American Mineralogist, Volume 65, pages 4gg_49g, Igg0 The crystal structure 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 fluorine-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-phosphate 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 space group 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 cleavage 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 iron 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 oxygen 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.