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Home , Alleghanyite, Chondrodite, Humite, Jerrygibbsite, Leucophoenicite, Monticellite

American Mineralogist, Volume 68, pages 951-959,1963

Mn'humites from Bald Knob, North Carolina: and phaseequilibriat

G. A. WrNren Tennessee Valley Authority P.O. Box 2957,Casper, Wyoming82606

E. J. EsseNE AND D. R. Peecon

Department of Geological Sciences The University of Michigan, Ann Arbor, Michigan 48109

Abstract

Alleghanyite,manganhumite and sonolitehave been found in the Bald Knob deposit of North Carolina.These are the manganeseanalogues of , and clinohumiterespectively. They are closeto end-memberMn mineralsin terms of cations but have about 25-30vo of hydroxyl replaced by fluorine. The observed partitioningof minor elementsamong coexisting -manganhumite and mangan- humite-sonolitesuggests that minor elementsubstitution does not stabilizeone manganese humiterelative to another.The Bald Knob humitescoexist with rhodonitesand manganese carbonates.Qualitative phase equilibria suggest that theseassemblages require water-rich conditionsto form in silica-undersaturatedrocks during regionalmetamorphism and that variations in XHzOI(XH2O+XCO) may account for their formation.

Introduction The humite minerals comprise a homologous series of nesosilicates general The Bald Knob manganesedeposit in AlleghanyCoun- havingthe formula nMgzSiO+.Mg(OH,F)z where n : ty, North Carolina has been shown by Simmonser a/. l, 2,3, and 4 for ,chondrodite, humite (1980) (1981)to have originally been a manganiferoussediment and ,respectively. Ribbe gives examples previously metamorphosedto the mid-amphibolitefacies. Winter er of the confusing and variable space group nomenclature al. (1981)estimated conditions of peak metamorphismof for the humite mineralsand recom- mends T = 575+40"C,P = 5-rl kbar, XH2O = 0.5, and./O2fS2 adoptionof the conventionsused by Jones(1969) which we locatedbetween l0-re/10-3 and l0-r6i 100based on analy- follow here. ,Mnq(OH)z(SiOr)e, sesof coexistingphases, experimental data and thermo- is isostructuralwith clino- humite dynamic calculations. The deposit is characterized by and is abundantat Bald Knob. Manganhumite, Mn7(OH)2(SiOa)3, both silica-rich and carbonate-richbulk compositionsand is isostructuralwith humite. It is rela- tively uncommon generally is host to a wide variety of unusualmanganese silicates, at Bald Knob and is identified carbonates,titanates, sulfides, and spinelgroup minerals; only in thin sectionand by microprobeanalysis. Allegha- nyite, three of these (,alleghanyite, and kellyite) were Mn5(OH)2(SiO4)2,is isostructuralwith chondro- dite. (1980) first described from this locality (Ross and Ke':, 1932; Momoi reports the synthesisof fluor-allegha- nyite, Peacoret al.,1974).Ofmuch interestis the occurrenceof fluor-sonolite, and fluor-norbergite; Francis and (1978) three manganesemembers of the humite group (allegha- Ribbe also synthesizedthe manganeseanalogue of norbergite (MAN), nyite, manganhumiteand sonolite, the respectiveana- although it has not yet been found in nature. logues of the magnesiumhumites chondrodite. humite and clinohumite) as well as , the manganese Two additional, closely-relatedmanganese end-mem- ber phases, (a . Although P-T-X relations between olivine and dimorph of manganhu- mite) (a the various humites at other localitiesare not clear-cut, and dimorph of sonolite), are only known well-defined equilibrium relationships are found at Bald from the Franklin areaofNew Jersey.They have no known Knob which permit a simple interpretationof olivine- equivalentsin Mg endmembers,and although humite relations. we have carefully searched for them, neither has been T found at Bald Knob. Moore (1978)determined the struc- ture of leucophoenicite and showed that it is character- t Contribution No. 386 from the Mineralogical Laboratory, ized by an unusual kinked and serrated octahedral chain The Universityof Michigan. arrangementwhich is unlike that of the humites. White 0003-004x/83/09I 0-095 I $02.00 951 9s2 WINTER ET AL: MN HUMITES FROM BALD KNOB and Hyde (1983,ms.) have further describedthis struc- (1977)experiments in the systemMgO-MgF2-SiO2-H2O ture and its relation to other phases.Jerrygibbsite has that humite may be metastable relative to chondrodite beendescribed recently as a new by Dunn et al. and clinohumiteat moderateto high temperatures.If so it (1984).It is orthorhombic and either is derived by unit- is difficult to reconcileoccurrences of humite unlessit is cell twinning of monoclinic sonoliteor, more likely, is a always a late, low-temperaturereplacement of other member of the leucophoenicitegroup. The equilibrium humites. It should be noted, however, that humite can relationsof leucophoenicitegroup mineralswith the Mn- only be demonstratedto be unstableif it can be madeto humitesremain unknown. decomposeto clinohumiteand chondrodite,and that its Someoccurrences of humitesand olivinesare enigmat- lack of growth in the laboratory indicatesnothing about ic in that apparentlyincompatible phases are reportedto its stability.Yoshinaga (1963), Chopin (1978), and Fukuo- coexist.Since most humiteslie on a binaryjoin between ka (1981)report alleghanyitewith sonolitewithout finding R2*(OH,F)2and R3+SiO4,only two contiguousphases interveningmanganhumite, and it is possiblethat under shouldcoexist unlessadditional components complicate some P-T conditions manganhumiteis less stable than the phase relations. However, many exceptions are alleghanyiteand sonolite, analogousto the relation in- known in reported occurrencesof humites and . ferred by Bourne (1974),Duffy and Greenwood (1979) Smith er al. (1944) reported parallel intergrowths of and Rice (1980) for humite. White and Hyde (1982) tephroite and alleghanyitefrom the Benallt manganese observe tephroite-alleghanyiteintergrowths from Hok- mine in Waleswithout observingintervening manganhu- keyino, Kyoto Prefecture,Japan using transmission elec- mite, leucophoeniciteor sonolite.Tilley (1951)described tron microscopy.The Bald Knob assemblagesinvolving parallel intergrowths of humites and olivines from a sonolite,manganhumite and alleghanyiteexhibit none of contactskarn in Skye, Scotland,in the sequenceforster- theseapparent inconsistencies. The detailedchemistries ite-chondrodite-humite-clinohumite-,where of thesephases, especially where two coexist, therefore forsteriteshould be found adjacentto clinohumiterather hold implicationsfor equilibriumassociations among hu- than chondrodite.Bourne (1974)found no humite in a mites and olivines. variety of assemblagesamong , clinohumite, Observations chondroditeand norbergitefrom the CanadianGrenville terrane. Duffy and Greenwood (1979)and Rice (1980) All sampleswere collected from the waste dumps at concludedfrom Bourne's observationsand from Duffv's Bald Knob and thereforewe have no direct knowledgeof

Fig. 1. Photomicrographsof Bald Knob Mn-humites, scale bar : 0.30 mm. Figure lA, sonolite (Son) porphyroblastsin a (Kt) matrix with a largealabandite (Abd) grain(uncrossed polars, sample BK14). FigurelB, twinnedalleghanyite (All) in a kutnohorire(Kr) matrix. (crossedpolars, sample635). Figure lC, tephroite(Tep) grainsin a tirodite (Tir)-(Rh) gneiss (crossedpolars, sample BK4). Figure lD, manganhumite(MH) intergrowthswith alleghanyite(All) (crossedpolars, sample BK22). WINTER ET AL: Mn HUMITES FROM BALD KNOB 953 their original relative positions. Ross and Kerr (1932) detailed characteization as being representative of the describedthe body, however, so that inferencescan rangeof bulk composition,mineralogy and texture. These be made about relative specimenposition by referenceto were studied using qualitative and quantitative electron their descriptions.Seventeen samples were chosen for microprobeanalysis (eure) and standardoptical methods

Table l. X-ray powderdiffractometer data for Bald Knob Mn-humites

Al leghsny i te ManganhuDi te sonol i te

lllo dobs dcalc hkt llIo dobs dcalc hkl IlIo dobg dcalc hkl

8 7.06 7.O4 002 13 5.06 5.07 020 7 5-27 5,26 020 4 4.7a 4.79 022 4 4.61 4.63 022 4.16 l0l 9 4.18 4.37 ll0 8 4.41 4.43 rl0 I 4.13 4.13 l0I 5 4.O5 4.05 103 1 4.0r 4.01 I02 7 1.90 3.91 002 t3 3.EE 3.89 2tl 3.89 Lt2 5 3.74 3.74 o2l 2t 3.78 3.79 113 5 3.78 ltanganhuDite 3,13 o22 6 3.67 3.63 006 76 3.63 3,62 lll 3.60 I20 49 3.63 3.63 lt2 8 3.56 3.55 tzl 8 t.t7 3.58 720 3.58 l2l 22 3.5r 1

93 2-515 2.6t5 l3l 9 2.502 2.614 107 28 2.6t3 2.609 I 33 2.6t3 003 2.59t O42 23 2.533 a.)J) 040 86 2.545 2.548 L17 2t 2.519 Manganhuui te 2.542 134 23 2.5t7 2.5t5 L32 I6 2.505 2.504 043 25 2-50'a 2.505 04r 44 2.464 2.463 ll5 53 2.428 200 9 2.411 2,440 200 t3 2.143 2.422 r3t 2.426 028 63 2.39t 2,392 ll3 L4 2.39t 2.399 r35 51 2.162 2.396 o44 64 2.350 2.346 l4l 2.355 r27 20 2.yt 2.341 140 2.y2 I 2.2A7 L2 2.2t7 2,219 2LL 2.2t7 4 2.259 22 2.209 04I 2,206 o43 IO 2.t22 ll3 2.t52 tl 2-t2E l0 2.M6 2.O39 r2l 1 2.O53 6 1.924 8 r.955 1.901 9 t. t65 8 r.EEE 13 I.EE7 4 t,873 1.864 r.866 r.E40 1.E39 100 I .807 100 1.603 87 t.808 l0 I .781 r3 1.777 t.757 l4 t.747 20 1.744 r4 l.7tl I t.721 t5 1.7(x, 27 1.591 t3 1.690 t.691 l0 1.652 9 1.657 4 1.559

1.638 I .631 8 1.63t 23 t.6ll 8 r.601 30 l -606 13 r -5E8 14 1.590 32 r -564 9 1.569 24 t.553 53 t.551 36 r .556 38 r.544 r.544 43 1.546 WINTER ET AL: Mn HUMITES FROM BALD KNOB

on polishedthin sectionscut normal to the obvious l-5 Table 2. X-ray unit cell and optical data for Bald Knob Mn- cm banding.Single-crystal and powder X-ray diffraction, humites

as well as universal-stageoptics, were usedto character- Mineral ize specific grains Locality MaDganhunite ManganhuDite Sonolite Alleghenyite which were chosen on the basis of sarple Nunber Brettfors Mine Bald Knob *".0 **o EN{peand optical characteristics. :;lo Silt Spececroup Pbm PbnE \zt/!- ?ztlL Alleghanyiteis easily identifiedin thin-sectionat Bald .(il ,,.azz(s)i 4.87s$, 4.s84(9) 4,866(6) Knob due to its striking polysynthetictwinning (Fig. lB) 6(8) 10.54(l) 10.670(6) ro.7oo(rr) 10,708(19) a(8)-(o) 2t.45Q) 2r,7E7o1) 14.3i4(19) E.260(16) andits occurrenceas small(0.05-l mm) roundedgrains in t00.44(r7) 108.77(19) manganesecarbonate matrix. Its X-ray powder diffrac- v(R) ro9o.2(27) 1t34.3(9) 735.6(14) 4o7.s(r?> tion data (Table l) are in good agreementwith the data of 1.707(3) 1.761(calc) 1.712(3) 1.772(2) Lee (1955)and Yoshinaga(1963). The unit cell parame- 1.732(3) 1.78r(2) 2v, 37(5)0 E4(5)o ters obtainedfrom thesedata (Table 2) agreewell with the dispersionrvrv

calculationsof Yoshinaga(1963), Rentzeperis (1970), and densiry 3.83(5) 4.053(carc) 4'023(calc) 4.000(celc) Momoi (1980).Cook (1969)reports X-ray datafor allegha- nyite from Franklin, New Jersey(which is that listed by oH/(F+OH) l.o o.73 0.7r (0.7) Mn/(Mn+ug+Fe)0.68 0.97 0.97 (0.95) rcros) with extra lines at 5.20, 4.32 and 3.264 which

cannotbe indexedwith our unit cell datafor alleghanyite, the settings rec@ended by Jones 4 al. (1969) ere used. indicatingcontamination phase(s). The sectings for oonoclinic sonolite and alleghenyite have elPhe with other The X-ray es the unique sngle. powder data of Lee (1955),Yoshinaga (1963), or that in These valuei wer€ obtained by Moore (1978) for tyPe mnganhuDite. Table I shouldbe used in place of Cook's data. we hev€ nodified his axial selting fr@ spece grouP Pbm. Manganhumite is found intimately intergrown with setting Pru to values estiMted by cmparison cith analyzed specinens either sonoliteor alleghanyiteat Bald Knob, althoughin froD Beld Xnob (Table 2). sampleDSl0 it forms largeporphyroblasts with only very minor intergrowths of alleghanyite. It was initially identi- fied due to its different extinction position in intergrowths (Fig. lD). Its 2V and indicesof refractionare both much to identify exceptin thin section.We have not yet found higherthan the type materialof Moore (1978),as would tephroitecoexisting with sonolitealthough such a relation be expectedconsidering its much greater Mn content. shouldoccur. The Bald Knob tephroites,carbonates and Manganhumite is very difficult to distinguish from the pyroxenoidsare often enriched in and otherhumites in thin-section,and even microprobe analy- relative to the humites (see Winter et al., 1981, for sis is insufficient to resolve the dimorphs manganhumite analysesof carbonatesand pyroxenoids). from leucophoeniciteand jerrygibbsite from sonolite. Assemblagesinvolving Bald Knob humites and teph- Single-crystalX-ray work was necessaryto unequivocal- roite are listed in Table 3. Although tephroite and quartz ly demonstrate the presence of manganhumite at Bald are both found in BK-4 they are located in different bands Knob. Its unit cell and spacegroup are comparedwith with no indicationof havingequilibrated. Common acces- type materials (Table 2) and powder diffractometer data sories are kellyite (Peacor et al., 1974)and galaxite with are given (Table l). less common and pyrophanite (Essene and Sonoliteoccurs as centimeter-sized,irregular porphy- Peacoro1983), fluorapatite and nickeloancattierite. roblasts at Bald Knob (Fig. lA) intergrown with other Characterization of the humites by powder X-ray dif- phases,especially manganhumite, which makes it diffi- fraction often leads to ambiguous results due to the cult to obtain sufficiently pure material for powder X-ray similarity in the patterns. This is in part caused by the diffraction (Table l). This sample contained a small difficulty in obtaining sampleswhich comprise only a quantity of manganhumite,as noted. Lattice parameters, singlespecies. Isolation and characterizationof individ- obtained by least-squaresrefinement of the diffractom- ual samples is even more difficult when studying Mn- eter data, are also given (Table 3). These data agree well humitesfrom Franklin, New Jersey,because leucophoen- with thoseof Yoshinaga(1963), Cook (1969),and Momoi icite and jerrygibbsite also occur and their diffraction fl980). patternsare similar to those of the Mn-humites.Indeed, Although originally reported by Ross and Kerr (1932), patterns of all five minerals have principal peaks in tephroite is not a common mineral at Bald Knob. It is common, and have similar distributions of weak and difficult to distinguishfrom the humites both optically and intensepeaks so as to appear equivalent at first sight. We chemically. Many powder X-ray diffraction patterns were have therefore taken great care to obtain high quality obtainedon possibletephroites in humite rocks without diffractometer patterns of single-phasematerials to serve positive results and we believe that some sonolite was as a guide for other investigators(Table l). These pat- mistaken as tephroite by Ross and Kerr (1932). Where terns were obtainedusing a l'lminute scanrate and were identified(Fig. lC), it occursas 0.2 mm anhedralgrains in standardized using quartz as an internal standard. The clusters with abundant carbonatesin -rhodo- final criteria for yielding pattern accuracy were: (l) the nite/-tirodite assemblagesand is impossible data were indexibleusing lattice parametersobtained by WINTER ET AL: Mn HUMITES FROM BALD KNOB 955

Table 3. Mineral assemblagesin Bald Knob rocks containing Mn-humite or tephroite

p!pooo60oooP .FPOO€c-p >EooqtoPP.F.-AO'; croppElo{locp{toE_ [email protected] o 4 E'Fo E Ei .- 6.; i o= -F o {J N €,F E o g I E :,- € F E € i € i fr z ; = E =- i E"" e ESP# r.z "63 E gi sF EodE E a a E

BK2 x X xxx BK4 XX X xxx BK6 X XXXXXXX 8K8 XX XX XXXX 8K ll x X XXXXX BK 13 X XXXX

BK 14 XXXX BK 18 XX XX XXXXX BK 19 x xx BK 22 XX X XXXXXXX BK 25 x X x BK 26 XX X X xxxx

BK 28 x XXX BK 50 X xx BK 28 X DS5 DSl0 X 635 xx

least-squaresrefinement (Table 2); and (2) a direct com- mogeneitiesand intergrowths. Another sonolite sample parisonof patternsverified a minimum number of inter- from Liingban, Sweden in part contained an ordered ferencesby characteristicpeaks of other phases.Only intergrowth of manganhumiteand sonolite. These obser- unambiguouslyindexed X-ray lineswere usedin the unit- vations imply that microprobe analysesof some Mn- cell refinements.The alleghanyitepattern was of single- humites may yield intermediatestoichiometries (as has phase material, that of sonolite contained significant been reported by Fukuoka, l98l) even where X-ray manganhumite and that of manganhumite contained a diffraction apparently shows the presence of a single trace of alleghanyiteas cited in Table l. phase. Referenceto the d-values listed in Table I shows that it is possibleto unambiguouslycharacterize patterns using Electron microprobe analyses peak d-valuesbetween 3.78_and 3.15A. Alleghanyitehas The electron microprobe proceduresfor analyzing Mn- an intensepeak at d: 3.154 which has no equivalentin humiteswere essentiallythe sameas those describedby either sonoliteor manganhumite(and one at d : 3.634 Winter et al. (1981)for pyroxenoids.The resultantanaly- which overlapsa peak in sonolitebut not in manganhu- ses (Table 4) permit the unambiguous identification of mite). Sonolite has a unique and intense peak at d : each specificMn-humite, but we note that in general,any 3.35A(and a more intensepeak at d = 3.$Aoverlapping two suchphases have very similarcompositions and care one of alleghanyite).Manganhumite has two characteris- mustbe takenin the identificationof specificMn-humites tic peakshaving d :3J8 and 3.434.We emphasizethat usinganalytical data alone.All ofthe analysesshow near even having reasonablywell-characterized patterns with end-membercompositions in terms of cations, having which unknownscould be directly compared,we experi- Mn/(Mn + Mg + Fe2*; : 95% (Fie.2). The elementsTi enced difficulty until these characteristic peaks were and Ca are presentonly as minor elementsin the Bald identified in samplespreviously well characterizedby Knob humites and presumably do not have a significant single-crystaland/or electronmicroprobe techniques. effect on the phaserelations. Dal Piaz et al. (1979)report White and Hyde (1982)report on TEM observationsof a calcian alleghanyitefrom the western Alps of Italy but Bald Knob Mn-humites.They found alleghanyite,man- our powder X-ray diffraction data on their sample shows ganhumite and sonolite to occur as relatively perfect that it is leucophoenicite.The lack of titanium in the crystals with no intergrowths and few faults. The Bald manganesehumites at Bald Knob may be caused by the Knob materialsmay be relatively perfect crystals because fact that pyrophanitescavenged much ofthe Ti presentin of the long annealingtimes attendant with regional meta- the protolith. The low values for Al given in Table 4 may morphism.This is in contrastwith a sonolitesample from not represent amounts that actually exceed background Hokkeyino, Kyoto, Japan which contained tephroite, values;Jones et al. (1969)conclude that Al is probably manganhumiteand alleghanyite with many faults, inho- not present in humite structures. Boron has been shown 956 WINTER ET AL: Mn HUMITES FROM BALD KNOB

Table 4. Microprobe analyses of Mn-humites and tephroites. Mn2SrOa Mg.SiOo Son : Sonolite,MH : Manganhumite,All = Alleghanyite,Tep = Tephroite.

Sample BK2 BK4 BK8 BKl4a 8K14b 8K22a BK22b wtX Son TeD TeD Son MH All lilH lvlongonhumrte Si 27.38 30.39 ?8.69 27.25 26.L9 24.65 26,2L 02 Alleghonyrle Ti02 0.09 0.0I n.a. O.42 0.41 0.14 0.17 Al20?- 0.09 0.07 0.08 0.19 0,21 0.00 0.00 tFed I .28 f. i9 5.63 1.69 1.30 0.84 0.94 Mn0 68.84 57.l3 64.39 67.84 68.87 i0.15 69.47 Mongones Anologue of Mso 1.08 2.06 0.45 L.23 0.81 0.78 0.73 Norbergile Ca0 0.09 0.06 0.02 0.14 0.06 0.i1 0.13 "Hzo r.57 r.37 1.84 2.60 1.89 F? 0.91 n.a. n.a. r.ll r.cr 2.20 I.46 tsim 100.95 99.91 99.26 100.78 100.51 100.54 100.39 qmole ratios si 4.O2 1.02 0.97 4.00 2.99 2.00 3.01 Ti 0.01 0.00 n.a. 0,05 0.03 0.0I 0.0I AI o.02 0.00 0,00 0.03 0.03 0.00 0.00 I Fez+ 0.16 0.26 0.16 0.21 0.12 0.06 0.09 4.82 l'ln 8.56 1.64 1.82 8.42 6,67 6.76 Pyrochroiie l.l9 O.24 0.t0 0.02 0.27 0.14 0.10 0.12 Ca 0.01 0.00 0.00 0.02 0.01 0.01 0.02 Mn(OH,F ). M9(OH,F). ,0H 1.54 0.00 1.44 i.40 1.41 1.45 F 0.42 n.a. n.a. 0,53 0.51 0.57 0,53 0 i6.08 4.04 3,94 16.13 t2.o7 8.02 12.04 Fig. 2. Humite, olivine, and brucite-group minerals projected Samp'leBK 8K22c BK22d BK22e BK22f W22g 635-a 635-b onto the plane MgzSiOa-Mn2SiOa-Mg(OH,F)rMn(OH,F)2. wt.* l4H A',t't Al l lilH Al l t',lH Son Only analyses with (Fe+Ti)/(Fe+Ti+Mg+Mn) < 5/o are Si02 26.21 24.29 23.96 25.73 24.t0 26.18 27.I3 plotted.Data are takenfrom Dana(1892), Hintze (1915),Rogers Ti02 n.a. n.a. n.a. n.a, n.a, 0.04 0.03 4'120? 0.10 0.10 0.11 0.10 0.11 0.08 0.09 (1935),Palache et al. (1944),Lee (1955),Hurlbut (1961),Deer er )Fed- l.l3 0.82 0,77 0.93 o.82 0,9t 0.89 al. (1962),Yoshinaga (1963), Peters et al. (1977),Moore (1978), Mno 69.75 69.27 69.41 68.62 68.90 71.L7 69.79 Chopin(1978), Francis (19E0), Ribbe (19E0), Nambu et al. (1980) MSo 0.78 0.86 0.82 0.76 0.80 0.48 0.47 Cao 0.13 0.11 0.13 0.09 0.17 0.09 0.i1 and Fukuoka (l9El). Solid dots from Bald Knob, open circles "Hzo 2.oo 2.73 ?.71 1.8i 2.70 t.52 1,41 (1981) plethoraofanalytical F2 1.30 1.89 1,93 r.47 1.91 I.88 r.24 from other localities.Fukuoka hasa 3sim 100.86 99.27 99.03 98.95 98.71 10i,56 100.64 data on Japanesealleghanyites, sonolites, and tephroites,and "moleratios only a few representativepoints are shown here. si 2.99 2,00 1.98 2,99 1.99 2,96 4,01 Ti n.a. n.a. n.a. n.a. n.a, 0.00 0.00 Al 0.01 0.0I 0.01 0.01 0.01 0.01 o.o2 I Fe2+ o. 11 0.06 0.05 0.09 0.06 0.09 0.11 l4n 6.73 4,81 4.83 6.76 4.83 6.83 8.75 Itlg 0.13 0.11 0.11 0.13 0.10 0.07 0.10 ca 0.02 0.01 0.01 0.0i 0.01 0.01 0.02 'on t.52 1.50 1.49 1.45 1.49 1.44 1.40 F 0.47 0.49 0.50 0.54 0.50 0.55 0.58 0 11.98 8.01 7.98 11.99 8.00 11.93 16.04 MnrSiOo rn.a. = not analyzed;all Fe assumedto be Fez+ 'zHzois calculated: mle oH = Z-F-2Ti-Al. -)um aoJusfeol0r I = u at{ormalizedto l,ln + M9 + Fe + Ca + Ti + Al = 9 for sonolite; 7 for manganhumite;5 for alleghanyite;2 for tephroite.

to substitute for Si in some humites (Hinthorne and Ribbe, 1974), but we were unable to analyze for it; however,the normalizedanalyses are consistentwith its absenceat Bald Knob. Comparedwith most previously publishedanalyses of Mn-humites (Smith et al., 1944; Yoshinaga,1963; Moore, 1978),those at Bald Knob are enrichedin F substitutingfor OH, with up to a third of the anion siteoccupied by F (Fig. 3). The F/(F + OH) ratio is remarkably constant (0.21-0.32), considering the variety of specimensanalyzed. lVln(OH ). Mn F. The analytical data for the Bald Knob manganese Fig. 3. Manganesehumites projected onto the plane Mn2SiOa- humites can be said to show simplicity in both solid Mn(OH)rMnF2. Only minerals with Mn > Mg and with solution in all phasesand in constancyof ratios of trace to (Fe+Ti)/(Fe+Ti+Mg+Mn) < 5Vo are shown. Referencesand minor elementswhere they do occur, suggestingthat such symbolsare given in Fig. 5. The data ofChopin (197E),Francis variablesdo not complicate the phaserelations. The Bald (1980)and Fukuoka(l9El) could not be plottedbecause they did Knob manganesehumites presentan ideal situationfor not analyze their humite group minerals for fluorine. WINTER ET AL: Mn HUMITES FROM BALD KNOB the evaluation of humite group phase relations in a leghanyite + sonolite may also be considered. Since chemicallysimple system. manganhumiteis 0.8Vodenser than an equimolar mixture of alleghanyite + sonolite, it should be favored at high Solid phase solutions and equilibria pressures.Thermal expansionand compressibilitydata There are no experimental or thermodynamic data on are neededto quantify volume changesat high pressures the stability of manganesehumites so any inferred phase and temperatures, but the sign of the calculated volume equilibria must be schematic and based solely on the changesis not expectedto changefor crustalconditions. assemblagesand mineral formulae. The phase equilibria Major element chemical variations for the humites may inferredby Duffy and Greenwood(1979) and Rice (1980) be shown on two graphs, one for Mg2SiOa-Mn2SiOa- for magnesiumhumites provide an imperfect analoguefor Mg(OH,F)rMn(OH,F)2, and one for Mn2SiOa-Mn(OH)r manganesehumites for two reasons: (l) norbergite is MnF2 (Figs. 2-3). The Bald Knob minerals show regular inferred to be stable in the magnesium system, but a variation in Mn:Mg and F:OH with no evidenceof one manganeseequivalent is not yet known at Bald Knob or humite systematicallypartitioning these elementsrelative elsewhere;(2) manganhumiteoccurs at Bald Knob, but to other humites. When data from other occuffences are humite is inferred to be unstable at high temperaturesin also plotted, evidencecan be seen for significantsolid the magnesiumsystem. solution along the binaries for pyrochroite-brucite, man- The cell volumes of the humite and leucophoenicite ganhumite-humite, alleghanyite--chondrodite, sonolite- group minerals offer some clue to their relative stabilities. clinohumite and tephroite-forsterite, but only insignifi- Using the cell volumes in Table 3 for the humites and the cant solution of manganesein norbergites (see also Fran- data of Dunn et al. (1984)for Franklin leucophoenicite cis, 1980).Fukuoka (1981)reports analysesof coexisting and jerrygibbsite, one can calculate that Bald Knob tephroite and sonolite which are shown in Figure 2. He manganhumiteis2.lVo denserthan Franklin leucophoeni- also gives data for coexistingalleghanyite and sonolite cite, while Franklinjerrygibbsite is0.77o denser than Bald which have been excluded from Figure 2 for clarity and Knob sonolite. Therefore, manganhumiteand jerrygibb- becausesome ofFukuoka's analysesappear to represent site are apparentlythe high pressurepolymorphs. The manganhumiterather than alleghanyiteor sonolite. Bald Knob humite group minerals contain significant If one considersthe Bald Knob assemblagesit is clear fluorine and the Franklin leucophoenicitescontain some that the activities of SiO2 and H2O are likely to be of and , and these elementsmay stabilizethe importance in controlling the specific manganesesilicate respective phases. However, analyses of coexisting assemblage.Humites are more undersaturatedthan oliv- Franklin jerrygibbsite and sonolite show little partitioning inesbecause they haveR2+/Si ) 2, but substitutionofthe of Zn and Ca betweenthe two phases(P. J. Dunn, pers. excessR2+ is paired to OH+F. We have chosento plot comm., 1982).It is judged that thesesolid solutionswill the activitiesof SiO2vs. H2O to further examinesilicate not changethe relative volumes for reactionsbetween the reactions (Fig. a) because OH is greater than F in Bald polymorphsleucophoenicite vs. manganhumiteand jerry- Knob humites.The effect of F could be consideredas a gibbsiteys. sonolite. The reaction manganhumite= al- diluentin the humites,extending their stabilitysomewhat with respect to pyroxenoids and olivines. A sample reactionin Figure 4 is:

Manganhumite* H2O : Alleghanyite+ SiO2 5Mnr(OH)z(SiO4)3+ 2H2O :7Mn(OH)z(SiO4)2 + SiO2

This reaction separatesthe manganhumitefield from the alleghanyitefield. The other field boundariesare generat- ed by similar reactions. This figure shows the five rele- vant two-phasemanganese silicate assemblagesat Bald Knob: rhodonite-tephroite, rhodonite-manganhumite, rhodonite-alleghanyite, manganhumite-alleghanyite,and manganhumite-sonolite(Table 3). The lack of manganese hydroxides and oxides at Bald Knob is apparentlyex- plained by a lack of extreme variations in a(SiO)/a(HzO) or becausetheir place is taken by manganesecarbonates due to the availability of CO2, suggestingthat equilibria amongmanganese carbonates and silicates should also be "Hzo considered. Fig. 4. Phase equilibria for manganesehumites in the space The inferred T-XCO2|(XCO2+XH2O) equilibria for a(SiO)--a(HrO). All humites are assumed to decompose manganesesilicates in the systemMnO-SiO2-H2O-CO2 regularly to the chemically adjacent phase. have been constructedto be consistentwith observed 958 WINTER ET AL: Mn HUMITES FROM BALD KNOB

certain minimum level of COz, by restricted values of aSiO2laH2O (Fig. 4), or possibly by its intrinsic instabil- ity. The apparent temperature difference implied in Fig- ure 5 for tephroite vs. alleghanyite + rhodonite may be minimized if the dehydration equilibria (lF(a) are close together or if additional solid solutions shift the curves. The significant solid solutions in Bald Knob minerals of 25-30% F(F + OH) in humites, 10-30% (Ca + Mg + Fe)/ (Ca + Mg * Fe * Mn) in pyroxenoids, and 10-70% (Ca + MH\ Mg + Fe)/(Ca + Mg + Fe + Mn) in carbonates (this paper coz

MH and data in Winter et al., l98l) clearly cannot be disre- Rc garded. A complete analysis of phase equilibria at Bald \ Hzo Knob would require the evaluation of these solid solu- tions after the simple phase diagram (Fig. a) is calibrated. Nevertheless, it seems clear that water-rich conditions are required for manganese humites, and that local varia- tions in X(H1OIX(CO) and a(SiOz) will ultimately prove to account for the various assemblages found at Bald Knob.

Acknowledgments We thank Prof. W. C. Bigelow and his staff of The University of Michigan Microbeam Laboratory for providing the facilities for microprobe analyses. We also thank Prof. W. B. Simmons, Jr. for introducing us to the interesting Bald Knob deposit and for conducting preliminary optical, X-ray and microprobe work on Bald Knob minerals. We are grateful for the helpful reviews xcoz providedby P. J. Dunn, C. A. Francisand P. H. Ribbe. Fig. 5. Phaseequilibria for manganesehumites in the spaceI- XCO2I(XCOI+XH2O).All humitesare assumed to bestable with References (Rc) and/or rhodonite (Rh) and/or the chemically Bourne, J. H. (1974) The petrogenesisof the humite group adjacentmanganese silicate as suggested by observationsat Bald minerals in regionally metamorphosedmarbles of the Gren- Knob. MAN : Manganese Analogue of Norbergite, ville supergroup.Ph.D. thesis,Queen's University. Alleghanyite(All), Manganhumire(MH), Sonolite (Son), Chopin, C. (1978) Les paragendsesreduites ou oxyd6es de Tephroite(Tep). concentrationsmanganesifdres des schisteslustr6s de Haute- Maurienne(Alpes franqaises).Bulletin de Min6ralogie,l0l, 5t4-53 I . Cook, D. (1969)Sonolite, alleghanyite and leucophoenicitefrom Bald Knob assemblages(Fie. 5). By extension,the stabil- New Jersey. American Mineralogist, 54, 1392-1398- ity of a hypothetical manganeseanalogue of norbergite DalPiaz" G. V., Battistini, G. di, Kienast,J. R., Venturelli,G. has been derived although it may always be less stable (1979)Manganiferous quartzitic schistsof the piemonte ophio- than alleghanyite+ Mn(OH,F)2. Any three coexisting lite nappe. Memorie degli Instituti di Geologia e Mineralogia manganesesilicates, or two manganesesilicates coexist- dell'Universitadi Padova.32. l-24. ing with rhodochrosite define a univariant equilibrium, Dana, E. S. (1892)The System of Mineralogy, 6th Ed. John e.9., Wiley and Sons,New York. Deer, W. A., Howie, R. A., and Zussman, J. (1962)Rock- Manganhumite * Rhodochrosite + H2O forming Minerals. Volume l. Ortho- and ring-silicates. Long- : Alleghanyite+ CO2 mans,Green and Company,Limited, London. Duffy, C. J. (1977)Phase equilibria in the system MgO-MgF2- 2Mnz(OH)z(SiO4)3+ MnCO3 + H2O : SiOrH2O. Ph.D. thesis,University of British Columbia. 3Mns(OH)2(SiO4)2+ COz. Duffy, C. J., and Greenwood,H. J. (1979)Phase equilibria in the Similar reactions for other assemblagesof manganese system MgO-MgFz-SiOz-HzO. American Mineralogist, 64, silicates, carbonatesand oxides can be balanced and lt56-rt74. W. 8., Jr. and Essene, placed on aT-X diagramrelative to simple decarbonation Dunn, P. J., Peacor,D. R., Simmons, E. J. (1984)Jerrygibbsite, a new polymorph of sonoliteand reactionsfor rhodochrosite(Fig. 5). If it is assumedthat memberof the humite-leucophoenicitegroups, from Franklin, this diagram appliesdirectly to Bald Knob assemblages, New Jersey.American Mineralogist, in press. variations in CO2/H2O alone may explain most of the Essene,E. J. and Peacor, D. R. (1983)Crystal chemistry and observedassociations. The lack of a known manganese petrology of coexisting galaxite and jacobsite from near Bald analogue of norbergite may be explained either by a Knob, N. C. AmericanMineralogist, 68,449-455. WINTER ET AL: Mn HUMITES FROM BALD KNOB 959

Francis,C. A. and Ribbe, P. H. (1978)Crystal structuresof the Peacor,D. R., Essene,E. J., Simmons,W. B. Jr., and Bigelow, humite minerals: V. Magnesian manganhumite. American W. C. (1974)Kellyite, a new Mn-Al memberof the serpentine Mineralogist, 63, 874-E77. group from Bald Knob, North Carolina and new data on Francis, C. A. (1980) Magnesium-manganesesolid solution in grovesite.American Mineralogist, 59, |153-1156. the olivineand humite groups. Ph.D. thesis,Virginia Polytech- Peters,Ij., Valarelli,J. V., Coutinho,J. M. V., Sommerauer,J. nic Institute and State University, Blacksburg,Virginia. and von Raumer, J. (1977)The manganesedeposits of Buritir- Fukuoka, M. (lgEl) Mineralogical and geneticalstudy on alaban- ama (Para, Brazil). SchweizerischeMineralogische und Petro- dite from the manganesedeposits of Japan. Memoirs of the graphischeMitteilungen, 57, 313-327. Faculty of Science,Kyushu University, SeriesD, Geology, Rentzeperis, P. J. (1970)The of alleghanyite, 24,207-251. Mn5(OH)2/(SiOo)2.Zeitschrift fiir Kristallographie, 132, l-18. Hinthorne,J. R. andRibbe, P. H. (1974)Determination of boron Ribbe, P. H. (1980)The humite series and Mn-analogs. In Paul in chondrodite by ion microprobe mass analysis. American H. Ribbe, Ed., Reviewsin Mineralogy,Volume 5, Orthosili- Mineralogist, 59, 1123-1126. cates, p. 231-274.Mineralogical Society of Arnerica, Washing- Hintze, C. (1915) Handbuch der Mineralogie. I(2). Leipzig, ton. D. C. Verlag Von Veit. Rogers,A. F. (1935)The chemical formula and crystal systemof Hurlbut, C. S, Jr. (1961)Tephroite from Franklin, New Jersey. alleghanyite.American Mineralogist, 20, 25-35. American Mineralogist, 46, 549-559. Ross,C. S. and Kerr, P. F. (1932)The manganeseminerals of a Jones, N. W., Ribbe, P. H. and Gibbs, G. V. (1969)Crystal vein near Bald Knob, North Carolina. American Mineralogist, chemistry of the humite minerals. American Mineralogist, 54, r7,l-r8. 391-4tl. Simmons,W. B. Jr., Peacor,D. R., Essene,E. J. and Winter, Lee, D. E. (1955)Mineralogy of someJapanese manganese . G. A. (1981)Manganese minerals of Bald Knob, North Caroli- Stanford University Publications,Geological Sciences, 5, l- na. Mineralogical Record, 12, 167-171. 64. Smith, W. C., Bannister,F. A., and Hey, M. H. (19,M)Banal- Momoi, H. (1980)Notes on hydrothermal synthes€sof allegha- site, a new barium-felspar from Wales. Mineralogical Maga- nyite group. Mineralogical Society of Japan Bulletin, 14, 179- zine,27,3l-46. 187. Tilley, C. E. (1951)The zonedcontact- ofthe Broadford Moore, P. B. (1970) Edge-sharing silicate tetrahedra in the area, Skye: a study of boron-fluorine metasomatismin dolo- crystal structure of leucophoenicite. American Mineralogist, mites. Mineralogical Magazine, 29, 621-666. 55. l146-1166. White,T. J. and Hyde, B. G. (19E2)Electron microscope studies Moore, P. B. (1978)Manganhumite, a new species.Mineralogi- of humite (chondrodite) seriesof minerals. II. Natural manga- cal Magazine, 42, 133-136. nese-rich specimens.Physics and Chemistry of Minerals, 8, Mossman, D. J. and Pawson, D. J. (1976)X-ray and optical 167-174. characterizationof the forsterite--tephroite serieswith White, T. J. and Hyde, B. G. (19E3)A description of the commentson knebelitefrom Bluebellmine, British Columbia. leucophoenicite family of structures and its relation to the CanadianMineralogist, 14, 479-4E6. humite family. Acta Crystallographica,in press. Nambu, M., Tanida, K., and Kitamura, T. (1980)Alleghanyite Winter, G. A., Essene,E. J. and Peacor,D. R. (1981)Carbon- from Hanawa Mine, Iwate Prefecture. Research Institute of ates and pyroxenoids from the manganesedeposit near Bald Mineral Dressingand Metallurgy,Tohobu University, 36,43- Knob, North Carolina. American Mineralogist, 66, 27E-2E9. 54. Yoshinaga,M. (1963)Sonolite, a new mangan€semineral. Mem- Palache,C., Berman,H. and Frondel,C. (1944)The Systemof oir of the Faculty of Sciences,Kyushu University, SeriesD, Mineralogy.Volume I. JohnWiley and Sons,Inc., New York. Ceology, 14, l-21. Palache, C. (1937) The minerals of Franklin and Sterling Hill, Sussex,New Jersey.United StatesGeological Survey Profes- Manuscript received, August 23, l9E2; sional Paper 180. acceptedforpublication, March 18, 1983.