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Mn'humites from Bald Knob, North Carolina: Mineralogy and Phaseequilibriat

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Mn'humites from Bald Knob, North Carolina: Mineralogy and Phaseequilibriat American Mineralogist, Volume 68, pages 951-959,1963 Mn'humites from Bald Knob, North Carolina: mineralogy 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 manganese deposit of North Carolina.These minerals are the manganeseanalogues of chondrodite, humite 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 alleghanyite-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 norbergite,chondrodite, humite (1980) (1981)to have originally been a manganiferoussediment and clinohumite,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. Sonolite,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 (galaxite,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 tephroite, the manganese Two additional, closely-relatedmanganese end-mem- ber phases, (a olivine. Although P-T-X relations between olivine and leucophoenicite dimorph of manganhu- mite) jerrygibbsite (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 mineral 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 olivines. 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-monticellite,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 forsterite, 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 kutnohorite(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)-rhodonite(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 ore 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 <r 56 3.43 3.44 4rr L2 3.45 ManganhuDite 3.37 I2l 32 3.35 3.36 r22 3.35 ll3 85 3.r5 3. 14 6 3. 14 Alleghanyite 9 3.ll 3.ll ll5 3 3.1I 3.11 3lt I 3.OO 3.00 t24 5 3..04 3.04 3r2 3,00 026 3.03 212 r00 2.866 2.465 5l 2.472 2.874 130 100 2.869 2.876 l3l 2,864 I t4 75 2.U9 2.849 131 50 2.81I 2.810 116 r7 2.E15 ManganhuDite 52 2.777 2.776 130 l8 2.779 2.779 L32 4 2.743 UanganhuEite 2,776 r25 71 2.739 lt2 I6 2.725 2.723 008 7 2.727 Hanganhumite 53 2.701 2.704 o22 )\ 2-649 2,688 027 68 2.7M 2.707 024 50 2,672 2.672 I 33 68 2.65E 2.659 114 2.667 040 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.
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