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Article
Paradimorphite, β-As
4
S
3
, a vintage new mineral from Solfatara
di Pozzuoli and Vesuvius, Napoli, Italy
Italo Campostrini1, Carlo Castellano1, Francesco Demartin1* , Ivano Rocchetti2, Massimo Russo3
and Pietro Vignola4
1
Università degli Studi di Milano, Dipartimento di Chimica, via Golgi 19, I-20133 Milano, Italy;
2
MUSE, Museo delle Scienze di Trento, Corso del Lavoro e della Scienza
3, I-38122 Trento, Italy;
3
Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Napoli | Osservatorio Vesuviano, Via Diocleziano, 328, I–80124 Napoli, Italy; and
4
CNR-Istituto di Geologia Ambientale e Geoingegneria, Via Mario Bianco 9 - 20131 Milano, Italy
Abstract
The new mineral paradimorphite corresponds to the high temperature polymorph of As
4
S
3
, whose existence was supposed by Arcangelo
Scacchi in 1850 in the fumaroles at the Solfatara di Pozzuoli, Campi Flegrei, near Napoli, Italy.
Crystals of paradimorphite are orange yellow, transparent or semitransparent, with adamantine lustre. Habit is prismatic and
observed forms are {110}, {101}, {111}, {100}, {010} and {001}. Tenacity is brittle, no distinct cleavage is observed and fracture is
conchoidal. The mineral does not fluoresce in long- or shortwave ultraviolet light. No twinning is apparent. The streak is saffron yellow.
Hardness (Mohs) = 1–2. The observed density is 3.510(3) g/cm
3
, calculated density is 3.500 g/cm
3
. The mineral is orthorhombic, space
group Pnma, with a= 9.1577(7), b= 8.0332(6), c= 10.2005(8) Å, V= 750.41(10) Å
3
and Z= 4. The eight strongest powder X-ray
diffraction lines are [d
obs
Å(I)(hkl)]: 6.299(48)(011), 5.186(100)(111), 4.174(31)(201), 3.133(34)(022), 3.116(58)(212), 2.980(41)(122),
1.846 (27)(413) and 1.808(23)(134). The structure was refined to R= 0.0229 for 979 reflections with I>2σ(I). Crystals of paradimorphite
contain As
4
S
3
molecules, of idealised C
3v
symmetry, with the four arsenic atoms in a triangular pyramidal arrangement, with sulfur atom
bridges on the three adjacent apical edges. Molecular dimensions and conformation are identical within standard uncertainties with
those of the low-temperature polymorph dimorphite. No substantial differences, neither in the molecular packing nor in the molecular
orientation, could be observed; minor differences being related to intermolecular distances only.
Keywords: paradimorphite, new mineral, arsenic sulfide, dimorphite, fumaroles, Solfatara di Pozzuoli, Vesuvius volcano
(Received 14 March 2022; accepted 6 May 2022; Accepted Manuscript published online: 12 May 2022; Associate Editor: Daniel Atencio)
Introduction
In his ‘Memorie geologiche sulla Campania’, a detailed report of
the mineralogical phases occurring in the fumaroles of the
Campi Flegrei, near Napoli, Italy, Arcangelo Scacchi (1850)
observed the presence of an arsenic sulfide, with probable com-
position As
4
S
3
, displaying two distinct crystalline forms. From
accurate goniometric measurements, he found two distinct
morphologies and axial ratios, corresponding to two possibly dis-
tinct phases, a more abundant form I with an a:b:cratio of
1:1.287:1.153 and a form II with the a:b:cratio 1:1.658:1.508.
(Fig. 1). Nevertheless, Scacchi considered his own analyses indeci-
sive for establishing a definitive chemical formula and the exist-
ence of two different mineral species, because of the small
quantity of material used in the analysis and because the method
used involved quantitative analysis only for sulfur. For these rea-
sons the name ‘dimorfina’or later dimorphite (from the Greek),
with reference to the two forms in which it was thought to exist,
was used with no distinction for the two minerals.
After Scacchi’s discovery, there has been considerable discus-
sion about whether dimorphite is indeed a distinct mineral or
instead a morphologically unusual orpiment, As
2
S
3
, not recog-
nised by Scacchi (Kenngott, 1870; Dana, 1885; Palache et al.,
1944). Schuller (1894) and Krenner (1907) showed As
4
S
3
can be
synthesised by direct combination of the elements mixed in stoi-
chiometric proportion, and their work allowed the existence of
the synthetic counterparts of one of the two dimorphites to be
demonstrated. In fact Krenner (1907) showed that the forms
and angles of the second type of dimorphite discovered by
Scacchi (form II) agree closely with those of synthetic As
4
S
3
pre-
pared by Schuller (1894) and that Scacchi’s second type of dimor-
phite should be considered a valid mineral species. In the
Commission on New Minerals, Nomenclature and Classification
of the International Mineralogical Association (IMA–CNMNC)
list of mineral species updated January 2022 (Pasero, 2022),
dimorphite is indeed considered a valid mineral and corresponds
to Sacchi’s form II, the polymorph, stable at room temperature,
with a= 11.21(2), b= 9.90(2), c= 6.58(1) Å, space group Pnma,
whose structure was determined by Whitfield (1973) on the
synthetic phase. Whitfield called this phase β-form, despite this
polymorph being stable at room temperature. The structure of
the other polymorph with a= 9.12(2), b= 7.99(2), c= 10.10(2) Å
and space group Pnma, had been reported earlier (Whitfield,
*Author for correspondence: Francesco Demartin, Email: francesco.demartin@unimi.it
© The Author(s), 2022. Published by Cambridge University Press on behalf of The Mineralogical Society of Great Britain and Ireland
Cite this article: Campostrini I., Castellano C., Demartin F., Rocchetti I., Russo M. and
Vignola P. (2022) Paradimorphite, β-As
4
S
3
, a vintage new mineral from Solfatara di
Pozzuoli and Vesuvius, Napoli, Italy. Mineralogical Magazine 1–7. https://doi.org/
10.1180/mgm.2022.47
Mineralogical Magazine (2022), 1–7
doi:10.1180/mgm.2022.47
https://doi.org/10.1180/mgm.2022.47 Published online by Cambridge University Press
1970) by the same author, who called this phase α-form. The
existence of the two distinct natural polymorphs was confirmed,
using powder X-ray diffraction and single-crystal precession
data by Frankel and Zoltai (1973), working on samples of dimor-
phite from Vesuvius, obtained by the Swedish Natural History
Museum. They also observed that in some crystals, having the
morphology of form I, both polymorphs were present, and they
concluded that the existence of dimorphite II crystals as pseudo-
morphs after dimorphite I suggests that dimorphite I is unstable
under conditions to which it was exposed after crystallisation, and
transforms to dimorphite II. The contemporary presence of both
polymorphs was also observed by us in some specimens. An
accurate structure refinement on both phases of natural origin
was later carried out by some of us (Gavezzotti et al.,2013)
together with a theoretical evaluation of the stability of both poly-
morphs at room temperature. In a crystallographic review of
arsenic sulfides by Bonazzi and Bindi (2008) the two forms are
labelled according to their field of stability, i.e. α-dimorphite
the phase stable at room temperature and β-dimorphite the
phase stable above 130°C.
It can be seen that much confusion exists in the literature for
the use of α- and β-descriptors of the As
4
S
3
polymorphs (see
Table 1). Following the rules in the nomenclature of temperature-
depending phase transitions, the low-temperature and the
high-temperature forms should be labelled as α- and β-form,
respectively, as reported by Bonazzi and Bindi (2008).
As a part of our studies on fumarolic minerals (Russo et al.
2017; Campostrini and Demartin, 2021; Campostrini et al.,
2019a,2019b; Demartin et al., 2014), we have investigated some
‘dimorphite’crystals from Solfatara di Pozzuoli and from
Vesuvius crater and the results of a crystallographic study on
both natural polymorphs of As
4
S
3
have been reported in
Gavezzotti et al. (2013). A recent complete characterisation of the
high-temperature phase, not yet recognised as a valid mineral species,
prompted us to submit a proposal to the IMA–CNMNC, to include
this phase in the list of the mineral species (IMA2020-101,
Campostrini et al., 2022). On April 2021 we received a communica-
tion from the chairman of the Commission reporting that the pro-
posed new mineral had major YES votes but, as the CNMNC was
going to consider revision of the guidelines for the nomenclature of
polymorphsandpolysomes,the votingresultfortheapprovalwassus-
pended. According to the new rules established by IMA-CNMNC
only recently, the mineral name was finally approved as paradimor-
phite, by analogy with the low temperature polymorph dimorphite.
This decision overcomes the confusion arisen using the α-and
β-descriptors. The approved mineral name abbreviation is Pdim
(Warr, 2021).
The holotype (from Solfatara di Pozzuoli) and cotype (from
Vesuvius) specimens of paradimorphite are deposited in the
Reference Collection of the Dipartimento di Chimica,
Università degli Studi di Milano, catalogue numbers 2020-03/
6121 and 2020-04/4226, respectively.
In Strunz classification paradimorphite is in 2.FA.10
(Dimorphite group) 2: SULFIDES and SULFOSALTS (sulfides,
selenides, tellurides; arsenides, antimonides, bismuthides; sulfar-
senites, sulfantimonites, sulfbismuthites, etc.) F: Sulfides of
arsenic, alkalies; sulfides with halide, oxide, hydroxide, H
2
O, A:
With As, (Sb), S. In the Dana classification paradimorphite
belongs to the 2.6.1.1 class, 2: SULFIDES 6: A
m
B
n
X
p
, with (m
+n):p = 4:3.
Occurrence
The Solfatara di Pozzuoli is an explosion volcano formed 4285
years ago, located in the central part of the Campi Flegrei caldera,
Napoli, Campania, Italy (40°49’41”N, 14°08’30”E). The deposits
of the Solfatara consist of pyroclastic products dispersed over an
area of about one square kilometre. Inside the crater the deposits
are strongly altered by the intense fumarolic activity. The mor-
phological characteristics of the volcano represent a unicum for
Table 1. History of the different naming of the two As
4
S
3
polymorphs.
Name a(Å) b(Å) c(Å) Reference
Paradimorphite
dimorphite type A Dana (1885)
dimorfina form I Scacchi (1850)
α-form (synthetic As
4
S
3
) 9.12(2) 7.99(2) 10.10(2) Whitfield (1970)
dimorphite I 9.07 8.01 10.30 Frankel and Zoltai (1973)
α-dimorphite 9.1577(7) 8.0332(6) 10.2005(8) Gavezzotti et al. (2013)
β-dimorphite 9.159(1) 8.033(1) 10.199(2) Bonazzi and Bindi (2008)
Dimorphite
dimorphite type B Dana (1885)
dimorfina form II Scacchi (1850)
β-form (synthetic As
4
S
3
) 11.21(2) 9.90(2) 6.58(2) Whitfield (1973)
dimorphite II 11.24(2) 9.90(3) 6.56(2) Frankel and Zoltai (1973)
β-dimorphite 11.2175(15) 9.9224(13) 6.6075(9) Gavezzotti et al. (2013)
α-dimorphite Bonazzi and Bindi (2008)
Fig 1. Morphology of the two As
4
S
3
forms observed by Scacchi (1850) and redrawn in
the ‘Atlas der Kristallformen’by Goldschmidt (1913). Form I (paradimorphite) left,
form II (dimorphite) right.
2 Italo Campostrini et al.
https://doi.org/10.1180/mgm.2022.47 Published online by Cambridge University Press
the Campi Flegrei area as it is a maar-diatreme. The holotype spe-
cimen of paradimorphite was collected here at the Bocca Grande
fumarole. Associated minerals are realgar, salammoniac, mascag-
nite, alacránite, adranosite and russoite (Russo et al.,2017).
The Somma–Vesuvius is a strato-volcano, whose oldest part is
represented by Monte Somma in whose interior the ‘Gran Cono’
of Vesuvius was formed. The volcano’s explosive activity took
place in some ‘Plinian’eruptions (e.g. Pomici di Avellino erup-
tion, ∼4000 years ago and Pompei eruption, 79 A.D.); subse-
quently the formation of the Vesuvius cone began. After the
great eruption of 1631, Vesuvius entered a state of ‘open conduit’
activity with frequent eruptions, on average one every seven years.
The most important eruptions of the last century were those of
1906 and the last one in March 1944. Currently, Vesuvius is in
state of quiescence with some seismic and fumarolic evidence.
The cotype specimen of paradimorphite was collected from an
active fumarole after the 1906 eruption and was found among
old specimens belonging to the, now dispersed, collection of the
Istituto Geomineralogico Italiano (Campostrini and Russo,
2012). Associated minerals in the cotype specimen are anhydrite
and sassolite. Realgar, lafossaite, anhydrite, bonazziite and an
unknown arsenic thallium chloride, probably related to lucabin-
diite are associated with paradimorphite in other specimens. In
both localities the mineral is a fumarolic sublimate.
Physical and optical properties
Crystals of paradimorphite are orange yellow, transparent or
semitransparent, with adamantine lustre. Habit is prismatic and
observed forms are {110}, {101}, {111}, {100}, {010} and {001}
(Figs 2–5). Tenacity is brittle, no distinct cleavage is observed
and fracture is conchoidal. The mineral does not fluoresce in
long- or shortwave ultraviolet light. No twinning is apparent. The
streak is saffron yellow. Hardness (Mohs) = 1–2. Vickers hardness
(micro-indentation): VHN
25
= 70 (range 59–80 kg/mm
2
) (obtained
using a Shimadzu type-M microhardness tester; average of five
indentation measurements). The density, measured by flotation in
a thallium malonate/formate solution (Clerici solution) for the
Solfatara sample (holotype), is 3.510(3) g/cm
3
. The calculated dens-
ity is 3.500 g/cm
3
(Solfatara), 3.520 g/cm
3
(Vesuvius), using the
empirical formula and single-crystal cell data.
Refractive indices were not measured conventionally, because
they were found to be higher than available reference liquids
(>1.9).The mineral is biaxial (+), dispersion is weak to very
weak, and r>v. Pleochroism is barely noticeable.
The Raman spectrum was obtained with an ANDOR 303 spec-
trometer equipped with a CCD camera iDus DV420A-OE and
using the 532 nm line of an OXXIUS solid state laser for excita-
tion. Figure 6 shows a comparison of the Raman spectrum of
paradimorphite with that of a dimorphite collected recently at
Vesuvius. Due to the similarity in the packing of the molecules
in the two polymorphs (see below), frequencies related to lattice
modes should essentially occur at very similar values. A broad
band at ∼52 cm
–1
,
is indeed observed in paradimorphite whereas
the corresponding band of dimorphite appears at 47 cm
–1
with a
shoulder at 56 cm
–1
. All the other bands fall in the range of vibra-
tional frequencies (125–386 cm
–1
) calculated by Gavezzotti et al.
Fig. 2. Back-scatter electron image of paradimorphite from Solfatara di Pozzuoli
(Field of view 120 μm, specimen #EDS-5955).
Fig. 3. Idealised drawing of a paradimorphite crystal similar to that on the right of
Fig. 2. Using Kristall 2000 (Klaus Schilling, http://www.kristall2000.de/).
Fig. 4. Paradimorphite crystals on mascagnite with realgar from Solfatara di Pozzuoli
(Field of view 3 mm).
Mineralogical Magazine 3
https://doi.org/10.1180/mgm.2022.47 Published online by Cambridge University Press
(2013) for the As
4
S
3
molecule in the gas phase. It should be pointed
out the lackof a shoulder at 337 cm
–1
in paradimorphite with respect
to dimorphite and the presence of a bandat 224 cm
–1
.
Chemical analysis
Quantitative chemical analyses (6) were carried out in wavelength
dispersive spectroscopy (WDS) mode using a JEOL JXA–8200
WDS electron microprobe (15 kV excitation voltage, 5 nA beam
current and 5 μm beam diameter). The following mineral and
pure elements served as standards: realgar (As and S), pure
elements (99.99% for Se, Sb and Te). X-ray intensities were con-
verted to wt.% by ZAF quantitative analysis software. Chemical
data for the Solfatara and Vesuvius samples are reported in
Table 2. The empirical formula, calculated on the basis of 7 atoms
per formula unit, for the Solfatara sample is: As
3.986
(S
3.011
Se
0.003
),
that of the Vesuvius crater sample is: (As
3.975
Sb
0.007
)
(S
2.982
Se
0.032
Te
0.004
). The simplified formula is: As
4
S
3
.
The ideal formula is As
4
S
3
which requires: As 75.70, S 24.30 wt.%,
total 100 wt.%.
X-ray crystallography and crystal structure determination
Powder X-ray diffraction data were collected for the Solfatara spe-
cimen with a Rigaku DMAX II powder diffractometer with graph-
ite monochromatised CuKαradiation. Data (in Å for CuKα)are
listed in Table 3. Unit cell parameters refined from the powder data
using UNITCELL (Holland and Redfern, 1997)area= 9.1596(9),
b= 8.0365(9), c= 10.1195(10) Å and V= 744.92(9) Å
3
.Noneof
the intense reflections corresponding to dimorphite were observed
in the powder pattern of this sample.
Single crystal data for paradimorphite were obtained from a
crystal fragment of the Solfatara sample. Details about the data
collection and refinement are summarised in Table 4, whereas
the final atom coordinates and anisotropic displacementparameters
and selected interatomic distances have already been reported by
Gavezzotti et al. (2013). Single-crystal data of the Vesuvius sample
gave the following unit-cell parameters: a= 9.155(3), b= 8.026(2),
c= 10.201(6) Å and V= 749.55(20) Å
3
. The a:b:cratio calculated
from the unit-cell parameters is 1.1400:1:1.2698 (single-crystal
data, Solfatara sample), 1.1407:1:1.2710 (single-crystal data,
Fig. 5. Paradimorphite crystals with anhydrite, cotype sample (#2020-04/4226), from
Vesuvius crater (Field of view 3 mm).
Fig. 6. Raman spectra of paradimorphite from Solfatara di Pozzuoli and dimorphite from Vesuvius.
4 Italo Campostrini et al.
https://doi.org/10.1180/mgm.2022.47 Published online by Cambridge University Press
Vesuvius sample). The crystallographic information file has been
deposited with the Principal Editor of Mineralogical Magazine
and is available as Supplementary material (see below).
Description of the crystal structure and discussion
A comparison of the structures of the two polymorphs, paradi-
morphite and dimorphite, is shown in Fig. 7. Crystals of both
phases contain cage-like As
4
S
3
molecules of C
3v
idealised sym-
metry located about a crystallographic mirror and packed together
by weak van der Waals interactions. Other cage-like covalently
bonded As
4
S
n
(n= 4 and 5) molecules are also the building blocks
of some other arsenic sulfides (see Bonazzi and Bindi, 2008 for a
general review), such as the As
4
S
4
molecules in realgar (Mullen
and Nowacki, 1972), pararealgar (Bonazzi et al., 1995) and
bonazziite (Bindi et al., 2015); the As
4
S
5
molecules in uzonite
(Bindi et al., 2003); and those in alacránite, As
8
S
9
(Bonazzi et al.,
2003) that contains both As
4
S
4
and As
4
S
5
molecules. In the As
4
S
3
molecules of paradimorphite and dimorphite, the four arsenic
atoms are arranged with a triangular pyramidal geometry, and the
sulfur atoms bridge the three apical edges of the pyramid.
Molecular dimensions and conformation are within standard
uncertainties the same for the molecules in the two polymorphs.
These are also very similar to those observed in the tetrakis(tris(μ
2
-
sulfido)-tetra-arsenic)-tetraphenylphosphonium chloride complex
(Siewert and Müller, 1992) where the As–As bonds are in the range
2.464(3)–2.494(3) Å and the As–S bonds in the range 2.229(6)–
2.243(6) Å. For paradimorphite, the As–As bonds of the triangular
base are in the range 2.4673(4)–2.4855(6) Å and are on average sig-
nificantly shorter than those observed for the As
4
S
4
molecules in
realgar (2.566(1)–2.571(1) Å), bonazziite (2.579(1) Å), pararealgar
(2.484(4)–2.534(4) Å) and alacránite (2.579(5) Å), and those in
the As
4
S
5
molecules in uzonite (2.527(1) Å) and alacránite
(2.566(6) Å). The As–S bonds fall in the range 2.2155(7)–2.2360
(10) Å and are again on the average shorter than those observed in
realgar (2.228(2)–2.247(2) Å), pararealgar (2.228(10)–2.261(8) Å)
Table 2. Analytical data (in wt.%) for paradimorphite (average of 6 analyses).
Constituent Mean Range S.D. (σ) Reference material
Paradimorphite, Solfatara di Pozzuoli*
As 75.38 74.65–76.15 0.54 Realgar
S 24.36 24.30–24.79 0.18 Realgar
Se 0.06 0.04–0.07 0.01 Se 99.99%
Total 99.80
Paradimorphite (cotype #2020-04/4226), Vesuvius crater**
As 75.45 74.85–76.35 0.60 Realgar
S 24.22 23.40–24.53 0.42 Realgar
Se 0.64 0.57–0.83 0.09 Se 99.99%
Sb 0.21 0.11–0.32 0.04 Sb 99.99%
Te 0.12 0.04–0.24 0.03 Te 99.99%
Total 100.64
*The empirical formula calculated on the basis of 7 atoms per formula unit is:
As
3.986
(S
3.011
Se
0.003
)
**The empirical formula calculated on the basis of 7 atoms per formula unit is:
(As
3.975
Sb
0.007
)(S
2.982
Se
0.032
Te
0.004
)
S.D. –standard deviation
Table 3. Powder X-ray diffraction data for paradimorphite.
I/Io (meas.) I/Io* (calc.) d(meas.) d** (calc.) hkl
16 17 6.788 6.791 1 0 1
48 50 6.299 6.293 0 1 1
100 100 5.186 5.187 1 1 1
1 2 4.571 4.580 2 0 0
4 3 4.444 4.429 1 0 2
31 37 4.174 4.172 2 0 1
7 6 4.005 4.018 0 2 0
21 25 3.889 3.879 1 1 2
16 17 3.457 3.458 1 2 1
3 5 3.401 3.395 2 0 2
19 10 3.152 3.165 1 0 3
34 30 3.133 3.147 0 2 2
58 65 3.116 3.127 2 1 2
11 8 3.015 3.020 2 2 0
41 46 2.980 2.978 1 2 2
15 27 2.921 2.923 3 0 1
14 23 2.733 2.717 2 0 3
18 12 2.568 2.573 2 1 3
5 9 2.453 2.439 1 0 4
3 10 2.362 2.364 3 2 1
1 5 2.320 2.333 1 1 4
3 13 2.308 2.312 2 3 0
6 8 2.287 2.290 4 0 0
4 12 2.270 2.264 3 0 3
6 14 2.256 2.250 2 2 3, 2 3 1
1 4 2.226 2.233 4 0 1
3 6 2.102 2.103 2 3 2
2 4 2.088 2.086 4 0 2, 1 2 4
2 5 2.050 2.045 1 3 3
4 12 1.867 1.867 0 4 2
27 16 1.846 1.844 4 1 3
23 20 1.808 1.803 1 3 4
7 6 1.758 1.759 5 1 1
8 18 1.718 1.715 4 3 1
1 8 1.690 1.687 3 0 5
3 8 1.655 1.656 3 4 1
1 5 1.617 1.624 1 1 6
1 4 1.577 1.576 3 3 4
*Calculated from the refined structure; **calculated from the unit cell a= 9.1596(9),
b= 8.0365(9), c= 10.1195(10) Å and V= 744.92(9) Å
3
obtained from least-squares
refinement of the above data using the program UNITCELL (Holland and Redfern, 1997).
The strongest lines are given in bold.
Table 4. Crystal data of the two As
4
S
3
polymorphs, data collection and
structure refinement details for paradimorphite.
Paradimorphite
$
(Solfatara di Pozzuoli)
Dimorphite
$
(Vesuvius)
Crystal data
Crystal system, space group Orthorhombic, Pnma (n. 62)
a(Å) 9.1577(7) 11.218(2)
b(Å) 8.0332(6) 9.922(1)
c(Å) 10.2005(8) 6.609(1)
V(Å
3
) 750.41(10) 735.44(17)
Z44
Data Collection
Radiation type MoKα
μ(mm
–1
) 18.377
2θ
max
(°) 63.16
Reflection range –13 ≤h≤13; –11 ≤k≤
11; –14 ≤l≤14
d
x
(g/cm
3
)* 3.500
Measured reflections 7553
Independent reflections, R
int
1284, 0.0399
Refinement
Observed reflections [I>2σ(I)] 979
Parameters refined 37
Final R[I>2σ(I)] and wR
2
(all data) 0.0229, 0.0474
Weighting scheme q = [max(0, Fo
2
)
+2Fc
2
]/3;
1/[σ
2
(Fo
2
) + (0.0218q)
2
]
Goof 0.995
Δρ
min
,Δρ
max
(e
–
/Å
3
)–0.71, 0.63
$
Data from Gavezzotti et al. (2013); *calculated using the empirical formula and
single-crystal cell data.
Notes: R=Σ||F
o
|–|F
c
||/ Σ|F
o
|; wR
2
={Σ[w(F
o
2
–F
c
2
)
2
]/Σ[w(F
o
2
)
2
]}
½
; Goof ={Σ[w(F
o
2
–F
c
2
)]/(n–p)}
½
where nis the number of reflections and pis the number of refined parameters.
Mineralogical Magazine 5
https://doi.org/10.1180/mgm.2022.47 Published online by Cambridge University Press
and uzonite (2.237(1)–2.261(1) Å), but closer to those found
in alacránite (2.205(9)–2.238(7) Å) and bonazziite (2.222(1)–
2.234(1) Å). These geometrical differences can be ascribed to the
different bonding pattern in the As
4
S
n
molecules. For n= 4 and 5,
instead of being arranged in a pyramidal fashion as in As
4
S
3
, the
As atoms are located at the vertices of a more-or-less regular disphe-
noid where there are (6 –n) disphenoidic edges corresponding to
As–As bonds and the sulfur atoms bridge namong the six disphe-
noidic edges. Thus, the number of As–S bonds formed by each
As atom, combined with the steric hindrance of the additional
S atom, determines the geometry of the As
4
framework.
From a comparison of the projections of the structure of both
polymorphs reported in Fig. 7 it seems that there are no substan-
tial differences in the molecular packing and orientation of the
As
4
S
3
molecules. A detailed study of the intermolecular interac-
tions, carried out by Gavezzotti et al. (2013), showed that slight,
but significant differences in the packing of the As
4
S
3
molecules
could be inferred from radial distribution curves of the centres
of mass of the molecules, that are essentially rigid cages. In
dimorphite the As
4
S
3
molecules are oriented with their pseudo
threefold axis along [102] and [10
2] whereas in paradimorphite
the threefold axes are along [101] and [10
1], but these directions
form similar angles with the crystallographic axes. These features
can account for the fact that the space group is maintained during
the transition from one polymorph to the other, due to slight
deformation of the lattice. The larger unit-cell volume of
paradimorphite with respect of that of the low-temperature
form dimorphite (see Table 4) is in line with its stability at higher
temperatures.
The lack of a distinct cleavage and the conchoidal fracture in
paradimorphite are in keeping with the absence of molecular
layers that are instead present in other molecular arsenic sulfides
such as realgar.
Acknowledgements. Valuable suggestions and constructing comments for
improving this paper have been given by the Principal Editor Dr. Stuart
Mills, Prof. Peter Leverett, Dr. Sergey M. Aksenov and by an anonymous
referee.
Supplementary material. To view supplementary material for this article,
please visit https://doi.org/10.1180/mgm.2022.47
Competing interests. The authors declare none.
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