The new mineral species graulichite-(La), ideally
LaFe33+(AsO4)2(OH)6, has been discovered in the
Patte d'Oie mine, Bou Skour mining district, Morocco. It occurs as yellow
rhombohedral crystals, up to 0.1 mm in size, with a resinous luster,
associated with malachite, agardite-(La), conichalcite, and a still
undetermined REE carbonate. Crystals are chemically zoned and two
homogeneous domains were identified, corresponding to the empirical chemical
formulae (calculated on the basis of 6 cations per formula unit, assuming
the occurrence of 14 O atoms)
(La0.34Ce0.20Ca0.11Sr0.07Pb0.05K0.04)Σ0.81(Fe2.163+Al0.84Cu0.20)Σ3.20(As1.23P0.39S0.37)Σ1.99O14H6.13
(domain #1) and
(La0.38Ce0.22Sr0.10Ca0.09Pb0.05K0.06)Σ0.90(Fe2.603+Al0.49Cu0.20)Σ3.29(As0.91P0.50S0.40)Σ1.81O14H6.53
(domain #2). Single-crystal unit-cell parameters are a=7.252(13), c=16.77(3) Å, V=764(3) Å3, space group R-3m. The eight strongest
reflections in the observed X-ray powder diffraction pattern are (d in Å,
visually estimated intensity): 5.86, medium; 3.045, strong; 2.511,
medium-weak; 2.239, medium; 1.960, medium-weak; 1.813, medium-weak; 1.689,
medium-weak; 1.478, medium. Graulichite-(La) belongs to the dussertite group
within the alunite supergroup. It is the La analogue of graulichite-(Ce) and
the Fe3+ analogue of arsenoflorencite-(La).
Introduction
The alunite supergroup is currently formed by more than 50 different mineral
species, characterized by the general chemical formula
DG3(TX4)2X6′, where D can be represented by monovalent,
divalent, trivalent, and tetravalent cations, as well as vacancy; G is
usually a trivalent cation and less frequently a divalent one; T is a
hexavalent or pentavalent cation, and rarely Si4+; and X and X′ are
O2-, (OH)-, (H2O), or F-. Species characterized by T = S6+ or As5+ are typically formed during the weathering of sulfide
deposits (e.g., Dutrizac and Jambor, 2000) and may play a relevant
environmental role in controlling the fate of potentially toxic elements
(PTE), including As. This element typically occurs in minerals belonging to
the dussertite and beudantite groups within the alunite supergroup (Bayliss
et al., 2010). Arsenic can also partially replace S in jarosite in acid mine
drainage systems, with important implications on the As mobility (e.g.,
Gieré et al., 2003; Martín Romero et al., 2010; Burton et al.,
2021). Moreover, the identification of alunite supergroup minerals may
explain other geogenic PTE anomalies in some localities (e.g., Mauro et al.,
2021).
In the framework of an ongoing study of the mineralogy of the Bou Skour
mining district, Morocco, carried out by the Association Française de
Microminéralogie, small yellow rhombohedral crystals were
observed on a centimeter-sized specimen of malachite grown on iron oxides and
associated with an agardite-like mineral. Preliminary chemical and
micro-Raman data showed that these crystals could correspond to the
La analogue of graulichite-(Ce). Further studies confirmed the preliminary
results, and the mineral and its name were approved by the Commission on New
Minerals, Nomenclature and Classification of the International Mineralogical
Association (CNMNC-IMA) under voting number 2020-093. The name highlights
its relations with graulichite-(Ce) (Hatert et al., 2003), in accord with
the nomenclature of REE-bearing minerals (Levinson, 1966; Bayliss and
Levinson, 1988). Holotype material is deposited in the mineralogical
collection of the Museo di Storia Naturale, University of Pisa, Via Roma 79,
Calci (Pisa), under catalogue number 19924.
This paper describes the new mineral species graulichite-(La), briefly
discussing its relations with other alunite supergroup minerals.
Occurrence and physical properties
Graulichite-(La) was found in the Patte d'Oie mine, Bou Skour mining
district, in the Djebel Saghro mountain range, about 50 km southeast of
Ouarzazate, Morocco. This mining district belongs to the Cu hydrothermal ore
deposits located in the Anti-Atlas domain. In the Patte d'Oie mine, ore body
is mainly formed by chalcopyrite with abundant bornite, chalcocite, galena,
and tetrahedrite-group minerals, in quartz veins. This locality is
well-known for its supergene minerals, e.g., agardite-(Y), first found at
this locality in 1964 (Dietrich et al., 1969), azurite, cuprite, and
wulfenite, the latter in bluish crystals (Dietrich, 1972; Dietrich and
Favreau, 2005). A description of the mineralogy of this locality was given
by Dietrich (1970).
A geological background can be found in Maacha et al. (2011) and El Azmi et
al. (2014). The studied material was originally collected by Jacques Émile Dietrich in the active underground workings of the Patte d'Oie mine in the
late 1960s, when he was a geologist in Morocco; the sample was then given to
one of the authors (Georges Favreau) and kept as a part of his systematic collection.
Graulichite-(La) occurs as rhombohedral crystals, up to 0.1 mm in size,
yellow in color (Fig. 1), with a light-yellow streak. Luster is resinous
and fluorescence was not observed. It is brittle, with an irregular
fracture. No cleavage or parting was observed. Hardness and density were
not measured, owing to the small size of available crystals. Mohs hardness
was estimated to be ∼ 3.5, in agreement with other alunite
supergroup minerals (e.g., Mills et al., 2010). As will be discussed below,
crystals of graulichite-(La) are chemically inhomogeneous and the calculated
density for the two observed homogeneous domains are 3.907 and 3.962 g cm-3 for domain #1 and #2, respectively. Notwithstanding its
euhedral morphology, crystals are internally porous, as confirmed by
polished sections showing only very thin laminae determining the external
morphology (Fig. 2). This feature greatly complicated the full
crystal-chemical investigation of this material, resulting in a low quality
of collected data.
Graulichite-(La), as yellow rhombohedral crystals with thin
needles of agardite-(La). Field of view: 0.5 mm. Collection Georges Favreau,
photo Pierre Clolus.
Backscattered electron images of polished crystals of
graulichite-(La) embedded in epoxy, showing their porous nature.
Owing to the small amount of available material and its nature, only very
basic optical observations were performed. Graulichite-(La) is transparent,
with a very weak pleochroism from light yellow to yellow. As in
graulichite-(Ce) (Hatert et al., 2003), birefringence was difficult to
observe because interference colors are masked by the yellow color of the
mineral. Mean refractive index, calculated according to the Gladstone–Dale
relationship (Mandarino, 1979, 1981), is 1.889 and 1.928 for domains #1
and #2, respectively.
Graulichite-(La) is associated with malachite, agardite-(La), conichalcite,
and a still undetermined synchysite-like REE carbonate. Its origin is
related to the oxidation of primary ores exploited at the Patte d'Oie mine;
the presence of REE in this secondary assemblage could be related to the
interaction between acidic solutions formed during sulfide oxidation and
minerals of the host rocks (andesite, granite).
Raman spectroscopy
Micro-Raman spectra of graulichite-(La) were collected in nearly
back-scattered geometry with a Horiba Jobin-Yvon XploRA Plus apparatus,
equipped with a motorized x–y stage and an Olympus BX41 microscope with a
50× objective. The 532 nm line of a solid-state laser was used. The
minimum lateral and depth resolution was set to a few micrometers. The system
was calibrated using the 520.6 cm-1 Raman band of silicon before each
experimental session. Spectra were collected through multiple acquisitions
(3) with single counting times of 30 s, with the laser power filtered at
25 % (6.25 mW). Backscattered radiation was analyzed with a 1200 g mm-1
grating monochromator.
The Raman spectrum of graulichite-(La) and the band positions are shown in
Fig. 3. The region between 100 and 1200 cm-1 can be divided into
three regions. Between 700 and 1200 cm-1, vibrational modes related to
the occurrence of (AsO4) and minor (PO4) and (SO4) groups can
be observed. The strongest band at 846 cm-1 is related to the ν1 mode of the (AsO4) groups. Bands at 995 and 1092 cm-1 (the
latter being broad) are likely due to the ν1 and ν3
modes of (SO4) groups, as well as to the occurrence of (PO4)
groups. For instance, Frost et al. (2011) assigned bands between 996 and
1000 cm-1 observed in beudantite to the ν1 mode of (SO4)
groups, and a series of bands between 1061 and 1155 cm-1 as due to the
ν3 modes of (SO4) groups. In florencite-(La), Frost et al. (2013) proposed to identify a band at 987 cm-1 as due to (PO4)
symmetric stretching modes, whereas antisymmetric stretching modes were
observed between 1064 and 1221 cm-1. The band at 714 cm-1 in the
Raman spectrum of graulichite-(La) may be interpreted as due to O–H
deformation modes, in agreement with Frost et al. (2013), who reported a band
at 716 cm-1 in florencite-(La). Between 350 and 700 cm-1, the
strongest band is at 466 cm-1, attributed to the ν4 mode of
(AsO4) groups; this band has a shoulder toward the lower wavenumbers,
interpreted as due to the ν2 mode of (SO4) groups. The bands
at 565 and 618 cm-1 may be interpreted as the ν4 modes of
(SO4) groups, as well as the ν2 modes of (PO4) groups.
The Raman bands below 350 cm-1 can be interpreted as M–O modes (e.g.,
the band at 330 cm-1 – Frost et al., 2013) and lattice vibrations.
Raman spectrum of graulichite-(La). The position of observed bands
is given (in cm-1).
The O–H stretching region, between 2900 and 3800 cm-1, is
characterized by a broad and weak band, with a relatively narrow spectral
and very strong feature at 2906 cm-1. Whereas the broad and weak band
may be attributed to several kinds of O–H environments (it may be possible
that both OH and H2O occurs in the studied mineral, and that
(As,P)O3(OH) groups may partially replace (As,P)O4 groups), the
band at 2906 cm-1 is not attributed. In addition, a band at 1441 cm-1 is observed. Similar features can be noted in the infrared
spectrum of florencite-(La) reported by Frost et al. (2013) but not
discussed. In addition, similar bands occur in the Raman spectrum of
florencite-(La) reported by Repina et al. (2011). These authors interpreted
the band at 2957 cm-1 as due to O–H stretching, whereas the bands in
the region between 1400 and 1500 cm-1 are interpreted as due to the
presence of CO3 groups. Other possible interpretations of this band may
be related to the possible occurrence of (NH4)+ groups or to the
possibility of overtone bands. The former hypothesis is unlikely, because
Raman spectroscopy is not very sensitive as regards the occurrence of N–H
bending modes (e.g., Kampf et al., 2016; Biagioni et al., 2020) and the
observed bands should indicate relatively high (NH4) contents that
should favor a significant expansion of the unit-cell volume with respect
to graulichite-(Ce). Such an expansion was not observed. The possible
interpretation of the band at 1441 cm-1 as due to overtones modes
agrees with that given by Plášil et al. (2014), who interpreted a
band at 1449 cm-1 as due to (SO4) overtones.
Chemical data
Preliminary chemical analysis, performed using energy-dispersive
spectrometry (EDS mode), revealed the occurrence of La, Ce, Ca, Sr, Fe, Al,
Cu, As, P, and S as the elements with Z>8 above the detection
limit.
Quantitative chemical analyses were carried out using a Superprobe JEOL JXA
8200 electron microprobe (WDS mode, 20 kV, 10 nA, 1 µm beam diameter)
at the Eugen F. Stumpfl laboratory (Leoben University, Austria). Standards
(element, emission line) were as follows: anhydrite (SKα), apatite (PKα,
CaKα), PtAs2 (AsLα), corundum (AlKα), hematite
(FeKα), monazite (LaLα, CeLα), chalcopyrite
(CuKα), strontianite (SrLα), galena (PbMα), and
sanidine (KKα). Counting times were 20 s for peaks and 10 s for left
and right backgrounds, respectively. Every attempt to collect chemical data
on the polished grains was unsuccessful, owing to the very small size of the
lamellae embedded in epoxy (Fig. 2). Finally, chemical data were collected
on the flat surface of an unpolished crystal of graulichite-(La). Low
analytical total, after the addition of calculated H2O, is probably
due to the porous nature of the studied material and the poor quality of
sample surface. Consequently, in addition to the measured values, chemical
data normalized to sum = 100 wt % are given (Table 1).
Note: n – number of spot analyses; e.s.d. – estimated standard deviation.
Two chemically homogeneous domains (labeled #1 and #2) were
identified. In agreement with Hatert et al. (2003), the chemical formulae of
graulichite-(La) were calculated on the basis of 6 cations per formula unit,
assuming the occurrence of 14 O atoms per formula unit (apfu); H content was estimated in order to
achieve the electrostatic balance. Domain #1 corresponds to the chemical
formula
(La0.34Ce0.20Ca0.11Sr0.07Pb0.05K0.04)Σ0.81(Fe2.163+Al0.84Cu0.20)Σ3.20(As1.23P0.39S0.37)Σ1.99O14H6.13,
whereas domain #2 has the composition
(La0.38Ce0.22Sr0.10Ca0.09Pb0.05K0.06)Σ0.90(Fe2.603+Al0.49Cu0.20)Σ3.29(As0.91P0.50S0.40)Σ1.81O14H6.53.
Both formulae led to the end-member composition
LaFe33+(AsO4)2(OH)6. The ideal formula of
graulichite-(La), LaFe33+(AsO4)2(OH)6, corresponds
to (in wt %) As2O5 33.49, Fe2O3 34.90, La2O3
23.74, H2O 7.88, total 100. Notwithstanding the low quality of chemical
data, the mineral stoichiometry is consistent with the crystal chemistry of
alunite-supergroup minerals.
X-ray crystallography
X-ray powder diffraction data were collected using a 114.6 mm Gandolfi
camera and Ni-filtered CuKα radiation. Table 2 gives the observed and
the calculated X-ray powder diffraction patterns, compared with that of
graulichite-(Ce) (Hatert et al., 2003). Unit-cell parameters, refined on the
basis of 13 unequivocally indexed reflections using the software UnitCell
(Holland and Redfern, 1997), are a=7.2469(9), c=16.767(2) Å, V=762.59(17) Å3.
X-ray powder diffraction data (d in Å) for graulichite-(La).
Intensity and dhkl were calculated using the software PowderCell 2.3 (Kraus and
Nolze, 1996) on the basis of the structural model given in Table 4. Only the
reflections with Icalc> 5 are given, if not observed. The
eight strongest reflections are shown in bold. For the sake of comparison, the X-ray powder diffraction data of graulichite-(Ce) (Hatert et al., 2003) are reported.
Note: observed intensities Iobs were visually estimated. s – strong; m – medium; mw – medium-weak; w – weak; vw – very weak.
Several crystals of graulichite-(La) were tested, but they usually gave
powder-like patterns, probably owing to the extreme fragility and porous
nature of the available sample that formed a powdery material during the
manipulation. Only one very thin lamina gave some spots suitable for
single-crystal X-ray diffraction study. Similar difficulties were also
encountered by Hatert et al. (2003) during their study of graulichite-(Ce).
They described powder-like patterns, with diffuse streaks, owing to
subparallel association of single individuals. Intensity data of
graulichite-(La) were collected using a Bruker Apex II diffractometer
equipped with a Photon II CCD area detector, and graphite-monochromatized
MoKα radiation (Dipartimento di Scienze della Terra, University of
Pisa). The detector-to-crystal distance was 50 mm. A total of 336 frames was
collected using ω scan modes, in 0.5∘ slices, with an
exposure time of 120 s per frame. The data were corrected for Lorentz and
polarization factors and absorption using the software package Apex3 (Bruker AXS
Inc., 2016). Owing to the very small crystal size, only 118 reflections were
measured up to 2θ=31.75∘, resulting in only 47 unique
reflections. Unit-cell parameters are a=7.252(13), c=16.77(3) Å,
V=764(3) Å3. The c : a ratio is 2.3125. These values can be compared
with those reported by Hatert et al. (2003) for graulichite-(Ce), i.e., a=7.288(2), c=16.812(9) Å, V=773.3(6) Å3. Systematic
absences agree with the space group symmetry R-3m. The crystal structure of
graulichite-(La) was refined using Shelxl-2018 (Sheldrick, 2015) starting from the
atomic coordinates of graulichite-(Ce) (Hatert et al., 2003). However, owing to
the very low number of unique reflections, several constraints were imposed.
Displacement parameters were refined isotropically, fixing their values to
0.030, 0.015, 0.020, and 0.025 Å2, for the A, B, X, O(1), O(2), and OH
sites, in agreement with values observed in graulichite-(Ce) (Hatert et al.,
2003); site occupancy factors for the A, B, and X sites were refined using the
following neutral scattering curves, taken from the International Tables for Crystallography (Wilson, 1992): La vs. □ at A, Fe vs. □ at B, and As vs. □ at
X. In the final stages of the refinement, site occupancy factors were fixed,
in order to reduce the number of refined parameters. Notwithstanding such an
approach, the data / parameter ratio is poor, i.e., 6.71 (47 unique
reflections and 7 refined parameters). After several cycles of isotropic
refinement, the R1 value converged to 0.0956 for 36 unique reflections
with Fo>4σ(Fo). Details of data collection and
refinement are given in Table 3. Atom coordinates and displacement
parameters are reported in Table 4, whereas Table 5 reports selected bond
distances.
Summary of parameters describing data collection and refinement for
graulichite-(La).
Crystal dataCrystal size (mm)0.040×0.035×0.030Cell setting, space groupTrigonal, R-3ma (Å)7.252(13)c (Å)16.77(3)V (Å3)764(3)Z3Data collection and refinement Radiation, wavelength (Å)Mo Kα, λ=0.71073Temperature (K)293(2)2θmax (∘)31.75Measured reflections118Unique reflections47Reflections with Fo>4σ(Fo)36Rint0.0563Rσ0.0726Range of h, k, l-5≤h≤5, -4≤k≤4, -12≤l≤12R [Fo>4σ(Fo)]0.0956R (all data)0.1222wR (on Fo2)*0.2162Goof1.237Number of least-squares parameters7Maximum and minimum residual peak (e Å-3)1.68 (at 0.68 Å from A) -1.50 (at 0.84 Å from A)
*w=1/[σ2(Fo2)+(0.0529P)2+514.5002P].
Sites, Wyckoff positions, site occupancy factors (s.o.f.),
fractional atomic coordinates, and isotropic displacement parameters (in
Å2) for graulichite-(La).
Selected bond distances (in Å) for graulichite-(La).
A-OH2.67(5) × 6B-OH1.958(19) × 4X-O(2)1.56(5) × 3-O(2)2.88(5) × 6-O(2)2.03(4) × 2-O(1)1.65(9)Mean2.78Mean1.98Mean1.58DiscussionCrystal chemistry of graulichite-(La)General features
As illustrated above, the crystal-chemical characterization of
graulichite-(La) suffered because of the low quality of available material.
Notwithstanding these shortcomings, all available data supported the
proposed identification. The crystal structure of graulichite-(La) (Fig. 4)
is isotypic with other alunite supergroup minerals, e.g., graulichite-(Ce)
and other members of the dussertite group. It can be described as formed by
layers of BO6 octahedra, bonded through corner-sharing, giving rise to a
planar network of triangular clusters delimiting hexagonal voids (such a
network is described as hexagonal tungsten bronze, HTB – e.g., Grey et al.,
2006). Such layers are decorated, on both sides, by XO4 tetrahedra. The
connection between successive layers, along c, is due to the 12-fold
coordination A sites.
Crystal structure of graulichite-(La), as seen down b. Brown and
violet polyhedra represent the B- and X-centered sites, whereas green and red
circles are the A and O(1), O(2), and OH sites, respectively.
Cation sites
The A site shows an average bond distance of 2.78 Å, to be compared with
2.74 Å reported by Hatert et al. (2003) for graulichite-(Ce) and 2.682 Å observed by Mills et al. (2010) in arsenoflorencite-(La). Crystal
structure refinement indicates a site scattering value corresponding to
∼ 54 electrons per formula unit (epfu). Chemical data show
similar La / (La + Ce) atomic ratios in both the observed chemically
homogeneous domains (i.e., ∼ 0.63), with the main difference
being related to the amount of vacancy, i.e., 0.19 and 0.09 apfu. However,
taking into account the low quality of chemical data, the occurrence of
vacancy and its amount cannot be quantified confidently. If one assumes that
no vacancy occurs, a hypothetical A site population could be
La0.43Ce0.25Ca0.11Sr0.10Pb0.06K0.05,
corresponding to 50.88 epfu, slightly less than the refined value. Assuming
such a hypothetical site population and using the bond parameters of
Gagné and Hawthorne (2015), the bond-valence sum at the A site would be
2.22 valence unit (v.u.), to be compared with a theoretical value of 2.63 v.u. (Table 6).
Weighted bond valences (in valence units) in graulichite-(La).
Note: left and right superscripts indicate the number of equivalent bonds
involving anions and cations, respectively. Σ′ is corrected for
H bonds.
The B site has an octahedral coordination, with four B–(OH) bonds (1.96 Å) and two slightly longer apical bonds with O(2) (2.03 Å). The
average bond length, 1.98 Å, is similar to the 〈B–O〉 distance observed in graulichite-(Ce), 1.99 Å (Hatert
et al., 2003). Using the ionic radii given by Shannon (1976) for
[VI]Fe3+, [VI]Al3+, [VI]Cu2+, and
[III]O2- and normalizing the sum of Fe3+, Al, and Cu2+
to 3 apfu, the calculated bond distances of 1.98 and 1.99 Å can be
obtained for chemical domains #1 and #2, respectively. Refined site
scattering (73.2 epfu) agrees with the chemical composition of domain #2;
indeed, normalizing the content of Fe3+, Al, and Cu2+ to 3 apfu,
one obtains (Fe2.38Al0.45Cu0.18), corresponding to a
calculated site scattering of 73.0 epfu. Domain #1 has a calculated B site
scattering of 68.4 epfu. Bond-valence sum at the B site is 3.20 v.u.,
larger than the theoretical value of 2.94 v.u.
The X site is a mixed (As, P, S) site. Its average 〈X–O〉
distance, 1.58 Å, is shorter than that observed in graulichite-(Ce)
(1.68 Å) and agrees with the partial replacement of As5+
(〈As–ϕ〉= 1.687 Å – Majzlan et al., 2014) with
the smaller cations P5+ (〈P–ϕ〉= 1.537 Å – Huminicki and Hawthorne, 2002) and S6+ (1.473 Å –
Hawthorne et al., 2000). Coupling the information about 〈X–O〉 distance and refined site scattering (46.1 epfu), one can
calculate the site population (As0.86P0.48S0.66)Σ2.00, which has more S than is suggested by the chemical data. Assuming
this occupancy, bond valence at the X site is 5.49 v.u., to be compared with
the theoretical value of 5.33 v.u. In alunite-structure type, the XO4
tetrahedron shows a short X–O(1) bond and three longer X–O(2) bonds. For
instance, in graulichite-(La), X–O(1) is 1.660(19) Å, whereas X–O(2) is
longer, i.e., 1.690(12) Å (Hatert et al., 2003). In the studied sample,
such a situation is apparently inverted, with X–O(1) longer than the three
symmetry-related X–O(2) bonds, i.e., 1.65(9) vs. 1.56(5) Å, respectively.
However, considering the large experimental uncertainty affecting the
structural data of graulichite-(La), due to the low quality of the structure
refinement, it is very probable that such an inversion in the bond length
distribution is an artifact.
Comparison between graulichite-(La) and other REE-bearing
dussertite group minerals.
Arsenoflorencite-(Ce)Arsenoflorencite-(La)Graulichite-(Ce)Graulichite-(La)Chemical formulaCeAl3(AsO4)2(OH)6LaAl3(AsO4)2(OH)6CeFe33+(AsO4)2(OH)6LaFe33+(AsO4)2(OH)6a (Å)7.0297.0327.2887.252c (Å)16.51716.51516.81216.77V (Å3)706.8707.2773.3764〈A–φ〉 (Å)No data available2.6822.7442.78Ce / (La + Ce)0.600.240.870.37*〈B–φ〉 (Å)No data available1.8941.9891.98Al / (Al + Fe)1.000.980.140.15*〈X–φ〉 (Å)No data available1.6681.6821.58As / (As + P + S)0.740.900.940.43*Ref.Nickel and Temperly (1987)Mills et al. (2010)Hatert et al. (2003)This work
* Using proposed site populations.
Anion sites
The three anion sites O(1), O(2), and OH have a bond-valence sum (BVS) of 1.14, 2.11, and 1.31 v.u. The O(1) site is an acceptor of H bonds from three symmetrically related
OH, with O⋯O distance of 2.81 Å,
corresponding to a bond strength, calculated according to Ferraris and
Ivaldi (1988) of 0.18 v.u. Consequently, the corrected BVS for O(1) and OH,
after considering the H bonds, are 1.68 and 1.13 v.u., agreeing with the
occurrence of O2- and (OH)- at these two sites. The underbonding
at O(1) could be related to the possible occurrence of (OH)- at this
position, as observed in other alunite supergroup minerals (e.g., Cooper and
Hawthorne, 2012). In graulichite-(La), several substitutions occur with
respect to the end-member formula
LaFe33+(AsO4)2(OH)6. At the A site, trivalent REE
are replaced by divalent (Ca, Sr, and Pb) and monovalent cations, whereas at
the B site, (Fe,Al)3+ is replaced by minor Cu2+; finally, at the
X site, As5+ and P5+ can be partially substituted by S6+.
Consequently, several heterovalent substitution mechanisms can be
hypothesized. Indeed, one could suggest mechanisms involving only cations,
e.g., A,BM3++XM5+=A,BM2++XM6+, or both cations and anions, e.g., A,BM3++O(1)O2-=A,BM2++O(1)(OH)-.
Relations with other species
Graulichite-(La) belongs to the dussertite group, a group of arsenate
minerals within the alunite supergroup (Bayliss et al., 2010). This new
mineral is the fourth REE-bearing member of this series, along with
arsenoflorencite-(Ce), arsenoflorencite-(La), and graulichite-(Ce). Table 7
compares these phases. In addition, “arsenoflorencite-(Nd)” was described
by Scharm et al. (1991) but never officially approved by the CNMNC-IMA.
Among studied REE-bearing arsenates belonging to the dussertite group,
graulichite-(La) has one of the lowest As / (As + P + S) atomic ratios. As
discussed above some discrepancies between chemical and structural data
occur; As is replaced by P and S, with the sum of As and P being larger than
the S content, and As being the dominant cation of the dominant valence at
the X site. Probably, the replacement of As by smaller P5+ and S6+
favors the contraction of unit-cell volume, ΔV=-1.20 % with
respect to graulichite-(Ce). Moreover, the occurrence of P5+ suggests
the possibility that Fe3+ analogues of florencite-(Ce) and
florencite-(La), both belonging to the plumbogummite group, may occur in
nature.
Data availability
The Crystallographic Information File data of
graulichite-(La) are available in the Supplement.
The supplement related to this article is available online at: https://doi.org/10.5194/ejm-34-365-2022-supplement.
Author contributions
MEC and GF collected preliminary data. CB and DM
carried out single-crystal X-ray diffraction and micro-Raman spectroscopy.
FZ collected electron microprobe data. CB and DM wrote the paper, with input
from MEC, GF, and FZ.
Competing interests
At least one of the (co-)authors is a member of the editorial board of European Journal of Mineralogy. The peer-review process was guided by an independent editor, and the authors also have no other competing interests to declare.
Disclaimer
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
Pierre Clolus is acknowledged for the photo of
graulichite-(La). The University Centrum for Applied Geosciences (UCAG) is
thanked for the access to the E. F. Stumpfl electron microprobe laboratory.
The comments of Associate Editor Edward Grew and the reviewers Juraj Majzlan and Frédéric Hatert helped us in improving the paper.
Financial support
This research received support from the Ministero
dell'Istruzione, dell'Università e della Ricerca through the project
PRIN 2017 “TEOREM – deciphering geological processes using Terrestrial and
Extraterrestrial ORE Minerals” (project no. 2017AK8C32).
Review statement
This paper was edited by Edward Grew and reviewed by Frédéric Hatert and Juraj Majzlan.
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