American Mineralogist, Volume 65, pages 4gg_49g, Igg0

The of althausite,Mg.(pOJ2(OH,O)(F,D)

CunIsrnN RoutvilNc Kjemisk Institutt, (Jniyersiteteti Oslo, Blindern, Oslo 3, Norway

AND GUNNEN RAEOS

Instituttfor Geologi, (lniversitetet i Oslo, Blindern, Oslo 3, Norway

Abstract

The crystal structure of althausitewas solved by direct methods and refined to a final R valueof 0'022for l164 high-anglereflections (sin d/I > 0.45A-r)with 1> 2.5o.Theposition of the hydrogenatom was found from a differenceFourier synthesis.The structureis ortho- : rhombic Pnma with a 8.258(2),b : 6.05aQ),c : 14.383(5)A.Magnesium atoms occur in both five- and six-fold coordination, and the coordination polyhedia are highly distorted. The Mg octahedraform chains along D by edge-sharing.Hydroxyl anA fluoine occur in a largely ordereddistribution among two diflerent structural siies 'channels'parallel and occupy alternatingposi- tions along to D.Partial vacancyin the (OH,F) sitesis dnfirmed, the pop- ulation factor for the F site being 8l percent.The crystal-chemicalformula of althausiteis thereforeMgo@Oo)r(OH,OXF,!) with Z: 4. The cleavagesin althausite, {00U perfect and {l0l} distinct, occur along planes crossing relatively few bonds and leave the chains of Mg octahedraunbroken. Bond-strengthcalcu- lations for althausiteand wagnerite are presentedand the OH content in wagner-iteis dis- cussed. Preliminary results of hydrothermal synthesesindicate the existenceof a seriesfrom F- wagneriteto OH,F-wagneriteand from OH-althausiteto OH,F-althausite.Infrared spectraof -free althausiteshow a splitting of the O-H stretchingfrequency, proving ih"t t*o distinct hydroxyl positionsare present.

Introduction agreement with the calculated value, (010) n (l l0) : 60.14". The optical orientation must be changed ac_ The new magnesium- mineral althausite cordingly to X: c, y : b, Z : a, OAp (010), a-nd the was described from serpentine-magnesite deposits in hardneis on {010} is 3vz in the c direction and 4 in Modum, south Norway, by Raade and rysseland the a direction. (1975). Based on a wet-chemical analysis,the empiri- The type locality for althausite is the Tin- cal formula was given as MgrPoo(oHo.3?Fo.25 gelstadtjern quarry, Modum, and the original de- Oo,nlo,r), with vacancy in the (OH,F) sites because scription was based on material from this occurrence. of a presumed substitution of the type 2(oH,F)- r= The mineral has also been found in a similar deposit o'z-tr' Althausite is orthorhombic : with a 8.258(2), farther to the north, the overntjern quarry. Here the b : 14.383(5),c : 6.054(2)A, : V:719.0(7)A3, Z 8. althausite has a reddish-brown color and occurs in a The was given as Pnar, but from the ex- somewhat different mineral association (talc, magne- tinctions, another possible space group is Pnam site, brown apatite, partly enstatite). Howevei, a which was omitted. Furthermore, the was more complete account of the mineral parageneses stated to be perfect along [001] and distinct along and the distribution of phosphate mineials *org 001) ' New investigations show that the correct the deposits is reserved fo, a separatepaper. cleavagesare {010} (perfect, pinacoidal) and {ll0} Amlcroprobeanalysisofthebverntjernalthausite (distinct, prismatic). A set of rough measurements of using Tingilstadtjern althausite as standard gave the the angle between the face normals of the perfect and Ottowingiesult (data calculated as direct ratios after the distinct cleavage planes gave 60-63o, in good background correction; values for the Tingelstadtjern w03-Mx/80/0506-o488$02.00 488 ROMMING AND RAADE: ALTHAUSITE

Table l. Weight loss vs. H2O + CO2 content for althausite and two lyzer (Table l). The internal consistency of the data apatrtes is excellent (HrO value for althausite from Raade

Weight loss Wt? of H2O anal CO2; and Tysseland, 1975).

from TGA calculated refative In the original description of althausite, the choice (reI. units) units in parentheses of axes was made according to the scheme c 1 a 1 b. description which follows, b and c Althausite 61 n:o 1.B7s (60.4) In the structural are interchanged in order to have the space-group 46 H2O+ CO2 l-.4'72 l4'7.s) symbol in the standard setting, Pnma. The two cleav- (OH,F)-apatite H2O+ CO2 1.2'72 (4r.0) ages thus become {001} and {l0l} in the trans- formed axial system.

ExPerimental material, after subtraction of apatite impurity and re- calculation to 100 percent, are given in parentheses): The cleavage fragment used for the structure de- MgO 50.5 (51.0), P,O, 44.4 (45.1), F 3.04 (2.99) termination came from the Tingelstadtjern quarry' weight percent. The content was determined Modum; it had approximate dimensions 0.2 x 0.2 x with the probe using a fayalite standard: total iron as 0.05 mm. The X-ray diffraction data were collected FeO is 0.40 and 0.80 weight percent in the Tin- on a Picker four-circle punched-card controlled dif- gelstadtjern and Overntjern althausites respectively. fractometer using graphite crystal monochromated The higher iron content of the Overntjern mineral MoKo radiation. A unique set of intensities was mea- should be noted. Its brown color is due to micro- sured, using tltre0/20 scan mode with a scan rate of scopic inclusions of iron oxides/hydroxides. 2omin-' (20). The scan range was from 0.9o below The Overntjern occurrence has provided a few 20(a,) to 0.9o above 20(ar); background counts were specimens with subhedral crystals of althausite im- taken for 30 sec at each of the scan limits. Out of the bedded in magnesite or talc. The crystals are elon- 163l reflections with sind/tr < 0.84-', 1456 were re- gated along c and flattened on D. Some natural sec- garded as observed, their intensity being larger than tions f c show the forms {010} and I l0]. One 2.5 times the estimated standard deviation. The vari- subhedral crystal has the approximate size a:b:c : ance of the intensity was taken as d(1) : G + (0.02 2: I :3 cm and shows the form {010} and four distinct C")'where G is the total number of counts and C" is terminal faces at one end of the crystal. Two of these the net count. Three standard reflections measured faces belong to the same form, identified as {131}. for every 100 reflections of the data set showed no The two other faces do not belong to the same form systematic variations. The intensity data were cor- and no simple indices could be attached to them. rected for Lorenz and polarization effects and for ab- : These must be regarded as vicinal faces, developed sorption fu(MoKa) 10.07 cm-']. because of imperfect growth of the crystal, which Structure determination may be ultimately due to defects in the crystal struc- tute, e.g. in this case partly empty (OH,F) sites (cl Intensity statistics gave convincing evidence of a Terpstra and Codd, 1961,p. 360-363). centric structure, indicating the space group Pnma The defect structure of althausite is clearly sub- (after reorientation according to International Tables stantiated by the present structure determination, but for X-ray Crystallography) rathet than Pna2, zs re- we would still like to mention other evidence which ported earlier (Raade and Tysseland' 1975). supports the correctness of the original chemical The positions of all non-hydrogen atoms were analysis. Firstly, the new mineral holtedahlite, found by direct methods using the program assembly (Mg,Na),(PO4,CO3OH)(OH,F), also from the Tin- MULTAN (Germain et al.,l97l) and refined by suc- gelstadtjern occurrence, was analyzed with the elec- cessive Fourier syntheses and least-squares calcu- tron microprobe using althausite as standard, with lations. So far the fluorine and hydroxyl HrO and CO, being determined separately with a atoms could not be differentiated because of the simi- Perkin-Elmer 240 elemental, atalyzer. In this way a larity in scattering power. A di-fference Fourier syn- perfectly stoichiometric formula was obtained for thesis revealed the position of a hydrogen atom close holtedahlite (Raade and Mladeck, 1979). Secondly, to one of these atoms, however, which could thus be the weight losses were obtained from TGA runs on identified as the oxygen atom. althausite and two associated apatites from Tin- The cornposition of althausite found from chem- gelstadtjern. The HrO and CO, contents of the apa- ical analysis indicates that the hydroxyl and fluorine tites were also determined with the elemental ana- positions are partially occupied and in part replaced 490 ROMMING AND RAADE: ALTHAUSITE

Table 2. Atomic coordinatesand thermal parametersfor althausite

u11 u33 UI3 u23

Ms(1) 0.13402(5) 0.00414(7) 0.168s8(3) 0,0067(2) 0.0049(2) o.oo85(2) 0.0009(r) 0.0004(r) 0.0009(1) wl2l 0.34763(71 0.25 0 .42143| 4l 0.00s4(2) 0.0066(3) 0.0I11(3) 0.0 0.002r(2) 0.0 Ms(3) 0.0771217t. 0.25 0. 5750s( 4) 0.0068(2) 0.0063(2) 0.0075(2) 0.0 -0.0007(2) 0.0 P{r) 0.48845(s) 0.25 0. 14583(3) 0.0037(2) 0.0041(2) 0.0048(2) 0.0 -0. 0001( I) 0.0 P(21 0.22456(51 0.25 0.84820(3) 0.0043(2) 0.004s(2) 0.005s(2) 0.0 0.0004(1) 0.0 0 (1r) 0.38234(10) 0.04123(t5) 0.14137(6) 0.006r(3) 0.0069(3) 0.0r12(3) -0.0013(3) 0.0006(2) -0.0006(3) 0(12) 0.60739(Is) 0.25 o.06273(91 0.0064(4) 0.0u2 (s) 0.0072(41 0.0 0.0012(3) 0.0 0(13) 0.59r47(16) 0.25 0.23ss4(8) 0 .0102(s) 0.0080(s) 0.0065(4) 0.0 -0.0029(3) 0.0015(3) 0 (2r) 0. r1s83(r0) 0. 04268(15) 0.85335(6) 0.0068(3) 0.0071(3) 0.0120(3) -0.0015(3) -0.0004(2) 0.0 0(221 0.32077(16) 0.25 0.7ss88(8) 0.0108(s) 0.0076(s) 0.0075(4) 0.0 0.c03s (4) 0.0 (23) 0 0.34092(rs) 0.25 0. 93021(9) 0.0068(4) 0.0187(6) 0.0085(4) 0.0 -0.0028(4) 0.0 F 0.33s00(15) 0.25 0.56627(8\ 0.0064(7) 0.0055(7) 0.0057(7) 0.0 -0.0015(3) 0.0 0 0.09124(1s) 0.25 0.0758s(8) 0.0050(4) 0.0082(5) 0.0071(4) 0.0 0.0008(3) 0.0 H 0. I54 (7) 0.25 o.035 ( 4) B=1.6(7)

y, The values of x, z a-re given jn fractional coordinates, the anisotJopic tenperature factor is of the form q(2r2 (url a*2h2ru22b*2k2*v33 c*212r2vr2.*b*hkt2ur3 b*"*kl)1. ustir-tea shndard deviations in pa-rentfpses refer "*.*hr*2u23 to the last diqits.

by oxygen ions. The further refinement by full-ma- A rigid-body analysisshowed that the thermal mo- trix least-squaresmethods included positional pa- tion of the phosphateions to a good approximation rameters,anisotropic thermal parameters(for all could be interpreted in terms of translational and li- atoms except for the hydrogen atom which was re- brational oscillation. The bond lengths used in the fined isotropically), and occupancy factors for hy- discussionof theseions are those correctedfor libra- droxyl and fluorine. Atomic scatteringfactors used tion. were thoseof Doyle and Turner (1968)for P, Mg and A differenceFourier map had its largest electron O, of International Tablesfor X-ray Crystallography densityat (0.442,0.250,0.560), 0.94A from the fluor- (1962)for F-, and ofStewart et al. (1965)for hydro- ine site. The integratedpeak amountedto about l/4 gen. In order to reduce systematic errors due to of an electron, and, although indicative of the re- faulty assessmentof atomic charge in the atomic placementof someOH- for the F- ion, it was not in- form factors, only diffraction data with sindltr > cluded as a partially occupiedhydrogen atomic posi- 0.454-' were applied for the last stageof refinement, tion in the refinement,even if the presenceof such an reducingthe number of applied observationsto 1164. atom is probably significant. No significantshift occurredin the hydroxyl occu- The final atomic positional and thermal parame- pancy factor; it was consequentlyfixed at its normal ters are listed in Table 2. Interatomic distancesand value for a fully occupiedsite. The fluorine site pop- bond anglesare reported in Table 3; estimatedstan- ulation factor convergedto 8l(l) percentofthe cor- dard deviationsare calculatedfrom the variance-co- respondingvalue. variance matrix and include uncertaintiesin cell di- The refinements converged to a conventional R mensions.Observed and calculatedstructure factors factor of 0.022 for the reflectionsemployed in the re- with estimatedstandard deviations are on deposit at finements(R : 0.025for all observedreflections), R. the BusinessOffice of the Mineralogical Society of : 0.031,and S : l2wAF/(m - n)],r, : 1.76.The America.' overdeterminationratio was 16.2. The quantity minimized in the least-squarescalcu- lations was XwAP, where w is the inverseof the vari- I To ance of the observedstructure factor. A description obtain a copy of this table, order Document AM-g0-133 from the Mineralogical Society of America, of the computer programs BusinessOffice, 2000 applied is given by Groth Florida Avenue, N.W., Washington, D. C. 20009.please remit (1e73). $1.00in advancefor the microfiche. ROMMING AND RAADE: ALTHAUSITE

Table 3. Interatomic distances(A) and angles(') for althausite(with estimatedstandard deviations in parentheses) r\41(r) -0 (rL) 2.100(1) 0 (11)-l'ls(1)-0 88.22(41 0 -Mg(1)-0(13) 83.08 ( 4) Ms(1)-o 2.031(r) 0(11)-Mg(1)-F 80.36(4) F -Ms(I)-o(21) 85.26(41 F19(1) -F 2.r4s(t) 0 (u) -Mg(r)-0 (21) 160.s8 (4) F -Iaq(I)-0(22) 81.80(4) Mg(I)-0(21) 2.107(1) 0 (11)-l'E (I) -0 (22) 90.97(5) F -Ms(I)-0(13) 176.8e ( s) -iql(r)-0 ( l,ls(1) -0 (22 ) 2-O22(rl 0 (u) -Mg(I)-0 (13) r02.35 ( 5) 0 (2I) (22) 99.87 s) r"q(r) -0 (13) 2.060(r) o -Ms(l)-r 9s.5s ( 4) 0 (21)-I'!l(1)-0(13) er.7s(s) 0 -w(1)-0(2r) 80.2r(4) 0 (22)-Mg(1) -0 (13) 99.s9(s) 0 -Mg(1) -0 (22) r'77.33 (4\

lag(2)-F 2.086121 F -Mg(2)-0(12) 80.s8 (s) 0(12)-Mg(2)-0 L72.33(61 Mg(2)-0(12) 1.9e7(1) F -Ms(2)-0 (21) 119.00(3) 0 (21)-I'19(2) -0 (21) 1r9.97(6) Mg(2) -0 (21) 2.048(r) F -Mg(2)-0 eI.75(s) 0 (21)-Mg(2) -0 82.07(3) Mg(2) -0 2.0L2(I't 0 (12)-trlg (2) -0 (2I) r01.59(3)

Ms(3)-F 2.133(1) F -Ms(3)-0(II) 82.rs(3) 0 (1r)-rE(3)-0 (23) r00.s3(3) Mg(3)-0(u) 2.034(1) F -r\E(3)-0(23) r74.38(6) 0 (11)-Ms(3) -0 (12) 116.41 ( 3) '79.42(51 -1,1s(3)-0 trlg(3 ) -o (23) 1.9s2(r) F -Mg(3) -0 (12) 0 (23) (12) 94.9'7(31 Mg(3) -0 (12) r.998(21 0 (11)-Mg(3)-0(1I) r20.38(6)

0 -ng(t) 2.03r (1) IE(1)-0 -Ms(1) 94.39(6) 0 -H -o (23) 176(s) 0-H 0.78 (6) Mg(I)-0 -H rL2(21 o -o (231-P(21 96.29(11 0 -l,tS(Z) 2.0I2(L',) Ms(I)-0 -Ms(2) 99.28(5) H -0 (23)-P(2) es(2) 0 -0 (23) 2.939(21 H -0 -Ms(2) 133(4)

F -rrs (1) 2.14s ( 1) MS(1)-F -LIE(1) 91.74(s) r4s(3) -F -Ms ( 2 ) 96.26(5\ r -Mg(3) 2.133(1) Ms(I)-r -Ms(3) 94.s0(4) F -Ms(2) 2.086(21 Ms(1)-F -P1q(21 I32.'78(31

P(1) -0(11) 1.s40 ( r) r.543 0(r1)-P(1)-0(12) I09.22(41 0(rr)-P(1)-0(11) I10.43(7) P(1) -0 (12) r. s47(1) 1.550 0(11)*P(r)-0(13) 1r0.36(4) P(r) -o (13) 1.s45 (1) 1. 549 0(12)-P(1)-0(13) 107.19(7)

P(2) -0(21) 1.s46 ( 1) I q4q o(2D-P(21 -o(22) 10e.84 ( 4) o(21)-P(2)-o(2r) 108.70(7) P(21 -0(22) r. s47(1) r. 551 o(21)-P(2)-o(23) 109.25 ( 4) P(2) -0 (23) r.522(rl 01221-P(21-0(23\ 109.94(8)

I4g(I) -0 (IU -Mg(3) 98.94 (4) I'E(1)-0 (11)-P (1) 129.48(5) Iq (3)-0 (11)-P (r) 12e.68(s) !q(2)-0 (12)-l4s(3) 103.74 (6) l4s(2) -0 (12)-P (U L22.86(81 M9(3)-0(12)-P(I) 133.39(8) Mg(1)-0(13)-wtg(t) 92.62(61 l4s(1) -0 (13)-P (1) 130.7s ( 4) Ms(r)-0 (21)-Ms (2) 94.'70(41 Ms(1)-0(2Il-P(2) 132.16(6) Mg(2)-0 (21)-P (2) r29.87(61 Mg(1)-0 (22)-rtg (U 9e.22(61 r'E(1) -0 (22)-P (2) 128.89(3) M9(3) -0 (23)-H 137( 1) r'E(3) -0 (23)-P(2) 126.94(81 H -0 (23)-P(2) es(r)

Descriptionof the structure There are three non-equivalentMg2* ions of which one is situated in a general eight-fold position A stereoscopicview ofthe structureis presentedin lMg(l)l and two are locatedon mirror planes tMg(2) Figure l; the numbering of the atoms may be seen and Mg(3)1. Two non-equivalent phosphate ions from Figures2,3, and 4. have the phosphorusand two of the oxygenatoms on ROMMING AND RAADE: ALTHAUSITE

Fig. l. Stereoscopic pair of the althausite structure. One unit cell is shown (b < a < c); some additional atoms outside the unit cell have been included in order to complete the Mg and P polyhedra. Phosphate groups and OH groups are indicated by fitled bonds. Fig. 2 (together with Figs. 3 and 4) can be used as a legend for identifying the different atomic positions.

mirror planes and two equivalent oxygen atoms in one Mg(2), and one Mg(3) ion, the fluorine site being general positions. The hydroxyl and fluorine ions are closeto the center of the Mg(l)-Mg(l)'-Mg(2) tri- situated on mirror planes. angle (Fig. 4c). The possiblehydrogen position for a Mg(l) (in general position) is coordinated in a dis- hydroxyl-substitutedfluorine site(cf. Structuredeter- torted octahedron to one fluorine, one hydroxyl, and mination) is completing a distorted trigonal bipy- four phosphate oxygen atoms. The coordinations of ramidal arrangementwith the Mg(3)-H direction as the two other magnesium ions are distorted trigonal the trigonal axis. bipyramidal, Mg(2) with ligands F-, OH- and three The hydroxyl ion, in addition to the hydrogen phosphate oxygen atoms and Mg(3) with ligands F- bond to a phosphateoxygen atom O(23), is coordi- and four phosphate oxygen atoms. The arrangemenrs nated to two Mg(l) ions and one Mg(2) ion in a tet- are visualized in Figure 3, which also shows the way rahedralarrangement. in which the polyhedra are connected. Each Mg(l) An important feature of the structure is that the octahedron shares edges with two other Mg(l) octa- OH- and F- anions occupy alternating positions hedra, one Mg(2) and one Me(3) trigonal bipyramid. along the b axis, such that a kind of 'double channel' The Mg(l) octahedra form straight chains parallel to is formed by two adjacentrows of OH-/F- (Fig. 2). the b axis (cf. Fig.2). The Mg(2) polyhedron shares Along b, the distancesF-O and F-H are 3.093 and edges with two Mg(l) octahedra and one Mg(3) trig- 3.06A respectively,whereas across the channels the onal bipyramid; the Mg(3) polyhedron shares edges correspondingdistances (between F and OH situated with two Me(l) octahedra and one Me(2) trigonal bi- at the same height above the plane ac) are a little pyramid. shorter,2.942 and 3.02A respectively. The arrangements around the anions are visual- ized in Figure 4. The two phosphate groups form Discussion nearly ideal tetrahedra with O-P-O bond angles in the range 107.2-110.4o. Sevenof the eight phosphate Relationto otherstructures oxygen atoms are coordinated to two magnesium Althausite belongs to a group of minerals with ions, the last oxygen atom O(23) being engaged in a general formula llz(){On)Z, where M is a divalent hydrogen bond to a hydroxyl ion and an ionic bond metal atom (Mg, Fe, Mn, Cu, Zn),X is P or As, and to a Mg(3) ion. These arrangements are all close to Z is mainly OH andlor F (cl Richmond, 1940).Four planar, as can be seen from the angle sums around diferent typesof crystal structuresare known among the , which vary between 354.1 and 360.0o. these minerals. Tarbuttite, ZnrPOoOH, is triclinic, The fluorine ion is coordinated to two Mg(l) ions, spacegroup Pl, Z: 2 (Cocco et al., 1966\and is ROMMING AND MADE: ALTHAUSITE

mite, ZoAsO4OH (Hawthorne, 19'16;Hill, 1976), and , MnrAsOoOH (Moore and Smyth, 1968)' which all have the spacegtoup Pnnm with Z: 4. To this seriesalso belongsolivenite, Cu'AsOoOH, space : group nt/n [with a as the unique axis and a 90.0(l)"1 and Z : 4 (Toman, 1977),whose mono- clinic structure deviates only slightly from the ap- proximate orthorhombic structure determined pre- viously (Heritsch, 1938). Interestingly, the silicate mineral andalusite,AlrSiOoO, Pnnm, Z : 4 (Burn- ham and Buerger, 196l) is isostructuralwith the last seriesof arsenatesand ,as was pointed out by Strunz(1936). The synthetic compoundsCdrPOoF and CurPOoF with space$oup C2/c (equivalent to I2/a) are iso- structural with triplite (Rea and Kostiner, 1974; 1976). CorAsOnOH is isostructural with (Riffel et al., 1975),whereas Sn,PO,OH with space grotp F2,/n and Z: 4 has a distinct structureunre- lated to the others(Jordan et al.,1976). Severalanal- ogous compounds with chlorine instead of F/OH display still other structural types, as could be ex- pectedfrom the larger sizeof the Cl- ion. Another re- lated group of minerals has the general formula AB(XO")Z, where A and B may be two divalent metal atoms of different size (Be, Mn, Mg, Fe, Zn, Cu, Ba, Pb) or one monovalent and one trivalent metal atom (Li, Na; Al, Fe). Included in this group Fig. 2. The crystal structure of althausite viewed down the D axis are also somevanadates. A number of distinct struc- (origin for the labelled axes is at the upper left corner of the ture types are known among thesecompounds. bottom plane). The P-O and O-H bonds are filled and the F The crystal structureof althausiteis unique among atoms are shown as filled ellipsoids. The Mg(l) octahedra form the structuresof these minerals and synthetic com- parallel to the D axis by edge-sharing, and the individual chains MgrPOoF,which is chemically octahedra are only sligbtly separated in this projection. Note the pounds. Wagnerite, two adjacent rows of alternating OH and F along the 6 axis, an closeto althausite,also has Mg in both 5- and 6-coor- 'double arrangement that results in a sort of channel' in the dination, but has a much more complicated struc- structure. This figure may serve as a legend to the stereoscopic ture, and its eight structurally distinct Mg polyhedra views of Fig. l. are connected differently. Another mineral that is chemically close to althausite is holtedahlite, isostructuralwith paradamite,ZnrAsOoOH. Triplite, hexagonaland must belong to yet another structure (Mn,Fe)rPOoF,and the isostructural iron analogue type. A structure determination of this mineral is in zwieselite are monoclinic, space Eroup I2/a, Z : 8 progress.The iron analogueof holtedahlite is satter- (Waldrop, 1969).The structureof thesetwo minerals lyite, recently describedby Mandarino et al. (1978). is closely related to that of wagnerite, MgrPOnF Althausite has some formal structural features in (Coda et al., 1967), /wolfeite, common with the mineralslibethenite--ada- (Mn,Fe),POoOH (Waldrop, 1970), and , mite-eveite-andalusite,in that they contain similar Mn AsOoOH (Dal Negro et al., 1974),which form a cation polyhedra with 5- and 6-coordinationand the seriesof monoclinic isostructuralminerals with space samekind of edge-sharingoctahedral chains. The al- ErolurpP2t/a anldZ : 16,i.e. with a doubling of the b thausite structure, with a cell volume about twice axis in comparisonwith triplite,/zwieselite.Another that of the others,has two different Mg" and OH/F structuretype is displayedby the orthorhombic min- sites,so that other aspectsof the structuresshow no erals libethenite, CuTPO.OH (Walitzi, 1963), ada- closerelationship. RAMMING AND MADE: ALTHAUSITE

t*$ &"n'

c Fig. 3. The three different Mg coordination polyhedra in althausite with their nearcst surroundings; oRTEp drawings, showing ellipsoids of thermal vibration. (a) The [Mg(I)O4(OH)F] distorted octahedra form chains parallel to D by edge-sharing, and are in turn connected with Mgv(2) and Mgv(3) polyhedra, also through edge-sharing. (b) The [Mg(2)Or(OH)F] distorted trigonal bipyramid shares edges with two Mgvr(l) polyhedra and one Mgv(3) polyhedron. (c) The [Mg(3)oaF] disto(ed trigonal bipyramid sharesidges with two Mgut(l) polyhedra and one Mgv(2) polyhedron.

Cleavage ken (based on the setting c < a < b, spacegroup Pnnm). However, for the isostructural minerals li- The perfect pinacoidal cleavage along {001} and bethenite,olivenite, adamite,and eveite,quite differ- the distinct prismatic cleavage along {l0l} in al- ent cleavagedirections are quoted in the literature, thausite are plausibly explained on the basis of struc- namely indistinct {100} and {010} for libethenite,in- tural information. These cleavages occur along distinct {011} and {ll0} for olivenite,good {lOU planes which cross relatively few bonds and are and poor {010} for adamite, and fair {l0l} for formed by breaking bonds between phosphate eveite, all cleavagesapparently given for the same groups and Mg polyhedra (Figs. I andz). The chains setting c < a < b (cf.e.g. Robertset al., 1974\.lt of [MgO"(OH)F] octahedra running parallel to the 6 should be expectedthat the cleavagesof a seriesof axis, and which apparently constitute a strong entity isostructural minerals were comparable, of course in the structure, are left intact. not in quality, but at leastin direction. Cleavagesre- Note that the two cleavages in andalusite, distinct ported for olivenite, adamite,and eveite,involving a along {ll0} and indistinct along {100}, are com- breaking of the octahedral chains running parallel to parable to the cleavages in althausite, and leave the the c axes,must therefore be regarded as question- AlOu chains which run parallel to the c axis unbro- able.

B"' il oMg3 -:...K. ]f ib lr ret i(" $"' a b c

Fig' 4' ORTEPdrawings, showing ellipsoids of thermal vibration. (a and b) The two non-equivalent POo tetrahedra in althausite and the connections between their oxygen atoms and the neighboring Mg atoms. (c) The bonding arrangement of the F atom. In synthetic althausite this site may also be occupied by hydroxyl, with the G-H pointing in the opposite direction to the G-Mg(3) bond. (d) The bonding arrangements around the OH site. ROMMING AND RAADE: ALTHAUSITE 495

Table 4. Bond-strengthcalculation for althausite(valence units)

Mg(l) Ms(2) Mg(3) P(t) P(2)

.l.958 0(lr) 0.330 0.379 x2+ 1.249 x 2+ x 2+ 0.409 0(12) 0.410 1.225 2.1?8 0.042x2+ x2+ 0.063 0.359 x 2+ 1.232 2.058 0(r3) 0.045

0.326 o(21) 0.043 0.368 x 2+ 1.229 x 2+ 2.003 x 2+ 0.037 0.052 (22) 0.388 x 2+ 1.225 2.101 o 0.048 x2+ 0(23) 0.041 0.452 I .314 0.10 1.948 x2+

0 0.38] x 2+ 0.397 0.90 2.059 F 0.236 x 2+ 0.266 0.242 0.980

'l.999 ,l9.'l96 2.100 x 2+ 2.045 4.955 4.997 1.00

Computedfrom the empirical bond-strength bond-length relations of Brown and }ju (.l976), Brown (1976) and Brown (1977),

Bondlengthsand bondangles . Bond- s trength calculations The mean P-O bond length (correctedfor libra- A bond-strengthtable for althausiteis presentedin 'ideal' tion) is 1.548A with a standard deviation from the Table 4, and is based on an structure with mean of 0.003A; the averageis taken over sevenof fully occupied OH- and F- sites. The P-O bond the P-O bonds.The P(2)-O(23) bond is significantly lengths utilized in the calculation are those uncor- shorter, 1.5264, indicating a smaller negative net rected for libration, sincethe empirical curves relat- chargeon this oxygenatom. The reasonis clearly in- ing bond strengthsand bond lengths are based on dicated by the coordination ofthe phosphateoxygen distances uncorrected for thermal motion (Brown atoms.O(23) is coordinatedto one Mg(3) ion and is and Shannon, 1973).As can be seenfrom the table, also engagedas acceptor in a hydrogen bond (Fig. the bond-strength sums for althausite are satisfac- 4b), whereasall the other phosphateoxygen atoms tory, with a maximum deviation of 6.4 percent for are each coordinatedto two Mg ions. O(12). However,to achievethis it was necessaryalso The O-H bond length (0.784) is shorter than the to considerthe contributionsfrom the second-closest acceptedO-H bond, which is 0.95A or longer if the neighbors at distances(Me-O) from 3.09 to 3.494, hydrogen is engagedin hydrogen bonding. This is although the calculated bond strengthsmay not be becausethe X-ray method measuresthe position of very accuratefor such long bonds. The large degree electron density maxima rather than the position of of distortion of the coordination polyhedra in al- the atomic nuclei. thausite makes the necessityof such an approach The distortion of the cation polyhedraconforms to quite reasonable. the generally acceptedrule for ionic structures,that We have also calculatedthe bond sumsfor wagne- sharededges tend to be shorter than unsharedones. rite (Table 5). The sums for Mg are systematically This may be verified by inspection of bond angles too low, and also in this caseit might be necessaryto with magnesiumions as the central atom as given in take into account the contributions from oxygen Table 3. atoms farther away. Similarly, highly distorted Mg RAMMING ,4ND RAADE: ALTHAUSITE

polyhedra occur in the wagnerite structure as in the Table 5. Calculated bond-strength sums (p) for wagnerite althausite structure.Note the significantly low sums around two of the F- ions, F(2) and F(3). Wagnerites Aton p (v.u.) A Aton p (v.u.) A may contain substantialamounts of OH- (seebelow), Its'(t) 1.94 3.0 0(l) 2.00 0.0 and possibly this hydroxyl is preferably substituted Ms"(3) t.9t 4.5 0(2) 1.93 3.5 in two of the four F- sites,so as to produce a partly 'I Me'(5 ) l 81 95 0(3) 99 0.5 ordered structure with regard to OH-/F-. However, 'l.93 r4s'(7) 1.82 9.0 0(4) 3.5 further discussionof the bond-strengthsums for wag- Msut(2) 1.97 1.5 0(s) I .96 2.0 nerite is better postponeduntil a refined structurehas tutsut(4) l 95 25 been determinedfor a wagneritewith known F:OH 0(6) 2.00 0 0 ugut(o) 1.86 ratio. We hope to undertakesuch an investigationat 7.0 0(7) 1.99 0.5 a later date. llsut(s) t.B5 7.5 0(8) 1.96 2.C P(l) 5 00 0.0 0(e) 1.97 ] 5

P(2) 5.01 4.2 0(10) 1.97 l 5

Distributionof OH- and F P(3) 5.08 1.6 0(lr) 1.99 0.5 The OH- and F- anions have approximately the P(4) 4 .97 0.6 o(r2) 2.02 I .0 same size and are frequently found to replace each F(t) 0.95 5.0 0(r3) 2.06 3.0 other completely.A well-known exampleamong the F(2) 0.86 14.0 0(r4) L86 7.0 phosphatesis the pair fluorapatite-hydroxyapatite. F(3) 0.86 r4.0 o(r5) 2.01 0.5 However,in someinstances the OH and F analogues F(4) 0.95 5.0 0(r6) I.94 3 0 may have different spacegroups and cell dimensions and show a limited degreeof solid solution, although Conputed from the enpirical bond-strength - bond-length the structures are commonly closely related. Ex- relations of Brown and |/Ju(1976) and Brown (1977), Eased amples, again taken from the phosphate group of on the structure determination of Coda et al . (1967). minerals, are the pairs zwieselite/triplite, Percertage difference between valence and p. (Fe,Mn)rPOoF, and wolfeite/triploidite, (Fe,Mn)rPOnOH,mentioned above. The main differ- encebetween these structures is that F- occurscom- ing has been found in the apatite structure by means pletely disorderedbetween two possiblesites in trip- of neutron diffraction (Nozik, 1978). lite, whereasOH- is ordered in triploidite, probably The availability of F, the temperature and pressure owing to the dipolar characterof the OH- (Waldrop, of formation, and also the oxygen fugacity of the en- 1970).The site splitting in natural triplite may be vironment are factors that control the degree of re- caused by the presenceof large amounts of the placement of F- and OH- in minerals. As discussed smaller Mg2* ion, since the pure compound below, a completely F-free althausite has been syn- MnrPOoFdoes not show this disorder (Rea and Kos- thesized, and this shows that F is not an essential tiner, 1972). constituent of the althausite structure. Apart from the similarity in sizebetween OH- and F-, these anions have otherwisevery di,fferentprop- The defect structure of althausite erties with regard to polarizability and electroneg- Chemical evidence presented in the Introduction ativity. Replacementand ordering of OH- and F- and the results of the structure determination have in crystal structuresare thus generally controlled by confirmed the original assumption made by Raade the coordination and kind of ligands of the structural and Tysseland (1975) that althausite has a defect sites.On this basis,the high degreeof ordering be- structure with partly empty (OH,F) sites. The struc- tween OH- and F- is readily explainedin the caseof tural data show that the OH- occurs mainly in one of althausite,where the two available sites have quite two distinct sites which has a full occupancy, and the different environmentsand one of them is preferred F- ions and empty positions are confined to the other by OH-, the other by F- (Fig. 4, c and d). In wagne- site [population factor 8l(l) percent]. The position of rite also, some ordering of OH- and F- is indicated the O'?- ions has not been proven, but considering the (seepreceding subchapter). These anions may be, to empirical coefficients obtained from the cherrical a large extent, ordered in many other structures,but analysis, (OHorrForroo,nEo,n),it is reasonable to as- in most casesthis is difficult to detect by X-ray dif- sume that both OH- and O'- occupy the position fraction methods.However, a OH-/F- anion order- that was found to be fully populated. The structural- ROMMING AND RAADE: ALTHAUSITE chemical formula for althausite should thus be ex- Further experimentsto elucidate the phase rela- pressed as Mg(POo),(OH,OXF,tr), with possibly tionships in the system MgrPOoF-MgrPOoOHare some OH in the F site. being undertaken. The preliminary results indicate 'double The occurrence of OH- and F- in chan- that there exist two series:from F-wagneriteto OH/ nels' along b may be a clue to the existence and for- F-wagnerite, and from OH-althausite to OH/F-al- mation of a defect althausite structure, in that these thausite, but the limits of the substitutionshave not channels may allow water to escape from the mineral yet been determined.Holtedahlite has only been ob- (perhaps caused by a strong thermal event which tained as a F-free phase. may have affected these polymetamorphic serpen- The partially orderedsite distributionsof OH- and tine-magnesite deposits), and further that the chan- F- in natural althausiteand possibly also in wagne- nels may provide a possibility for adjustments of rite were discussedabove. In synthetic OH-althaus- atomic positions to compensate for the electrostatic ite, the OH ions must occur in two different struc- imbalances brought about by the O'- ions and empty tural positions. This is clearly corroborated by the positions. An analogy of great importance is pro- infrared spectroscopicstudy. vided by the apatite structure, which also contains Infrared spectroscopy 'channels' (along c) filled with OH-/F-/CI- and which has also been described with partially as well The O-H stretchingfrequency of natural althaus- as completely empty halide sites (e.9. McConnell, ite occursat 3510cm-' (Raadeand Tysseland,1975). 1973 for a review). Using the Lippincott-Schroedercurve for the theo- retical relation betweenO-H frequencyshift and hy- Synthetic work drogen bond length (Lippincott and Schroeder, Some preliminary results of hydrothermal syn- 1955),this givesan O-H "' O distanceof ca.2.944 thesesperformed by G. Raade at Mineralogisches In- in althausite, in good agreementwith the value stitut der Universitiit Karlsruhe are presented here, 2.939(2) A obtainedfrom the structuredetermination. since they have some bearing on the discussion of the Synthetic OH-althausite formed at 4 kbar and althausite structure. 600"C showsa distinct splitting of the O-H stretch- At present, results from runs made at the following ing band, with sharppeaks at 3510and 3530 cm-'. pressures and temperatures are available: I kbar The latter frequency correspondsto a slightly larger (650'C), 2kbar (550, 650'C), 4 kbar (400, 500, 550, O-H ... O distanceof ca. 2.97A accordingto the Lip- 600, 650'C), and'7 kbar (600'C). From mixtures of pincott-Schroederrelation. This must imply an ad- MgHPO.'3H,O, MgF,, and Mg(OH), with the ratio justment of distancescompared to the structure of Mg:P:F :2:l:1, only wagneritehas beenobtained natural althausite,where F-O(21) is 2.87A and F- as a pure phase with just a trace of OH as shown by O(H) is 2.94L.If OH weresubstituted for F- in the infrared spectroscopy. From a similar mixture with althausite structure with the oxygen occupying ex- OH:F : l: l, a pure wagnerite phase is obtained in actly the sameposition as fluorine, a bifurcated hy- some cases, and mixtures of wagnerite and varying drogen bonding to the two nearest oxygens O(21) amounts of althausite in others. The formation of al- and O(H) would be the most likely result (Fig. 3c). thausite is clearly favored by higher temperatures/ This meansthat the O atom is both a donor and an lower pressures compared to wagnerite, which is con- acceptor of a hydrogen bond. However, an adjust- sistent with the denser packing of the wagnerite ment of distancescould result in one of the bifur- structure (Table 3 in Raade and Mladeck, 1979). catedbonds being preferred.As an ultimate corollary This wagnerite contains considerable amounts of OH of these changes,it is even possiblethat the space and has a somewhat larger cell volume than the group could be different for the OH-althausite,e.g.7f nearly pure F-wagnerite. Also the althausite is a OH/ the symmetry center is lost. F-bearing variety. Natural wagnerite is often assumedto represent More interesting in the present context are the runs the pure F end-member,and HrO is rarely reported made with F-free reactants, l.e. with Mg:P:OH : in the analyses.However, wagnerite from Bamble 2:l:l.In thesecases mixtures of varying amounts of showsimportant amountsof OH, accordingto the in- OH-althausite and holtedahlite are produced, to- frared spectrum (cl Raade and Tysseland, 1975), gether with several unknown phases. This althausite and wagnerite from Sweden has a ratio F: OH : has a somewhat larger cell volume than the OH/F l:0.81 (Henriques,1956). The substitutionof OH- variety. for F- has a substantialinfluence on the refractivein- 498 ROMMING AND RAADE: ALTHAUSITE dices,which are increased,and this is why the points Heritsch, H. (1938) Die Struktur des Olivenites Cu2(OH)(AsO)o. for the Swedishwagnerite fall offthe lines on the dia- Z. Kristallogr., 99, 466479. gram presentedby Propach(1976) to relate valuesof Hill, R. f . 0976) The crystal structur€ and infrared properties of adamite.Am. Mineral., 6l, 979-986. refractive indices and Fe,Mn contents for wagnerites. Jordan, T. H., L. W. Schroeder,B. Dickens and W. E. Brown The O-H stretchingfrequency for natural as well as (1976)Crystal structur€ofstannous hydroxide phosphate,a re- synthetic wagneritesoccurs at 3570-3580cm-', in- action product of stannous fluoride and apatite. Inorg. Chem., dicating an O-H ..' O bond distanceof ca. 3.03A, 15,l8l0-1814. Lippincott, E. which is (Raade R. and R. Schroeder(1955) One-dimensional model similar to that of holtedahlite and ofthe hydrogenbond. J. Chem.Phys.,23,1099-1106. Mladeck, 1979). Mandarino, J. 4., B. D. Sturman and M. L Corlett (1978) Satter- lyite, a new hydroxyl-bearing ferrous phosphate frorn the Big Acknowledgments Fish River area,Yukon Territory. Can. Mineral., 16,4ll4l3. The synthetic work from which some preliminary results are McConnell, D. (1973)Apatite. Springer-Verlag,New York. presentedhere was made possible through a fellowship to G. Moore, P. B. and J. R. Smyth (1968)Crystal chemistryof the basic Raade from the Alexander von Humboldt-Stiftung. The infrared arsenates:III. The crystal structure of eveite, spectrawere recordedat Institut ftir Kristallographie,Universitiit Mn2(OH)(AsOa\.Am. Minerat, JJ, 184l-1845. Karlsruhe.We thank W. Smykatz-Klossof MineralogischesInsti- Nozik, Yu. Z. (1978) Neutron diffraction mcthods for investiga- tut, Universitet Karlsruhe for the TGA runs and C. J. Elliott at tion in the structure of minerals (abstr.). Phys. Chem. Minerals, Departmentof Mineralogy, British Museum (Natural History) for J, 83-84. the H2O and CO2 determinations.W. L. Grimn at Mineralogisk- Propach, G. (1976) Brechungsindizesund Dichten zum Bestim- Geologisk Museum, University of Oslo gave assistancewith the men von Wagneriten (Mg,Fe)2POaF. Neues Jahrb. Mineral. microprobe analyses.A crystal of althausitewas put at our dis- Monatsh.,159-161. posal for measuremcntby R. Hansen.We thank ProfessorsE. Al- Raade,G. and M. H. Mladeck (1979)Holtedahlite, a new magne- thaus and H. Wondratschekfor their comm€nts on the rnanu- sium phosphatefrom Modum, Norway. Lithos, 12,283-287. script. - and M. Tysseland(1975) Althausite, a new mineral from Modum, Norway. Lithos, 8,215-219. Rea, J. R. and E. Kostiner (1972)The crystal structureof manga- References nesefluorophosphate, Mn2(PO )F . Acta Crystallogr.,828,2525- 2529. Brown, I. D. (1976)On the geometryof O-H...O hydrogenbonds. - and - (1974)Cadmium fluorophosphate,Cdr(POa)F. Acta Crystallogr.,A 32, 2+31. Acta Crystallogr.,830, 2901-2903. - (1977)Predicting bond lengthsin inorganic crystals.Acta - and - (1976)The crystal structureof copper fluoro- Crystallogr.,r-l-3, 1305-1310. phosphate,Cur(POa)F. Acta Crystallogr.,832, 1944-1947. - and R. D. Shannon(1973) Empirical bond-strength-bond- Richmond, W. E. (194())Crystal chemistryof the phosphates,ar- length curvesfor oxides.Acta Crystallogr.,A29,266-282. senatesand vanadatesof the type A2XO4(Z).Am. Mineral., 25, - and K. K. Wu (1976)Empirical parametersfor calculating 4414?9. cation-oxygen bond valences.Acta Crystallogr., 832, 1957- Ritrel, H., F. Zettler and H. Hess(1975) Die Krisrallstruktur von 1959. Co2(OH)AsOr.N euesJahrb. M ineral. M onatsh.,5 l+5 17. Burnham, C. W. and M. J. Buerger(1961) Refinement of the crys- Roberts,W. L., G. R. Rapp, Jr. and J. Weber (1974)Encyclopedia tal structure of andalusite. Z. Kristallogr., I I S, 269-290. of Minerals. Van Nostrand Reinhold, New York. Cocco,G., L. Fanfani and P. F. Zanazzi (1966)The crystal struc- Stewart,R. F., E. R. Davidson and W. T. Simpson(1965) Coher- ture of tarbuttire, Zn2(OH)POo.Z. Kristallogr.,123,321-329. ent X-ray scattering for the hydrogen atom in the hydrogen Coda, A., G. Giuseppettiand C. Tadini (1967)The crystal struc- molecule.J. Chem.Phys., 42, 3175-3187. ture of wagnerite.Rend. Accad. Naz. Lincei, 43,212-224. Strunz, H. (1936) Vergleichendertintgenographische und mor- Dal Negro, A., G. Giuseppertiand J. M. Martin Pozas(19?4) The phologische Untersuchung von Andalusit (AlO)AlSiO4, Li- crystal structure of sarkinite, Mn2AsOa(OH). TschermaksMin- beth€nit(CuOH)CuPOa und Adamin (ZnOH)ZnAsOa.Z. Kris- eral. Petrogr.Mitt., 21,246-260. tallogr., 94, 6{u--73. Doyle, P. A. and P. S. Turner (1968)Relativistic Hartree-Fock X- Terpstra, P. and L. W. Codd (1961) Crystallometry. Longmans, ray and electronscattering factors. Acta Crystallogr.,A24,390- London. 39'7. Toman, K. (1977)The symm€tryand crystalstructure of olivenite. Germain, G., P. Main and M. M. Woolfson (1971) The appli- Acta Crystallogr., 8 33, 2628-2631. cation ofphase relationshipsto complexstructur€s. III. The op- Waldrop, L. (1969) The crystal strucrure of triplite, timum use of phaserelationships. Acta Crystallogr.,A27,368- (Mn,Fe)rFPOo.Z. Kristallogr., 130,l-14. 376. - (1970)The crystalstructure oftriploidite and its relation to Groth, P. (1973)Crystallographic computer programs for cynrn- the structuresof other mineralsof the triplite-triploidite group. 74.Acta Chem.Scand.,27,1837. Z. Kristallogr., I3l, l-20. Hawthorne, F. C. (1976)A refinementof the crystal structure of Walitzi, E. M. (1963) Strukturverfein€rung von Libethenit adamite. Can. Mineral., 14, 143-148. Cur(OH)POo. TschermaksMineral. Pa rogr. Mitt., 8, 614-624. Henriques, A. (1956) An iron-rich wagnerite, formerly named talktriplite, from Hillsjriberget (Homsjdberget),Sweden. lrk. Manuscript received,October 18, 1979; Mineral. Geol.,2, 149-153. acceptedfor publication, December28, 1979.