American Mineralogist, Volume 69, pages 961-966, 1984 The crystal structure of viitaniemiite AenNE PeruNBN The University of Helsinki Department of Inorganic Chemistry Vuorikatu 20, SF-00100Helsinki 10, Finland eNo Sr,ppo L Lenrr Geological Survey of Finland Kivimiehentie l, SF-02150ESPOO 15, Finland Abstract The crystal structureof viitaniemiiteNa(Ca,Mn)AIPO4F2OH a : 5.457(2),b -- 7.151(2), c: 6.336(2)A,B: 109.36(3)"V :251.68A3, Z: 2, spacegroup P2/m, hasbeen solved by Pattersonand Fourier methodsand refinedby the least-squaresmethod to an R index of 0.037for 728observed (>2o) reflections.The structurecontains two setsof infinite chains parallel to the b-axis, one composedof A|O2(OH)2F2octahedra sharing opposite OH cornersand the other of (Ca,Mn)O+Fzoctahedra sharing opposite O-O edges.These chains alternatelaterally sharingF cornersto form a set of parallelsheets held togetherby POa tetrahedraand NaOaFagable disphenoids. The sheet structure of viitaniemiitecontaining octahedrally coordinated atoms in two separatepositions resemblesthat of montebrasiteand eosphorite.These three related phosphateminerals are associatedwith each other in the type locality of viitaniemiite, Viitaniemipegmatite, Orivesi, southernFinland, where they crystallizedduring hydrother- mal replacementprocesses caused by residualfluids of the pegmatitemelt. Introduction encounteredas very smallcrystals in vesiclesofsilicocar- bonatitetogether with cryolite, calcite,quartz, and welo- Viitaniemiite occurs as a rare hydrothermal mineral in ganite. Becausethe powder diffraction data of viitanie- the phosphate-richViitaniemi pegmatite,Orivesi, south- miite resemble those of an unnamed mineral from ern Finland.One of the authors(SIL) hasdescribed it asa Greifenstein,Sachsen, given on JCPDScard l3-0587,the new mineralin a study on the mineralogyand petrology of presentauthors studied several museum specimens taken the graniticpegmatities of the Eriijiirvi area(Lahti, l98l). from this locality and identified viitaniemiite from the The mineralwas encounteredas an inclusionin eosphor- sampleslabeled as lacroixite(cf. Mrose, l97l). According ite aggregateand is associatedwith morinite, another to the descriptinsby Slavik(1914, l9l5), lacroixite(and aluminum-bearingphosphate mineral. Structureanalysis therefore also viitaniemiite) occurs there in druses of confirmedthe ideal formula Na(Ca,Mn)AIPO4F2OHwith lithiogranitetogether with jeZekite(sodian morinite), apa- Z :2, althottghwet chemicalanalysis indicates that some tite, childrenite (Fe end member of the isomorphous of the fluorine may be replaced by OH groups. The serieschildrenite-eosphorite), roscherite, and tourmaline. crystal data measuredduring the structure analysisare Preliminary microanalyzerdeterminations showed that given in Table 1. The X-ray powder data, the optical Greifensteinviitaniemiite is rich in calcium and contaiirs properties,the chemicaldata, and the mineralogicalde- only a few percent of manganese;the fluorine content, scriptionof the mineral have beengiven by Lahti (1981) however,is equalto that of the Finnishviitaniemiite. Due andare not reproducedhere. A preliminarydescription of to its different chemical composition, the-Greifenstein the structurehas been given by Lahti and Pajunen(1982). viitaniemiitehas a largerunit cell (a -- 5.484, b = 7.184, To date viitaniemiitehas been identified positively in only c : 6.85A, F : 109.00',v = 254.8443;based on the three localities:at Viitaniemi. in museum specimens precessionfilms from sampleno. 86746in Harvard Min- collectedfrom drusesof granitein Greifenstein,Sachsen eralogicalMuseum). Detailed studies on the Greifenstein (East Germany),and from Franconquarry, northeastern viitaniemiiteand lacroixiteare in progress,because some Montreal, where its occurrencehas been confirmed by of the data cited for lacroixite obviously derive from Ramik et al. (1983). In this quarry viitaniemiite was viitaniemiite. 0003-004x/84/09I 0-096 l $02.00 961 962 PAJUNEN AND LAHTI: STRUCTURE OF VIITANIEMIITE Table l. Crystal data for viitaniemiite ed in the refinement with a fixed isotropic temperature factor (I : 0.0642. a = 5,45-ll2\A V = zSt.OAA3 The final refinement included positional parameters, b = 7.1.5I(2)A anisotropic thermal parameters, and occupancy factors = e 6.836(2) A D(calc.) = 2.242 S/q3 for calcium and manganese.The initial values for the B = 109.36(3)o ,(ms) = 3.245 g/m3 occupancyfactors were obtained from chemical analysis. Space gnp P2a,/n u(I4o(e) = 2.42 q-L The occupancy factors convergedto 0.592(5)for calcium (id@l) (Ca,th)ALPO4F2OH Fomla Na and 0.401(4)for manganese.Final R was 0.037, R* : 0.M3 and S = (IwAF2lm-111rrz: 4.12. The difference Experimental map had its largest electron density 0.61 eA-3 in the vicinity of the (Ca,Mn) atom. The calculations were For the structure analysis a transparent crystal plate of performedon a Univac I108 computerusing programs of viitaniemiitemeasuring about x x 0.1 0.2 0.3 mm was removed the xnay76 system (Stewart, 1976). The final atomic from the type material. Roundish crystals suitable for study are positions and thermal parameters are listed in Table 2. difficult to obtain, because the mineral forms thin, sometimes The interatomic radial, crystal plates (0.02-0.2 mm thick) flattened parallel to the distancesand bond anglesare reported in (l0T) plane. Table 3. Table 4, giving observedand calculatedstructure Single crystal X-ray precession photographs of this crystal factors, is on deposit at the Business Office of the l exhibit monoclinicsymmetry with systematicabsences 0k{, k: Mineralogical Society of America. 2n l- I consistentwith the spacegroups PTlm andP2,. The unit cell dimensions(298 K) given in Table I were obtainedby least- Description of the structure squaresrefinement of angular settingsof 20 reflections centered A stereoscopicview of the structure is presentedin on a Nicolet P3 automatic four-circle diffractometer -using Figure l. Al and (Ca,Mn) are in the centersof symmetry, graphite monochromatedMoKo radiation (I = 0.71074). A phosphorusand two of the oxygen atoms of the phos- uniqueset of intensitieswas measuredusing the r+scanmethod phate plane. with variablescan speed. Of the 788 reflectionswith 3. < 20 < ion are situated on the mirror The sodium 50",728were regardedas observed,their intensitybeing larger and hydroxyl ions are also on the mirror plane. Fluorine than 2.0 times the estimatedstandard deviation. Three standard and one oxygen of the phosphate ion are in general reflections measured for every 50 reflections showed no positions. systematic variation. The intensity data were corrected for Aluminum is surroundedby two oxygens,two hydrox- Lorentz and polarization efects and an empirical absorption yls and two fluorines in octahedral arrangement. The correction basedon the (ffscanmethod was applied (North et al., A1O2(OH)2F2octahedra form chains parallel to the b-axis l96E),with the coemcientranging from 0.707to 1.0. by sharing opposite OH- corners. In this 7A corner- sharing OH-AI-OH-AI chain (Moore, 1980) the OH Structure determination bridges are in trans configuration. (Ca,Mn) is surrounded The spacegroup, assumedto be PZ/m on the basisof by four oxygens and two fluorines in distorted octahedral diffraction intensity statistics, was later confirmed by the arrangement.The octahedra shareopposite O-O edgesto structuresolution. The symmetryin the spacegroup P2rl form a setofchains parallelto the b-axis.The sharededge m requiresthat most of the atomsbe in specialpositions. is considerablyshorter than the others, suggestingrepul- The structure was solved by Pattersonand Fourier meth- sion between neighboring (Ca,Mn) cations. The two ods. The Pattersonsynthesis showed that most of the chains alternate by sharing F corners to form a set of -r9.25. atomsare situatedat y : A solutionwas found for parallel sheets. Adjacent sheets are held together by Al and (Ca,Mn)in the centersof symmetry,for F and one phosphate ions. The phosphate ion is a nearly ideal O in general positions and for other atoms on the mirror tetrahedronwith O-P-O anglesin the range 108.3-112.2'. plane. Additional linkagebetween the sheetsis provided by the The atomic scattering factors used were those of Cro- NaOaFapolyhedron (Fig. 2). Five Na-O and Na-F dis- mer and Mann (1968)for non-hydrogenatoms with anom- tancesare between2.3-2.54, defininga distortedsquare alous dispersion coefficients from the International Ta- pyramid, but three additional distancesof 2.8A are in- bles for X-ray Crystallography (Ibers and Hamilton, cluded,resulting in a polyhedronwith coordinationnum- 1974), and, of Stewart et al (1965) for hydrogen. The ber eight. The next larger distancesare >3.14. This structure was refined by the full-matrix least-squares polyhedron, gable disphenoid, has been discussedin - rnethod.The function minimizedwas lwllF.l l,f'"||'zwith detail by Moore (1981).Previously it hasonly beenfound : unit weights.R 0.071was obtainedby refinementwith in wyllieite (Moore and Molin-Case, 1974)and fillowite isotropic temperature factors for non-hydrogen atoms. (Araki and Moore. l98l). The temperaturefactors were converted to anisotropic in the form given in Table 2; refinementreduced R to 0.057. ' To obtain a copy of Table 4, order Document Am-84-251 A diference map calculated at this stagerevealed a peak from the Mineralogical Society of America, Business Office, on the mirror plane lA from O(4). This peak was assumed 2000Florida Avenue, N. W., Washington,D. C. 20009.Please to be the hydrogenatom. The hydrogenatom was