Eur. J. Mineral.
2001,13,769-777

Ekatite, (Fe3+Fe2+,Znh2(OH)6[As03]6[As03,HOSi03h,
a new mineral,from Tsumeb, Namibia, and its crystal structure
PAUL KELLER
Institut ftir Mineralogie und Kristallchemie, Universitat Stuttgart, Pfaffenwaldring 55,
D-70569 Stuttgart, Germany. - e-mail: paul.keller@po.uni-stuttgart.de

Abstract:
Ekatite, ideally (Fe3+,Fe2+,Zn)I2(OHMAs03MAs03,HOSi03]z,
is a new mineral from Tsumeb, Namibia.
It is associated with chalcocite and occurs as small sprays of fine needles, up to 2 mm long and less than 0.2 mm indiameter. Ekatite is brownish black, translucent
with a brown streak and vitreous lustre. It is brittle without cleavage
and its Mohs' hardness is -3. The mineral is non-fluorescent,optically uniaxial positive with indices ofrefraction
OJ
analysis revealed-1.99 and E -2.08, pleochroic
with OJ = dark brownish black and E = medium brown. Microprobe
the following chemical composition(in wt%): (total Fe given as) Fe203 50.79, ZnO 3.78, ASP3 42.50, and Si02 2.10.

Theempirical

formula

based

on

20

cationsandtheresultsofstructure

determinationis:(Fe3+s9sFe2+S.l4Zn081hll.90(OH)600(As1.0l03)6.00[(As03h43(HOSi03)0.6th:
204' Single crystal X-ray studies give thefollowing parameters: hexagonal, space group P63mc; a = 12.773(2), c= 5.051(1) A, V = 713.7(4) A3 with Z= 1; Deale= 4.061 g/cm3. The strongest reflections of the powder pattern are [dobsin A, (Ivis)and (hklheJ]: 11.11 (3) (100), 6.37
(5) (110),3.220 (10) (211, 220), 2.766 (3) (400), 2.420 (7) (401, 410),1.867 (3) (402),1.672 (3)(521), and 1.508 (3)(701,531).
The crystal structure was solved by direct methods and refined to R = 0.043 using 980 observed uniquereflections. It contains dimers of face-sharing (Fe3+,Fe2+,Zn)04(OHh octahedra, which are linked together bycommon edges to form infinite double chains. They are connected by comers to form a three-dimensional frameworkwith two types of channels running along the c-axis. The larger is hexagonal and lined by six As03-groups, in sucha way that the lone-pairs of AS3+point to its centre. Trigonal pyramids of As03 and HOSi03 tetrahedra, in random
distribution, occupy the smaller triangular channel, with either the lone pairs or the OH oriented along the c-axis.
Ekatite is structurally related to phosphoellenbergerite,
ellenbergerite and holtedahlite, as well as to severaltransition-metal tellurites, selenites, phosphites and Zn7(OHh(S04)(V04)3'
The name is in honour of Dieter Ekat (1935-1996), a Namibian mining engineer.
Key-words:

ekatite, new mineral, crystal structure, Fe arsenite/silicate,

Introduction
The new mineral species was brought to our attention by Michael Grieser, Mannheim, who submitted it for identification. During the course of description, Herbert Nagele, Windhoek, donated asecond specimen with the new mineral.
The new mineral name honours Dieter Ekat
(1935-1996), a mining engineer and former owner ofthe Rubicon mine, Namibia. Both name and species
have been approved by the Commission on New
Minerals and Mineral Names (CNMMN) of 1MA as

DOl: 10.1127/0935-1221/2001/0013-0769

Tsumeb, Namibia.

No. 98-024. The type material is deposited at the Museum of the Institut fUrMineralogie und Kristallchemie, Universitat Stuttgart, catalogue No. NM20.
Keller et at. (1999) have presented some structuraldetails of the species fully described here.

Occurrence
Ekatite is a new species from Tsumeb, Namibia,that can be added to the long list of type minerals
from this exceptional locality (Keller, 1984; Geb@ 200] E. Schweizerbart'sche0935-1221/01/0013-0769
Verlagsbuchhandlung,$ 2.25

D-70176 Stuttgart

P. Keller

770

Physical properties
Ekatite forms small sprays of striated, fine needles.
They are elongated along the c-axis, up to 2 mmlong but less than 0.2 mm in diameter (Fig. 1).
There are indistinct (hkO) forms, but no termination
could be detected. The mineral is brittle without
distinct cleavage. The Mohs' hardness is about 3.
Ekatite is brownish black, with a brown streak,translucent and displays bright vitreous lustre. It is
optically uniaxial positive with indices of refraction OJ- 1.99 and E - 2.08. According to the Gladstone-Dale relationship (Mandarino, 1981), thecompatibilityindexl-(K/Ke)= 0.007 is superior

for the mean measured and the calculated index ofrefraction nmeas.-2.02 and neale.= 2.013, respectively. The mineral is non-fluorescent, strongly pleochroic with OJ= dark brownish black and E =medium brown, both with a greenish tint. Although thecolours are not typical, Fe3+-Fez+ charge transfer
between face-sharing octahedra may account forthe pleochroism.

ChemistryFig. 1. Ekatite fonning a small spray of fine needles on amatrix of corroded quartz.

hard, 1991). The secondary mineral was found on amatrix of etched quartz and chalcocite. Unfortunately, there is no exact information about the sampling location in the mine. Both specimens havebeen acquired from the same dealer in Tsumeb. According to him, they were collected at "deep levels". This fits well with the occurrence of other
As3+-bearing minerals from the "second oxidationzone" like leiteite, Zn(As3+z04), schneiderhohnite, Fez+Fe3+3(As3+03)(As3+zOsh. ludlockite,
PbFe3+4(As3+SOII)Z' claudetite, AsZ03, arsenolite,
AsZ03, etc. (Keller, 1984; Keller & Innes, 1986).
Table 1. Chemicalcomposition

Scam scan
Stuttgart

Ekatite was initially analysed using a Microspec
WDX-400 wavelength-dispersive
system of a
Scamscan CS44 with an operating voltage of 30 kV
and a beam current of20 nA (Table 1). The following standards were used for calibration: SiOz (Si),
FeSz (Fe), Zn (Zn) and GaAs (As). To verify thesmall amount of SiOz, independent analyses werecarried out with a Cameca Camebax SX50 electron
microprobe (University Paul Sabatier, Toulouse,
France) operated at 15 kV and 20 nA (for all elements), and the standards wollastonite (Si), FeZ03(Fe), ZnS (Zn) and As (As) (Table 1). The presenceof OR has been proved by IR-spectroscopy, whichshows two absorption bands with wavenumbersof ekatite.

Atomicproport. I)

CamecaToulouse

Atomicproport. I)

Mean
wt%

Atomicprop. I)

Requiredcompos.

Ideal.
compos.

27.17
27.45
27.41
5.93
27.11
5.96
27.26
5.95
Fe02)
21.19
5.14
21.12
21.41
21.18
5.09
21.20
5.18
ZnO
3.94
0.84
3.66
0.79
3.80
0.81
3.79
3.73
42.41
7.50
42.16
7.48
42.56
7.49
41.94
42.95
As203
2.21
0.59
2.12
0.61
2.11
2.07
0.63
2.02
SiOz
Hz03)
6.64
6.61
3.41
3.40
3.46
3.38
6.59
3.42
Sum
101.15100.35100.00100.0099.53
I): On the basis of 20 cations; 2):Fe3+and Fe2+calculated on the basis of the structure detennination; 3):from OH(4) andOH(5) according to the structure detennination
FeZ032)

771

Ekatite, a new mineral and its crystal structure

Table 2. X-ray powder diffraction data for ekatite.
hkl

deale.

dobs.

leak.

Ivis.

100
110
200
101
210
201

11.062
11.11
56
30
6.37
11
6.386
50
5.52
10
5.531
6
4.60
4.595
30
20
4.18
4.181
3
10
3.730
3
}
}
3.69
20
301
3.687
9
211
3.221
100
}
}
3.220
100
220
3.193
16
310
3.068
3.070
I
5
301
2.975
10
10
2.978
400
2.766
23
2.765
30
311
2.620
2.622
9
10
320
2.538
5
}
}
2.531
20
002
2.526
5
401
29
2.426
}
}
2.420
70
410
2.414
8
112
2.349
2.349
7
10
202
2.297
2.300
14
10
321
2.262
2.267
5
10
2.210
4
500
2.212
10
411
2.179
2.178
5
5
501
2.027
2.028
7
10
222
1.981
1.982
18
10
421
1.931
4
1.932
20
402
1.865
1.867
3
30
511
1.849
1.849
6
20
1.815
430
1.819
3
5
322
1.790
1.788
7
20
520
1.771
1.771
I
10
412
1.745
1.746
I
10
431
1.711
10
610
1.687
1.689
2
10
521
1.672
10
1.672
30
440
1.595
1.595
8
20
530,700
1.580
1.582
10
3
521,213
1.562
1.561
I
10
701,531
1.507
1.508
3
30
Indexing and deak based on the unit cell dimensions a
12.773, and c 5.051 A obtained from single crystal study.
dobs.determined with a Debye-Scherrer camera, 57.3 mm,
and Mn-filtered Fe-Ka radiation. The relative intensities
Ivis.and leak.are visually estimated and calculated fromthe crystal structure, respectively.

3484 and 3465 cm-I. No other elements than thosementioned above have been detected. The oxidation states of As and Fe, as well as H20 content are
given according to stoichiometry
and the results ofstructure determination,
as described below. The

empirical formula of ekatite based on 20 cations
IS (Fe3+ S.9sFe2+ S.14ZIlo.81h: 11.90(OH)6.00( As 1.0103koo[(As03)1.43 (HOSi03)O61h:204 (Table 1).

X-ray crystallography

and structure

determination
An indexed powder diffraction pattern is given in
Table 2. It was obtained with a 57.3 mm DebyeScherrer camera and FeKa radiation, filtered with
Mn. No internal standard was used. The relative intensities are visually estimated. For comparison,
the powder pattern calculated on the basis of structure determinationis added.
A fragment of a single crystal, showing (hkO)
faces, was measured on a Stoe IPDS equipmentwith monochromatized
MoKa radiation. Two hundred images with~<I>
1 ° were exposed at a durationof 3 minutes each. The image plate was set to 60
mm, which resulted in a maximum of 28 66°. The
absorption was corrected using the HABITUS program (Herrendorf,1997) for 671 equivalents
and81 unique reflections. Transmission
factors ranged
between 0.5621 and 0.8251. All other essential
measuring parameters are given in Table 3.
Systematic absences and reflection statistics indicate the non-centrosymmetricspace groups P63
and P63mc without any systematic absence violations. Since P63 is a subgroup of P63mc, the latter
was initially used for structure determination
by direct methods with the SHELXTL program package(SHELXTL,
1996). It yielded the position ofM(I)== Fe3+, As(1), As(2), 0(1) to 0(3) and OH(4).
Structure refinement on F2 with the SHELXL97
program (SHELXL-97,
1997) was performedinseveral steps. The position M(1) on (12d) was refined for Fe3+, Fe2+ and Zn by applying constraints
and restraints of the SHELXL97 program: the sameatomic coordinates were used for all three atoms by
the constraint EXYZ and the same displacement
parametersby the constraint
EADP; SUMP restraints were applied to fix the average charge to
2.5 and the sum of occupancy to 1.0. The results are
in relatively good agreementwith the chemical
analyses (Tables I and 4). In a second refinement
step the position of As(2) was split by introducing
Si(2). Both could be refined anisotropicallywithout any constraint or restraint. Then OH(5) was introduced by selecting a peak from the differenceFourier map. It was isotropically refined with fixed
occupancy according to the result of chemical analyses. Finally, two H were added with coordinatesfrom the difference-Fourier
map. They could be re-P. Keller

772

Table 3. Experimental
ekatite.

details and crystallographic

Crystal data
Space group
a [A]
erA]
v(A3]

P63me
12.773(2)5.051(1)713.7(4)
1
4.061
14.82
0.3,0.15,0.1

zDeale [g/cm3]
11[mm-I]
Size [mm]Intensity measurements
Diffractometer
Monochromator
Radiation
28max [0]~in., ~ax
kmin., kmax.
Imin., Imax.

Observed reflections
Unique reflections~nter
Ro
TemperatureF(OOO)

rOC]

Structure determination
and refinement
Reflections used
Reflections with F 0>4o(F 0)Parameters refined
Extinction
R 1(FH F 0>4o(F 0) }Ri(all)wR2(all)
Weightingparameters

GooF
Flack -x -parameter(~p)max [e/A3](~p)min [e/A3]
Rms [e/A3]

data forStoe IPDS
graphite
Mo-Ka
65.65
-18, 19-19,19
-7,7
10026
982
0.069
0.026
25
819.2

980
938
65
0.0064
0.043
0.046
0.116
0.026513.27
1.268
0.0108
2.58
-1.49
0.33

fined isotropic ally, but only with fixed occupancy
parameters. More details of the structure determination and refinement are given in Table 3. The final atomic coordinates and equivalent displacement parameters are given in Table 4.
Structure determination and refinement were also performed in the subgroup P63, mainly to checkthe ordering ofthe di- and tri-valent cations. The final R-values for P63, however, are slightly worse
than those obtained for space group P63mc (Table3); RJ(F){Fo>4o(Fo)} 0.045; R1(all) 0.049; wR2(all)0.1136; GooF 1.315.

Fig. 2. Crystal structure of ekatite projected onto the (001)plane.
The polyhedra
represent
the octahedraofM(l)04(OHhand the tetrahedra of HOSi(2)03' which envelop the As(2)03 groups. The hexagonal and triangularchannels are indicated with I and II, respectively.

Description and discussion of the crystal
structure
The most impressive features of the ekatite framework structure are (i) densely packed trigonal rods
centred on the three-fold axes, consisting of
M(1)04(OH)2' As(2)03 and HOSi(2)03 groups, aswell as (ii) large hexagonal channels along the 63
screw axes that contain only As(1)03 groups (Fig.
2, 5a). Ekatite represents a new mineral structure
type. Some interesting structural relations to few
other minerals, especially phosphoellenbergerite,
P63mc (Brunet & Schaller, 1996; Raade et al.,1998), ellenbergerite, P63 (Chopin et al., 1986; Comodi & Zanazzi, 1993), holtedahlite, P3lm
(R0mming & Raade, 1989) and a number of chemical compounds, will be discussed in the last chapter.

M(l)OiOH}zframework

and its three-dimensional

According to the results of chemical analyses,
structure refinement, bond valence calculation andmean M(1)-(O,OH) distances, Fe3+ and (Fe,Zn)Z+
are randomly distributed in about equal amounts
over the site M(1) (Tables 5 and 6). As already described, an attempt to prove the ordering of the diand tri-valent cations, like in the ellenbergerite
structure, was without success. Structure determination and refinement in the possible subgroup P63

Ekatite,

Table 4. Atomic coordinates

Atom
Fe(1)
Fe(2)
Zn(2)

a new mineraland its crystal

parameters(xl0-4) of ekatite.

and displacement

Wyck. Occup. x
12d 0.508 0.91549(8)12d 0.337 0.91549(8)12d 0.150 0.91549(8)Y
0.57285(9)0.57285(9)0.57285(9)U11z0.5935(2) 127(4)0.5935(2) 127(4)0.5935(2) 127(4)773

structure

U22
U23
U33
U13
8(4) -2(4)98(4) 101(5)8(4) -2(4)98(4) 101(5)8(4) -2(4)98(4) 101(5)

U12
59(3)
59(3 )
59(3 )

Ueq
107(3)
107(3)
107(3)70(3)85(5)
74(11) 64(31)
76
202

38(2)
37(6)
38

77(3)
71(14)
77

As(1) 6cAs(2) 2bSi(2) 2b

0.50
0.128
0.041

0.71631(8)0.66667
0.66667

0.85816(4) 0.6246(3)0.895(1)0.33333
0.837(1)0.3333375(4)74(11)
760(1) 6c0(2) 12d0(3) 6c
OH(4) 6c
OH(5) 2b

0.5
1.0
0.5
0.5

0.6048(8)0.6559(6)0.5968(4)0.0490(7)0.886 (2)0.437(1)0.723 (2)0.778(2)0.166(4)

164(43) 252(39) 109(40) 27(15) 54(30)187(29) 76(24) 127(25) -11(18) -61(20)215(38) 43(35) 334(52) -70(33) -35(16)104(33) 117(26) 163(37) 20(13) 40(27)0.0411) 0.66667

0.8024(4)0.9339(5)0.1936(8)0.5245(3)0.33333

H(1)H(2)0.52) 0.01(1)0.0411) 0.66667

0.502(6)0.33333

0.60 (3)0.388102)

6c
2b

-2(2)
0
0

-5(4)
0
0

Tbe anisotropic displacement factor exponent is defined as: _2Jt2[h2a'2UIl +... + 2hka'b'Ud.
1):fixed according to the Si content; 2):fixed

Table 5. Selected
M(1)
0(2)
M(1)-O(1)
M(1)
0(3)
M(1)
- 0(2)'
M(1)
OH(4)
M(1)
OH(4)
mean

atomic distances
1.984(5)1.985(5)2.012(5)2.072(5)2.202(6)2.284(6)2.090(A) and angles CO)for ekatite.
0(2)
As(1)
As(1)
0(1)
mean

1.783(3)As(2) - 0(3)1. 773(9)

Si(2)
0(3)
Si(2)
- OH5

1.65(2) x3
1.66(19)1.65

x21.807 (3)1.791
x3

H(1) - OH(4)
1.01
mean
H(1) - OH(4)'1.75
H(2) - OH(5)1.12
H(2) - 0(3)2.29
0(1) - M(1) - 0(2)104.8(3)0(1) - As(1) - 0(1)'98.7(4)0(1) - M(1) - 0(2)'0(1) - As(1) - 0(2)97.7(3)92.6(3)0(1) - M(1) - 0(3)102.5(3)0(3)As(2) - 0(3)'0(1)-M(1)-OH(4)96.4(3)98.1 (5)0(3)0(1)-M(1)-OH(4)'171.9(3)119.3(4)- As (2) - OH50(2) - M(1) - 0(2)'92.0(2)0(3)0(2)-M(I)-0(3)97.1(3)Si(2)108.6- 0(3)'
0(2) - M(1) - OH(4)0(3) - Si(2) - OH5
158.5(3)110.30(2) - M(1) - OH(4)'83.3(2)160.0(3)0(2)'- M(1) - 0(3)OH(4) - H(1) - OH(4)'0(2)'-M(1)-OH(4)83.4(2)
170.3
OH(5)5 - H(2) - 0(3)0(2)'-M(1)-OH(4)'85.9(2)137.6 x30(3) - M(1) - OH(4)81.6(3)0(3) - M(1) - OH(4)'77.5(3)
OH(4) - M(1)- OH(4)'75.6(1)
M(1) = 0.5 Fe3++ 0.42 Fe2++ 0.08 Zn, derived from the chemical composition

82(21) 185(26)76(21) 125(15)21(18) 216(26)52(17) 129(19)94(53)

P. Keller

774

Table 6. Bond valence table for ekatite.
0.5Fe3+

0.42F e2+0.08Zn =L M2.5+As3'(1)

0(1)

0.27

0.21

0.04

0.52 2x~

0.96

2.00

0(2)

0.27
0.21

0.21
0.17

0.04
0.03

0.52
0.41

1.02 2x-l.

1.95

0(3)

0.25

0.20

0.04

0.49 2x~

OH(4)

0.15
0.12

0.12
0.10

0.02
0.02

0.292x~0.242x-l.

0.7As(2)0.73

0.3Si(2)

0.28

0.30
0.90~
2.47

1.013x-l.

3.00

0.30-1.
0.90
3.33

The partial values of the cations are given in italics: M = Fe3+, Fez+ and Zn2+ at the second and X(2)the third

column,= As3+ and Si4+ at

respectively.

give neither better R-values nor significantly different mean M(l)3+_(0,OH) and M(lf+-(O,OH)
distances 2.079 and 2.097 A, respectively. Therefore, it is assumed that Fe3+, Fez+ and Zn are randomly distributed over the octahedral site hence thespace group P63mc. Should sufficient natural orsynthetic material be available in the future, thiscan be proved easily by spectroscopic investigation(e.g. Mossbauer).
M(l) is situated in general position and forms
strongly distorted M(l)04(OH)z octahedra thatbuild up M(l )z07(OH)z dimers by common OH( 4)0(3)-OH(4) faces (Fig. 2, 4). The remarkablebond-lengths
variation, M(l)-O
1.984(5) to
2.072(5)

1.99
1.06

OH(5)
LM,X

L X33-(2) LO

A and M(l)-OH

2.202(6)and 2.284(6)A(Table 5 and Fig. 4), can be interpreted in terms ofthe distinct anion bond-valence requirements of Ozor OR- (Table 6). Cation-cation repulsion due to theshortM(l)-M(l) distance, Fe(3+.z+)_Fe(3+.z+)= 2.940A, may have supported the arrangement of two OR
at the shared octahedral face (Fig. 4).
The dimeric octahedral units form zigzag double chains extending along [001] by 0(2)-OR(4)edges like a ribbon of the NiAs structure type parallel [110] (Fig. 3). Finally, such double chains link
together by O(l) vertices to give a three-dimensional (M(l)dOH)60Z4)Z4- framework of densely
packed trigonal rods with small triangular and large
hexagonal channels along the three-fold and the 63
screw axes, respectively (Fig. 2 and 5a).

As03 groups, substitution

Both the As03 groups form unpolymerized trigonal pyramids with stereoactive lone-pair electronson each AS3+atom. While the trigonal pyramids ofAs(2) display ideal 3m site symmetry, those ofAs(l) is slightly distorted. In accordance with its
point symmetry m, the As(l )03 group has a longer
bond on the mirror plane, As(l)-O(l) 1.807(3) A,
and two shorter

Fig. 3. Crystal structure of ekatite seen along [001]. A and
B indicate a perpendicularand c1inographic view, respectively, on the zigzag double chains of octahedral dimers.
The 0(5) vertices of the HO(5)Si(2)0(3)3
tetrahedra all
point into the same direction.

by HOSi03

bonds,

As(l)-0(2)1.783(3)A(Fig. 4). Whereas the first oxygen is the common
comer of two trigonal rods, each of the latter belongs to two different zigzag double chains (Fig. 2
and 3). The As-O distances and angles (Table 5) fit
well with data given in the literature (ICSD,
2000).
As(1)03 groups are located along the walls of
the large hexagonal channels and the lone-pairelectrons of As(l) are directed toward the channel
centre. It is noteworthy that As(l) occupies the position of face-sharing trigonal antiprism, with anempty centre (Fig. 5a). Its dimensions are As(l)-Ekatite,a new mineral

a)

0(3)
As(1)
'1..8 ~I
,\'0)

\. .

.

0(2)

00

."1
0(1)

c)
/
''>6' '.]0(2)and its crystal

structure775

metry 3m replaces about one third of the As(2 )03, in
random distribution. The OH apex points in the same
direction as the lone-pair electrons of the As(2)03
group. However, due to the large isotropic displacement factor of OH (Fig. 4) and the uncertain result of
anisotropic refinement, its position on the three-foldaxis may not be ideally realized, but deviations along
the x- and y-axis may occur. The Si-O and Si-OH
bond distances fit well with mean literature values
and with the bond-valence calculations (Table 6).

OH(4)'
Hydrogen bonds

0(1)Fig. 4. ORTEP drawings of some coordination polyhedra ofekatite: (a) HOSi(2)03 and (b) As(2)03' both parallel to thec-axis;(c) AS(l)03'
andthestrongly
deformedM(1h07(OHhdimer (d), viewed along the c-axis. Whereas
OH(5) displays an unusually large isotropic displacement
parameter, all others show usual values of anisotropy.

As(1) 4.028 and 5.435 A and As(1)-D 3.201 and3.666 A, respectively.
The As(2)03 pyramids are situated on the threefold axes in the centre of the small trigonal channel.
Its As and their lone-pair electrons point all in thesame direction (Fig. 2) and cause the polarity of thestructure, proved by an ideal Flack parameter (Table 3). As a peculiarity, HOSi(2)03 with point sym-~

The site of H( 1) is evidently proved by structure determination and the resulting bond distances (Table5) as well as by the result of empirical bond-valencecalculations (Table 6). H(1) belongs with its shortbond, H(1)-0(4) 1.01 A, to the OH(4) (Fig. 3) like inother structurally related minerals and compounds(see below). There is only one possible acceptor distance with H(I)-OH(4) 1.75 A to another OH(4).
H(2) is part of OH(5) (Fig. 3), but the exact positionof H(2) and its hydrogen bonds are rather uncertain.
Due to the large isotropic displacement factor of
OH(5) (Fig. 4, Table 4), both atoms might be distributed randomly around the three-fold axis (for H(2) toa larger extent). However, refinements on 6c positions instead of 2b end with worse results.

Bond-valence calculation
Bond-valence calculation using the empirical valuesof Brese & O'Keeffe (1991) gave further evidence
for several topics discussed above (Table 6), espe-~

Fig. 5. Structural relationships between the ekatite (a) and phosphoellenbergerite(b) structure types. The trigonal rods arecircled. I and II indicate the hexagonal and triangular channels, respectively. (a). In the hexagonal channel I, the edges ofthe empty trigonal prisms with As(l) at their comers are delineated. As(2)03 and HOSi(2)03 are randomly distributed inchannel II. (b) The hexagonal channel is completely filled with face-sharing M(2) octahedra. The Al-Mg ordering schemeof M(1) in the structure of ellenbergerite is indicated by italics.

776

P. Keller

cially for the existence of OH(4) (~ 1.06 v.u.) andOH(5) (~ 0.90 v.u.). The distribution of di- and trivalent cations on site M(l) fits also very well withthe theoretical and calculated values 2.50 and 2.47,
respectively, as well as with the charge sum ofO(l)to 0(3), within the usual uncertainties. The replacement of about 1/3 AS3+by Si4+on the site of As(2)produces excellent values ~X(2) 3.33 v.u. and~0(3) 1.99 v.u.. No additional arguments, however, could by given for the hydrogen-bond system.

Structural relationships
Typical for all crystal structures related with ekatite
are densely packed zigzag double chains formed by
M(l)04(OHh octahedra dimers, as already described for ekatite (Fig. 3). They are the bones of athree-dimensional skeleton with two types of parallel
channels. The small triangular channel in the centreof three comer-sharing double chains is always occupied by complex anions. The result are trigonal rods
(Fig. 5 and 6), which are a common feature of all related structure types discussed above. Distinctive
differences are developed with respect to the large
hexagonal channels, either by the occurrence of coordination polyhedra - different in size and/or degreeof occupancy - or the existence or non-existence ofsuch channels (Fig. 5 and 6).
Ekatite, with the general formula M(l)12X(2)2
X(lMOHMO,OH,Dh6
represents an open structure type, only with X(l)03 in the hexagonal channel (Fig. 5a). It is also adopted by hydroxyphosphites, P63mc, (M(I)jjD)(OH)6 (HP03)g, of Co, Ni

Fig. 6. The crystal structure ofholtedahlite:a dense packing
of II-rods known from the ekatite and the phosphoellenbergerite structures. The ordering ofM(l) cations ofholtedahlite is given as M(I.1) and M(1.2). Note that only one halfof the X( I) polyhedra of ekatite occurs in holtedahlite, theother half is replaced by X(3) in different orientation.

and Zn (Marcos et al., 1993a and b), as well as of
Mn and Fe (Attfield et aI., 1994). The rather small
differences are: (i) HX(I)03 instead ofX(I)03' and(ii) one empty M(l) site in random distribution, according to stoichiometry. These compounds became of interest as a new family of microporous
materials that involve infinite channels with window sizes up to about 4 A. Of the same interest aresynthetic hydroxytellurites (Perez et aI., 1976) andhydroxyselenites (Amoros et aI., 1996) of Ni2+and
C02+, with the general formula M(I)12(OH)6[X(I)03MX(2)03]6(OH)2 and space group P63mc.
They are structurally very close to ekatite, but withadditional (OHh in the hexagonal channel.
The structural relationships between ekatite andholtedahlite,
Mg(I)6Mg(2MOH,OMHOP(I)03'
C03] [P(2)04h [P(3)04b P31m (RjZlmming& Raade, 1989), is mainly characterized by loss of thelarge hexagonal channels, so typical for the ekatite
and phosphoellenbergerite structure types. The triangular rods (II) of the structures discussed so far
are densely packed repeating-units in holtedahlite(Fig. 6). The zigzag double chains consist of facesharing Mg( 1.1) octahedra dimers and Mg( 1.2) octahedra dimers alternating along the c-axes. In the
sheet of Mg(1.1) octahedra the double chains areconnected by vertices of X(2) anions, HOP(l)03
and C03 in random distribution, and three X(l) anions P(3)04' which is one half of the complex X(I)anions of ekatite. Common vertices and two X(3)tetrahedra P(2)04' not known from the ekatite
structure type, are the linking elements in the sheetof Mg(l.2) octahedra.
The structure type with the most densely packed
hexagonal channels is represented by synthetic
ZnjiOHMS04MV04)6(Kato et aI., 1998) andphosphoellenbergerite,[Mg,Fe,DhMgdOH)6(HOP03,C03MP04,HOP03,As04)6(Brunet &Schaller, 1996; Raade et aI., 1998), with space group
P63mc and general formula M(2)2M(I)12X(2)2
X(lMOH)6(0,OH,D)32 (Fig. 5b). Their hexagonal
channel is filled by the coordination polyhedra ofM(2) and X(l). Ellenbergerite, [Mg,(Ti,Zr),Dh
Mg6(Al,MgMSi,P)g02g(OH)1O' (Chopin et al. 1986;
Comodi & Zanazzi, 1993) belongs to the same formula type. However, the M(l) cations of the facesharing octahedra are ordered with one Mg and one
Al in each dimer (Fig. 5b). The consequence is achange in symmetry from space group P63mc to P63.
The structure comparison of ellenbergerite withmagnesiodumortierite, idealized as (Mg,Ti,D)(Al,
DMAl,Mg,DMOHMB03)[(Si,D)04b
is fullydescribed by Ferraris et al. (l995). Both structures
are built up by two slabs. One type is common toEkatite,a new mineralboth structures, but the second differs to such an extent that a transformation of dumortierite minerals
into an ellenbergerite-type hexagonal polymorph,as suggested by Moore & Araki (1978), would be
far from straightforward (Ferraris et at., 1995).
Therefore, the term "dumortierite family" should
not be used for the synthetic compounds as introduced by Marcos et at. (1993a and b), but ekatite orphosphoellenbergerite are appropriate.
There are obvious relations between the conditions of formation and features of the structure
types. Ekatite, with its large empty hexagonal channels, and the structurally related chemical compounds have been formed from aqueous solutions
at low temperature and, if at all, low pressure. The
cations are mainly transition-metals. The densely
packed phosphoellenbergerite structure type, however, represents mainly Mg-Al minerals, which
originate from high- to ultra-high-pressure metamorphic rocks.
Acknowledgements:
I am very grateful to all colleagues who have supported the description of the
new mineral with important contributions, especially Prof. Dr. Th. Schleid and Dr. F. Lissner, Stuttgart. X-ray data were collected and the IR-spectra
performed by Dr. F. Lissner and M. HauBlein, respectively, Institut fUr Anorganische Chemie, Universitat Stuttgart. Drs. F. Fontan and Ph. de Parseval, Universite Paul Sabatier, Toulouse, have verified the microprobe data. Mrs Sumati Panicker-Otto has improved the English text. The manuscript
has also benefited from the comments of the referees Paola Comodi and Gunnar Raade.

References
Amoros, P., Marcos, M.D., Roca, M., Beltran-Porter,
A.,
Beltran-Porter,D. (1996): Synthetic pathway for new
tubular transition-metalhydroxo- and fluoro-selenites:
crystal structure of M,z(X}z(Se03MOH)6(M = C02+,
Ni2+; X OH-). l. Solid State Chern., 126, 169-176.
=
Attfield, M.P., Morris, R.E., Cheetham, A.K. (1994): Synthesis and structures of two isostructural
phosphites,
FeI1(HP03MOH)6and Mnll(HP03)g(OH)6'
Acta crystallogr., C50,981-984.
Brese, N.E. & 0' Keeffe, M. (1991): Bond-valence parameters for solids. Acta crystallogr., B41, 192-197.
Brunet, F. & Schaller, T. (1996): Protons in the magnesium
phosphates phosphoellenbergeriteand holtedahlite: an
IR and NMR study. Arner. Mineral., 81, 385-394.
Chopin, C., Klaska, R., Medenbach, 0., Dron, D. (1986):
Ellenbergerite,a new high-pressure Mg-AI-(fi,Zr)-silicate with a novel structure based on face-sharing octahedra. Contrib. Mineral. Petrol., 92, 316-321.

and its crystal

777

structure

Comodi, P. & Zanazzi, P. F. (1993): Structural study of ellenbergerite. Part I: Effects of high temperature. Eur. l.
Mineral., 5, 819-829.
Ferraris, G., Ivaldi, G., Chopin, C. (1995): Magnesiodumortierite,a new mineral from very-high-pressure
rocks (Western Alps). Part I: Crystal structure. Eur. l.
Mineral., 7,167-174.
Gebhard, G. (1991): Tsumeb, eine deutsch-afrikanische
Geschichte. C. Gebhard, GieBen, 239 p.
Herrendorf,
W. (1997): Programm
HABITUS,
GieBen,1995; Fa. STOE & Cie GmbH: Programm X-SHAPE,
Version 1.02, Darmstadt, 1997.
ICSD (2000): Release 2000/1. Fachinformationszentrum
Karlsruhe and National Institute of Standards and Technology Gaithersburg.
Kato, K., Kanke, Y., Oka, Y., Yao, T. (1998): Crystal structureofzinchydroxide
sulfatevanadate(V),
Zn7(OHh(S04)(V04hZ. Kristallogr.
- New Cryst.
Struc., 213, 26.
Keller, P. (1984): TsumeblNamibia - eine der spektakuliirsten
Mineralfundstellen
der Erde. Lapis, 9, H. 7&8,13-62.
Keller, P. & Innes, J. (1986): Neue Minerale von Tsumeb.
Lapis, 11, H. 9, 28-32.
Keller,P.,
Lissner,F.,
Schleid,Th.
(1999):(Fe3+,Fe2+MOH,0)3[As03h[As03,
HOSi03]:Ekatit,einhydrogensilicathaltiges
Eisen(IIIIII)-HydroxidOxoarsenat(III).
Z. Kristallogr., Supl. 16, 84.
Mandarino, J.A. (1981): The Gladstone-Dalerelationship:
Part IV. The compatibility
concept and its application.
Canad. Mineral., 19,441-450.
Marcos, M.D., Amoros, P., Le Bail, A. (1993a): Synthesis
and crystal structure of tubular hydroxyphosphite:
Zn1, D(HP03MOH)6.J.
Solid State Chern., 107,250-257.
Marcos, M.D., Amoros, P., Beltran-Porter, A., Martines, R.,
Attfield, P. (1993b): Novel crystalline microporous transition-metal phosphites Mll(HP03MOH)6(M = Zn, Co,
Ni). X-ray powder diffraction structure determination of
the Co and Ni derivates. Chern. Mater., 5, 121-128.
Moore,
P.B.
& ArakiT. (1978):Dumortierite,
Si3B[AI6.7sDo.250172s(OH)o.75]:
a detailed
structure
analysis. N. lb. Mineral. Abh., 132,231-241.
Perez, G., Lasserre, F., Moret, J., Maurin, M. (1976): Structure cristalline des hydroxytellurites
de nickel et de cobalt. l. Solid State Chern., 17, 143-149.
Raade, G., RfI!mming, c., Medenbach, 0. (1998): Carbonat-substituted
phosphoellenbergerite
from Modum,
Norway: description
and crystal structure. Mineral.
Petrol., 62, 89-101.
RfI!mming, C. & Raade, G. (1989) The crystal structure ofnatural and synthetic holtedahlite. Mineral. Petrol., 40,
91-100.
SHELXTL (1996): Version 5, Siemens Analytical Instruments Inc., Madison, Wi.
SHELXL-97(1997): G. M. Sheldrick, Univ. Gottingen,Copyright @ George M. Sheldrick, 1993-7.

Received
ModifiedAccepted

26 Septernber 2000
version received 6 February
14 February 2001

2001