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AmericanMineralogist, Volume 63, pages 91 3-917, 1978 Compositionaland refractive index variations of the herderite-hvdroxvl-herderiteseries PnrnnB. LrevnNs Departmentof Geology,Uniuersity of Delaware Newark.Delaware l97l I Pnrr J. DunN D epartmentof GeologicalS ciences, S miths onian Institution lhashington,D. C. 20560 ,arNpRIcgAno V. Genrs R.D.] Potts t own,P ennsy luania I 9464 Abstract The compositionalrange of the herderite-hydroxyl-herderiteseries, CaBePOo(F,OH), has not beenwell-established. Microprobe analysis of 41 herderiteseries samples from 19 local- itiesgave compositions between 98 and 53 mole percenthydroxyl-herderite. The only speci- menof trueherderite known to us is a cut gemat the SmithsonianInstitution with 60 mole percentherderite. Nineteenth-century analyses of specimensfrom Stoneham,Maine, report- ing highfluorine contents and widely cited, must be considered doubtful. Indices ofrefraction of 12 samplesshow a linear declinewith increasingF content, and least-squareslinear regressionanalysis of the data gave a = 1.615- 0.00059(7oH) r: -0.991 p: t.634-0.00056(7oH) r: -0.994 I = 1.644-0.00055(7oH) r: -0.994 where7oH is the mole percentherderite in the specimen,and r is the correlationcoefficient of the indiceswith compositionfor the fitted lines.The X-ray diffractionpattern of hydroxyl- herderiteshows no perceptibleshifts with composition. Introduction rine, but Genth (1884) felt that Winkler's methods were totally inadequate for the detection of fluorine. Herderite, CaBePOnF,and hydroxyl-herderite, Ca- The division of the seriesinto herderite and hydroxyl- BePO.(OH), are end-membersof an isomorphous herderite was made by Palacheet al. (1951,p. 820). series. Herderite was first discovered in the tin veins Most analyses of herderite datb from the nine- of Ehrenfriedersdorf, Saxony (Haidinger, 1828); hy- teenth century and suffer from the difficulties of droxyl-herderite was first describedfrom a pegmatite fluorine analysis by the wet-chemical methods in use at Paris,Maine, ashydro-herderitebyPenfield(1894). at the time. Optical data on analysedherderites are The actual composition of the Ehrenfriedersdorf sparse. This paucity of recent and reliable data herderite has never been established.Winkler, in prompted us to analysea large number of herderites Weisbach(1884), gave two analyses,with 6.59 weight and to determine indices of refraction for a number percent and 6.54 weight percent HrO, greater than of these. A summary of our results, with a detailed the theoretical amount of water in pure hydroxyl- discussion of hydroxyl-herderite from two Brazilian herderite, and only a "doubtful reaction" for fluo- localities, is presentedelsewhere (Dunn et al.,1979). 0003404x/'r8/09r0-0913$02.00 913 914 LEAVENS ET AL,,' HERDERITE.SERIES Occurrence Maine, Penfield(1894) noted that a prism of the The herderiteseries minerals occur typically in peg- mineral was cut to determinethe index of refraction matites,as crystalsin cavitiesformed during the late, by the methodof minimumdeviation. This prism and replacementstage of crystallization,Crystals are usu- a specimenlabelled "hydro herderite,Paris, Maine" ally only a few mm across,but in oneBrazilian local- (Brush#1973) are preserved in theBrush collection of ity reachl5cm (Dunn et al., 1979).The most com- Yale University and were made available for this monly associatedminerals are crystalline qvartz, study,Penfield (1894) found a "Irace"offluorine, but albite,muscovite, and clays.Microcline, if present,is microprobeanalysis shows that this hydroxyl-herd- etched,as at Topsham,Maine (Yatsevitch,1935), erite actuallycontains about I percentF by weight, and Virgem da Lapa, Minas Gerais,Brazil (Dunn el and is 9l mole percenthydroxyl-herderite, rather al., 1979).Hydroxyl-herderite may form by the alter- thanthe pureend-member. Penfield was a meticulous ationof beryl(Yatsevitch, 1935; Perham, 1964) or of analyst;the presenceof I percentF undetectedin the beryllonite(Palache and Shannon,1928). It rarely sampledespite his careis evidenceof the difficultyof showsalteration, but occursas bright, euhedralcrys- F analysisby the methodshe used. talsimplanted on the otherminerals. Ford (1911)described a herderitemineral from Auburn, Maine.He includeda chemicalanalysis for water and fluorine,which yieldedF : 6.04percent, Chemistry HrQ = 3.62 percent,corresponding to 48 mole per- Forty-onesamples of herderiteseries minerals were cent and 65 mole percenthydroxyl-herderite respec- analysedwith an Anl-Snlrq electronmicroprobe us- tively. A twinned crystal of the describedmaterial ing an operatingvoltage of l5kV, a beamcurrent of (Yale rt3772)was examined in this study.It hasone 0.l5pA, and a lOp beam.The standardswere fluo- end sawnand is presumablythe analysedspecimen. rapatitefor Ca, P, and F; hornblendefor Si, Al, Fe, Microprobe analysisgave 4.20 percent F, corre- and Mg; and manganitefor Mn. The datawere cor- sponding to 64 mole percent hydroxyl-herderite, rectedfor background,backscatter, absorption, and compatiblewith the HrO valuereported by Ford. fluorescenceusing a computerprogram employing Hydroxyl-herderite from the Golconda Mine, the,Bence-Albee factors. The analysesare presented MinasGerais, Brazil (NMNH #121024)was used by in Table l, alongwith the fluorineanalysis of a fac- Lagerand Gibbs (1974)for a detailedcrystal-struc- eted green gem herderitein the SmithsonianGem ture analysis.They treatedit as purehydroxyl-herd- Collection(Dunn and Wight, 1976). erite, but our analysisof fragmentsfrom the same The analysesare averages;the fluorinecontent of specimen,a singlecrystal 7 X 5 X 4cm,showed 3.39 singlecrystals varied by asmuch as 20 percentrela- percent F, correspondingto 7l mole percenthy- tive at differentsample points. Indices of refractionof droxyl-herderite. different fragments of single crystals show corre- Completeanalyses of herderiteminerals for which spondingvariation. In addition, differentsamples F > OH werepublished by Hiddenand Mackintosh from the samelocality are in somecases quite differ- (1884)and by Genth (1884),both on materialfrom ent in averagecomposition. No attemptwas made to Stoneham,Maine. Since all six samplesfrom Stone- mappossible zonal distribution of fluorinein anyof hamwhich we analysedare hydroxyl-herderite, these the specimens. early analysesmust be reconsidered.Hidden and The analyticaldata indicate very little replacement Mackintosh(1884) gave 11.32percent F and stated of Ca by Fe, Mg, or Mn in the herderite-hydroxyl-only that "the fluorinewas calculated from theexcess herderiteseries. Most samplesconform quite well to of lime." They did not statethat they madedirect the theoreticalcontent for Ca and P. The analytical testsfor fluorine or for water. Although they stated data indicatethat herderiteis a very rarespecies. All that they had no doubt that the formula calculated the samplesstudied herein are hydroxyl-herderite; from their analysisrepresented the true composition nonewere found with a fluorinecontent which would of themineral, it is in factuncertain with respectto F. indicateF > OH. The only herderiteknown to us is Genth (1884)in summarizinghis analyticaldata, the gemstonedescribed by Dunn and Wight (1976). stated,"somewhat doubtful is the exactamount of A number of the analyzedspecimens have been fluorinewhich it contains."Although Genth thought studiedin the past,and somecomments about these hisF content(8.93 percent) too low,it isclear that he arein order. wasdoubtful of the analyticalprocedures for F deter- In his descriptionof hydro-herderitefrom paris, mination in use at that time. Penfieldand Harper LEAVENS ET AL.: HERDERITE SER/ES 915 Table l. Partialchemical analyses for the herderitesertes # NMNII unless Legs Locality noted sio2 CaO Pzos o=F Total Mol8oH Theoretical hydroxyl-herderite 34.82 44.06 0,00 0. 00 78. 88 100t1 Dunton Gen Mine, Newry, Maine * R4109 r.40 30.60 38.52 0.22 0.09 72.60 98+ Dunton cem Mine, Newry, Maine * L27895 0.83 30.s4 40.00 0.07 0.03 73.5L 999 Foote Mine, Kings Mountain, N.C. * 132503 3.35 30.29 37.59 0.08 o. 03 72.L6 99/ Keyes +l Mine, Orange, New Hampshire Private 0.67 34.04 42.80 0.24 0.10 77.6s 98 (" ) R16743 0.87 34.04 42.07 0.22 0.09 77.!0 98 Palemo *1 Mine, North croton, N.H. 106076-r 0. 00 34.L7 43.L4 0.19 0.08 7'1.32 98 (" ) 106076-t 0.03 33.26 43. 55 0.47 0.20 77.2L 96 Paris, Maine * Brush 1973 0.00 34.45 44.08 1.01 0.43 79.LL 9I Golconda Mine, Minas cerais, Brazil L2LO29 0.04 34.49 44.09 r.16 0.49 79.29 90 121033 0. 16 34. 42 44.24 1.69 0.7r 79.80 86 r2L032 0. 07 34.56 44.32 r.69 0.71 79.93 UO 121030 o.23 34.25 43.94 1.73 0,73 79.42 85 L2L034 o.32 34.22 44.01 L.82 0.77 79.60 84 * L2L024 0.00 34.59 42.93 3.39 1.43 79.48 ?l Fletcher Mine, North croton, N.H. 10615 7 0.00 34.78 43.97 0. 36 0. 15 75.96 97 (') 10616 7 0. 30 34.24 43.89 2.82 t-.19 80.05 76 (") 106191 0.00 34.47 43.89 L.72 0.72 79.36 85 Poland, Maine K)ZUI 0. 15 34.09 43.73 2.33 n oa ?q ?? 80 (") B13685 0.10 33.86 43.8s 3.32 1.40 79,73 Auburn, Maine 814r60 0.48 34.50 43.25 r.65 0. 69 79.L9 86 (") RI0289 0.00 34.14 43.99 2.90 L.22 79.7L 75 (") 46845 0.00 34.79 43.9? 3.78 l. 59 80.95 68 (") * laLe 37'12 0.00 33.90 43-34 4.20 L.79 79.65 64 Poland, Maine R 5201 0.00 33.8r 43.24 3.85 L.62 79.28 67 Bennett Mine, Buckfield, Maine LJ 1J6 L 0.14 34.54 44.02 2.98 I.25 80.43 (') L32682 0.14 33.89 43.52 3.41 L.44 79.52 7L Greenwood, Maine 123395 0.33 34.'lO 43.49 3.85 L.62 80.75 55 Stoneham, Maine 120 817 0. l0 34.23 43.9L 3.29 1.