Mineralogical Magazine, February 2015, Vol. 79(1), pp. 171–189

As-bearing new mineral species from Valletta mine,
Maira Valley, Piedmont, Italy: II. Braccoite,
NaMn2+
5 [Si5AsO17(OH)](OH), description and crystal
structure
FERNANDO CÁMARA1,2,*, ERICA BITTARELLO1,2, MARCO E. CIRIOTTI3, FABRIZIO NESTOLA4, FRANCESCO RADICA5
6
AND MARCO MARCHESINI
1

Dipartimento di Scienze della Terra, Università degli Studi di Torino, via Tommaso Valperga Caluso 35,
I-10125 Turin, Italy
2
CrisDi, Interdepartmental Center for Crystallography, via Pietro Giuria 7, I-10125, Turin, Italy
3
Associazione Micromineralogica Italiana, via San Pietro 55, I-10073 Devesi-Cirié, Turin, Italy
4
Dipartimento di Geoscienze, Università degli Studi di Padova, via Giovanni Gradenigo 6, I-35131 Padova, Italy
5
Dipartimento di Scienze Geologiche, Università degli Studi Roma Tre, largo San Leonardo Murialdo 1,
I-00146 Rome, Italy
6
EEEP house, UNIR, Basing View, Basingstoke, Hampshire RG21 4YY, UK[Received 15 May 2014; Accepted 27 July 2014; Associate Editor: S. J. Mills]
ABSTRACTThe new mineral species braccoite, ideally NaMn2+
5 [Si5AsO17(OH)](OH), has been discovered in the Valletta
mine dumps, in Maira Valley, Cuneo province, Piedmont, Italy. Its origin is probably related to the reaction
between ore minerals and hydrothermal ﬂuids. It occurs as subhedral crystals in brown-red coloured thinmasses, with a pale-yellow streak and vitreous to resinous lustre. Braccoite is associated with tiragalloite, forwhich new data are provided, as well as gamagarite, hematite, manganberzeliite, palenzonaite, quartz,
saneroite, tokyoite, unidentiﬁed Mn oxides, organic compounds, and Mn arsenates and silicates under study.
Braccoite is biaxial positive with refractive indices a = 1.749(1), b = 1.750(1), g = 1.760(1). It is triclinic,
space group P1̄, with a = 9.7354(4), b = 9.9572(3), c = 9.0657(3) Å, a = 92.691(2), b = 117.057(4), g =105.323(3)º, V = 740.37(4) Å3 and Z = 2. Its calculated density is 3.56 g/cm3. The ten strongest diffractionlines of the observed powder X-ray diffraction (XRD) pattern are [d in Å, (I), (hkl)]: 3.055 (69)(22̄1), 3.042(43)(102), 3.012 (65)(32̄1̄), 2.985 (55)(23̄1̄), 2.825 (100)(213̄), 2.708 (92)(220), 2.627 (43)(23̄2̄), 2.381(58)(41̄1̄), 2.226 (25)(214̄) and 1.680 (433̄)(36). Chemical analyses by wavelength-dispersive spectroscopy
electron microprobe gave (wt.%): Na2O 4.06, CaO 0.05, MnO 41.76, MgO 0.96, Al2O3 0.04, CuO 0.02, SiO2
39.73, As2O5 6.87, V2O5 1.43, SO3 0.01 and F 0.04. H2O 2.20 was calculated on the basis of 2OH groups
p.f.u. Raman spectroscopy conﬁrmed the presence of (SiO4)4 , (AsO4)3 and OH groups. The empiricalformula, calculated on the basis of S cations-(Na,K) = 11 p.f.u., in agreement with the results of the crystal
3+
3+
5+
structure, is Na1.06(Mn2+
4.46Mn0.32Mg0.19V0.01Al0.01Ca0.01)[Si5(As0.48Si0.37V0.15)O17(OH)](OH0.98F0.02); thesimpliﬁed formula is Na(Mn,Mg,Al,Ca)5[Si5(As,V,Si)O17(OH)](OH,F).
Single-crystal XRD allowed the structure to be solved by direct methods and revealed that braccoite is the
As-dominant analogue of saneroite. The structure model was reﬁned on the basis of 4389 observed
reﬂections to R1 = 3.47%. Braccoite is named in honour of Dr Roberto Bracco (b. 1959), a systematic
minerals collector with a special interest in manganese minerals. The new mineral was approved by theInternational Mineralogical Association Commission on New Minerals, Nomenclature and Classiﬁcation(IMA 2013-093).

* E-mail: fernando.camaraartigas@unito.it
DOI: 10.1180/minmag.2015.079.1.14# 2015 The Mineralogical Society

FERNANDO CÁMARA ET AL.

K EY WORDS : braccoite, saneroite, arseno-silicates, tiragalloite, new mineral species, crystal structure, Raman,
Valletta, Piedmont, Italy.

Introduction

The rock hosting braccoite is compact,
granular, dark red verging on black quartzite.
Blocks of this material have been dug up andpiled up in a small landﬁll where they have been
mixed with calcareous rocks also from the
excavated material.
Braccoite is strictly associated with tiragalloite,
and with gamagarite, hematite, manganberzeliite,
palenzonaite, quartz, saneroite, tokyoite, unidentiﬁed Mn oxides, organic compounds, and Mn
arsenates and silicates under study. These ﬁndings
make, in terms of mineralogical variety, the small
dump of the old Valletta mine one of the richest
Italian deposits of arsenate and silicoarsenate
mineral phases, similar to those of Graveglia
Valley (Antoﬁlli et al. 1983; Borgo andPalenzona, 1988; Palenzona, 1991, 1996;
Marchesini and Pagano, 2001). Other As-richminerals found in the rock samples collected in
the dump, although not strictly associated withbraccoite are: adelite CaMg(AsO4)(OH), arseniopleite-caryinite series (Ca,Na)NaMn2+(Mn 2 + ,Mg,Fe 2 + ) 2 (AsO 4 ) 3 –(Na,Pb)(Ca,Na)C a M n 22 + ( A s O 4 ) 3 , b a r io p h a r m a c o s i d e r i t e
Ba0.5Al4(AsO4)3(OH)4·4H2O, berzeliite
NaCa2Mg2(AsO4)3, grandaite Sr2Al(AsO4)2(OH)(IMA2013-059) and tilasite CaMg(AsO4)F; theseare found along with aegirine, albite, azurite,
baryte, braunite, calcite, diopside, ﬂuorapatite,
ganophyllite, gypsum, ilmenite, hollandite, malachite, magnesio-arfvedsonite, magnesioriebeckite, magnetite, mimetite, muscovite, neotocite, opal, orthoclase, phlogopite, ranciéite,richterite, rutile, rhodonite, talc, tetrahedrite,titanite and some other unknown phases underinvestigation.

THIS is the second in a series of new-mineraldescriptions of As-bearing minerals from Valletta
mine (Cámara et al., 2014). The sample
containing braccoite, the As-analogue of saneroite, was collected by one of the authors (MM) in2012 in the dumps of Valletta mine, Vallone della
Valletta, Canosio municipality, Maira Valley,
Cuneo province, Piedmont, Italy (44º23’54’’N,
7º5’42’’E, 2562 m asl).
The name is in honour of Dr Roberto Bracco(b. 1959), a systematic minerals collector with a
special interest in manganese minerals (Barresi etal., 2005; Bracco and Balestra, 2014). Dr Bracco
has authored or coauthored several publications on
systematic mineralogy, especially those devoted to
new occurrences in Liguria (Bracco et al., 2006;2012). The new mineral was approved by theInternational Mineralogical Association
Commission on New Minerals, Nomenclature
and Classiﬁcation (IMA 2013-093).
A fragment of the holotype material has been
deposited in the mineralogical collections of theMuseo Regionale di Scienze Naturali di Torino,
Sezione di Mineralogia, Petrograﬁa e Geologia,
Torino, Italy, catalogue number M/15939.
Braccoite is intergrown with tiragalloite, which
is a rare mineral. For this reason the chemical and
Raman spectrum of tiragalloite
5+[Mn2+
4 As Si3O12(OH)] from Valletta mine isalso provided.
Geological setting and mineral occurrence
Brief geological and historical information wasprovided in Cámara et al. (2014). The deposit atValletta mine has never been studied from agenetic point of view and available geological
data for the area provide limited detail. Other thanthe historical texts, there is no mention in theliterature of the occurrence of metalliferous
mineralization in this locality. Preliminary work
carried out during sampling showed that it is asmall iron deposit with subordinate manganese, inquartzites with quartz veins that contain a large
variety of mineral phases rich in arsenic,
vanadium, barium and strontium. The volume of
the mineralized body is, however, rather limited at
the surface.

Mineralogical characterization
Appearance and physical properties
Braccoite occurs as subhedral equant crystals, a
few hundred microns across, with unevenfracture, grouped in thin masses, a few centimetres in size (Fig. 1), on granular red-brownquartzite with reddish-brownish-black K-feldspar
and compact quartz. In rare cases the mineralforms rims around the remnants of protolithic
quartz clasts. Individual crystals are brown-red incolour and translucent. Braccoite has a paleyellow streak, a vitreous to resinous lustre and172

BRACCOITE, AN As-BEARING NEW MINERAL FROM PIEDMONT, ITALY

FIG. 1. (a) The rocks containing braccoite; (b) rare red crystalline masses with brown hue of braccoite holotype
intergrown with orange tiragalloite forming a thin layer on hematite and quartz (ﬁeld of view: 5 mm). Photo:
R. Bracco.

of ﬁve analyses are given in Table 1a. Low totals
are related to the difﬁculty with preparing good thinsections of polymineralic aggregates, and have also
been reported for saneroite samples (Nagashima
and Armbruster, 2010a).
The empirical formula, calculated on the basisof 19 O atoms per formula unit (a.p.f.u.) andconsidering 2(OH) is, within rounding errors,
N a 1 . 0 6 ( M n 24 .+4 6 M n 30 .+3 2 M g 0 . 1 9 A l 0 . 0 1
Ca 0.01 ) S4.99 [(Si 5.36 As 0.48 V 0.15 ) S5.99 O 17 (OH)](OH 0.98 F 0.02 ). Alternatively, the empiricalformula, calculated on the basis of S cations(Na,K) = 11, Mn2+/Mn3+ratio calculated in order toobtain [2(OH) (Na 0.5)] groups p.f.u. [Mn3+/(total Mn) = 0.066] and tetrahedral V5+ calculated
as 6 (Si+As), and with excess V assigned to thedoes not ﬂuoresce under shortwave or longwave
ultraviolet light. Braccoite is optically biaxial
positive, with a 2Vmeas = 26(2)º and 2Vcalc = 35º.
The measured refractive indices are a = 1.749(1),b = 1.750(1) and g = 1.760(1) (589 nm). Braccoite
is weakly pleochroic with X = brownish yellow,
Y = dark yellow, Z = yellow. The mineral is brittle
and no cleavage or parting is observed. Hardness
and density were not measured due to the small
crystal size and because it occurs intimatelyintergrown with tiragalloite. The calculated
density obtained from the empirical formula andunit-cell parameters of the single crystal used forthe crystal-structure determination is 3.56 g/cm3.
Chemical data
The chemical composition of braccoite wasdetermined using a Cameca SX-50 electron microprobe (wavelength-dispersive spectroscopy mode,
WDS) at the Department of Geosciences(Università di Padova) on a thin section obtained
from the holotype close to the place where thecrystal used for the diffraction study was extracted.
Major and minor elements were determined at
20 kV accelerating voltage and 20 nA beam
current (beam size 2 mm), with 40 to 20 s counting
time on both peak and background. X-ray counts
were converted to oxide wt.% using the ‘PAP’correction program supplied by Cameca (Pouchou
and Pichoir, 1984; 1985). The crystals studied in
the thin section (Fig. 2) were found to behomogeneous. Fe, Sb and Pb were analysed but
were below detection limits. H2O was calculated on
the basis of 2OH groups per formula unit (p.f.u.)(Nagashima and Armbruster, 2010a). The averages

FIG. 2. Backscattered electron image of a section of aquartz (qtz) vein showing braccoite (brac) and tiragalloite (tirag) used during the WDS analyses. The small
spot within quartz is baryte (bary).

173

FERNANDO CÁMARA ET AL.

TABLE 1a. Chemical data for braccoite (ﬁve analytical points).
Wt.%
Na2O
CaO
MgO
MnO
Mn2O3***
Al2O3
CuO
SiO2
As2O5
V2O**
5
SO3
F
H2O*
O = F
Total

Range

4.06
0.05
0.96
41.76
3.07
0.04
0.02
39.73
6.87
1.43
0.01
0.04
2.20
0.02
97.44

3.72
0.03
0.90
40.94
2.55
0.01
0.01
38.70
6.10
1.35
0.01
0.00
2.12
0.08
96.85

4.22
0.06
1.01
42.46
3.87
0.12
0.04
40.21
7.79
1.61
0.02
0.19
2.24
0.00
98.26

SD

Probe standard (line)

0.20
0.01
0.05
0.41
0.53
0.04
0.01
0.59
0.61
0.11
0.01
0.00

albite Amelia (NaKa)
diopside (CaKa)
synthetic periclase (MgKa)
MnTiO3 (MnKa)
corundum (AlKa)
metallic Cu (CuKa)
diopside (SiKa)
synthetic AsGa (AsLa)
vanadinite (VKa)sphalerite (SKa)fluorite (FKa)* H2O calculated in order to have 2(OH) p.f.u.
** total V is reported as V2O5 but tetrahedral V5+ is calculated as 6 (Si + As) and excess V is assigned to theoctahedral sites as V3+, following Nagashima and Ambruster (2010a)*** Mn2+/Mn3+ ratio calculated (Mn3+/total Mn = 0.066) in order to obtain 2(OH) groups p.f.u. and V distributed asreported.

Table 1b. Comparison of chemical data available for tiragalloite from other localities.
Wt.%

Valletta mine, Italy(1)

Molinello mine, Italy(2)

Ködnitz Valley, Austria
(3)

As2O5
V2O5
Sb2O5
SiO2
TiO2
Al2O3
FeO
MnO
CaO
MgO
PbO
SO3
Na2O
F
O=F
Total

16.91
0.59
0.01
31.45

16.07
1.67

18.35

32.38

31.91
0.02
0.02
0.56
46.02
0.75
0.00

0.17
48.34
0.75

46.88
0.27
0.39
0.04
0.03
0.01
0.11
0.05
96.59

0.03

99.38

97.66

(1) This work (average of three analytical points); (2) Gramaccioli et al. (1980); (3) Albrecht (1990).

octahedral sites as V3+, following Nagashima andArmbruster (2010a), within rounding errors, is3+
3+
Na1.06(Mn2+
4.46Mn0.32Mg0.19V0.01Al0.01Ca0.01)S=5.00
174

5+[Si5.37As 5+
0.48 V 0.15 O 17 (OH)](OH 0.98 F 0.01 ). The
simpliﬁed formula can be written as:
NaMn2+
5 [Si 5AsO 17 (OH)](OH), which requires

BRACCOITE, AN As-BEARING NEW MINERAL FROM PIEDMONT, ITALYNa2O 3.78, MnO 43.31, SiO2 36.68, As2O5 14.03
and H2O 2.20, total 100 wt.%. The presence of OH
was conﬁrmed by micro-Raman spectroscopy. The
mean refractive index n of braccoite, the calculated
density and the empirical formula yielded a
Gladstone-Dale compatibility index (Mandarino
1979, 1981) of 0.020, rated as excellent.
Braccoite is unreactive and insoluble in 2 M and
10% HCl and 65% HNO3.
The chemical data of tiragalloite
5+[Mn2+
4 As Si3O12(OH)] from Valletta mine andtiragalloite from the type-locality of Molinello
mine (Ne, Graveglia Valley, Liguria, Italy)
reported by Gramaccioli et al. (1980) arecompared in Table 1b. Considering a stoichiometric H2O content in order to have one (OH)group p.f.u., i.e. 1.46 wt.% H2O, and 13 oxygena.p.f.u., the formula corresponding to the averageof three analyses is (Mn2+
3.92Mg0.06Na0.03)S4.01
5+
4+
(As5+
0.87V0.05Si0.09)S1.01Si3O12(OH0.96F0.04).
Micro-Raman spectroscopy
The Raman spectrum of braccoite (Fig. 3) was
obtained at the Dipartimento di Scienze della
Terra (Università di Torino) using a micro/macro
Jobin Yvon LabRam HRVIS, equipped with amotorized x–y stage and an Olympus microscope.
The backscattered Raman signal was collectedwith 506 objective and the spectrum was
obtained for a non-oriented crystal. The
632.8 nm line of an He-Ne laser was used as

excitation; laser power (20 mW) was controlled bymeans of a series of density ﬁlters. The minimum
lateral and depth resolution was set to a few mm.
The 532 nm line of a Nd laser was also used as
excitation; laser power (80 mW) was dosed bymeans of a series of density ﬁlters. An aperture of
200 mm was used to reduce the beam dose. The
lateral and depth resolution were ~2 and 5 mm,
respectively. The system was calibrated using the520.6 cm 1 Raman band of silicon before eachexperimental session. Spectra were collected withmultiple acquisitions (2 to 6) with single counting
times ranging between 20 and 180 s. The spectrum
was recorded using the LabSpec 5 software
package (Horiba Jobin Yvon, 2004, 2005) from
200 to 4000 cm 1. Spectra collected with bothlasers were equivalent. The spectrum reported inFig. 3 was collected with the 632.8 nm line of the
He-Ne laser.
There is a close match between the braccoite
spectrum and that of saneroite from the type
locality of Molinello mine (Graveglia Valley,
Liguria, Italy) in the RRUFF database (R060488)(Downs, 2006). All the bands observed between
700 and 1000 cm 1 are characteristic of the twogroups present in braccoite, SiO 44 andAsO 3 (OH ) 2 (Myneni et al., 1 9 98 a ,b;Nakamoto, 1986). The spectrum shows intense
bands at ~829, 907 and 932 (823, 909 and936 cm 1 for saneroite R060488 in the RRUFF
database, respectively, and weak peaks at 706 and748 cm 1 (700 and 729 cm 1 for saneroite

FIG. 3. Raman spectra of braccoite in the 200 4000 cm 1 region and (inset) between 200 and 1200 cm 1.

175

FERNANDO CÁMARA ET AL.

R060488). The intense peak at 1017 cm 1 with aweak shoulder at 1040 cm 1 may be assigned to
the n1 symmetric stretching mode of the SiO4
units (Mills et al., 2005) (1011 and 1022 cm 1 forsaneroite R060488) while the region assigned in
the pyroxenes to the stretching modes of the
Si O bonds is present in the braccoite spectrum
at 665 cm 1 (660 cm 1 for saneroite R060488).
Bending modes of O Si O are observed at
525 cm 1 and 563 cm 1 for braccoite, while theRaman spectrum of saneroite R060488 shows asingle weak band around 523 cm 1. Cation–oxygen vibration modes appear in the low regionof the spectrum below 460 cm 1: weak and broad
peaks are observed at 226, 261, 291, 360, 390 and
451 cm 1 (228, 281, 343, 376 and 436 cm 1 forsaneroite R060488, respectively). The Raman
spectrum of braccoite shows a broad envelopeof overlapping bands centred upon 3361 and
3507 cm 1, which are characteristic of OH
stretching modes, in accordance with the presenceof hydroxyl groups in the structure (the spectrumof saneroite R060488 was only collected for<1200 cm 1).
Tiragalloite is intergrown with braccoite inrocks from Valletta mine. There is no available
Raman spectrum for tiragalloite and therefore aspectrum was also collected for this mineralphase (Fig. 4). The spectrum shows a strong
absorption centred at 869 cm 1 with threeshoulders at 803, 836 and 902 cm 1, twointense peaks at 661 and 647 cm 1, weakerpeaks at 960, 975 cm 1, and a broad band at~1004 cm 1. As for braccoite and saneroite, thefrequency separations between the bands due tothe asymmetric and the symmetric stretches of
the anionic groups (SiO4)4 and (AsO4)3 intiragalloite vary strongly from one structure toanother, and cannot be assigned with conviction(Hawthorne et al., 2013). Bands with frequencies between 250 and 600 cm 1 correspond to(SiO4)4 and (AsO4)3 vibrations (286, 320,
364, 398, 481, 508 and 549 cm 1), while weak
and broad bands <250 cm 1 correspond tolattice modes (153, 181 and 218 cm 1). In theregion between 1200 and 3000 cm 1 thespectrum displays a considerable amount ofnoise (a broad envelope of overlapping bands
centred upon 1635, 1702 and 1799 cm 1); thisis a result of the low intensity of the bands. In
accordance with the presence of hydroxyl
groups in the structure, a wide and weak band
was found at ~3100 cm 1 . Based on the
Libowitzky (1999) correlation, the band at~3100 cm 1 can be possibly assigned to the
O11 H(11)_O1 bond present in tiragalloite(O11_O1 = 2.725 Å corresponding to3257 cm 1, using crystal data provided by
Nagashima and Armbruster 2010b).
X-ray diffraction (XRD)
The powder XRD pattern of braccoite wasobtained at CrisDi (Interdepartmental Center for
Crystallography, Torino, Italy) using an Oxford
Gemini R Ultra diffractometer equipped with aFIG. 4. Raman spectra of tiragalloite in the 150 4000 cm 1 region and (inset) between 150 and 1200 cm 1.

176

BRACCOITE, AN As-BEARING NEW MINERAL FROM PIEDMONT, ITALY

CCD area detector, with graphite-monochromatized MoKa radiation. Indexing of the reﬂections
was based on a calculated powder patternobtained from the structural model, using thesoftware LAZY PULVERIX (Yvon et al., 1977).
Experimental and calculated data are reported inTable 2. The unit-cell parameters reﬁned from thepowder data with the software GSAS (Larson and
Von Dreele, 1994) are a = 9.756(6), b = 9.961(7),c = 9.087(7) Å, a = 92.23(5), b = 117.27(5), g =105.21(4)º, V = 742.2(9) Å3.
Single-crystal XRD data were collected using
an Oxford Gemini R Ultra diffractometerequipped with a CCD area detector at CrisDi
with graphite-monochromatized MoKa radiation(l = 0.71073 Å). A crystal fragment showing
sharp optical extinction behaviour was used forcollecting intensity data. No crystal twinning wasobserved. Crystal data and experimental details
are reported in Table 3. The intensities of 7946
reﬂections with 13 < h < 14, 14 < k < 14, 13< l < 13 were collected to 64.4º2y using a 1ºframe and an integration time of 20 s. Data were
integrated and corrected for Lorentz and polarization background effects, using the package
CrysAlisPro (Agilent Technologies, Version
1.171.36.20, release 27-06-2012
CrysAlis171.36.24). Data were corrected for
empirical absorption using spherical harmonics,
implemented in the SCALE3 ABSPACK scaling
algorithm in CrysAlisPro. Reﬁnement of the unitcell parameters was based on 4389 measured
reﬂections with I > 10s(I). At room temperature,
the unit-cell parameters are a = 9.7354(4), b =9.9572(3), c = 9.0657(3) Å, a = 92.691(2), b =117.057(4), g = 105.323(3)º, V = 740.37(4) Å3,

TABLE 2. Observed and calculated powder XRD data for braccoite. The ten strongest reﬂections are reported inbold*.
hk ldobs (Å)

dcalc (Å)

Int.(obs)

Int.(calc)

h k ldobs (Å)

dcalc (Å)

Int.(obs)

Int.(calc)

1
0
2
2
1
0
2
2
0
1
1
1
1
1
2
1
1
3
2
2
1
2
2
1
1
3
2
0

1̄
2
2̄
1̄
1
2
2̄
1
1
2
3̄
2̄
1̄
3̄
2̄
3̄
0
2̄
3̄
3̄
0
1
2
1
3
0
3̄
1

4.785
4.723
3.850
3.836
3.785
3.763
3.741
3.522
3.438
3.337
3.310
3.212
3.143
3.055
3.055
3.054
3.042
3.012
2.998
2.985
2.974
2.825
2.708
2.699
2.673
2.655
2.627
2.433

4.798
4.710
3.842
3.820
3.767
3.741
3.748
3.516
3.420
3.341
3.308
3.192
3.147
3.042
3.064
3.063
3.037
3.010
3.002
2.979
2.967
2.822
2.696
2.687
2.661
2.647
2.614
2.422

8
8
7
21
7
16
9
8
10
19
8
9
19
6
69
17
43
65
6
55
8
100
92
6
20
12
43
15

7.8
10.2
6.1
7.6
6.5
14.7
7.3
1.6
1.1
10.0
8.1
3.5
27.2
2.4
55.4
18.2
15.2
26.9
4.7
31.5
4.7
100.0
72.7
10.0
10.8
17.8
29.4
18.1

2
0
4
0
2
2
0
3
1
2
3
1
3
5
3
1
4
4
4
3
0
4
0
5
3
6
1
6

2.393
2.388
2.381
2.361
2.283
2.226
2.218
2.204
2.186
2.185
2.091
2.082
2.067
1.779
1.738
1.693
1.680
1.680
1.655
1.595
1.595
1.545
1.537
1.495
1.485
1.440
1.434
1.434

2.399
2.388
2.378
2.355
2.271
2.224
2.223
2.202
2.181
2.172
2.084
2.084
2.060
1.773
1.732
1.694
1.683
1.676
1.648
1.599
1.586
1.542
1.540
1.488
1.480
1.436
1.431
1.434

9
22
58
11
12
25
20
21
6
8
9
10
12
7
7
9
24
36
14
7
13
7
6
9
7
6
15
15

2.3
6.1
18.7
2.9
14.8
13.6
11.3
24.8
4.1
4.5
5.5
14.1
13.9
6.5
7.0
5.5
15.3
25.9
14.7
5.7
13.7
6.1
5.1
7.8
2.9
2.4
13.2
12.7

1
0
1̄
2̄
1
1
0
2̄
2
2̄
0
2̄
2
1̄
1
1
2
1̄
0
1̄
3̄
3̄
0
2
0
3̄
2̄
3

2̄
4
1̄
4
2
1
4
4̄
0
3̄
4̄
2
1
1̄
1
2
5̄
3
2̄
3̄
5
0
3
3̄
1̄
0
0
3̄

2
1̄
1̄
0
1
4̄
2̄
1̄
4̄
3̄
2̄
4̄
1
4̄
2
5̄
0
3̄
5̄
3
2
2
5̄
5̄
6̄
5̄
5
0

* Only reﬂections with Irel > 6s(Irel) are listed; differences in observed and calculated intensities are related topreferred orientation.

177

FERNANDO CÁMARA ET AL.

TABLE 3. Crystal data and summary of parameters describing data collection and reﬁnement for braccoite.
Crystal system
Space group
Unit-cell dimensionsa (Å)b (Å)c (Å)
a (º)
b (º)
g (º)
V (Å3)Z
m (mm 1)F(000)
Dcalc (g cm 3)
Crystal size (mm)Radiation typey range for data collection (º)
Rint (%)
Reflections collected
Independent reflections
Fo > 4s(F)Refinement method
No. of refined parameters
Final Robs (%) all data
R1 (%) Fo > 4s(F)wR2 (%) Fo > 4s(F)Highest peak/deepest hole (e Å 3)
Gof on F2

Triclinic
P1̄
9.7354(4)9.9572(3)9.0657(3)92.691(2)117.057(4)105.323(3)740.37(4)2
5.62
758.783.56
0.2060.1560.17
MoKa (0.71073 Å)
3.5 32.3
3.13
18,0394911
4389
least-squares matrix: full
300
4.14
3.47
8.61
+0.81 / 0.66
1.191

space group P1̄ and Z = 2. The a:b:c ratio is0.978:1:0.910. A total of 4911 independent
reﬂections were collected and the structure wassolved and reﬁned using the SHELX set ofprograms (Sheldrick, 2008).
Description of the structure
Structure model
The crystal structure of braccoite (Fig. 5) is
topologically identical to that of the hydropyroxenoid saneroite. Both crystals have a ﬁve repeat
single isolated chain of SiO4 tetrahedra whichhave a sixth tetrahedron as an appendix. In thislatter tetrahedron the Si1 xAsx substitution occurs(Si1 xV5+
x in saneroite). Chains repeat laterally bya centre of symmetry forming a layer of tetrahedra
parallel to (11̄1). Five octahedral sites occupied
by Mn (mostly Mn2+, with some Mn3+) form aband which runs parallel to two single chains oftetrahedra attached above and below. Laterally
the bands are separated by channels occupied
partially by two independent Na sites, one
178

occupied completely and another with partial
occupation. The structure of braccoite was therefore reﬁned starting from the atom coordinates ofsaneroite excluding H sites (Nagashima andArmbruster, 2010a). The nomenclature of thesites therefore follows that of the aforementioned
authors. Scattering curves for neutral and ionizedatoms were taken from the International Tables
for Crystallography (Wilson, 1992). Site-scattering values were reﬁned for the cation sites
using two scattering curves contributing proportionally and a constrained sum to full occupancy:
Mn2+ and Mg were used for the sites Mn(1 5);
Si4+ full occupancy was ﬁxed at the T(1 5) sites,
while Si4+ and As were used at the T(6) site; Na+
was used for the Na(1) and Na(2) sites, although
the occupancy was held ﬁxed at Na(1) and reﬁned
at Na(2). After converging, the positions of two H
atoms [H(7) and H(19) sites] were located indifference Fourier maps and added to the model;
atom coordinates of H sites were reﬁned andisotropic thermal parameters were constrained tobe 1.2 times the isotropic equivalent of theBRACCOITE, AN As-BEARING NEW MINERAL FROM PIEDMONT, ITALY

FIG. 5. Detail of the braccoite structure showing the bands of Mn octahedra and the silicate chains. Blue: Si
tetrahedra; green: As Si tetrahedron; yellow: Mn octahedra; light blue: Na; white: H. Violet double arrow shows theshort Na(2)_H(7) distance. Approximate vector of projection is [51̄5]. Design obtained with VESTA 3 (Momma andIzumi, 2011).

oxygen atom of the hydroxyl group assuming ariding motion model, while a soft constraint of
0.98 Å (Franks, 1973) was applied to theH(19) O(19) distance. Structure reﬁnement
converged to R1 = 0.0347 for 4389 reﬂections
with Fo > 4s(Fo) and 0.0413 for all 4911 data.
Tables 4, 5 and 6 report atomic coordinates, thedisplacement parameters and selected bonddistances, and angles, respectively, for braccoite.
Bond-valence calculations using the parametersof Brown (1981) are reported in Table 7. The cifand structure factor list ﬁles have been deposited
with the Principal Editor of Mineralogical
Magazine and are available from http://www.
minersoc.org/pages/e_journals/
dep_mat_mm.html.
Site occupancies
Cation sites
There are 13 cation sites in the braccoite
structure: six sites are 4-coordinated, ﬁve sites are
6-coordinated and two are 8-coordinated. One of
the six 4-coordinated sites, the T(6) site, has alarger mean atomic number [24.17(6) electrons
per site (e.p.s.) vs. 14 e.p.s. for the other ﬁve sites,
Table 4], and <T O> is larger than that of theother ﬁve sites (1.675 Å vs. a mean of 1.624 Å for179

the other ﬁve sites, Table 6). Chemical analyses
report the presence of both V5+ and As5+ that canorder in a site with tetrahedral coordination. The
reﬁned site scattering is >23 e.p.s. and therefore
implies dominance of As in the presence of
sufﬁcient amounts of Si. The latter is conﬁrmed
by electron microprobe (EMP) analyses(Table 1a). In the presence of a concomitant
Si V As solid solution in a cation site withtetrahedral coordination, the size of the tetrahedron is not sufﬁcient to provide the actual
dominance of As5+ vs. V5+ because they havevery similar ionic radii (0.335 and 0.355 Å,respectively, Shannon, 1976). The distances
observed in the crystal studied (Table 6) arecompatible with a Si V substitution (values of
1.68 169 Å are usually found for saneroite,
Nagashima and Armbruster, 2010a,c; 1.70 Å formedaite, Nagashima and Armbruster, 2010b).
Note that, besides the chemical strain due to athree component solid solution, T(6) is not themost distorted 4-coordinated site in the structure.
The T(5) site shows the greatest angle variance(s2 = 41.72, computed according to Robinson etal. 1971, Table 6) as observed for saneroite (s2 =40.00, Basso and Della Giusta, 1980).
With regard to the ﬁve 6-coordinated sites, all
are Mn2+ dominant. However, site Mn(3) isFERNANDO CÁMARA ET AL.

TABLE 4. Multiplicities, fractional atom coordinates and equivalent isotropic displacement parameters (Å2) for
braccoite*.
Site occupancy
Na(1)
Na(2)
Mn(1)
Mn(2)
Mn(3)
Mn(4)
Mn(5)
T(1)
T(2)
T(3)
T(4)
T(5)
T(6)
O(1)
O(2)
O(3)
O(4)
O(5)
O(6)
O(7)
O(8)
O(9)
O(10)
O(11)
O(12)
O(13)
O(14)
O(15)
O(16)
O(17)
O(18)
O(19)
H(7)
H(19)**+

1 Na
0.521(6) Na+
0.953(5) Mn2+
0.047(5) Mg2+
0.917(5) Mn2+
0.083(5) Mg2+
0.843(5) Mn2+
0.157(5) Mg2+
0.944(5) Mn2+
0.056(5) Mg2+
0.953(5) Mn2+
0.047(5) Mg2+
1 Si4+
1 Si4+
1 Si4+
1 Si4+
1 Si4+
0.465(3) Si4+
0.535(3) As
1 O
1 O
1 O
1 O
1 O
1 O
1 O
1 O
1 O
1 O
1 O
1 O
1 O
1 O
1 O
1 O
1 O
1 O
1 O
1 H
1 H

x/a

y/b

z/c

Uiso


0.1912(3)00.5340(2)
0.4432(3)0.0300(5)0.0151(7)0.74388(5)0.97982(5)0.29542(6)0.01168(14)0.99723(5)0.21282(5)0.22004(6)0.01045(15)0.86298(5)0.88807(5)0.02950(6)0.00981(16)0.57398(5)0.66844(4)0.09774(5)0.00999(15)0.71912(5)0.55370(5)0.85346(6)0.01242(15)0.87449(9)0.03102(9)0.20746(9)0.47814(9)0.61959(9)0.27556(8)0.23894(8)0.54939(8)0.75745(8)0.07757(8)0.82439(10)0.60955(9)0.77372(10)0.73037(9)0.89560(9)0.00920(15)0.00861(15)0.00943(15)0.00925(15)0.00829(15)0.60697(5)0.71401(4)0.48355(5)0.00903(12)0.7048(2)0.8853(2)0.1110(2)0.3022(2)0.4987(2)0.4645(2)0.8556(3)0.9645(2)0.9535(2)0.8287(2)0.0805(2)0.6649(2)0.3746(2)0.4961(2)0.2404(2)0.6809(3)0.5172(2)0.2568(3)0.0897(2)0.937(5)0.179(3)0.1654(2)0.2267(2)0.4094(2)0.6692(2)0.9265(2)0.7114(2)0.4331(2)0.7248(2)0.1532(2)0.8130(2)0.6058(2)0.4918(2)0.2765(2)0.1480(2)0.9490(2)0.8807(2)0.6042(2)0.3469(2)0.0502(2)0.493(4)0.050(4)0.7947(3)0.6567(3)0.6204(3)0.7062(3)0.7529(3)0.5465(3)0.8222(3)0.0081(3)0.4220(2)0.2469(2)0.8035(3)0.0594(2)0.1181(2)0.9102(3)0.9367(2)0.4750(3)0.2977(3)0.3699(3)0.1728(3)0.820(5)0.276(3)0.0132(4)0.0145(4)0.0133(4)0.0124(4)0.0128(4)0.0142(4)0.0144(4)0.0118(4)0.0123(4)0.0110(4)0.0143(4)0.0114(4)0.0113(4)0.0127(4)0.0112(4)0.0148(4)0.0137(4)0.0149(4)0.0134(4)0.017***
0.016**** The temperature factor has the form exp( T) where T = 8 (p2)U(sin(y)/l)2 for isotropic atoms.
**Atom coordinates reﬁned with a soft constraint to O H = 0.98 Å.
***Uiso reﬁned and constrained to be 1.2, the isotropic equivalent of the oxygen atom of the hydroxyl group.

signiﬁcantly smaller (2.183 Å vs. 2.20 2.25 Å,Table 6). This can be interpreted as ordering of alighter and smaller Mg cation, which is present in
the chemical analyses. Yet ordering all the Mg at
the Mn(3) site would require a site-scatteringvalue smaller than that observed. On the other180

hand, a small quantity of Mn3+ has been inferredin the chemical formula (see Chemical data
section) in order to achieve charge balance,
assuming full occupancy of H at the H(7) andH(19) sites. In addition, Nagashima andArmbruster (2010a) conﬁrmed the presence of a

BRACCOITE, AN As-BEARING NEW MINERAL FROM PIEDMONT, ITALYTABLE 5. Anisotropic displacement parameters for braccoite (Å)*.

Na(1)
Na(2)
Mn(1)
Mn(2)
Mn(3)
Mn(4)
Mn(5)
T(1)
T(2)
T(3)
T(4)
T(5)
T(6)
O(1)
O(2)
O(3)
O(4)
O(5)
O(6)
O(7)
O(8)
O(9)
O(10)
O(11)
O(12)
O(13)
O(14)
O(15)
O(16)
O(17)
O(18)
O(19)

U11

U22

U33

U12

U13

U23

0.0487(13)0.0233(13)0.0098(2)0.0103(2)0.0098(2)0.0093(2)0.0125(2)0.0087(3)0.0085(3)0.0086(3)0.0097(3)0.0081(3)0.0098(2)0.0107(9)0.0118(9)0.0159(10)0.0112(9)0.0138(9)0.0129(9)0.0134(10)0.0112(9)0.0137(9)0.0107(9)0.0148(10)0.0115(9)0.0128(9)0.0128(9)0.0115(9)0.0175(10)0.0142(9)0.0133(9)0.0125(9)0.0380(12)0.0130(12)0.0107(2)0.0107(2)0.0093(2)0.0091(2)0.0113(2)0.0102(3)0.0092(3)0.0088(3)0.0090(3)0.0091(3)0.00979(19)0.0153(9)0.0220(10)0.0102(9)0.0131(9)0.0087(8)0.0199(10)0.0108(9)0.0132(9)0.0134(9)0.0121(9)0.0140(9)0.0114(9)0.0120(9)0.0140(9)0.0110(9)0.0130(9)0.0142(9)0.0159(10)0.0153(9)0.0215(10)0.0087(11)0.0128(2)0.0104(2)0.0112(2)0.0111(2)0.0153(2)0.0101(3)0.0079(3)0.0110(3)0.0093(3)0.0086(3)0.00859(19)0.0142(9)0.0113(9)0.0104(9)0.0129(9)0.0121(9)0.0099(9)0.0214(11)0.0112(9)0.0087(9)0.0091(9)0.0200(10)0.0103(9)0.0103(9)0.0142(9)0.0104(9)0.0136(10)0.0105(9)0.0165(10)0.0112(9)0.0280(10)0.0032(9)0.00149(16)0.00265(16)0.00304(17)0.00244(16)0.00397(17)0.0031(3)0.0025(3)0.0021(3)0.0025(3)0.0029(3)0.00275(14)0.0024(7)0.0053(8)0.0016(7)0.0020(7)0.0012(7)0.0037(8)0.0042(8)0.0056(7)0.0022(7)0.0040(7)0.0063(8)0.0035(7)0.0051(7)0.0062(7)0.0034(7)0.0031(8)0.0017(8)0.0068(8)0.0049(8)0.0230(10)0.0090(10)0.00516(17)0.00545(17)0.00566(18)0.00485(17)0.00813(18)0.0055(3)0.0039(3)0.0050(3)0.0049(3)0.0044(3)0.00542(15)0.0072(8)0.0069(8)0.0049(8)0.0065(7)0.0045(8)0.0064(8)0.0101(8)0.0046(7)0.0056(7)0.0036(7)0.0120(8)0.0044(7)0.0059(7)0.0078(8)0.0048(7)0.0081(8)0.0060(8)0.0066(8)0.0044(8)0.0175(9)0.0023(8)0.00090(16)0.00342(16)0.00310(16)0.00261(15)0.00284(16)0.0040(3)0.0027(3)0.0039(3)0.0038(3)0.0038(3)0.00280(13)0.0071(8)0.0043(8)0.0020(7)0.0052(7)0.0036(7)0.0032(8)0.0058(8)0.0034(7)0.0019(7)0.0023(7)0.0069(8)0.0038(7)0.0040(7)0.0047(7)0.0044(7)0.0048(8)0.0003(7)0.0069(8)0.0034(7)* The temperature factor has the form exp( T) where T = 2p2Sij(h(i)h(j)U(i,j)a6(i)a6(j)).

limited quantity of Mn3+ in saneroite from
Molinello (Graveglia Valley, Italy) by using theratio of the X-ray intensities of the MnLb and
MnLa lines after the method of Albee and Chodos(1970) and Kimura and Akasaka (1999).
Therefore, a limite d amount o f Mn 3 +(0.066Mn3+/Mntotal) was also assumed for braccoite. Incidentally, other Mn3+ phases have beenfound at the Valletta mine (e.g. grandaite, Cámara
et al., 2014) and all the Fe-bearing phases containFe3+ only. Because there is no large bond-valence
contribution to the Mn(3) site (Table 7) it isprobable that Mn3+ is also distributed in the othertwo smaller sites, Mn(2) and Mn(4). In the
structure of braccoite there is also one octahedronthat is slightly larger than the others, the Mn(1)181

site. It would appear that this site should host thevery small amount of Ca reported in the analyses,
although that amount is not enough to justify theobserved size enlargement. However, Ca could
also be distributed at the Na(1) or Na(2) sites. The
Mn(1) site is also more distorted (s2 = 170.51,
compared to values ranging between 55.18 and86.77, Table 6). This is possibly due to the factthat the oxygen at O(16) acts as bond donor to theproton at H(19) and that it is the only octahedronthat shares an edge with a tetrahedron, the T(5)site, which is also the most distorted tetrahedron.
The O(5) O(14) edge involves two anion sites
with among the highest and the lowest bondvalence contribution, respectively (Table 7), andis also the shortest. Hence there is a possible

182

2.285(3)2.292(3)2.293(3)2.355(3)2.484(3)2.513(3)2.753(3)2.944(3)2.490
25.2102.069(2)2.150(2)2.177(2)2.190(2)2.308(2)2.627(2)2.253
14.087170.515
1.0617

Na(2) O(18)
O(4)
O(7)
O(3)
O(6)
O(11)
O(2)
O(3)
<Na(2) O>
V (Å3)

Mn(1) O(9)
O(10)
O(16)
O(14)
O(15)
O(5)
<Mn(1) O>
V (Å3)s2 *l*2.119(2)2.136(2)2.161(2)2.189(2)2.222(2)2.346(2)2.196
13.71167.5451.02072.116(2)2.156(2)2.163(2)2.191(2)2.199(2)2.272(2)2.183
13.52355.1771.01742.107(2)2.190(2)2.199(2)2.220(2)2.246(2)2.287(2)2.208
13.93967.4781.0204

Mn(2) O(11)
O(9)
O(19)
O(15)
O(18)
O(8)
<Mn(2) O>
V (Å3)s2*l*Mn(3) O(19)
O(8)
O(19)
O(13)
O(15)
O(10)
<Mn(3) O>
V (Å3)s2*l*Mn(4) O(14)
O(12)
O(17)
O(10)
O(12)
O(13)
<Mn(4) O>
V (Å3)s2*
l*1.613(2)1.617(2)1.626(2)1.632(2)
1.622
2.178
15.7611.00381.596(2)1.626(2)1.635(2)1.647(2)
1.626
2.200
9.140
1.0021

T(2) O(9)
O(10)
O(2)
O(3)
<T(2) O>
V (Å3)s2 *l*2.108(2)2.159(2)2.178(2)2.231(2)2.270(2)2.374(2)2.220
14.06686.772
1.0260T(1) O(8)
O(1)
O(7)
O(2)
T(1) O>
V (Å3)s2 *l*Mn(5) O(7)
O(17)
O(13)
O(12)
O(8)
O(18)
<Mn(5) O>
V(Å3)s2 *l*T(5) O(14)
O(15)
O(1)
O(5)
<T(5) O>
V (Å3)s2 *l*T(4) O(13)
O(4)
O(5)
O(6)
<T(4) O>
V (Å3)s2 *l*T(3) O(11)
O(12)
O(4)
O(3)
<T(3) O>
V (Å3)s2 *l*1.592(2)1.607(2)1.634(2)1.679(2)
1.628
2.183
41.722
1.00981.602(2)1.612(2)1.633(2)1.643(2)
1.623
2.181
14.0641.00330.86(4)1.62(4)2.48(1)
176.37(2)º1.646(2)1.667(2)1.670(2)1.719(2)
1.675
2.403
12.9451.0032

H(19) O(19)0.95(2)
H(19)_O(16)1.99(2)O(19)_O(16)2.855(10)O(19) H(19)...O(16) 150.34(18)ºH(7) O(7)H(7)_O(11)O(7)_O(11)O(7) H(7) _O(11)1.604(2) T(6) O(16)1.618(2)O(17)
1.628(2)O(18)
1.642(2)O(6)1.623<T(6) O>
2.188
V (Å3)7.846
s2*1.0019l** Mean quadratic elongation (l) and the angle variance (s2) were computed according to Robinson et al. (1971); ** V = polyhedral volume.

2.444(2)2.451(2)2.613(2)2.877(2)2.596
26.007

Na(1) O(5) (62)O(16) (62)O(1) (62)O(6) (62)<Na(1) O>**V (Å3)

TABLE 6. Main interatomic distances (Å) and geometrical parameters for braccoite.

FERNANDO CÁMARA ET AL.

183

4.000

F.C.* 4.000

4.000

4.000

3.999

1.056

1.026
0.972
0.946

T(4)

4.000

3.956

1.084
1.041

0.863

0.968

T(5)

4.630

4.609

1.240
1.173
1.167

1.029

T(6)

2.000

1.870

0.332
0.248
0.344

0.457
0.369

0.120

M(1)

2.070

1.975

0.353

0.302

0.328

0.393

0.223
0.376

M(2)

2.240

1.996

0.392
0.347

0.317

0.323

0.264

0.353

M(3)

2.030

1.917

0.323

0.331
0.287
0.260
0.410

0.306

M(4)

0.047
0.116
0.030
0.134

Na(2)

0.166 62;2.000 1.000

0.560

0.763

0.136

0.081

0.133

0.169 62;
0.063 62; 0.086

0.115 62;Na(1)1.888 1.026

0.358
0.211

0.341

0.298

0.271

0.409

M(5)

1.274
0.950
1.000

0.255
0.475

1.019
0.475

H(7)

Anion sites coordination reported only for coordination other than 3. *F.C: = formal charge at site on the basis of chemical formula.

3.963

4.000

O(19)

3.994

1.010

0.985

0.949

O(12)

1.073
0.988

0.937

0.965

T(3)

1.050

1.024

0.988

1.013
0.975

T(2)

O(11)

O(8)
O(9)
O(10)

O(7)

O(13)
O(14)
[IV]
O(15)
[IV]
O(16)
O(17)
[IV]
O(18)

[IV]

[IV]

[IV]

[IV]

[IV]

O(4)
[IV]
O(5)
[IV]
O(6)

O(3)

O(1)
O(2)

T(1)

TABLE 7. Bond-valence calculations for braccoite (Brown, 1981).

1.000

0.986

0.824

0.162

H(19)

37.96

1.091

1.980
1.826
1.934
1.749
1.855
1.815

1.927

1.524

1.871
1.906
1.928

1.531

2.145
2.123
2.124

2.032

2.096
1.987

37.38

1.915

1.911

1.779
1.999

2.550
2.006

+ H
contrib.

BRACCOITE, AN As-BEARING NEW MINERAL FROM PIEDMONT, ITALYFERNANDO CÁMARA ET AL.

charge-shielding mechanism operated by theelectronic clouds of both oxygen atoms.
The 8-coordinated sites host Na atoms. The
Na(1) site has full occupancy and bond distances
are compatible with 1 a.p.f.u. of Na (Table 8),
while the Na(2) site shows a reﬁned site scattering
which indicates approximately half occupancy ofNa (Table 4). This site shares four edges with four
Si tetrahedra and two edges with two Mn
octahedra [Mn(2) and Mn(5)]. This is probably
impeding the full occupancy of this site andproduces a rather distorted bonding environment.
Taking into consideration the observed sitescattering values and those obtained from EMP
analyses, the agreement for all cations sites iswithin 2% relative error, with slightly lighter values
from diffraction data than obtained from chemical
analyses (230.8 electrons per formula unit, e.p.f.u.,
vs. 233.9 e.p.f.u., respectively). Site-distribution
according to the structure reﬁnement (site scatteringand bond distances) and electron microprobe data
results give full occupancy of Si at the T(1 5) sites,5+2+
T6 (As5+
0.48Si0.37V0.15), Mn1 (Mn0.98Mg0.01Ca0.01),
3+
3+
Mn2 (Mn2+
Mn
Mg
),
Mn3
(Mn2+
0.06
0.87
0.07
0.66Mn0.22
3+
2+
3+
V0.01Al0.01Mg0.10), Mn4 (Mn0.96Mn0.03Mg0.01),
Mn5 (Mn2+
0.99Mg0.01), Na1 (Na1.00) and Na2(Na0.56), with an overall positive charge of 36.53.
The agreement between observed values and those
calculated from chemical composition after site
assignment is reported in Table 8.

Anion sites
There are 19 anion sites in the structure of
braccoite, ten are 3-coordinated and the rest are 4coordinated (Table 7). There are three anion sites
with a bond-valence incidence signiﬁcantly
greater than 2 vu: O(4), O(5) and O(6). The
same atoms show also high bond-valence
incidence for saneroite (Basso and Della Giusta,
1980; Nagashima and Armbruster, 2010a), in
particular O(4), which is 3-coordinated; at theO(4) site, the contribution from T(3) and T(4) isalready 2.011 vu and therefore the contribution
from the Na(2) site (0.134 vu, Table 7) oversaturates this anion site. This is in fact anotherstrong restriction preventing full occupancy of theNa(2) site (see above).
Two anion sites are actually 3-coordinated[O(11) and O(16)] but act as donors of two
respective H bonds at O(7) and O(19). Chemical
analyses show a very limited amount of ﬂuorine.
While it is not possible to assess in which site theﬂuorine orders, it is highly probable that it orders
at the O(19) site: this site receives a bond-valence
contribution of 1.091 vu (Table 7) and therefore
hosts an (OH) group, which belongs to threeoctahedra of two Mn(3) and one Mn(2) site. The
Raman spectrum in Fig. 3 shows a broadenvelope of overlapping bands centred upon
3361 and 3507 cm 1, which reﬂect the two
essential next-neighbour conﬁgurations:

TABLE 8. Reﬁned site-scattering and assigned site-populations for braccoite.
Site

Refined sitescattering(e.p.f.u.)

Mn(1)
Mn(2)
Mn(3)

24.39(7)23.93(7)22.96(7)

Mn(4)
Mn(5)
T(6)
Na(1)
Na(2)

24.28(7)24.39(7)24.17(6)11.00
5.72(6)

[IV]

O(19)

Calculated <X j>calc.*site-scattering(Å)(e.p.f.u.)

Assigned site-population(a.p.f.u.)
Cations
0.98 Mn2+ + 0.01 Mg + 0.01 Ca
0.87 Mn2+ + 0.07 Mn3+ + 0.06 Mg
0.56 Mn2+ + 0.32 Mn3+ + 0.10 Mg +0.01 V3+ + 0.01 Al3+
0.96 Mn2+ + 0.03 Mn3+ + 0.01 Mg
0.99 Mn2+ + 0.01 Mg
0.48 As + 0.37 Si + 0.15 V5+
1.00 Na
0.56 Na + 0.44 &<X j>obs.
(Å)

Ideal
composition(a.p.f.u.)

24.82
24.22
23.53

2.191
2.170
2.130

2.253
2.196
2.183

Mn2+
Mn2+
Mn2+

24.87
24.87
24.47
11.00
6.16

2.183
2.189
1.671
2.560
2.560

2.208
2.220
1.675
2.596
2.490

Mn2+
Mn2+
As
Na**
Na,o

Anions0.98 OH + 0.02 FOH

X = cation, j = O, OH, F* calculated by summing constituent ionic radii; values from Shannon (1976)** site in special position, half multiplicity.

184

BRACCOITE, AN As-BEARING NEW MINERAL FROM PIEDMONT, ITALY

Mn2+Mn2+Mn2+ and Mn3+Mn3+Mn2+, while otherconﬁgurations are also possible, i.e. MgMgMn2+,
Mn3+Mn3+Mn3+ or even MgMgMg, yielding
overall a broad band. Hydrogen bonding is alsopresent at the O(7) anion site. However, in thiscase, a short distance with another oxygen atom at
the O(11) anion site (2.48 Å) along with a similarbond-valence contribution, of 1.531 for O(7) and
1.524 valence units (vu) for O(11), is probably
responsible for a very strong H bond (see later).

2000) and for which the IR O H stretchingmode was found at 1386 cm 1 (Hammer et al.,1998). Another topologically related structure is5+scheuchzerite (NaMn2+
9 [Si9V O28(OH)](OH)3;
Brugger et al., 2006), which also has a verystrong H bond among O(26) and O(29) anion sites,at a distance of 2.35 Å (Brugger et al., 2006). In
this case a band is observed at 1466 cm 1 in the
FTIR spectrum, which can correspond to thestrong H bond. It should also be taken into accountthat the H(7) site is at a distance of 2.09(5) Å fromthe Na(2) site, which is not far of the Na H
distance in NaH (1.913 Å; Chen et al., 2005) andthis surely stresses the bonding environment of the
proton at around half of the H(7) sites.

Hydrogen bonding
Strong H bonding is present in the braccoite
structure as also observed in saneroite. A close
inspection of Table 7 shows that there are fouroxygen sites with bond-valence incidence <1.8 vu:
O(7), O(11), O(16) and O(19). There is one very
short acceptor donor distance corresponding to avery strong H bond [O(7)_O(11) = 2.48 Å,Table 6], and another longer distance corresponding to a medium-strength H bond(O(19)_O(16) = 2.855 Å, Table 6). Using therelation v (cm 1) = 3592 3046109·
exp( d(O_O)/0.1321) (Libowitzky, 1999),
bands should be expected at 1456 and3467 cm 1. While frequencies at ~3500 cm 1
are observed in the Raman spectrum of braccoite(Fig. 3) the expected band around 1400 cm 1 is
not visible in the spectrum. The positions of two H
atoms were observed in the Fourier-difference
maps at convergence and were added to the model.
In particular, the position observed for the H(7)atom shows a bond with oxygen at the O(7) anionsite with a short H(7)_O(11) distance of
1.62(4) Å. The position of the corresponding H
atom in saneroite was not detected by Basso andDella Giusta (1980) but was found with very
similar atom coordinates by Nagashima andArmbruster (2010a) (x = 0.937(5) y = 0.493(4)z = 0.820(5) for braccoite and x = 0.940(3) y =0.506(3) z = 0.815(4) for saneroite specimen 1 ofNagashima and Armbruster, 2010a, table 3), and aband at ~1400 cm 1 was observed in the Fouriertransform infrared spectrum (FT-IR) collected onsaneroite from Molinello by Brugger et al. (2006).
The fact that both O(7) and O(11) show anequivalent bond-valence contribution (Table 7),
suggests a plausible disordered environment forthis proton. Such a situation, with a disordered
position for H, has been observed in anotherrelated pyroxenoid structure, serandite(NaMn2[Si3O8(OH)]), which shows an O_O
distance of 2.464 2.468 Å (Jacobsen et al.,Related minerals

185

Braccoite, NaMn2+
5 [Si5AsO17(OH)](OH), is the
As-dominant analogue of saneroite,
5+
NaMn2+
5 [Si5V O17(OH)](OH) (Basso and Della
Giusta, 1980; Lucchetti et al., 1981; Nagashima
and Armbruster, 2010a). For the dominant cationin the T6 site Nagashima and Armbruster (2010a)
proposed adding a sufﬁx, i.e. ‘‘saneroite-(V)’’,‘‘saneroite-(Si)’’ and ‘‘saneroite-(As)’’. In the
recent IMA guidelines, Hatert et al. (2013) haveallowed the use of any other name conﬁrming that‘‘mineral names are chosen by the authors of newmineral species, according to guidelines established by Nickel & Grice (1998)’’. A new namewas chosen to avoid sufﬁxing saneroite so as topreserve in toto this ‘‘well-established name’’ andalso to meet with the preferences of the collectors’community.
Braccoite also has a structural similarity with
5+
scheuchzerite, NaMn2+
9 [Si 9 V O 28 (OH)](OH) 3(Brugger et al., 2006; Palenzona et al., 2006;
Roth, 2007): while saneroite/braccoite have asilicate single chain with ﬁve tetrahedra in therepeating unit
with an additional tetrahedron
branching sideways (Fig. 5)
scheuchzerite hasa chain that consists of the branched saneroite
chain with additional attached silicate tetrahedra,
forming ‘‘loops’’ (Brugger et al., 2006). These‘‘loops’’ are also present in a new Na-Mn
borosilicate, steedite NaMn2+
2 [Si3BO9(OH)](OH)(IMA2013-052), of which the crystal structure
resembles closely those of the serandite–pectolite
pyroxenoids and is also broadly similar to the
crystal structure of scheuchzerite (Haring and
McDonald, 2014).
Braccoite is the ﬁrst As member of thesaneroite family and a comparison of the186

Saneroite(2, 3)
NaMn2+
5 [Si5VO17(OH)](OH)
Triclinic
P1̄
9.741(5)9.974(7)9.108(5)92.70(4)117.11(4)105.30(4)744.162
0.977:1:0.913
3.47
3.51
3.06(s), 2.83(s), 2.70(s), 3.01(m),2.98(m), 2.62(m), 2.20(m)biaxial ( )bright orange
X = deep orange; Y = lemon-yellow;
Z = yellow-orange
n.d.
white
resinous to greasy
tabular-prismatic crystals

quartz, baryte, caryopilite,
ganophyllite, medaite, palenzonaite,
pyrobelonite, fianelite, parsettensite,rhodochrosite, kutnahorite, aegirine

Braccoite(1)

NaMn2+
5 [Si5AsO17(OH)](OH)
Triclinic
P1̄
9.7354(4)9.9572(3)9.0657(3)92.691(2)117.057(4)105.323(3)740.37(4)20.978:1:0.910
n.d.
3.56
3.774(30), 3.514(30), 3.042(60),3.005(60), 2.973(80), 2.821(100),
2.696(90), 2.620(30), 2.676(50),
1.673(30)biaxial (+)
brown-red
X = brownish yellow, Y = dark yellow,
Z = yellown.d.
pale-yellow
vitreous to resinous
subhedralaegirine, hematite, tiragalloite, quartz,
unidentified Mn oxides
and Mn silicates

NaMn2[Si3BO9](OH)2
Triclinic
P1̄
6.837(1)7.575(2)8.841(2)99.91(3)102.19(3)102.78(3)424.81(1)20.9026:1:1.1671
n.d.
3.104
8.454 (100), 7.234(39),3.331(83), 3.081(38),2.859(52), 2.823(80)Steedeite(5)

saneroite, tiragalloite

2 3yellow-orange
vitreousacicular and prismatic crystals

aegirine, analcime, catapleiite,
eudialyte, microcline
nepheline,
natrolite, pyrrhotite,sérandite,sodalite, thermonatrite

n.d.
white
vitreousacicular crystals

biaxial (+)
biaxialyellow-orange
pale pink to colourless
X = brown yellow; Y = pale yellow Not observed

NaMn2+
9 [Si9O25(OH)(VO3)](OH)
Triclinic
P1̄
9.831(5)10.107(5)13.855(7)86.222(10)73.383(9)71.987(9)1254.2(10)20.973:1:1.371
3.50(2)3.47
2.71(100), 3.09(80),7.91(70),8.68(50), 2.92(40),
3.22(40), 3.94(30), 4.83(30)Scheuchzerite(4)

Refs: (1) this work; (2) Lucchetti et al. (1981); (3) Nagashima and Armbruster (2010a); (4) Brugger et al. (2006); (5) Haring and McDonald (2014).

Association

Hardness (Mohs)
StreakLustreHabit and forms

Optical character
Colour
Pleochroism

FormulaCrystal system
Space groupa (Å)
b
c
a (º)
b
g
V (Å3)Z
Axial ratios (a:b:c)Dmeas (g cm 3)
Dcalc (g cm 3)
Strongest lines in the
powder pattern: dobs(Å)(I)

Reference

TABLE 9. Comparison of minerals related to braccoite. References are given in brackets.

FERNANDO CÁMARA ET AL.

BRACCOITE, AN As-BEARING NEW MINERAL FROM PIEDMONT, ITALYTABLE 10. Comparison of chemical data between saneroite from Molinello mine (Italy) and Fianel(Switzerland) and braccoite from Valleta (this work).
Saneroite (Molinello mine, Italy)1

SiO2
Al2O3
MnO
Mn2O3
MgO
CaO
Na2O
K2O
CuO
NiO
V2O5
As2O5
SO3
F
Total

Saneroite (Fianel,
Switzerland)1

Braccoite (Vallettamine, Italy)2

Specimen 1
Wt.%
SD

Specimen 2
Wt.%
SD

Wt.%

SD

Wt.%

SD

39.99
0.02
42.2
–
0.01
0.13
4.34
0
0.1
0.03
7.15
1.22
–
–
95.19

39.06
0.01
40.33
–
0.00
0.11
4.36
0.01
0.2
0.03
7.78
1.92
–
–
93.81

41.03
0.01
41.53
–
0.03
0.33
4.52
0.01
0.14
0.02
6.05
1.31
–
–
94.98

0.98
0.02
1.12
–
0.04
0.12
0.25
0.01
0.2
0.03
1.23
1.85
–
–

39.73
0.04
39.00
3.07
0.96
0.05
4.06
–
0.02
–
1.43
6.87
0.01
0.04
95.28

0.59
0.04
0.41
0.53
0.05
0.01
0.2
–
0.01
–
0.11
0.61
0.01
0

1.06
0
1.38
–
0.02
0.05
0.28
0.01
0.14
0.04
1.8
1.27
–
–

0.65
0.02
1.06
–
0.00
0.04
0.27
0.01
0.24
0.03
0.7
1.65
–
–

Refs: (1) Nagashima and Armbruster (2010a); (2) this work.

properties of the members is reported in Table 9along with other related structures. A comparisonof published analyses of saneroite-group minerals
is reported in Table 10. In the Strunz System(Strunz and Nickel, 2001) braccoite ﬁts into
subdivision 9.D.K, inosilicates with 5-periodic
single chains. Its equivalent synthetic compound
is not known.
Acknowledgements
The authors are indebted to Bruno Lombardo,
who passed away some days after the identiﬁcation of this new species, for his assessment
support in ﬁeldwork at the Valletta. Mariko
Nagashima, Gerald Giester, Peter Leverett andAssociate Editor Stuart Mills are thanked for theirconstructive comments on the manuscript. FC andEB thank MIUR and AMI for the co-funding of a
research contract for EB for the year 2013. Raul
Carampin (CNR-IGG, Padova, Italy) is thanked
for his support with the WDS analysis.
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