American Mineralogist, Volume 95, pages 487–494, 2010

Arsenic-rich fergusonite-beta-(Y) from Mount Cervandone (Western Alps, Italy):
Crystal structure and genetic implications
AlessAndro GuAstoni,1 FernAndo CámArA,2 And FAbrizio nestolA1,3,*
1Department of Geoscience, University of Padova, via Giotto 1, 35137 Padova, Italy
CNR-Institute of Geosciences and Georesources, U.O.S. Pavia, via Ferrata 1, 27100 Pavia, Italy
3
CNR-Institute of Geosciences and Georesources, U.O.S. Padova, via Giotto 1, 35137 Padova, Italy
2

AbstrACt
An As-rich variety of fergusonite-beta-(Y) occurs as greenish yellow pseudo-bipyramidal crystals
up to 1 mm in length in centimeter-sized secondary cavities within sub-horizontal pegmatite dikes atMount Cervandone (Western Alps, Italy). The mineral is associated with quartz, biotite, potassium
feldspar, and orange-yellow barrel-shaped hexagonal crystals of synchysite-(Ce) up to 2 mm in length.
Fergusonite-beta-(Y) crystallized during the Alpine metamorphism under amphibolite-facies conditions,as a result of interaction between As-enriched hydrothermal fluids, circulating through the pegmatite
dikes, and precursor accessory minerals in the pegmatites enriched in high-field-strength elements.
These pegmatites are of NYF (niobium-yttrium-fluorine) geochemical type and served as the principal
source of Be, Y, Nb, Ta, and rare-earth elements (REE) that were liberated and redeposited as rare
Be-As-Y-REE minerals, including the As-rich fergusonite-beta-(Y). The latter mineral crystallizes
with monoclinic symmetry [a = 5.1794(14), b = 11.089(3), c = 5.1176(14) Å, β = 91.282(8)°, V =293.87(14) Å3, space group I2/a] and has the empirical formula (Y0.70Dy0.07Er0.05Ca0.05Gd0.02U0.02Yb0.01
Tb0.01Th0.01Nd0.01)Σ0.95(Nb0.68As5+
0.27W0.06Ta0.01Si0.01)Σ1.03O4. The crystal structure of fergusonite-beta-(Y)has been refined using a thermally untreated single crystal to R1 = 6.6% for 441 observed reflections
with Fo/σFo > 4. The incorporation of As in the structure of monoclinic fergusonite-type phases isdiscussed in the context of the data available for synthetic analogs.
Keywords: Fergusonite, single crystal, X-ray diffraction, EMPA

introduCtion And previous work
The compound YNbO4 (fergusonite) occurs as different
polymorphs depending on the temperature of crystallization(Wolten 1967): a high-T polymorph (space group I41/a, no. 88,
Z = 4) isostructural with scheelite (CaWO4; Hazen et al. 1985)
and powellite (CaMoO4; Crichton and Grzechnik 2004); and a
low-T polymorph with the fergusonite-type structure (known
also in the mineralogical literature as fergusonite-beta, spacegroup I2/a, no. 15, Z = 4). The structures of both scheelite- andfergusonite-type polymorphs were first determined by Komkov(1959), who proposed the non-centrosymmetric space group I2
for the fergusonite type. This author observed the monoclinic
polymorph after annealing a natural tetragonal fergusonite crystal
at 1000 °C for 15 min. Neutron diffraction studies of synthetic
YNbO4 have shown that it actually crystallizes with space group
I2/a (no. 15, Weitzel and Schröcke 1980). The standard setting
for space group I2/a is C2/c, which can be obtained from theformer by applying the transformation matrix (001/010/101).
However, to facilitate comparison with the scheelite-type structure, we will use the body-centered setting in the present work.
Wolten and Chase (1967) observed in synthetic YTaO4 that the
low-T fergusonite-type structure forms only as a consequenceof a displacive second-order transformation of the scheelitetype structure. For the same compound, a different monoclinic* E-mail: fabrizio.nestola@unipd.it
0003-004X/10/0004–487$05.00/DOI: 10.2138/am.2010.3239

487

polymorph (space group P2/a, no. 13, Z = 2) forms at T above
the tetragonal-monoclinic transition and transforms very slowly
on cooling to a scheelite-type structure through a discontinuoustransition. Sugitani and Nagashima (1975) claimed to have observed the P2/a monoclinic structure also in synthetic YNbO4.
Recently, Hori et al. (2006) have described the natural occurrence of YTaO4, approved as the new mineral iwashiroite-(Y),whose structure has the same topology as wolframite (spacegroup P2/c, no. 13, with Z = 2). Due to the structural similarity
between iwashiroite-(Y) and wolframite and the extensive Nb for
Ta substitution in iwashiroite-(Y), the existence of a wolframitetype phase is expected also for YNbO4.
The stoichiometry of the above-mentioned three polymorphs
is of type ABX4, where the B sites are fourfold coordinatedand the A sites are eightfold coordinated. However, these threepolymorphs are not the only possible topologies for ABX 4
compounds with B = (Nb,Ta) and X = O. If A = Sb3+ or Bi3+, thestructure is orthorhombic and can be either polar (space group
Pc21n) or centrosymmetric (Pcnn), as seen in the stibiotantalitestibiocolumbite and bismutotantalite-bismutocolumbite series(Galliski et al. 2001 and references therein). Even though the
charge distribution and lattice parameters of these phases are
similar to fergusonite (A = R3+, B = R5+), the topologies ofthese minerals are completely different because in the Sb andBi oxides, both A and B sites are sixfold coordinated. A different, yet monoclinic, polymorph has been described at high P
for BaWO4 (BaWO4-II type, space group P21/n, no. 14, Z = 8),

488

GUASTONI ET Al.: STRUCTURE OF ARSENIC-RICH BETA-FERGUSONITE-(Y)which has the scheelite structure at room P (see Manjon and
Errandonea 2009, for a review of high-P transformations in the
ABX4 compounds). last, if Nb occupies the A site and boron theB site (as in behierite, NbBO4), the symmetry becomes I41/amd(no. 141, Z = 4) corresponding to a zircon-type structure topology (e.g. Mrose and Rose 1961). In fact, members of the solid
solution between xenotime-(Y) (YPO4; Krstanović 1965) andchernovite-(Y) (YAsO4; Goldin et al. 1967), which also preserve
the same stoichiometry and charge distribution, also crystallize
with the zircon-type structure and can transform to a scheelitetype structure at high P (Zhang et al. 2008). Phosphates andarsenates of rare-earth elements (REE) with ABX4 stoichiometry
also occur as monoclinic monazite-type structures (space group
P21/n, no. 14, Z = 4).
Unit-cell data for an unheated sample of fergusonite-type
YNbO4 from granitic pegmatites at Batou, Inner Mongolia, werefirst reported by Gorzhevskaya et al. (1961). The Ce-dominantfergusonite-type CeNbO4, formerly known as brocenite, from the
Chernigovskiy carbonatite in Ukraine, upon heating to 700 °C
gave an X-ray pattern identical to that of synthetic CeNbO4,
NdNbO4, and PrNbO4 (Chashka and Marchenko 1976). A surveyof the literature shows that no crystal structure refinements ofnatural fergusonite-type YNbO4 have been published so far. The
structure topology of the fergusonite-type structure has recently
attracted interest because it is characteristic of stable high-pressurepolymorphs of ABX4 compounds with the scheelite structure.
Materials with the ABX4 stoichiometry are of technological importance as solid-state lasers (X = F), solid-state scintillators (X= O) (Errandonea et al. 2005, 2008 and references therein), andsuper-hard materials (e.g., reidite, ZrSiO4, Reid and Ringwood
1969; liu 1979; Scott et al. 2002). Recently, a fergusonite-type
structure has been observed for a quenched high-P phase of(Zr,Ti)O2 (Troitzsch et al. 2007), which is used as a temperaturestable dielectric material in ceramic capacitors and as a stableoscillator at microwave frequencies in satellite communication.
The present study is the first report of the crystal structure ofunheated naturally occurring fergusonite-type YNbO4, as exemplified by As-rich fergusonite-beta-(Y) from Mount Cervandone(Verbano-Cusio-Ossola, Devero valley, Western Alps, Italy). We
also provide the first description of the crystal chemistry of an Asrich fergusonite-group mineral. Mount Cervandone and the Swiss
Wannigletscher (also referred to as Cherbadung) are among thebest-known mineral localities in the Alps. A plethora of rare minerals from this locality has been investigated in detail (Graeserand Albertini 1995), including several arsenate minerals that werediscovered here: asbecasite [Ca3(Ti,Sn4+)As3+
6 Si2Be2O20] and cafarsite [Ca8(Ti,Fe2+,Fe3+,Mn)6–7(As3+O3)12·4H2O] (Graeser 1966);
cervandonite-(Ce) [(Ce,Nd,la)(Fe3+,Fe2+,Ti4+,Al)3O2(Si2O7)1–x+y(AsO3)1+x–y(OH)3x–3y] (Demartin and Gramaccioli 2008); fetiasite[(Fe2+,Fe3+,Ti)3O2(As3+2 O5)] (Graeser et al. 1994); gasparite-(Ce)[(Ce,la,Nd)AsO4] (Graeser and Schwander 1987); and paraniite(Y) [Ca2Y(AsO4)(WO4)2] (Demartin et al. 1994).

yellow color under sunlight. The mineral occurs in centimetersized secondary cavities hosted by sub-horizontal pegmatite dikes,
and is associated with quartz, biotite, potassium feldspar, andorange-yellow barrel-shape hexagonal crystals of synchysite-(Ce)up to 2 mm in length. The Mount Cervandone locality was previously described as a typical example of Alpine-type quartz-bearing fissures (Graeser and Albertini 1995). However, recent studies
indicate that the mineralized fissures of Mount Cervandone andWannigletscher are mainly found in, and related to, pegmatitic
dikes (Guastoni et al. 2006). These dikes are strongly deformed
and contorted, exhibiting boudinage-like textures, but they are
generally concordant with the foliation of fine-grained two-mica
leucocratic gneiss, which can be interpreted as leucograniticaplitic rocks metamorphosed under amphibolite-facies conditions(Dal Piaz 1975). The dikes have a thickness of several decimeters
and are composed of coarse vitreous and smoky quartz, potassium
feldspar, and greenish micas (probably muscovite). locally, thepegmatite dikes are crosscut by Alpine-type quartz veins, which
are generally subvertical, discordant with respect to the foliationof the gneissic host rocks and commonly contain open fissures
lined with quartz crystals. Several accessory minerals are found inthese quartz fissures, including cafarsite, agardite-(Y), asbecasite,
chernovite-(Y), gasparite-(Ce), rutile, anatase, and tennantite.
The latter sulfide mineral is frequently associated with several
arsenates, carbonates, sulfates, and vanadates of Fe, Cu, Pb, andZn (Guastoni et al. 2006).

oCCurrenCe And desCription oF thepArAGenesis

As-rich fergusonite-beta-(Y) from Mount Cervandone occurs
as pseudo-bipyramidal crystals up to 1 mm in length (Fig. 1),
which have a greenish yellow color under incandescent light and

FiGure 1. Secondary-electron image of two pseudo-bipyramidal
crystals of As-rich fergusonite-beta-(Y) from Mount Cervandone.

GUASTONI ET Al.: STRUCTURE OF ARSENIC-RICH BETA-FERGUSONITE-(Y)The pegmatite dikes show a strong NYF (niobium-yttriumfluorine) geochemical signature manifested in the presenceof several Be-As-Nb-Y rare-earth minerals, which include
aeschynite-(Y), agardite-(Y), Nb-rich anatase, asbecasite,cervandonite-(Ce), chernovite-(Y), crichtonite-group minerals,
As-rich fergusonite-beta-(Y), fluorite, gadolinite-(Y), monazite(Ce), paraniite-(Y), Nb-rich rutile, synchysite-(Ce), and xenotime-(Y). The assemblages of arsenates, carbonates, sulfates, andvanadates of Fe, Cu, Pb, and Zn are mainly hosted by quartz veins
deposited by hydrothermal fluids during the Alpine event. We
interpret these assemblages as products of crystallization from
As-enriched hydrothermal fluids generated during the Alpine
metamorphism under amphibolite-facies conditions. Interaction of the fluids circulating through the pegmatite dikes withthe aforementioned accessory minerals enriched in high-fieldstrength elements resulted in the breakdown of these minerals,
remobilization of Be, Y, Nb, Ta, and REE, and crystallization ofseveral rare Be-As-Y-REE minerals, including the As-rich varietyof fergusonite-beta-(Y) studied in the present work.

AnAlytiCAl methods
The chemical composition of As-rich fergusonite-beta-(Y) was determinedusing a Cameca-Camebax SX50 electron microprobe operating in wavelengthdispersive mode with a focused beam (~1 µm in diameter), an acceleration voltage
of 20 kV, and a beam current of 20 nA, with 10 and 5 s counting times for peak
and background, respectively. X-ray counts were converted to oxide wt% using
the PAP correction program supplied by Cameca (Pouchou and Pichoir 1985).
The following natural and synthetic standards, spectral lines and detector types
were used in the analysis: wollastonite (SiKα, CaKα, TAP), corundum (AlKα,TAP), apatite (PKα, TAP), Fe2O3 (FeKα, liF), AsGa (AsLα, TAP), Zr-Y-REEsilicates (ZrLα, YLα, REELα, and NdLβ, liF), synthetic UO2 and ThO2 (UMαand ThMα, PET), metallic Nb (NbLα, PET), and Ta and W (TaLα and WLα, liF).
The concentrations of Al, P, Fe, and Zr were found to be below the detection limitof the electron microprobe (ca. 0.05 wt%). The average composition and formulaof fergusonite-beta-(Y) are reported in Table 1.
Diffraction data were obtained from a very small single crystal (0.13 × 0.10 ×
0.07 mm3) mounted on a Bruker AXS SMART APEX diffractometer equipped with

Table 1.

Average composition of As-rich fergusonite-beta-(Y) from
Mount Cervandone and formula calculated on the basis of
4 O atoms

Oxide
SiO2
As2O5
Ta2O5
Nb2O5
WO3
Ce2O3
Nd2O3
Sm2O3
Gd2O3
Tb2O3
Dy2O3
Er2O3
Yb2O3
ThO2
UO2
Y2O3
CaO
Total

wt%
0.17
12.05
0.71
35.23
5.04
0.21
0.41
0.16
1.66
0.59
5.01
3.61
0.90
0.58
2.28
30.79
1.01
100.41Range
0.09–0.25
10.19–16.15
30.31–37.38
0.51–1.18
4.24–6.70
0.06–0.38
0.17–0.51
0.10–0.45
1.52–1.91
0.38–0.73
4.73–5.20
3.32–3.83
0.55–1.30
0.45–0.66
1.51–3.23
29.45–31.50
0.81–1.15

Element
Si
W6+
As5+
Ta5+
Nb5+
ΣB

apfu
0.007
0.056
0.270
0.008
0.683
1.024

Y
Ca
Ce
Nd
Sm
Gd
Tb
Dy
Er
Yb
Th
U
ΣA

0.702
0.046
0.003
0.006
0.002
0.024
0.008
0.069
0.049
0.012
0.006
0.022
0.949

s.s. Bcalc
41.75
s.s. Acalc
42.29
s.s. Bobs
41.82∆B (%)–0.1
s.s. Aobs
41.51∆A (%)
1.8
Note: s.s. obs is the observed site scattering obtained from the refinement of
the crystal structure, here reported in electrons per formula unit (epfu); s.s. calc
is the site scattering calculated from the chemical composition obtained byelectron-microprobe analysis.

489

a graphite-filtered MoKα X-ray source and a CCD detector. Despite the small sizeof the sample, the diffraction spots showed a significant mosaicity. The intensitiesof 3053 reflections with –8 < h < 8, – 17 < k < 17, – 8 < l < 8 were collected to 70°2θ with an acquisition time of 15 s per 0.2° frame. An empirical absorption correction (SADABS, Sheldrick 1998) was applied to the acquired data. The refined
unit-cell parameters (Table 2) were obtained from 928 reflections. On the basis of
441 unique observed reflections (Fo > 4σF), the crystal structure was refined inspace group I2/a (no. 15, setting b3), starting from the coordinates of Santoro etal. (1980), transformed for this setting with the SHElXl 97 program (Sheldrick1997) using the WinGX integrated system (Farrugia 1999) to R1 = 6.6% and agoodness-of-fit value of 1.16. Scattering curves for ionized atoms were takenfrom the International Tables for Crystallography (Wilson 1992). The R indicesand all other pertinent details of the refinement are summarized in Table 2. Site
occupancies for the A and B sites were refined with the scattering curves of Dy andNb, respectively. The chemical analysis (Table 1) obtained from the same crystal
indicates that X-ray scattering at the A site is significantly higher than that fromY. Accordingly, we decided to use the scattering factor of the stronger scatterer
Dy, which has an ionic radius similar to that of Y. Agreement between observed(X-ray) and calculated (i.e., based on the electron microprobe analysis) scattering
per site is satisfactory (Table 1). Final atomic parameters are given in Table 3, andselected atomic distances and angles in Table 4. A table of observed and calculated
structure factors is available from the authors upon request.

results And disCussion
Composition
Back-scattered electron images (BSE) of polished fragmentsof the As-rich fergusonite-beta-(Y) crystal used for the structurerefinement show that it is devoid of zoning and does not containany exsolved phases (Fig. 2a). However, the electron microprobe
analyses (Table 1) indicate significant variations in the occupancyof the B site: 30.31–37.38 wt% Nb2O5, 10.19–16.15 wt% As2O5,
4.24–6.70 wt% WO3, and 0.51–1.18 wt% Ta2O5. From Figure
2b, it is clear that the highest (Nb+Ta) contents (0.704–0.748
atoms per formula unit, apfu) are found at the core, whereas thehighest As contents (0.300–0.394 apfu) are confined to the rim of
the crystal. The W content is almost constant across the crystal,and As is never a dominant species at the B site. At the A site,the mineral shows a more homogeneous composition (Table 1)
dominated by Y and heavy REE.
Table 2.

Crystal data and structure refinement details for As-richfergusonite-beta-(Y)Formula weight
Temperature
Wavelength
Crystal system
Space group
Unit-cell dimensions
VolumeZ
Density (calculated)
Absorption coefficientF(000)
Crystal size
Theta range for data collection
Index ranges
Reflections collected
Independent reflections
Completeness to theta = 35.15°
Absorption correction
Max. and min. transmission
Refinement method
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]R indices (all data)
Largest diff. peak and hole

960.34298(2) K
0.71073 Å
Monoclinic
I2/aa = 5.1794(14) Å
α = 90°b = 11.089(3) Å
β = 91.282(8)°c = 5.1176(14) Å
γ = 90°293.87(14) Å3
85.427 Mg/m3
26.250 mm–1432
0.13 × 0.10 × 0.07 mm33.67 to 35.15°–8 ← h ← 8, –17 ← k ← 17, –8 ← l ← 83053
655 (Rint = 0.0469)
99.7%
Semi-empirical from equivalents
0.160 and 0.080
Full-matrix least-squares on F2
655 / 0 / 31
1.157
R1 = 0.0663, wR2 = 0.2019
R1 = 0.0874, wR2 = 0.2189
5.139 and –1.495 e·Å–3490
Table 3.

GUASTONI ET Al.: STRUCTURE OF ARSENIC-RICH BETA-FERGUSONITE-(Y)Atomic coordinates and equivalent isotropic displacement parameters (Å2 × 103) for As-rich fergusonite-beta-(Y)

Atom
Wyck
Occ.*
x
y
z
Ueq
U11
U22
U33
U23
U13
U12
A
4e
41.51
¼
0.1244(1)
0
23(1)
27(1)
14(1)
27(1)
0
–1(1)
0
B
4e
41.82
¼
0.6302(1)
0
25(1)
21(1)
33(1)
20(1)
0
1(1)
0
O1
8f
0.0039(16) 0.7108(7) 0.1760(20)
43(3)
39(5)
25(4)
66(6)
10(4)
2(4)
4(3)
O2
8f
0.9105(16) 0.4580(6) 0.2508(15)
32(2)
35(4)
21(3)
41(4)
1(3)
–4(3)
4(3)
Notes: Ueq is defined as one third of the trace of the orthogonalized Uij tensor. The anisotropic displacement factor exponent takes the form: –2 π2[ h2a*2U11 + ... +2hka*b*U12].
* Occupancies obtained using the scattering curves of Dy and Nb at the A and B sites, respectively.

Table 4.

Selected distances and angles for As-rich fergusonitebeta-(Y)A-O2 (×2)
2.335(8)A-O2 (×2)
2.402(7)A-O1 (×2)
2.344(9)A-O1 (×2)
2.414(8)
<A-O>
2.374
V A (Å3)
23.56
A-B
3.600(1)A-A
3.783(1)
B-O2 (×2)
1.802(8)B-O1 (×2)
1.813(9)
<B-O>
1.807
3
V B (Å )
2.98
AV*
46.8
QE*
1.0123O1-B-O1
120.9(5)O2-B-O2
114.2(4)O1-B-O2 (×2)
107.6(4)O1-B-O2 (×2)
103.4(4)O2-A-O2
133.9(4)O1-A-O1
131.8(4)O1-A-O1
81.6(4)O2-A-O2
79.6(4)O2-A-O1 (×2)
101.6(3)O2-A-O1 (×2)
96.8(3)O2-A-O2 (×2)
71.02(17)O2-A-O2 (×2)
74.0(3)O1-A-O2 (×2)
153.6(3)O1-A-O2 (×2)
74.5(3)O2-A-O1 (×2)
153.8(3)O2-A-O1 (×2)
72.3(2)O1-A-O1 (×2)
74.7(3)O1-A-O1 (×2)
69.21(19)O2-A-O1 (×2)
126.0(3)O2-A-O1 (×2)
125.1(3)*AV = angle variance, QE = quadratic elongation (Robinson et al. 1971).

Crystal structure
The observed site scattering value at the B site agrees wellwith the average value obtained from the chemical composition.
The substitution of As for Nb in the B site reduces the averageobserved <B-O> distance from 1.883 Å in synthetic fergusonitetype REENbO4 compounds to 1.808 Å in the Mount Cervandone
sample (Table 5). Complete substitution of Nb with As in theB site would lead to a <B-O> distance of 1.740 Å observed inchernovite-(Y) (Strada and Schwendimann 1934). At the sametime, this substitution reduces deformation of the B-site geometry from an angle-variance (AV, Robinson et al. 1971) value of
141.4 to 46.9 for fergusonite-type ErNbO4 (Keller 1962) and thestudied sample, respectively. In the scheelite-type structure, AVvalues are in the range 22.4–26.14 (see Table 5). Nonetheless, inthe zircon-type structure of chernovite-(Y), the AsO4 tetrahedron
is highly deformed. An increase in As content is probably also
responsible for the reduction of the β angle from 94.6 to 91.3°
from REENbO4 compounds to As-rich fergusonite-beta-(Y),respectively (see Table 5). Interestingly, the compound BiAsO4,

FiGure 2. Back-scattered electron image of the studied crystal (a)showing the location of electron-microprobe analyses plotted in b, interms of the main constituents at the B site.

which can crystallize with both the scheelite-type structure (tetrarooseveltite, Mooney 1948) and the monazite-type structure(rooseveltite, Bedlivy and Mereiter 1982), show smaller <As-O>
distances of 1.619 and 1.682 Å, respectively. The structure ofsynthetic SmAsO4 has been recently reported to also have ascheelite-type structure (Kang and Schleid 2006) and shows<As-O> = 1.697 Å. The difference in the observed <As-O>
distances between SmAsO4 and tetrarooseveltite may be due tothe lower quality of structural data available for the latter. The
AV value of rooseveltite (54.68) is closer to that observed in
the studied crystal.
In the literature, there is a significant volume of crystallo-GUASTONI ET Al.: STRUCTURE OF ARSENIC-RICH BETA-FERGUSONITE-(Y)Table 5.

491

Selected average bond distances, distortion parameters, average ionic radii (<ri>), and lattice parameters of ABX4 compounds with
A = REE, B = Nb or As, and X = O

AStructure type
ABX4
<A-O> (Å) <B-O> (Å)
AV*
QE*<ri> (Å) B<ri> (Å)
a (Å)
b (Å)
c† (Å)
β (°)(a+c)/2 (Å) Ref.
Fergusonite-type LuNbO4
2.321
1.886
120.91 1.033
0.977
0.48
5.229(3) 10.822(8) 5.042(2) 94.38(2)5.136
1Fergusonite-type YbNbO4
2.329
1.877
128.43 1.034
0.985
0.48
5.239(1) 10.834(1) 5.044(1) 94.47(1)5.142
2Fergusonite-type TmNbO4
2.331
1.894
119.83 1.033
0.994
0.48
5.262(2) 10.876(5) 5.049(2) 94.52(5)5.156
1Fergusonite-type ErNbO4
2.346
1.888
141.40 1.036
1.004
0.48
5.280(1) 10.916(5) 5.064(2) 94.48(8)5.172
1Fergusonite-type HoNbO4
2.358
1.880
129.47 1.035
1.015
0.48
5.299(1) 10.947(1) 5.072(1) 94.53(1)5.185
2Fergusonite-type YNbO4
2.363
1.894
121.79 1.035
1.019
0.48
5.317(1) 10.999(1) 5.090(1) 94.53(1)
5.204
3Fergusonite-type DyNbO4
2.360
1.899
139.40 1.036
1.027
0.48
5.320(1) 11.000(8) 5.074(4) 94.57(7)5.197
1Fergusonite-type TbNbO4
2.382
1.881
136.27 1.035
1.040
0.48
5.349(1) 11.036(9) 5.085(1) 94.62(3)5.217
1Fergusonite-type GdNbO4
2.394
1.889
135.06 1.035
1.053
0.48
5.374(3) 11.095(9) 5.106(5)
94.58
5.240
4
Fergusonite-type EuNbO4
2.407
1.884
134.89 1.036
1.066
0.48
5.395(1) 11.135(1) 5.116(1) 94.62(1)5.253
1Fergusonite-type SmNbO4
2.417
1.888
134.45 1.036
1.079
0.48
5.422(5) 11.178(9) 5.121(5)
94.69
5.272
4
Fergusonite-type PrNbO4
2.466
1.875
134.62 1.035
1.093
0.48
5.499(2) 11.342(7) 5.157(2) 94.57(5)5.328
1Fergusonite-type NdNbO4
2.447
1.879
129.09 1.034
1.109
0.48
5.467(1) 11.279(1) 5.146(1) 94.50(1)5.307
2Fergusonite-type CeNbO4
2.476
1.883
132.79 1.035
1.143
0.48
5.535(1) 11.399(1) 5.159(1) 94.60(1)5.347
5Fergusonite-type LaNbO4
2.505
1.874
121.41 1.032
1.160
0.48
5.565(1) 11.519(1) 5.202(1) 94.10(1)5.383
2Fergusonite-type (As-rich) YNbO4 2.374
1.808
46.94
1.012
1.024
0.39
5.179(1) 11.090(3) 5.118(1) 91.28(1)5.149
6Scheelite-type
SmAsO4
2.427
1.697
26.14
1.007
1.079
0.335
5.066(1) 11.461(1) 5.066(1)
90.00
5.066
7
Scheelite-type
CeNbO4
2.497
1.854
22.43
1.006
1.143
0.48
5.377(1) 11.595(1) 5.377(1)
90.00
5.377
8
Scheelite-type
YNbO4
2.500
1.892
25.41 1.007
1.014
0.48
5.160
10.890
5.160
90.00
5.160
9
Note: References: [1] Keller (1962), [2] Tsunekawa et al. (1993), [3] Weitzel and Schröcke (1980), [4] Trunov and Kinzhibalo (1982), [5] Santoro et al. (1980), [6] thisstudy, [7] Kang and Schleid (2006), [8] Skinner et al. (2004), [9] Komkov (1959).
* AV = angle variance and QE = quadratic elongation (Robinson et al. 1971).
† c refers to b in scheelite-type structures.

graphic data on synthetic fergusonite-type niobates with different
cations in the A site (REE and Y). Using these data, we confirm
a linear correlation between the average distance <A-O> and theionic radius of the A-site cation A<ri>, as first noted by Kinzhibalo
et al. (1982): A<ri> = 0.943*<A-O> (Å) – 1.2057 (R² = 0.97).
The sample examined in the present study and published dataon scheelite-type structures (with As or Nb at the B site) plot on
the same trend (Fig. 3a). The only outliers are synthetic PrNbO4
studied by Keller (1962) and scheelite-type fergusonite studied
by Komkov (1959), who reported a rather unusual A-site occupancy for this natural sample, Y0.85Yb0.15NbO4. With the above
equation, Komkov’s data give a calculated ionic radius of 1.15 Å,which is much closer to that of la. It is clear that deviation ofthis sample from the above trend results from the inadequacyof its chemical analysis.
Another interesting relationship was observed between the alattice parameter and the mean ionic radius of the A-site cation.
Fergusonite-type compounds define a linear trend with A<ri>= 0.521*a (Å) – 1.7468 (R² = 0.98) because changes in A-site
occupancy (with fixed B-site occupancy) have a very limited
effect on the β angle. Scheelite-type polymorphs plot outsidethat trend and so does the As-rich monoclinic sample examinedin this study (Fig. 3b). If we ignore the monoclinic distortionby averaging the a and c lattice parameters, the scheelite-type
compounds with Nb in their B site plot along the trend (Fig.
3c), but not the As-rich sample. This deviation implies that theincorporation of As reduces the length of the a lattice parameter,or has an important effect on the β angle, thus reducing the magnitude of monoclinic distortion. This can be also seen in Figure
3d, where the AV value of Robinson et al. (1971) is plotted vs.
A<ri>: while monoclinic phases with light to medium rare-earth
cations at the A site show a progressive decrease of the AV value
with increasing A<ri>, tetragonal phases show a much smaller
distortion of the B tetrahedron, and so does the As-rich sample.
Considering the fact that only about 25% of the B positions areoccupied by As and that fergusonite-type structures undergo aphase transition to the scheelite-type structure with decreasing T,

it is plausible that the presence of As reduces the temperature of
the tetragonal-to-monoclinic transition. Therefore, in principle, itwould be possible to expect that an increase in As content would
lead to a change in symmetry from I2/a to I41/a, the distortion
being proportional to the As content. Clearly, experimental dataon intermediate Nb-As compositions will be necessary to explore
the compositional dependence of the monoclinic-to-tetragonalphase transition in fergusonite.
It is well known that increasing P leads to phase transitions
from scheelite-type to fergusonite-type structures (Grzechnik
et al. 2005) and from zircon-type to scheelite-type structures(Stubican and Roy 1963; Zhang et al. 2008). It has been also
proposed that a wolframite-type phase is the post-fergusonitetype structure with increasing pressure (li et al. 2004), whereas
the scheelite to wolframite high-P phase transition has been
predicted for CaMoO4 and CaWO4 by Nicol and Durana (1971)
and confirmed by Errandonea et al. (2003). These phase transitions have been modeled by Fukunaga and Yamaoka (1979)
and Bastide (1987) as a function of the ratio of the ionic radii of
cations at both A and B sites and of the ionic radius of the anion.
Recently, transformation from a zircon-to-scheelite structure and,at higher P, to a fergusonite-type structure has been described for
luVO4 (Mittal et al. 2008).
Kolitsch and Holtstam (2004) reviewed the crystal chemistryof REEXO4 compounds (with X = As, V, and P), which show
both the monazite- and zircon-type topologies. Depending on theionic radius of the lanthanide considered, for a fixed compositionof the B site, the larger the REE3+ cation, the more stable themonazite-type structure. At the same time, an increase in sizeof the XO4 group destabilizes the monazite structure in favor of
the zircon-type structure. loskutov et al. (1977) even proposedthat an increase in ionic radius of the X cation ultimately leads
to the stabilization of a scheelite structure and, for very large X,
a wolframite-type structure. Interestingly, SmAsO4 falls close
to the region in which the structural topology of REE arsenates
changes from that of monazite (REE = la, Ce, Pr, and Nd and
REE = Pm-lu, Y, and Sc; see Kolitsch and Holtstam 2004 and492

GUASTONI ET Al.: STRUCTURE OF ARSENIC-RICH BETA-FERGUSONITE-(Y)FiGure 3. Relations
between the average
ionic radius at the A siteand (a) the average bonddistance at the A site,(b) a lattice parameter,(c) average a+c lattice
parameter, and (d) anglevariance value (Robinson
et al. 1971) for thestudied sample (opencircle), SmAsO4 of Kangand Schleid (2006) (graycircle), T-fergusonite(Ce) of Skinner et al.
(2004) (gray diamond),
T-fergusonite-(Y) of
Komkov (1959) (cross)and synthetic fergusonitetype REENbO 4 withvariying compositionof the A site (references
as in Table 5) (black
circles).

references therein). According to Kang and Schleid (2006),
scheelite-type structures are stabilized at high P, and become
metastable at room P. However, scheelite-type NdAsO4 has also
been reported from room-P synthesis at 550 °C (Mazhenov etal. 1988).
In Figure 4, three ABO 4 structure types with different
topologies but the same coordination for A and B cations(fergusonite-type, scheelite-type, and zircon-type) are compared. From a descriptive point of view, a shift along the c axis
within (110) planes would change the zircon-type topology[as found in chernovite-(Y)] to the scheelite topology. The
latter structure type can easily undergo a symmetry reduction
to monoclinic by softer distortion. Figure 4 shows only the
topologies stable at room P and T; the actual mechanism of
the zircon-scheelite transformation at high pressure is much
more complex and involves a change in coordination of the O
atoms, which undergo a displacement of ca. 1 Å (Smirnov etal. 2008). Structural changes are required to effect a transition
to the fergusonite topology, as can be seen from the results of
Mittal et al. (2008), who observed a negligible volume change
during the scheelite-to-fergusonite phase transition, and proposedthat it is a second-order displacive phase transition. In addition,
Kolitsch and Holtstam (2004) proposed that the presence of
impurities (Ca, Th, U, and Si) can stabilize one or the other

FiGure 4. Sketch comparing the structures of fergusonite topology(a), scheelite topology in two different orientations (b), and zircon
topology (c). The open squares in the fergusonite and scheelite topologies
help in visualizing the tetragonal-to-monoclinic deformation. White
spheres are A sites. Tetrahedra are B sites. The arrows in c show how
a differential shift along [001](110) relates the zircon and scheelite
topologies.

GUASTONI ET Al.: STRUCTURE OF ARSENIC-RICH BETA-FERGUSONITE-(Y)polymorph. The sample studied in the present work contains allof the above elements, which may explain the stability of the
fergusonite structure relative to the scheelite structure.
To conclude, although we cannot rule out the possibility thata complete series of scheelite-type compounds involving the
As-for-Nb substitution exists, it is highly probable that there
is at least one change in topology across that series, controlled
mainly by atomic substitutions in the B site. Paraniite-(Y)[Ca2Y(AsO4)(WO4)2], which is also present at Mount Cervandone and may be viewed as an intermediate member of the solid
solution between scheelite and chernovite-(Y), has an overall
scheelite topology, but a tripled lattice periodicity along [001]owing to cation ordering in the symmetrically non-equivalent
B sites along the c axis. We are conducting a detailed studyof chernovite-(Y) samples from the same locality to constrain
the chemical variations and possible changes in symmetry innaturally occurring Y arsenates.

ACknowledGments
We thank the fruitful comments of W. Crichton, an anonymous reviewer,and Associate Editor A. Chakhmouradian that helped to improve the manuscript.
We also thank R. Carampin of CNR-IGG Padova for access to the WDS electron
microprobe facility and R. Gastoni of CNR-IGG Pavia for sample preparation.
Fernando Cámara was supported by funding by CNR-IGG through the project
TAP01.004.002. Fabrizio Nestola was supported by funding by MIUR PRIN
2006047943 to A. Dal Negro.

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Manuscript received March 11, 2009
Manuscript accepted noveMber 10, 2009
Manuscript handled by anton chakhMouradian