American Mineralogist, Volume 96, pages 878–887, 2011

Ambrinoite, (K,NH4)2(As,Sb)8S13.H2O, a new mineral from Upper Susa Valley, Piedmont,
Italy: The first natural (K,NH4)-hydrated sulfosalt
Cristian Biagioni,1 ElEna BonaCCorsi,1,2,* MarCo PasEro,1 YvEs Moëlo,3
MarCo E. Ciriotti,4 Danilo BErsani,5 athos Maria CallEgari,6 anD MassiMo BoioCChi7
Dipartimento di Scienze della Terra, Università di Pisa, Via Santa Maria 53, I-56126 Pisa, Italy2
Istituto di Geoscienze e Georisorse, CNR, Via Moruzzi 1, I-56124 Pisa, Italy
3
Institut des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS, 2, rue de la Houssinière, BP 32229, F-44322 Nantes Cedex 3, France
4
Associazione Micromineralogica Italiana, Via San Pietro 55, I-10073 Devesi-Ciriè, Italy
5
Dipartimento di Fisica, Università di Parma, Viale G.P. Usberti 7/a, I-43100 Parma, Italy
6
Dipartimento di Scienze della Terra, Università di Pavia, Via Ferrata 1, I-27100 Pavia, Italy
7
Centro Grandi Strumenti, Università di Pavia, Via Bassi 21, I-27100 Pavia, Italy
1aBstraCt
Ambrinoite, ideally (K,NH4)2(As,Sb)8S13⋅H2O, occurs as a rare sulfosalt species in the Triassic
evaporitic formation of Gessi (gypsum) outcropping near the hamlet of Signols (Oulx, Susa Valley,
Torino, Piedmont, Italy). The new species is associated with sulfur and orpiment; in the same occurrence
galkhaite, stibnite, and enargite were also identified. Ambrinoite occurs as aggregates of tabular crystals
up to 1 mm in length. The color is red, with an orange-red streak; the luster is vitreous to resinous. The
mineral is transparent; its microhardness VHN(10 g) = 30 kg/mm2, corresponding to a Mohs hardnessof about 2. Electron microprobe analysis gives the empirical formula [K1.43(NH4)0.42Na0.02Tl0.01]Σ=1.88(As5.82Sb2.18)Σ=8.00S13.22⋅1.2H2O, close to stoichiometric [K1.5(NH4)0.5]Σ=2(As6Sb2)Σ=8S13⋅H2O; the calculated
density is 3.276 g/cm3. Micro-Raman spectroscopy confirmed the presence of water and ammonium
cation. Ambrinoite is triclinic, space group P1, with a = 9.704(1), b = 11.579(1), c = 12.102(2) Å,
α = 112.82(1), β = 103.44(1), γ = 90.49(1)°, V = 1211.6(3) Å3, Z = 2. The strongest X-ray powder
diffraction lines [d in Å (I) (hkl)] are: 10.78 (100) (001), 5.79 (55) (021), 4.23 (35) (102), 5.31 (34)(102), 5.39 (32) (002). Its crystal structure has been solved by X-ray single-crystal diffraction on thebasis of 2667 unique reflections, with a final R = 0.035. It is formed by two kinds of modules: slabs(110)PbS of modified PbS archetype (type A slabs) and openwork slabs with channels accomodating(K,NH4)+ cations and H2O molecules (type B slabs). Its structure can be described as an order-disorder(OD) structure, built up by two different kinds of layers. Taking into account only the short (As,Sb)-S
bonds, (As,Sb)S3 triangular pyramids form double chains similar to those described in other naturaland synthetic compounds, among which its homeotype gillulyite, as well as gerstleyite. Ambrinoite
belongs to the hutchinsonite merotypic family. It is probably the product of late-stage hydrothermal
fluid circulation. The name of this new mineral species (IMA 2009-071) honors Pierluigi Ambrino (b.
1947), the mineral collector who kindly provided us with the studied specimens.
Keywords: Ambrinoite, sulfosalt, potassium, ammonium, crystal structure, gillulyite, Signols,
Upper Susa Valley, Torino, Piedmont, Italy

introDuCtion
The Upper Susa Valley (Piedmont, Italy) is characterized by
the relative abundance of evaporitic outcrops, belonging to theTriassic “Gessi” (gypsum) formation. These evaporites, which
are part of the Pennidic Domain, are usually associated withdetachment horizons separating ophiolitic and oceanic units fromcontinental units. In particular, in the area of Signols, the Gessi
formation is located between the ophiolitic units of Roche del’Aigle and of Vin Vert, and the continental units of Vallonetto
and Ambin (Polino 1999).
The mineralogy of these evaporitic rocks was studied by
Colomba (1898, 1909), who described the presence of anhydrite,* E-mail: elena@dst.unipi.it
0003-004X/11/0506–878$05.00/DOI: 10.2138/am.2011.3723

878

dolomite, gypsum, halite, hematite, a (Li,Mg)-rich mica, pyrite,
quartz, sphalerite, sulfur, and tourmaline. Damarco and Barresi(2005) reported also the occurrence of fluorite, orpiment, andstibnite. Finally, Biagioni et al. (2010) described the findings ofenargite and of the very rare sulfosalt galkhaite.
The aim of this paper is the description of ambrinoite, a new
sulfosalt from Upper Susa Valley. The new species and its namehave been approved by the CNMNC of the IMA (no. 2009-071).
It is named after Pierluigi Ambrino (b. 1947), the mineral collector who provided us with the studied specimens. The typematerial is deposited in the mineralogical collection of the Museodi Storia Naturale e del Territorio, University of Pisa, under thecatalog number 19500; the cotype specimen is deposited in themineralogical collection of the Museo Regionale di Scienze
Naturali, Torino, with catalog number M/15824.

BIAGIONI ET AL.: AMBRINOITE, THE FIRST NATURAL (K,NH4)-HYDRATED SULFOSALT

oCCurrEnCE anD ParagEnEsis
Ambrinoite was found in the Cumbë Sûrdë quarry, Signols,
Oulx, Upper Susa Valley, Torino, Piedmont, Italy. This quarry
had been exploited for gypsum since 1883 and was abandonedin the early sixties of the 20th Century (Guiguet et al. 2003).
The first specimens of ambrinoite were found in 1998 by
the mineral collectors P. Ambrino, A.A. Barresi, and P. Brizio;
unfortunately, the very low amount of material did not allow its
identification as a new species. In 2007, the mineral collector
R. Cola found a new specimen in which the phase was abundant
and well crystallized, but he did not recognize the red mineral asa potential new species. He presented the sample by chance to P.
Ambrino, who immediately brought it back to the same findingsof 1998, and provided us with the sample for a full description.
In hand specimen, ambrinoite occurs as cinnabar-red lamellar
aggregates, up to 1 mm thick, scattered in the saccharoidal gypsum matrix, together with sulfur and orpiment. Tabular crystals of
this new mineral, elongated on [100], are up to 100 µm long andless than 10 µm across. The paragenesis suggests a low temperature of formation. Besides, as stated above, Biagioni et al. (2010)
reported the presence of galkhaite at Signols, in association withorpiment; according to Pekov and Bryzgalov (2006), galkhaite
is a low-temperature phase. Therefore, ambrinoite was probably
deposited by highly alkaline low-temperature hydrothermal fluids, similarly to the only other hydrated alkaline sulfosalt known
in nature, i.e., gerstleyite (Frondel and Morgan 1956), describedin the Kramer borate district (California, U.S.A.).

oPtiCal anD PhYsiCal ProPErtiEs
Ambrinoite is cinnabar-red in color, very similar to getchellite;
it has a reddish streak and a vitreous to resinous luster. Minute fragments are transparent. In plane polarized transmitted light, ambrinoite
shows a strong pleochroism, being yellow along [100] and orange-red
in a normal direction. Between crossed polars, the mineral shows aparallel extinction to the cleavage traces and a negative elongation.
Birefringence is hidden by the reddish color of the crystals. The
mean refractive index was calculated using the method proposed by
Korotkov and Atuchin (2008); according to these authors, the average possible error of the calculations for non-oxide compounds isabout 12%. Taking into account the chemical formula derived by the
structural study (see below), the mean refractive index of ambrinoite
should be 2.5(3), but could not be measured.
Density was not measured because of the scarcity of available
material; the calculated density, with the same formula used for mean
refraction calculation, is 3.276 g·cm−3.
Ambrinoite is brittle and its fracture is splintery. It shows two
perfect cleavages on {001} and {010}, whereas on {100} the cleavage is poor. Mohs hardness could not be directly measured becauseof the small crystal size. It is less than 2, according to a measured
value VHN (10 g load) of 30 kg/mm2. This value corresponds to a
microhardness of 0.32 GPa, to be compared with 0.14 for talc and0.61 for gypsum (Broz et al. 2006).

ChEMiCal anD sPECtrosCoPiC stuDiEs
In search of an unexpected component: The ammonium cation
The presence of ammonium in ambrinoite was first suspected, then proven, by a feedback process combining electron

879

microprobe analysis, crystal structure solution and micro-Raman
spectroscopy. At a first step, preliminary EDS analysis with SEM
showed the presence of K, As, Sb, and S as the only significant
elements with Z > 9. The crystal structure study then indicateda formula close to stoichiometric K2(As,Sb)8S13⋅H2O, but withan unexplained deficit on the occupancy of the K sites (and thus
a deficit of positive charges), suggesting the additional presence of another (monovalent) cation, with a lower Z [higher Z
would have overpassed a full site occupancy: significant Rb or
Cs contents are excluded, despite the occurrence of galkhaite,(Cs,Tl,)(Hg,Cu,Zn)6(As,Sb)4S12, at Signols].
Lithium, Na, or NH4 were possible candidates. In Signols
evaporites, lithium was detected in a (Li,Mg)-rich mica (Colomba
1898); its substitution to K in ambrinoite would contract themean volume of K sites, but preliminary bond valence calculations on K positions indicate a valence deficit that is inversely
an expanded volume, which excludes any significant substitutionof K by a smaller cation, Li or Na.
Finally, it was hypothesized that K+ was partially replaced by
NH4+, a larger but lighter monovalent cation, as it was reported by
Zelenski et al. (2009) for tazieffite, a complex chloro-sulfosalt
from Kamtchatka (Russia). In addition, it is known that inevaporitic environments high concentration of ammonium ion,
formed by deamination of organic matter during early diagenesis,
is common in fluid inclusions (Bogomolov et al. 1970; Pironon
et al. 1995a, 1995b). The presence of ammonium was indirectly
inferred in tazieffite (Zelenski et al. 2009), but could not be detected, due to its very low expected content (0.03 wt%).
On this basis, ambrinoite was analyzed by a microprobe
equipped with WDS, with a program including N, together with
Na and Tl. Despite an analytical artifact (see below), N could
be detected (with only traces of Na and Tl). Simultaneously, aRaman study of ambrinoite permitted to confirm qualitatively
the presence of H2O molecules together with NH4+ ions.
Electron microprobe analysis
Three crystal fragments of ambrinoite were prepared asa polished section. Chemical analysis was performed with a
CAMEBAX SX 100 electron microprobe (West Microprobe
Laboratory, IFREMER, Plouzané, France). The operating conditions
were: accelerating voltage 20 kV (10 kV for oxygen and nitrogen),beam current 20 nA, beam size 5 µm, a short counting time (10 s)
for all elements (to reduce the risk of sample degradation under thebeam); standards (element, emission line): orthoclase (KKα), albite(NaKα), lorandite (TlMα), GaAs (AsLα), stibnite (SbLα), pyrite(SKα), and cassiterite (OKα). Selenium, Cl, and Hg were planned,
but not detected. Nitrogen analysis was performed according to
the conditions described by Huneau et al. (2000): synthetic VN asa standard, with NKα line; the short counting time induced a highvariability of the N concentration for one spot analysis.
The presence of N was initially overlooked, as the measureof its background was overestimated due to a Sb secondary
peak close to the NKα peak. Fortunately, the analysis of orpiment, As2S3, getchellite, AsSbS3, and stibnite, Sb2S3, as internal
standards, allowed to interpolate the true background for N at its
peak position in ambrinoite, taking into account its As/Sb ratio.
This permitted to reveal a positive difference between peak andbackground for N in ambrinoite, corresponding to a significant

880

BIAGIONI ET AL.: AMBRINOITE, THE FIRST NATURAL (K,NH4)-HYDRATED SULFOSALT

N content, despite its low values (around 0.5 wt%).
Table 1 gives weight concentrations obtained on the main fragment. For getting the true total, the H content corresponding to H2O
and NH4+ was added. There are only traces of Na (mean ∼0.05 wt%)
and Tl (mean ∼0.13 wt%), whereas N is close to 0.5 wt%. Oxygen
reaches 1.5 wt%. Table 1 (lower part) gives atom ratios on the basisof (As + Sb) = 8 atoms. Taking into account the N content as NH4+
cation, the valence equilibrium Ev appears good (mean: –1.9%).
The sum of monovalent cations is in agreement with the total of
2 apfu, as suggested by the structural results; it thus excludes asignificant content of Li (which cannot be detected by EPMA).
Potassium concentration is very homogeneous (1.43 ± 0.02 apfu),
indicating that fluctuations in N atom ratio (0.42 ± 0.14 apfu) are
due exclusively to a poor count statistic. Although N concentrationis only about the tenth of that of K, the NH4/K atom ratio is very
high, close to 0.3. The As/Sb ratio is close to 6:2, with a significant
As-for-Sb substitution, from (As6.1Sb1.9) to (As5.45Sb2.55).
The mean structural formula of the analyzed fragment is[K1.43(NH4)0.42Na0.02Tl0.01]Σ=1.88(As5.82Sb2.18)Σ=8.00S13.22⋅1.2H2O.

[K1.5(NH4)0.5]Σ=2(As6Sb2)Σ=8S13⋅H2O, or (K,NH4)2(As,Sb)8S13⋅H2O.
Micro-Raman spectroscopy
Nonpolarized micro-Raman spectra were obtained on an
unpolished fragment of ambrinoite in nearly backscattered geometry with a Jobin-Yvon Horiba “Labram” apparatus, equippedwith a motorized x-y stage and an Olympus microscope with a
50× objective. The 632.8 nm line of a He-Ne laser was used; laser
power was controlled by means of a series of density filters. The
minimum lateral and depth resolution was set to a few microm-

wt%
1
2
3
4
5
6
7
8
9
10
mean
σ

X-raY DiffraCtion stuDiEs
X-ray powder diffraction

Taking into account relative errors, especially on light elements O
and N, this formula can be idealized as the stoichiometric one

Table 1.

eters. The system was calibrated using the 520.6 cm–1 Raman
band of silicon before each experimental session. Spectra werecollected with multiple acquisitions (2 to 6) with single counting times, ranging between 20 and 180 s. The Raman spectra
confirmed the presence of both H2O and NH4+ in the ambrinoite
structure. The following lines were observed:(1) In the region 200–1200 cm–1: 207, 216, 294, 324, 341,
352, 364, 371, and 393 cm–1. These lines can be attributed to As-Sand Sb-S stretching and bending vibrations, which occur between
200 and 400 cm–1 (Forneris 1969; Kharbish et al. 2007) (Fig. 1a).
(2) In the region between 1200 and 1900 cm–1: 1423 cm–1
(N-H bending) and 1595 cm–1 (O-H bending) (Fig. 1b).
(3) In the region 2600–3800 cm–1: 3150 cm–1 (N-H stretching)and 3475 cm–1 (O-H stretching) (Fig. 1c).
These complementary results of the microprobe and Raman
studies thus permitted to refine the crystal structure of ambrinoite
with a full occupancy of the K sites. (CIF on deposit1.)

The powder X-ray diffraction pattern for ambrinoite (Table 2)
was obtained using a 114.6 mm diameter Gandolfi camera, with
Ni-filtered CuKα radiation. Indexing of the reflections was doneusing the calculated powder pattern obtained by the structural model
described below, using the software POWDERCELL (Kraus andNolze 2000). The unit-cell parameters refined through least-square
methods of all the 37 univocally indexed reflections with CELREF
Deposit item AM-11-029, CIF. Deposit items are available two ways: For a paper
copy contact the Business Office of the Mineralogical Society of America (seeinside front cover of recent issue) for price information. For an electronic copy visit
the MSA web site at http://www.minsocam.org, go to the American Mineralogist
Contents, find the table of contents for the specific volume/issue wanted, and thenclick on the deposit link there.

1

Microprobe analyses of ambrinoite: chemical composition as wt% (upper part) and number of atoms on the basis of 8 (As + Sb)(lower part)
K
4.51
4.71
4.66
4.60
4.54
4.51
4.56
4.60
4.48
4.52
4.57
0.07

Na
0.02
0.06
0.07
0.06
0.04
0.06
0.05
0.03
0.02
0.05
0.05
0.02

Tl
0.01
0.16
0.14
0.17
0.09
0.07
0.20
0.16
0.20
0.07
0.13
0.06

N
0.32
0.21
0.52
0.44
0.33
0.47
0.73
0.66
0.54
0.56
0.48
0.16

As
37.87
37.49
37.16
37.54
36.57
35.01
34.85
34.27
33.30
32.84
35.69
1.87

Sb
19.23
19.31
19.42
19.87
20.48
22.63
22.64
23.55
24.77
25.04
21.69
2.30

S
35.02
34.85
34.97
35.20
34.73
34.82
34.36
34.71
34.22
34.05
34.69
0.37

O
1.30
1.53
1.40
1.47
1.47
1.58
1.62
1.50
1.59
1.69
1.52
0.11

Hcalc
0.25
0.25
0.32
0.31
0.28
0.33
0.41
0.38
0.35
0.37
0.33
0.05

Total
98.53
98.58
98.67
99.67
98.53
99.48
99.42
99.85
99.47
99.19
99.14
0.51

Atoms
K
Na
Tl
N
As
Sb
S
O
Hcalc
Ev*
0.01
0.00
0.28
6.10
1.90
13.17
0.98
3.06
–2.3
1
1.39
0.03
0.01
0.18
6.07
1.93
13.19
1.16
3.06
–2.4
2
1.46
0.04
0.01
0.45
6.05
1.95
13.31
1.07
3.95
–2.2
3
1.45
0.03
0.01
0.38
6.03
1.97
13.22
1.11
3.73
–2.0
4
1.42
0.02
0.01
0.29
5.95
2.05
13.20
1.12
3.39
–2.3
5
1.42
0.03
0.00
0.41
5.72
2.28
13.30
1.21
4.06
–2.5
6
1.41
0.03
0.01
0.64
5.72
2.28
13.17
1.24
5.06
–0.7
7
1.43
0.02
0.01
0.58
5.62
2.38
13.31
1.15
4.62
–1.8
8
1.45
0.01
0.01
0.48
5.49
2.51
13.18
1.23
4.36
–1.5
9
1.41
0.03
0.00
0.50
5.45
2.55
13.19
1.31
4.62
–1.4
10
1.44
0.02
0.01
0.42
5.82
2.18
13.22
1.16
3.99
–1.9
mean
1.43
0.14
0.25
0.25
0.06
0.10
0.69
0.6
σ
0.02
0.01
0.00
* Relative error on the valence equilibrium (%), calculated as [Σ(val+) − Σ(val−)] × 100/Σ(val−). The last decimal digits for the N and O wt% are purely “aesthetic,”
because the absolute error corresponds to few decimals percentages..

BIAGIONI ET AL.: AMBRINOITE, THE FIRST NATURAL (K,NH4)-HYDRATED SULFOSALT

figurE 1. Raman spectra of ambrinoite.

(Laugier and Bochu 1999) are a = 9.70(1), b = 11.58(2), c = 12.12(3)Å, α = 112.8(1), β = 103.1(2), γ = 90.7(2)°, and V = 1215(4) Å3.
Single-crystal X-ray diffraction
Preliminary Weissenberg photographs suggested a triclinic
symmetry and indicated that even very small fragments of ambrinoite actually consist of multiple crystals. However, it waspossible to obtain the triclinic unit cell in a Bruker-AXS three881

circle diffractometer working with graphite monochromated
MoKα X-radiation and equipped with a Smart-Apex CCD detector. At room temperature, the triclinic unit cell was: a = 9.716(1),b = 11.581(1), c = 12.103(2) Å, α = 112.71(1), β = 103.48(1),γ = 90.48(1)°, and V = 1214.3(3) Å3.
The intensity data collection was performed at the Elettra
synchrotron facility (Basovizza, Trieste, Italy) by using a verysmall crystal, with dimensions 0.08 × 0.01 × 0.005 mm3. Crystal data and experimental details are reported in Table 3. The
wavelength of the radiation was set to 1.0 Å, and the crystal
was placed at 36 mm from the 165 mm MarCCD detector. In
this experimental setup, 73 frames were collected with a rotation
angle ∆ϕ = 5°. Data were integrated and corrected for Lorentz,
polarization, and background effects using the HKL package(Otwinowski and Minor 1997) on the basis of the intensities of
equivalent reflections. Reflections point to a triclinic cell, witha = 9.704(1), b = 11.579(1), c = 12.102(2) Å, α = 112.82(1),β = 103.44(1), γ = 90.49(1)°, and V = 1211.6(3) Å3. The a:b:c
ratio calculated from the unit-cell parameters is 0.838:1:1.045.
The solution and refinement of the structure were performed bymeans of the SHELX set of programs (Sheldrick 2008).
A Patterson map calculated in the space group P1 revealed
the coordinates of 12 electron density maxima, identified as
Sb, As, and S on the basis of their heights and interatomic
distances. The remaining atomic positions were deduced fromdifference Fourier syntheses. At this point, two couples of atoms,
related by the inversion center, showed odd short distances,and the reliability index remained quite high (nearly 22%).
After decreasing the symmetry to P1, the R index dropped to
10% and the difference Fourier map listed additional electron
density maxima, which were attributed to H2O molecules andto alkaline cations. After some least-square refinement cycles,in which anisotropic displacement parameters were introduced
for all the atoms, and the ratio Sb/As was refined for the Msites, the refinement smoothly converged to R1 = 0.040 for 2554
reflections with Fo > 4σ(Fo) and 0.042 for all 2667 reflections.
However, a careful scrutiny of the structure revealed that no
deviations from a centrosymmetric atomic arrangement were
present, and that a shift of the origin was needed to correctly
describe the structure in the actual space group P1. After a few
cycles of refinement in this space group, and the refinement of
the fraction of NH+4 substituting K+ in the interlayer sites, thereliability index dropped to 0.035.
Atomic coordinates, occupation factors, and equivalentdisplacement parameters are shown in Table 4, anisotropic
displacement parameters are listed in Table 5, whereas selectedbond distances are reported in Table 6.
The refined occupancies for the M sites indicated that As
and Sb statistically occupy M1 and M2 sites (Sb0.54As0.46 andSb0.57As0.43, respectively), whereas the M5, M6, and M7 sites
are mainly occupied by As with only minor Sb, and finally M3,
M4, M8 are Sb-free (Table 4).

DEsCriPtion of thE struCturE
Cation coordination
M cations are bound exclusively to S, with the three shortest
bonds ranging between 2.2 and 2.5 Å (Table 6). M1 and M2 sites,
Sb-dominant, present the longest bond lengths (2.39 to 2.46 Å);

882

BIAGIONI ET AL.: AMBRINOITE, THE FIRST NATURAL (K,NH4)-HYDRATED SULFOSALT
Table 2.

X-ray powder diffraction for ambrinoite

h k ldcalc†
Icalc‡
dobs Iobs§
h k ldcalc†
Icalc‡
dobs
Iobs§
10.784100
10.7* vs3 0 3
2.73026
0 0 1
10.60870 4 3
2.716714
2.721*
mw
0 1 0
9.769
8
9.6*
w
1 4 1
2.6927
5
0 1 1
9.381
3
0 4 0
2.6520
4
1 0 0
7.446
2
7.4* vw
2 0 4
2.648223
2.646*
m
1 1 0
7.255
3
7.2* vw
1 2 32.6104
4
1 1 1
6.672
10
6.7* vw
3 2 0
2.572621
2.571*
m
1 1 0
6.390
6
2 0 3
2.5550
20
0 1 1
6.365
10
0 2 3
2.5401
3
2.537
s
1 1 1
6.322
10
6.29* w
3 2 2
2.5267
21
1 0 1
5.914
3
3 2 2
2.4903
5
2.502
w
1 1 1
5.845
5
2 2 3
2.4903
6
0 1 2
5.761
52
5.75*
s
0 4 4
2.4423
6
0 2 1
5.527
6
2 4 3
2.4375
3
2.436
m
1 1 2
5.392
29
5.33
m
2 4 0
2.4258
6
0 0 2
5.296
33
4 0 1
2.424011
1 0 2
4.946933 2 42.41166
1 2 1
4.872254.872* w
2 2 2
2.407741 2 1
4.851542 4 1
2.385082.386*w
1 2 0
4.584054.556* w
0 4 1
2.364842.371*
vw
1 2 2
4.475943 0 4
2.362452 1 0
4.46123
1 2 3
2.319032.329*vw
2 1 1
4.230526
4.264* mw
2 4 1
2.25613
2.261*w
1 0 2
4.146511
4.155* m
3 3 02.224022.211*
vw
0 2 1
4.11824
1 2 4
2.152022.151*
vw
1 2 2
4.081043 0 3
2.10747
2.123*
vw
1 2 1
3.94801
3.932* vw
2 4 5
2.086932.089*
vw
2 0 1
3.755829
2 4 4
2.05913
2.064*
vw
0 2 3
3.7229103.721 m4 0 4
2.04801
2.047*
vw
2 2 0
3.71823
3 0 5
2.035352.039*
w
0 3 2
3.686493 4 3
1.9736
5
1.978*w
1 0 3
3.6681
8
1.969
vw
2 2 1
3.6276
7
3.639 mw
3 4 5
1.92457
2 2 2
3.607350 6 3
1.9203
4
1.922
mw
2 2 1
3.55774
3 4 2
1.89653
1.898*
vw
1 2 1
3.535943.533* vw
5 0 0
1.8762
4
1.870
w
0 3 0
3.43601
3.435* vw
5 0 3
1.87115
1 3 0
3.2878193.285* m
2 0 6
1.84326
1.845*w
2 0 3
3.247843 4 4
1.813551.820*vw
1 2 2
3.161026
3.170* mw
0 0 6
1.7973
4
1.802
w
2 0 2
3.1270112 0 5
1.79325
3 0 0
3.066716
3.075* mw
0 2 5
1.7661
4
1.767
m
3 0 2
2.880522
3 4 1
1.76607
0 4 2
2.8541
4
2.875
s
3 4 3
1.73064
1.726*
vw
0 4 1
2.84125
2 4 7
1.6674
3
1.666*w
1 2 2
2.816713
3 6 2
1.618053 0 12.770055 4 01.6179
3
1.621
mw
1 4 1
2.766813
3 6 0
1.61723
1 4 2
2.7525
5
2.762
s
6 0 2
1.61637
3 2 1
2.74054
5 4 3
1.60543
1 4 2
Notes: The asterisks indicate the 37 univocally indexed reflections, which were used to refine the unit cell. The
experimental error is estimated to about 0.04 °2θ. This corresponds to an uncertainty in observed d-spacingsin the order of magnitude of the last reported digit.
† The distances were calculated on the basis of the unit cell refined by using synchrotron data.
‡ Intensities were calculated on the basis of the structural model.
§ Observed intensities were visually estimated. s = strong; vs = very strong; m = medium; mw = medium-weak;
w = weak, vw = very weak.

they form additional weak bonds with S13 and S12, respectively,
at distances >2.90 Å. The other sites, M3 to M8, As-dominant,are characterized by shorter bond lengths, ranging from 2.198
Å (for M8, occupied exclusively by As) up to 2.376 Å for M6
site. M6 and M7 sites, in which up to 30 at% As is replaced bySb, show an additional weak bond at ~3 Å.
K+ and NH+4 occupy two sites, K1 and K2, with slightly different K/NH4 s.o.f., 0.83/0.17 and 0.78/0.22, respectively. The
smallest K1 site is eightfold coordinated; the shortest bond isformed with W1 (2.81 Å). The mean K1-S bond length is 3.40
Å. K2 site is 10-fold coordinated; the shortest bond is also withW1 (2.78 Å), while the mean bond length of the nine K2-S
bonds is 3.598 Å.
Table 7 shows bond-valence analysis of cations and anions,according to Brese and O’Keeffe (1991). For K1 and K2 site,
there is no bond valence parameter for NH4+-S bond, but, as
NH+4 has almost the same size as Tl+ (compare isotypic NH4Cl
and TlCl; Roberts et al. 2006), bond-valence balance was calculated considering NH+4 as Tl+ and then using the known (Tl,S)and (Tl,O) parameters. This approach was applied recently by
Zelenski et al. (2009), to infer indirectly the presence of minor
NH4+ in a slightly expanded Pb site in the crystal structure of thecomplex sulfosalt tazieffite.
According to Table 7, the total bond valence of M atoms fits
very well with the ideal value of 3 (from 2.89 to 3.07). While K1
shows also a good total (0.96), there is a significant lower total forK2 (0.87), which may be due to distinct sub-positions of NH4+ andK+, whereas the crystal structure study gave only a mean position.

BIAGIONI ET AL.: AMBRINOITE, THE FIRST NATURAL (K,NH4)-HYDRATED SULFOSALT
Table 3. Crystal data and summary of parameters describing datacollection and refinement for ambrinoite

Table 4.

Crystal data
X-ray formula[K1.61(NH4)0.39] (As6.36Sb1.64)S13·H2O
0.08 × 0.01 × 0.005
Crystal size (mm3)
Cell setting, space group
Triclinic, P1
a, b, c (Å)
9.704(1), 11.579(1), 12.102(2)α, β, γ (°)
112.82(1), 103.44(1), 90.49(1)1211.6(3) Å3
V (Å3)
Z
2
Data collection and refinement
Radiation, wavelength (Å)
synchrotron, λ = 1 ÅTemperature (K)293
Detector to sample distance
36 mm
16.5 × 16.5
Active detection-area (cm2)
Number of frames
73
Rotation width per frame (°)
5
Maximum observed 2θ
63.5
Measured reflections
9705
Unique reflections
2667
2554
Reflections Fo > 4σ(Fo)0.0283
Rint after absorption correction
Rσ0.0234
Range of h, k, l
−9 ≤ h ≤ 9, −12 ≤ k ≤ 12, −12 ≤ l ≤ 12
0.0341
R [Fo>4 σ Fo]
R (all data)
0.03530.0913wR (on Fo2)
Goof
1.141
Number of least-squares parameters
225

Site
M1
M2
M3
M4
M5
M6
M7
M8
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
S11
S12
S13
K1
K2
W1

Anisotropic displacement parameters for ambrinoite

U11
U22
U33
U23
U13
U12
M1
0.0361(6) 0.0278(4) 0.0425(7) 0.0142(3) 0.0154(3) 0.0030(3)M20.0341(6) 0.0298(4) 0.0500(5) 0.0192(3) 0.0109(3) 0.0023(3)M30.0374(6) 0.0280(5) 0.0294(5) 0.0143(4) 0.0071(4) 0.0025(4)M40.0369(7) 0.0269(5) 0.0280(5) 0.0135(4) 0.0062(4) 0.0012(4)M50.0396(7) 0.0292(5) 0.0350(6) 0.0108(4) 0.0088(4) 0.0018(4)M60.0327(7) 0.0285(5) 0.0451(6) 0.0143(4) 0.0087(4) 0.0024(3)M70.0357(7) 0.0302(5) 0.0476(6) 0.0180(4) 0.0153(4) 0.0060(4)M80.0409(7) 0.0282(5) 0.0377(5) 0.0110(4) 0.0091(4) 0.0014(4)0.027(1)0.027(1)0.014(1)0.005(1)0.001(1)S1
0.044(1)0.033(1)0.054(1)0.024(1)0.023(1)0.006(1)S2
0.047(2)0.035(1)0.031(1)0.015(1)0.006(1) −0.002(1)S30.031(1)0.031(1)0.045(1)0.021(1)0.017(1)0.004(1)S40.044(2)0.029(1)0.028(1)0.010(1)0.010(1) −0.002(1)S50.037(1)0.032(1)0.045(1)0.021(1)0.005(1)0.004(1)S6
0.036(1)0.029(1)0.031(1)0.010(1)0.005(1)0.004(1)S7
0.033(1)0.041(1)0.030(1)0.018(1)0.010(1)0.011(1)S8
0.038(1)0.030(1)0.040(1)0.010(1)0.012(1)0.002(1)S90.035(1)0.032(1)0.035(1)0.008(1)0.009(1)0.001(1)S10
0.033(1)0.030(1)0.049(1)0.021(1)0.001(1)0.003(1)S110.039(1)0.053(1)0.031(1)0.023(1)0.007(1)0.005(1)S120.040(1)0.033(1)0.030(1)0.014(1)0.009(1)0.004(1)S13
0.042(1)0.058(2)0.095(2)0.044(2)0.007(1)0.003(1)K1
0.048(2)0.061(2)0.075(2)0.020(2)0.044(2)0.007(2)K2
0.088(3)0.046(4)0.050(4)0.014(3) −0.004(3)0.008(3)W1
0.087(6)Note: The anisotropic displacement factor exponent takes the form: –2π2[ h2a*2U11+ ... + 2 h k a* b* U12].

General organization
Considering only the three shortest M-S bonds, the crystal
structure of ambrinoite can be described on the basis of MS3
triangular pyramids sharing S corners to form two types of chains[(As,Sb)4S7]∞ running along [100]; within each chain, trigonal
pyramids arranged in M3S5 triangular groups alternate with asingle MS3 pyramid (Fig. 2a). The two different chains are connected each other by sharing the S corner of the single pyramidsof each chain, giving rise to double chains [(As,Sb)8S13]∞ (Fig.
2b). In Figure 2c these double chains are seen along [100]. Twodouble chains are interpenetrated in a zigzag fashion into more
complex columns forming layers parallel to (010). In ambrinoite,

Atomic positions and equivalent displacement parameters
for ambrinoite

Site populationx
Sb0.54As0.46
0.39230(7)0.01055(7)Sb0.57As0.43
0.66142(10)As1.000.66440(9)As1.000.24245(10)As0.92Sb0.08
0.00896(9)As0.80Sb0.20
0.38933(9)As0.75Sb0.25
0.23678(10)As1.000.6161(2)0.3786(2)0.8204(2)0.3974(2)0.4723(2)0.0114(2)0.8220(2)0.4694(2)0.1611(2)0.1587(2)0.0192(2)0.2637(2)0.2578(2)0.6680(3)K0.83(NH4)0.17
0.9242(4)K0.78(NH4)0.22
0.2172(8)H2O

Table 6.
Table 5.

y0.64339(5)0.14677(6)0.98014(7)0.70477(7)0.92642(7)0.64686(7)0.14671(7)0.42874(8)0.7814(2)0.3401(2)0.0599(2)0.8510(2)0.5620(2)0.3439(2)0.5719(2)0.0630(2)0.0573(2)0.5625(2)0.8485(2)0.2992(2)0.7763(2)0.3331(2)0.7592(3)0.7496(6)z0.83575(6)0.83795(6)0.71276(7)0.70910(7)0.96400(7)0.82178(7)0.82502(7)0.94031(8)0.5576(2)0.8069(2)0.6437(2)0.8382(2)0.6432(2)0.8169(2)0.6362(2)0.6408(2)0.6991(2)0.6931(2)0.8258(2)0.0350(2)0.0381(2)0.6791(3)0.4774(3)0.5286(6)Ueq0.0344(3)0.0369(3)0.0309(3)0.0302(3)0.0354(4)0.0358(4)0.0362(4)0.0365(3)0.0323(5)0.0404(5)0.0321(5)0.0371(5)0.0314(5)0.0367(5)0.0323(5)0.0348(5)0.0368(5)0.0345(5)0.0392(5)0.0398(5)0.0348(5)0.065(1)0.073(1)0.066(2)M2 Sb0.57As0.43 −S6
−S9
−S3
−S12
−S13
−S11

2.393(2)2.418(2)2.462(2)2.962(2)3.262(2)3.402(2)

−S5
−S7
−S1
−S12

2.254(2)2.258(2)2.288(2)3.031(2)M6 As0.80Sb0.20 −S11
−S10
−S7
−S13
−S12
−S6
M8 As1.00
−S12
−S6
−S2

2.317(2)2.321(2)2.379(2)2.981(2)3.423(2)3.486(2)2.198(2)2.299(2)2.307(3)Selected bond distances (Å)

M1 Sb0.54As0.46 −S4
−S10
−S5
−S13
−S12
−S2

2.392(2)2.426(2)2.463(2)2.944(2)3.281(2)3.390(2)

−S8
−S3
−S1
−S13

2.253(2)2.260(2)2.294(2)3.150(2)M5 As0.92Sb0.08 −S13
−S4
−S11

2.236(2)2.323(2)2.327(2)M7 As0.75Sb0.25 −S2
−S9
−S8
−S12
−S13
−S4

2.334(2)2.339(2)2.378(2)3.009(2)3.381(2)3.490(2)

−W1
−S1
−S6
−S5
−S7
−S3
−S8
−S2

2.815(8)3.299(3)3.338(3)3.379(3)3.400(3)3.420(3)3.474(3)3.495(4)M3 As1.00

883

M4 As1.00−W1
2.779(8)−S1
3.334(4)−S10
3.464(3)−S6
3.500(4)−S9
3.534(4)−S3
3.571(4)−S7
3.668(3)−S2
3.736(4)−S93.770(4)−S11
3.806(4)Note: The actual content of the M1–M8 and K1–K2 sites is reported.

K1
K0.83(NH4)0.17

K2
K0.78(NH4)0.22

these layers are held together by (K, NH4)-S bonds; H2O molecules are coordinated by two K+ cations, whereas K+ cations
are coordinated only by one H2O, besides seven or nine S atoms.
As the strong bonds, corresponding to short M-S bonds, occur
exclusively within the complex columns, weak bonds between
these columns could be easily broken: this explains the perfect{001} and {010} cleavages observed under the microscope. Two
different orientations of the crystal structure of ambrinoite are

884

BIAGIONI ET AL.: AMBRINOITE, THE FIRST NATURAL (K,NH4)-HYDRATED SULFOSALT

Table 7. Bond-valence balance in ambrinoite
S1
M1
M2
M3
M4
M5
M6
M7
M8
K1
K2
Σanions

S2
0.06

S3

S4
0.95

S5
0.79

0.80
1.00

0.91
0.93

S6

S7

S8

S9

0.97

S10
0.87

0.90

0.06

1.02

1.00
0.04

0.15
0.14
2.13

1.88

0.85
0.12
0.08
2.01

0.90
0.13
0.09
2.13

0.11
0.06
1.99

S13
0.21
0.09
0.09

W1

0.12

0.82

0.04
0.11
0.09
2.00

S12
0.09
0.21

1.02
0.89

0.96
0.88
0.09
0.05
2.04

S11

0.96

0.88
0.97

0.10
1.93

0.04
1.95

0.94

0.05
0.15
1.18

1.12
0.16
0.06

0.09
0.04
1.88

1.96

1.80

1.73

0.16
0.18
0.34

Σcations
2.97
3.03
3.02
3.07
2.89
3.00
3.00
2.96
0.96
0.89

figurE 2. Chains of MS3 pyramids in ambrinoite as seen: (a) down[010], a horizontal; (b) normal to (001), a horizontal; (c) down [100].

sketched in Figure 3.
Taking also into account the longer M-S bonds, the column layer becomes a complex slab with negative charge,[(As,Sb)8S13]2– (Fig. 4). Big, weakly bound cations (K,NH4)+,
together with H2O molecules, fill the inter-slab space that permits
to relate ambrinoite structure in the field of solid-state chemistry
to the family of intercalation compounds.
Ambrinoite as an OD structure
The structural refinement of ambrinoite allows us to recognize
the details of its structural modules, at difference from what happens in the closely related sulfosalt gillulyite (Foit et al. 1995),
which shows a high degree of disorder in both the shape of thechains formed by (As,Sb)S3 pyramids, and the actual distribution of the Tl+ cations between the chains. On the basis of theaverage structure refined by Foit et al. (1995), Makovicky and
Balić-Žunić (1999) proposed two possible ordering models forgillulyite. Moreover, they used the OD theory to derive the four
possible subcells of gillulyite (Table 1 in Makovicky and BalićŽunić 1999), by stacking two different kinds of layers, one ofthem being disordered.
Also, the structure of ambrinoite can be described as built up
by two kinds of layers, both of them ordered. From a geometricpoint of view, the inter-slab space above can be considered as aseparate layer; the two kinds of layers are represented in Figure
5 and 6, respectively. The first one (denoted as “layer A,” inagreement with the description of minerals belonging to thehutchinsonite merotypic family in Makovicky 2005) is formedby double chains of (As,Sb)S3 pyramids and corresponds to a
distorted slab (110)PbS of modified PbS archetype, whereas thesecond one (“layer B”) is formed by K+ and NH4+ cations andH2O molecules.
The two layers have the same a and b parameters, namely afigurE 3. Crystal structure of ambrinoite as seen down [100] in (a)and down [010] in (b).

= 9.70 Å and b = 11.58 Å, and different widths: the layer groupsymmetry of the layer A is P21/m1(1), whereas the layer groupsymmetry of layer B is P11(1). The parentheses indicate thedirection of missing periodicity.
The OD structure presents two distinct ρ-planes, which areboth λ-ρ-planes, so that the structure belongs to the IV category(Ferraris et al. 2004). The OD groupoid family symbol for ambrinoite may be written as
P

21

m

1

(1)

P 1 1
 −0.147... − 0.203...



(1)

BIAGIONI ET AL.: AMBRINOITE, THE FIRST NATURAL (K,NH4)-HYDRATED SULFOSALT

885

figurE 4. Ball and stick model of the complex slab [(As,Sb)8S13]2– in ambrinoite. Dark and white balls are (Sb,As) and S atoms, respectively.

figurE 5. Layer A in three different orientations.

where the numbers within square brackets are the r and s components of the projection of the vector connecting the origins of
the two subsequent layers. The r and s values are referred to thecommon a and b translations, respectively.
If we assume an arbitrary position for the layer B2n, thepositions of the adjacent layers A2n–1 and A2n+1 are uniquely determined. They are related by the inversion center, which is a λoperation of the layer B. It means that only one variety of triples

figurE 6. Layer B in three different orientations. Black and grayballs represent K cations and H2O molecules, respectively. For sake ofclarity, the MS3 pyramids forming the layer A are outlined in white color.

A2n–1B2nA2n+1 exists. On the contrary, if we assume an arbitrary
position for the layer A2n+1, there are two possible positions foreach of the adjacent layers B2n and B2n+2. Layers B2n and B2n+2 canbe related by either the 21 axis parallel to a or by the inversion
center, both being λ operations of the A layer.
There are two possible structures that present only two
kinds of triples of layers, and they are called polytypes withmaximum degree of order (i.e., MDO polytypes) in the OD
theory. To characterize them it is useful to introduce the con-

886

BIAGIONI ET AL.: AMBRINOITE, THE FIRST NATURAL (K,NH4)-HYDRATED SULFOSALT

cept of generating operation, namely the “τ-operation witha translational component parallel to the stacking direction,with magnitude equal to the distance between the two closest
τ-equivalent layers” (Ďurovič 1997).
In the former MDO polytype (MDO1), the inversion centerin A is active. The generating operation is the translation t1 =c0+2ra+2sb, where c0 = 10.78 Å corresponds to the distance
between the two nearest-neighbor equivalent λ-planes, and r
and s are defined above. The inversion center is valid for thewhole structure, which is triclinic P1, and has cell parameters
a = 9.70, b = 11.58, c = 12.10 Å, α = 112.8, β = 103.4, and γ =90.5°. It corresponds to the structure of ambrinoite, as describedin the previous paragraphs and drawn in Figure 3.
In the second MDO polytype (MDO2), the twofold screw axis
21 along [100] in the layer A is active. The constant applicationof this operation generates a monoclinic structure. Its generating
operation is a glide normal to a with translation component c0,
which becomes a glide c in a structure with c = 2c0+4sb. The
corresponding structure has space group P21/c11, cell parameters
a = 9.70, b = 11.58, c = 23.52 Å, α = 111.8°.
The observed X-ray diffraction patterns of ambrinoite do notshow any indication about the occurrence of this MDO2 polytype
in our sample. However, it could occur in other crystals from thesame or other possible localities.

rElationshiP with othEr natural or sYnthEtiC
sulfosalts

In nature, alkaline sulfosalts are very rare: the two hydrated
oxysulfosalts cetineite, NaK5Sb14S6O18(H2O)6 (Sabelli et al.
1988), and its dimorph ottensite, Na3(Sb2O3)3(SbS3)⋅3H2O (Sejkora and Hyršl, 2007); galkhaite, (Cs,Tl,)(Hg,Cu,Zn)6(As,Sb)4S12(Divjaković and Nowacki 1975; Chen and Szymański 1981),
and the hydrated Na sulfosalt, gerstleyite, Na2(Sb,As)8S13⋅2H2O(Nakai and Appleman 1981). Ambrinoite is the first natural(K,NH4)-hydrated sulfosalt.
According to its crystal structure, ambrinoite is the last
discovered member of a wide group of natural and synthetic
sulfosalts of As and/or Sb combined with large mono- or divalentcations (Tl+, NH4+, Cs+, Na+, Pb2+, Rb+, organic cations), belonging
to the hutchinsonite merotypic series (Makovicky 1997). The
structures of these phases consist of regular 1:1 intergrowthsof two kinds of slabs, one of which is common to all members,
whereas the second one may differ.
Taking into account the chains of corner-sharing MS3 pyramids, this kind of chain is already known to occur in two natural
sulfosalts belonging to the hutchinsonite family: gillulyite, andgerstleyite. In particular, ambrinoite is strictly related to gillulyite, Tl2(As,Sb)8S13, described by Wilson et al. (1991) from
the Mercur gold deposit (Utah, U.S.A.). The crystal structure of
gillulyite was determined by Foit et al. (1995) and reinterpreted,in the light of the order-disorder theory, by Makovicky and
Balić-Žunić (1999). Considering one of the ordered polytypes(P-ordering) of gillulyite, hypothesized by the latter authors,
the chains formed by MS3 pyramids are topologically identicalto those in ambrinoite; gillulyite differs from the latter in thesubstitution Tl+ = (K,NH4)+ and in the absence of H2O. Raman
spectra collected during this work on a sample of gillulyite,with the aim to verify the presence of H2O, did not show any

evidence of the presence of this molecule. As in cetineite andgerstleyite, also in ambrinoite the H2O molecules are bound only
to the alkali cations. The MS3 chains of ambrinoite and gillulyite are similar to that of gerstleyite; in this mineral, (Sb,As)S3
pyramids are linked to form double chains, running along [100](Nakai and Appleman 1981). However, whereas in gerstleyite(space group Cm) the single chains within the double ones are
related by a mirror plane, such symmetry element is not present
in ambrinoite and gillulyite.
In addition, several other synthetic sulfosalts show the samekind of MS3 chains, i.e., (NH4)2Sb4S7 (Dittmar and Schäfer 1977),[C4H8N2][Sb4S7] (Parise and Ko 1992), [C2H8N]2[Sb8S12(S2)] (Tanet al. 1996), and Rb2Sb8S12(S2)⋅2H2O (Berlepsch et al. 2001).
Other compounds with a similar stoichiometry were hydrothermally prepared by Wang et al. (2000), who described
the synthesis and crystal structure of [(CH3NH3)0.5(NH4)1.5]
Sb8S13⋅2.8H2O and Rb2Sb8S13.3⋅3.3H2O. Even if they display a
stoichiometry similar to that of ambrinoite, these two synthetic
compounds show 12-membered rings of SbS3 pyramids, linked
into one-dimensional complex chains, very different from thecrystal structure of ambrinoite.
As pointed out by Makovicky (2005), K is a “channelbuilding” element in sulfosalts; in fact, ambrinoite has a microporous character, with the presence of channels, filled with H2O
molecules and (K,NH4)+ cations, clearly underlining the zeolitic
aspect of this new mineral phase.
Thus, ambrinoite constitutes the first natural sulfosalt witha significant content of ammonium, substituting about 1/4 ofpotassium atoms in the crystal structure. The crystal structure ofambrinoite is homeotypic with that of gillulyite. In the light ofthe present investigation and the work of Makovicky and BalićŽunić (1999), ambrinoite, gillulyite and gerstleyite, belonging
to the hutchinsonite merotypic series (Makovicky 1997; Moëlo
et al. 2008), could be included in the new gerstleyite mineralgroup, according to the definition given by Mills et al. (2009).
Following the Strunz classification (Strunz and Nickel 2001),
ambrinoite belongs to the 2.HE subgroup (sulfosalts with alkalies and H2O).
According to the original geochemistry revealed by theparagenesis described at Signols, the metallogenic process forming ambrinoite may be the result of the interaction of post-acidmagmatism fluids generated within the crystalline basement,
and bringing Li (in mica), B (in tourmaline), F (in fluorite), Cs(in galkhaite), As, Sb, and Tl (in sulfides and sulfosalts), withoverlying Triassic formations, where reducing conditions due toorganic matter favored a high ammonium concentration as well as
the precipitation of sulfur as sulfides. These peculiar geochemical conditions may favor the discovery of other characteristic
minerals at Signols.

aCknowlEDgMEnts
We thank Pierluigi Ambrino, mineral collector, who provided us with thefirst samples of ambrinoite for study, and Marco Lezzerini for the measurementof micro-hardness. Thanks are also due to Bob Downs for the preliminary Raman
test of the RRUFF specimen of gillulyite. Microprobe analysis was performed owing to the skilled advice of M. Bohn (CNRS engineer, Département Géosciences
Marines, IFREMER, Centre de Brest). E.B. thanks Stefano Merlino for the helpful
suggestions and discussions about the OD nature of ambrinoite and gillulyite. Boththe referees, Francesco Demartin and Stefan Graeser, are warmly thanked for theirconstructive revision of the paper. This research was financially supported by MIUR
through project PRIN 2007 “Compositional and structural complexity in minerals

BIAGIONI ET AL.: AMBRINOITE, THE FIRST NATURAL (K,NH4)-HYDRATED SULFOSALT(crystal chemistry, microstructures, modularity, modulations): analysis and applications,” and was made possible by the municipality of Oulx and the Department ofMineralogical and Petrological Sciences of the University of Torino, which allowed
us to collect the investigated samples at the Signols quarry.

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Manuscript received OctOber 15, 2010
Manuscript accepted January 26, 2011
Manuscript handled by andrew McdOnald