Eur. J. Mineral.
2008, 20, 95–100
Published online November 2007

Cassagnaite, a new, V-bearing silicate mineral from the Cassagna mine,
northern Apennines, Italy
Riccardo BASSO1, *, Cristina CARBONE1 and Andrea PALENZONA2
1

Dipartimento per lo Studio del Territorio e delle sue Risorse, Università di Genova, Corso Europa 26,
16132 Genova, Italy*Corresponding author, e-mail: mineral@dipteris.unige.it
2
Dipartimento di Chimica e Chimica Industriale, Università di Genova, via Dodecaneso 31, 16146 Genova, Italy
Abstract: Cassagnaite occurs at the Cassagna manganese mine (Eastern Liguria, Italy), filling fractures in braunite + quartz layered
mineralizations together with piemontite. It occurs as very rare isolated prismatic to tabular {001} crystals, usually elongatedalong [100], and as entangled aggregates of a few crystals. Few aggregates consist of very small cassagnaite crystals (maximum
size up to 0.05 mm) closely associated with much larger piemontite, quartz, and braunite crystals. The crystals are generally verysmall, with a maximum size up to 0.1 mm, golden brown in colour, transparent with vitreous lustre. The crystal structure, refinedin the space group Cmcm with cell parameters a = 6.066(1) Å, b = 8.908(1) Å, c = 18.995(2) Å and Z = 2, may be described as alayer stacking along [001] of a fundamental building block of composition [M12 (OH)2 (SiO4 )2 ]4− that alternates with intersheets,
randomly occurring in a ratio ideally 1:1, of type 1 [(Ca, Mn2+ )2 SiO2 ]4+ and of type 2 (Ca, Mn2+ )2 M22 (OH)2 O2 ]4+ , where Fe3+
and Mn3+ populate 3/4 of the M1 site and Al the remaining 1/4, while V3+ , Mg and Al occupy in nearly equal proportions the M2
site. The simplified formula, inferred from chemical analyses, structure refinement and crystal-chemical considerations, may be
written as (Ca, Mn2+ )4 (Fe3+ , Mn3+ , Al)4 (OH)4 (V3+ , Mg, Al)2 (O, OH)4 (SiO4 )2 (Si3 O10 ). From the composition of the “dominant”
3+
end-member of the complex solid-solution the end-member formula Ca4 Fe3+
4 (OH)4 V2 O2 (OH)2 (SiO4 )2 (Si3 O10 ) may be proposedfor cassagnaite. So, cassagnaite may be classified as sorosilicate with insular and triple tetrahedral groups and belongs to theardennite group in Dana’s classification.
Key-words: cassagnaite, new mineral, chemical data, X-ray powder pattern, crystal structure, vanadium, ardennite group.

Introduction
In the Val Graveglia (Northern Apennines, Liguria, Italy)
rare and new minerals are widespread in manganese ore deposits and in silicified woods associated to metacherts (“Diaspri di M. Alpe” Formation) that occur at Molinello, Gambatesa, Statale, and Cassagna mines (Cortesogno & Galli,
1974; Cortesogno et al., 1979; Cabella et al., 1998). In particular, in the Cassagna mine some of these rare minerals(such as medaite, palenzonaite, strontiopiemontite, sursassite, tinzenite, and tiragalloite) were found. In this paper
we report the occurrence of cassagnaite, a new mineral
species, (approved by the Commission on New Minerals,
Nomenclature and Classification of the International Mineralogical Association, Ref. 2006 – 19a), whose name derives from the Cassagna mine where the mineral has beenfound. The holotype sample is deposited in the collectionof the Dipartimento per lo Studio del Territorio e delle sue
Risorse (Dip.Te.Ris), Università di Genova, Italy.

Occurrence and physical properties
In the Val Graveglia mines the Mn-ores occur as
braunite-bearing layered mineralizations, massive lenses
DOI: 10.1127/0935-1221/2007/0019-1777

and boudins near the stratigraphic base of the cherts of
the “Diaspri di M. Alpe” Formation, overlying the ophiolitic basement (Marescotti & Cabella, 1996; Cabella
et al., 1998). The mineralizations are characterized bybraunite + quartz association equilibrated under prehnitepumpellyite facies conditions (Cortesogno et al., 1979;
Lucchetti et al., 1990) during the tectono-metamorphic
evolution that aﬀected both ophiolites and cherts. Hydrothermal fluid circulation, along later extensional fractures under decreasing P-T metamorphic conditions, induced the concentration of commonly dispersed elementsthat were involved into the genesis of As, Ba, Cu, Sr, andV-rich mineral assemblages. While at the Gambatesa andMolinello mines some vanadium minerals were discovered
for the first time, representing new minerals species (suchas cavoite, medaite, palenzonaite, reppiaite, saneroite, tiragalloite, vanadiocarpholite, and vanadomalayaite), cassagnaite represents the first new V-rich mineral found at the
Cassagna mine as type locality. Cassagnaite occurs filling fractures in braunite + quartz layered mineralizations
together with piemontite. It appears as very rare isolated
prismatic to tabular {001} crystals, usually elongated along[100] (Fig. 1) and as entangled aggregates of a few crystals.
The crystals are generally very small, with a maximum sizeof 0.06 mm. Only two crystals with higher dimensions (up0935-1221/07/0019-1777 $ 2.70
c 2007 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart
96

R. Basso, C. Carbone, A. PalenzonaTable 1. Chemical composition for cassagnaite.

Fig. 1. SEM image of [100] elongated prismatic and {001} tabular
crystals of cassagnaite.

to 0.04×0.06×0.1 mm) have been found, one of which wasused for X-ray analyses and the other was unfortunately
lost. Few aggregates, one of which is up to 1 mm in diameter, consist of very small cassagnaite crystals (maximum
size up to 0.05 mm) closely associated with much larger
piemontite, quartz, and braunite crystals.
Cassagnaite is transparent with vitreous lustre, goldenbrown in colour and the streak is nearly white. The morphology of the crystals is generally constituted by theprism {011} and the pinacoids {001} and {100}. It is brittle and exhibits {001} cleavage (or parting). Due to theunusual scarcity of available material and to the reduced
size of the crystals, the determinations of physical and optical properties are not complete. The calculated density is
3.22 g/cm3 . The mineral does not fluoresce under shortand long-wave ultraviolet light. Optically, cassagnaite isbiaxial and some optical measurements were performedon the largest face, likely parallel to the {001} cleavage,of a prismatic crystal elongated along [100], with dimensions 0.01 × 0.02 × 0.04 mm. The observed section showsparallel extinction, positive elongation and low interference colours. The refractive index values are N (parallelto a): 1.810 (± 0.005) and n: 1.800 (± 0.005), measured by
Cargille’s liquids at 25 ◦ C (λ = 589 nm). The pleochroism is very weak: N = golden yellow-brown and n = pale
golden yellow-brown.

Chemical composition
Quantitative analyses (seven points on two crystals) were
performed by means of an electron microprobe (PHILIPSSEM 515 electron microscope equipped with EDAX
PV9100 spectrometer) in energy-dispersion mode, with anaccelerating voltage of 15 kV and a beam current of about
2 nA. Pargasite (Mg, Al, Si, Ca), metallic vanadium (V),rhodonite (Mn), and fayalite (Fe) were used as reference

Oxide

range (wt%)

CaO
MnO
Mn2 O3Fe2 O3Al2 O3
MgO
V2 O3
SiO2
H2 O by
diﬀerence
Total

17.85–18.93
14.61–15.20
12.37–13.02
6.71–8.49
2.07–2.61
5.04–5.84
29.25–33.75

mean (wt%) recalculated to obtain
Ca+Mn2+ = 4 (wt%)
18.42
18.42
14.98
5.20
10.88
12.61
12.61
7.55
7.55
2.36
2.36
5.42
5.42
31.69
31.69
5.87
100.00standards for the elements in brackets. Raw data were reduced using the ZAF algorithm and the standard softwareof the EDAX PV9100. Spot analyses show neither significant variations of composition nor compositional zoning
in individual crystals. Other elements are below detection
limits. The mean chemical composition is reported in Table 1 along with an indication of variability. H2 O content
could not be directly determined because of the lack ofmaterial but a micro-Raman analysis has been carried out
with a Renishaw 2000 Ramascope equipment using a laser
operating at 633 nm; the large band in the range 3500–
3000 cm−1 of the Raman shift shows the presence of (OH)−ions in the structure of cassagnaite.
The empirical formula, based on 26 oxygen atoms pfu
taking into account crystal-chemical considerations and theindications of the structure refinement later discussed, is3+
3+
3+
Ca3.3 Mn2+
0.7 Fe1.6 Mn1.4 Al1.5 V0.7 Mg0.6 Si5.2 O26 H6.5 .

X-ray powder-diﬀraction analysis
The X-ray powder-diﬀraction pattern was obtained by aGandolfi camera (diameter 114.6 mm), using a V-filtered
CrKα radiation. Relative intensities were visually estimated and the reflections were indexed on the basis of thepattern calculated by means of the LAZY PULVERIX program (Yvon et al., 1977), using the structural model resulting from the refinement as input data. A comparison
between these patterns is reported in Table 2. The unit-cellparameters refined from the measured powder data are a =6.07(1) Å, b = 8.89(1) Å and c = 18.96(2) Å.

X-ray structure analysis
Severe diﬃculties were encountered to find a single crystal suitable for the X-ray analysis. All the checked crystals exhibit very poor reflection quality, almost all a large
quantity of streaked reflections, likely related to a highdegree of structural disorder, and generally weak reflections likely due to the small size. Finally, the data collection was performed on a prismatic crystal, with dimensions 0.04 × 0.04 × 0.10 mm, by means of anCassagnaite

97

Table 2. X-ray powder-diﬀraction data for cassagnaite (d in Å).

ENRAF-NONIUS MACH-3 automatic diﬀractometer using graphite-monochromatized MoKα radiation. The cellparameters were determined and refined using 25 reflections, measured in the 12◦ < θ < 19◦ range. A semiempirical absorption correction was applied using the ψscan method (North et al., 1968). The crystal structure wasrefined in the space group Cmcm (no forbidden reflections
were detected) using as starting values the positional parameters of the orientite structure refined by Moore et al.
(1985), as suggested by the similar stoichiometry of chemical composition and unit cell.The refinement was carriedout by a modified version of the ORFLS program (Busing
et al., 1962), using the scattering factors for neutral atoms
taken from Ibers & Hamilton (1974). The numbering of
cations and oxygens was assumed according to those reported for orientite in the data base ICSD 201629. M1 andM2 refer to the atom sites populated by Mn3+ and Mn2+ , respectively, in the structure refinement of orientite reported
by Moore et al. (1985). According to these authors, themultiplicity of the sites M2 and Si1 was reduced to a half.
The structure refinement of cassagnaite required a half multiplicity also for the O6 oxygen. Scale factor, occupancies
of Ca/Mn on the Ca site, Fe/Al on the M1 site and V/Mg
on the M2 site, positional and thermal parameters were simultaneously derived, except for the O6 atom, whose temperature factor was fixed to a value of 1 Å2 . Tables 3–5
summarize experimental details and results of the crystal
structure refinement.
In consideration of the complicated scheme of solid solution and site ordering, the cationic populations of the Ca,
M1, and M2 sites were inferred following this method:– a fraction of the total manganese as Mn2+ to fill up the
Ca site (Ca + Mn2+ = 4 pfu);– full occupancy of the M1 site with trivalent cations (Σ =4 pfu), taking into account the composition resulting
from the empirical formula and the crystal-chemical

characters of the minerals with analogous 6 × 9 Å sheet
structures emphasized by Moore et al. (1985);– the remaining cations used to fill the M2 site, on thebasis of the stoichiometry of the empirical formula, toobtain ideally Σ = 2 pfu;
– best agreement for each of the Ca, M1, and M2 sites
among the electron numbers and the mean polyhedral
distances derived from the structure refinement andthose calculated from the cationic population, respectively.
The resulting cationic populations of the Ca, M1, and M2
3+
3+
sites may be expressed as (Ca3.3 Mn2+
0.7 ), (Fe1.6 Mn1.4 Al1.0 ),and (V3+
0.7 Mg0.6 Al0.5 ) pfu, respectively. Final refinement occupancies of the Ca (Ca/Mn = 0.88/0.12), M1 (Fe/Al =0.68/0.32) and M2 (V/Mg = 0.44/0.56) sites correspondto 20.6, 21.8, and 16.8 electrons, respectively, in agreement with the electron numbers (20.9, 22.4, and 16.6) required from the above cationic distribution. Also the mean
polyhedral distances from refinement (Table 5) agree withthose required from the above cationic distribution. If a linear contribution by each fraction of cation is supposed forthe calculus of the mean polyhedral distances and the single cation-oxygen is reproduced by sum of both cation andoxygen eﬀective ionic radii (Shannon, 1976), the following
calculated distances are obtained: Ca, Mn − O = 2.41 Å,
M1 − O = 2.00 Å, and M2 − O = 2.02 Å.
The electrostatic valence balance, reported in Table 6, issimplified by using simple Pauling bond strengths obtained
dividing the formal charge of cations by the coordination
number and assigning coordination number 6.5 to Ca, owing the half multiplicity of O6 and formal charge 3 to M2,
neglecting the fraction of Mg. The bond strength sums foroxygens, although roughly estimated, clearly indicate that
O5 and O6 are oxygens pertaining to hydroxyl groups. Thishypothesis agrees with possible hydrogen bonds owing to
the too short O . . . O distances, between oxygen atoms not

98

R. Basso, C. Carbone, A. PalenzonaTable 3. Crystal structure data for cassagnaite.
Space groupZa, b, c, V

Cmcm
2
6.066(1) Å, 8.908(1) Å, 18.995(2) Å, 1025.45 Å3

Scan mode
Measured reflections
Unique reflections
Observed reflections (I > 2σI )ω−θ2430 (2 ≤ θ ≤ 28◦ , −8 ≤ h ≤ 8, −11 ≤ k ≤ 11, −25 ≤ l ≤ 0)689
299

RW (observed)
R (observed)
R (all)

0.146
0.181
0.289

Table 4. Site occupancy fraction (sof), final atomic positions, and isotropic temperature factors (Å2 ) for cassagnaite.
Atom
Ca
M1
M2
Si1
Si2
O1
O2
O3
O4
O5 (OH)O6 (OH)

site
8 f
8 d
8 g
4 c
8 f
8 f
8 g
16 h
8 f
8 f
8 f

sof
1
1
0.5
0.5
1
1
1
1
1
1
0.5

x
0
1/4
0.2448(54)0000.2280(65)0.2166(35)000y0.6834(11)1/40.4411(18)0.1066(32)0.0289(13)0.9555(26)0.1864(29)0.1182(17)0.8666(36)0.3681(32)0.4214(65)z0.1571(5)0
1/4
1/4
0.0948(6)0.1796(15)1/40.0808(8)0.0540(16)0.0326(15)0.1799(34)221.90(3)
1.56(4)B
1.06(24)0.45(14)0.23(45)0.62(81)0.40(26)0.33(56)
2.10(67)
0.54(38)
1.93(71)
0.91(62)
1.0

Table 5. Selected interatomic distances (Å) in cassagnaite.
2
2
2

M1-O3
-O4
-O5

1.94(2)2.10(2)
1.95(2)2

2
average

—–
2.00

2
2
2
2
2
2

2.85(3)2.88(3)2.74(3)2.76(3)2.66(3)3.06(1)

O3-O4
-O4
-O5
-O5
O4-O5
-O5

average

2
2

Ca-O1
-O2
-O3
-O4
-O6

average—–
2.83
2.46(2)2.42(3)2.32(2)2.55(3)2.37(6)
—–
2.41

2
2
2
2
2

M2-O1
-O2
-O2
-O6

O1-O1
-O2
-O2
-O6
O2-O6
-O6
O6-O6

2.05(3)2.27(3)2.19(3)2.00(5)—–2.09
2.67(6)2.82(3)3.20(4)3.05(1)2.84(6)
3.17(6)2.66(13)—–
2.96

Si1-O1-O2

2

Si2-O1
-O3
-O4

—–
1.73

4

O1-O1
-O2
O2-O2

2.67(6)2.82(3)2.77(8)

—–
1.63
2

2

—–
2.79

O5-O3
O6-O5

2.96(3)2.84(7)1.74(3)
1.56(2)1.64(3)

O1-O3
O1-O4
O3-O3
O3-O4

2.71(3)2.51(4)2.63(4)2.65(3)—–
2.64

Cassagnaite

99

(b)

(c)

(d)
(a)
Fig. 2. Clinographic view of the crystal structure of cassagnaite: a) whole polyhedral framework where intersheets, inclusive of both the two
randomly occurring types 1 and 2, are reported; b) fundamental building block; c) intersheet of type 1; d) intersheet of type 2.

Table 6. Electrostatic valence balance for cassagnaite.
O1
O2
O3
O4
O5
O6

Ca
0.31
2 × 0.31
0.31
0.31
0.31

M1

0.5
2 × 0.5
2 × 0.5

M2
1/2 × 2 × 0.5
1/2 × 2 × 0.5

Si1
1/2 × 1.0
1/2 × 1.0

Si2
1.0
1.0
1.0

2 × 0.5

Σ
2.31
1.62
1.81
2.31
1.00
1.31

count for a disordered stacking, ideally in the ratio 1:1,
of intersheets of type 1 [(Ca, Mn2+ )2 SiO2 ]4+ , where only
Si1 tetrahedra are present, and intersheets of type 2[(Ca, Mn2+ )2 M22 (OH)2 O2 ]4+ , where only M2 octahedra
are present, tacking into account that the coexistence of theface-sharing M2 octahedra and Si1 tetrahedra is forbidden
for the unacceptable too short distance M2-Si1 = 2.14(3) Å.

Conclusions
linked to the same cation, O5-O3 = 2.96(3) Å and O6-O5 =2.84(7) Å (Table 5) and with the hydration degree inferredfrom the chemical composition.
The refinement results and the above considerations giverise to a possible structural model for cassagnaite similar to that proposed by Moore et al. (1985) for orientite, in which may be recognized an analogous sheet,
named “fundamental building block” (f.b.b.) of composition [M12 (OH)2 (SiO4 )2 ]4− , where M1 is populated bytrivalent cations as Fe3+ , Mn3+ , and Al, that alternatesalong [001] with an “intersheet” of whole composition[(Ca, Mn2+ )4 M22 (OH)2 SiO4 ]8+ per unit formula, where
M2 is populated by cations, ideally trivalent, as V3+ , Mg,
and Al. The half-occupied sites M2, Si1, and O6 ac-

The above described structural model infers the presenceof isolated (SiO4 ) tetrahedra, pertaining to the f.b.b., when
the intersheet is of the type 2 and the formation of (Si3 O10 )groups, linking adjacent f.b.b., when the intersheet is of thetype 1 (Fig. 2).
To account for this crystal-chemical aspect, the unit
formula has been doubled compared with that assigned to orientite by Moore et al. (1985). So thesimplified formula of cassagnaite may be written
as (Ca, Mn2+ )4 (Fe3+ , Mn3+ , Al)4 (OH)4 (V3+ , Mg, Al)2 (O,OH)4 (SiO4 )2 (Si3 O10 ), where the hydroxyl groups per formula unit account for the valence balance and are positioned next to the cations to which they are bonded withthe highest strength.

100

R. Basso, C. Carbone, A. Palenzona

A slight deviation from the simplified ratio 1:1 of intersheets of the two types may be expected taking into accountthe chemical composition of cassagnaite, as inferred fromthe empirical formula, which shows a small excess of the
Si content as well as a small deficiency of cations to fill up
the M2 site.
According to Hawthorne (2002), the appropriate composition for the “dominant” end-memberof the complex solid-solution may be written as
3+
Ca4 Fe3+
4 (OH)4 V2 O2 (OH)2 (SiO4 )2 (Si3 O10 ) and it may be
assumed as the end-member formula of cassagnaite.
The poor quality of the X-ray single-crystal reflections
and the too low number of observed reflections prevent any
attempt to introduce anisotropic temperature factors in therefinement and to refine the structure of cassagnaite according to the model proposed for orientite by Mellini et al.
(1986), which involves a great number of parameters to be
derived. The proposed structure of cassagnaite represents
at present an averaged model of a high disordered structure
and attempts will be made to improve it.
On the basis of the structural characters, cassagnaite belongs to the group of minerals with 6 × 9 Å sheet structurereported by Moore et al. (1985).
From the point of view of the mineral classification,cassagnaite may be ascribed to the Dana Class: 58.3.1.2
Sorosilicate insular, mixed, single, and larger tetrahedralgroups – with insular single and triple groups (n = 1, 3) –ardennite group (ardennite and orientite), and to the Strunz
and Nickel Class 9.BJ, Sorosilicates with Si3 O10 , Si4 O11 ,etc. Anions; Cations in Octahedral [6] and/or Greater Coordination.
Acknowledgements: The authors wish to thank G.
Corallo and G. Ghia which found and furnished the sample. The authors are grateful to Dr. M. Pani for the singlecrystal X-ray data collection and to Dr. M. M. Carnasciali
for the micro-Raman analysis. The authors would like tothank Prof. M. Pasero and Prof. A. Barresi for helpful
reviews. The Italian MIUR – Ministero dell’Istruzione,dell’Università e della Ricerca (PRIN-COFIN2003: “Crystal chemistry of ore minerals and their alteration products
from polymetallic ore deposits in Ligurian ophiolites” andPRIN-COFIN2006: “The role of mineral phases in the mobilization and storage of contaminant elements within mining sites of eastern Liguria”) is acknowledged for financial
support.

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Received 30 May 2007
Modified version received 26 July 2007
Accepted 24 September 2007