Mineralogical Magazine (2020), 84, 653–661
doi:10.1180/mgm.2020.50

ArticleIsselite, Cu6(SO4)(OH)10(H2O)4⋅H2O, a new mineral species from
Eastern Liguria, Italy
Cristian Biagioni1*, Donato Belmonte2, Cristina Carbone2, Roberto Cabella2, Nicola Demitri3, Natale Perchiazzi1,

Anthony R. Kampf4 and Ferdinando Bosi5,61
Dipartimento di Scienze della Terra, Università di Pisa, Via Santa Maria 53, I-56126 Pisa, Italy; 2Dipartimento di Scienze della Terra, dell’Ambiente e della Vita
(DISTAV), Università degli Studi di Genova, Corso Europa 26, I-16132 Genova, Italy; 3Elettra – Sincrotrone Trieste S.C.p.A., S.S. 14 km 163,5 in Area Science Park,
Basovizza, I-34149 Trieste, Italy; 4Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA 90007,
USA; 5Dipartimento di Scienze della Terra, Sapienza Università di Roma, Piazzale Aldo Moro 5, 00185 Rome, Italy; and 6CNR-Istituto di Geologia Ambientale eGeoingegneria, Sede Secondaria di Roma “Sapienza”, Piazzale Aldo Moro 5, I-00185 Roma, Italy

Abstract
The new mineral isselite, Cu6(SO4)(OH)10(H2O)4⋅H2O, has been discovered in the Lagoscuro mine, Monte Ramazzo mining complex,
Genoa, Eastern Liguria, Italy. It occurs as sprays of blue acicular crystals, up to 0.1 mm long, associated with brochantite and posnjakite.
Streak is light blue and the lustre is vitreous. Isselite is brittle, with irregular fracture and good cleavage on {001} and {100}. Measured
density is 3.00(2) g/cm3. Isselite is optically biaxial (–), with α = 1.599(2), β = 1.633(2) and γ = 1.647(2) (determined in white light). The
measured 2V is 63.6(5)°. Dispersion is moderate, with r > v. The optical orientation is X = b, Y = c and Z = a. Isselite is pleochroic, with
X = light blue, Y = blue, Z = blue; X << Z < Y. Electron microprobe analyses give (wt.%): SO3 11.45(21), MgO 0.31(7), CoO 1.07(14), NiO
9.41(90), CuO 51.29(126), ZnO 1.10(20), H2Ocalc 24.21, total 98.84. The empirical formula of isselite, based on Σ(Mg,Co,Ni,Cu,Zn) = 6
atoms per formula unit, is (Cu4.80Ni0.94Co0.11Zn0.10Mg0.06)Σ6.00(S1.06O4.19)(OH)10⋅5H2O. Isselite is orthorhombic, space group Pmn21,
with unit-cell parameters a = 6.8070(14), b = 5.8970(12), c = 20.653(4) Å, V = 829.0(3) Å3 and Z = 2. The crystal structure of isselite
was refined from single-crystal X-ray diffraction data to R1 = 0.067 on the basis of 2964 reflections with Fo > 4σ(Fo). It shows a layered
structure formed by zig-zag {001} layers of Cu-centred polyhedra. Sulfate groups occur in the interlayer along with one H2O group.
Isselite is chemically related to redgillite and montetrisaite.
Keywords: isselite, new mineral species, sulfate, copper, crystal structure, Monte Ramazzo mining complex, Eastern Liguria, Italy(Received 11 March 2020; accepted 15 June 2020; Accepted Manuscript published online: 19 June 2020; Associate Editor: David Hibbs)

Introduction
The ore deposits of Eastern Liguria, Northern Apennines, Italy,
are well-known among mineralogists for the occurrence of several
rare species from the Mn ore deposits hosted in middle Jurassic
metacherts of the Graveglia Valley, Genoa Province (Cabella
et al., 1998; Bindi et al., 2013; Biagioni et al., 2019a) and the
Cerchiara Mine, La Spezia Province (Kampf et al., 2013;
Kolitsch et al., 2018). In addition, Cu–Fe–Ni–Co sulfide ore
deposits are also hosted in ophiolitic rocks of the Northern
Apennines and the Sestri–Voltaggio Zone, where several typesof Volcanic Massive Sulfide (VMS) deposits occur in mafic–
ultramafic rocks at different stratigraphic positions within theophiolitic sequences (Ferrario and Garuti, 1980; Zaccarini andGaruti, 2008; Schwarzenbach et al., 2012). Among them,stockwork-vein and seafloor stratiform orebodies are associatedwith serpentinised mantle peridotites and serpentinite breccias(e.g. Ferrario and Garuti, 1980), like in the Monte Ramazzo–*Author for correspondence: Cristian Biagioni, Email: cristian.biagioni@unipi.it
Cite this article: Biagioni C., Belmonte D., Carbone C., Cabella R., Demitri N., Perchiazzi
N., Kampf A.R. and Bosi F. (2020) Isselite, Cu6(SO4)(OH)10(H2O)4⋅H2O, a new mineral
species from Eastern Liguria, Italy. Mineralogical Magazine 84, 653–661. https://doi.org/
10.1180/mgm.2020.50

Lagoscuro area, where a pyrrhotite-dominated mineralisation
had been actively exploited since the beginning of the 19th
Century (Rolandi, 1974; Pipino, 1977). The primary mineralisation underwent subsequent fluid–rock interaction, hydrothermal
mobilisation, and multi-stage alteration processes which led, on
the one hand, to sulfide reconcentration and recrystallisation
along tectonic structures, on the other hand to metal reworking
and the formation of secondary phases, such as (Ni,Co)-bearing
oxy-hydroxides (asbolane and heterogenite), carbonates (kolwezite, Co-rich malachite and spherocobaltite) and silicates (pecoraite). This peculiar mineral assemblage occurring in the Monte
Ramazzo–Lagoscuro ore deposit is quite different from otherhydrothermal sulfide mineralisation described so far in ophiolites
from the Eastern Liguria, where (Ni,Co)-enrichment occurs
mostly in pyrite and/or accessory minerals like millerite, siegenite
and pentlandite (e.g. Cortesogno et al., 1977; Schwarzenbach
et al., 2012; Moroni et al., 2019).
During the examination of secondary minerals from theMonte Ramazzo mining complex, blue acicular crystals fromthe Lagoscuro mine were encountered. Crystallographic studies
and chemical analyses showed this phase to be a new mineral species, which we describe herein and name isselite. The mineral andits name have been approved by the International Mineralogical© Mineralogical Society of Great Britain and Ireland 2020

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Association (IMA) Commission on New Minerals, Nomenclature
and Classification (CNMNC) (IMA2018-139, Biagioni et al.,2019b). Holotype material for isselite is deposited in the mineralogical collection of the Museo di Storia Naturale, Università diPisa, Via Roma 79, Calci, Pisa, Italy, under catalogue number
19904, and of the Dipartimento di Scienze della Terra,dell’Ambiente e della Vita (DISTAV), Università di Genova,
Corso Europa 26, Genova, Italy, under catalogue number
MO484. Cotype material is preserved in the collections of the
Natural History Museum of Los Angeles County, 900 Exposition
Boulevard, Los Angeles, CA 90007, USA, catalogue number
67195. The new mineral is dedicated to Arturo Issel (1842–1922)
for his pioneering work on the geology, palaeontology and mineralogy of Italy, with a special reference to Liguria (Issel, 1892). He wasprofessor of Geology and Mineralogy at the University of Genoa
beginning in 1866 and Director of the Institute of Geology and
the Geological Museum until 1917. Arturo Issel was the authorof important scientific monographs on a variety of topics (notonly geology and palaeontology, but also anthropology and ethnology) and contributed to several geological maps of Liguria.

Cristian Biagioni et al.

Fig. 1. Isselite, sprays of acicular blue crystals. Field of view: 2 mm. Monte Ramazzo
mining complex, Genoa. Photo M. Esposito. The crystal morphology is shown to theright.

Table 1. Electron microprobe analysis of isselite (average of six spot analyses –in wt.%).

Occurrence and physical properties

Oxide

Mean

Range

S.D.

Isselite occurs as a secondary phase in the Fe–Ni–Co sulfide mineralisation formerly exploited at the Lagoscuro mine (44°28′ 35’’N,
8°51′ 35′′ E), Ceranesi, Genoa Province, Liguria, Italy. This mine,
which belongs to the Monte Ramazzo mining complex, operated
from 1815 to 1825, mainly for magnesium sulfate (Pipino, 1977).
The Co-rich pyrrhotite + pentlandite ± sphalerite sulfide ore,
hosted within serpentinite and serpentinite breccia in contact
with basaltic rocks belonging to the Figogna Unit in the
Ligurian Alps (Galli, 1954 and 1963; Capponi and Crispini,2008), underwent multi-stage alteration processes and hydrothermal mobilisation, which developed a mineral assemblage
formed by violarite + valleriite + andradite ± magnetite, alongwith other Ni–Co secondary minerals (e.g. pecoraite, heterogenite
and asbolane) and hydrated Mg–Cu(–Ni) carbonates and sulfatessuch as malachite, brochantite, langite, posnjakite and hydromagnesite. The transformation of pentlandite to violarite in the sulfide
ore seems to constrain the first stage of hydrothermal alteration to
T < 210°C under strongly acidic to neutral (i.e. 1 < pH < 6) andreducing (i.e. logfO2 (g) < –35) conditions (Tenailleau et al., 2006;
Xia et al., 2009).
Isselite was identified in only a few specimens as sprays of blue
acicular crystals, striated parallel to [100] and tabular on {001}, up
to 0.1 mm long (Fig. 1). Observed forms are {001}, {010}, {103}and {103}. The streak is light blue. Isselite is vitreous and transparent. The mineral is brittle, with an irregular fracture andgood cleavage on {001} and {100}. Hardness was not measured,owing to the small crystal size. The density measured by sink–float in methylene iodine – toluene is 3.00(2) g/cm3, comparedwith a density of 2.946 g/cm3, calculated on the basis of theempirical formula and the unit-cell volume refined from singlecrystal X-ray diffraction data at room temperature.
Isselite is optically biaxial (–), with indices of refraction α =1.599(2), β = 1.633(2) and γ = 1.647(2) (determined in whitelight). The measured 2V of 63.6(5)°, obtained using extinction
data analysed with EXCALIBR (Gunter et al., 2004), compares
well with the calculated 2V of 64.2°. Dispersion is moderate,with r > v. The optical orientation is X = b, Y = c and Z = a. Isseliteis pleochroic, with X = light blue, Y = blue, Z = blue; X << Z < Y.
The compatibility index, calculated using the Gladstone–Dale

SO3
MgO
CoO
NiO
CuO
ZnO
H2Ocalc
Total

11.45
0.31
1.07
9.41
51.29
1.10
24.21
98.84

11.30–11.73
0.20–0.40
0.89–1.22
8.63–11.01
49.03–52.55
0.86–1.42

0.21
0.07
0.14
0.90
1.26
0.20

S.D. – standard deviation

relationship (Mandarino, 1979, 1981), is 0.003 (superior), using
the empirical formula and the calculated density; it is 0.021(excellent) using the empirical formula and the measured density.
Isselite is a late-stage, secondary mineral that crystallised from a
low-temperature, aqueous solution. In holotype material, it is associated closely with brochantite and posnjakite. Other associatedminerals are pyrrhotite, pentlandite, allophane, chrysocolla andlangite. The source of the Cu in isselite may be valleriite, alongwith minor chalcopyrite, covellite and digenite. These sulfides arewidespread in the Monte Ramazzo–Lagoscuro ore deposit, though
not in close association with isselite in the samples studied.
Chemical and spectroscopic analyses
Preliminary energy-dispersive spectroscopy (EDS) chemical analyses showed that isselite contains Cu, Ni, Co and S as the only
elements with Z > 8. Quantitative chemical analyses were performed using a CAMECA SX50 electron microprobe (Istituto diGeologia Ambientale e Geoingegneria, CNR, Rome) operatingin wave-length dispersive mode. Operating conditions were: accelerating voltage 15 kV, beam current 15 nA, and beam diameter
5 μm. Standards (element, emission line) were: periclase(MgKα), baryte (SKα), synthetic Co (CoKα), synthetic Ni(NiKα), synthetic Cu (CuKα) and synthetic Zn (ZnKα). Iron
and Pb were sought but found to be below detection limits. The
small amount of available material precluded the determinationof H2O by direct measurement; consequently, the H2O content
was inferred from the crystal structure analysis. Its presence isconfirmed by Raman spectroscopy (see below).

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Fig. 2. Raman spectrum of isselite in the range 200–1200 cm–1 (a) and 2700–4000 cm–1 (b).

The electron microprobe analysis results are given in Table 1.
The empirical formula of isselite, based on Σ(Mg,Co,Ni,Cu,Zn) =6 atoms per formula unit, is (Cu4.80Ni0.94Co0.11Zn0.10Mg0.06)Σ6.00(S1.06O4.19)(OH)10⋅5H2O. The ideal formula of isselite is Cu6(SO4)(OH)10⋅5H2O, which requires SO3 10.86, CuO 64.71, H2O 24.43,
total 100.00.
Micro-Raman spectra were obtained on an unpolished sample
of isselite in nearly back-scattered geometry with a Jobin-Yvon
Horiba XploRA Plus apparatus, equipped with a motorised x–ystage and an Olympus BX41 microscope with a 10× objective(Dipartimento di Scienze della Terra, Università di Pisa). The
532 nm line of a solid-state laser was used. The minimum lateraland depth resolution was set to a few μm. The system was calibrated using the 520.6 cm–1 Raman band of silicon before eachexperimental session. Spectra were collected through multiple
acquisitions with single counting times of 60 s. Back-scatteredradiation was analysed with a 1200 gr/mm grating monochromator. The Raman spectrum of isselite is shown in Fig. 2.
The strongest band occurs at 984 cm–1 and can be related to thesymmetrical stretching ν1 of the SO4 group. The bands at 386,
440 and 495 cm–1 can be interpreted as due to the symmetricalbending ν2 of the SO4 group, whereas the weak band at
630 cm–1 could be related to the antisymmetrical bending ν4.
Finally, the band at 250 cm–1 may be attributed to latticemodes. The broad band at 3484 cm–1 can be attributed to the
O–H stretching modes. No evident bending of H–O–H bonds,
in the typical range between 1595–1700 cm–1 (e.g. Kolesov,

Table 2. X-ray powder diffraction (d in Å) data for isselite.*
Iobs

dobs

Icalc

dcalc

hkl

s
m
mw
w
vs
vw
vw
vw
w
vw
mw
vw
vw
w
w
s
vw
w
vw
w
m
mw
mw
mw
w
w
vw
mw
mw

10.3
6.4
5.67
5.14
4.84
4.47
4.32
4.14
3.742
3.462
3.400
3.237
3.035
2.919
2.838
2.708
2.628
2.589
2.516
2.406
2.225
2.179
2.129
2.065
2.044
1.943
1.886
1.796
1.701

92
29
25
11
100
9
4
1
7
4
8, 11
4
4
7
10
4, 16, 22, 14, 12
3
10
3
6
5, 13
11
7
10
2, 1
2
2, 1
2, 2, 5, 6, 6
6
3
6

10.3
6.46
5.67
5.12
4.84
4.48
4.36
4.09
3.741
3.442
3.404, 3.383
3.232
3.029
2.918
2.842
2.710, 2.710, 2.708, 2.706, 2.683
2.638
2.582
2.518
2.420
2.229, 2.216
2.178
2.120
2.065
2.046, 2.040
1.931
1.871, 1.868
1.810, 1.799, 1.798, 1.791, 1.778
1.698
1.698
1.687

002
101
011
012
103
013
111
112
113
006
2 0 0, 0 1 5
202
115
211
204
0 2 3, 2 1 3, 1 0 7, 1 2 0, 1 2 1
017
008
123
206
2 2 0, 2 2 1
222
223
0 0 102 2 4, 1 1 9032
2 2 6, 1 2 8
1 0 11, 3 0 7, 3 2 0, 3 2 1, 2 2 7
400
324
228

*Notes: the dhkl values were calculated on the basis of the unit cell refined by using single-crystal data. Intensities were calculated on the basis of the structural model reported in Table 4.
Observed intensities were visually estimated: vs = very strong; s = strong; m = medium; mw = medium–weak; w = weak; vw = very weak. Only reflections with Icalc > 5 are listed, if not observed.
The strongest reflections are shown in bold.

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Cristian Biagioni et al.

Table 3. Crystal data and summary of parameters describing data collection
and refinement for isselite.

Table 4. Sites, Wyckoff positions, atom coordinates and equivalent isotropic
displacement parameters (Å2) for isselite.

Crystal data
Crystal size (mm)Cell setting, space group
a (Å)
b (Å)
c (Å)
V (Å3)Z
Data collection
Radiation, wavelength (Å)
Temperature (K)2θmax (°)
Measured reflections
Unique reflections
Reflections with Fo > 4σ(Fo)RintRσRange of h, k, lRefinement
R [Fo > 4σ(Fo)]R (all data)wR (on Fo2)
Gof
Number of least-squares parametersΔρmax, Δρmin (e –/Å3)

Site

Wyk.

x

y

z

Ueq

Cu(1)
Cu(2)
Cu(3)
Cu(4)
Cu(5)
S
OH(1)
OH(2)
OH(3)
OH(4)
OH(5)
OH(6)
Ow(7)
Ow(8)
O(9)
O(10)
O(11)
Ow(12)
Ow(13)
Ow(14)

2a
2a
2b
4c
2b
2b
2b
2a
4c
4c
4c
4c
2b
2a
2b
2b
4c
2a
2b
2a

0
0
½
0.24886(18)½½½
00.3117(10)0.3064(11)0.1864(11)0.1940(12)½0½½
0.328(3)0½
00.0315(3)0.5234(3)0.8539(3)0.68698(18)0.3447(3)0.9276(7)0.8412(17)0.5310(16)0.5960(11)0.1076(12)0.7802(11)0.2701(12)0.804(3)0.612(3)0.814(3)0.172(2)0.858(3)0.067(2)0.306(2)0.563(2)0.01412(8)0.00062(8)0.22094(7)0.11180(8)0.20795(8)0.88577(19)0.1134(7)0.1093(7)0.2008(4)0.2213(4)0.0236(4)0.0003(4)0.3252(9)0.8958(9)0.9481(7)0.8931(8)0.8504(8)0.1407(6)0.0840(6)0.2837(6)0.0249(4)0.0239(4)0.0228(3)0.0230(2)0.0227(3)0.0345(7)0.028(2)0.0264(18)0.0237(12)0.0268(14)0.0242(13)0.0288(14)0.054(4)0.054(4)0.041(3)0.059(4)0.096(6)0.034(2)0.036(2)0.037(2)0.080 × 0.010 × 0.005
Orthorhombic, Pmn21
6.8070(14)5.8970(12)20.653(4)829.0(3)2Synchrotron, 0.59040
298(2)
59.46
18,2074088
2964
0.0651
0.0488−9 ≤ h ≤ 10, −9 ≤ k ≤ 9, −34 ≤ l ≤ 34
0.06690.09210.17901.013
1406.19 [at 1.31 Å from O(3)]–1.38 [at 0.67 Å from Cu(4)]Wyk. = Wyckoff position

Table 5. Selected bond distances (in Å) for isselite.

2006), was observed. The interpretation of the spectral bands is inagreement with Frost et al. (2004).

Cu(1)–OH(6)Cu(1)–OH(5)<Cu(1)–OH>
Cu(1)–Ow(12)1.950(8) ×2
1.961(7) ×2
1.956
2.623(13)Crystallography

Cu(2)–OH(6)Cu(2)–OH(5)Cu(2)–Ow(8)Cu(2)–OH(2)<Cu(2)–OH>

1.994(8) ×2
2.032(7) ×2
2.23(2)2.244(14)2.088

Cu(3)–OH(4)Cu(3)–OH(3)Cu(3)–Ow(7)Cu(3)–OH(1)<Cu(3)–OH>

1.994(7) ×2
2.032(7) ×2
2.173(18)2.222(14)2.074

The powder X-ray diffraction pattern of isselite was obtainedusing a 114.6 mm diameter Gandolfi camera, with Ni-filtered
CuKα radiation. The observed powder X-ray diffraction patternis compared with the calculated one (obtained using the software
PowderCell; Kraus and Nolze, 1996) in Table 2. Unit-cell parameters, refined from the powder data on the basis of 22 unequivocally indexed reflections through the software UnitCell (Holland
and Redfern, 1997), are a = 6.789(1), b = 5.923(1), c = 20.651(3) Å,V = 830.4(2) Å3 and Z = 2.
The single-crystal X-ray diffraction study was carried out at the
XRD1 beamline, ELETTRA synchrotron facility (Lausi et al.,2015). A monochromatic wavelength of 0.59040 Å was used on
a 50 μm × 50 μm beam size, using a Dectris Pilatus 2M hybrid
pixel area detector at a distance of 85 mm. A total of 500 frames
was collected using w scan mode in Δw = 0.5° slices, with an exposure time of 1 s per frame. The diffraction data, collected at room
temperature, were indexed, integrated, scaled, and corrected for
the Lorentz-polarisation factor using the software XDS (Kabsch,2010). The refined unit-cell parameters are a = 6.8070(14),b = 5.8970(12), c = 20.653(4) Å and V = 829.0(3) Å3. The a:b:c
ratio calculated from unit-cell parameters is 1.1543:1:3.5023.
The statistical tests on the distribution of |E| values (|E 2 – 1| =0.703) suggest the acentric nature of isselite. The examination of
the systematic absences indicated the possible space groups
P21nm and Pmn21. Only the choice of this latter space group
allowed the solution of the crystal structure through direct methods
using Shelxs-97 (Sheldrick, 1997). After having located the heavier
elements, the structure was completed through successive
difference-Fourier maps; owing to the quality of available material,
H atoms were not located. After several cycles of anisotropic refinement, performed using Shelxl-2018 (Sheldrick, 2015) and using

Cu(4)–OH(2)Cu(4)–OH(1)Cu(4)–OH(5)Cu(4)–OH(3)<Cu(4)–OH>
Cu(4)–Ow(12)Cu(4)–Ow(13)1.928(5)1.937(5)1.950(7)1.963(7)1.944
2.871(11)2.883(10)Cu(5)–OH(4)Cu(5)–OH(3)<Cu(5)–OH>
Cu(5)–Ow(13)1.941(7) ×2
1.965(7) ×2
1.953
2.571(13)

S–O(11)
S–O(10)S–O(9)<S–O>

1.441(13) ×2
1.448(12)1.452(14)1.446

neutral scattering curves taken from the International Tables for
Crystallography (Wilson, 1992), the refinement converged to R1 =0.0669 for 2964 unique reflections with Fo > 4σ(Fo) and 140 refinedparameters. The occurrence of high electron density residuals (up to
6.19 e –/Å3) is related to the less-than-ideal quality of the available
crystals of isselite. An attenuation of the residuals can be obtainedby truncating the intensity data to low angular values. For instance,
the truncation of the dataset to 30° in 2θ results in a highest residualof 2.3 e –/Å3. However, the ratio between observed reflections andleast-square parameters significantly decreases (from ∼21 to ∼ 4.5).
An attempt at truncating the dataset at 2θ = 50° resulted in a slightly
better R value (0.0625 for 2269 reflections) and slightly lower residuals (up to 5.4 e –/Å3). However, the estimated standard uncertainties on bond lengths slightly increased. For this reason, the full
dataset is here presented, with the awareness of the non ideal qualityof the structure refinement (in particular as regards the high residuals) but supported by the crystal-chemical soundness of the proposed structural model.
Details of the data collection and crystal-structure refinement
are given in Table 3. Atom coordinates and equivalent isotropic

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Table 6. Bond-valence sums (BVS – in valence units) for isselite*.
Cu(1)
OH(1)
OH(2)
OH(3)
OH(4)
OH(5)
OH(6)
Ow(7)
Ow(8)
O(9)
O(10)
O(11)
Ow(12)
Ow(13)
Ow(14)
Σcations
Theoretical

Cu(2)

Cu(3)
0.23

0.22
0.47↓×2
0.48↓×2

0.39↓×2
0.43↓×2

Cu(4)

Cu(5)

S

2×→

0.50
0.51
0.46
2×→

0.39↓×2
0.43↓×2

0.46↓×2
0.49↓×2

0.48
0.26

0.23
1.59
1.61
1.64↓×2
2×→

0.08

2×→

1.98
2.00

2.09
2.00

2.13
2.00

0.04
0.04

2.03
2.00

0.09
1.99
2.00

Σanions
1.23
1.24
1.31
0.92
1.34
0.91
0.26
0.23
1.59
1.61
1.64
0.16
0.17
–

6.48
6.00

*Left and right superscripts indicate the number of equivalent bonds involving anions andcations, respectively.

displacement parameters are reported in Table 4, whereas Table 5
gives selected bond distances. Bond-valence calculations, shownin Table 6, were performed using the bond parameters of Breseand O’Keeffe (1991). The crystallographic information file has
been deposited with the Principal Editor of Mineralogical
Magazine and is available as Supplementary material (see below).

Crystal structure description
General organisation
The crystal structure of isselite (Fig. 3) includes five independent
Cu sites, one S site, and fourteen O positions. It can be describedas formed by {001} zig-zag layers of CuΦ4–6 polyhedra, whereΦ = (OH or H2O). Sulfate groups occur in the interlayer, together
with one H2O group. No H positions were located from the structure determination; the assignments of OH and H2O groups werebased on the bond-valence analysis (see Table 6).

Cation coordination and site occupancies
In isselite, the Cu sites display different kinds of coordination(Fig. 4). The Cu(2) and Cu(3) sites show the typically distorted(4 + 2) octahedral coordination of Cu2+, related to the Jahn-Teller
effect (e.g. Burns and Hawthorne, 1996). The average value ofthe four shorter distances is 2.013 Å for both sites, somewhat
longer than the equatorial bond length of 1.97 Å reported by Eby
and Hawthorne (1993). The two apical Cu–O distances have average values of 2.237 and 2.197 Å at the Cu(2) and Cu(3) sites,
respectively, shorter than the average value of 2.44 Å reported by
Eby and Hawthorne (1993). The smaller than typical octahedral
distortions observed for the Cu(2) and Cu(3) sites suggest thatminor Ni and Co detected in isselite are located at these twosites. Taking into account the nature of the ligands, Cu(2) andCu(3) octahedra can be defined both as Cu(OH)5(H2O) polyhedra.
The Cu(1) and Cu(5) sites have a square pyramidal coordination,with four equatorial bonds ranging between 1.941 and 1.965 Å(average value of 1.954 Å); the fifth bond is at a longer distance,
i.e. 2.571 Å [Cu(5)–Ow(13)] and 2.623 Å [Cu(1)–Ow(12)].
Taking into account the ligand nature, the cations at the Cu(1)and Cu(5) sites can be related to the Cu(OH)4(H2O) polyhedra.

Fig. 3. Crystal structure of isselite as seen down b. Symbols: green polyhedra = Cusites and yellow polyhedra = S site. Circles: red = O2–, blue = OH– and light blue = H2O.

A square-planar coordination is shown by the Cu(4) site, having four ligands at an average distance of 1.944 Å, in agreementwith the ideal mean value of 1.94 Å for square-planar Cu2+
(Eby and Hawthorne, 1993). Taking into account the two very
long distances Cu(4)–Ow(12) and Cu(4)–Ow(13), ≈2.9 Å, avery distorted octahedron Cu(OH)4(H2O)2 can be identified,instead of a square-planar group Cu(OH)4.
Bond-valence sums (BVS) at the Cu sites range between 1.98
and 2.13 valence units (vu), in agreement with the occurrenceof Cu in the divalent state. The higher values occur at theCu(2) and Cu(3) sites and can be related to the admixture ofminor Ni2+ and very small amounts of Co2+, Zn2+ and Mg2+.
The SO4 group is hosted in the interlayer and is bonded to the{001} Cu layers through hydrogen bonds, as revealed by theoccurrence of several underbonded O in the bond-valence calculation (Table 6) and the occurrence of short O⋅⋅⋅O distances(Table 7). The BVS at the S site is 6.48 vu, considerably greaterthan expected for S6+. In this regard, it should be noted thatthe position of O(11) seems to be affected by disorder, as indicated by a Ueq value larger than those of other O atoms belonging
to the SO4 group.
Hydrogen bonds
The short O⋅⋅⋅O distances are given in Table 7, confirming theimportance of H bonds in the crystal structure of isselite, in agreement with the undersaturation of all O atoms. Taking into
account the results of BVS given in Table 6, three distinct groupsof O atoms can be found: (1) O atoms having BVS close to ∼1.6
vu; (2) O atoms with BVS of ∼1.2–1.3 vu; and (3) O atoms showing BVS of ∼ 0.2 vu. These three different O groups correspond toO2–, (OH)– and H2O groups, respectively.
Oxygen atoms belonging to the isolated SO4 groups, that isO(9), O(10) and O(11), are acceptors of H bonds. Oxygen atom

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Cristian Biagioni et al.

Fig. 4. Coordination environments of Cu2+ in the crystal structure of isselite. The colour scheme is the same as in Fig. 3.

hosted at O(9) is an acceptor of two H bonds from twosymmetry-related (OH)– groups hosted at OH(5); the oxygen
atom at O(10) is an acceptor of H bonds from two symmetry
related (OH)– groups hosted at OH(6) and from an H2O groupoccurring at the Ow(14) site. Finally, O(11) is involved in amore complex H-bond system. Indeed, O(11) is an acceptor of
H bonds from OH(4) and it is at H-bond distances with
Ow(7), Ow(8), and Ow(14). In addition, Ow(14) is an acceptorof H bonds from two symmetry-related OH(3) groups, whereas
a still more complex situation involves the H2O groups hosted
at Ow(7) and Ow(8). These two H2O groups are bonded to theCu atoms hosted at Cu(3) and Cu(2) sites, respectively. Bothare at short O⋅⋅⋅O distances with O(11). In addition, the distanceOw(7)⋅⋅⋅Ow(8), 2.85 Å, suggests another H-bond contact. Owing
to the lack of knowledge about the H positions and the occurrenceof different possible configurations, the actual H-bond system
involving these atoms has to be considered only speculative andit will not be detailed here. As a matter of fact, the possibilityof different configurations involving the oxygen atom hosted at
O(11) could be the reason for its relatively high Ueq value. Its
position could be an average position between different H-bondconfigurations. Moreover, this uncertainty could affect the BVSof S, as discussed briefly above.
Oxygen atoms hosted at OH(4) and OH(6) are both donor (inthe H bonds described above) and acceptor from H2O groups
hosted at Ow(12) and Ow(13), respectively. It is worth notingthat Ow(12) and Ow(13) act as donor to two symmetry-related
OH(4) and OH(6), respectively, whereas they are acceptors of H
bonds from OH(2) and OH(1), respectively.
Discussion
Isselite and relations with other Cu sulfates
Fifteen different mineral species are currently known in the system CuO–SO3–H2O (Table 8). From a chemical point of view,

Table 7. O⋅⋅⋅O distances (in Å) and corresponding bond strengths (in valence
units, vu).
O⋅⋅⋅O
OH(1)⋅⋅⋅Ow(13)OH(2)⋅⋅⋅Ow(12)OH(3)⋅⋅⋅Ow(14)OH(4)⋅⋅⋅O(11)Ow(12)⋅⋅⋅OH(4)OH(5)⋅⋅⋅O(9)OH(6)⋅⋅⋅O(10)Ow(13)⋅⋅⋅OH(6)Ow(7)⋅⋅⋅Ow(8)Ow(7)⋅⋅⋅O(11)Ow(8)⋅⋅⋅O(11)Ow(14)⋅⋅⋅O(10)Ow(14)⋅⋅⋅O(11)

d (Å)

vu*

2.805(15)2.813(15)2.733(11)2.825(16)2.679(12)2.650(11)3.094(14)2.714(12)2.85(2)3.04(2)2.82(2)2.75(2)3.07(2)

0.18
0.18
0.21
0.17
0.24
0.25
0.11
0.22
0.17
0.12
0.18
0.20
0.12

*Calculated using the relationships of Ferraris and Ivaldi (1988).

the most simple species are those having the chemical formula
Cu(SO4)⋅nH2O (0 < n < 7): chalcocyanite (n = 0), poitevinite(n = 1), bonattite (n = 3), chalcanthite (n = 5) and boothite (n =7). Their crystal structures are reviewed by Eby and Hawthorne(1993). Chalcocyanite typically occurs in fumarolic environments.
Indeed, it was first described from Vesuvius, Italy, by Scacchi(1873), along with another anhydrous copper oxy-sulfate,
dolerophanite, Cu2O(SO4). The other hydrated copper sulfates
listed above are usually related to the weathering of Cu ores,
although in some case they can be found also as sublimates infumaroles (e.g. Balassone et al., 2019).
The remaining species, commonly associated with weathered
Cu ores (e.g. Zittlau et al., 2013), are represented by the twobasic copper sulfates antlerite and brochantite and seven hydrated
basic copper sulfates. Isselite belongs to this latter group of copper
sulfates, showing the presence of (OH)– and H2O groups. In
particular, it is chemically related to redgillite, Cu6(SO4)

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659

Table 8. Natural Cu sulfates in the system CuO–SO3–H2O.
MineralChemical formula

Antlerite
Bonattite
Boothite
Brochantite

Cu3(SO4)(OH)4Cu(SO4)(H2O)3
Cu(SO4)(H2O)6⋅H2O
Cu4(SO4)(OH)6

Chalcanthite
Chalcocyanite
Dolerophanite
Isselite
Kobyashevite
LangiteMontetrisaite
Poitevinite
Posnjakite
Redgillite
Wroewolfeite

Cu(SO4)(H2O)4⋅H2O
Cu(SO4)Cu2O(SO4)
Cu6(SO4)(OH)10(H2O)4⋅H2O
Cu5(SO4)2(OH)6(H2O)4
Cu4(SO4)(OH)6(H2O)⋅H2O
Cu6(SO4)(OH)10⋅2H2O
Cu(SO4)(H2O)
Cu4(SO4)(OH)6(H2O)
Cu6(SO4)(OH)10(H2O)
Cu4(SO4)(OH)6(H2O)⋅H2O

a (Å)

b (Å)

c (Å)

α (°)

β (°)

γ (°)

Space group

8.24
5.59
14.19
13.141
12.782
6.14
8.41
9.37
6.81
6.07
7.14
2.99
5.12
10.58
3.16
6.04

6.04
13.03
6.54
9.861
9.872
10.74
6.71
6.32
5.90
11.06
6.03
16.97
5.16
6.34
10.44
5.65

11.99
7.34
10.82
6.021
6.032
5.99
4.83
7.64
20.65
5.51
11.22
14.81
7.54
7.86
19.44
14.34

90
90
90
901
90.22
82.3
90
90
90
102.9
90
90
107.1
90
90
90

90
97.1
106.0
103.21
902
107.4
90
122.3
90
92.3
90
90
107.4
118.0
90.1
93.4

90
90
90

Pnma
Cc
P21/c
P21/a 1
P21/n 2
P1
Pnma
C2/m
Pmn21
P1
Pc
Cmc21
P1
Pa
P21/c
Pc

902
102.7
90
90
90
92.6
90
90
92.7
90
90
90

Reference
Hawthorne et al. (1989)
Zahrobsky and Baur (1966)
Leverett et al. (2004)
Merlino et al. (2003)
Bacon and Titterton (1975)
Wildner and Giester (1988)
Effenberger (1985)
This work
Pekov et al. (2013)
Gentsch and Weber (1984)
Orlandi and Bonaccorsi (2009)
Giester et al. (1994)
Mellini and Merlino (1979)
Pluth et al. (2005)
Hawthorne and Groat (1985)

1

MDO1; 2MDO2.

Fig. 5. Comparison between the crystal structure of isselite (a) and those of montetrisaite (b) and redgillite (c). The colour scheme is the same as in Fig. 3.

(OH)10(H2O) (Pluth et al., 2005) and montetrisaite, Cu6(SO4)(OH)10(H2O)2 (Orlandi and Bonaccorsi, 2009). However, notwithstanding the chemical similarities, the crystal structure ofisselite is different from those shown by redgillite and montetrisaite (Fig. 5). In addition, isselite could be the Cu analogue of
guarinoite, Zn6(SO4)(OH)10⋅5H2O (Sarp, 1993). However, Mills
et al. (2019) noted that guarinoite might have been misidentified
in its original description and is more likely to be Na(Zn,Co)4(SO4)(OH)6Cl⋅5–6H2O and equivalent to Co-rich gordaite-2H.

Nickel partitioning in isselite
After the first identification of isselite on the type specimen fromthe Lagoscuro mine, this mineral was identified, through powder
X-ray diffraction, also on a sample from the Monte Ramazzo
mine, as thin flattened acicular crystals associated with brochantite. Only EDS chemical data are available for this new occurrence; however, these data clearly indicated the occurrence ofNi, whereas Co and other elements are below the detection

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660

limit. The Ni/(Ni + Cu) atomic ratio is close to 0.19, similar to thevalue observed in the type material, where the Ni/(Mg + Co + Ni
+ Cu + Zn) atomic ratio is 0.16. In both cases, the ideal composition is close to NiCu5(SO4)(OH)10⋅5H2O.
The similar scattering factors of Ni (Z = 28) and Cu (Z = 29) precluded the reliable description of the distribution of Ni and Cu atoms
among different cation sites. Nickel and Cu are ordered in someminerals. Gillardite and the related mineral paratacamite-(Ni),both ideally Cu3Ni(OH)6Cl2, are atacamite-group minerals where
Ni is partitioned in the octahedrally coordinated sites not showing
the Jahn-Teller effect, whereas Cu2+ is hosted at the distorted octahedrally coordinated sites (Clissold et al., 2007; Sciberras et al.,2013). Similarly, Ni2+ is preferentially partitioned in the lessdistorted octahedrally-coordinated Me2 site of the crystal structureof glaukosphaerite, (Cu,Ni)2(CO3)(OH)2, whereas the Me1 site
shows the typical 4 + 2 distorted octahedral coordination of Cu2+
(Perchiazzi and Merlino, 2006). Another example is represented
by hloušekite. This mineral, having the ideal formula
NiCu4(AsO4)2(AsO3OH)2(H2O)9, is a member of the lindackerite
supergroup and Ni is hosted at a mixed (Ni,Co) site not showing
the typical distortion of Cu2+-centred polyhedra (Plášil et al., 2014).
In isselite, the Cu(2) and Cu(3) sites have octahedral coordinations, showing the typical (4 + 2) distortion due to the Jahn–Teller
effect for Cu2+. As discussed above, these sites have slightly longer
equatorial bonds and shorter apical distances with respect tothose reported by Eby and Hawthorne (1993) for a typicalCu2+-centred octahedron. This could suggest the substitution of
Cu2+ by Ni2+ at these two positions. However, as there is no certainty about the quantification of Cu–Ni site distribution, for classification purposes the Cu(2) and Cu(3) sites could be merged, as
occurs in the nomenclature of other minerals (e.g. M1 + M2 + M3
in amphibole; Hawthorne et al., 2012). In accord with the
IMA–CNMNC guidelines (Bosi et al., 2019), the end-member
formula of isselite can be considered Ni-free. However, the actualrole of Ni in stabilising this phase deserves further study.

Conclusion
Isselite is a new secondary Cu mineral showing a novel crystal
structure and a complex hydrogen-bond system. Its Ni content,
observed at both of its occurrences, opens the question about
the role of this element in favouring the crystallisation of thisnew mineral species. It is likely that isselite could be more common than thought as an alteration product of Cu–Ni ores.
Along with ramazzoite (Kampf et al., 2018), isselite is thesecond new mineral discovered in the assemblages from theMonte Ramazzo mining complex, stressing the necessity forfurther investigation of this locality. In particular, a deep understanding of the geochemical relations between the Cu–Fe–Ni–Coprimary mineralisation and their secondary alteration products
would shed light on the possible role of hydrothermal fluids in
Ni vs. Co selective mobilisation and enrichment as well as on the
degree of hydrothermal alteration of the Monte-Ramazzo–
Lagoscuro ore deposit.
Supplementary material. To view supplementary material for this article,
please visit https://doi.org/10.1180/mgm.2020.50.
Acknowledgements. Marco Esposito and Fabrizio Castellaro are acknowledged for providing the first specimen of isselite and the photo used in thispaper. The comments of three anonymous reviewers improved the quality ofthe original manuscript.

Cristian Biagioni et al.

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