refined lattice parameters are summarized in
Table 2.

Special attention was paid to the hydration state of Sb2O5 ·
nH2O as the amount of H2O strongly influences the calori-
metric results. The color of the initial product changed after
the TG analysis from yellow to white, indicating H2O loss
and possible partial reduction. The total weight loss mea-
sured for our Sb2O5 · nH2O sample was 9.83 wt %. Accord-
ing to Kovalenko et al. (2019), Sb2O5 · nH2O gradually loses
water and becomes H2O-free at around 843 K. Additional
weight loss was attributed to partial reduction of antimony
and release of O2 gas. Using the same interpretation of our
TG data, a weight loss of 9.28 wt % is attributable to H2O,
and the remaining weight loss of 0.55 wt % is due to the par-
tial reduction of the Sb2O5.

https://doi.org/10.5194/ejm-33-357-2021

Eur. J. Mineral., 33, 357–371, 2021

360

J. Majzlan et al.: Thermodynamics of chapmanite

Figure 1. Fragment of the chapmanite structure, showing the tetrahedral sheets (cyan), dioctahedral sheets (brown) populated by Fe3+, and
the Sb3+ atoms (gray) bonded to three oxygen atoms in the dioctahedral sheets. Red balls represent oxygen atoms. Constructed from the
data in Ballirano et al. (1998).

Table 2. Lattice parameters of the studied antimony phases. Lattice parameters constrained by symmetry are not listed. Only the chapmanite
sample was natural. Nominal chemical formulae are listed in Table 1.

Phase

Space group

a (Å)

b (Å)

c (Å)

β (◦)

V (Å3) References

Pccn
Valentinite
Pna21
Cervantite
Sb2O5 · nH2O Fd3m
Byströmite
Brandholzite
Chapmanite

P42/mnm
P3
Cm

4.9166(3)
5.4435(4)
10.365(1)
4.6528(3)
16.1061(5)
5.2172(7)

12.4783(6)
4.8092(4)

5.4141(3)
11.7693(7)

9.2384(7)
9.8640(5)
7.7613(8)

9.001(1)

101.71(1)

332.16(3) Whitten et al. (2004)
308.10(4) Amador et al. (1988)
1113.5(8) Kovalenko et al. (2019)
199.99(2) Byström et al. (1942)
Friedrich et al. (2000)
2215.9(2)
Ballirano et al. (1998)
356.89(4)

The H2O content determined for Sb2O5 · nH2O was taken
into account for the reduction of calorimetric data. For the
calorimetric experiments, the sample Sb2O5 · nH2O with
9.28 wt % H2O was used. The presence of H2O in the sample
was corrected with the assumption that this water is loosely
bound and its heat content is equal to the heat content of free
(unbound) H2O (H1073 K − H298.15 K = 73 kJ mol−1). There-
fore, Tables 3 and 4 contain reactions and reaction enthalpies
that refer to Sb2O5 and not Sb2O5 · nH2O.

The chemical reactions considered in the calorimetric ex-
periments are summarized in Table 3. The drop solution
enthalpies of antimony oxides in lead borate and sodium
molybdate are listed in Table 4. The final oxidation state of
the antimony in the lead borate melt under oxygen flushing
and bubbling is assumed to be 5+. There are no experimental
data to confirm this assumption, but the experience with sim-
ilar systems (e.g., arsenic in oxide melts, Majzlan, 2017) and
the magnitudes of the measured enthalpies support the as-
sumption. No difficulties or irregularities were encountered
in the experiments with lead borate. The dissolution of the
antimony oxides in sodium molybdate results in more scat-
tered data than in lead borate due to baseline shifts. A possi-
ble explanation is a reaction of the antimony with the sodium
molybdate solvent and the formation of an unknown refrac-
tory compound. Nevertheless, consistent drop solution en-
thalpy data can be obtained if the number of drops is in-
creased. For sodium molybdate, the results for Sb2O5 are

more consistent (smaller baseline shifts) than for the Sb3+-
containing oxides.

5.2 Thermodynamics of Mg(Sb(OH)6)2 · 6H2O

(brandholzite)

The drop solution enthalpies of brandholzite were measured
in both oxide melt solvents, and the enthalpies of formation
from oxides and from elements were calculated via a thermo-
chemical cycle, using reactions in Table 3 and their respec-
tive enthalpies (Table 4). The cycle can be expressed by the
equation

(cid:49)f H o(brandholzite) = (cid:49)H7 + (cid:49)H3 − (cid:49)H4 + 12(cid:49)H11

+ (cid:49)H17 + (cid:49)H13 + 12(cid:49)H21.

(22)

The enthalpies of formation calculated from the measure-
ments in the two solvents (Table 5) are consistent within their
uncertainties.

Low-temperature heat capacity data (Fig. 2a) for brand-
holzite show no anomalies, as expected for a phase with
the elements Mg, Sb, O, and H. Integration with polyno-
mials gave the standard entropy at T = 298.15 K of So =
571.0 ± 4.0 J mol−1 K−1. DSC data were measured between
280 and 300 K and agree well with the PPMS data. The DSC
data are ≈ 0.5 % higher than the PPMS data.

Eur. J. Mineral., 33, 357–371, 2021

https://doi.org/10.5194/ejm-33-357-2021

J. Majzlan et al.: Thermodynamics of chapmanite

361

Table 3. Reactions considered in the thermochemical cycles used to calculate the enthalpies of selected reactions in the text and enthalpies of
formation. cr: crystalline; l: liquid; g: gas; sol: solution in the molten calorimetric solvent. The numbers in parentheses indicate temperature
in kelvin (K).

1
2
3
4
5
6

Sb2O3(cr, 298) + O2(g, 1073) → Sb2O5(sol, 1073)
Sb2O4(cr, 298) + 0.5O2(g, 1073) → Sb2O5(sol, 1073)
Sb2O5(cr, 298) → Sb2O5(sol, 1073)
Mg[Sb(OH)6]2 · 6H2O(cr, 298) → MgO(sol, 1073) + Sb2O5(sol, 1073) + 12H2O(g, 1073)
MgSb2O6(cr, 298) → MgO(sol, 1073) + Sb2O5(sol, 1073)
(Fe1.88Al0.12)Sb(Si2O5)O3(OH)(cr, 298) + 0.5O2(g, 1073) →
0.94Fe2O3(sol, 1073) + 0.06Al2O3(sol, 1073) + 0.5Sb2O5(sol, 1073) + 2SiO2(sol, 1073) + 0.5H2O(g, 1073)
MgO(cr, 298) → MgO(sol, 1073)
Fe2O3(cr, 298) → Fe2O3(sol, 1073)
Al2O3(cr, 298) → Al2O3(sol, 1073)
SiO2(cr, 298) → SiO2(sol, 1073)

7
8
9
10
11 H2O(l, 298) → H2O(g, 1073)

2Sb(cr, 298) + 1.5O2(g, 298) → Sb2O3(valentinite, 298)
2Sb(cr, 298) + 2.5O2(g, 298) → Sb2O5(cr, 298)

12
13
14 Mg(cr, 298) + 2Sb(cr, 298) + 12H2(g, 298) + 9O2(g, 298) → Mg(Sb(OH)6)2 · 6H2O(cr, 298)
15 Mg(cr, 298) + 2Sb(cr, 298) + 3O2(g, 298) → MgSb2O6(cr, 298)
16

1.88Fe(cr, 298) + 0.12Al(cr, 298) + Sb(cr, 298) + 2Si(cr, 298) + 4.5O2(g, 298) + 0.5H2(g, 298) →
(Fe1.88Al0.12)Sb(Si2O5)O3(OH)(cr, 298)
17 Mg(cr, 298) + 0.5O2(g, 298) → MgO(cr, 298)
18
19
20
21 H2(g, 298) + 0.5O2(g, 298) → H2O(l, 298)

2Fe(cr, 298) + 1.5O2(g, 298) → Fe2O3(hematite, 298)
2Al(cr, 298) + 1.5O2(g, 298) → Al2O3(cr, 298)
Si(cr, 298) + O2(g, 298) → SiO2(quartz, 298)

Table 4. Drop solution enthalpies ((cid:49)dsH ) of antimony compounds and reference phases in lead borate and sodium molybdate melt at 1073 K.
“No.” is the reaction number of the reactions in Table 3. Data for the drop-solution enthalpies of reference Sb-free phases from Navrotsky
(2014), with the exception of MgO (Lilova et al., 2019). H1073–H298.15 is heat content between T = 298.15 and T = 1073 K. Heat content
is calculated from heat-capacity polynomials in Robie and Hemingway (1995). All data are in kJ mol−1.

Phase

No.

(cid:49)dsH , lead borate (cid:49)dsH , sodium molybdate H1073–H298.15

Valentinite
Cervantite
Sb2O5
Brandholzite
Byströmite
Chapmanite
Periclase
Hematite
Corundum
Quartz
H2O

1
2
3
4
5
6
7
8
9
10
11

−351.17a ± 2.08b (8c)
−148.31 ± 4.69 (16)
−58.00 ± 2.81 (8)
1127.8 ± 22.0 (8)
172.13 ± 3.43 (8)
158.88 ± 3.28 (16)
42.09 ± 0.41
182.29 ± 1.34
120.12 ± 0.17
47.79 ± 0.32

−235.72 ± 2.76 (8)d
−34.80 ± 0.63 (8)
59.13 ± 0.45 (9)
1209.6 ± 24.2 (8)
246.89 ± 3.52 (7)

0.44 ± 0.47

a Mean. b Two standard deviations of the mean. c Number of measurements. d Mielewczyk-Gryn and Lilova (unpublished
data).

73

The relatively large uncertainties (in kJ mol−1) of the
(cid:49)f H o values from the calorimetric work are related to the
high molecular weight of the samples and the large mag-
nitude of heat effects of the primary high-temperature ox-
ide melt solution calorimetric data. The usual uncertainty on
these data is about 1 % of the measured signal. In the case
of brandholzite, the signal is large because of the appreciable

amount of H2O in this phase and the heat effect caused by
this H2O. Hence, the uncertainties are not a sign of any prob-
lems in calorimetry. Despite these uncertainties, the data are
very useful in evaluating the equilibria below.

https://doi.org/10.5194/ejm-33-357-2021

Eur. J. Mineral., 33, 357–371, 2021

362

J. Majzlan et al.: Thermodynamics of chapmanite

Figure 2. Low-temperature heat capacity of (a) brandholzite and (b) byströmite.

Table 5. Summary of the enthalpies of formation for the reference phases and the enthalpies of formation calculated from the calorimetric
data in lead borate or sodium molybdate in this work. “No.” is the reaction number of the reactions in Table 3. All data are in kJ mol−1.
Enthalpies of formation for reference phases from Robie and Hemingway (1995), with the exception of the datum for Sb2O5 (data from this
work and Abramchuk et al., 2020).

Phase

No. (cid:49)f H o, determined

in lead borate

(cid:49)f H o, determined
in sodium molybdate

(cid:49)f H o, reference
phases

−5252 ± 25
−1742.6 ± 5.0
−3164.9 ± 4.7

−5258 ± 27
−1742.0 ± 4.3

Valentinite
Sb2O5
Brandholzite
Byströmite
Chapmanite
Periclase
Hematite
Corundum
Quartz
H2O(l)

12
13
14
15
16
17
18
19
20
21

−708.6 ± 2.9
−953.0 ± 2.4

−601.6 ± 0.3
−826.2 ± 1.3
−1675.7 ± 1.3
−910.7 ± 1.0
−285.8 ± 0.1

5.3 Thermodynamics of MgSb2O6 (byströmite)

ted to a Maier–Kelley polynomial. The results are listed in
Table 6.

The drop solution enthalpy of byströmite was measured in
the two molten oxide solvents, and the enthalpy of forma-
tion from oxides and elements was calculated via a thermo-
chemical cycle, using reactions in Table 3 and their respec-
tive enthalpies (Table 4). The cycle can be expressed by the
equation

(cid:49)f H o(byströmite) =
(cid:49)H7 + (cid:49)H3 − (cid:49)H5 + (cid:49)H17 + (cid:49)H13.

(23)

The standard entropy, calculated from the low-temperature
heat capacity (Fig. 2b), is 139.3±1.0 J mol−1 K−1. DSC data
were measured between 280 and 560 K. In the region of over-
lap, the DSC data are 1.3 % lower than the PPMS data. The
DSC data were shifted to match the PPMS data and then fit-

5.4 Chapmanite: sample characterization

A full-profile refinement of the powder XRD data (Fig. 3a)
quantified the fractions of minerals in the sample. This anal-
ysis gave 85.2 % chapmanite, 9.9 % quartz, and 4.9 % cal-
cite. Small crystals of pyrite, visible during the separation of
the fine fraction, were not captured by this analysis. Inspec-
tion of the sample in a scanning electron microscope did not
reveal the presence of any other crystalline phases or grains
which could be suspected to be amorphous. It has to be noted,
however, that the carbonates were also not seen and are sus-
pected to form microcrystalline coatings on the sheet sili-
cates. Chapmanite forms book-like aggregates of platy crys-
tals (Fig. 3b). Tiny (less than a few micrometers) hexahedral
crystals used to be pyrite. In the EDX analyses, they gave

Eur. J. Mineral., 33, 357–371, 2021

https://doi.org/10.5194/ejm-33-357-2021

J. Majzlan et al.: Thermodynamics of chapmanite

363

Table 6. Coefficients for the Maier–Kelley heat-capacity polynomial Cp = a + bT + cT −2 for byströmite and chapmanite.

a (×10−2)

b (×10)

c (×10−6)

Temperature range (K)

Byströmite
Chapmanite

1.6122
2.2895

1.0579
3.4527

−2.7795
−2.5940

280–560
280–460

strong Fe and S but also O signal, suggesting that they are
pseudomorphs of iron oxides with sulfate after pyrite. This
observation agrees with the fact that pyrite was not captured
by PXRD. It is not clear if pyrite weathered naturally or dur-
ing the initial sample handling. Quartz occurred as larger
grains with typical conchoidal fracture.

This sample was subjected to electron microprobe analy-
ses. The analyses were done on flakes of the chapmanite sam-
ple obtained by the first fine-fraction separation as polish-
ing of this sample turned out to be impossible. These flakes
provide a relatively, but not perfectly, flat surface and are
porous, thus diminishing the quality of the analyses. Hence,
these results can be considered only as semiquantitative.
They are scattered – for example CaO averages at 1.3 wt %
with a standard deviation of 0.5 wt % (n = 30), and MgO is
1.0 ± 0.6 wt %. These results agree well with the fraction of
carbonate refined from the PXRD data. The SO3 signal is
ubiquitous, giving 3.7 ± 4.3 wt %. It agrees with the suppo-
sition that pyrite in the sample weathered to X-ray amor-
phous iron oxides (perhaps schwertmannite) with elevated
sulfur content. The P2O5 concentration of 2.0 ± 0.4 wt %
is attributable to the previous treatment with sodium hex-
ametaphosphate. The only impurity that can be assigned to
chapmanite is Al2O3, with 1.4 ± 1.3 wt %.

Obviously, calorimetric results from such a sample would
be difficult to interpret. Therefore, the sample underwent sev-
eral cycles of cleaning. The powder was re-dispersed three
times in 5 % HCl overnight and then rinsed, filtered, and
dried. Furthermore, a finer fraction of the sample was ob-
tained by re-dispersing the sample, using the same procedure
as described in the methods section but allowing it to sedi-
ment for 48 h, thus eliminating the larger quartz grains. After
this procedure, the sample was rinsed extensively in an at-
tempt to remove the phosphate.

Quartz and carbonates were removed from the sample, as
evidenced by the PXRD data after the treatment. We recog-
nized that not all phosphate was removed, and this impurity
is not easy to correct for. There remained 0.4 wt % P2O5 and
0.2 wt % SO3 in the sample (recalculated ICP-MS data after
the total digestion). Experience with high-temperature ox-
ide melt calorimetry shows, however, that impurities under
1 % can generally be neglected if there is no way to cor-
rect for them. This is true if the contribution of such im-
purities to the measured heat effect is likely to be within
the experimental error of the measurement and as long as
their heat effects are comparable to that of the major phase.
This is the case for P2O5 and SO3. Pyrite, on the other hand,

would be an example of an impurity with much different,
in this case much higher, signal. Our analyses showed that
pyrite was not present, either in the initial or purified sam-
ple. The molar Al / (Al + Fe) ratio of 0.06 was considered
in the chemical formula used for calorimetric calculations.
The molar Sb / (Al + Fe) ratio was 0.98, which is slightly
less than 1 but within the uncertainty of the analysis. A slight
Sb deficiency in chapmanite was also detected by Zhukhlis-
tov and Zvyagin (1977). They assumed that some of the va-
cant octahedra were occupied by Fe although there was no
evidence of Fe2+ in the sample that could support such an
assumption. The method selected (ICP-MS) is considered to
be more accurate and precise than the electron microprobe
analysis, but the downside is that Si is partially volatilized
during the digestion as SiF4. Hence, Si cannot be determined,
and the complete stoichiometry cannot be fixed. The for-
mula constructed for the reduction of the calorimetric data
is (Fe1.88Al0.12)Sb(Si2O5)O3(OH), with molecular mass of
431.1537 g mol−1. All thermodynamic data presented in this
work refer to this formula and molecular mass.

The sample contained adsorbed water that manifested it-
self strongly during the DSC measurements. During the dy-
namic DSC measurement, the endothermic signal from the
adsorbed H2O (at its peak at T ≈ 620 K) exceeded the intrin-
sic heat capacity of the sample by ≈ 70 %. To remove the ad-
sorbed H2O prior to the calorimetric experiments, the sample
was treated statically (i.e., not under continuously increasing
temperature) at 470 K for 3 min and cooled. The loss of struc-
turally bound H2O begins at 620 K and continues to 900 K,
when it is interrupted in the data by a mass gain. This mass
gain is probably related to oxidation of Sb. The nature of
high-temperature products was not investigated. The PXRD
data indicated no structural changes in chapmanite after the
thermal treatment at 470 K.

In addition, the purification led to preferential enrichment
of the sample in smaller particles. Small particle size could
also influence thermodynamic properties. It would be desir-
able, in the future, to repeat the calorimetric experiments on
a synthetic, pure sample of chapmanite. However, synthesis
protocols for such a phase are unknown at this time. Despite
these complexities, we believe that our results are an accu-
rate representation of the thermodynamic properties of this
phase.

https://doi.org/10.5194/ejm-33-357-2021

Eur. J. Mineral., 33, 357–371, 2021

364

J. Majzlan et al.: Thermodynamics of chapmanite

Figure 3. (a) Powder X-ray diffraction pattern of the chapmanite sample after the first treatment step. All peaks belong to chapmanite, with
the exception of two small peaks that are assigned to quartz (Q) and calcite (C). (b) Secondary-electron image of this chapmanite sample,
showing the small book-like aggregates of chapmanite but also small fragments of quartz (Q) and pseudomorphs of iron oxides after pyrite
(P) with the cubic morphology.

5.5 Thermodynamic properties of chapmanite,

(Fe1.88Al0.12)Sb(Si2O5)O3(OH)

From the two solvents commonly used for high-temperature
oxide melt solution calorimetry (see Navrotsky, 2014), lead
borate is the only option for the calorimetry on chapmanite.
This restriction is caused by its ability to dissolve silicates, in
contrast to sodium molybdate. For this reason, we have cross-
checked the two solvents and established that lead borate is a
suitable solvent for calorimetry of antimonous and antimonic
compounds.

The drop solution enthalpy of chapmanite was measured
in lead borate, and the enthalpy of formation from elements
and oxides was calculated via a thermochemical cycle, using
reactions in Table 3 and their respective enthalpies (Table 4).
The cycle can be expressed by the equation

(cid:49)f H o(chapmanite) = 0.94(cid:49)H8 + 0.06(cid:49)H9 + 0.5(cid:49)H1

+ 2(cid:49)H10 + 0.5(cid:49)H11 − (cid:49)H6
+ 0.94(cid:49)H18 + 0.06(cid:49)H19 + 0.5(cid:49)H12
+ 2(cid:49)H20 + 0.5(cid:49)H21.

(24)

(25)

The enthalpy of formation from oxides, for the reaction

0.94Fe2O3 + 0.06Al2O3 + 0.5Sb2O3 + 2SiO2 + 0.5H2O
→ (Fe1.88Al0.12)Sb(Si2O5)O3(OH),

is