American Mineralogist, Volume 100, pages 608–614, 2015

Balestraite, KLi2VSi4O10O2, the first member of the mica group with octahedral V5+
Giovanni O. Lepore1, Luca Bindi1, Alberto Zanetti2, Marco E. Ciriotti3, Olaf Medenbach4,
and Paola Bonazzi1,*
Dipartimento di Scienze della Terra, Università di Firenze, Via La Pira 4, I-50121 Firenze, Italy2
Istituto di Geoscienze e Georisorse, CNR, UOS di Pavia, Via Ferrata 1, I-27100 Pavia, Italy
3
Associazione Micromineralogica Italiana, via San Pietro 55, I-10073 Devesi-Ciriè, Italy
4
Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, D-44780 Bochum, Germany
1

Abstract
A mica-group mineral characterized by a high V content and free of Al was found in the manganesiferous beds within the metacherts of the ophiolitic sequences at the Cerchiara mine, Eastern Liguria(Italy), in association with hematite, quartz, and calcite. Chemical and structural characterization
supported by Raman data defines this phase as a new mineral species, which is named balestraite after
Corrado Balestra, a prominent Italian amateur mineralogist. Balestraite, ideally KLi2V5+Si4O10O2, is
a 1M trioctahedral mica crystallizing in the C2 space group, with a = 5.2024(5), b = 8.9782(7), c =9.997(2) Å, b = 100.40(2)°, V = 459.3(1) Å3, Z = 2. The reduction of symmetry from the “ideal” spacegroup C2/m is related to the ordering of V at only one of the two pseudo-symmetric octahedral sites.
Vanadium forms very distorted octahedra with a [2+2+2] geometry characteristic of the valence state+5. The Li,V composition of the octahedral sheet, the pure tetrasilicic character of the tetrahedral sheet,and the anhydrous character produce unusual geometrical features for this mica. The occurrence of 5+as the dominant valence state of V and the virtually complete O2– → OH- substitution at the O4 site
indicate strongly oxidizing conditions of crystallization, which are consistent with balestraite occurringat the boundary between carbonate-bearing veins and hematite bands.
The new mineral and name were approved by the Commission on New Minerals, Nomenclature
and Classification, IMA (2013-080).
Keywords: Balestraite, new mineral, Li-mica, V-mica, Cerchiara mine, pentavalent vanadium

Introduction
Preliminary chemical analyses of a micaceous mineral found in
the manganesiferous beds within the metacherts of the ophiolitic
sequences at the Cerchiara mine, Eastern Liguria (Italy), showed
a composition suggesting a new member of the mica group. In
particular, data obtained via energy-dispersive spectrometry (EDS)
analyses on a polished section indicated an unusually high V content, and no other transition metals or Al. Muscovite and phlogopitewith high V contents have been occasionally reported [e.g., Pan andFleet (1992), Ankinovich et al. (2001), and Giuliani et al. (2008)].
Until now, however, the only known V-members of the mica groupwere roscoelite, ideally KV2[AlSi3O10(OH)2] (Brigatti et al. 2003),
and chernykhite, ideally BaV2[Al2Si2O10(OH)2] (Ankinovich et al.
1973). Thus, owing to the absence of Al, this mineral was worthyof further investigations.
The present paper reports the results of chemical, structural,and spectroscopic studies to define and describe this new mica,
which was named balestraite after Corrado Balestra (b. 1962), a
prominent Italian amateur mineralogist and an expert of Ligurian
minerals. The mineral and its name have been approved by theCommission on New Minerals, Nomenclature and Classification,
IMA (2013-080). The holotype material is deposited in the min* E-mail: paola.bonazzi@unifi.it
0003-004X/15/0203–608$05.00/DOI: http://dx.doi.org/10.2138/am-2015-4972

608

eralogical collections of the Museo di Storia Naturale, Università
di Firenze (Italy), under catalog number 3133/I.

Geological setting, occurrence, andparagenesis

Balestraite was found in the ore body of the Cerchiara mine(~44°11ʹ58″N, 9°42ʹ1″E), which belongs to the well-known Mn
district of Eastern Liguria named “Gambatesa district.” The ore
body is located near the base of chert sequences (“Diaspri diMonte Alpe” Formation) overlaying Jurassic ophiolites (Cortesogno et al. 1979; Lucchetti et al. 1988). The ore consists ofrhythmic interlaying of braunite-bearing metasediments (5–15
cm thick) and hematite-rich cherts. According to Cabella et al.
(1998), the primary Mn-oxide and hematite-rich cherts formedby fractionation from hydrothermally derived metalliferous
siliceous muds during turbiditic re-sedimentation; subsequently,
the sedimentary-diagenetic deposits were re-equilibrated under
prehnite-pumpellyite facies conditions leading to a braunite +quartz stable assemblage. During this stage, reactions triggeredby mobilized fluids along fractures produced Mn-silicate andMn-carbonate assemblages at the expense of braunite + quartz.
Successive decompressional tectonic evolution under decreasing P-T metamorphic conditions induced further concentrationof dispersed elements, such as Ba, Sr, As, and V, to allow thegenesis of a great variety of new and rare minerals in later exten-

LEPORE ET AL.: BALESTRAITE, THE FIRST MICA WITH OCTAHEDRAL V5+

sional fractures. The carbonate veins exhibit boundaries, which
are locally enriched in alkaline and alkaline-earth elements,
such as Li, Na, and Ba (Cabella et al. 1990). Sugilite (Cabella
et al. 1990), together with other unusual Li-rich phases such as
norrishite and nambulite and small amounts of vanadiferous
species like gamagarite, tokyoite, and pyrobelonite, have beenfound (Balestra 2006).
Balestraite was found at the boundary between carbonatebearing veins and hematite bands. The mineral forms a layered
cluster, about 2 × 1 cm in size, of randomly packed crystals incontact with quartz and calcite (Fig. 1).

Physical and optical properties
Balestraite exhibits a tabular morphology, and does not show
any inclusions of, or intergrowths with, other minerals. The
maximum grain size of balestraite is about 400 mm. The mineralis pale yellow in color, transparent with silky luster and does not
fluoresce under short- and long-wave ultraviolet light. It is brittle(Mohs hardness between 2½ and 3) with uneven fracture andshows a perfect {001} cleavage. The calculated density is 2.946
g/cm3, based on the empirical formula and the unit-cell dimensions determined by single-crystal X‑ray diffraction (see below).
Balestraite is biaxial, optically negative with a = 1.642(2), b= 1.664(2), and g = 1.676(2); the measured 2V is 84.4(2)° (2Vcalc= 72°), and it has a distinct dispersion (r < v). The mineral does
not show pleochroism and its color is pale gray in thin section.
Optical properties were measured using monochromatic Na light(l = 589 nm) at room temperature.

Experimental methods
X‑ray diffraction and structure refinement
Several platy crystals with variable lateral dimensions and thickness (volumes
ranging from 1 to 400 ×104 mm3, approximately) were mounted on a 8 mm diametercarbon fiber and examined on a CCD-equipped Oxford Diffraction Xcalibur 3 diffractometer to avoid twinned and/or deformed crystals. In general, crystal quality
was poor, with weak and very broad reflections, usually not sufficient even for
unit-cell determination. After dozens of trials, a crystal (30 × 220 × 250 mm) suitablefor full data collection, although far from optimal, was found. The refined unit-celldimensions are: a = 5.2024(5), b = 8.9782(7), c = 9.997(2) Å, b = 100.40(2)°, and V= 459.3(1) Å3, suggesting that the mineral is a 1M polytype. Intensity integration and609

standard Lorentz-polarization corrections were performed with the CrysAlis RED
software package (Oxford Diffraction 2006). The program ABSPACK in CrysAlis
RED (Oxford Diffraction 2006) was used for the absorption correction, and thestructure refinement was performed using the full matrix least-squares program
SHELXL-97 (Sheldrick 2008). Initial attempts to refine the structure in the ideal
space group C2/m did not produce convergence. Ordering of the octahedral cations
was considered with a decrease of symmetry to the C2 space group, starting fromatomic coordinates given by Brigatti et al. (2000) for polylithionite. The refinementin this subgroup led to an improvement of the R factor (with R1 decreasing from
0.261 to 0.223 for the isotropic model), without significant correlation between
parameters of the pseudo-mirror related atoms. Attempts to refine a Cm model,
which produces disorder of the octahedral cations, led to higher R factor (R1 = 0.240
for the isotropic model) with an unreliable isotropic thermal parameter for O4.
Scattering curves for ionized Li, O, Si, and V as well as Dfʹ, Df″ coefficientswere taken from Wilson and Prince (1999). The site occupancy factors were initiallyallowed to vary (Li+ vs. V5+) for the three independent M sites; M1 was foundto be fully occupied by Li and fixed accordingly. Refinement of the anisotropic
atomic displacement parameters was possible only for T1, T2, and M2 (nearly
totally occupied by V), whereas for M1 and M3 (nearly totally occupied by Li)
and the oxygen atoms an isotropic model was maintained. The final R1 indices of
0.127 [for 1302 reflections with Fo > 4s(Fo)] and 0.138 (for all 1587 independent
reflections) were considered acceptable given the quality of the crystal. The refined
Flack parameter (Parsons and Flack 2004) of 0.08(2) is consistent with a highly
asymmetrical distribution of the enantiomorphic components and indicates that theacentric model is correct. Details of the data collection and refinement are given in
Table 1, final atomic coordinates and isotropic displacement parameters are listed
in Table 2, and anisotropic displacement parameters are in Table 3. Table 41 lists
the observed and calculated structure factors. (CIF1 available.)
The diffraction rings from few grains of balestraite were collected with a CCDequipped diffractometer Xcalibur PX Ultra using CuKa radiation (50 kV and 40mA) and then converted into a conventional XRD pattern. The crystal-to-detector
distance was 7 cm. Data were processed using the CrysAlis software package
version 1.171.31.2 (Oxford Diffraction 2006) running on the Xcalibur PX control
PC. Observed and calculated X‑ray powder diffraction data are listed in Table 5.
The unit-cell parameters from powder data are: a = 5.2087(2), b = 8.9991(3), c =10.0304(4) Å, b = 100.354(3)°, and V = 462.51(2) Å3, in fair agreement with thevalues obtained from single-crystal data (Table 1).
Deposit item AM-15-24972, Table 4 and CIF. Deposit items are stored on the
MSA web site and available via the American Mineralogist Table of Contents. Find
the article in the table of contents at GSW (ammin.geoscienceworld.org) or MSA(www.minsocam.org), and then click on the deposit link.
1

Table 1.

Crystallographic data and refinement parameters forbalestraite

Ideal formula
Crystal system
Space group
Unit-cell parameters (Å; °)Unit-cell volume (Å3)
Z
Crystal size (mm)Crystal data

KLi2V5+Si4O12
monoclinic
C2
5.2024(5), 8.9782(7), 9.997(2); 100.40(2)459.3(1)20.030 × 0.220 × 0.250

Data collection
Diffractometer
Oxford Diffraction Xcalibur 3
Temperature (K)293(3)
Radiation, wavelength (Å)
MoKα, 0.71073
2q max for data collection (º)
63.98
Crystal-detector dist. (mm)50h, k, l ranges–7 to 7, –13 to 13, –14 to 14
Axis, frames, width (º), time per frame (s)w, 515, 1.00, 10
Total reflections collected14541
Unique reflections
1587
Unique reflections I > 3s(I)1302
Data completeness to qmax (%)
98.3
Absorption correction method
ABSPACK (Oxford Diffraction 2006)

Figure 1. Balestraite from Cerchiara mine associated with quartz
and calcite (R. Bracco photo); field of view is 2 mm. (Color online.)

Structure refinement
Refinement method
Full-matrix least-squares on F2
Data/restraints/parameters
1587/1/60
R1 [Fo > 2s(Fo)], wR2 [Fo > 2s(Fo)]0.127, 0.342
R1 all, wR2 all
0.138, 0.345
Goodness-of-fit on F2
1.025
–
3
Largest diff. peak and hole (e /Å )1.09, –1.48

610
Table 2.
Atom
K
M1
M2
M3
T1
T2
O1
O21
O22
O31
O32
O4

Table 3.

LEPORE ET AL.: BALESTRAITE, THE FIRST MICA WITH OCTAHEDRAL V5+
Atoms, site occupancy factors (s.o.f.), fractional atom coordinates, and equivalent isotropic displacement parameters(Å2) for balestraites.o.f.
K1.00
Li1.00V0.93(2)Li0.07
Li0.97(2)V0.03
Si1.00Si1.00
O1.00
O1.00
O1.00
O1.00
O1.00
O1.00

x/a
0
0
0
0
0.0766(6)0.5820(6)0.056(2)0.307(2)0.806(2)0.123(2)0.646(2)0.106(2)y/b0.5021(5)0.012(3)0.3612(3)0.681(2)0.1706(3)0.3354(3)0.003(1)0.2535(9)0.2524(9)0.1712(9)0.332(1)0.474(1)

z/c
0
½
½
½
0.2320(4)0.2290(3)0.1682(8)0.1689(9)0.1601(9)0.3906(8)0.3902(8)0.3901(9)Uiso
0.043(1)0.019(4)0.031(1)0.018(8)0.030(1)0.029(1)0.032(2)0.029(2)0.031(2)0.024(2)0.030(2)0.041(2)Anisotropic displacement parameters of the atoms forbalestraite

Atom
U11
U22
U33
U23
U13
K
0.036(2)0.025(2)0.069(3)00.012(2)M20.017(1)0.011(1)0.067(3)00.012(1)T1
0.018(2)0.014(1)0.058(3)0.001(1)0.010(1)T2
0.019(2)0.017(2)0.050(2)0.001(1)0.009(1)Note: M1, M3, and the oxygen atoms refined as isotropic.

U12
0
0−0.001(1)0.001(1)Chemical analyses
A crystal of about 450 × 180 × 50 mm was embedded in epoxy and polishedfor electron microprobe analysis, which was obtained with a JEOL 8200 electron
microprobe at the Dipartimento di Scienze della Terra of the University of Milan(Italy) in wavelength-dispersion mode at 15 kV, 5 nA beam current, and 10 mm as
the beam size. The following standards were used: wollastonite (SiKa), K-feldspar(KKa), pure vanadium (VKa), and rhodonite (MnKa). Five point analyses on different spots were performed. Na, Mg, Ca, Ba, Al, Ti, F, and Cl were below 0.01 wt%.
The crystal fragment was found to be homogeneous within analytical uncertainty.
In situ analysis of the content of Li was performed using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The laser probe consistsof a Q-switched Nd:YAG laser, model Quantel (Brilliant), whose fundamental
emission in the near-IR region (1064 nm) was converted into 266 nm wavelength
using two harmonic generators. Spot diameter was near 40–50 mm. The ablatedmaterial was analyzed by using an Elan DRC-e quadrupole mass spectrometer.
Helium was used as the carrier gas and mixed with Ar downstream of the ablation
cell. Data reduction was performed using the Glitter Software. NIST SRM 610 was
analyzed as an external standard, whereas the SiO2 was used as an internal standard.
Precision and accuracy were assessed from repeated analyses of the BCR-2g, NIST
SRM 612, and 610 standards and resulted in better than ±3%. Full analytical details
are reported in Tiepolo et al. (2005) and Miller et al. (2007).
Table 6 reports the chemical analyses (means and ranges in wt% of oxides),standard deviations and atomic ratios calculated on 12 oxygen atoms per formula
unit (pfu) assuming balestraite as completely anhydrous and all vanadium at
the pentavalent state, as indicated by the spectroscopic and structural analysis,
respectively (see below).

Raman spectroscopy
A Raman spectrum was obtained using a micro/macro Jobin Yvon LabRam
HRVIS, equipped with a motorized x-y stage and an Olympus microscope. The
backscattered Raman signal was collected with 50× objective and the Raman
spectrum was obtained for a randomly oriented crystal. The 632.8 nm line of an
He-Ne laser was used as excitation; laser power was controlled by a series of density filters. The lateral and depth resolution were about 2 and 5 mm, respectively.
The system was calibrated using the 520.6 cm–1 Raman band of silicon before
the experimental session. Spectra were collected with multiple acquisitions (2 to6) with single counting times ranging between 20 and 180 s. The spectrum wasrecorded using the LabSpec 5 program from 150 to 4000 cm–1.

Electron paramagnetic resonance
The EPR measurement was carried out on a Bruker ER 200D-SRC spectrometer operating at X-Band (~9.5 GHz) interfaced with DS/EPR software to a
PC for data acquisition and handling; the actual operating frequency value wasTable 5.

Observed and calculated X-ray powder diffraction data forbalestraite

(a)
(b)
hk l
d (Å)
Iobs/I100
d (Å)
Icalc/I100
001
9.9
50
9.8328
41
002
4.96
20
4.9164
14
020
4.51
100
4.489140
110		 4.4456
22
111
4.34
40
4.3127
49
021
4.11
25
4.0836
29
111
3.85
10
3.8315
17
112
3.60
40
3.5893
100
022
3.34
30
3.3151
66
003
3.30
30
3.2776
32
112
3.08
35
3.0668
93
113
2.877
10
2.8611
26
023
2.662
15
2.6471
29
201
2.592
70
2.5927
15
130
2.574
70
2.583339
2 0 0			2.5585
23
1 3 1			2.5565
38
113
2.460
10
2.46024004
		 2.4582
4131		 2.4444
8
132
2.385
70
2.377839
201
		 2.3738
25
114
–
–
2.3101
7
221
2.251
10
2.2451
9
220
2.232
10
2.2228
12
041
2.198
20
2.1883
22
222
2.167
10
2.1563
10
133
2.133
15
2.1252
18
202
2.13
s
2.1183
11
221
2.10
s
2.0985
8
005
1.976
25
1.966615204––
1.9579
5
133
1.957
10
1.9446
12
224
1.817
5
1.7947
4
311
1.703
<5
1.7026
4
044
1.657
<5
1.65756151
		 1.6533
4
135
1.642
15
1.6384
26
204
1.63
s
1.631512241
		 1.6309
8
313
1.614
5
1.6124
4
243
1.583
5
1.578611
152
		 1.5732
4
153
1.546
5
1.5432
6
314
1.512
<5
1.51534
206
		 1.5093
5
312
1.503
50
1.50666
3 3 1			1.5004
23
1 3 5			1.5002
8
060
1.486
5
1.496410225––1.3701
5
207
1.338
5
1.33746
136
		 1.3301
12
401
–
–
1.29894402––
1.2964
4
261
1.299
10
1.296010260		 1.2917
7
335
1.284
5
1.2842
4
Notes: (a) = observed diffraction pattern obtained by converting the diffraction
rings into a conventional XRD pattern; (b) = calculated diffraction patternobtained from structural data (only reflections with Icalc/I100 ≥ 4 are listed); s =shoulder of a peak.

determined by using DPPH radical [2,2-di(4-tert-octyl-phenyl)-1-picrylhydrazyl,
g = 2.0037] as external standard. The spectrum was registered inserting the crystal
into an amorphous silica tube, at room temperature. Under the adopted operating
conditions (100 KHz modulation frequency, 4 db modulation amplitude), theminimum amount of detectable V can be estimated in 5 × 1013 ions (Burns andFlockhart 1990). This value corresponds to ~0.01% of the total V content in thesample, given the experimental volume (about 19 × 106 mm3, approximately) andthe density of the investigated sample.

LEPORE ET AL.: BALESTRAITE, THE FIRST MICA WITH OCTAHEDRAL V5+

Results
The crystal structure of balestraite is topologically similar tothat of the other trioctahedral micas belonging to the 1M polytype;however, the mineral is characterized by unusual geometrical features related to its Li,V-rich chemical composition. Consistently
with many Li-rich micas (e.g., Brigatti et al. 2007 and references
therein), balestraite crystallizes in the C2 space group. The reduction of symmetry from the ideal space group C2/m is related
to the ordering of V at one (M2) of the two cis-octahedral sites(M2 and M3), whereas Li almost fully occupies the other twosites (Table 2, Fig. 2). This ordering scheme is consistent withthe site scattering (21.6 e- in M2 vs. 3.0 and 3.6 in M1 and M3,
respectively) and the geometry of the three octahedra.
The analysis of the Raman spectrum (Fig. 3) shows no peaksin the region of the O-H stretching band (3300–3800 cm–1) thus
suggesting balestraite to be anhydrous. In this light, chargebalance would require all vanadium at the pentavalent state.
This feature is indeed confirmed observing the geometry of theM2 polyhedron, which exhibits a V-O bond-length distribution
characteristic of the valence state +5 (Schindler et al. 2000),
showing a [2+2+2] coordination with two short vanadyl bonds(1.66 Å) with O4 in a cis arrangement, two longer equatorial
bonds (1.985 Å) with O32, and the two longest bonds (2.184 Å)with O31 trans to the vanadyl anions (Fig. 4). Furthermore, theEPR spectrum shows no absorption signal due to V4+ (the only
V species with odd number of unpaired electrons), thus allowing to exclude significant amounts of V4+ and making unlikely
the presence of V3+.
Table 6.

Electron microprobe analyses (means and ranges in wt%of oxides) and atomic ratios (on the basis of 12 O atoms)of balestraite

Mean
Range
Atom
Atomic ratios
K2O
11.24
11.19–11.28
K
0.99
Li2Oa
7.20
7.04–7.46
Li
2.00
V2O5
21.15
21.03–21.28
V
0.97
SiO2
58.46
58.06–58.76
Si
4.04
Total
98.05
Σcations
8.00
a
Li determined by LA-ICP-MS (three point analyses on different spots); Mn2O3
up to 0.12 wt% corresponding to 0.006 atoms per formula units.

Figure 2. The octahedral sheet of balestraite viewed down the caxis. M1 and M3 octahedra host Li+, V5+ occupies M2.

611

The octahedral V5+ is also supported by a high-wavenumber
Raman band (973 cm–1), consistent with what reported by Frostet al. (2005) for symmetric vibrational modes from V5+O6 units
in decavanadate minerals, including pascoite (three bands at 991,
965, 958 cm–1) and hummerite (two bands at 999–962 cm–1).
The empirical formula (based on V as V5+ and O4 occupiedby O2–) is K0.99Li2.00V5+
0.97Si4.04O12 and this result is consistent withthe structure refinement and close to the simplified formula ofKLi2V5+[Si4O10O2]. The electron count calculated on the basis of
the empirical formula for the octahedral sites (28.3) is in excellent accord with that obtained by the structure refinement (28.2).

Discussion
Octahedral sheet<M1-O> and <M3-O> bond lengths (2.08 and 2.10 Å,
respectively) are similar to those found for Li-rich octahedra(e.g., Tyrna and Guggenheim 1991; Brigatti et al. 2000, 2007).
M1 and M3 octahedra are flattened and less distorted in comparison with M2, which is strongly influenced by the presenceof V5+ (see Tables 7 and 8 for details). The trioctahedral sheet
in balestraite is affected by the ordering of cations with markedly different size and charge, whereas the “oxy” component
only marginally affects the octahedral bond lengths. Divalentanions (O2–) at O4 would be expected to increase the electrostatic attraction with all M cations, such as in “oxybiotite”(Ohta et al. 1982), thus inducing a shortening of all the M-O4
distances. However, this trend occurs only for M2, whereas
M1-O4 and M3-O4 are longer than the others (Table 7). Thisfeature, observed also in norrishite, KLiMn3+2 [Si4O10O2] (Tyrna
and Guggenheim 1991), may be related to the presence of Li+.
Thus a high and positive off-center shift value, usually indicating sensible amounts of “oxy” component in trioctahedral
C2/m micas (Cesare et al. 2003; Mesto et al. 2006; Gianfagna
et al. 2007; Matarrese et al. 2008; Scordari et al. 2008, 2010),
is only observed for M2 (0.316 Å), whereas a negative value
is shown by M3 (–0.198 Å).
Bond valence sums (BVS) for O1, O21, O22, O31, andO32 are all slightly higher than 2.0 v.u. (Table 9), whereas

Figure 3. Raman spectrum of balestraite.

612

LEPORE ET AL.: BALESTRAITE, THE FIRST MICA WITH OCTAHEDRAL V5+
Table 7.

Bond distances (Å) for balestraite

M1
M2
M3
–O31i, ii
1.97(2) 	  –O4i, ii
1.66(1)	 –O32v, vi
1.98(2)
iii, iv
vii, xiii–O32
2.17(2) 	  –O32
1.985(9)	 –O31v, vi
2.067(8)iii, iv
i, ii
i, ii
–O4
2.17(1) 	  –O31
2.184(8)	 –O42.27(2)
mean
2.10
mean
1.943
mean
2.10
T1
T2
K
i
i
viii, xiv
–O31
1.560(8)	 –O32
1.586(9)	 –O22
3.031(9)–O22vii
1.63(1)	 –O1xii
1.620(9)	 –O22vi, ix
3.038(9)
i
i
i, x
–O1
1.63(1)	 –O21
1.624(9)	 –O21
3.068(9)iiii, xi–O21
1.632(9)–O22
1.637(9)–O1
3.074(8)ix, xv
mean
1.613
mean
1.617
–O21
3.092(9)ix, xv
				–O1
3.092(8)
				
mean
3.066
Notes: Symmetry codes: (i) x, y, z; (ii) −x, y, −z+1; (iii) x−½, y−½, z; (iv) −x+½,
y−½, −z+1; (v) −x+½, y+½, −z+1; (vi) x−½, y+½, z; (vii) x−1,y, z; (viii) −x+1, y, −z;(ix) −x+½, y+½, −z; (x) −x, y, −z; (xi) −x−½, y+½, −z; (xii) x+½, y+½, z; (xiii) −x+1,y, −z+1; (xiv) x+1, y, z; (xv) x+½, y-½, z.

Table 8.
Figure 4. [2+2+2]-coordination of V5+ in the M2 octahedron.

the BVS for O4, despite the very short M2-O4 bonds, is 1.74
v.u. A low value, however, does not require the presence ofH, as documented for other anhydrous or partially anhydrous
members of the mica group (norrishite, BVSO4 = 1.69 v.u.,
Tyrna and Guggenheim 1991; oxykinoshitalite, BVSO4 = 1.54
v.u., Kogarko et al. 2005; oxyphlogopite, BVSO4 = 1.61 v.u.,
Chukanov et al. 2011).
Tetrahedral sheet
The tetrahedral sheet of balestraite is characterized bynearly perfect hexagonal rings; the tetrahedral rotation angle
a of 0.2° is the lowest value found in natural micas (Brigatti
and Guggenheim 2002). This feature is probably related to thesmall lateral dimensions of the Si4 tetrahedral sheet comparedto the lateral dimensions of the octahedral sheet, which, owing
to the Li,V composition, is one of the most contracted among
the trioctahedral micas leading to very short a and b cell edges.
Basal oxygen plane is corrugated in a similar manner to thatobserved in other Li-rich micas (Brigatti and Guggenheim 2002
and references therein) with O1 and O21 atoms drawn toward
the octahedral sheet (Dz = 0.087).
Tetrahedra in balestraite are elongated parallel to csinb withthe mean pyramidal edges length longer than the basal edges(2.670 vs. 2.591 Å for T1 and 2.671 vs. 2.603 Å for T2). Thisdifference in length reduces the basal surface and contributes tothe decrease of the lateral dimensions of the sheet. Tetrahedra
in balestraite are distorted, as indicated by the high values of
BLD (bond length distortion) and t (defined as the mean ObasalT-Oapical angle) parameters (Table 8). The high BLD values are
related to the short T-Oapical distances: whereas the mean Si-O
bond lengths (1.61 and 1.62 Å for T1 and T2, respectively) areclose to the expected tetrahedral Si-O bond length of 1.623 Å
on the basis of predictive equation proposed by Baur (1978),the Si-Oapical are significantly shorter (1.560 and 1.586 Å forboth T1 and T2, respectively). The displacement of Si toward
the apical O atoms may be related to high Si content. The high
BVS on O1, O21, and O22 (Table 9) is not capable of being
directly balanced by any other substitutional mechanism. In
contrast, for the apical oxygen atoms, the presence of Si alone

Selected structural parameters for balestraite

Whole layer
Oct. sheet
ΔTM (Å)0.298
VM1 (Å3)12.03
βideal99.989
VM2 (Å3)
9.54
intralayer shift–0.347a
VM3 (Å3)
12.10
		ψM1 (°)
59.147
InterlayerψM2 (°)
56.279V (KO12) (Å3)
57.76
ψM3 (°)
59.185tint (Å)
3.259
OAVM1
70.90
<K-O>inner (Å)
3.066
OAVM2
58.76
<K-O>outer (Å)
3.066
OAVM3
71.22
		
OQEM1
1.0229Tet. sheet
OQEM2
1.0295
α (°)
0.15
OQEM3
1.0251Δz (Å)
0.087
BLDM1
4.227
τT1 (°)
113.54BLDM29.712
τT2 (°)
112.50
BLDM3
5.204
TAV T1
21.62
ELDM1
5.187
TAV T2
11.66
ELDM2
1.671
TQET1
1.0047
ELDM3
5.238
TQET2
1.0026
ShiftM2(Å)+0.316BLDT1
1.643
ShiftM3(Å)–0.198BLDT2
0.951
tM(O3) (Å)
2.155
VT1 (Å3)
2.14
tM(O4) (Å)
2.161
VT2 (Å3)
2.16
tM(O3-O4) (Å)
2.157
ttet (Å)
2.221
Notes: ttet = tetrahedral sheet thickness calculated from z coordinates of basal
and apical O atoms; TQE = tetrahedral quadratic elongation (Robinson et al.
1971); TAV = tetrahedral angle variance (Robinson et al. 1971); τ = tetrahedral
flattening angle; α = tetrahedral rotation angle (Hazen and Burnham 1973);
Δz = departure from co-planarity of the basal O atoms (Güven 1971); ΔTM =2√3<O-O>basal – 3√2(<M1-O> + <M2-O> + <M3-O>)/3; ψ = octahedral flattening
angles (Hazen and Burnham 1973); BLD and ELD = octahedral bond-length andedge-length distortion parameters (Kunz et al. 1991); ShiftM2,3 = off-center shift(Laurora et al. 2007) “+” and “–” related respectively to a migration towards oraway from (010) plane; OQE = octahedral quadratic elongation (Robinson et al.
1971); OAV = octahedral angle variance (Robinson et al. 1971); tM(O3-O4), tM(O3), tM(O4)= octahedral sheet thickness calculated from the z coordinates, respectively, of all
oxygen atoms bonded to octahedral cations (O3 and O4), of only the tetrahedral
apical oxygen atoms (O3), and of only oxygen atoms bonded to hydrogen atoms(O4) (Toraya 1981); tint calculated from the z coordinates of basal oxygen atoms;<K-O>inner/outer = mean bond length for inner/outer K-O bonds: K-O22viii,xiv, -O1ii,xi,-O21ix,xv and K-O21ix,xv, -O22vi,ix, -O1ix,xv, respectively.

in the tetrahedra and V5+ in the adjacent octahedron is balancedby the concomitant entry of a monovalent cation (Li).
The magnitude of the tetrahedral distortion, t, is among thehighest ever found in natural micas (Brigatti and Guggenheim
2002): this observation, together with the displacement of Si
toward the apical oxygen atom, describes the elongated shapeof the tetrahedra. This feature is observed also in norrishite(Tyrna and Guggenheim 1991) and in synthetic polylithionite,
KLi2AlSi4O10F2 (Takeda and Burnham 1969) and, although

LEPORE ET AL.: BALESTRAITE, THE FIRST MICA WITH OCTAHEDRAL V5+
Table 9.

Bond-valence (v.u.) arrangement for balestraite

T1
T2
SO
O1
0.078×2↓/0.075×2↓
1.027 1.056
2.24
O21 0.080×2↓/0.075×2↓
		1.022 1.044
2.22
O22 0.086×2↓/0.088×2↓
1.016 1.008
2.20
O31
0.256×2↓ 0.357×2↓ 0.197×2↓ 1.241
2.05
O32
0.149×2↓ 0.611×2↓ 0.248×2↓
1.157
2.17
O4
0.148×2↓ 1.476×2↓ 0.114×2↓
1.74
0.96
1.11
4.89
1.12
4.31
4.27
Note: Calculated from the bond-valence curves of Brese and O’Keeffe (1991)
assuming the ideal formula KLi2V5+Si4O10O2.
K

M1 (Li) M2 (V5+) M3 (Li)to a lesser extent, in tainiolite, KLiMg2Si4O10F2 (Toraya et al.
1977) and synthetic cesian tainiolites (Mariychuk et al. 2007;
Baumgartner et al. 2009; Koch and Breu 2013).
Interlayer
The interlayer is occupied by very regular KO12 polyhedrawith a short mean bond length of 3.066 Å. One of the moststriking feature is the homogeneity of the K-O distances with
<K-Oinner> matching <K-Oouter> (Table 8). This geometrical arrangement indicates nearly perfect hexagonal symmetry of thetetrahedral sheet and is also related to the small size of the tetrahedra. The interlayer thickness of only 3.26 Å, together with thoseobserved in norrishite (3.27 Å; Tyrna and Guggenheim 1991)
and oxykinoshitalite (3.221 Å; Kogarko et al. 2005), is among
the lowest values found in trioctahedral micas (Brigatti and Guggenheim 2002) and contributes to the very short length of the ccell edge (9.997 Å). The small thickness is related to the absenceof electrostatic repulsion usually occurring between the interlayer
cation and the hydrogen bonded to O4 and here occupied by adivalent anion alone as first suggested by Cruciani and Zanazzi(1994) and then observed in hydrogen depleted micas of recent
investigation (e.g., Schingaro et al. 2011; Scordari et al. 2013).
Constraints on petrogenesis
In terrestrial geological systems, vanadium exists as V3+,
V4+, and V5+, with V5+ relative content increasing with oxygen
activity (fO2) (Canil 2002; Zanetti et al. 2004). The occurrenceof 5+ as the dominant valence state of V in balestraite is thus
consistent with strongly oxidizing conditions of crystallization,
which are supported also by the complete O2– → OH- substitution at the O4 position and by the occurrence of balestraite at
the boundary between carbonate-bearing veins and hematite
bands. Oxidizing conditions of formation were also argued formineral assemblages containing the dehydrogenated end-member
3+
amphibole, NaNa2(Mn2+
2 Mn3 )Si8O22O2 (Hawthorne et al. 1995).
The latter, renamed mangano-mangani-ungarettiite according to
the new nomenclature rules (Hawthorne et al. 2012), crystallized,
like balestraite, during low-T metamorphic recrystallization ofsubmarine Mn-rich deposits at the Hoskins mine (Grenfell, NewSouth Wales, Australia). A further linking between the formationof balestraite and mangano-mangani-ungarettiite, besides theconcomitant occurrence of calcite and quartz bands, is represented by their common Mn- and Li-rich mineral assemblage
characterized by the presence of braunite, norrishite, and sugilite.
As a whole, such a match suggests that both the compositionof the manganese deposit and its hydrothermal alteration weresimilar in the Hoskin and Cerchiara occurrences.
Balestraite and mangano-mangani-ungarettiite may be as-613

sumed to form where host rocks enriched in transition elements
reacted with alkaline fluids at low-T conditions, thus producing
unusual alkali-rich amphiboles and micas completely dehydrogenated and containing highly oxidized transition elements (Mnand V). Further investigations are needed to determine whether
the strongly oxidizing conditions controlling the crystallizationof balestraite and mangano-mangani-ungarettiite were the resultof a buffering effect exerted by the transition-element-rich hostmatrix or a primary feature of the flowing fluids, the latter possibly CO2 rich.

Implications
Vanadium in minerals may be present in variable oxidation
states forming polyhedra with different coordination number[e.g., tetrahedral (V5+), trigonal bipyramidal (V4+, V5+) andoctahedral (mainly V3+ and, to lesser extent, V4+ and V5+)], andtherefore its presence in complex solid solutions adds difficulties
in the determination of cation distribution. For the V-membersof the mica group, i.e., chernykhite (Ankinovich et al. 1973) androscoelite (Brigatti et al. 2003), vanadium is nominally present
as V3+ (i.r. = 0.64 Å, Shannon 1976). However, variable amountsof V4+ and V5+ cations, which are similar in radii size (0.58 and0.54 Å, respectively) may coexist in octahedral sites. According
to Ankinovich et al. (1973), in the Ba-bearing muscovites andchernykhite from the Karatau carbonaceous-siliceous schists,
micas hosted in rocks enriched in organic matter contain V3+,
whereas V4+ occurs in micas from rocks poor in organic matterand may sometimes replace V3+. Balestraite, a near V5+ endmember, allows the evaluation of the structural effect of V5+
within the octahedral sheet in micas.
Moreover, balestraite enlarges the number of dehydrogenatedrock forming minerals (amphiboles and micas) containing highly
oxidized transition elements. The incorporation in the mica
structure of V5+ allows for a mechanism to charge balance the“oxy” component in micas.

Acknowledgments
The manuscript benefited from the revisions of Stephen J. Guggenheim and an
anonymous reviewer and from demanding and perceptive comments of the membersof the IMA-CNMNC. Thanks are due also to Fabrizio Castellaro for providing thesample and Roberto Bracco for the photo of balestraite. Francesco Di Benedetto
is thanked for the acquisition of the EPR spectrum. X‑ray intensity data were collected at CRIST, Centro di Cristallografia Strutturale, University of Florence, Italy.

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Manuscript received March 12, 2014
Manuscript accepted August 31, 2014
Manuscript handled by Fernando Colombo