Canadian Mineralogist Yol.24, pp. 599-604(1986)

BOBFERGUSONITE:A NEW PRIMARYPHOSPHATE MINERAL FROM CROSSLAKE, MANITOBA

T. SCOTTERCITI, ALAN J. ANDERSON2,PETRTnnn.f ANDFRANK C. HAWTHORNE Department of Geological Sciences, tlniversity of Manitoba, Winnipeg, Manitoba R3T 2N2

ABSTRACT Na2Mn5Fd+Al(POl6Q:4').les sixraies les plus inten- sesdu clichdde poudre[d in A, 1>40] sont:3.05(100), The new mineral speciesbobfergusonite occurs as abun- 2.87(66), 2.7 | (49),2. 5 I (53),2.08(67 ) et I .5 8(43) ; Les para- dant, Iargeanhedral crystals in a granitic pegrnatiteat Cross pbtresr6ticulaires sont: c 12.773Q),b 12.46Q),c 11.038(2) Lake, Manitoba. It rangesin color from red-brown(holo- 4., 9 97.15', groupeP2/n, La bobfergusoniteressemble type) to green-brown, has ayellow-brown streak, resinous beaucoupaux membres du groupede la wyllieite,mais elle and is nonfluorecent, The measureddensity is 3.54(l) s'endistingue tant par sastructure que par sacomposition. g/cm3; the hardnessis 4. It is biaxial (+), a 1.694(l), g Le nom honoreRobert Bury Ferguson. 1.698(l),1 l.7l5Q), 2V 46.4(2)o,pleochroism ,Y: Fyellow- orange,Z orange, f[D and a.\X= + l0' in the 0 angle. (Iraduit par la Rddaction) The averagechemical composition for the holotype (wt.9o) is: Na2O6.8, MgO 0.3, CaO 1.2, MnO 31.?, FeO 0.3, Mots-cl6s:bobfergusonite, nouvelle espice min6rale, phos- ZnO 0,1,Al2O3 7.5, Fe2O36.9, PzOs45.2,H2O 0.3, total phate, pegpatite granitique, Manitoba, wyllieite, 100.3. The semiempiricalformula,. (Na2.sMns.53Cas.2s alluaudite. tr r.uo)za(Mnr.eaAl1.39Fe3+6.s1Mgg .u.,F e+ o.uzno.u)roPs.ss INTRODUCTION (OB.6eOHo.3l)D24considered yrith structural data, gives the ideal formula Na2MnrFe3+AI(PO)6 six Q=4). ^The phosphate strongestlines of the X-ray powder pafrern td in A, /> 4()l Large crystals of a resinous brown are 3.05(100),2.87 (66), 2.71(49), 2.51 (53), 2.08(67)and mineral were found (by AJA) in a granitic pegma- 1.58(43). The unit-cell.parameters are a t2.773Q), b tite at Cross Lake, Manitoba. An initial electron- 12.486Qr,c 11.038(2)4,, B 97.15.; the spacegroup is microprobe and X-ray-diffraction investigationof P21/n. lt is most closely related to wyllieite-group the mineral showedit to be closely.relatedto, but minerals, from which it differs in both structure and com- distinct from, the wyllieite-group and alluaudite- position. The mineral is named after Dr. Robert Bury Fer- group birth- guson. minerals. With the retirement and 65th day of ProfessorRobert Bury Fergusonat hand, and Keywords: bobfergusonite, new mineral, phosphate, gra- preparations underway for this special issueof Zfte nitic pegmatite, Manitoba, wyllieite, alluaudite. Canadian Minerologist, we chose to honor our friend, colleagueand teacherby naming this mine- gives gxeatpleasure Souvarps ral after him. It us to announce the new speciesbobfergusonite. It is particularly fit- ting that this new pegmatitic phosphate from Mani- La bobfergusonite,nouvelle espbce min6rale, forme de larges cristaux idiomorphes en abondancedans la pegma- toba is named after Dr. Ferguson, doyen of modern tite granitique du lac Cross, au Manitoba. Sa couleur va mineralogyin Manitoba, in honor of his contribu- du rouge-brun (holotype) au vert-brun; sa rayure estjaune- tions to mineralogy and crystallography, many of brun, et son dclat, r€sineux.Elle n'est pas fluorescente.Den- which havefocused on granitic pegmatites(e.g., Can. sit6 mesurde3.54(l), duretd 4. Elle est biaxe positive, o Minerol. 11, Part 3). The new mineral and its name r.694(r),B 1.698(1),7 r.715(2),2v46.4Q)", pl6ochroiime have been approved for publication by the IMA X= Y jaune-or4nge,Z orange, fll0, aAX: + 10. dans Commission on New Minerals and Mineral Names. I'angle B. Sa compositionmoyenne (90 en poids) d'aprbs Holotype material (grams) is preserved in the col- les rdsultats analytiques sur l'6chantillon holotype, est: lections of the Mineralogical Museum, University of Na2O6.8, MgO 0.3, CaO 1.2,MnO 31.7,FeO 0.3, ZnO 0.1,Al2O3 7,5, Fe2O3 6.9, P2Os45,2, H2O 0.3, total 100,3. Manitoba, Winnipeg (M6083),andthe Royal Onta- La formule semi-empirique, (Na2.s7Mn9.53Ca6.rorio Museum, Toronto 0i/.42687).More abundant, + preserved n r,20)E4(Mn3.6sAll.39Fe3+o.8tM g6.stF & s.aiZno.or)>oPs.seideotype material is also at these institu- (OB.6eOH0jt)r2a considdrdei la lumibre des donndes tions. structurales, mbne i la formule id6alisde LocauoN AND OCCURRENCE lPresent addressi Mineral SciencesDivision, National Museum of Natural Sciences,Ottawa, Ontario KIA 0M8. The new phosphatemineral occursin an unnamed 2Presentaddress: Department of Geological Sciences, pegmatite at Cross Lake in central Manitoba, about Queen'sUniversity, Kingston, Ontario K?L 3N6. 5 km north-northwest of the Cross Lake settlement. 599

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The pegmatite,denoted #22by Anderson(1984), is sity and extreme Mn-enrichment. Assemblagesof located at the southeasternshoreline of an unnamed primary phosphate minerals in granitic pegmatites islandin the westernpart of the lake (Long. 54" 4l' are typically much lessdiverse, and not as intimately N, Lat. 97"49' W), associated,the minerals occurring most typically as The parent pegmatiteis a memberof the Northern large isolated crystals.All of the primary phosphates pegmatite seriesof the Cross Lake pegmatite field. present are the examplesrichest in Mn known for This field is hostedby the CrossLake greenstonebelt iheir species, and have very high values of in the SachigoSubprovince, at the northwesternedge Mn/(Mn+Fe2+). The assemblagewill be dealtwith of the Superior Province of the Canadian Shield. in greaterdetail in a later paper, when all of its mem' Although no agedetermination is available for these bers have been more completely characterized. pegmatites,they probably are of the samelate Keno- ran ageas other fields of rare-elementpegmatites in PHYSIcALAND OPTIcAL PnoPpntms the Superior Province. Preliminary Rb/Sr isotopic data indicate I minimu6 ageof 2.5 Ga for the leu- Bobfergusonite typically occurs as anhedral, cogranitesof the CrossLake field (G.S. Clark & R.E. equant single crystals' or :rs aggregatesof relatively Meintzer, pers. comm. 1985). few crystals. Crystal sizeranges from lessthan I mm Most pegmatitesof the Northern seriesbelong to to I cm long. The mineral is variablein color, rang- the beryl-columbite type of the rare-element class ing from green-brownto red-brown. The red-brown of pegmatites. However, the overall geochemical variety was initially perceived to be volumetrically signature of this seriesis rather exceptional among the most significant, so representativered-brown the peematite populations of the Superior Province. material was designated as the holotype. All data The seriesis poor in rare alkali metals, particularly presentedin this paper pertain to the holotype, unless Li and Rb relative to Cs. In contrast, enrichment in otherwisestated. Mn andF h noticeablein the most fractionated mem- All color varieties are nonfluorescent and have a bersof the series,such as the#22peg$atrte(Ander- yellow-brown streak. The mineral is brittle' shows son 1984). perfect {010} cleavageand has a prominent {10{} parting. The lustre is resinousexcept on the parting PARAGENESIS surface,which showsa characteristicbronzy lustre. The measureddensity is 3.54(1) g/cm3, based on The #22 pegmatiteis emplacedin metagreywacke, five repeatedweighings with a Bermanbalance; the which gradesinto metaconglomeratesouth of the mean ialculated density of 3.57 g/un3 agreeswell dyke. The main part of the pegmatite is 40 m long with the measuredvalue. The hardnessis 4. and up to 6.7 m wide in outcrop, with prominent Small chips of the mineral are transparent.Bob- but asymmetric zoning. The wall zone, adiacent to fergusoniteis biaxial positive. Indicesofrefraction the northwestern contact, consists mainly of sac- were measured by the Becke line method with the charoidal albite and qtJarlz, with scattered garnet, useof a spindle stageand a Na-light source(589 nm): schorl, muscovite and K-feldspar. The samemineral a L.694(l),B 1.698(l),y l3l5Q). The 2V ansle' assemblagedominates the outer intermediate zone, determined from extinction curves refined from in which it is accompanied by columbite-tantalite, spindle-stagedata using the program EXCALIBR microlite, , and beryl. Large crystalsof brown (Bloss1981), is 46.4Q)", which compareswell with K-feldspar and book muscoviteform the inner inter- 2V(calc) 52o. The optic orientation is Y[b and mediatezone, separatedfrom a segmentedquartz oAX: + 10oin the 0 angle.Pleochroism is moder- "gore" by core-margin muscovite and columnar ate: X= I yellow-orange,Z orange.Absorption: greenberyl. The quartz "core" is in contact with the Z>X- L Using the Gladstone-Daleconstants of wallrock along the southeastern flank of the peg- Mandarino (1981),the compatibility index is 0.014' matile. indicating superior agreementbetween physical and Phosphateminerals most cornmonly occur as nod- optical data. ules in the intermediatezones of the pegmatites; however,large, isolatedcrystals of bobfergusonite X-Rev CnYstanlocnnPgY with no associatedphosphates are alsopresent. The nodules consist of intimately associated primary Precessionphotographs of bobfergusoniteare very phosphateminerals, which show little or no replace- similar to those of the wyllieite-group minerals, ment by secondaryphosphates. These primary phos- exceptfor the presenceof largenumbers of diffrac- phate minerals are bobfergusonite,beusite, fillowite, tion spots at 1/zh and t/z/ (wyllieite cell), suggesting , apatite, and an unknown Mn-phosphate. that the bobfergusonite structure is a derivative of Very minor alluaudite is also present, and is proba- the wyllieite structure. The cell volume is four times bly secondary in origin. that of the wyllieite structure; however, a and c can The phosphateassemblage is unique for its diver- be resetto give a unit cell with only twice the volume

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of the wyllieite structurp. This cell has dimensions involves no changein the cell volume. On the bob- a=12.8,b12.5, cl1.0A, PgT.S",andisrelatedto fergusonitesetting,thecells forthethreestructure the setting chosenfor the wyllieite cell (Moore & Ito types are of comparable dimensions except for a: 1973)by: o(B)=29(W), b(B):66y1, c(B):a.sin q(A):a(\,t/)=/za@).However,onthiscommonset- P/l(l0l)* AQ00)'elC0\D, and B(B): [(101)'&n(200),',] ting, the spacegroups of thesestructures are differ- (W). B and W stand for bobfergusoniteand wyllieite, ent: the alluaudite structure has I2/o, the wyllieite respectively. Systematic absences are h0l with structure has P2t/c, and bobfergusonitehas P2r/n. h+l=Zn+ I and 0k0 with k:2n+ l, uniquely giv- This setting was used in indexing the patterns of ing F21/n as the spacegroup. Table 2. Unit-cell dimensionsfor bobfergusonite, obtained The structure types can be distinguished on the by single-crystalX-ray diffractometry (MoKo radi- basisof a single (h0l)* precessionphotograph, which ation), are given in Table l. One green-brown and is quite easyto obtain becauseall three groups of two red-brown crystals were investigated. Unit-cell minerals have only {010} as a perfect .A dimensions vary slightly with color: the red-brown schematiccomposite (&0/)* precessionphotograph variety tends to have smaller a and b cell edgesthan is shown in Figure l. Minerals with the wyllieite the green-brownvariety, yet c and B ofboth varie- structure(upper right-hand quadrant of Fig. l) can ties are similar. be distinguishedfrom those with the alluaudite struc- X-ray powder diffractograms of bobfergusonite ture (upper left-hand quadrant of Fig. l) by the are very similar to those of the wyllieite-group and presenceof extra diffraction-maxima with h odd, alluaudite-group minerals (Table 2). The wyllieite which halve the apparent 0-level repeat period for structure is a more highly ordered derivative of the alluaudite along X. In turn, bobfergusonite Qower alluaudite structure, and the bobfergusonite$truc- half of Fig. l) can be distinguished from wyllieite- ture is a more highly ordered derivative of the wyl- group minerals by the presence of additional lieite structure @rcit et al. 198Q. Consequently, it diffraction-maxima that halve the apparent 0level is of little surprise that diffraction maxima cor- repeatperiods along both Xand Z of wyllieite (and responding to the alluaudite-structure subcell are thus quarter and halve the apparent Olevel X and strong in the powder patterns of all three types of Z repeatperiods of alluaudite). structure and dominate their diffraction records. From the data of Table 2, the supercell diffraction- maxima of wyllieite have a mean relative intensity Cttst\,Itsrnv of only 2r/o and, except for (100), with an intensity of.6t/0, are difficult to detect without modern X-ray Energy-dispersion X-ray spectra were obtained equipment (1.5 kW X-ray tubes or better). The with a MAC 5 electron microprobe using a Kevex diffraction maxima of bobfergusonitethat violate Micro-X 7000spectrometer. Spectra were collected the symmetry of wyllieite are necessarilyeven weaker for 200live secondswith an operatingvoltage of 15 than the supercelldiffraction-maxima of the wyllieite kV and a samplecurrent of l0 nA (measuredon syn- structure, and cannot be observed by powder thetic fayalite), and were corrected for current and methods.Consequently, in X-ray-diffraction inves- voltage drift. NaInSirOu (Na), chromite tigations, bobfergusonitecan only be confidently dis- (Mg,Al,Fe), apatite (P,Ca), MnF, (Mn) and willem- tinguishedfrom wyllieite-group and alluaudite-group ite (Zn) were used as standards. minerals by single-crystal methods. Data were reducedwith Kevexsoftware utilizine To simplify distinction by single-crystalmethods, the program MAGIC V (Colby 1980).PKa overlai the cell chosen by Moore & Ito (1973) for the onto AIKcr,B due to incompletecharge-collection, alluaudite-group and wyllieite-group minslals \flas ZnLou! overlap onto NaKa,B, and MnKB overlap transformed to the bobfergusonite setting, which onto FeKa were dealt with by stripping techniques using library spectra acquired.at the same time as the samplespectra. 'I. 57Fe- IABLE UNIT.CELLDINENSIONs FOR BOBFEREUSONITE The Fd+/Fe3+ ratio was determined by Mdssbauerspectroscopy. Spectra were collectedwith Red-broxn Green-broM a 5l2-register multichannel analyzeron samplescon- P-2 taining an Fe-concentiation of 5 mg/crrf , and were 't2.776(2\ folded to give 245 data points over approximate u (B) r2.7s7(2) r 2.843(3) an b 12,488(2) 12.478(2) 12.5r 8(3) lange of -4 to + 4 mm,/s(relative to a-Fe foil). Data c ^ 11.035(2) 10.996(2) 11.0r4(2) B (:) s7.2r(r) s7.47(1) 97,37 (2) were collectedon red-brown holotype material and v (x3) 1746.714) r74r.0(5) 1756.0(6) on a representativesample ofthe green-brownvari- ety. Resulting values of the Fe2+,/Fe3+ratio were Detemlned usJng a Nicolet R3n four-circle dlffractoneter wlth gra- phite-nonochromted MoKq radiatlon. determined from areas of the spectral components @rcit et c/., in prep.); 0.05 was found for the red-

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TABLEZ. POIdDER-DIFFRACTIONDATA FOR BOBFERGUSONITE AND brown variety and 0.54 for the green-brownvariety. RELATEDI'IINERALS Examination of a 23-mg sampleof the holotype Bobfefgusonite(P-1 ) \ Ferrcwyllleltel Hagendorfite2 by thermogravimetryindicated the presenceof minor amounts of H2O and CO2. H2O was evolved hklddl hkldI hktdI peaking (caIc) (obs) (obs) (obs) between70 and 400oC, at 90 and 300'C. CO2was evolvedbetween 400 and 600oC,peaking 011 8.23 8.24 0il 8.09 13 011 B.16 2 at 540"C. The data for H2O evolution are similar 200 6.34 6,33 18 100 6.26 6 al' 020 6.24 6.26 020 6,14 20 020 6.26 21 to those for the mineral O'Danielite KelTer et oo2 5.48 5.47 002 10 002 5,40 6 1981a),a relative of alluaudite-groupminerals that 211 5.27 5,26 Tr'l 2 ot2 5,0l 5.01 contains O-H bonds (Keller et al. l98lb). Conse- 211 4.81 4.81 lil 4.74 1 220 4.447 4.445 120 4,388 quently, the H2O evolved from bobfergusonite 221 4.?5? 4.254 121 4,286 2 (0.28 wt.Vo) is interpretedto representOH in the 212 4.171 4.171 112 4.120 4 Tl2 4.188 I 022 4.117 4.120 4.052 6 022 4.094 .I3 structure.The very minor CO2(0.17 wt.9o) is most t21 4.003 122 probably due to contamination. 013 r.soe|:.sool6 0'13 3.452 30 or:| r.aoz of five spots acrossa 3-mm 230 3.479) 130J Resultsof analyses 231 3.385 3.383 2 131 3,337 4 cleavage fragment of the red-brown holotype are 122 3.263 I 1r3 3.193 2 given in Table 3. The data show that the mineral is 3.054 3.054 100 211 3.025 22 zlr 3.093 44 132 2.993 6 I32 3.042 9 homogeneousover the scaleof a crystal. Only Zn 223 2,9s1 2.9s2 4 123 2.9'12 3 appearsto violate this observation;it is presentat 4oz 2,904 2.902 37 202 2.A75 9 20? 2.937 16 411 2.868 2.859 66 211 2.833 27 211 2.81 41 only one of the five spots. The detectionlimit for 420 2.825 2.824 36 220 2.791 I 9 220 2.849 15 possible 241 2.750l| 2.698 57 141 2.762 15 Zn is approximately0.25 wt,$/oZnOi it is 004 2.7*l 2.739 36 004 oo4}2.707 loo that Zn may also vary only slightly' but being near t42 2.711 2.712 49 042 2.711 100 o42t 222'2.660 422 2.633 2.633 '172a 222 2.605 its detection limit, its variation may seemto be more 402 2.606 2.607 202 2.572 7 202 2.601 lr 214 2.578 2.579 2 114 2.564 3 extremethan is actually the case.Results of two ana- 242 2.551\ 2.55't 2 142 2.515 3 of a typical sample 73ll2.47e 27 2:rIz.s:+ lysestaken from a l-mm crystal 024 2.50712.508 53 213) 2131 of the greeq:brown variety are also given in Table 413 2.504J 024 2.480 yary 422 2.405 2.405 l0 222 2.371 7 '150222 2.403 3. They show that the mineral does in chemis- 250 2.323 2.324 2 150 2.289 3 2.333 grain, intragranular 2$ 2.283 2.283 2 try from grain to despite 234 2.227 2.?24 3 l34I 2.r96 4 134 2.220 The green-brown variety has a higher 2r3l homogeneity. 433 2.178 2.17A l0 23i 2.1s2 233 2.192 F&+ /Fe3+ ratio, higher tolal Fe and slightly lower 015 2.157.2.158 7 015 2.r26 ll 015 2.128 lt 610 2.083f2.032 67 310 2.061 l5 310 2.103 34 Al than the red-brown varietY. 060 2.081I 060 2.053 5 060 2.088 5 Formula contents for averaged compositions 053 2.061 2.061 6 l0 053 2.056 5 242 2.002 (Table calculatedon a basisof 24 (O + OH). 442 2.000'1,985 2.001 4) were 433 1.98s I 233 1.956 7 233 1.976 9 062 Considerationof the density and unit-cell volume 404 'I1.954 1.954 4 | 062 .947 1,945 4 o6al1.911 8 2041 1 .946 Cation sumsfor both var- 035' 1.918 givesZ=4 for this basis. 035 '1.8841.938 1.936 4 03sJ 630 1.883 4 '125330 1.899 ieties are appreciably lower than 16, the maximum 1 .876 wyllieile and allu- 424 1,866 r.865 4 224 1.839 6 224 1.850 number of cations possible in the 254 1.813 1.8r3 4 T54 1.787 4 T54 1.811 structureson a basisof 24 (O + OII). The sums 2't6 1.796 1.796 4 1j6\- t .7iz 6 Tr5 I .780 audite 453 l 786 1 .786 2 2531 closely approach 15, suggestingthat bobfergusonite 34l l.736 3 34tI 641 ',r.74lI1.756 1.755 4 435 1.741 l3 23511.718lt 26011 .744 ideally has one vacancy per formula unit. 460 1.739) 2601 235) empirical formulae are difficult to calcu- 270 1.717 1.717 3 170 1,691 3 170 1.724 5 Strictly 462 1.69111.690 2 zbzF |,0o5 late for alluaudite-groupand wyllieite-g;roupminerals 415 1.690J '145t-215) 2r5l 245 r.68111.678 4 I .653 3 '145t323F r.670 4 and bobfergusoniteowing to disorder of someof the 623 1.673) tzs) severalsites, e.9., Mn 444 1.657 1.657 1 244 1.632 6 334Ir.648 cationsin theseminerals over 334 1.623 4 244) over severalof the M and Xsites. Conse- 055 disorder 426 r.61711.617 226Ir.se7 6 350 1.625 4 quently, semiempiricalformulae must be calculated. 650 I.6t3J 350J 226 1.615 3 r.57r 3 tE2) Results of the crystal-structure analysis @rcit e/ 652 1.589 1.s88 352 15 8lt 1.563 1.583 6 4111r.561 9 iii Fr.sse al. 1D9A give the structural formula of bobfer- 046 1.57611.575 43 4oz) 4o2) 802 1.575' 046 I.553 15 046 I.561 l0 gusonite as:

SG: P21ln 5G: P21lc SGI 12/a 12.773(2)(E) a: 6.323(1 ) 6.464(1) a: (11 lx54x44llx32ndx14 b: 12.486(2) * 12.29312) D: 12.527'ro.e07( (lrttoi,nowptlA444lM54146+(PO+)u c: ll.038(2) ^ c: 10.880(2) r) B: 97.15(1). (") B: 97,35(l ) B, 97.930)

1. Vlctory tillne, south Dakota,ljsA. whereX2-X5 are occupiedby Na, n, Xl by Mn' 2. Hagendorf-southl4lne' I'1.6emany. (Cal, Ml andluf2 by Mn, (a), Iut3 andM4 bv Mn, PhlllDs Ptfl710autonated powder diffractoneter, automatlcdiver- q;"""'itii, qraphlte-nonoihrmted cuKdradlation' Tnternal iF.j*,Al), M5 by Fe3*, Al, (Mg, zn,F&"), andM6 itandard: NBsslllcon, batch640a. by Al, @e3+).

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/24/4/599/3445950/599.pdf by guest on 28 September 2021 BOBFERCUSONITE, A NEW MINERAL SPECIES FROM MANITOBA 603 Positions over which there is complete disorder are enclosedin brackets. Mr and X-site constituents of lesserabundance are enclosedin parentheses.Group- ing the contentsof the M sites versusthe X sitesgives A (Z:4): the semiempiricalformula a

.o (Na2.eaMns.53Cao.roE,.ro)"a(Mn3.6sAl,.rrFe,* o.r, Mge.67Fd*o.oaZno.orbePs.ee(Or.6eoHojl)E24 a. .a which, -.8. t disregardingminor M-site vacancies,is cal- ,, ..-X* culatedby assigningallAl, Fe, Mg,Znto theMsites, ,., .a.a a'a.a O.o togetherwith enoughMn to sum to 6 cations. The 1e111siningMn plus all Na and Ca are assignedto the a. a.4.. a X sites. a.a a The presenceof a large number of vacancies at a.a the X sitepis a responseto excesspositive chargeat the X and M sitesowing to the presenceof Mn at .a Xl and minor Al3* at M3 arrd M4, a Ideal bobfergusonite has only Mn at Ml-M4 and Xl, Fe3*at M5, N at M6, only Na at.X2-X3, and B 1:l Na:n at X4-X5 et sl. 1980. The idealfor- @rcfi Frc. 1, A schematiccomposite (&0I)* precessionphoto- mula is thus: Na2MnrFe3+Al(PO4)6Q:4). graph showingthe differencesin the diffraction patterns of the alluaudite structure (,4, upper left-hand quad- rant), the wyllieite structure (lZ, upper right-hand quad- rant) and the bobferg;usonitestructure (,B, lower half). DISCUSSIoN Spot sizemimics diffraction intensity; the smallestspots cover the l-2590 intensity range, intermediatespots Bobfergusoniteis closelyrelated to the alluaudite- coverthe 26-50V0and 5l-7590 ranges,and the largest group and wyllieite-group minerals. The three spots cover the 76-10090range. structure-types differ principally in terms of cation ordering within and between chains of M-calion TABLE3. CHF4ICALCOIIPOSiTION OF BOBFEROU5ONITE octahedra et al. l9BA. The alluaudite struc- @rcrt (P-2) ture showsthe leastamount of cation ordering, rang- Red-Brom(P-l) Green-Brom ing from having completeM-cation disorder (e.g., end-member varulite) to two-site M-cation chain order (e.g., end-memberalluaudite). The wyllieite lla20 rt.% 6,7 7,O 6.8 6,5 6.1 structure shows greater intrachain cation order; l'lg0 0.3 0.3 0,4 0.3 'L30.4 0.4 0.5 Ca0 1.2 1.1 1.3 1.3 0.6 0,6 cation ordering takes place over two sets of M sites iln0 116 1ti 31.9 31.3 31.8 32.7 3l,8 Fe0 0.3 0.3 0.3 0.3 3,4 3.7 (e.g., end-memberferrowyllieite) to a maximum of zn0 0.5 0.0 0.0 0.0 0.0 0.2 0.4 threeMsites (e.9., end-memberwyllieite). The bob- A1203 7.5 7.5 7.5 7.4 Fe203 6.7 6;8 7.1 6.6 7.0 7,1 7,6 fergusonitestructure showsordering of wyllieite-like Pzos 45.3 45.7 45.4 44.7 45.3 43,8 43.7 Hzo 0.3 0.3 0.3 -3ti-.8'0.3 nt 0.3 -3970.3 intrachain cations over four sets of i4 sites, with a TOIT3 MT.O- mr6 m0-3 T6it:z potential maximum of up to six sites. Moore & Ito (1979)have subdividedthe alluaud- For sample P-1, I to 3 pertain to the core' and 4r5 to the rlm. ite and wyllieite groups on the basis of cation parti- For P-2, 1 pertalns to the core, and 2 to the rim. tioning betweencertainMsites. Table 5 summarizes All elenents wlth Z>10 mre detemlned by electron nlcrcp1obe. H20 is based on a slngle TG analysls for sample P-I. Fez'lFef ls ob- the method of classificationand lists presentlyknown talned by l46ssbauer spectrcscopy for one sMple each of P-l and species.Extending the classification criteria to bob- P-2. fergusonite,the population of Ml + M2 corresponds to the A criterion, and the population of M3 + M4 correspondsto the .B criterion of Table 5. Bobfer- TABLE4. AVERAGMFOR}IULA COI|IENTS OF BOBFERCUSOI{ITE

gusonite has Mn dominant for both criteria, thus its Sample ila llg Ca iln Fe2+ Zn Al Fe3a P 0r0 alluaudite-group analogue is varulite, but it has no P-l 2.07 0.07 0.20 4.21 0.04 0.01 1.39 0,81 5.99 0.31 23.69 known wyllieite-groupanalogue. Although bobfer- P-2 1.98 0.10 0.10 4.41 0.49 0.04 0.93 0.89 6.00 0.32 23.68 gusonite would seem to be very similar to varulite cdlculated on a basls of 24(O'0H)fbn averagesof the dnalysesof Table i, accordingto the criteria of Table 5, it has major CdtJonsums: P-l 14,79, P-2 14.94. amount$of Al, unlike varulite.

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TABLE5. CLASSIFICATION.OFALLUAUDITE- AND I{YLLIEITE-GROUP MINERALS* tometry. Financial support for this work was provided by Natural Sciences and Engineering Donlnant Al luaudite [y]1 lelte grants to rc CatJon Group Grcup ResearchCouncil of Canada operating and FCH, and by the Canada-Manitoba Interim a -e. Agreement on Mineral Development' 1982-1984. l,ln2+ Fe2+ Hagendorfite llyl l ielte ie_ te.' Ferrcrlylllelte REFERENCES I'ln2r llo2+ |'|aghagendorfite Qinghellte Fe2+ ri2*

I,in2+ Mn2t Varul lte AnornsoN,A.J. (1984):The Geochemistry, Mineralogy Fe2+ Mnz* (us'1 r"i.r and Petrology oi the CrossLake PegmatiteField, lln2+ Fe3* Alluaudl te Rosmrylte CentralManitoba. M.Sc. thesis,Univ. Manitoba' Fe2a Fe3+ Ferroal 1uaudite Winnipeg, Manitoba.

A: Ml slte, B: l.l2slte for alluaudlte-grcup mlnerals. (1981\: The SpindleStage: Principles and [: l'{1 slte, E: Mza site for i{yllielte-iroup mlnerals. Bross,F.D. Practice. CambridgeUniversity Press,Cambridge, The generlc nane ls glven on the basis of the ldentlty of g. The ldentlty of A detemlnes the preflx: lf l'|n ls domlnant, no preflr; England. lf Fe2+ls domlnont, "feno' ls used. Maghagendorflte ls an excep- tlon. Colav, J.W. (1980):MAGIC V - a computerprograrn ---: not yet knom, us.r predlcted to be unstable, * mdlfled for quantitative eleltron-excrtedenergy dispersive after l,loofe & Ito (1979). analysis. QUANTEX-Ray Insfruction Manual, KevexCorporation, Foster City, California. Encn, T.S., HewtronNr,f.C. AtERNf' P. (198Q:The It would seem at the presenttime that bobfer- of bobfergusonite.Can. Mineral, gusonite can be distinguished from all known a.605-614, wyllieite-group and alluaudite-group minerals on the H., DwN, P.J. & NBwsuRv,D. of chemistry alone. Although thesedifferences Krlrer, P., Hms, basis (1981a): O'Danielite, NaZn3H2(AsOa):,a r€w presently known, it is possible that in chemistry are mineral from Tsumeb, Namibia. Neues Jahrb, a wyllieite-group analogueof bobfergusonitemay be Mineral. Monatsh., 155-160. found in the future. Becauseof the similar Al_ contents of bobfergusonite and the wyllieite-group +, RFrsL,H., Zerrmn, F. & HEss,H. (1981b): minerals(c/. Moore & Ito 1979),distinction on the Agco3H2(Aso)3 und AgZn3H2(Asor3Darstellung basisof chemistryalone may eventuallynot be pos- und Krislallstruktur. Ein weiterer neuer Arsenat- sible; however,the use of both chemicaland struc- Strukturtyp. Z. Anorg. allg. Chem. 474, 123'134- rural(i,e., single-crystalXRD) data will alwaysgive J.A. (1981):The Gladstone-Dalerelation- an un,unbiguous identification. Marpennrlo, ship. IV. The compatibilityconcept and its appli- cation. Cun. Mineral. 19' M1450, ACKNOWLEDGEMENTS Moonn, P.B. & Iro, J. (1973): Wvllieite' We thank R.A. Ramik of the Royal Ontario Na;r*+zAl(POlr & rlew species'Mineral' Record Museumfor the TG analysis,and C.A. McCammon, 4, 13l-136. formerly of the Dept. of Physics, University of Manitoba for the collection of Mdssbauerspectra. & - (1979): Alluaudites, wyllieites, We extend thanks to the following members of the arroiadites: crystal chemistry and nomenclature: -235. Department of Earth Sciences, University of Minbral. Mas, 43, 227 Manitoba, for their contributions: R. Chapman obtained some of the first ED X-ray spectraof the mineral, and R. Eby obtained unit-cell partuneters Received October 23, 1985,revised manuscript accepted for the green-brownvariety by sineile-crystaldiffrac- September 17' 1986.

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