THE AMDRICAN MINERALOGIST, VOL. 56, SEP'IEMBER-OCTOBER, 1971

CRYSTAL CHEMISTRY OF THE BASICMANGANESE ARSENATES:

V. MIXED MANGANESE COORDINATION IN THEATOMIC ARRANGEMENT OF ARSENOCLASITE

Paur- B. MoonB aNl JoANN Morrx-Case , Department oJ theGeophysical Sci,ences, The Uni,uersity of Chicago,

Chicago, Illino'is, 60637.

Arsrnec'r

A rsenoc las i t e ,Mn r (OH) r (AsO)2 ,a18 .29 (2 ) , b5 .75 (1 ) , c9 .31 Q)L ,Z :4 , spaceg roupP2 t2 rZ r , i sbasedon "doub le "hexagona l c l ose -packed ( . . . ch . . . ) oxygenan ionswhoselayers are normal to the c-axis. Its structure was deciphered from direct methods, and

R(hkl):0.07 for 1800 independent reflections.The cationic occupancies are very complicated. Three non-equivalent octahedra,

Mn(2)-O, Mn(3)-O and Mn( )-O, link by edge-sharing to form walls alternately one- and

two-octahedra wide which run parallel to the 6-axis. Also running parallel to this axis are

chains of corner-sharing Mn(l)-O trigonal bipyramids which further link by corner-sharing

to As(2)-O tetrahedra forming three-membered rings; and chains of cornerlinked As(l)-O

and Mn(S)-O tetrahedra.

The interatomic Me-O averages are Mn(1)v-O 2.19, Mn(2)vLQ 2.25,Mn13)vr-O 2.22,

Mn( )vt-g 2.19, Mn(S)w-O 2.13, As(l)w-O 1.67 and As(2)rv-O 1.69 A. Althoush the

Mn(S)Iv-O polyhedron is of distorted tetrahedral shape, the Mn(5) cation resides nearly in

the plane oI the base and the polyhedron is more properly a trigonal pyramid.

INrnooucrtoN

Systematic structural investigations on members of the seriesMn2+"(OH)2"-:,(AsOa)3-, by Moore (1967 a, 1967b, 1968a, 1968b, 197 0a)shall lead to novel insights concerning the interaction between crystal-chemical homology and paragenesis. We report studies on the atomicarrangement of yet another member, arsenoclasite.

Arsenoclasite, Mn6(OHr)(AsO+)2, was first described by Aminoff(1931) as a new species from the famous Lingban mines, in the provinceof Vdrmland, Sweden. It occurs in the paragenesis adelite-sarkinite-arsenoclasite in calcite-filled fissures cutting hausmannite impregnateddolomitic marble from the "Irland" drift. Both arsenoclasite and sarki-nite crystallized later than adelite but it is difficult to decipher the rela-tive positions in time for the first two species. Flink (192a) discussedthe occurrences of sarkinite at Lingban and noted some specimens withthe characteristic flesh-red color of sarkinite but with one perfect cleav-age. As shown by Aminoff, "sarkinites" with perfect cleavage proved tobe arsenoclasite. Moore (I967b) re-investigated type arsenoclasite, con-firmed the crystal .cell data of Aminoff, and established the uniquelydetermined space group P 2Qrh. This present investigation was per-formed on the same crvstal used in that studv.

1539

1540 MOORE AND MOLIN-CASE

ExpBnrurNrlr,

Crystal cell parameters are presented in Table 1, citing the paper of Moore (1967b)which also includes indexed powder data. A crystal of 0.0003 mms volume was selectedfor three-dimensional structure analysis utilizing a PAILRED difiractometer and graphite"monochromatized" MoK" radiation. With r as the rotation axis, 4800 reflections includ-ing tlre s1'rnmetry equivalent pairs l(hhl) and l(ilkl) from the l:0- to 12-levels were ob-tained using a2.2o halL-angle scan and a background counting time on both sides of eachpeak oI 20 seconds.

At first, the individual reflections were processed to obtain raw F(oDs) data. Sincep:137 cm-r, polyhedral transmission correction was applied using a local version of tleGNABS program (Burnham, 1963) and symmetry equivalent reflections were averaged.Remaining were 1800 independent reflections above background error and 620 independentreflections of. "zero" intensity. Qnly the Don-zero data were used in tlre following study.

Sor,urron oF rrrE Srnucrunp

Solution of the arsenoclasite atomic arrangement was a miserableproblem. At first, attempts were made to decipher the structure on thebasis of a structural relationship with pseudomalachite but several trialsof heavy atom refi.nement failed. One key to the solution of the structurewas the subsequent assumption that arsenoclasite is based on oxygenclose-packing, a typical feature among the known basic manganesearsenate structures. Accordingly, the assumed structural relationship

TABLE I. CRLSTAL CELL DATA FOR ARSENOCTASTTE,

COR}IWALLITE AND PSEI'DOI\AIACHITE.

e (8)

E (8)

s (8)

pspace group

z

fomla

O cbe

p carc 1snlffi3)

reference

Arsenoclasite

18.2s (2)

s. 7s (r)

e .31 (2)

9oo

P21212r

4

!er5 (oI0 q (Aso+) z

4.161

4 .21

l{oore (1967b)

Cowallite

17 .61

s . 8 I

+ .60

9zor5'

PZy/a

2

1r.68

Berry(19sI)

PBildomlachite

17 .06

5 . 7 6

l { . 4 9

9roo2'

P2y'a

2

4.30- r l .35

{ .3 I1

Berry (I950)

cu, (olD u (Aso4), cu, (olD q(Poq) 2

IAninoff(I93I)

COORDINATION IN ARSENOCLASIT E 1541

with pseudomalachite had to be abandoned. The relationship a/6-b/1/3-3.24 suggested that the a-axis is the sum of six Mn2+-O octahedraledges, and the relationship with D automatically defines the orientationof the octahedral voids. Furthermore, cf 4-2.34, the typical oxide layerseparation in close-packed structures. Tests of several electrostaticallyplausible heavy atom models were undertaken, but all failed.

It was then decided to attempt solution by direct methods using atangent routine adapted for non-centrosymmetric crystals, as describedby Dewar (1970). Normalized structure factors were calculated fromthe observed structure magnitudes applying a scale factor and an overallisotropic temperature factor obtained by the method of Wilson (1942).The four two-dimensional reflections initially chosen to select the originand the enantiomorph led to a meaningless solution. On the secondattempt, the reflections (160), (041), (671), (144), and (511) were as-signed the values rf 2, 0, r, y, and. z respectively. Symbolic additionapplied to these reflections with the program MAGIC (Dewar, 1970)yielded 118 additional phases with a variance <0.65 and indicated that

!:-zr/2, r-z:r/2, and r: -rf4 or 3r/4. Setting *:3r/4 and'z:zr/4 satisfied the criterion for origin and enantiomorph selection.The 123 phases from MAGIC were submitted to tangent formula refine-ment. After two cycles, a total ol 263 reflections were determined withE>1.3 and the phase shifts indicated reasonable convergence. An.E-map calculated from these reflections appears in Figure 1 with loca-tion of the final heavy metal positions superimposed.

It is apparent that the .B-map contained the sufficient heavy atominformation for successful atomic parameter location and convergencethrough Fourier synthesis. The correct ascription of the initial atomicpositions and species was facilitated by the previous assumption ofoxygen close-packing. Approximate metal-metal distances which werecompatible with sensible crystal chemistry expected for a basic mangan-ese arsenate combined with relative peak heights led to peaks A, B,

C, D, E, F:As(1), As(2), Mn(5), Mn(4), Mn(2) and Mn(3) respectively(Fig. 1). The reliability index,

R(hkt) :>ll n1oasl | - | n1r'tr1ll

: 0.39> | nloa'y I

for this arrangement. A Fourier synthesis of the phases calculated

revealed the Mn(1) atom which corresponds to the feeble peak labelle{('G" in Figure 1. Further Fourier syntheses followed by parameter

refinement for the metals led, to R(hhl):0.20. At this stage, eight non-

equivalent oxygen atoms were located on the Fourier map and a double

r542

+

MOORI' AND MOLIN-CASE

Fro. 1. Projection of the three-dimensional .E-map dorvn the b-axis resulting from thedirect determination of 263 symmetry independent phase angles. The approximate Icoordinates obtained from the three-dimensional map as well as the final heavy metals andtheir positions refined by least-squares are included.

hexagonal close-packed motif became apparent. The remaining fouratoms were approximatelv located by interatomic distance calculation5.Typical for a close-packed slructure, some of the Iight atoms had to be"coaxed" along even at the stage of the relatively low reliabil i ty indexinentioned above.

RnntNrvorqt

The arsenoclasite asymmetric formula unit constitutes one molecularunit of HaMn5As2O1z. Atomic coordinates of the five manganese, twoarsenic and twelve oxygen atoms were refined using a local version ofthe familiar ORFLS least-squares program of Busing, Martin and Levy(1962) for the IBM 7094 computer. Finally, scale factor, atomic coor-dinate and isotropic thermal vibration parameters were refined on thebasis of full-matrix inversion until the parameter shifts were within theirl imits of error. The final reliabil i tv index was R(hhl\:0.070 for 1800

C

Mn (4 )+0 .38

Mn(5 )+0 .40

Mn (3 )+0.07

Mn( l )+0.27

COORDLNATION IN A RSENOCLASITE I J4.t

TABLE 2. ARSENOCIASITE: ATOMIC COORDINATES, CATIoN CO0RDIMTI0N NUMBERS

AND I SOTROPIC THERI'IA L V] BRATION PARAMETERS .

Gsl]Iateq standard eryors in parentheses) .

Mn(1) V

I"lI'(2) VI

wr(3) vI

Mn(r+) VI

Mn(s) IV

As ( I ) IV

As (2 ) IV

o (1) = (ol0 -

0 ( 2 ) = ( 0 D -

o (3 ) = ( oD-

o ( 4 ) = ( o D -

o (s)

0 (6)

o ( 7 )

o (8)

o (e)

o (10)

o (rr)

o ( r2)

x

0 " 4323 (1 )

. 0303 ( r )

. 22 r4 ( t )

. 124 r ( 2 )

.316'r (2)

.37r+r+ (r )

_1243 (1)

. 0s re ( 7 )

.202s (7)

.0 '+r2 (7)

"2L7L (7 )

. f 46 r ( 6 )

. I 27s (e )

. L 7 6 2 ( 7 )

"0380 (7 )

. 37ss (8 )

. 2S3 r+ (7 )

.+3t+2 (7)

. 3e16 (7 )

v

0 . 2 6 e 7 ( s )

. os4' l (5)

. o7 L2 (5 )

" s s 8 7 ( s )

. s80 r+ (5 )

. l l r 7 ( 3 )

. 19 28 (3 )

. 7 3s0 (26 )

. 3 8 4 4 ( 2 6 )

. 3 6 7 6 ( 2 s )

. 746 r ( 2s )

trtrtr] r"?)

.232+(2+)

- . 0277 Gq )

.0e80 (26 )

. r r 03 (23 )

.2L87 (24)

. 3007 (24 )

. 84 r+8 (2s )

Z

o . 7 5 2 2 ( 3 )

. 8886 (3 )

" 8s82 (3 )

. 8 7 e 6 ( 3 )

. 000e (3 )

. 07 rs ( 2 )

. 1838 (2 )

.021s (r .6)

. 73e7 (16 )

. 7s69 (16 )

- " 0282 ( f s )

. 2643 (1s )

"00 r2 (17 )

.2332 (L6)

.2282 (16)

" 2s08 ( r s )

.0206 (r -s)

- . 000q (16 )

. 0 l ss ( 16 )

s (82)

0 . 6s ( 3 )

. 6 r ( 3 )

. se ( 3 )

. 76 (3 )

. 7 r ( 3 )

. 08 (2 )

. 01 (2 )

. e8 (2 r )

. e6 (18 )

. e0 (17 )

. e0 (17 )

. 62 ( r 6 )

r . 0 4 ( r 8 )

. 87 ( f 6 )

1 . 0 0 ( 1 9 )

.es ( r7)

.6e (17)

. 8 7 ( r e )

r .o7 (17)

reflections, constituting a data to variable parameter ratio of about 25: 1.Final atomic and isotropic thermal vibration parameters are l isted in

Table 2. The oxygen thermal vibration parameters range from 0.6 tol. l 1^2, typical values for oxygen close-packing. The Mn2+ parametersrange from 0.6 to 0.8 and As5+ from 0.0 to 0.1 Ar. The lf (aDs) | - p(catc)data appear in Table 3.1

r ro obtain a copy of rable 3, order NApS Document No. 01521 from National Auxil-iary Publications Service of the A.s.I.s., c/o ccM rnformation corporation, 909 ThirrlAvenue, New York, New York, 10022; remitting $3.00 for microfiche or g5.00 for photo-copies, payable in adva,nce to CCMIC-NApS.

1544 MOORE AND MOLIN-CASE

DrscussroN oF TrrE Srnucruns

Arsenoclasite has been generally regarded as structurally related to

pseudomalachite, Cu6(OH)4(PO4), and cornwallite, Cu6(OHr)(AsO+)2,

and their crystal cell relationships are summarized in Table 1. Palache,

Berman, and Frondel (1951) classify arsenoclasite with pseudomalachite

and Strunz (1970) places all three compounds together and suggests that

arsenoclasite may possibly be derived from the others on the basis of

unit cell twinning.Despite the marked similarity in compositions and crystal cells,

arsenoclasite is not closely related structurally to the other compounds

except for the presence of the same kind of octahedral wall revealed in

the structure of pseudomalachite investigated by Ghose (1963). Pseu-

domalachite is based on sheets of distorted Cu2+-O octahedral arseno-

clasite is based on walls and chains of three distinct coordination poly-

hedra: Mnvr-O octahedra, MnY-O trigonal bipyramids and Milv-O

tetrahedra (trigonal pyramids).Arsenoclasite is constructed of a "double" hexagonal close-

packed array ol oxygen atoms whose layers are stacked perpen-

dicular to the c-axis. It shares this f eature in common with

fl inkite, ry[1r+rffn3+(OH)n(AsOu); allactite, Mnz(OH)s(AsOr)z; and

synadelphite, Mne(OH)e(HzO)r(AsOa)(AsOr)z and we conclude that

the oxygen stacking sequence ( ' ' ' .h ' ' ') i. one of the more persis-

tent underlying aSpects of the basic manganese arsenate crystal chem-

istry. Figure 2 illustrates the population sequence along the c-axis'

Oxygen atoms are situated at z-0, +, +, Z.The trigonal bipyramidally

coordinated Mn(1) cations are located at z-i and {. The octahedrally

coordinated cations, Mn(2), Mn(3) and Mn(4) are distributed at z-*,

3,8, +.The tetrahedrally coordinated cations As(1) and Mn(S), are

located above and below a-0, j and tetrahedrally coordinated As(2)

above and below z-1, *. The crystallochemical formula can be written

MnTYIMnYMnIY(OH)r(AsrvOr)zi thus, I of the available octahedral voids

are occupied. The trigonal bypyramid is constituted of two tetrahedral

voids joined by face-sharing with Mn(1) at the center of the shared

face. Consequently, z% of the available tetrahedral voids are utilized by

cations.It is instructive to examine the oxygen packing density in the arseno-

clasite structure. All close-packed layers are fully o-ccupied with respect

to anions and the volume per oxygen atom is 20.4L3. This value can be

compared with cubic close-packed manganosite, MnO, which has a

volume of 21.8 At, the major oxygen volume difference between the two

doubtlessly arising from the more tightly bound As5+-O tetrahedra in

arsenoclasite.

COORD] N ATION I N ARS ENOCLASIT E 1545

o9.2 A

3rq 6.9 A h

trz 4.6 i c

9-

0-,

AsM

7/B

5te

,8

,/^ o2.3A h

tte

z= O 0'OA cFrc. 2. rdealization of the population sequence of cations and their coordination

numbers and anions along the z-direction in arsenoclasite. Heights are given as lractionaland absolute coordinates with respect to the c-axis. The letters ((c"

and ,.h', refer to thecenters of the cubic and hexagonal close-packed blocks respectively.

1546 MOORE AND MOLIN-CASE

To describe thoroughly the very complex yet aesthetically pleasing

arsenoclasite structure requires careful dissection. The Mn(2)-, Mn(3)-

and Mn(4)-oxygen octahedra join by edge-sharing to form a wall of

octahedra, alternately one- and two-cctahedra wide which runs parallel

to the 6-axis and which is oriented parallel to [001]' This wall defines

the D:5.754 repeat and is the compcnent also commcn to the pseudo-

malachite structure. It is featured in Figure 3a' Ghose (1964) has com'

mented on the occurrence of this wall in other first transition series

oxysalt structures as well, including its presence in hydrozincite,

Zns(OH)e (COs)2, and lindgrenite, Cus(OH)r(MoOa)2, the latter structure

determined by Calvert and Barnes (1957). It is also the component in

the kotoite, Mge(BOa)2, crystal structure, as discussed by Moore (1970b)'

Projected down the 6-axis, the walls are stacked like brickwork as a

result of the two-fold screw operations (Fig. 4) and possess no obvious

stacking relationship to pseudomalachite.The Mn(l)-O trigonal bipyramids l ink along two corners to form

infinite chains which also run parallel tc the D-axis. One of these corners

also joins to the As(2)-O tetrahedron forming chains of Mn(1)-As(2)-

Mn(1) three-membered rings (Figure 3b). This composite chain l inks

through vertices at a common level along z by corner-sharing to the

octahedral wall.A third type of chain, also running parallel to the b-axis, is present

in arsenoclasite and consists of corner-sharing Mn(5)-As(1) tetrahedra

(Figure 3c). This chain l inks laterally to an octahedral wall on either

s ide and through i ts apical ox) 'gen atoms to another symmetry equiva-

Ient octahedral wall. The tetrahedral vertex, O(11), is also joined to

vertices of the Mn(1)-O trigonal bipyramids.Assembled, the arsenoclasite structure is shown idealized in Figure 5.

It is remarkable in possessing three distinct Mn-O coordination poly-

hedra and it is gratifying that direct methods succeeded in unravelling

an arrangement that would have been very difficult to decipher by other

procedures. The solution of the arsenoclasite structure also adds confi-

dence to the application of direct methods to noncentrosymmetric

mineral structures.The perfect cleavage parallel to [ 100 ] is explicable in a relative sense

since this plane is parallel to the axis of the chains (6) and the axis nor-

mal to the close-packed layers (c), the directions of strong Me-O-l\{e

bonds bridging between the chains and the oxygen layers.

INtnnnroltlc DTsTANCES

The arsenoclasite analysis of Blix in Aminoff (1931) indicates that the

compound is nearly pure stoichiometric Mns(OH)a(AsOn)2. The reason-

COORDIN AT ION IN ARSENOCLASI T I' t.s47

.':'.t:;".::l:,i

:.:'..

Ifrc.3. rdealized polyhedral chains which are the components of the arsenoclasitestructure projected down the c-axis. Ali chains run parallel to the 6_axis.

a The wall of alternate one- and two-octahedra, comprised of the Mn(2)-, Mn(3)_and Mn(4)-oxygen octahedra.

b. The corner-sharing chains of Mn(l)v-g trigonal bipyramids (stippled) and As(2)-otetrahedra. Note the presence of Mn(1)-Mn(1)-As(2) three-membered rings.

c. The corner-sharing chain of alternate Mn(5)rv-g trigonal pyramids (stippled) andAs(1)-O tetrahedra.

able isotropic thermal vibration parameters for all ionic species attestto an ordered structure and the essential absence of substitutins cationsof different atomic number. we shall discuss the interatomic Jistu.rceson the basis of an electrostatic model.

1548 MOORE AND MOLIN-CASE

FIt

-l

Icl

Frc.4. Orientation of the geometrically equivalent octahedral walls in arsenoclasite

(A) and pseudomalachite (B). The walls run perpendicular to the plane of the drawing

and the connections between centers of the two-octahedra in the wall are drawn as bold

Iines.

According to such a model, effects on the Me-O distances should be

noted if the anions are undersaturated or oversaturated electrostatically

with respect to cations. Table 4 reveals that O(12), O(5), and O(7) are

considerably undersaturated with A2: -.25, - '35, and -'42 respec-

tively; whereas O(6) and O(9) are considerably oversaturaled both with

A>:+0.25. The remaining cations deviate only slightly from electro-

static neutrality.Table 5 presents the Me-O and O-O' polyhedral distances in the

arsenoclasite crystal structure. The polyhedral averages are Mn(1)v-O

2.19, Mn(2)vr-O 2.25, Mn(3)vr-g 2.22, Mn(4)vr-O 2 '19, Mn(S)w-O 2 '13,

As(1)N-O 1.67 and As(2)rv-O 1.694. T'he Mn2+vI-O and As5+Iv-O aver-

ages are typical for their polyhedra and have been recorded for numerous

slructures. T\he Mn2+v-O trigonal bipyramidal average is considerably

longer than the Mnv-O 2.t2L distance in eveite, Mnz(OH)(AsO+) (an

isotype of andalusite) reported by Moore and Smyth (1968), but the

environments in the two structures are quite different' The Mn2+rY-O

distance is considerably longer than the Milv-O 2.04 h tetrahedron re-

port ed by Moor e ( 1 970c) f or manganostibite, M nl+vt1'1tt3+tvsb 5+As5+O1z'

It is difficult to suggest reasons for these differences without much

speculation since the number of refined structures with Mn2+ in tetra-

hedral or five-fold oxvgen coordination are few and no survey is prac-

ticable. Geometrically, the trigonal bipyramid is capable of continuous

distortion and passes into the square pyramid. As for Mn(S)N-O in

IB

COORDI N AT ION I N ARSENOCLASIT IJ 1549

F_b________rFrc. 5. Idealized polyhedral diagram of the arsenoclasite atomic arrangement down

the c-axis, built Irom the components in Figure 3. AII Mn-O polyhedra are stippled andthe As-O tetrahedra are unshaded. The Mn(2)-, Mn(3)-, Mn( )-O octahedral wall is cen-tered at s-f. Labelling of the atoms conlorms to trre convention in Table 5. comprehen-sion of the three-dimensional polyhedral arrangement is further assisted by Figure 2.

arsenoclasite, the coordination polyhedron deviates considerably from acation-centered tetrahedron since Mn(5) resides nearly in the plane ofthe base. For geometrical discussion, the oxygen polyhedron is tetra-hedral in shape, but from an electronic standpoint, it is more properly

1550

TABLE II.

MOORE AND fuIOLIN-CASE

ARSENOCIASITE: ELECTROSTATIC VALENCE BAIANCES (E) ABOTN ANIONSA

0H(1) lfrt(1) ++.{n(2) +}art(4)

oH(2) Mn(3) +Mn(q) +l4n(s)

oH(3) I'4n(2) +I4n(2) +Mn(t+)

oH(4) I"h (3) +l4n (+) +Mn(s)

0(s) As(2)+I f r r ( I )

o(6) As(2)+t ln(2) f lan(3)+Mn(t+)

o(7) As(2)+!an(3)

o (8) As (2) +[,ln (1) +!h (])

o (9) As (l) +r'h (2) +I&t (3) +Mn (r+)

o(10) As(1) +lan(3) +Ifrt(s)

o(11) As (1) +Ian(l-) +lan(2)

o(12) As(1)+[an(s)

) / \ + 2 / R + ) / 6

7/6+2/6+2/_+I

2/6+u6+2/6

2//6+2/6+u4

s/++2/s

5/4+u6+2/6+2/6

5/++2/6

5/++Us+U5

5/++U6+U6+?/6

s/4+U6+U+

V4+Us+U6

5/++2/4

tr

1 . 0 7

I . L 7

r . 0 0

1 . 1 7

f . 6 5

1 . 5 8

" n q

2 . 08

1 . 9 8

r . 7 5

^E+ . o 7

+ .00

+ . I 7

- . 3 5

- . + 2

+ . 0 5

+ ) <

+ . 0 8

_ . 0 2

+0 .00

uTh" ,AE for severely oversaturated or undersaturated. anions are italicized-

a trigonal pyramid. The distinction between Mn(1)Y-O and Mn(5)Iv-O re-

s u l t s f r o m t h e i r c a t i o n l o c a t i o n s i n t h e s t a c k i n g s e l l u e n c e ( ' ' ' c h ' ' ' ) .Mn(1)v is located at z-|- and t, which in Figure 1 are the centers of the

hexagonal close-packed blocks and the cation must coordinate to apical

anions above and below. Mn(S)Iv however, is located at z-0, |, which

are centers of the cubic close-packed blocks' Consequentlv, Mn(5)rv

coordinates to an apical anion above (below) but the apical anion

below (above) is not present.The only polyhedra which share edges are the Mn(2)-, Mn(3)- and

Mn(4)-oxygen octahedra. As shown in Table 5, shared edges tend to be

among the shortest for their polyhedra. The effects of electrostatic devia-

tions from neutrality about anions are also apparent: the undersaturated

anions show short Me-O distances on the average, whereas the over-

saturated anions have longer Me-O distances than average. These

accord with generally observed trends in oxysalt crystals add yet further

confidence to the contention that three distinct coordination polyhedra

for Mn2+ occur in arsenoclasite.

l J . ) I

u

d od t r @

. 8 . 3 8q t r a

do d

r f o

o g dP ) g_ P 2H d !o o d

H qo o gt s t

q a o

r o. d o i if r i l d do o d d. ! r a o o

o t p !p o d da t u ! !o d a 6

! o E !o ! !E - o o' r t o o o

A O T L i

d F o d

o o o

3 3 I l l 3 E R F R R $ l R: : : : t : . . . . . { .

d - - - t H N N N N N N I N

? Sa9? Saagag; ??? i Y* * { *Y, - > ^ t t ^ r ^

? 3003034 o o o o o o

N ^ ^ ^ -

; S g 9 9 - - - - - ^o < o o o o g g D L g e lF a r P l . ? q c e N ! s , q , l ' ' '

? I ? I ? ?* - : -1 l l_1 i : : i : : l : P sooooo< o o o o o o

. - - - - : a ^ ^ aY I . e F o o l r J I S c ,5 d d F ; - ; e ? ; ; ?

| | | ^ t r ' i

^- a N a a a N( , g 3 g g g c ,5 o o a o o o

qq q iq 'q lq k t t q? qs d * K h s teJ J J J " i J I " ; . , i J J J J ; ; ; ; ; ; J I ;ai - S . i a F - ac u i f i / V u t j a - ^ ^ N ^ ^ ^E - o o - - 5 g l c . $ . l v . e . $ . H d v i o oi r i . ' - i i . ? { yy {Fy : yyy

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1 552 MOORE AND MOLIN-CASE

AcKNowr-EDGEMENTS

This study was supported by a Dreyfus Foundation grant and the NSI' grant GA-

10932 awarded to the senior author.

Rllrpnxcrs

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valences in the crystal structure of synadelphite . Amer. Minerd.55,2023-2037 .

-- (1970b) Edge-sharing silicate tetrahedra in the crystal structure of leucophoenicite.

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Vol. 2. John Wiley and Sons, New York, 800-802.

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Manwscript receh.td, February 22, 1971; acceptd Jor publi.calion, April 14, 1971'