817
The Canadian Mineralogist
Vol. 38, pp. 817-830 (2000)

HIGHLY UNDERSATURATED ANIONS IN THE CRYSTAL STRUCTURE
OF ANDYROBERTSITE – CALCIO-ANDYROBERTSITE, A DOUBLY ACID ARSENATE
OF THE FORM K (Cd,Ca) [Cu2+5 (AsO4)4 {As(OH)2O2}] (H2O)2
MARK A. COOPER AND FRANK C. HAWTHORNE§
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada

ABSTRACTThe crystal structure of andyrobertsite, ideally K Cd [Cu2+5 (AsO4)4 {As(OH)2O2}] (H2O)2, monoclinic, a 9.8102(9),b 10.0424(6), c 9.9788(7) Å, ␤ 101.686(7)°, V 962.6(1) Å3, P21/m, Z = 2, has been solved by direct methods and refined to an R
index of 2.3% for 2140 observed (Fo > 5␴F) reflections measured with MoK␣ X-radiation. There are three crystallographicallydistinct As sites, each occupied by As5+ in tetrahedral coordination. One of the As sites is coordinated by two O atoms and two(OH) groups to form a doubly acid arsenate group, {As(OH)2O2}. There are four crystallographically distinct Cu sites, each
occupied by Cu2+ in square-pyramidal coordination. There is one crystallographically distinct M site occupied by Cd, Ca and
Mn2+ coordinated by four O atoms and two (H2O) groups in a trigonal-prismatic arrangement. Variation in the occupancy of thissite gives rise to the two related species andyrobertsite (Cd dominant) and calcio-andyrobertsite (Ca dominant). There is one
distinct K site occupied by K and coordinated by four O atoms, two (OH) groups and two (H2O) groups in a distorted-cube
arrangement. Four (CuO5) polyhedra link to a central anion to form a [Cu4O13] group. The anions at the base of this group cornershare to four (AsO4) tetrahedra that link to a fifth (CuO5) square pyramid. A single (As␾4) (␾ : unspecified anion) tetrahedron
links to the central anion of the [Cu4O13] group to form a [Cu5 (AsO4)4 (As␾4) O9] cluster. These clusters are arranged at thevertices of non-coplanar 44 nets parallel to (100) and corner-link to form a heteropolyhedral framework with interstitial M (= Cd,
Ca, Mn2+) and K (= K) sites between adjacent 44 nets. Two of the anions in the structure show anomalously low incident-bondvalence sums (1.60 and 1.69 vu: valence units).
Keywords: andyrobertsite, crystal structure, electron-microprobe analysis, arsenate, Tsumeb.

SOMMAIRE
Nous avons résolu la structure cristalline de l’andyrobertsite, dont la composition idéale est K Cd [Cu2+5 (AsO4)4{As(OH)2O2}] (H2O)2, monoclinique, a 9.8102(9), b 10.0424(6), c 9.9788(7) Å, ␤ 101.686(7)°, V 962.6(1) Å3, P21/m, Z = 2, parméthodes directes, et nous l’avons affinée jusqu’à un résidu R de 2.3% en utilisant 2140 réflexions observées (Fo > 5␴F) mesurées
avec rayonnement MoK␣. Il y a trois sites As cristallographiquement distincts, chacun contenant As5+ en coordinence tétraédrique.
Un des sites As est lié à deux atomes O et deux groupes (OH) pour former un groupe arsenate doublement acide, {As(OH)2O2}.
Il y a quatre sites Cu cristallographiquement distincts, chacun étant occupé par Cu2+ en coordinence pyramidale carrée. Il y a unsite M cristallographiquement distinct, dans lequel se trouvent Cd, Ca et Mn2+ en coordinence avec quatre atomes O et deuxgroupes (H2O) dans un agencement trigonal prismatique. Les variations dans ce site mènent aux deux espèces andyrobertsite (Cddominant) et calcio-andyrobertsite (Ca dominant), présents dans le même cristal. Il y a un site distinct K occupé par K et coordonnépar quatre atomes O, deux groupes (OH) et deux groupes (H2O) dans un agencement cubique difforme. Quatre polyèdres (CuO5)
entourent un anion central pour former un groupe [Cu4O13]. Les anions à la base de cet agencement sont les coins de quatre
tétraèdres (AsO4), qui sont connectés à une cinquième pyramide carrée (CuO5). Un seul tétraèdre (As␾4) (␾ : anion non spécifié)
est lié à l’anion central du groupe [Cu4O13] pour former une agrégation de composition [Cu5 (AsO4)4 (As␾4) O9]. Ces
agglomérations définissent les coins de réseaux 44 non coplanaires parallèles à (100), qui partagent leurs coins pour former une
trame hétéropolyédrique ayant des sites interstitiels M (= Cd, Ca, Mn2+) et K (= K) entre réseaux 44 adjacents. Deux des anions
dans cette structure font preuve de valences de liaison incidentes anormalement faibles (1.60 et 1.69 unités de valence).
(Traduit par la rédaction)
Mots-clés: andyrobertsite, structure cristalline, analyse à la microsonde électronique, arsenate, Tsumeb.

§

E-mail address: frank_hawthorne@umanitoba.ca

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THE CANADIAN MINERALOGISTINTRODUCTION
Andyrobertsite, ideally K Cd [Cu 2+ 5 (AsO 4 ) 4{As(OH)2O2}] (H2O)2, and calcio-andyrobertsite, ideally K Ca [Cu2+5 (AsO4)4 {As(OH)2O2}] (H2O)2, are
two new minerals from Tsumeb mine, Namibia (Cooper et al. 1999). Both minerals form a lamellarintergrowth that is crystallographically continuous andin which the lamellae are only a few micrometers wide.
However, composite crystals show high-quality diffraction patterns with small sharp diffraction-maxima, despite the composite nature of the crystals. The crystal
structure of andyrobertsite – calcio-andyrobertsite wassolved in order to properly characterize these minerals;
the results are presented here.

EXPERIMENTAL
Andyrobertsite – calcio-andyrobertsite from the type
locality was generously made available for this work by
Mr. William W. Pinch. A thin cleavage fragment(|| {100}) was attached to a glass fiber and mounted ona Siemens P4 automated four-circle diffractometer. Cell
dimensions (Table 1) were determined from the setting
angles of 48 automatically aligned reflections in therange 40 < 2␪ < 60°. Intensities were measured from 4
to 60°2␪ (1̄3̄ ≤ h ≤ 13, 0 ≤ k ≤ 14, 0 ≤ l ≤ 13) with scans
speeds varying between 2.0 and 29.3°2␪/min; a total of
3127 reflections was measured over one asymmetricunit. Psi-scan data were measured for 17 reflections
uniformly distributed between 4 and 60°2␪ at increments of psi of 4°. A thin-plate correction with a glancing angle of 5° reduced R(azimuthal) from 11.9 to 1.3%.
Intensities were corrected for Lorentz, polarization andbackground effects, and then reduced to structure factors; of the 2520 remaining unique reflections, 2140
were classed as observed (| Fo | > 5␴Fo).
Subsequent to the diffraction experiment, the crystal
used for the collection of the X-ray intensity data wasanalyzed with a Cameca SX–50 electron-microprobe
according to the procedure described by Cooper et al.
(1999). Back-scattered-electron (BSE) imaging of thecrystal showed a fine lamellar zoning on a scale of mi-

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crometers. A traverse of 40 equidistant (20 ␮m beamsize) points was acquired perpendicular to the lamellae.
Each successive electron-microprobe point overlappedsignificantly (~60%) with a neighboring point to provide continuous chemical sampling. The compositionsof five selected points (A → E) are given in Table 2 andshown in Figure 1 in terms of the occupancy of the Msite; H2O values were calculated from the H2O content
established by crystal-structure refinement, and unit
formulae were calculated on the basis of 22 anions (including two OH and two H2O groups). The average(bulk) composition of the crystal was estimated byweighting each of the forty compositions according to
the shape of the crystal and the resulting length of thelamellae analyzed; the result is shown as + in Table 2
and Figure 1. The bulk composition of the crystal lies
exactly on the boundary between andyrobertsite andcalcio-andyrobertsite; for clarity of expression, we willrefer to the structure as andyrobertsite.

STRUCTURE SOLUTION AND REFINEMENT
All calculations were done with the SHELXTL PC(Plus) system of programs; R indices are of the form
given in Table 1 and are expressed as percentages. The
E statistics indicate a centrosymmetric structure, and thesolution was found in the space group P21/m. All cations and anions were located by successive cycles ofleast-squares refinement and difference-Fourier synthesis, and refinement converged to an R index of 2.4% fora model with anisotropic-displacement parameters for
all atoms (except H). Hydrogen atoms were located bybond-valence analysis and difference-Fourier synthesis,

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THE STRUCTURE OF ANDYROBERTSITE

and were included with constrained isotropic-displacement parameters and soft constraints on the O–H and
OW–H distances (≈ 0.98 Å) in the final cycles ofrefinement, which converged to an R index of 2.3%.
Site-occupancy refinement for the cation sites showscomplete chemical order over the As, Cu and K sites,
accounting for all As + Cu + K measured by chemical
analysis (Table 2). The M site gives an electron sum
intermediate between Ca (20 electrons) and Cd (48 electrons). The chemical analysis indicates that the remaining Cd + Ca + Mn must occur at the M site. To compare
the Cd + Ca + Mn content from the unit formula(Table 2) with that from the site-occupancy refinement,
the occupancy of Mn at the M site was fixed at the value
from the unit formula, and the remaining scattering wasthen refined in terms of Cd and Ca. The occupancy of
the M site refined to 0.428(4) Ca + 0.426(4) Cd + (0.146Mn), in exact agreement with the unit formula of thecrystal (Table 2). Final atom positions and displacement
factors are given in Table 3, selected interatomic distances and angles for non H-atoms are listed in Table 4,interatomic distances and angles involving H atoms are
given in Table 5, and a bond-valence analysis is shownin Table 6. Observed and calculated structure-factors
may be obtained from The Depository of Unpublished
Data, CISTI, National Research Council, Ottawa,
Ontario K1A 0S2, Canada.

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FIG. 1. Variation in chemical composition of the crystal of
andyrobertsite used for crystal-structure work (marked bythick black line); the bulk composition (+) lies exactly on
the boundary between andyrobertsite and calcioandyrobertsite.

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THE CANADIAN MINERALOGIST

FIG. 2. Cation coordination in andyrobertsite; blue circles: Cu2+ atoms; grey highlighted circles: O atoms; small black circles:
H atoms; large green circle: M-site atom; large yellow highlighted circle: K atom.

DESCRIPTION OF THE STRUCTURE
Coordination of the cations
There are three crystallographically distinct As sites,
each occupied by As5+ coordinated by four O atoms ina tetrahedral arrangement. The relative dispersion of thebond lengths in each tetrahedron is not large; despite
their unusual connectivity, the As(3)␾4 tetrahedron issomewhat more distorted than the As(1)O4 and As(2)O4
tetrahedra, but none of the distortions are particularly
large. As the bond topology of this structure is rather
unusual, we also list the ␾–␾ and ␾–As–␾ angles inTable 4.
There are four crystallographically distinct Cu sites,
each occupied by Cu2+ coordinated by five O atoms in
distorted square-pyramidal arrangements (Fig. 2). In allcases, the four meridional Cu2+–O bonds are short(1.92–1.98 Å), and the apical bond is long (2.27–
2.60 Å). The deviation from equal bond-lengths in the(CuO5) polyhedra is much larger than is normal in such

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polyhedra in minerals (Eby & Hawthorne 1990, Burns& Hawthorne 1995). The square-pyramidal geometriesof the (CuO5) polyhedra show significant bond-elongation to the apical oxygen [O(9)], in addition to large
angular deviations between the apical and equatorial
oxygen atoms (Fig. 2). The geometry for Cu(4)O5 shows
only marginal deviation from that normally observed for
Cu2+ in square-pyramidal coordination. It should be
noted that this distortion does not seem to be inducedby the bond-valence requirements at the O(9) and O(10)sites (those sites involved in the apical bonds) as theanions at these sites show anomalously low incident
bond-valence sums (Table 6).
There is one crystallographically distinct M site thatis occupied by Cd, Ca and Mn2+. Indeed, the chemicalcomposition of the crystal used to collect the X-ray intensity data (Fig. 1) indicates that both the Cd and Ca
species (andyrobertsite and calcio-andyrobertsite, respectively) are present in the crystal, and in places, thecomposition approaches fairly closely to the (as yet notfound) Mn2+-dominant species. The M site is surrounded

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THE STRUCTURE OF ANDYROBERTSITE

by four O atoms and two (H2O) groups in a trigonalprismatic arrangement (Fig. 2), and the dispersion of theM–␾ (␾ = O2–,H2O) bonds is fairly small (Table 4).
There is one crystallographically distinct K site, occupied by K and coordinated by four O atoms, two (OH)groups and two (H2O) groups in a distorted-cube arrangement (Fig. 2), with significant dispersion (2.73–
3.33 Å) in the individual K–␾ bond-lengths.

STRUCTURE TOPOLOGY
The key element of the andyrobertsite structure is a
[Cu5 (AsO4)4 (As␾4) O9] cluster that is illustrated byball-and-stick and polyhedral diagrams in Figure 3. Four(CuO5) groups [involving two Cu(1), one Cu(2) and one
Cu(3) sites] link to a central O(9) anion, sharing edges

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821

to form a [Cu4O13] group (Figs. 3a, b). The base of thisgroup is decorated by four (AsO4) tetrahedra [involving
two As(1) and two As(2) sites] that corner-share to thefour basal oxygen atoms of the Cu(4)O5 square pyramid. A single (As␾4) tetrahedron attaches to the central
O(9) anion of the [Cu4O13] group to form the complete[Cu5 (AsO4)4 (As␾4) O9] cluster (Figs. 3c, d).
These clusters are arranged at the vertices of aslightly corrugated 44 net parallel to (100) to form asheet (Fig. 4). Adjacent clusters are displaced from eachother by (b/2 + c/2), and adjacent [Cu5 (AsO4)4 (As␾4)O9] clusters are inverted in the [100] direction (Fig. 4).
Clusters link directly by sharing peripheral vertices ofthe outer arsenate groups of one cluster with meridionalanions of (CuO5) polyhedra in adjacent clusters (Fig. 4).
The result is a thick slab that resembles a chess board
with one color occupied by [Cu5 (AsO4)4 (As␾4) O9]
clusters.
The stacking of these slabs is shown in Figure 5. The
slabs are linked by sharing of apical Cu(4)O5 vertices

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THE CANADIAN MINERALOGIST

FIG. 3. The stereochemistry of the [Cu5 (AsO4)4 (As␾4) O9] cluster in andyrobertsite; (a) ball-and-stick model of the [Cu4O13]
group, legend as in Figure 2, (b) polyhedral model of the [Cu4O13] group viewed down [100]; (CuO5) polyhedra are blue; (c),(d) polyhedral model of the [Cu5 (AsO4)4 (As␾4) O9] cluster viewed down [100] and perspective view, respectively; legend
as in Figure 3(b) with the As␾4 tetrahedra shown in pink.

with O vertices of the As(3)␾4 tetrahedra to form a continuous Cu␾5–As␾4 framework. Further interslab linkage is provided by H-bonding between the (OH) groupsof the doubly acid arsenate tetrahedron and two meridional O atoms of the Cu(4)O5 polyhedron.
In the view of the structure shown in Figure 4,
andyrobertsite appears orthorhombic; Figure 5 shows
why the structure is actually monoclinic. The Cu(4)O5
polyhedron links clusters adjacent in the a direction via
the Cu(4)–O apical bond. This bond is coaxial with the817 38#4-août00-2170-04

822

central axis of one cluster, but links to the basal O atomof the As(3)␾4 tetrahedron; as the latter anion is off thecentral axis of the adjacent cluster, adjacent clusters are
systematically offset along –c as they link along a(Fig. 5).
The overall structure is a highly complicated framework of (As␾4) tetrahedra and (CuO5) square pyramids,
with Cd, K and H2O occupying the interstices andstrengthening the framework (Fig. 6). Above, it wasnoted that the [Cu5 (AsO4)2 (As␾4) O9] clusters occupy

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THE STRUCTURE OF ANDYROBERTSITE

FIG. 4.

The heteropolyhedral (CuO5)–(As␾4) slab in andyrobertsite, projected down an axis 6° from [100]; legend as in Figure 3.

alternate vertices of a 44 net. In Figure 6, it can be seenthat the other set of alternate vertices is occupied by the
M and K sites. In Figure 6, only the M and K sites proximal to the upper surface of the slab are seen. Attachedto the lower surface (not shown) is an M site beneathevery K site, and a K site beneath every M site. Linkage
between adjacent slabs occurs via (1) the shared H2O–
H2O edge between adjacent M␾6 and K␾8 polyhedra and817 38#4-août00-2170-04

823(2) the shared OH–OH edge between adjacent As(3)␾4
and K␾8 polyhedra.
Hydrogen bonding
The bond-valence analysis (Table 6) indicates that
there is one (OH) group and one (H2O) group in the
andyrobertsite structure; details of their stereochemis-

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THE CANADIAN MINERALOGIST

FIG. 5.

The (CuO5)–(As␾4) framework in andyrobertsite projected down an axis 6° from [010]; legend as in Figure 3.

try (Table 5) lie in the ranges typical for H-bonding(Ferraris & Franchini-Angela 1972, Chiari & Ferraris
1982, Ferraris & Ivaldi 1984) and are illustrated in
Figure 7.
The (OH) group is bonded to As at the As(3) site toform a doubly acid arsenate group: {As(OH)2O2}. Thisis the third example of a doubly acid arsenate group to817 38#4-août00-2170-04

824

be found in a mineral. Kaatialaite, Fe3+ {As(OH)2O2}3(H 2 O) 5.5 (Boudjada & Guitel 1981, Schmetzer &Medenbach 1986), and the unnamed phase Ca {As(OH)2O2}2 described by Ondruš et al. (1997) each contain an {As(OH)2O2} group.
Ferraris & Ivaldi (1984) examined the stereochemistry of protonated arsenates and showed that there arestrong correlations between the As–OH and O–H distances and between the As–(OH) and H…O distances;
despite the fact that soft constraints on the H-atom positions were used in the refinement of the structure ofandyrobertsite, the stereochemical data for andyrobertsite are in almost exact accord with the As–OH versus
H…O trend of Ferraris & Ivaldi (1984): inandyrobertsite, the H atom at H(3) H-bonds to the O(3)atom of the adjacent Cu(4)O5 polyhedron (Fig. 7a). The
Cu(4)O5 polyhedron also links directly to the As(3)␾4
tetrahedron through the O(10) atom.
The (H2O) group links directly to the M-site cation(Fig. 7b), and hence must H-bond to adjacent anions tosatisfy its own bond-valence requirements. There is asingle H-bond by the H atom at H(1) to an O(7) anion

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THE STRUCTURE OF ANDYROBERTSITE

FIG. 6.

The structure of andyrobertsite projected onto (100); legend as in Figures 2 and 3.

of an adjacent As(2)O4 group, and a bifurcated H-bondby the H atom at H(2) to adjacent O(3) and O(7) anions(Fig. 7b). The resulting bond-valence sums at the O(3)and O(7) anions are close to the ideal value of 2 vu
(Table 6).

HIGHLY UNDERSATURATED ANIONSIN THE ANDYROBERTSITE STRUCTURE
Inspection of Table 6 shows that all cations havebond-valence sums near their ideal expected values, and817 38#4-août00-2170-04

825that the magnitude of the grand sum for all bonds (44.25vu) is very near the ideal total (44 vu) pfu. Most valencesums at the anions are near their expected ideal value(2 vu); however, sums at O(9) and O(10) are anomalously low at 1.69 and 1.60 vu, respectively. Valencesums calculated for well-refined structures normally fallwithin 10% of their ideal value, whereas the valencesums at O(9) and O(10) are low by 15 and 20%, respectively. Bonding in this region of the structure thus needs
to be examined further. The O(9) and O(10) sites occur
on a mirror plane, whereas all other anions occur at gen-

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THE CANADIAN MINERALOGIST

FIG. 7. Perspective views of the H-bond arrangement in andyrobertsite: (a) the (OH) group; (b) the (H2O) group, designated
OW; proposed H-bonds are shown as broken lines.

eral positions; thus, on a pfu basis, the small bond-valence excesses (> 2 vu) at several of the anions morethan offset the large bond-valence deficits at O(9) andO(10). The bond-valence sums in Table 6 indicate thefollowing with respect to bonding in the andyrobertsite
structure: (1) anomalously low bond-valence sums at
O(9) and O(10), (2) compensatory small excesses ofbond-valence at the other anion sites, and (3) normalbond-valence sums at all cation sites.
The local environments of O(9) and O(10)The O(9) anion is bonded to four Cu2+ cations [Cu(1)⫻ 2, Cu(2), Cu(3)] and one As5+ cation [As(3)] withbond-valence contributions of 4 ⫻ ~0.10 and 1.30 vu,
respectively. The O(9) anion occurs at the core of the[Cu5 (AsO4)4 (As␾4) O9] cluster (Fig. 3). No other cations (including H) are sufficiently close to bond to O(9);moreover, the neighborhood of O(9) is too spatiallyconstricted to accommodate another cation.
The O(10) anion is bonded to one As5+ cation [As(3)]and one Cu2+ cation [Cu(4)], with bond-valence contributions of 1.39 and 0.21 vu, respectively (Figs. 5, 7).
No additional cations (including H) are sufficiently
close to bond to O(10), although there is considerable
space around O(10) in the structure. We conclude thatthe bond-valence deficiencies at O(9) and O(10) do not
result from unrecognized chemical bonds in the struc-817 38#4-août00-2170-04

826

ture. The O(9) and O(10) anions form an edge of theAs(3)␾4 tetrahedron, and the individual As(3)–␾ bondsand ␾–As(3)–␾ angles are in the usual range (Table 4).
However, the connectivity of the As(3)␾4 tetrahedronin the andyrobertsite structure is very unusual; one vertex [O(9)] of the As(3)␾4 tetrahedron is coordinated by
five cations, another vertex [O(10)] is coordinated by
two cations, and the remaining vertices are [1]-coordinated OH groups. As a result, there is no single strong
bond (>0.2 vu) that forms between any of the verticesof the As(3)␾4 tetrahedron and the neighboring cations(Table 6).
Anisotropic displacementsin the andyrobertsite structure
A cursory glance at Table 3 shows rather prominent
anisotropy at many of the sites. Without going into minutiae, we can summarize the observed isotropy–anisotropy relations in the following manner: at the core of
the [Cu5 (AsO4)4 (As␾4) O9] cluster is a rigid cube ofatoms defined by isotropic anions O(4), O(6), O(3) andO(7) at its eight corners. Joined to the top surface of thiscube along each of the four edges are four distorted(CuO5) polyhedra. Each (CuO5) of the [Cu4O13] groupis securely anchored at its base but has the freedom tobend and flex. The loosely attached As(3)␾4 tetrahedronsits atop the [Cu4O13] group, exhibiting lateral displace-

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THE STRUCTURE OF ANDYROBERTSITE

ment with mutually responsive distortions in the underlying [Cu4O13] group that shares the O(9) vertex withthe tetrahedron. The following structural features allow
this displacement: (1) the O(9) anion forms four weak
and flexible bonds to Cu2+ atoms of the [Cu4O13] group;(2) the O(10) anion is coordinated by only two cationsites [As(3), Cu(4)], and moderate flexure in the Cu(4)–O(10)–As(3) angle is allowed; (3) as mentioned above,
there is considerable space around O(10), and therefore

minimal restriction is imposed on displacement ofO(10); (4) the O(10) anion is the long apical ligand of
the Cu(4)O5 polyhedron, stereochemically a condition
that allows for considerable flexibility in the position of
the O(10) atom.
For the O2– anions [O(1) → O(10)], there is a reasonably strong negative correlation between anisotropy
and incident bond-valence (Fig. 8). We refer to this as
the “rattle” effect, whereby greater anisotropy is com-

FIG. 8. The equivalent-isotropic displacements of the simple anions versus their incident
bond-valence sums in the structure of andyrobertsite; the line is drawn merely as a
guide to the eye.

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THE CANADIAN MINERALOGIST

monly associated with longer average bond-lengths,
resulting in lower incident bond-valence sums. On a pfu
basis, the excess bond-valence (>2 vu) at O(1) throughO(8) is exactly offset by the deficit in bond-valence atO(9) and O(10). The mean valence for O(1) → O(10)[36.09 vu / 18 atoms of oxygen pfu = 2.005 vu] is shownas the vertical dashed line.
What causes the large positional anisotropy associated with the As(3)␾4 tetrahedron? Would relaxing themost anisotropic atoms off the mirror plane lead to asimple ordered arrangement with less deviant O(9) andO(10) bond-valence sums? To test this, we tried refining the structure in the space group P21; however, weobserved no significant departure from the P21/m structural model. We can therefore rule out a long-range-or-

dered lower symmetry. A more likely explanation for
the anisotropy may involve chemically driven bondlength adjustments associated with extreme Ca ↔ Cd
↔ Mn chemical variations at the M-site (Fig. 1). The
M␾6 trigonal prism shares its O(2)–O(2) and O(5)–O(5)edges with the Cu(3)O6 and Cu(2)O6 polyhedra, respectively (Fig. 6). We can consider the As(3)␾4 tetrahedronof the andyrobertsite structure as the weakly bonded“shock-absorber” between the strongly bonded Cu–As
slabs (Fig. 5). Extreme chemical variation at the M-siteinduces positional disorder (anisotropy) in the [Cu4O13]
group and the As(3)␾4 tetrahedron. This argument suggests that the resulting atom displacements are dominantly static, rather than dynamic.

FIG. 9. The structure of Ba [Cu2+ (V4+O) (PO4)4]; (a), (b): the [Cu4 (VO) (PO4)4 O9] cluster viewed down [001] and in perspective, respectively; (c) the structural unit viewed
down an axis 4° from [100]; blue: (CuO5) polyhedra; yellow: (PO4) tetrahedron; orange: (VO5) polyhedra.

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THE STRUCTURE OF ANDYROBERTSITE

Comparison with the structure ofBa [(V4+O) Cu2+4 (PO4)4]The only other occurrence of the [Cu4O13] group is
in Ba [(V 4+ O) Cu 2+ 4 (PO 4 ) 4 ] (Meyer & MüllerBuschbaum 1997) (Fig. 9a). Phosphate tetrahedra extend from the base of the [Cu4O13] group and attach tofour corners of a V4+O5 polyhedron (Fig. 9b); this unitof the structure is topologically the same as the lower
three-quarters of the [Cu5 (AsO4)4 (As␾4) O9] cluster inandyrobertsite (cf. Fig. 3d). Adjacent [Cu4O13] groups
link via corner-sharing with PO4 tetrahedra in the samemanner as in andyrobertsite (compare Figs. 9c and 5).
The [Cu4O13] groups in adjacent Cu–P slabs are centered over one another along [001], with a direct linkage provided through the VO5 polyhedron (Fig. 9c).
Hence, Ba [(V4+O) Cu2+4 (PO4)4] contains no counterpart of the As(3)␾ 4 tetrahedron in andyrobertsite.
Table 7 compares several features of the [Cu4O13] groupin the two structures. In both structures, the Cu–Cu distances are the same. Andyrobertsite shows slight elongation of <Cu–Oeq> and concomitant shortening of<Cu–Oap>. In andyrobertsite, the apical O(9) anion
bonds to As5+ (1.30 vu); in Ba [(V4+O) Cu2+4 (PO4)4],
the analogous anion forms a short (1.622 Å) vanadyl
bond to V4+ (1.55 vu) (Figs. 5, 9c). In each structure,
the apical anion of the [Cu4O13] group has a low incident-bond-valence sum (1.69 vu: andyrobertsite; 1.79
vu: Ba [(V4+O) Cu2+4 (PO4)4]).
The anisotropic-displacement parameters for the atoms of the [Cu4O13] group in Ba [(V4+O) Cu2+4 (PO4)4]
are much more isotropic; all atoms have (Umax / Umin)between 1 and 1.8. Of great interest is the apical anionof the [Cu4O13] group, which is isotropic [(U11 = U22) =U33] and shows a small overall displacement (Ueq =0.014 Å2). We can summarize these observations for theBa [(V4+O) Cu2+4 (PO4)4] structure in the following
manner: (1) there is complete chemical order at all sites;(2) all sites show long-range positional order (isotropic
behavior); (3) the low bond-valence sum (1.79 vu) at
the apical anion of the [Cu4O13] group must be regarded
as a characteristic feature of its very unusual stereochemistry.

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We conclude that the low incident bond-valencesums in andyrobertsite and Ba [(V4+O) Cu2+4 (PO4)4]
are actual features of the structure, rather than artifactsof the structure refinement. The O(9) anion in
andyrobertsite and the analogous anion in Ba [(V4+O)
Cu2+4 (PO4)4] have incident bond-valence sums of 1.69
and 1.79 vu, respectively, and the similarity of theircoordinations indicates that these low sums must be afunction of the local stereochemistry. The O(9) anion in
andyrobertsite is associated with considerable surrounding anisotropic displacements, but these displacements
seem to be connected to extreme substitution of differently sized cations (Ca = 1.00, Cd = 0.95, Mn2+ = 0.83
Å) at the neighboring M-site. The O(10) anion in
andyrobertsite (to which there is no analogue in Ba[(V4+O) Cu2+4 (PO4)4] is only [2]-coordinated, with nopossibility of acting as a H-bond acceptor, and again,
the low incident bond-valence sum of 1.60 vu is the result of this connectivity. The existence of these two
structures emphasizes one aspect of the valence-sum
rule: in some (presumably very stable) structural arrangements, major deviations from the valence-sum rulecan occur for a small number of the anions in the structure. The destabilizing effect of this feature must be
outweighed by the stability of the rest of the structure.
This feature is prominent also in the amphibole andpyroxene structures, where the O(4) and O(2) anions,
respectively, may show major deviations (>0.25 vu)from the valence-sum rule.

ACKNOWLEDGEMENTS
We thank Bill Pinch for his generosity in providing
the material for this work and for his enthusiasm andencouragement. This work was supported by Natural
Sciences and Engineering Research Council of Canada
Research and Major Equipment Grants to FCH.

REFERENCESBOUDJADA A. & GUITEL, J.C. (1981): Structure cristalline d’unorthoarséniate acide de fer(III) pentahydraté: Fe(H2AsO4)3•5H2O. Acta Crystallogr. B37, 1402-1405.
BROWN, I.D. & ALTERMATT, D. (1985): Bond-valence parameters obtained from a systematic analysis of the inorganic
crystal structure database. Acta Crystallogr. B41, 244-247.
BURNS, P.C. & HAWTHORNE, F.C. (1995): Coordination-geometry structural pathways in Cu2+ oxysalt minerals. Can.
Mineral. 33, 889-905.
CHIARI, G. & FERRARIS, G. (1982): The water molecule in crystalline hydrates studied by neutron diffraction. Acta
Crystallogr. B38, 2331-2341.
COOPER, M.A., HAWTHORNE, F.C., PINCH, W.W. & GRICE, J.D.
(1999): Andyrobertsite and calcioandyrobertsite: two newminerals from the Tsumeb mine, Tsumeb, Namibia. Mineral. Rec. 30, 181-186.

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830

THE CANADIAN MINERALOGISTEBY, R.K. & HAWTHORNE, F.C. (1990): Clinoclase and the geometry of [5]-coordinated Cu 2+ in minerals. Acta
Crystallogr. C46, 2291-2294.
FERRARIS, G. & FRANCHINI-ANGELA, M. (1972): Survey of thegeometry and environment of water molecules in crystalline hydrates studied by neutron diffraction. Acta
Crystallogr. B28, 3572-3583.

O NDRU Š , P., V ESELOVSK Ý , F., S KÁLA , R., C ÍSA ŘOVÁ , I.,
HLOUŠEK, J., FRÝDA, J., VAVŘIN, I., ČEJKA, J. & GABAŠOVÁ,
A. (1997): New naturally occurring phases of secondary
origin from Jáchymov (Joachimsthal). J. Czech Geol. Soc.
42, 77-108.
SCHMETZER, K. & MEDENBACH, O. (1986): Kaatialaite from
Nieder-Beerbach, Odenwald – a second occurrence. Neues
Jahrb. Mineral., Monatsh., 337-342.

________ & IVALDI, G. (1984): X–OH and O–H…O bondlengths in protonated oxoanions. Acta Crystallogr. B40, 1-6.
M EYER , S. & M ÜLLER -B USCHBAUM , H. (1997): Cu 4O 12Baugruppen aus planeren CuO4-Polygonen im Barium–
Vanadyl–Oxocuprat(II)–phosphat Ba(VO)Cu4(PO4)4. Z.
Anorg. Allg. Chem. 623, 1693-1698.

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Received March 13, 2000, revised manuscript accepted June 7,
2000.

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