Mineralogical Magazine, October 2016, Vol. 80(6), pp. 1077–1088Tvrdýite, Fe2+Fe3+
2 Al3(PO4)4(OH)5(OH2)4·2H2O, a newphosphate mineral from Krásno near Horní Slavkov,
Czech Republic
J. SEJKORA1,*, I. E. GREY2, A. R. KAMPF3, J. R. PRICE4 AND J. ČEJKA1
1

Department of Mineralogy and Petrology, National Museum, Cirkusová 1740, Praha 9, 193 00, Czech Republic
CSIRO Mineral Resources, Private Bag 10, Clayton South, Victoria 3169, Australia
3
Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los
Angeles, CA 90007, USA
4
Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia
2[Received 30 June 2015; Accepted 22 October 2015; Associate Editor: Juraj Majzlan]
ABSTR ACTTvrdýite, Fe2+Fe3þ
2 Al3(PO4)4(OH)5(OH2)4·2H2O, is a new phosphate mineral from the abandoned Huber
open pit, in the Krásno ore district near Horní Slavkov, western Bohemia, Czech Republic. It was foundalong with Al-rich beraunite, fluorapatite and pharmacosiderite in a cavity of quartz gangue. Tvrdýite forms
acicular to fibrous crystals with diameters in the range 0.5–5 μm and lengths up to 300 μm, partly grouped inradiating aggregates up to 3 mm in size. It has a silvery to olive, greyish green colour with pearly lustre,
greyish-white streak and is very brittle with an uneven fracture; individual fibres are somewhat flexible.
Cleavage on {100} is good; the Mohs hardness is ∼3–4. The calculated density is 2.834 g cm–3. Tvrdýite is
optically biaxial (–), with α = 1.650(2), β = 1.671(1) and γ = 1.677(1) (white light); 2V = 56(1)°; dispersion:r > v, strong; optical orientation: Z = b, X ≈ a, Y ≈ c; pleochroism: X = greenish blue, Y = yellowish orange,
Z = yellowish orange (X >> Y > Z ). Tvrdýite is monoclinic, space group C2/c, a = 20.564(4), b = 5.101(1), c= 18.883(4) Å, β = 93.68(3)° and V = 1976.7(7) Å3, Z = 4, a:b:c = 4.031:1:3.702. The strongest eight lines
in the powder X-ray diffraction (XRD) pattern [d in Å (I)(hkl)] are 10.227 (100) (200), 9.400 (6) (002),
7.156 (14) (202), 5.120 (7) (400), 3.416 (11) (600), 3.278 (6) (602), 2.562 (5) (800) and 2.0511 (3) (10,0,0).
Chemical analyses by electron microprobe yielded MnO 0.01, ZnO 5.08, FeO 4.31, Fe2O3 21.16, Al2O3
16.71, P2O5 32.64, As2O5 2.56, F 0.53, H2O (calc.) 17.84, O = F –0.22, total 100.62 wt.%. The resulting
empirical formula, calculated on the base of 27 anions, obtained from the crystal structure, is3þ
2+ 3þ
Zn0.52Fe2þ
0:50 Fe2:21Al2.75(PO4)3.86(AsO4)0.19OH4.60F0.23(OH2)4·2H2O. The ideal formula, Fe Fe2 Al3(PO4)4(OH)5(OH2)4·2H2O, requires FeO 8.75, Fe2O3 19.44, Al2O3 18.62, P2O5 34.56, H2O 18.64, total
100.00 wt.%. The crystal structure of tvrdýite was solved from single-crystal data (synchrotron beamline)
and refined to R1 = 0.038 for 2276 reflections with I > 2σ(I). Tvrdýite is isostructural with beraunite, but
contains dominant Al in two of the four independent M sites, which are all occupied by Fe in beraunite.
K E Y WO R D S : tvrdýite, new mineral, phosphate, beraunite, crystal structure, Krásno near Horní Slavkov,

Czech Republic.
Introduction
THE mineral beraunite, ideally Fe2+Fe3þ
5 (PO4)4(OH)5(OH2)4·2H2O, was described for the first time from

*E-mail: jiri_sejkora@nm.cz
DOI: 10.1180/minmag.2016.080.045© 2016 The Mineralogical Society

the Hrbek mine at Svatá Dobrotivá (St Benigna)
near Beroun (Beraun), central Bohemia, Czech
Republic, by Breithaupt (1841). More recently,
beraunite was studied by Fanfani and Zanazzi(1967), Marzoni Fecia di Cossato et al. (1989) and
Moore and Kampf (1992). It is known from many
localities worldwide (Anthony et al., 2000).

J. SEJKORA ET AL.

During our investigations of late hydrothermaland supergene F-rich phosphate assemblages from
Krásno ore district a number of rare and unusual
minerals have been discovered. One of this suite
was an interesting ‘beraunite’ with significant
contents of Al and Zn (Sejkora et al., 2006c),
which have not been previously described for thismineral phase. In this paper, we report thedescription of a new mineral, tvrdýite, found at
Krásno. The new mineral is an analogue of
beraunite in which Al3+ replaces Fe3+ as thedominant cation in two of the four Fe sites in the
beraunite structure, providing the ideal formula
Fe2+Fe3þ
2 Al3(PO4)4(OH)5(OH2)4·2H2O. The newmineral honours the Czech mineralogist andgeologist Dr Jaromír Tvrdý (b. 1959) from
Liberec, northern Bohemia, Czech Republic, for
his contributions to mineralogy and economic
geology. Dr Tvrdý is the author of papers on themineralogy and geology of the Krásno – Horní
Slavkov area (e.g. Plášil and Tvrdý, 2008; Sejkora
and Tvrdý, 2009) and surrounding occurrences(Sejkora et al., 2008; Tvrdý and Plášil, 2010;Pažout et al., 2015). The pronunciation of tvrdýite
is /twe:(r) ’di: ait/, or using special characters /twɜːʳ
ˈdiː aɪt/. This would be similar to ‘twerdeeite’.
The new mineral and name have been approvedby the Commission on New Minerals, Nomenclature
and Classification of the International Mineralogical
Association (IMA 2014-082). The holotype specimen (one sample 5 cm × 4 cm × 2 cm) is depositedin the collections of the Department of Mineralogy
and Petrology of the National Museum in Prague,
Cirkusová 1740, Praha 9, Czech Republic, catalogue number P1P 11/2014. Parts of the holotype
are also deposited at Museum Victoria, Melbourne,
Australia, specimen number M53361 and in the
Natural History Museum of Los Angeles County,
Los Angeles, USA, catalogue number 65560.

stock is bell-shaped in section, similar to a bluntcone. At 100 m under the present surface, the size is
200 m × 100 m, at 150 m under the surface, thestock extends 400 m NE-SW and is ∼250 m wide.
The Huber stock consists basically of autometamorphosed Li-mica-topaz granite with a variable
degree of greisenization. It is assumed that thewhole apical part of this cupola was formed bygreisen and quartz with rich ore mineralization(Beran and Sejkora, 2006). Detailed descriptions of
the mineralogy (more than 230 mineral species) of
the Krásno ore district were published by Beran andSejkora (2006) and Sejkora et al. (2006a–c). Morerecently, the new minerals krásnoite (Mills et al.,2012) and iangreyite (Mills et al., 2011) were
described from this locality, which also hosts thesecond world occurrence of kunatite (Mills et al.,2008) and plimerite (Sejkora et al., 2011).
Tvrdýite was found in late hydrothermally andsupergene altered phosphate accumulations in theabandoned Huber open pit in the Krásno ore district(50°07’22"N 12°48’2"E). The pit exposes the top
part of Huber stock; it is completely greisened andcontains quartz bodies with disseminated cassiterite, wolframite, sulfides and phosphates and, in thepast, was mined for Sn and W (Sejkora et al.,2006c). Tvrdýite was found in a cavity in quartz
gangue in association with older, greenish brown,
Al-rich (1–1.5 atoms per formula unit (apfu) Al)
beraunite, white fluorapatite and yellowish green
pharmacosiderite. It is a late hydrothermal orsupergene mineral formed from the breakdown ofprimary triplite (in association with isokite andfluorapatite) in contact with acidic waters.

Occurrence
The Krásno ore district near the Horní Slavkov,
Slavkovský les area (Czech Republic) is one of themost important areas of tin and tungsten mining incentral Europe (Beran and Sejkora, 2006). The
district is characterized by greisen mineralization inseveral granite cupolas of the large Krušné hory
(Erzgebirge) granite batholith underlying metamorphic rock (mainly gneisses). The largest Sn-W
deposit in this district is the Huber stock; in the past,
it was mined from the Huber open pit and Huber(Stannum) shaft up to 200 m under the surface. The

FIG. 1. Photomicrograph of multiply intergrown fibrous
crystals of tvrdýite from Krásno. Secondary electron
image (Hitachi 3700N).

1078

TVRDÝITE, NEW PHOSPHATE MINERAL

Physical and optical propertiesTvrdýite occurs as thin lath crystals elongated on[010] and flattened on {100} with diameters in therange 0.5–5 μm and lengths up to 300 μm (Fig. 1),
commonly grouped in radiating aggregates up to
3 mm in size (Fig. 2). Aggregates are silvery toolive greyish green with a pearly lustre. Tvrdýite
aggregates are opaque, but individual fibrous
crystals are translucent to transparent. It has alight olive-green streak. Aggregates are very brittle,
but individual fibres are somewhat flexible. The
Mohs hardness is estimated to be 3–4 by analogy
with beraunite and the mineral has a good cleavageon {100}. A density of 2.834 g cm–3 was calculatedusing the empirical formula and unit-cell parameters obtained from single-crystal XRD data.
Density measurements were not undertaken due topaucity of the pure material and its fibrous nature.
Tvrdýite is non-fluorescent under shortwave or
longwave ultraviolet radiation. It is optically biaxial(–), with indices of refraction measured using whitelight α = 1.650(2), β = 1.671(1), γ = 1.677(1). The
measured 2V is 56(1)° based on extinction datausing EXCALIBR (Gunter et al., 2004) and is
identical to the calculated value. Observed dispersion is strong with r > v. Optical orientation is Z = b,
X ≈ a, Y ≈ c. Tvrdýite is pleochroic with X =greenish blue, Y = yellowish orange, Z = yellowish
orange (X >> Y > Z ). The Gladstone-Dalecompatibility index 1 – (KP/KC) is 0.022 for theempirical formula and unit-cell parameters fromsingle-crystal XRD data, indicating excellent compatibility (Mandarino, 1981).

FIG. 2. Olive-grey to green fibrous aggregates of tvrdýite
from Krásno. Field of view is 5 mm; photo by Jiří Sejkora.

Water could not be analysed directly because of theminute amount of material available; it wascalculated on the basis of 17 H apfu, from thestructure analysis, corrected for partial substitutionof F for OH.
The significant contents (in apfu) of Fe (2.61–2.83), Al (2.55–2.80), Zn (0.46–0.56) and P (3.75–3.85) are characteristic for the holotype sample oftvrdýite. Results of electron probe microanalyses(EPMA) show strong negative Al-Fe correlation(Fig. 3); contents of Zn do not correlate with Fe or
Al. A strong positive correlation was observed for
Al-F (Fig. 4). This suggests that Al replacement for
Fe in beraunite resulted from a reaction with F-richacidic solutions; however, the maximum amount of
F in tvrdýite is only 0.25 apfu. A small content of
As (0.16–0.22 apfu) was found in the tetrahedral
sites, but there is only a weak correlation between
As and P (Fig. 5). The empirical formula of tvrdýite
3þ
(based on 27 anions pfu) is Zn0.52Fe2þ
0:50 Fe2:21Al2.75(PO4)3.86(AsO4)0.19(OH)4.60F0.23(OH2)4·2H2O. Based on the structure analysis (see below), Zn is not
dominant at any specific site; at M1 Fe2+ >> Fe3+,
Zn; at M2 Al >> Fe3+; at M3 Al > Fe3+,Zn and at M4
Fe3+ >Al,Zn. The simplified formula (M1–M42–
M2–M32) is thus (Fe2+ ,Fe 3+,Zn)(Fe3+ ,Al,Zn)2(Al,Fe3+)(Al,Fe3+)2(PO4)4(OH)5(OH2)4·2H2O. The
ideal formula is Fe2+Fe3þ
2 Al3(PO4)4(OH)5(OH2)4·2H2O, which requires FeO 8.75, Fe2O3
19.44, Al2O3 18.62, P2O5 34.56, H2O 18.64, total
100.00 wt.%.

Chemical composition
The chemical composition of tvrdýite (Table 1) was
determined using a Cameca SX100 electron
microprobe (wavelength-dispersive spectroscopy
mode, 15 kV, 10 nA, 8 µm beam diameter) fromnine analytical points. The following X-ray lines
and standards were selected; Lα line: As (lammerite); Kα lines: P, Ca (fluorapatite), Al, K (sanidine),
Zn (gahnite), Fe (almandine), Mg (syn. Mg2SiO4),
Na (albite), Si, Mn (spessartine), Ti (titanite), Cl(vanadinite) and F (topaz). Contents of Na, K, Ca,
Mg, Ti, Si and Cl were below detection limits(∼0.05 wt.%). Peak counting times (CT) were 10–
20 s and the counting time for background was 50%of the peak CT. The measured intensities were
processed for matrix effects using the ‘PAP’correction routine (Pouchou and Pichoir, 1985).

Vibrational spectroscopy

1079

The ideal formula of monoclinic tvrdýite, spacegroup C2/c, Z = 4, is Fe2+Fe3þ
2 Al3(PO4)4(OH)5

J. SEJKORA ET AL.

TABLE 1. Chemical composition of tvrdýite in comparison with associated Al-rich beraunite.
TvrdýiteAl-rich beraunite

Mean (n = 9)

Range

SD

Mean (n = 7)

MnO
ZnO
FeO*
Fe2O3*
Al2O3
P2O5
As2O5
F
H2O**
O=F

0.01
5.08
4.31*
21.16*
16.71
32.64
2.56
0.53
17.84
–0.22

0–0.06
4.56–5.45

0.02
0.30

15.6–17.3
32.1–33.0
2.19–3.01
0.45–0.58

0.53
0.25
0.32
0.04

0.02
6.01
4.14
31.70
7.54
31.39
2.19
0.17
17.16
–0.07

Total

100.62Range

SD

0–0.095.29–6.44

0.04
0.42

6.59–8.68
30.1–31.9
1.78–3.88
0.12–0.24

0.77
0.64
0.75
0.04

100.25

Calculation based on 27 anions pfu obtained from the crystal structure
Mn
0.00
0.00
Zn
0.52
0.65
0.50
0.50
Fe2+
Fe3+
2.21
3.50
Al
2.75
1.31
P
3.86
3.90
As
0.19
0.17
F
0.23
0.08
OH
4.60
4.81
H2O
6.00
6.00
*Calculated from total Fe as FeO of 23.35 wt.%, range 22.75–24.42, SD [standard deviation] = 0.54
(tvrdýite) and mean 32.66 wt.%, range 31.74–33.59, SD = 0.73 (Al-rich beraunite), on the basis of
0.5 Fe2+ pfu, from bond-valence calculations. **Water was calculated on the basis of 17 H apfu,
from the structure analysis, corrected for partial substitution of F for OH.

(OH2)4·2H2O. The crystal structure of tvrdýite
contains two symmetrically distinct phosphategroups, five symmetrically distinct hydroxyls andtwo symmetrically distinct water molecules. The
free (PO4)3– ion, possessing Td point symmetry, ischaracterized by four fundamentals, the ν1 (A1)

FIG. 3. The contents Fe vs. Al (apfu) for tvrdýite andassociated Al-rich beraunite.

FIG. 4. The contents Al vs. F (apfu) for tvrdýite andassociated Al-rich beraunite.

1080

TVRDÝITE, NEW PHOSPHATE MINERAL

FIG. 5. The contents P vs. As (apfu) for tvrdýite andassociated Al-rich beraunite.

symmetric stretching vibration, the ν2 (δ) (E)doubly degenerate bending vibration, the ν3 (F2)triply degenerate antisymmetric stretching vibration, and the ν4 (δ) (F2) triply degenerate bending
vibration. Corresponding band wavenumbers arelocated approximately in the range 410–490 (v2),
510–670 (ν4), 930–990 (ν1) and 975–1140 (ν3) cm–1,
respectively (Pechkovskii et al., 1981). All
vibrations are Raman active; the ν3 and ν4
vibrations are also infrared active. Any symmetry
lowering may cause infrared activation of all
vibrations and splitting of degenerate vibrations(Nakamoto, 1986). Molecular water, point symmetry C2v is characterized by three fundamentals,

the symmetric stretching vibration ν1 (A1) OH, andthe antisymmetric stretching vibration ν3 (B1) OH,
both in the range of ∼3600–2900 cm–1, and thebending vibration ν2 (A1) H2O (δ H2O) in the rangeof ∼1700–1590 cm–1 and libration modes in therange ∼900–300 cm–1 (Nakamoto, 1986). All
vibrations are Raman and infrared active;
however, there are sometimes problems in therecording and resolution of the Raman bands.
Hydroxyl ions, OH–, ( point symmetry C∝v) areindicated commonly by sharp bands between 3700
and 3450 cm–1, but sometimes at lower wavenumbers, if any appreciable amount of hydrogenbonding is involved, δ M–OH at ∼1500 cm–1 andover a wide range below this value, and OH
libration modes in the spectra range from 1000 to
300 cm–1 (Lutz, 1995).
The infrared vibrational spectrum of tvrdýite wasrecorded by the attenuated total reflection method
on a Nicolet iS50 spectrometer (range 4000–400 cm–1,
resolution 4 cm–1, 64 scans). The main bands(Fig. 6) observed are (cm–1) at 3610, 3394, 3255,
1631, 1191, 1058, 1017, 994, 936, 843, 613 and485. A band at 3610 cm–1 is assigned to the ν OH
stretching vibrations of hydroxyl ions. Bands at
3394 and 3255 cm–1 are attributed to the ν OH
stretching vibrations of symmetrically distincthydrogen-bonded water molecules. O–H···Ohydrogen bond lengths vary in the range fromapproximately >3.2 to 2.73 Å (Libowitzky, 1999).
A band at 1631 cm–1 is assigned to the ν2 (δ)

FIG. 6. The infrared spectrum of tvrdýite; the x axis was rescaled at 2200 and 1000 cm–1 to emphasize selected features inthe spectrum.

1081

J. SEJKORA ET AL.

FIG. 7. The Raman spectrum of tvrdýite; the x axis was rescaled at 1000 cm–1 to emphasize selected features in the
spectrum.

bending vibrations of water molecules. Bands at
1191 and 1058 cm–1 are assigned to the ν3 (PO4)3–
antisymmetric stretching vibrations, and those at
1017 and 994 cm–1 to the ν1 (PO4)3– symmetricstretching vibrations. Bands at 936 and 843 cm–1
may be connected with δ M–OH bending vibrations
or with librations modes of water molecules orhydroxyls. Bands at 613 and 485 cm–1 are assigned
to the ν4 (δ) (PO4)3– and to the ν2 (δ) (PO4)3–
bending vibrations, respectively. The basic
characteristics of the experimental IR spectrum oftvrdýite correspond to published infrared spectra ofisostructural beraunite (Chukanov, 2014; Frostet al., 2014).
Tvrdýite was also investigated with a confocal
Nicolet DXR Raman microscope. The Raman
signal was excited by a 532 laser and detectedwith a multichannel air-cooled CCD camera. The
laser power at the sample was limited to 1 mW toavoid possible thermal destruction of the samples.
Spectra were recorded between 100 and 2000 cm–1
with a resolution of 4 cm–1 and a minimum lateral
resolution of ∼1 μm on the sample (Fig. 7). The
main bands observed are (cm–1) at 1623, 1194,
1102, 1023, 860, 698, 637, 586, 496, 415, 303, 281,
233 and 143. The broad band at 1623 cm–1 isrelated to the ν2 (δ) bending vibrations of watermolecules. Bands at 1194 and 1102 cm–1 are
attributed to the ν3 (PO4)3– antisymmetric stretching vibrations, and that at 1023 cm–1 to the ν1(PO4)3– symmetric stretching vibrations. A band at

860 cm–1 is related to the δM–OH bending
vibrations or to the libration modes of watermolecules or hydroxyls. The ν4 (δ) (PO4)3–
bending vibrations are connected with the bands
at 698, 637 and 586 cm–1, and the ν2 (δ) (PO4)3–
bending vibrations with the bands at 496 and415 cm–1. According to Frost et al. (2014), bands at
303 and 281 cm–1 may be related to the metal
oxygen stretching vibrations, and those at 233 and143 cm–1 may be described as external or lattice
vibrations.
According to Pechkovskii et al. (1981), it may beconcluded that only the presence of phosphate ions,(PO4)3– in the structure of tvrdýite, may be inferredfrom the infrared and Raman spectra. This conclusion differs from that of the infrared and Raman
spectra of isostructural beraunite taken from Frostet al. (2014), in which not only (PO4)3–, but also
(OHPO3)2– ions should be observed in the infrared
and Raman spectra. However, splitting of doubly
and triply degenerate (PO4)3– and (HOPO3)2– (orfragments PO3 and POH)) stretching and bending
vibrations and overlapping of related bands makeany unambiguous assignment of (HOPO3)2– groups
in the presence of (PO4)3– difficult.

Powder diffraction

1082

Powder XRD data for tvrdýite were collected on a
Bruker D8 Advance diffractometer (National

TVRDÝITE, NEW PHOSPHATE MINERAL

program package (Ondruš, 1993) and the unit-cellparameters were refined by the least-squares
algorithm implemented by Burnham (1962). The
experimental powder pattern was indexed inaccordance with the calculated values of intensities
obtained from the crystal structure refinement,
based on the Lazy Pulverix program (Yvon et al.,1977). The experimental powder data given in
Table 2 agree with the pattern calculated fromsingle-crystal data; experimental intensities areaffected by preferred orientation caused by the good(100) cleavage and by the fibrous nature of studied
mineral. The refined unit-cell parameters of tvrdýite
are a = 20.543(2), b = 5.101(1), c = 18.877(4) Å,
β = 93.64(1)o and V = 1974.1(6) Å3.

TABLE 2. Powder XRD data (dhkl in Å) for tvrdýite.
Iobs

Icalc

dobs.

dcalc

100.0
5.5
13.8
1.0
7.0
1.0
0.7
0.5
0.6
0.3
0.7
0.8
11.4
0.5
5.6
0.6
2.4
0.3
1.4
2.3
1.3
0.8
0.3
5.0
0.6
0.3
0.8
0.6
0.7
0.8
3.1
0.5
0.3
0.2
0.2
0.3
0.3
0.6
1.1
0.5
0.4
0.8

100.0
38.2
20.4
4.6
5.9
18.8
3.6
1.0
13.1
3.3
9.4
0.9
9.4
6.1
3.2
6.4
8.2
6.8
5.8
32.7
1.1
8.2
7.4
5.3
4.5
0.3
0.6
3.6
1.4
8.9
3.7
5.3
2.4
5.9
2.3
0.3
2.1
1.5
0.9
7.7
0.7
1.4

10.227
9.400
7.156
6.716
5.120
4.769
4.707
4.383
4.353
4.087
3.701
3.579
3.416
3.364
3.278
3.196
3.140
3.123
3.057
3.031
2.853
2.708
2.670
2.562
2.540
2.513
2.31382.28902.19412.07572.05111.98091.94181.90201.83601.70791.6954
1.66321.64051.59861.46391.426810.251
9.419
7.166
6.726
5.125
4.770
4.710
4.387
4.354
4.088
3.699
3.583
3.417
3.368
3.280
3.196
3.140
3.125
3.057
3.031
2.853
2.708
2.670
2.563
2.540
2.514
2.31362.28882.19332.07492.05011.98191.94231.9015
1.83511.7084
1.69591.6626
1.63981.59791.4644
1.4266

h k l
2
0
2
2
4
1
0
2
1
3
3
4
6
3
6
5
0
5
2
3
6
5
1
8
7
8
8
7
4
3
10
6
5
6
3
12
6
12
12
10
14
12

0 0
0 2
0 2
0 2
0 0
1 1
0 4
0 4
1 2
1 0
1 2
0 4
0 0
1 3
0 2
1 0
0 6
1 1
0 6
1 4
0 4
1 4
1 6
0 0
1 0
0 2
0 4
1 4
0 8
1 8
0 0
2 2
1 8
2 4
1 9
0 0
0 10
0 2
0 4
2 0
0 0
0 8

Single-crystal X-ray diffraction

Museum, Prague) with a solid state 1D LynxEyedetector using CuKα radiation and operating at
40 kV and 40 mA. The powder pattern wascollected using Bragg–Brentano geometry in therange 4–70°2θ, in 0.01° steps with a counting timeof 30 s per step. Positions and intensities ofdiffractions were found and refined using thePearsonVII profile-shape function with the ZDS
1083

A needle-shaped crystal 0.005 mm in diameter and0.07 mm long was separated carefully from thesurface of a high-Al fibrous sheath for study at
the microfocus macromolecular beamline MX2 ofthe Australian Synchrotron. Data were collected at
100 K using an ADSC Quantum 315r detector andmonochromatic radiation with a wavelength of
0.71073 Å. A phi scan was employed with a frame
width of 1° and a counting time per frame of 2 s.
The image plate data were processed using XDS
software to produce intensity data that were refinedusing SHELXL (Sheldrick, 2008). A total of 19,435
reflections was collected to a maximum θ = 30°,
giving 2744 unique reflections with Rint = 0.057.
Details of data collection and refinement are given
in Table 3.
A structure solution was obtained using SHELXT(Sheldrick, 2008), which confirmed the beraunite
structure as reported by Moore and Kampf (1992),
but with an origin shift of (½,0,½). The origin was
relocated to conform to previous studies (Fanfani
and Zanazzi, 1967; Marzoni Fecia di Cossato, et al.,
1989; Moore and Kampf, 1992) and the structure
was refined using SHELXL (Sheldrick, 2008). Site
occupancy refinements were made with zinc
substitution for iron at M1, for which the iron was
confirmed to be predominantly Fe2+ by Moore andKampf (1992). Aluminium substitution for trivalent iron was incorporated at the sites M2 throughM4. Substitution of As for P in the P1 and P2 sites
was also tested, but the site occupancy factor for As
refined to zero. A difference-Fourier synthesis
showed the positions of all nine expected H
atoms based on previous studies. The O−H
distances were restrained to be 0.95(2) Å and the

J. SEJKORA ET AL.

TABLE 3. Summary of data collection conditions and refinement
parameters for tvrdýite.
Simplified formulaTemperature
Wavelength
Crystal system
Space group
Unit-cell dimensions

VolumeZ
Density (calculated)
Absorption coefficient
Absorption correction
Crystal size (mm)Theta range for data collection
Index ranges
Reflections collected
Independent reflections
Completeness to θ = 30.01°
Refinement method
Data / restraints / parameters
Goof on F2
Final R indices [I>2σ(I )]R indices (all data)
Extinction coefficient
Largest diff. peak and hole

Fe2+Fe3þ
2 Al3(PO4)4(OH)5(OH2)4·2H2O
100 K
0.71073 Å
Monoclinic
C2/c
a = 20.564(4) Å
b = 5.1010(10) Å
c = 18.883(4) Å
β = 93.68(3)°1976.7(7) Å3
4
2.76 g cm–3
3.009 mm–1
SADABS, Tmin/Tmax = 0.707/0.746
0.005 × 0.005 × 0.05
2.16 to 30.01°.
–28 ≤ h ≤ 28, –6 ≤ k ≤ 6, –26 ≤ l ≤ 26
19,4352744 [Rint = 0.0571]
95.3%
Full-matrix least-squares on F22744 / 12 / 204
1.053
R1 = 0.0384
R1 = 0.0506, wR2 = 0.0958
0.0033(3)0.73 and –0.62 e – Å–3

H−H distances for the water molecules wererestrained to be 1.45(3) Å. An overall isotropic
displacement parameter was refined for the Hatoms, while all other atoms were refined withanisotropic displacement parameters. The final
agreement factors are R1 = 0.038 for 2276 observedreflections with I > 2σ(I ) and 0.051 for allreflections. Refined atom coordinates, equivalentisotropic displacement parameters and site occupancies are reported in Table 4. Polyhedral bonddistances and valence sums for the metal atoms are
given in Table 5. Hydrogen-bonding details arereported in Table 6.

Crystal structure of tvrdýite
The refined structure for tvrdýite is shown inprojection along the b axis in Fig. 8. A key featureof the structure, as described in detail by Moore(1970), is the presence of linear face-sharedoctahedral trimers, called h-clusters by Moore(1970). These trimers, with M1 as the central
1084

atom and M4 as the terminal atom, are aligned
along [101] and are interconnected by cornersharing with pairs of M3 octahedra along [001]and with PO4 tetrahedra along [010] to form dense
slabs parallel to (100). The connection between theslabs is through corner-sharing of the terminaloctahedra of the trimers with M2O2(OH)2(OH2)2
octahedra. This linkage creates an open framework
structure with large 10-sided channels runningparallel to [010], which are occupied by watermolecules (O14 in Table 4). The structure also
contains smaller channels with hexagonal crosssection running parallel to [101], as shown in
Fig. 9. This shows that the Al-dominant sites, M2
and M3, and the Fe-dominant sites, M1 and M4, form
separate bands, which alternate parallel to (101).
Almost full ordering of Al occurs in theoctahedra centred on M2 (92%Al + 8%Fe), while
the M3 site is two thirds occupied by Al. The M1
and M4 octahedra of the face-sharing trimers
contain predominately Fe. The bond-valence sum
at site M1 is 2.46, indicating that this site contains
approximately equal amounts of Fe3+ and Fe2+.

TVRDÝITE, NEW PHOSPHATE MINERALTABLE 4. Atom coordinates (x 104) and equivalent isotropic displacement parameters (Å2 ×103) for tvrdýite. Uiso is defined as one third of the trace of the orthogonalized Uij tensor.
Atoms.o.f.

M(1)
M(2)
M(3)
M(4)
P(1)
P(2)
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)
H(1)
H(2)
H(3)
H(4)
H(5)
H(6)
H(7)
H(8)
H(9)

0.92(2) Fe + 0.08(2) Zn
0.919(7) Al + 0.081(7) Fe
0.670(5) Al + 0.330(5) Fe
0.680(6) Fe + 0.320(6) Al

x/a

y/b

z/c

Uiso

0
¼
442(1)
1092(1)1062(1)4083(1)
1797(1)4261(1)4219(1)917(1)
4807(1)994(1)
3794(1)1310(1)65(1)1935(1)03879(1)2488(1)2302(1)0(30)1970(30)04050(30)3850(30)
2840(20)2420(30)2530(20)1876(11)0¼
2775(1)371(1)
4727(1)429(2)
4758(5)2497(4)2396(5)4328(5)465(5)
4779(5)3162(5)3450(5)477(5)
143(4)
1211(6)4893(5)972(5)
3578(6)–1140(60)1580(80)–520(40)6480(60)
4070(110)–10(100)2110(100)5060(70)
3970(110)001733(1)4154(1)260(1)
1814(1)171(1)
204(1)
5004(1)1029(1)1659(1)2401(1)1658(1)3652(1)3926(1)4645(1)¼
3173(2)948(1)
2160(2)3740(30)
4910(30)¼
3090(30)2754(19)1050(30)
1300(20)2260(40)2190(4)

13(1)
12(1)
12(1)
12(1)
11(1)
13(1)
18(1)
16(1)
19(1)
21(1)
17(1)
22(1)
20(1)
17(1)
20(1)
17(1)
17(1)
29(1)
18(1)
26(1)
71(7)
71(7)
71(7)
71(7)
71(7)
71(7)
71(7)
71(7)
71(7)

s.o.f. = site occupancy factor

This was expected to be the site in which the Zn wasincorporated, but the site refinement gave only 8%
Zn in M1, which corresponds to ∼20% of theamount of Zn measured by EPMA. A considerationof bond valences in Table 5 suggests that the most
likely sites for the majority of the zinc are theterminal octahedra of the face-sharing trimers, M4.
This was checked by a refinement in which a fixedamount of Zn was included in M4 and the Al and Fe
occupancies refined. The result was inconclusive,with no improvement in the R factor and with anincrease in the Al/Fe ratio in M4 that was notconsistent with the EPMA results.
The H-bonding scheme is reported in Table 6.
The bonds involve O−O distances in the range 2.69
to 3.34 Å, which correspond to medium to weakbonds. The stronger of the bonds are associated1085with the coordinated H2O molecules bonded to M2(O13) and M3 (O12) as donors. The acceptors are
the channel water molecules (O14) and phosphateoxygen (O6), respectively.
Electron probe microanalyses show strong
element-element correlations for Al-Fe (negative)
and for Al-F ( positive). The latter is consistent with
Al replacement for Fe in beraunite being due toreaction with F-rich acidic solutions. The averageamount of F is only ∼0.2 F pfu. It could not belocated in the structure analysis, but would be
expected to substitute at the hydroxyl sites O10 andO11 coordinated to the Al-dominant sites M2 andM3, respectively. The EPMA data show only weakcorrelation between As and P, consistent with thestructure analysis, and suggests that As may be fully
disordered at the tetrahedral sites.

J. SEJKORA ET AL.

TABLE 5. Polyhedral bond lengths (Å) and cation
valence sums (valence units) for tvrdýite.
M(1)−O(2) x2
M(1)−O(9) x2
M(1)−O(3) x2
Average
Valence sum
M(3)−O(4)M(3)−O(5)M(3)−O(11)M(3)−O(6)M(3)−O(9)M(3)−O(12)Average
Valence sum
P(1)−O(4)P(1)−O(1)P(1)−O(3)P(1)−O(2)Average
Valence sum

2.040(2)2.055(2)2.084(2)2.060
2.46
1.875(3)1.893(2)1.931(2)1.934(3)1.961(2)2.027(3)1.937
3.11
1.514(3)1.531(2)1.549(2)1.557(2)
1.538
4.96

M(2)−O(10) x2
M(2)−O(1) x2
M(2)−O(13) x2
M(4)−O(8)M(4)−O(10)M(4)−O(7)M(4)−O(2)M(4)−O(3)M(4)−O(9)P(2)−O(8)P(2)−O(7)P(2)−O(6)P(2)−O(5)1.875(2)1.893(2)1.954(2)1.907
3.09
1.903(2)1.917(2)1.928(3)2.064(2)2.090(2)2.128(2)2.005
2.92
1.535(2)1.536(3)1.537(3)1.537(2)
1.536
4.98

Tvrdýite is isostructural with the basic iron
phosphate mineral beraunite, Fe2+Fe3þ
5 (PO4)4(OH)5(OH2)4·2H2O (Fanfani and Zanazzi, 1967; Marzoni
Fecia di Cossato, et al., 1989; Moore and Kampf,
1992). The most important difference is in the
crystal chemistry, with two of the four independent
M sites containing dominant Al in tvrdýite.
Comparative data for the two minerals are given
in Table 7. The replacement of the majority of Fe by
Al in the M2 and M3 sites results in a relatively
large unit-cell volume decrease of 5.1%, withthe largest decrease associated with the c axis
(2.0% decrease).

FIG. 8. Projection along [010] of the structure for tvrdýite.
Metal atoms are labelled. H atoms are small grey spheres.

FIG. 9. View of the structure of tvrdýite along [101].

Acknowledgements
The constructive comments of Associate Editor Juro
Majzlan, Structure Editor Peter Leverett and thereviewers helped to improve the quality of the earlier

TABLE 6. Hydrogen bonding in tvrdýite (Å, o).
D–H···Ad(D–H)

d(H···A)d(D···A)<(DHA)

O(9)–H(1)···O(5)O(10)–H(2)···O(1)O(11)–H(3)···O(5)O(12)–H(4)···O(6)O(12)–H(5)···O(7)O(13)–H(6)···O(8)O(13)–H(7)···O(14)O(14)–H(8)···O(14)O(14)–H(9)···O(6)0.90(2)0.89(2)0.88(2)0.90(2)0.895(19)0.899(19)0.893(19)0.899(19)0.90(2)

1.94(3)
1.97(3)2.608(16)1.92(4)2.12(3)
1.96(2)
1.83(3)2.12(4)
1.92(2)2.804(3)2.806(3)3.345(4)2.737(4)2.989(4)2.845(3)2.695(4)2.946(3)2.824(3)

159(6)
156(6)
141.8(3)
150(6)
164(5)
169(6)
164(6)
152(6)
172(6)

1086

TVRDÝITE, NEW PHOSPHATE MINERALTABLE 7. Comparative data for tvrdýite and beraunite (Moore and Kampf, 1992).
Tvrdýite
FormulaSymmetry
Cell
Z
Strongest
powderpattern lines
d, I, hkl
Optics2+

BerauniteFe3þ
2 Al3(PO4)4(OH)5(OH2)4·2H2O

Fe
Monoclinic, C2/c
a = 20.564(4), b = 5.101(1),c = 18.883(4) Å, β = 93.68(3)°V = 1976.7 Å3
4
10.227, 100 (200)7.156, 14 (202)
3.416, 11 (600)
5.120, 7 (400)
3.278, 6 (602)9.400, 6 (002)
Biaxial (–)α = 1.650, β = 1.671, γ = 1.677

version of the manuscript. We thank Radek Škoda
(Masaryk University, Brno) and Ján Pásztor (Nicolet,
Prague) for their kind assistance with the collection of
the EPMA and spectroscopy data. This work was
supportedfinanciallybyproject
NAKIDF12P01OVV021 of the Ministry of Culture of the
Czech Republic for JS and DKRVO 2016/02(00023272) to JČ. Further support was from theJohn Jago Trelawney Endowment to the Mineral
Sciences Department of the Natural History Museumof Los Angeles County for ARK.

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