Hydroromarchite is a mineral that so far has been found only in a
few locations in the world and recognized as a common product of submarine
corrosion of pewter artefacts. Here we report a new locality for this rare
mineral found at the Saint James Church archaeological site in Creussen,
Germany. There it appeared to be a product of weathering of a tin artefact
(a tin button) buried in soil of the churchyard for about 300 years. The
mineral, found in paragenesis with romarchite and cassiterite, was
identified using single-crystal X-ray diffraction.
Introduction
The rare mineral hydroromarchite is the natural occurrence of hydrous tin
(II) oxide. It is approved by the International Mineralogical Association
(IMA) and remains in the IMA List of Minerals providing the following
information:
approved formula: Sn32+O2(OH)2
IMA status: A (approved)
IMA no./year: 1969-007
country: Canada
first
reference: Canadian Mineralogist 10 (1971), 916 (Organ and Mandarino,
1971)
second reference: Canadian Mineralogist 41 (2003), 649 (Ramik et
al., 2003).
Organ and Mandarino (1971) claimed to find natural hydrous tin (II) oxide
on tin artefacts in a freshwater environment, which they named
hydroromarchite, on the basis of its powder X-ray diffraction (XRD) pattern
matching that reported by Donaldson (1961) for synthetic crystalline hydrous
tin (II) oxide. As the powder XRD pattern was not indexed (Donaldson, 1961),
the first available crystallographic information was limited by the
triclinic lattice parameters. The crystal data of Donaldson (1961) were superseded by
the data from single-crystal X-ray diffraction (SCXRD) measurements, also on
synthetic crystals (Howie and Moser, 1968), which revealed their
tetragonal symmetry. Howie and Moser (1968) were the first who suggested a structure
model for the tin (II) oxyhydroxide based on the Sn6O8 structural
unit, consisting of an approximately regular cube of oxygen atoms (side 2.82 Å) superimposed on an octahedral cluster of tin atoms (side 3.59 Å).
The following crystallographic data were reported: the unit cell parameters
a=7.93(1) and c=9.13(1) Å; the space group (from systematic absences)
P4/mnc or P4nc (Howie and Moser, 1968, 1973); the number of formula units
(3SnO ⋅ H2O) per unit cell Z=4. The atomic coordinates were not provided
in the papers, and the quality of the structural refinement was very
moderate (R-factor equal to 16 %). A group of co-authors (Ramik et al.,
2003), including the authors of the first report of hydroromarchite in 1971
(Organ and Mandarino, 1971), re-examined their powder XRD data and accepted
tetragonal symmetry and the space group P4‾21c (Ramik et al.,
2003). This space group was suggested by Abrahams et al. (1996), who refined
the structure of synthetic tin (II) oxyhydroxide (JCPDS file number 46-1486)
by Rietveld analysis, using in the starting model the atomic parameters of
the isostructural lead analogue (Hill, 1985). The unit cell parameters
reported by Abrahams et al. (1996) are a=7.9268(4) and c=9.1025(5) Å.
The latest report on the low-temperature SCXRD of synthetic hydroromarchite
appeared as a short abstract for a crystallographic meeting (Reuter and
Schröder, 2010). High-quality and large enough single crystals were
found by Reuter and Schröder (2010) by chance, in a more than 10-year-old sample of nBuSnH3 in toluene. The unit cell parameters and the
space group (a=7.8809(2), c=9.0595(4) Å, and P4‾21c)
provided there (Reuter and Schröder, 2010) agree with those reported by
Abrahams et al. (1996). Unfortunately, the abstract by Reuter and
Schröder (2010) does not contain information about the atomic
coordinates, atomic displacement parameters, and experimental temperature,
and since then the complete data of the structure refinement have not been
published or deposited to any crystallographic database. Thus, the
reported structural data based on the SCXRD analysis are either incomplete
and insufficiently accurate (Howie and Moser, 1968, 1973) or their details
are unavailable (Reuter and Schröder, 2010). Moreover, they disagree
with regard to the space group: Howie and Moser (1968) suggested either
P4/mnc or P4nc, whereas Reuter and Schröder (2010) found it to be
P4‾21c, which matches the results of powder XRD analysis reported
for both the synthetic samples (Abrahams et al., 1996) and mineral
hydroromarchite (Organ and Mandarino, 1971; Ramik et al., 2003; Dunkle et
al., 2003).
All researchers who have studied synthetic tin (II) oxyhydroxide have been concerned that the
preparation of single crystals of this compound had proven to be
particularly difficult. The controversy in the crystal symmetry could likely
be resolved if single crystals of hydroromarchite could be found in nature
and studied using modern methods of SCXRD analysis. That appeared to be the
case in the present work. Here we report a new natural occurrence of the
anthropogenic mineral hydroromarchite after only a few unambiguously
confirmed instances so far (Organ and Mandarino, 1971; Ramik et al., 2003; Dunkle et
al., 2003, 2004; Berger et al., 2019; Di Martino et al., 2019).
It was found not in a water environment but in soil. The results of the
hydroromarchite's structural investigation and its SCXRD data analysis are
described below.
Hydroromarchite: an anthropogenic mineral originating from tin
weathering
Hydroromarchite is a substance formed from man-made materials (tin or its
alloys with a minor amount of alloying metals, Cu, Fe, As, and Pb, called
pewter) and found mainly on archaeological artefacts. Since 1998 such
substances have not been considered by the IMA Commission on New Minerals,
Nomenclature and Classification (CNMNC) as minerals (Nickel and Grice, 1998;
Hazen et al., 2017). The hydroromarchite, however, although not matching the
current IMA CNMNC's definition of a mineral (Nickel, 1995), still preserves
the “title”, as it was approved as a mineral a long time before. As
suggested by Hazen and co-authors (Hazen et al., 2017), it should be
considered among natural phases in the category “II. Minerals associated with
archaeological artefacts” in the group “C. Alteration of tin artefacts”
(Hazen et al., 2017). In this light hydroromarchite is certainly qualified to
be called an anthropogenic mineral (also in light of its origin and locality identified in this
work; see below) but qualifying it as a natural mineral (Hazen et al., 2017) based on its
finding in Cantiere Speranza (Corchia mine), Italy (Garuti and Zaccarini,
2005), is likely not quite justified yet. The mineral from Corchia mine was
identified only on the basis of its chemical composition deduced from
results of several electron-microprobe analyses, and the authors Garuti and
Zaccarini (2005) themselves wrote that “since no Sn-mineral precursor has
ever been reported from the Northern Apennine copper deposits, a natural
origin of the hydroromarchite from Corchia could be seriously challenged”
and “…a complete characterization is needed to confirm its
mineralogical nature” (Garuti and Zaccarini, 2005).
Hydroromarchite was found only in a few locations in the world and is
recognized as a common product of submarine corrosion of pewter artefacts.
Along with romarchite (SnO), it was first identified on tin pannikins, which
were exposed to fresh water of Winnipeg River at Boundary Falls, Ontario,
for about 150–170 years (Organ and Mandarino, 1971; Ramik et al., 2003). Then
it was found on plates and other pewter artefacts (made of tin with a minor
amount of alloying metals Cu, Fe, As, and Pb) recovered from the shipwreck Queen Anne's Revenge archaeological site (Beaufort, North Caroline) after
being exposed to salty oceanic water for about 280 years (Dunkle et al.,
2003). In both cases the identification was made on the basis of X-ray
diffraction data. Dunkle et al. (2004) reported on a few other findings in
submarine environments (see Dunkle et al., 2004, and references therein).
Finding hydroromarchite in a non-underwater environment is even rarer.
During an excavation in the Mochlos settlement (Crete, Greece) in 2004, a
tin ingot was unearthed, and some hydroromarchite could be identified among
the products of the ingot's disintegration (Berger et al., 2019).
Investigations of fragments of a tin-rich alloy organ pipe (mid-18th
century), coming from an instrument placed in the church of San Giovanni
Battista in Chieti (Abruzzo) in central Italy (Di Martino et al., 2019),
revealed hydroromarchite among products of the organ pipe's corrosion due to
moisture. Present work has added one more location to the currently quite
short list due to finding hydroromarchite as a product of a tin artefact
weathering in soil for about 300 years.
New locality for anthropogenic hydroromarchite: the description of the Saint James Church (St. Jakobus-Kirche) archaeological site in Creussen,
Germany, and the tin artefact
The tin artefact, whose alteration led to the formation of hydroromarchite, was
found at an archaeological site in the old city of Creussen, located in the
state of Bavaria, 12 km south of Bayreuth (Germany). The Bavarian
cultural heritage preservation legislation (das Bayerische
Denkmalschutzgesetz) has ensured that no valuable site will be destroyed by
construction without study. The archaeological excavation in the area of the
former cemetery both at the south and the north sides of Saint James
Church (Fig. 1) (latitude: 49.843975/49∘50′38.31′′ N,
longitude: 11.623794/11∘37′25.657′′ E) started in relation to
the church's general renovation that began in 2017.
Saint James Church (St. Jakobus-Kirche) in the old town of Creussen. The oldest altar
part of the church is dated to 1477. It has appeared in its present form since 1700.
The excavations conducted by ReVe Büro für Archäologie GbR
recovered human remains and artefacts from burials dated to as far back as the
9th century (personal communication with Claus Vetterling, chief of the
ReVe Büro), thus providing a more secure estimate for the age of the Creussen settlement, which was first mentioned in written historic chronicles in 1003
(Kröll, 2003). The analysis of skeletal remains by advanced modern
methods contributes to historical anthropology, palaeopathology, and many
other archaeological disciplines seeking better understanding of prehistoric
human health and diseases, as well as moving from bones to social behaviour
(Katzenberg and Grauer, 2019). Examining material remains enables
patterns of past human behaviour and cultural practices to be deduced.
During the previous excavations in the rear churchyard (der hintere Kirchhof at the northern side of Saint James Church) in 1989, about 20
burial places were recovered, which were dated to the 18th century
(Kröll, 2003). In the dig along the northern wall of the church (Fig. 2), explored in August 2021, the archaeologists excavated dozens of burials
and recovered numerous artefacts, such as ceramics, clothes fastening hooks, buckles, fragments of leather girdles, many sets of garment
buttons, and others. Here we describe only 1 button from a set of 13 tin
buttons (Fig. 3) of the same kind found in the burial located in the very
eastern part of the dig, near the wall of the altar part of the church.
The excavation site at the northern side of Saint James Church,
where the tin buttons were found.
The set of 13 tin buttons as found at the excavation site on the
northern side of Saint James Church.
This is a one-piece cast button with a cast eye shank (Fig. 4). The face of
the button has a floral decoration (Fig. 4a). The face and shank are
cast as one unit, and the mold seam, well seen on the back (Fig. 4b, c), is passing through the shank also. Materials, basic construction
techniques used for buttons manufacturing, and approximate dates for these
techniques help archaeologists to more accurately assign termini post quem for buttons from
archaeological deposits. According to the typology of 18th century
metal buttons (Hughes and Lester, 1981, 1991; Hinks, 1988; Luscomb, 1967),
the one we describe here can be regarded as type 1A2, metal waistcoat-size
(19 mm) button, with the dates ranging from ca. 1720 to ca. 1800. Thus, 1720
can be assigned as the terminus post quem.
It is interesting to mention that visiting the Fränkische-Schweiz Museum
in Pottenstein (Upper Franconia, Germany), the authors came across the
information about men's clothes in the region in the 18th–19th
centuries. Waistcoats worn at that time by men (Fig. 5) featured about a dozen
buttons (Kretschmer, 1860/1870).
Images of the button taken under an optical microscope. (a) The face
of the button with a floral decoration. (b, c) The back of the button with
the shank which reveals that this is a one-piece cast button
with the eye separated from the back by a short stem: the face and shank are
cast as one unit. A seam mold on the back is passing through the shank.
Men's clothes in Upper Franconia in the 18th–19th
centuries (Kretschmer, 1860/1870), courtesy of the Fränkische-Schweiz
Museum. Waistcoats typically had about a dozen buttons.
The terminus ante quem (1796) could also be set due to the historical records and the church
documents. According to Kröll (2003), the rear churchyard was in
use until 1772 and closed in 1806; see Kröll (2003) and references
therein. According to the four archival documents (files of the Creussen
parish archive PfA Creußen 207, 566, 567, and 568; Fig. 6), which were
examined on the authors' request (personal communication with Daniel Schönwald, State Church Archive of the Evangelical Lutheran Church in
Bavaria – Landeskirchliches Archiv der Evangelisch-Lutherischen Kirche in
Bayern (LAELKB)), considering also the church registers of the
Evangelical Lutheran Creussen parish, the front and rear cemeteries were
closed in 1796. Hence, the button recovered from the Saint
James Church archaeological site in Creussen in 2021 could have been buried in the graveyard's soil for as long as
300 years. The material of the button and products of its weathering
were studied by analytical methods.
Materials and methods
The material of the button itself and products of its weathering found on
the back of the button (Fig. 4b) were extracted and investigated
using powder and single-crystal XRD, scanning electron microscopy, and
energy-dispersive X-ray spectroscopy (SEM/EDX). SnO2 was used as a
standard for quantitative EDX.
Powder and single-crystal XRD datasets were collected using a
diffractometer equipped with a Bruker D8 platform (the 3-axis goniometer),
an APEX detector, and an Ag Kα Incoatec I µS source (beam
size of ∼ 50 µm FWHM (full width at half maximum)). The powder XRD data were
collected in transmission geometry; the sample was rotated 360∘
during 60 to 300 s depending on the quality of material and sample size
(ranging from 10 to 50 µm in linear dimensions). For studies of
pre-selected single-crystal samples, we acquired half-sphere data with the
exposure time of 60 s per frame with a step of 0.3∘ and 1265 frames in total.
Lorentz and polarization corrections, as well as an analytical absorption
correction based on the crystal shape, were taken into account for the
correction of the reflection intensities using the CrysAlis package
(CrysAlisPro Software System, 2019). All crystallographic data refinements
were performed based on F2 using the SHELX97 programme package (Sheldrick,
2015) in the WinGX System (Farrugia, 1999). The chemical composition of the
button was studied using scanning electron microscopy (ZEISS SEM, Leo Gemini
1530 with a Schottky field emission gun employing an accelerating voltage of
15–20 kV).
Results and discussion
Two flakes of an approximate size of 0.07×0.07×0.03 mm3 were cut with a
scalpel from a metallic part of the button and another one (∼0.3×0.3×0.03 mm3) from an interface between the metallic and weathered
parts of this artefact. Powder X-ray diffraction of the metallic flakes
corresponds to pure β-Sn (a tetragonal unit cell, lattice
parameters a=5.831(2) and c=3.181(1) Å). SEM/EDX analysis
confirms that the samples are pure tin. In the diffraction pattern of the
flake taken from the interface, there are lines of other phases, apart from those of tin, which could be identified as cassiterite (SnO2) and romarchite
(SnO). However, their lattice parameters were difficult to determine
because their diffraction lines are broad and of low intensity. SEM/EDX
analysis of the sample imaged in Fig. 7 shows the presence of tin oxides.
About 20 isometric particles of black or dark-yellowish colour of 20 to 30 µm in diameter were picked with a tungsten needle from the weathered part
of the button (Fig. 8). Their examination using XRD shows the presence of
continues Debye rings. Diffraction patterns of a few particles (richer in
yellowish material) contain numerous spots apart from diffraction rings.
Powder materials are cassiterite, SnO2 (tetragonal, typical lattice
parameters a=4.735(1), c=3.184(1) Å), and romarchite, SnO
(tetragonal, lattice parameters in different samples vary within 0.01 Å
with typical values of a=3.801(2) and c=4.835(2) Å). SEM/EDX
analysis confirms the presence of tin oxides in a very fine intergrowth (so
it is impossible to characterize composition of individual minerals).
A photograph of some of the documents from the State
Church Archive of the Evangelical Lutheran Church in Bavaria
(Landeskirchliches Archiv der Evangelisch-Lutherischen Kirche in Bayern
(LAELKB)) inspected to establish terminus ante quem.
SEM image in back-scattered electrons of a sample taken from the
interface between the metallic and the weathered parts of the button. The gray area is
the tin metal, and the black area corresponds to tin oxides.
Optical microscope image of the weathered part of the artefact. The
shiny silver colour areas are tin; the black or dark-yellowish colour areas are
weathered material. The scale (white bar) is 500 µm.
Four single-crystal datasets were collected for particles giving a spotty
diffraction pattern. Apart from a cassiterite powder, these samples contain a
tetragonal phase with unit cell parameters (a=7.9173(11) and
c=9.0860(14) Å) that are in good agreement with those previously
reported for hydroromarchite, Sn3O4H2, from SCXRD (a=7.93(1) and c=9.13(1) Å from Howie and Moser, 1968; a=7.8809(2)
and c=9.0595(4) Å from Reuter and Schröder, 2010) and powder XRD
(a=7.9268(4) and c=9.1025(5) Å) (Abrahams et al., 1996). Each
particle contains dozens of randomly oriented domains of hydroromarchite
that indicate that the size of individual crystals is just a few
micrometres. The particle that gave the best SCXRD data was polished and
investigated by SEM/EDX. SEM image of this sample in back-scattered
electrons (Fig. 9) looks quite homogeneous with darker dots that may
correspond to cassiterite. Chemical analysis (EDX) confirms that only two
elements, tin and oxygen, are present in an approximate atomic proportion of 3:4.
SEM image in back-scattered electrons of a particle, which was
studied by SCXRD. It contains dozens of randomly oriented domains of
hydroromarchite and a powder of cassiterite.
Slice of the (0kl) reciprocal space of a hydroromarchite single
crystal. Reflection condition k+l=2n corresponds to the space groups
P4/mnc or P4nc. The systematic absences do not support the P4‾21c space
group, which was suggested by Ramik et al. (2003) and Abrahams et al. (1996)
on the basis of their powder diffraction data. Powder lines are due to
cassiterite.
Only one SCXRD dataset has sufficient quality for structure solution and
refinement (Table 1). The quality of natural crystalline material is moderate
(it scatters only to ∼ 0.9 Å) but satisfactory for
reliable structural analysis. The structure of hydroromarchite has the space
group P4/mnc(#128) with two Sn and one O atoms on crystallographically
distinct positions (see Table 1 and the Crystallographic Information File (CIF) for the full crystallographic
data). The space group P4/mnc was chosen between the two possible ones (P4/mnc or
P4nc, based on systematic absences; Fig. 10) because the structure refinement
in the lower-symmetry group does not improve the R-factor. The space group
determined in our study agrees with that reported by Howie and Moser (1968,
1973), also based on SCXRD at room temperature. The systematic absences
(Fig. 10) do not support the lower-symmetry P4‾21c (#114) space
group, which was suggested by Abrahams et al. (1996) on the basis of their
powder diffraction data and the Rietveld refinement of the structure of tin
(II) oxyhydroxide. Abrahams et al. (1996) used “a starting model based on
the isostructural lead compound in space group P4‾21c”; thus the
space group was chosen a priori. The same P4‾21c space group was reported
by Reuter and Schröder (2010) based on low-temperature SCXRD.
Considering group–subgroup relations, we cannot exclude that the lower
symmetry found by Reuter and Schröder (2010) is due to the
low-temperature conditions of their SCXRD measurements. However, in the
absence of the structure refinement details in the publication by Reuter and
Schröder (2010) – temperature, atomic coordinates, and atomic
displacement parameters to compare with – we would rather refrain from any
further speculations concerning the discrepancy.
Crystal structure, data collection, and refinement details of
hydroromarchite, Sn3O2(OH)2.
Crystal data Chemical formulaSn3O2(OH)2Mr840.14 Crystal system, space groupTetragonal, P4/mncTemperature (K)293 Pressure (GPa)Ambient a, c (Å)7.9173(11), 9.0860(14) V (Å3)569.54(18) Z2 Radiation typeAg Kαμ (mm-1)6.86 Crystal size (mm)0.005×0.005×0.005Data collection DiffractometerBruker, custom made Absorption correctionMulti-scan, CrysAlisPro 1.171.40.54a (CrysAlisPro Software System, 2019) Empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm Tmin, Tmax0.966,0.966 No. of measured, independent,and observed [I>4σ(I)]reflections2414, 605, 502 Rint0.058 Refinement R[F2>2σ(F2)], wR(F2), S0.069, 0.145, 0.98 No. of reflections605 No. of parameters22 Δρmax, Δρmin (e Å-3)2.27, -2.15 Crystal structure Wyckoff siteCoordinates (x, y, z)Uiso (Å2)Sn14e0.5 0.5 0.2233(2)0.0192(3)Sn28h0.7913(2) 0.6223(2) 0.50.0196(3)O1 single position:O116i0.598(2) 0.725(3) 0.359(2)0.085(6)O1 split position:O1′16i, 50 % occupancy0.612(7) 0.718(8) 0.339(3)0.042(3)O1′′16i, 50 % occupancy0.588(7) 0.730(8) 0.381(3)0.042(3)
The structural unit of Sn3O4H2 is a Sn6O8 cluster
(Fig. 11a) consisting of oxygen atoms in the corners of an approximately
regular cube (the cube's edge length of 2.56 Å vs. 2.82 Å in Howie and
Moser, 1968) superimposed on an octahedral cluster of tin atoms (the
octahedron's edge length of 3.55 Å vs. 3.59 Å in Howie and Moser, 1968). An average Sn–O distance (2.22 Å) in hydroromarchite is the
same as in romarchite (Izumi, 1981), supporting the Sn(II) oxidation state of
tin. Assuming Sn(II), the bond valence sum on oxygen atoms is equal to 1.5;
then one H+ linked to two oxygen atoms provides the balance of valences
(note that for Sn(III) or Sn(IV) a formal balance is not possible). Using
the bond valence parameters from the dataset deposited at the IUCr web page
(IUCr resources, 2020) and considering hydrogen to be at the middle between
two oxygen atoms, one gets the bond valence sum on oxygen equal to 1.89
(with the same parameters the corresponding sum for romarchite is equal to
1.88). Layers of Sn6O8 clusters, oriented differently in adjacent
layers, alternate in the c direction (Fig. 11b). Centres of the clusters form
a body-centred tetragonal bct lattice. The distance between oxygen atoms of
adjacent Sn6O8 clusters in successive layers, connected by
hydrogen bonds, is 2.80 Å vs. 2.60 Å in Howie and Moser (1968) and is
considerably less than the oxygen–oxygen distance between clusters in the
same layer (4.62(5) vs. 4.60 Å in Howie and Moser, 1968). It should be
pointed out that the length of the shortest inter-cluster contacts (2.60 Å) in Howie and Moser (1968) was found to be significantly shorter than
the oxygen–oxygen distance within a single cluster (2.82 Å), which is not
logical from a crystal–chemical point of view. In our model the inter-cluster
O–O distance is equal to 2.56 Å and can become even less (down to
∼ 2.3 Å) in metal–organic analogues of a tin hydroxide
(Suslova et al., 2007). The precise position of the hydrogen atoms,
“symmetrically or statistically placed between oxygen atoms”, was not
discussed in Howie and Moser (1968) but advised to be studied. Our
investigation suggests a statistical distribution of hydrogen atoms in the
structure of hydroromarchite, as we see that the Sn6O8 cluster is
not distorted. In contrast, an ordered distribution of ligands in metal–organic
analogues leads to considerable distortion of the Sn6O8 clusters
(Suslova et al., 2007).
The crystal structure of hydroromarchite. (a) A
Sn6O8 cluster representing a structural unit and consisting of
oxygen atoms (red balls) in the corners of an approximately regular cube
superimposed on an octahedral cluster of tin atoms (grey balls); c axis is
vertical in the figure. (b) Ball-and-stick model of the
Sn3O4H2 structure viewed along the c direction. Centres of the
Sn6O8 clusters form a body-centred tetragonal bct lattice. This
model corresponds to the one proposed by Howie and Moser (1968) with a
single O1 position. (c) Ball-and-stick model in which O1 position is split
into two, O1′ and O1′′, with the
occupancy equal to 0.5 (see Table 1). The splitting leads to the improvement
of the thermal parameter of oxygen (Uiso ∼ 0.04 Å2),
thus suggesting that this model correctly represents the structure of
Sn3O2(OH)2.
Note that the structural model of hydroromarchite proposed by Howie and Moser (1968) and described above contains only one type of oxygen (i.e. all O
atoms are in the same crystallographic position), and two hydrogen atoms per
formula unit have to be added for a charge balance. Thus, in this structural
model the formula Sn3O4H2 would have to be chosen because a
choice of Sn3O2(OH)2 would mean two types of oxygen in the
crystal structure. Both formulas were, however, suggested by Howie and
Moser (1968) to interpret the analytical formula of hydroromarchite,
probably based on their chemical intuition. If we consider the shortest
contacts between clusters we found, they are ∼ 2.80 Å, which is too long for a symmetric hydrogen bond. As currently understood, in
hydrous compounds with a common oxygen–hydrogen–oxygen sequence, a
hydrogen bond is asymmetric (O–H⋯O) due to thermally activated
hydrogen mobility at low compression, which often symmetrises (O–H–O) under
increasing pressure (Meier et al., 2022). This hydrogen mobility causes an
effective displacement of O atoms involved in the hydrogen bond. Indeed,
the thermal ellipsoid of the oxygen atom is elongated, and in isotropic
approximation it is quite large (Uiso∼ 0.08 Å2;
Table 1). Splitting the O1 position into two, O1′ and
O1′′ with the occupancy equal to 0.5 (Table 1),
leads to the improvement of the thermal parameter of oxygen (Uiso∼ 0.04 Å2), and thus the structure of
Sn3O2(OH)2 should be correctly presented, as shown in Fig. 11c.
Conclusions
In this work we report a new locality for the rare mineral hydroromarchite,
Sn3O2(OH)2. It was found not in a submarine environment but
in soil, at the Saint James Church archaeological site in Creussen, Germany.
A tin artefact (a tin button) was exposed to weathering in soil for
about 300 years. The mineral was identified using single-crystal X-ray
diffraction. Its structure was solved and refined with the R-factor of
6.9 % providing more accurate crystallographic data than currently available
for hydroromarchite.
Data availability
All data derived from this research are available upon request from the
corresponding author.
Author contributions
ND initiated the project. ND and MM selected the artefact. ND and LD
performed the data collection and evaluation. MM oversaw archaeological
interpretation. All authors discussed the results. ND wrote the paper with
input from LD and MM.
Competing interests
The contact author has declared that none of the authors has any competing interests.
Disclaimer
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
We thank the following persons who kindly supported this study: Claus
Vetterling (ReVe Büro für Archäologie GbR, Bamberg) for granting
Natalia Dubrovinskaia and Leonid Dubrovinsky admission to the archaeological site in Creussen and his team,
especially Verena Stürner, for sharing their knowledge in history and
archaeology during the excavation time in Creussen in August 2021; Achim Peter (pastor of Saint James Church, Creussen) for helpful communications;
Daniel Schönwald (Landeskirchliches Archiv der Evangelisch-Lutherischen
Kirche in Bayern, LAELKB, Nürnberg) for his help in searching for
documents in the church archive; Fabian Wittenborn and Jens Kraus
(Fränkische-Schweiz Museum, Pottenstein) for their help in ethnographic
questions; Alexander Wölfel (Schnabelweid) and Gernot Gebauer (Creussen)
for sharing with the authors their collections of ancient buttons found in
the fields around Creussen in Upper Franconia; and Dorothea Wiesner
(University of Bayreuth), who helped in SEM analysis. Natalia Dubrovinskaia thanks
Vadim Kessler and the Department of Molecular Sciences (Swedish University
of Agricultural Sciences (SLU), Uppsala, Sweden) for involving her in the
August T. Larsson Guest Researcher Program. Natalia Dubrovinskaia is also grateful to Vadim
Kessler and Gulaim Seisenbaeva for sparking her interest in weathering
processes, for fruitful discussions during the preparation of her lectures on
mineral weathering in the course on environmental geochemistry taught
at the SLU in the winter semester of 2021, and for helpful references concerning
crystal chemistry of metal–organic analogues of a tin hydroxide.
Financial support
This open-access publication was funded by the University of Bayreuth.
Review statement
This paper was edited by Cristian Biagioni and reviewed by Andrew Locock and two anonymous referees.
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