Fe K edge XAS study of amethyst. Phys Ch

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/225241818

Fe K-edge XAS study of amethyst. Phys Chem
Mineral
Article in Physics and Chemistry of Minerals · May 2009
DOI: 10.1007/s00269-009-0332-0

CITATIONS

READS

16

156

8 authors, including:
Francesco Di Benedetto

F. D’Acapito

101 PUBLICATIONS 800 CITATIONS

251 PUBLICATIONS 3,389 CITATIONS

University of Florence

SEE PROFILE

Italian National Research Council

SEE PROFILE

Giordano Montegrossi

Italian National Research Council
119 PUBLICATIONS 815 CITATIONS
SEE PROFILE

Some of the authors of this publication are also working on these related projects:
Advanced methods for XAS data collectrion/analysis View project
Structural studies on materials for microelectronics View project

All content following this page was uploaded by F. D’Acapito on 29 November 2016.
The user has requested enhancement of the downloaded file. All in-text references underlined in blue
are linked to publications on ResearchGate, letting you access and read them immediately.

Phys Chem Minerals
DOI 10.1007/s00269-009-0332-0

ORIGINAL PAPER

A Fe K-edge XAS study of amethyst
Francesco Di Benedetto • Francesco D’Acapito • Gabriele Fornaciai
Massimo Innocenti • Giordano Montegrossi • Luca A. Pardi •
Silvia Tesi • Maurizio Romanelli

•

Received: 25 March 2009 / Accepted: 18 September 2009
Ó Springer-Verlag 2009

Abstract An X-ray absorption spectroscopy (XAS) study
of the Fe local environment in natural amethyst (a variety
of a-quartz, SiO2) has been carried out. Room temperature
measurements were performed at the Fe K-edge
(7,112 eV), at both the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure
(EXAFS) regions. Experimental results were then compared with DFT calculations. XANES experimental spectra
suggest Fe to occur mainly in the trivalent state, although a
fraction of Fe2? is identified. EXAFS spectra, on the other
hand, reveal an unusual short distance for the first coordi˚ , the coordination
nation shell: \Fe–O[ = 1.78(2) A
number being 2.7(5). These results allow to establish that
Fe replaces Si in its tetrahedral site, and that numerous
local distortions are occurring as a consequence of the
presence of Fe3? variably compensated by protons and/or
alkaline ions, or uncompensated. The formal valence of Fe,
on the basis of both experimental and DFT structural features, can be either 4? or 3?. Taking into account the
XANES evidences, we suggest that Fe mainly occurs in the
trivalent state, compensated by protons, and that a minor

F. Di Benedetto (&)
M. Innocenti
S. Tesi
M. Romanelli

Department of Chemistry, University of Florence, Florence, Italy
e-mail: DiBeneFr@Geo.UniFi.It
F. D’Acapito
CNR-INFM-OGG c/o ESRF, Grenoble, France
G. Fornaciai
Department of Florence, ARPAT, Florence, Italy
G. Montegrossi
IGG, CNR, Florence, Italy
L. A. Pardi
IPCF, CNR, Pisa, Italy

fraction of Fe4? is stabilised by the favourable local
structural arrangement.
Keywords Amethyst
Fe
XANES
EXAFS

DFT calculations
Fe bioavailability

Introduction
The presence and the crystal chemistry of Fe species in
amethyst (the purple variety of quartz, SiO2) has been the
matter of investigations lasting almost half century (e.g.

Bappu 1953; Tumuklu et al. 2008). One of the major
sources of interest in this subject was related to the identification of the colouring centres of this quartz variety
(Lehmann and Moore 1966; Hutton and Troup 1966;
Cohen and Hassan 1974; Cox 1976, 1977; Cohen 1985;
Adekeye and Cohen 1986; Halliburton et al. 1989;
Rossman 1994; Burkov et al. 2005). However, the
knowledge of chemical and structural relationships
between Fe and quartz has a fundamental interest in toxicology, owing to the fact that Fe has been revealed to be
able to modulate health effects due to surface radicals of
silica polymorphs (Donaldson and Borm 1998; Fubini and
Otero Area`n 1999).
On the basis of the available crystal chemical information, Fe was recognised to occur in three valence states (?2,
?3 and ?4) as the result of investigations performed with
several different techniques. Through electron paramagnetic resonance (EPR) spectroscopy Fe3?, in particular, was
found to occur in at least six different substitutional centres,
either charge uncompensated or compensated by a monovalent cation (H?, Li? or Na?; Mombourquette et al. 1986,
1989; Halliburton et al. 1989; Minge et al. 1990; Weil
1994). Fe2?, on the contrary, was identified as an interstitial

123

Phys Chem Minerals

impurity, presumably located in the channels parallel to the
c-axis of the quartz structure (Fig. 1; Kihara 1990).
In the last 15 years, three relevant spectroscopic (XAS,
EPR, Mo¨ssbauer) contributions were aimed to finally solve
the question concerning the colour centres in amethyst
(Cressey et al. 1993; Schofield et al. 1995; Corteza˜o et al.
2003; Dedushenko et al. 2004). Moreover, these studies
attempted to describe the whole crystal chemistry of the Fe
content in amethyst.
Cressey et al. (1993) and Schofield et al. (1995) investigated several amethyst samples by means of XAS
working at the Fe–L2,3 absorption edge. The obtained
absorption structure evidenced the unambiguous presence
of both Fe2? and Fe3? species, in a more or less constant
ratio. As a consequence, these authors suggested the possible existence of a coupled crystal chemical and crystallographic control in the relative abundances of Fe valence
states in natural crystals.
A successive study by Corteza˜o et al. (2003) reported
the results of a thermally stimulated depolarisation currents

and X-band EPR investigation, identifying a prominent
abundance of a substitutional Fe3? impurity coupled with
an oxygen trapped hole. The same authors first analysed the
EPR spectra of powdered samples, evidencing the disappearance of the features of Fe3? as isolated impurity in
quartz, and the contextual appearance of a broad signal,
attributed to interacting Fe3? ions in an amorphous
surrounding.

Fig. 1 Quartz structure, c-axis projection (mod. from Lehmann and
Moore 1966, and from Rossman 1994). Substitutional Fe can occur in
all the Si tetrahedral sites (small dark grey spheres). Interstitial
(octahedral and large tetrahedral) sites are indicated by medium grey
spheres

123

Dedushenko et al. (2004), in a recent study where they
provided a unequivocal Mo¨ssbauer evidence of the existence of Fe4? in c-irradiated quartz (as already described,
by means of electronic spectroscopy by Cox 1976, 1977),
attributed to substitutional positions the impurity Fe ions,

and suggested that this constrained site should have a
peculiar bond distance and electronic density. Although
Dedushenko et al. (2004) did not express the relative
abundance of Fe4?, the Mo¨ssbauer spectra presented in
their study allowed to point out that Fe4? and Fe3? have
comparable abundance.
The present study on amethyst was undertaken with the
aim of ascertaining the chemical and structural characteristics of Fe impurities at the local scale, and relate them to
its reactivity and bioavailability. The present investigation
was performed through Fe K-edge X-ray absorption spectroscopy, coupled with chemical analysis and DFT
calculations.

Experimental and computing procedures
A large single crystal of Brazilian amethyst was cut parallel
to the (1, 1, -2, 0) natural surface, realising a flat sample
(approximate dimensions, 20 9 35 9 4 mm3), suitable for
X-ray absorption measurements both in the conventional
and in the reflectance modes. The fresh surface has been
successively polished down to 1 lm.
Trace element analysis of the investigated sample was

obtained by induced coupled plasma-atomic emission
spectroscopy (ICP-AES), using an ICP/OES varian
instrument. Sample was dissolved in HF, HNO3 and HCl in
microwave bomb. Detection limits for the investigated
elements were 3, 8, 1, 3, 10 and 10 mg kg-1 for Fe2O3,
Al2O3, Li2O, Na2O, K2O and TiO2, respectively.
XAS measurements were carried out at the European
synchrotron radiation facility (ESRF), at the Italian
beamline BM08 ‘‘Gilda’’ (d’Acapito et al. 1998). The
monochromator was equipped with a pair of Si(111)
crystals and was run in the so-called dynamically focusing
mode (Pascarelli et al. 1996). The experimental energy
resolution was estimated to be about 0.6 eV at the Fe Kedge to be compared with a core hole linewidth of 1.2 eV
(Krause and Oliver 1979). The harmonic rejection was
achieved by using a pair of Pd-coated mirrors having a
cutoff energy of about 18 keV. Spectra have been collected
at the Fe K-edge (7.112 keV) in fluorescence mode making
use of a high-purity Ge solid-state detector. All spectra
were collected at room temperature on a continuously
rotating sample holder in order to minimize the effect of

coherent diffraction from the crystalline matrix. Energy
calibration was checked by measuring a Fe metallic foil
before and after each spectrum. The energy calibration was

Phys Chem Minerals

achieved by defining 7,112.0 eV the first inflection point of
the absorption spectrum of metallic Fe following the data
of Bearden and Burr (1967). The model compounds used as
reference of the edge position in the various Fe valence
states were hematite, a-Fe2O3, and wustite, FeO (Fig. 2).
Model compounds were measured at the same time as a
metallic Fe foil for an accurate energy calibration.
EXAFS data were extracted from the raw absorption
coefficient spectrum with the ATHENA code and analysed
with the ARTEMIS code (Ravel and Newville 2005).
Theoretical EXAFS paths for the Fe–O pair were generated
with the feff8.0 code (Ankudinov et al. 1998) using Muffin
Tin potentials and the Hedin–Lundqvist approximation for
their energy-dependent part. Data were analysed in the k

˚ -1, and were Fourier transformed
range, k = [2.5–10.0] A
˚ (see Fig. 3). In the
in the R interval, R = [0.8–2.9] A
Fourier spectrum shown in Fig. 3, the two main peaks
marked by arrows correspond to the Fe–O and Fe–Si
coordination shells. The analysis was carried out by using
single scattering paths to reproduce these two features.
Ab initio calculations based on the density functional
theory (DFT) were carried out by using the VASP code
(Kresse and Hafner 1993). The full potential projected
augmented-wave (PAW) method with local spin density
generalized gradient approximation (LSD-GGA) was used
as implemented in VASP. The calculation was based on a
(2*2*2) supercell of 72 atoms taken from the quartz structure (Kihara 1990) where a Si atom was substituted with a Fe
ion. Different calculations considering Fe as 4?, 3?, ‘High’
and ‘Low’ spin as well as a complex iron ? hydrogen were
carried out. The reciprocal (k) space was sampled with a
single point (C point). In order to account for the strong

Fig. 3 Fourier transform of the EXAFS data (shown in the inset).
˚ -1 with
The transform was carried out in the interval k = [1.1–8.0] A
a k2 weight. The arrows mark the Fe–O and Fe–Si coordination shells

localization of the Fe d states, the Hubbard model for the onsite Coulomb interactions was considered with the values of
U = 4.5 eV and J = 0.9 (Jiang and Guo 2004). A plane
wave cutoff of 480 eV was used and convergence criteria
were 10-6 eV on the energies in the electronic self-consis˚ -1 for forces acting on the atoms
tent loops, and 10-3 eV A
in the ionic relaxation loops. Geometry optimizations were
carried out by minimizing the atomic forces to the convergence values previously specified.

Results
Trace element composition
The results of the ICP-AES investigations (Table 1) confirm the presence of the most ubiquitous replacing and
interstitial trace ions. Fe content in the sample is comparable with other investigated amethyst samples (Schofield
et al. 1995). Al content is relevant, as well as the amounts
of alkaline ions, which act as charge compensators for Al
and other trivalent species.

Table 1 Trace element content of the analysed sample

Fig. 2 XANES spectra of amethyst sample compared with model
compounds for Fe2? (wustite) and Fe3? (hematite) and Fe in Sodium–
Silica glass (Gliozzo et al. 2009). In the inset the pre-edge peak of
amethyst sample is reported (dots) together with the fit with 2 lines
(continuous line) and the residual (dashed line)

Oxide

Amount (mg kg-1)

Fe2O3

74(1)

Al2O3

232(1)

Li2O

31(1)

Na2O

\3

K2O

33(1)

TiO2

\10

123

Phys Chem Minerals

X-ray absorption near-edge structure (XANES)
The X-ray absorption near-edge structure (XANES) spectra
of the sample, compared with two crystalline and one glass
model compounds, is shown in Fig. 2. The crystals represent
models for the edge position in the case of Fe2? (wustite)
and Fe3? (hematite) valence states. The glass sample comes
from an investigation on Roman glasses (slab labelled FN5
in Gliozzo et al. 2009) for which electron micro probe
analysis revealed a dominant sodium-silicate composition
(SiO2 68%, Na2O 17%) and EXAFS revealed Fe in the 3?
state, in agreement with literature on highly sodic glasses
˚ . This
(Burkhard 2000) with 4 oxygen neighbours at 1.90 A
compound is used as a reference for the intensity of the preedge peak. The edge of the amethyst sample is at a slightly
lower energy than that of hematite (Fe3?) and well above
that of wustite (Fe2?) and the height of the pre-edge peak is
about at the same energy position and of the same intensity
of that observed in the glass sample. This suggests that Fe is
predominantly in the 3? valence state. The energy shift with
respect to hematite could be due to different modulations
coming from the structure as well as from a minor content of
Fe2? to which XANES is more sensitive than EXAFS
(Rovezzi et al. 2009).
The peaks appearing in the pre-edge region (7,108–
7,118 eV, inset of Fig. 2) were best fitted with a model
comprising an arctangent function (modelling the edge
jump) plus a double pseudo-Voigt line. We found evidence
of two lines at 7,114.6(2) eV with an amplitude (relative to
normalized edge jump) of 11 (1)%, and at 7,112.9(3) with
an amplitude of 2(1)%. The integrated intensity of this
peak is 0.34(1) and its centroid is at 7,114.4(1) eV. Considering that Fe(II) and Fe(III) ions in tetrahedral environment are reported to exhibit peaks, respectively at 7,112
and 7,114 eV with amplitudes of about 10% (Galoisy et al.
2001), we can derive that Fe is present predominantly in
the 3? state with a possible contribution from Fe2? estimated to be less than 20%. This is further confirmed by
comparing the values of the integrated intensity and centroid values with the reference table established by Wilcke
et al. (2001), where the values reported in our sample fall in
the region of tetrahedral Fe3? compounds [an energy shift
of 1 eV is apparent due to the fact that the Fe edge was
defined in that paper at 7,111.08 instead of 7,112 eV, as in
the present case: see the same Table in Giuli et al. (2002)].

tetrahedral Fe3? in silicate glasses (Farges et al. 2004) or
crystals (Giuli et al. 2001). The related Debye–Waller
factors are, respectively r2Fe–O = 0.0077(4) and r2Fe–Si =
˚ 2. r2Fe–O is in the same range as that observed for
0.004(1) A
Fe-bearing silicate glasses (e.g. Giuli et al. 2002; Farges
et al. 2004) suggesting the presence of a multiplicity of
Fe–O distances.
Density functional theory calculations
DFT calculations were performed to compare and interpret
the EXAFS experimental findings. In particular, we have
simulated the environment of a Fe4? and a Fe3? ion
substituting for Si in its site in the structure of quartz
(Corteza˜o et al. 2003; Dedushenko et al. 2004). In the latter
case the extra positive charge was compensated either with
a neutralizing background charge on the cell or by creating
a complex with an H? ion (Mombourquette et al. 1986). In
this latter case, Fe is inserted in a O tetrahedral site and H
compensates one of the O dangling bonds in the same site.
A test on the reliability of the pseudopotentials used
by DFT was carried out on the structures of fayalite,
Fe2?SiO4, magnetite, Fe2?Fe23?O4, and hematite,
Fe23?O3, finding the first shell Fe–O bond length data
expanded of about 0.7%, averaging all data on these
structures (note that DFT calculation of model structures
considered the same potentials, energy cutoff and convergence criteria of amethyst calculations; only the k-space
mesh considered was made up of 3*3*3 points determined
with the Monkhorst–Pack scheme). The data presented
here account for this average expansion factor. In both
Fe3? and Fe4? pure substitutional models DFT finds two
pairs of distances with a small difference in length
˚ ) with average values of 1.84 and 1.78 A
˚
(\0.02 A
(Table 1; Fig. 4). A more disordered environment is found
for the Fe–H complex, where however, the first shell dis˚ for the 3 shortest bonds. For the Fe–
tance is about 1.81 A

Extended X-ray absorption fine structure (EXAFS)
The EXAFS data provide quantitative results on the local
geometry around Fe. The best fit result indicates that Fe is
˚ and 1.3(7) Si
coordinated to 2.7(5) O atoms at 1.78(2) A
˚
atoms at 3.14(5) A (Fig. 3). Moreover, Fe–O bond length
is considerably shorter than that observed in literature for

123

Fig. 4 Structural models of the Fe substitutional sites, obtained from
DFT calculations: the Fe3?:H? (left) and Fe4? (right) are shown with
the channels along the c-axis projection. In the left model, Fe bonds
˚ and the fourth (bonded to the proton) at
three O ions at 1.81 A
˚ ; in the right model, Fe bonds two O ions at 1.78 and two at
2.06 A
˚ . SiO4 tetrahedra are shown as polyhedra, for clarity
1.80 A

Phys Chem Minerals

Si shell we note a double distance in Fe4?, a single distance
in Fe3? and multiple distances in the Fe–H complex. In all
cases the Fe ions were found in high spin (HS, electron spin
quantum number S = 5/2 for Fe3? and S = 2 for Fe4?)
state: for pure substitutional Fe3? also a calculation forcing
a low spin state (LS, S = 1/2) was carried out but the cell
energy was found to be considerably higher (2.2 eV) than
the HS case. The data on Fe–O bond distances are in good
agreement with the experimental findings on the average
Fe–O bond lengths in tetrahedral coordination (1.89 and
˚ for Fe3? and Fe4?, respectively) in Na5FeO4 and
1.80 A
Na4FeO4 crystals (Jeannot et al. 2002) and for the Fe–O
distance in tetrahedral Fe3? sites in tetra-ferriphlogopite
(Giuli et al. 2001). Note the reduction of the Fe–O bond
length in the Fe3?–H? complex that can be attributed to the
Coulomb repulsion between these two positive ions that
pushes Fe towards the O neighbours.

intensity and amplitude of the pre-edge peak) as well as
˚ , and the coordiEXAFS data: Fe–O distance, 1.78(2) A
nation number, 3.1(5), allow to selectively assign Fe to a
tetrahedral (TD) site. The possible occurrence of Fe in the
interstitial octahedral (OH) site (Rossman 1994) appears by
far lower than the population of TD sites: the subordinate
occurrence of Fe in interstitial OH cannot, in fact, be ruled
out at all, due to the detection limits of the XAS technique
under the present conditions. The obtained results provide a
robust indication as far as the nature of the TD site is
concerned. Note in particular that the occurrence of Fe with
a peculiar reduced distance to the first neighbour oxygen
shell (if compared with literature on Fe3? ions in glass and
crystals; Table 2) was predicted by Dedushenko et al.
(2004) and perfectly agrees with their Mo¨ssbauer data. On
the basis of the present results, therefore, a replacement
mechanism involving Fe for Si in their tetrahedral sites can
be identified as accounting most of the Fe impurities in
amethyst.

Discussion
Fe valence
On the basis of the obtained results a detailed insight of the
Fe speciation within amethyst can be derived.
Fe coordination
Remarkably, bulk Fe results largely concentrated in a
single type of structural site: the XANES data (integrated

The comparison of the computational and EXAFS experimental data suggest that the first shell Fe–O and the second
shell Fe–Si distances in amethyst compatible with the fourvalent state and with the 3? state coupled to H. The
occurrence of Fe in low spin state could be another possible
solution, but the kind of environment (low spin states are

Table 2 Valence, coordination number and mean Fe–O distance of Fe in amethyst and in some reference compounds
Mineral/compound

Formula

Valence

˚)
N, \RFe–O[ (A

˚)
N, \RFe–Si[ (A

Amethyst

SiO2

Mostly 3?

2.7(5), 1.78(2)

1.3(7), 3.14(5)

This study, exp.

4?

4, 1.78

2, 3.18

This study, DFT

Source

2, 3.25
?

3 , HS

4, 1.84

4, 3.24

This study, DFT

3?, LS

4, 1.82

2, 3.17

This study, DFT

3? H?, HS

3, 1.81

2, 3.18

1, 2.03

3.27

2, 3.26
This study, DFT

3.39
Sodium ferrate (III)

Na5FeO4

?3

1.890 [TD]

1

Sodium ferrate (IV)
Magnetite

Na4FeO4
Fe3O4

?4
?2, ?3

1.804 [TD]
2.06 [OH]

1
2

1.89 [OH]
Wustite

FeO

?2

2.15 [OH]

3

Hematite

Fe2O3

?3

1.95–2.12 [OH]

4

Fayalite

FeSiO4

?3

2.12–2.24 [OH]

5

2.06–2.29 [OH]
Tetra-ferriphlogopite

?3

1.86 [TD]

?2

2.22 [OH]

6

1 Jeannot et al. (2002), 2 Hamilton (1958), 3 Bragg and Claringbull (1965), 4 Blacke et al. (1966), 5 Fujino et al. (1981), 6 Giuli et al. (2001)

123

Phys Chem Minerals

typical of octahedral sites where the crystal field is stronger) and the higher energy of the cell found from DFT
make this situation unlikely. Nonetheless, the present
XANES and most of the literature agree to point out that Fe
occurs mainly in the trivalent state. The experimental Fe–O
distance, conceivably smaller than that expected for trivalent Fe in pure substitutional coordination (Table 2), could
be related to the presence of complexes involving charge
compensating cations (Li?, H?) that were proposed in
literature (Mombourquette et al. 1989; Halliburton et al.
1989) to realize charge neutral complexes with Fe3?.
Indeed the calculation on the Fe3?–H? complex matches
well the experimental data both for the first and the second
shell providing a strong evidence of the realization of this
structure in amethyst.
The agreement of the DFT calculation performed
assuming four-valent Fe with the experimental EXAFS Fe–
O distance is also particularly interesting. These results, in
fact, fully agree with the Mo¨ssbauer results of Dedushenko
et al. (2004). Moreover, it is noteworthy to recall that the
present data are not in contrast with the available EPR
literature (e.g. Corteza˜o et al. 2003; Weil 1994) which
point to a variety of Fe3? species in quartz. Indeed, Fe4?
being an integer spins species, is usually silent at room
temperature and X-band EPR spectroscopy.
As noted by Rossman (1994), the controversial data
about valence states of Fe in amethyst deserves further
experimental confirmations to be fully unravelled. We are
aware that, on the basis of the present data, a final assessment of the existence of Fe4? in natural amethyst cannot
unambiguously be proposed. Nevertheless, the most relevant aspects of the present study allow us to constrain the
hypotheses on Fe speciation in amethyst. Accordingly, we
suggest that most of Fe occurs in the trivalent state in
replacement of Si. The availability of several charge compensating ions (Mombourquette et al. 1986, 1989; Halliburton et al. 1989; Minge et al. 1990; Weil 1994), allows
Fe3? to be accommodated in several ways, with own local
structural distortions. Fe4?, responsible of the appearance
of the purple colour in amethyst, occurs in subordinate
amount (natural crystals can, of course, present different
ratios with respect to the synthetic analysed by Dedushenko
et al. 2004). The c irradiation, indicated as oxidising agent
producing Fe4? (e.g. Rossman 1994; Dedushenko et al.
2004), can act on trivalent Fe ions presenting a very
favourable local arrangement, with metal–oxygen distances
perfectly suitable for four-valent Fe (Fig. 4).
Further high-frequency high-field EPR investigation at
very low temperatures are in progress, to verify if appropriate spectra for Fe4? can be detected. The combination of
high magnetic fields and low temperature, in fact, allows to
specifically investigate spin systems with even number of

123

unpaired electrons (e.g. Telser et al. 1998; Nesterova et al.
2008).
A qualitative relationship can be established between the
present experimental data and the results proposed by
Corteza˜o et al. (2003) on the Fe behaviour when amethyst
crystals are crushed or ground. These authors, in fact,
observed the complete disappearance of Fe(III) EPR signals of amethyst, and the corresponding appearance of a
new broad signal attributed to Fe-oxide clusters. This
behaviour could be easily interpreted on the basis of the
local structural features of Fe presented here. Fe, replacing
Si in a very small site, can act as local centre of crystal
fragility when mechanical energy is provided. Accordingly, Fe ions would be preferably enriched at the newly
generated surfaces, and thus available for further chemical
evolution towards thermodynamically more stable phases
(as Fe-oxides).
On the basis of the present results, structural and crystal
chemical features of Fe in quartz severely modulate its
bioavailability, especially when dust inhalation is associated with mechanical and/or thermomechanical treatments.
Acknowledgments The authors acknowledge the Tuscany Administration for funding this research under the programme ‘‘Progetto di
ricerca per l’individuazione delle cause di variazione della reattivita`
superficiale della silice cristallina, nei principali comparti di lavoro
toscani, in relazione alla sua potenziale patogenicita`’’. Italian CNR is
also acknowledged for support. Authors acknowledge the European
Synchrotron Radiation Facility for provision of synchrotron radiation
facilities during experiments SI1593 and SI1773. Authors are also
indebted to L. Pellicci of the Ce.Ri.Col Lab. for the ICP-AES
investigations, and to N. Capolupo and P. A. Pozzi of the University
of Florence and G. Saviozzi for sample preparation. F. dA.
acknowledges E. Gliozzo for kindly permitting the publication of the
data relative to the glassy sample. The manuscript benefited of the
stimulating review by Y. Pan and an anonymous reviewer to whom
authors express their warmest thanks.

References
Adekeye JID, Cohen AJ (1986) Correlation of Fe4? optical anisotropy, Brazil twinning and channels in the basal plane of amethyst
quartz. Applied Geochem 1(1):153–160
Ankudinov AL, Ravel B, Rehr JJ, Conradson SD (1998) Real-space
multiple-scattering calculation and interpretation of X-rayabsorption near-edge structure. Phys Rev B58:7565–7576
Bappu MKV (1953) Spectroscopic study of amethyst quartz in the
ultraviolet and infrared regions. Indian J Phys 27:385–392
Bearden JA, Burr AF (1967) Re-evaluation of X-ray atomic energy
levels. Rev Mod Phys 39:125–142
Blacke RZ, Hessevick RE, Zoltai T, Finger LW (1966) Refinement of
the hematite structure. Am Mineral 51:123–129
Bragg L, Claringbull GF (1965) Crystal structure of minerals. In:
Bragg L, Claringbull GF (eds) The crystalline state, vol 4. Bell
and Sons, London
Burkhard DJM (2000) Iron bearing silicate glasses at ambient
conditions. J Non-Cryst Solids 275:175–188

Phys Chem Minerals
Burkov VI, Egorysheva AV, YuF Kargin, Mar’in AA, Fedotov EV
(2005) Circular dichroism spectra of synthetic amethyst crystals.
Crystallogr Rep 50(3):461–464
Cohen AJ (1985) Amethyst color in quartz, the result of radiation
protection involving iron. Am Mineral 70(11–12):1180–1185
Cohen AJ, Hassan F (1974) Ferrous and ferric ions in synthetic
a-quartz and natural amethyst. Am Mineral 59:719–728
Corteza˜o SU, Pontuschka WM, Da Rocha MSF, Blak AR (2003)
Depolarisation currents (TSDC) and paramagnetic resonance
(EPR) of iron in amethyst. J Phys Chem Solids 64:1151–1155
Cox RT (1976) ESR of an S = 2 centre in amethyst quartz and its
possible identification as the d4 ion Fe4?. J Phys C Solid State
Phys 9:3355–3361
Cox RT (1977) Optical absorption of the d4 ion iron(4?) in
pleochroic amethyst quartz. J Phys C Solid State Phys
10(22):4631–4643
Cressey G, Henderson CMB, van der Laan G (1993) Use of L-edge
X-ray absorption spectroscopy to characterize multiple valence
states of 3d transition metals; a new probe for mineralogical and
geochemical research. Phys Chem Miner 20:111
d’Acapito F, Colonna S, Pascarelli S, Antonioli G, Balerna A, Bazzini
A, Boscherini F, Campolungo F, Chini G, Dalba G, Davoli I,
Fornasini P, Graziola R, Licheri G, Meneghini C, Rocca F,
Sangiorgio L, Sciarra V, Tullio V, Mobilio S (1998) GILDA
(Italian Beamline) on BM8. ESRF Newsl 30:42–44
Dedushenko SK, Makhina IB, Marin AA, Mukhanov VA, Perfiliev
YD (2004) What oxidation state of iron determines the amethyst
colour? Hyperfine Interact 156(157):417–422
Donaldson K, Borm PJA (1998) The quartz hazard a variable entity.
Ann Occup Hyg 42(5):287–294
Farges F, Lefrere Y, Rossano S, Berthereau A, Calas G, Brown GE Jr
(2004) The effect of redox state on the local structural
environment of iron in silicate glasses: a combined XAFS
spectroscopy, molecular dynamics, and bond valence study.
J Non-Cryst Solids 344:176–188
Fubini B, Otero Area`n C (1999) Chemical aspects of the toxicity of
inhaled mineral dusts. Chem Soc Rev 28:373–381
Fujino K, Sasaki S, Take´uchi Y, Sadanaga R (1981) X-ray
determination of electron distributions in forsterite, fayalite
and tephroite. Acta Crystallogr B37:513–518
Galoisy L, Calas G, Arrio MA (2001) High-resolution XANES
spectra of iron in minerals and glasses: structural information
from the pre-edge region. Chem Geol 174:307–319
Giuli G, Paris E, Wu Z, Brigatti MF, Cibin G, Mottana A, Marcelli A
(2001) Experimental and theoretical XANES and EXAFS study
of tetra-ferriphlogopite. Eur J Mineral 13:1099–1108
Giuli G, Pratesi G, Cipriani C, Paris E (2002) Iron local structure in
tektites and impact glasses by extended X-ray absorption fine
structure and high-resolution X-ray absorption near-edge structure spectroscopy. Geochim Cosmochim Acta 66:4347–4353
Gliozzo E, Santagostino Barbone A, D’Acapito F, Turchiano M,
Turbanti Memmi I, Volpe G (2009) The sectilia panels of
Faragola (Ascoli Satriano, Southern Italy): a multi-analytical
study of the green, marbled (green and yellow), blue and
blackish glass slabs. Archaeometry (in press). doi:10.1111/
j.1475-4754.2009.00493.x
Halliburton LE, Hantehzadeh MR, Minge J, Mombourquette MJ,
Weil JA (1989) EPR study of Fe3? in a-quartz: a reexamination
of the lithium-compensated center. Phys Rev B40:2076–2081
Hamilton WC (1958) Neutron diffraction investigation of the 119°K
transition in magnetite. Phys Rev 110:1050–1057

Hutton DR, Troup GJ (1966) Paramagnetic resonance centres in
amethyst and citrine quartz. Nature 211:621
Jeannot C, Malaman B, Ge´rardin R, Oulladiaf B (2002) Synthesis,
crystal, and magnetic structures of the sodium ferrate (IV)
Na4FeO4 studied by neutron diffraction and Mo¨ssbauer techniques. J Solid State Chem 165:266–277
Jiang X, Guo GY (2004) Electronic structure, magnetism, and optical
properties of Fe2SiO4 fayalite at ambient and high pressures:
a GGA ? U study. Phys Rev B69:155108, 6 pp
Kihara K (1990) An X-ray study of the temperature dependence of the
quartz structure. Eur J Mineral 2:63–77
Krause MO, Oliver JH (1979) Natural widths of atomic K and L
levels, Ka X-ray lines and several KLL Auger lines. J Phys
Chem Ref Data 8:329–338
Kresse G, Hafner J (1993) Ab initio molecular dynamics for liquid
metals. Phys Rev B47:558–561
Lehmann G, Moore WJ (1966) Color center in amethyst quartz.
Science 152:1061–1062
Minge J, Mombourquette MJ, Weil JA (1990) EPR study of Fe3? in
a-quartz: the sodium-compensated center. Phys Rev B42:33–36
Mombourquette MJ, Tennant WC, Weil JA (1986) EPR study of Fe3?
in a-quartz: a reexamination of the so-called/center. J Chem Phys
85:68–79
Mombourquette MJ, Minge J, Hantehzadeh MR, Weil JA, Halliburton
LE (1989) EPR study of Fe3? in a-quartz: hydrogen-compensated center. Phys Rev B39:4004–4008
Nesterova OV, Petrusenko SR, Kokozay VN, Skelton BW, Jezierska
J, Linert W, Ozarowski A (2008) Structural, magnetic, highfrequency and high-field EPR investigation of double-stranded
heterometallic [{Ni(en)2}2(l-NCS)4Cd(NCS)2]n
nCH3CN polymer self-assembled from cadmium oxide, nickel thiocyanate and
ethylenediamine. Dalton Trans 2008:1431–1436. doi:10.1039/
b713252b
Pascarelli S, Boscherini F, D’Acapito F, Hrdy J, Meneghini C,
Mobilio S (1996) X-ray optics of a dynamical sagittal-focusing
monochromator on the GILDA beamline at the ESRF. J Synchrotron Rad 3:147–155
Ravel B, Newville M (2005) ATHENA, ARTEMIS, HEPHAESTUS:
data analysis for X-ray absorption spectroscopy using IFEFFIT.
J Synchrotron Radiat 12:537–541
Rossman G (1994) Coloured varieties of the silica minerals. In:
Heaney PJ, Prewitt CT, Gibbs GV (eds) Rev Mineral 29:433–
467
Rovezzi M, D’Acapito F, Navarro-Quezada A, Faina B, Li T,
Bonanni A, Filippone F, Amore Bonapasta A, Dietl T (2009)
Local structure of (Ga,Fe)N and (Ga,Fe)N:Si investigated by
X-ray absorption fine structure spectroscopy. Phys Rev
B79:195209. doi:10.1103/PhysRevB.79.195209
Schofield PF, Henderson CMB, Cressey G, van der Laan G (1995) 2p
X-ray absorption spectroscopy in the earth sciences. J Synchrotron Radiat 2:93–98
Telser J, Pardi LA, Krzystek J, Brunel L-C (1998) EPR spectra from
‘‘EPR-silent’’ species: high field EPR spectroscopy of aqueous
Chromium(II). Inorg Chem 37:5769–5775
Tumuklu A, Gumus H, Sen S (2008) Role of trace elements in natural
amethysts in colouring. Asian J Chem 20(5):4138–4140
Weil JA (1994) EPR of iron centres in silicon dioxide. Appl Magn
Reson 6:1–16
Wilcke M, Farges F, Petit P-E, Brown GE Jr, Martin F (2001)
Oxidation state and coordination of Fe in minerals: An Fe
K-XANES spectroscopic study. Am Mineral 86:714–730

123