Eur. J. Mineral., 33, 165–174, 2021
https://doi.org/10.5194/ejm-33-165-2021© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Occurrence of silesiaite, a new calcium–iron–tin sorosilicate
in the calcic skarn of El Valle-Boinás, Asturias, Spain
Antonia Cepedal, Mercedes Fuertes-Fuente, and Agustín Martin-Izard
Department of Geology, University of Oviedo, Arias de Velasco s/n, Oviedo, 33005, Spain
Correspondence: Antonia Cepedal (mcepedal@uniovi.es)
Received: 7 October 2020 – Revised: 4 March 2021 – Accepted: 8 March 2021 – Published: 19 April 2021

Abstract. Silesiaite(Ca2 Fe+3 Sn(Si2 O7 )(Si2 O6 OH)),
the
Fe3+
analogueofkristiansenite(Ca2 ScSn(Si2 O7 )(Si2 O6 OH)), has been found in the calcic Cu–Au skarn of El Valle-Boinás, in the northof Spain, which is the second occurrence of this mineral in the world. The study under optical microscopyshows crystals with a distinct pleochroism, from uncoloured to yellowish, high relief and imperfect cleavage
under plain polarized light. Under polarized and analysed light, the mineral shows anomalous colours ofinterference and hourglass and sector optical zoning. Backscattered electron images reveal compositional zoning
mimicking optical zoning with light grey (Sn-rich) and dark grey (Fe-rich) zones. The electron microprobe
analyses showed that Fe-rich zones are also the richest in Al and Ti, whereas the Sn-rich zones are richestin Mn. The Fe+3 and Fe+2 proportions calculated by stoichiometry suggest a couple substitution such as
+3
+4
+2
2(Fe,
 Al) ⇔ (Sn, Ti) + (Fe, Mn, Mg) . According to this, the formula of the silesiaite can be written as+2
Ca2 Fe+3
1−2x Fex Snx Sn (Si2 O7 ) (Si2 O6 OH), where x is between 0 and 0.4.

1

Introduction

Silesiaite,
with
an
ideal
formulaofCa2 Fe+3 Sn(Si2 O7 )(Si2 O6 OH), was first found in apegmatite of the Karkonosze granite in Lower Silesia,
Poland (Pieczka et al., 2017). According to these authors, the mineral is the Fe3+ analogue of kristiansenite(Ca2 ScSn(Si2 O7 )(Si2 O6 OH)), a triclinic (C1) Sc-bearing
sorosilicate also found in a granite pegmatite in Norway(Raade et al., 2002). Pieczka et al. (2017), in their communication, gave the cell parameters, refined from powder
data, and space-group symmetry of silesiaite (a = 10.028(1),b = 8.408(1), c = 13.339(2) Å, α = 90.01◦ (1), β = 109.10◦(1), γ = 90.00◦ (1); C1), in addition to the main reflections(9.147(100), 8.408(12), 6.607(64), 5.195(6), 4.413(10),3.312(7), 3.151(12), 3.095(9)).
The aim of this paper is to shed light on the occurrenceof silesiaite in the El Valle-Boinás Cu–Au deposit, Asturias,
in the northern part of Spain. The deposit, unlike the otheroccurrences, is a skarn-type deposit overprinted by a lowtemperature hydrothermal alteration (Cepedal et al., 2000,
2003, 2008). The presence of silesiaite is related to an iron-rich calcic skarn, in association with other minerals including
andradite (Adr96−100 ), clinopyroxene (Hd30−47 ), actinolite,babingtonite, apophyllite-(KOH) and hydroandradite, among
others (Cepedal et al., 2003). In this paper, we present an optical and electronic microscopy study of the mineral, whichappears as optical twinned and sector-zoned crystals. Electron microprobe analyses performed on the crystals revealed
that there is also a compositional zoning, based on tin andiron contents. In addition to this, Raman analyses were performed on chemically different areas of some crystals. This
work will contribute to increase the knowledge of this mineral, whose second occurrence in the world is described here.

2

Geological setting

The El Valle-Boinás Cu–Au skarn is related to the intrusionof two different stocks, from quartz-monzonite to monzogranite, at 300 ± 5 Ma (Martin-Izard et al., 2000) in a preorogenic Palaeozoic sequence (Fig. 1). The intrusion of thesegranitoids led to the formation of two different Cu–Au skarn
types, calcic and magnesian, in the limestones and dolostones

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

166

A. Cepedal et al.: Occurrence of silesiaite in the calcic skarn of El Valle-Boinás

Figure 1. Schematic geology of El Valle-Boinás Cu–Au deposits where silesiaite was found (modified from Cepedal et al., 2003).

of the Láncara Formation (Cambrian in age), in addition toendoskarn mineralization in the igneous rocks. Three mainmineralized areas were defined around the intrusive body:Boinás West, Boinás East and El Valle (to the north; notshown in Fig. 1). Magnesian and calcic skarns occurred in
El Valle and, particularly, in Boinás East. In Boinás West,
only calcic skarn was developed. The silesiaite was foundin the calcic skarn from Boinás West. This skarn consists
mostly of garnet (Adr20−100 ), pyroxene (Hd3−97 ), wollastonite and lesser amounts of vesuvianite. Skarn zonation isexpressed in terms of the mineralogy and changing chemical compositions of the main calc-silicate minerals. Proximalskarn consists of coarse-grained grossular garnet (Gr20−50 )and intermediate pyroxene (Hd28−52 ); the pyroxene / garnet
ratio increases towards distal parts of the skarn. Wollastonite
and an early Fe-poor pyroxene (Hd3−28 ) occur close to thecontact with marble. Iron enrichment during skarn formation is indicated by the presence of andradite (Adr70−100 ) andhedenbergite (Hd70−97 ). This Fe-rich pyroxene continued toform during the first retrograde stage, which is characterized
by epidote (Ps22−27 ), amphibole (Tr20−58 ), quartz, calcite,
K-feldspar, apatite and titanite. Hedenbergite is locally altered to babingtonite, a hydrated calcic-ferrous-ferric-silicate(Cepedal et al., 2003), in association with quartz, calcite andFe-rich epidote (up to Ps48 ). The second retrograde stage
is characterized by the formation of chlorite and prehnite,in addition to quartz and calcite. Chalcopyrite, bornite andpyrite are the main sulfides with chalcocite, molybdenite,
tetrahedrite-tennantite, arsenopyrite and lesser amounts ofpyrrhotite. Gold occurs as native Au, electrum or tellurides

Eur. J. Mineral., 33, 165–174, 2021

(petzite and sylvanite), in association with hessite and bismuthinite, among others (Cepedal et al., 2006).

3

Materials and methods

The polished thin section where silesiaite occurs was studied at the University of Oviedo under optical and scanningelectron microscopy (SEM) using a JEOL-6610LV in association with chemical microanalysis by means of energy
dispersive X-ray (EDAX). Backscattered electron (BSE) images of the silesiaite crystals were used to perform the electron microprobe analysis (EMPA), also at the University of
Oviedo, using a Cameca SX100. The analytical conditions
were an accelerating voltage of 15 kV and a beam currentof 30 nA for an acquisition time of 20 s for X-ray peak andbackground, except for some elements (Sc, Zr, Nb, Cu, Zn),
for which the conditions were 20 kV and 100 nA of beam
current. Standards, diffracting crystals, analytical lines andmean detection limits (in wt. %) were as follows: albite-Na(TAP, Kα, 0.07), MgO-Mg (LTAP, Kα, 0.02), jade-Si (TAP,Kα, 0.03), andradite-Ca (LPET, Kα, 0.04), Al2 O3 -Al (TAP,Kα, 0.03), MnTi-Ti (LPET, Kα, 0.02), MnTi-Mn (LLIF, Kα,0.03), magnetite-Fe (LLIF, Kα, 0.05), cassiterite-Sn (LPET,
Lα, 0.09), orthoclase-K (LPET, Kα, 0.03), pure Sc-Sc (LLIF,
Kα, 0.05), pure Nb-Nb (LPET, Lα, 0.04), zircon-Zr (TAP,
Lα, 0.05), pure Cu–Cu (LLIF, Kα, 0.02) and sphalerite-Zn(LLIF, Kα, 0.02),
The Raman spectra were obtained at room temperature
with a Horiba Jobin Yvon RPA-HE 532 Raman spectrographwith a multichannel air-cooled (−70 ◦ C) charge-coupled device (CCD) detector, using an argon-ion laser (514.5 nm)
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A. Cepedal et al.: Occurrence of silesiaite in the calcic skarn of El Valle-Boinás
equipped with a 1800 g mm−1 grating and a nominal powerof 100 mW. The spectral resolution was 1 cm−1 and the spectral range between 200 and 3400 cm−1 . The Raman system
includes an Olympus 50X long working distance (LWD) visible objective (NA = 0.50 and WD = 10.6 mm). The silicon
standard was used for the calibration of the frequencies of
the Raman spectra. The acquisition time was set in such a
way that the intensity reached at least 1/3 of the maximum
level of the CCD (i.e. 20 000 counts per second). Spectrum
manipulation consists of a basic treatment, such as baseline
adjustment and peak fitting (Lorentz function). The Raman
analyses were performed at CREGU (Nancy, France).

4

Results

Silesiaite was found in the distal skarn close to the marble front, where centimetre-scale babingtonite crystals developed in a retroskarn cavity along with hydroandradite,apophyllite-(KOH), fibrous actinolite, quartz, calcite and sulfides, mainly chalcopyrite (Fig. 2). Silesiaite occurs as subhedral crystals of less than 0.5 mm, forming aggregates atthe contact with chalcopyrite crystals. The mutual contact
between both minerals is irregular, indicating that chalcopyrite was corroded by the silicate minerals that form a surrounded band of replacement. The EMPA analyses carriedout on chalcopyrite show up to 0.25 wt. % of Sn as impurity(range of values between 0.01 wt. % and 0.25 wt. % of Sn,
with a limit of detection of 0.003 wt. %) that was released
during the chalcopyrite replacement, allowing the formationof silesiaite just at the contact with the sulfide. Further fromthe contact, hydroandradite developed as aggregates of finegrained euhedral crystals which show a zonation with a core
and a narrow rim (Fig. 2a). The EMPA analyses in garnet
crystals show rims richer in Fe than the cores are and Sn contents up to 3.0 wt. % SnO2 (Cepedal et al., 2003). The central
part of the cavity is filled by apophyllite-(KOH), quartz, calcite and babingtonite that can form crystals on the scale ofcentimetres.
In optical microscopy, under plane polarized light, the silesiaite crystals are uncoloured to yellowish, showing a distinctive pleochroism (Fig. 2c). They have high relief, and the
Becke line test indicates that the mineral grains have a lower
refractive index than the hydroandradite does (refractive index, n, of andradite is 1.887) and higher than that of theCanada balsam (n = 1.55). Some crystals exhibit traces ofspaced cleavage planes as parallel thin black lines (Fig. 2c).
Under polarized and analysed light, the crystals show opticalzoning and twinning. One of the sections shows an hourglass
zoning, whilst other sections show less defined sector zoning. In addition to this, most of the crystals show a kind ofpolysynthetic twinning with thin plates. The birefringence islow, but instead of the grey to white colours in the first order of the interference colour spectrum, leather brown andhttps://doi.org/10.5194/ejm-33-165-2021

167

ink blue to grey-blue colours are observed. This behaviour
indicates a high dispersion of birefringence (Fig. 2d).
The backscattered electron images show the crystals are
chemically inhomogeneous and the chemical zoning reproduces the optical one (Fig. 2e). The chemical composition of
the section with hourglass zoning mimics this optical zoning with dark and light grey areas. The qualitative analyses by EDAX showed that light grey areas are richer in Sn
and poorer in Fe, whilst dark grey areas are richer in Fe andpoorer in Sn. From now on, we will refer to light and darkgrey areas as Sn-rich and Fe-rich areas, respectively. In addition, a fine oscillatory zoning occurs in the areas of thecrystals where polysynthetic twins occur. This zonation also
consists of Sn-rich and Fe-rich zones richer or less rich in Sn.
The BSE images were used to select the points of analyses inthe electronic microprobe.
4.1

EMPA analyses

Several EMPA analyses were carried out in Sn-rich andFe-rich areas. These analyses are reproduced in Table S1and are available in the Supplement. An average of theresults is shown in Table 1, where all the iron is considered as ferric. Other elements analysed were K (limit
of detection, LOD = 325 ppm), Na (LOD = 660 ppm),
Sc (LOD = 540 ppm), Zr (LOD = 514 ppm), Nb(LOD = 380 ppm) and Zn (LOD = 194 ppm), but all
were always below their respective detection limits. All theanalyses were below 100 wt. % due to the presence of (OH)−groups in the structure. The structural formula was obtained
based on 13.5 anhydrous oxygen atoms and 8 cations, inaccordance with the kristiansenite (the Sc-analogue) crystal
structure data of Ferraris et al. (2001).
Microprobe analyses confirm that the dark grey areas havea lower amount of Sn (average of 28.24 wt. % SnO2 ) thanlight grey areas (average of 33.10 wt. % SnO2 ) (Table 1). Ironis the main substitute for Sn (Fig. 3), almost reaching 1 apfu
(atoms per formula unit) in some analyses. In the Fe-rich areas there are also higher amounts of Al and Ti and lower
amounts of Mn (Table 1). In the case of Al, all the analyses
were above the limit of detection (LOD = 250 ppm), and therange of values is from 1.35 to 0.55 wt. % Al2 O3 . Only in
six analyses was Ti detected (LOD = 240 ppm) with a range
from 690 to 1010 ppm TiO2 . In the case of Mn, it was only
detected in four analyses (from 345 to 390 ppm MnO). In
the Sn-rich areas, the analyses showed higher amounts ofMn, which was detected in 17 analyses with a range from
364 to 694 ppm MnO. Titanium was always below its LOD.
Aluminium was always detected with a range of values from0.56 wt. % to 0.27 wt. % Al2 O3 , lower than in the Fe-rich areas.
In Fig. 3, the atomic contents from the calculated formula
are plotted in binary diagrams, in which the chemical differences between Sn-rich and Fe-rich areas are graphicallyshown. The atomic contents of Si and Ca have almost theEur. J. Mineral., 33, 165–174, 2021

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A. Cepedal et al.: Occurrence of silesiaite in the calcic skarn of El Valle-Boinás

Figure 2. (a) Microphotography showing the silesiaite (sil) and hydroandradite (had) crystal aggregates surrounding and replacing chalcopyrite (cpy). Apophyllite-(KOH) and babingtonite (bab) appear to be filling the cavity under plain polarized light. (b) The same image
under polarized and analysed light. The mineral has anomalous ink blue and leather brown interference colours. (c) Detail of the silesiaite
aggregates in which a weak pleochroism from uncoloured to yellowish is observed. (d) The same image under polarized and analysed light.
One of the sections has an hourglass zoning and something resembling polysynthetic twins with thin plates. (e) Backscattered electron image that shows the compositional zoning mimicking the optical zoning. Light grey areas are rich in Sn, and dark grey areas are rich in Fe.
(f) Secondary electron image of a tiny fragment of the sample. It shows an aggregate of some silesiaite crystals with a tabular habit and animperfect cleavage of the crystals.

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A. Cepedal et al.: Occurrence of silesiaite in the calcic skarn of El Valle-Boinás

169

Figure 3. Binary diagrams showing the correlation between elements (in atoms per formula unit, apfu) from silesiaite electron microprobe
analyses.

same range of values in both areas, and no correlation between Al and Si (apfu) has been observed. However, a slightnegative correlation between Fe+3 and Si (apfu) is observedin the Sn-rich areas. This could indicate that a slight substitution of Si by Fe+3 exists in the tetrahedral positions. In thebinary diagram of Al (apfu) vs. Sn (apfu), the higher amountsof Al in the Fe-rich areas becomes more evident, in which Al
is positively correlated with Sn. However, no correlation has
been observed in the Sn-rich areas. On the contrary, there isa strong inverse correlation between Fe+3 and Sn+4 (apfu)

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in both areas. This negative correlation is greater in the binary diagram of Fe+3 + Al (apfu) vs. Sn (apfu) (Fig. 3),
indicating that the substitution of Fe+3 and Al by Sn+4 ismost likely. As no univalent cations such as Na+ or K+ arepresent, charge balance is assumed to have been achieved dueto the presence of Fe+2 instead of Fe+3 .
We performed a tentative discrimination between divalentand trivalent iron with the aid of the stoichiometry of themineral formula and charge-balance considerations, in accordance with Droop (1987). The results are presented in Ta-

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A. Cepedal et al.: Occurrence of silesiaite in the calcic skarn of El Valle-Boinás

Table 1. Electron microprobe analyses of silesiaite (number of analyses in brackets).
Light grey areas
Wt. %Dark grey areas

Average (27)Range

Average (23)

Range

SiO2
Al2 O3TiO2
Fe2 Oa3
SnO2
MnO
CaO
CuO
Total
Formulab
Si
Al
Ti
Fe
Sn
Mn
Ca
Cu

37.93
0.35
–
8.89
33.10
0.05
17.63
0.12
97.53

38.39–37.44
0.56–0.27

38.95
1.04
0.06
10.95
28.24
0.04
18.19
0.08
97.44

39.58–38.38
1.35–0.55
0.10–0.07
12.68–10.11
29.65–26.71
0.04–0.03
18.37–17.91
0.19–0.03

3.93
0.04

3.97–3.90
0.07–0.03

0.69
1.37
0.00
1.96
0.01

0.74–0.66
1.40–1.32
0.01–0.00
1.98–1.93
0.02–0.01

3.93
0.12
0.00
0.83
1.14

3.97–3.89
0.16–0.06
0.01–0.00
0.97–0.78
1.20–1.07

1.97
0.01

2.00–1.95
0.01

Fe+3 cFe+2 c0.37
0.32

0.48–0.22
0.44–0.24

0.73
0.10

0.90–0.61
0.19–0.01

9.58–8.43
33.88–32.24
0.07-0.04
17.89–17.35
0.25–0.02– Below the detection limit. a Total iron as ferric. b Calculated based on 13.5 O. c Calculated by chargebalance.

Figure 4. Binary diagrams showing the correlation between elements in atoms per formula unit (apfu) from silesiaite electron microprobe
analyses considering Fe+3 and Fe+2 .

ble 1 and Fig. 4. Ferric iron remains the main cation in the
Fe-rich areas, where the Fe+2 proportion reaches an averageof only 0.1 apfu. In the Sn-rich areas, however, the proportions of Fe+2 and Fe+3 are quite similar (Table 1). In bothareas, Sn correlated positively with Fe+2 and negatively with
Fe+3 (Fig. 4). This could suggest a coupled substitution of
the following type:2(Fe+3 , Al+3 ) ⇔ Fe+2 + Sn+4 .
In the diagram of Fig. 4 we include Ti+4 and the other divalent cations present, Mg+2 and Mn+2 . The correlation isslightly improved (R 2 ∼ 1).
Eur. J. Mineral., 33, 165–174, 2021

4.2

Raman spectrum

Several Raman spectra were obtained from Sn-rich and Ferich areas of the silesiaite crystals. A selection of these spectra is shown in Fig. 5, in which the Raman spectra of kristiansenite (Ruff ID = R090022) is also shown in order tocompare them. The raw data of Raman spectra are included
as supplementary files (Table S2). Similarly to the Raman
spectra of kristiansenite, all the spectra are characterized by
two sharp bands at around 544 and 592 cm−1 and anothersharp band at around 735 cm−1 . The spectra also reproduce
the broader, less intense Raman peaks at around 843, 989 and1034 cm−1 , the first one masked by the signal of the glass
slide. In relation to the peak at around 989 cm−1 , there arehttps://doi.org/10.5194/ejm-33-165-2021

A. Cepedal et al.: Occurrence of silesiaite in the calcic skarn of El Valle-Boinás

171

Figure 5. Raman spectra from Fe-rich and Sn-rich areas of silesiaite crystals. Raman spectrum of kristiansenite is also shown (Ruff
ID = R090022). Glass signifies intensities corresponding to glass slide.

subtle differences between the spectra of silesiaite: whereas
the Raman spectra from Sn-rich areas show rounded andless intense peaks, those from Fe-rich areas show sharper
peaks (Fig. 5). The lowest wavenumber region (from 100 to
500 cm−1 ) is quite complex. The spectra from silesiaite are,in general, similar to those of kristiansenite, although somepeaks are sharper (e.g. peaks at around 366 and 490 cm−1 )and/or are slightly shifted to a higher wavenumber (e.g. peaks
at around 291 and 193 cm−1 ).
5Discussion

The specific structure of silesiaite has not been published yet,and the crystals occurring in El Valle-Boinás are too small tobe isolated for X-ray diffraction. However, taking into account the fact that silesiaite is considered the Fe3+ analogueof kristiansenite (Pieczka et al., 2017), we can use the structure of the latter mineral as refined by Ferraris et al. (2001)
and modified by Evans et al. (2018). According to these authors, the structure contains Si2 O7 double tetrahedra (sorosilicate or disilicate groups), and half of the disilicate groups
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carry a hydrogen atom, representing a new type of silicate
structure with protonated Si2 O6 OH and normal disilicate
groups (Ferraris et al., 2001). The rest of the cations are in
isolated polyhedra of 6-coordinated Sn, Sc and Fe and (7+2)coordinated Ca cations. There are four octahedral positions,labelled M1, M2, M3 and M4, which are isolated from eachother and alternate with the disilicate groups along the plane(101̄) (Fig. 6). Calcium and silicon positions are completely
occupied by these cations in the structure of kristiansenite(Evans et al., 2018), whereas the octahedral positions havevery diversified occupants, the dominant ones being tetravalent (Sn, Ti, Zr)+4 and trivalent cations (Sc, Fe, Al)+3 , andthey also have trace amounts of divalent (Mn+2 ) and pentavalent (Nb, Ta+5 ) cations. According to Evans et al. (2018)
in the studied kristiansenite from Szklarska Porȩba, the M1
and M2 sites are mainly occupied by Sc, and the M3 and M4
sites are chiefly occupied by Sn. The distribution of cations
among the sites is highly disordered, with a notable dominant preference of Sn–Sc disorder at the M3 and M4 sites
and Sc-X disorder at the M2 and M1 sites. In silesiaite, theEur. J. Mineral., 33, 165–174, 2021

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A. Cepedal et al.: Occurrence of silesiaite in the calcic skarn of El Valle-Boinás

Figure 6. Structure of silesiaite based on the structure of kristiansenite (Ferraris et al., 2001; Evans et al., 2018).

latter octahedral sites will be primarily occupied by Fe, in
substitution of Sc, so M2 and M1 are labelled as Fe in Fig. 6.
The possible existence of a cleavage in kristiansenite wasalready pointed out by Raade et al. (2002) as an explanation of an enhancement of the (004) reflection in the diffraction patterns. According to these authors, this could be due tosome degree of preferred orientation resulting from a {001}cleavage, which is reasonable to assume because this cleavage plane exactly intersects the hydrogen bonds and cuts
only a small number of the cation-oxygen coordination bonds(Raade et al., 2002) (Fig. 6).
The electron microprobe analyses of the silesiaite
Ca2 Fe+3 Sn(Si2 O7 )(Si2 O6 OH) crystals from El Valle-Boinás
calcic skarn show that the mineral is strongly zoned
in terms of iron abundance with zones where ironreaches 1 apfu and zones where Sn is the dominantcation (Table 1). The mineral formula can be written as
Ca2 (Fe+3
1−x Snx )Sn(Si2 O7 )(Si2 O6 OH), where x is between 0
and 0.4. The Sn content of silesiaite is higher than that ofkristiansenite, its Sc analogue (Ca2 ScSn(Si2 O7 )(Si2 O6 OH))
with mean values of between 0.97 Sn apfu (Evans et al.,2018) and 1.06 Sn apfu (Raade et al., 2002). These authors
show Sc contents for kristiansenite of around 0.7 apfu, theother cations in the structure being Fe, Mn, Ti and Al, among

Eur. J. Mineral., 33, 165–174, 2021

others. In the silesiaite from El Valle-Boinás, Al was themain trace element detected with values of up to 1.4 wt. %Al2 O3 (Table 1) and being more abundant in the Fe-richzones. Other cations detected in lesser proportions were Mn
and Ti, in addition to Cu in the proximity of chalcopyrite.
The Sc content was always below the detection limit, which
could indicate that there is no solid solution between silesiaite and kristiansenite or simply due to the different growthenvironment of silesiaite in this work (calcic skarn instead ofpegmatite). Similarly, neither Zr nor Nb or Ta were detected,
which is probably due to the same reason.
A substitution between Fe+3 and Sn has been observed in
Sn-bearing silicates as in epidote, clinozoisite or andradite(Mulligan and Jambor, 1968; Van Marcke de Lumen, 1986;
Ordosch et al., 2019). The first authors describe an unusually high Fe+2 content (1.5 wt % FeO) in their tin-bearing
epidote. In the case of allanite, the substitution of Fe+3 andAl+3 by Fe+2 is used to correct the charge imbalance arising from the replacement of Ca+2 by trivalent rare-earth elements (Deer et al., 1962). As no univalent cations suchas Na+ or K+ are present in the silesiaite, charge balanceis assumed to have been achieved by the presence of Fe+2
instead of Fe+3 , as was previously mentioned. The calculated Fe+2 and Fe+3 cationic proportion (by stoichiometry

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A. Cepedal et al.: Occurrence of silesiaite in the calcic skarn of El Valle-Boinás
based on Droop, 1987) indicates higher Fe+2 content (up to
0.4 apfu) in Sn-rich areas than in Fe-rich areas where the ferric cation is dominant (up to ∼ 1 apfu, Table 1) and a coupled
substitution of2(Fe, Al)+3 ⇔ (Sn, Ti)+4 + (Fe, Mn, Mg)+2 ,so  the mineral  formula can be written as+2
Ca2 Fe+3
where x1−2x Fex Snx Sn (Si2 O7 ) (Si2 O6 OH),is between 0 and 0.4. The substitution of the larger
cations Sn+4 (r = 0.77 Å) and Fe+2 (r = 0.73 Å) for Fe+3
(r = 0.64 Å) and Al+3 (r = 0.61 Å) produces the variationin the lattice constants which could be the factor responsible
for the optical zonation observed (Fig. 2).
Chemical zonation in kristiansenite has been mentioned
by Raade et al. (2002) who reported backscattered electron
images with irregular or oscillatory zoning, with light areas
richest in Sn and dark areas richest in Sc, although sector
zoning had not been mentioned until now. The sector andhourglass zoning observed in the silesiaite crystals from El
Valle-Boinás may have a similar origin to that of titanaugite(Downty, 1976), e.g., due to a preferred enrichment in Sn+4 ,Fe+2 , Fe+3 and Al+3 in faces belonging to different prismatic forms as a result of a metastable growth process during the replacement of chalcopyrite, favoured by the supplyof Sn and reduced conditions. However, this point needs tobe studied in more detail.

6

Conclusions

The presence of silesiaite in the calcic Cu–Au skarn of El
Valle-Boinás, which is the second known occurrence of thismineral in the world, confirms the existence of this Fe+3 analogue of kristiansenite (Ca2 ScSn(Si2 O7 )(Si2 O6 OH)). The
mineral occurs as aggregates of small crystals (< 500 µm)
but of sufficient size to allow for their study under optical microscopy and to carry out electron microprobe analyses. However, its size and quantity have not allowed us
to perform X-ray diffraction studies. This work presents
the first optical and electronic images of silesiaite in theworld, in addition to the first electron microprobe analyses. These chemical analyses indicate that silesiaite fromEl Valle-Boinás has a higher amount of Sn than kristiansenite and does not contain Sc. Moreover, the crystals show a strong zonation both optically and chemicallywith Sn-rich and Fe-rich zones. The calculated structural
formula of the mineral, and the Fe+3 and Fe+2 proportion determined by stoichiometry, suggests a couple substitution of 2(Fe, Al)+3 ⇔ (Sn, Ti)+4 + (Fe, Mn, Mg)+2 . We
propose that the formula of the silesiaite can be written+3+2
as Ca2 Fe1−2x Fex Snx Sn (Si2 O7 ) (Si2 O6 OH), where x is
between 0 and 0.4.
https://doi.org/10.5194/ejm-33-165-2021

173

Data availability. Raman and EMPA data are available in the Supplement.

Supplement. The supplement related to this article is available online at: https://doi.org/10.5194/ejm-33-165-2021-supplement.

Author contributions. AC was responsible for conceptualization,
methodology and data curation. AC and MFF made the validation of
the research. AC prepared the manuscript with contributions fromall co-authors. MFF managed and had the coordination responsibility for the research activity planning and execution. AC, MFF and
AMI acquired the financial support for the project leading to thispublication.

Competing interests. The authors declare that they have no conflictof interest.

Acknowledgements. We thank the reviewers and the associate editor for their suggestions, which have improved significantly the content and clarity of the paper.

Financial support. This research has been supported by the Ministerio de Economía, Industria y Competitividad, Gobierno de España(grant no. CGL2016-76532R).

Review statement. This paper was edited by Sergey Krivovichev
and reviewed by two anonymous referees.

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