American Mineralogist, Volume 73, pages 632-636' 1988

Holdawayiteoa new manganesehydroxyl-carbonate from the Kombat mine' Namibia

DoN.lLo R. Pnacon, Entc J. EssnNe, Ror..lt"lo C. Rousn Department of Geological Sciences,University of Michigan, Ann Arbor, Michigan 48109-1063, U.S.A. Pnrn J. DuNN, JosnPn A. Nnr,nN Department of Mineral Sciences,Smithsonian Institution, Washington, D.C. 20560, U.S.A- Jonr, D. Gnrcr Division of MineralScience, National Museum of NaturalHistory, Ottawa, Ontario KIA 0M8' Canada JoHN INNns 182Bielby Road, Kenmore Hills, 4069,Brisbane, Queensland, Australia Or,nc voN KNoRRTNG 2l ChurchwoodAvenue. Leeds, LS 16 5LF' England

Ansrru,cr Holdawayite is a new mineral occurring at the Kombat mine, Namibia. The chemical analysisis MgO 4.4, MnO 64.6, CaO 0.5, FeO 0.2, B2O31.2, CO2 l4'2, rI2O ll'47, Cl 4.4, less O = Cl 1.0, sum 100.1 wto/0,corresponding to the ideal formula Mnu(COr)(OH),(Cl,OH). Holdawayiteis monoclinic, spacegroup C2/ m, with a : 23'437 (5), b:3.3137(3), c: 16.618(6)A, P : 1ll.l5(2)', Z: 4. The most intensepowder X-ray diffraction lines are (d(A),t,hkl) 10.93(100)(201,200), 5.459(80) (400,202), 3.879(70)(602,004), 1.789(70)(1I.1.3,1 l7), 2.690(60)(51I,l13,512,312), 2.589(50) (606,513;006,3r4),7.77(40)(202,002),2.s26(40)(802,602),2.792(40)(313),and 1.657(40) (020). It occurs principally as subhedral grains in manganeseores and rarely as fibrous, silky aggregates.It is pink and translucent and has a vitreous luster. The observed and calculateddensities are 3.19(4) and 3.24(3) g/cm3,respectively. Optical properties include biaxial(-);2V:12(3)'(meas.),18"(calc.);a:1.644(l),P:l'719(l)'7:l'721(l); moderate dispersion, r < v; X : b, Z n c : 45(3)'. Holdawayite forms coarse veins cutting silicate-faciesMn-bearing rocks with assem- blagesincluding humite- and/or leucophoenicite-groupminerals. Idealized Cl-free holda- wayite, Mn'(COTXOH)0,may form by hydration of rhodochrosite and in turn may oxidize to high". oxides and hydroxides of Mn. It has been named in honor of Dr. Michael J. Holdaway, professorof petrology at Southern Methodist University'

INrnonucrroN National Museun of Natural History, Ottawa, under NMNS no' 51510' A locally abundant pink carbonatewas found in the Kombat mine in Namibia in 1982 but could not be iden- occunnBNCs tified as a known mineral. The hypothesis that it might be a new specieshas been borne out by subsequentin- Holdawayite occurs at the Kombat mine, 37 km east vestigations,and its characteization is presentedherein. of Otavi and 49 km south of Tsumeb, Namibia. The We have named this new mineral holdawayite in honor geologyof the mine was describedby Innes and Chaplin of Dr. Michael J. Holdaway, professor of petrology at (1986). All known samplesof holdawayite were collected SouthernMethodistUniversiiy,inrecogrritionofhismany betweenthe 1238- andl24l-melevationsintheEl5-11 researchcontributions to mineralogy ind petrology, and South stope on I I level of the Asis West sector of the especiallyfor his efforts on behalf of the society of min- mine. The original sampleswere collected on August 2, .tutogitti as editor of the American Mineralogisl. It is 1982, and the exposurewas accessibleuntil early in Sep- partiJuhrly appropriate that this mineral occursas a ma- tember, 1982. Holdawayite was restricted in occurrences jor rock-forming mineral of petrologic significance.The to the footwall zone of a complexly deformed body of ipecies and the name were approvedby the Commission tectonically intercalated manganeseand iron ores, which on New Minerals and Mineial Names, IMA, prior to are locally dominated by carbonate- and silicate-facies publication. Type material is deposited at the Smithson- rocks rather than by the more usual hausmannite ores. ian Institution under catalogue no . 163209 and at the Holdawayite occurred typically as an anastomosingmesh 0003404x/88/0506-0632$02.00 632 PEACORET AL.: HOLDAWAYITE 633

of veinlets, I to 8 mm in thickness,mainly in a hematite- has more Mg than other analyzed holdawayite, but still carbonatematrix and also as patchesand blebs I to 4 cm has Mn >> Mg. The fibrous holdawayite occursonly with in diameter. It locally formed up to 40 volo/oof the ore. the ribbeite * galaxite assemblagedescribed above. It is estimatedthat ca. 200-300 metric tons of holdaway- pRopERTIES ite-bearing material was removed from this stope during Prrvsrcar- AND oprrcAr, the course of mining. Holdawayite occursas subhedralcrystals approximate- The mineral was not encounteredelsewhere within this ly 2 cm in diameter in the holotype sample. It is trans- manganeseorebody nor within the other lenses in the lucent and light to bright pink in color with a vitreous Kombat mining field. However, it is noteworthy that at luster; fresh, glassy material resemblessome rhodonite. lower elevationsin the El5-11 S body, the relatedmin- It gradually changesto a brown color on exposureand in eral defernite (Sarp et al., 1980)is abundant within haus- some caseshas developeda thin black sooty coating that mannite-barite ores. masks the true color. In an unusual occurrence,fibrous Occasionally, the veins of holdawayite mimic the ore holdawayite with a silky luster occurs as a coating on foliation and the tectonically transposedmineral protore massive material. The streak is light pink, and there is layering. The veins clearly postdate the major foliation no discernible fluorescencein ultraviolet radiation. The event but are also deformed. There is an abundance of hardness(Mohs) is approximately 3, and the mineral is curvilamellar cleavages developed in the holdawayite moderatelybrittle. Cleavageis perfecton { 100}, and there crystal grains. In one instance, a crudely encrusted vein is an irregular fracture. The density is 3. l9(4) g/cm3, mea- of coarselycrystalline magnesiansiderite carrying blebby sured with a Berman balance,compared with the calcu- pyrite and chalcopyrite was clearly observed to crosscut lated value of 3.24 g/cm3.Holdawayite efervescesweakly the ore foliation in a l5-m-thick zone of tectonically in- in dilute HCl. It is stained light pink with an alizain- tercalatedfeldspathic sandstone,calcitic dolostone,black red-dilute HCI mixture. argillite, and layered iron and manganeseores. Where Optically, holdawayite is biaxial negative, .uv.rrh2V : this l0-cm-thick vein cuts the zone of manganeseores, l2(3)' (meas.),I 8' (calc.),and a : | .644(l), P : 1.7l9(l), holdawayite was the only vein mineral present.Only he- and 7 : 1.721(l), measuredin Na light. Dispersionis matite and carbonates(siderite, dolomite) have been not- moderate r < v. No pleochroism or color was observed ed in immediate associationwith holdawayite. Lensesof in thin section.The orientation is X : b, Z n c : 45(3)" silicate-faciesmanganese ores within a hematite + hold- in the obtuse angle ofbeta. awayite matrix contain the assemblagesribbeite + py- rochroite + calcite * mcgovernite-like mineral (Peacor X-n-q.v CRYSTALLoGRAPHY et al., 1987a),ribbeite * clinochlore + jacobsite * galax- Weissenbergand precessionphotographs obtained us- ite + calcite * mcgovernitelike mineral, alleghanyite + ing cleavagefragments showed that holdawayite is mono- galaxite + jacobsite * calcite, and alleghanyite + jacob- clinic, with extinctions consistentwith spacegroups C2, site + kutnahorite + galena + hydroxylapatite. Although Cm or C2/m. A determination of the crystal structure it is difficult to infer the relationship betweenholdawayite (Peacorand Rouse, 1988)shows C2/m to be the correct and any of these assemblages,there is little evidence of choice.Lattice parametersas determined by least-squares replacementor reaction with the matrix minerals at the refinement of powder X-ray difraction data are a : edgesof the holdawayite veins and patches. Layers of 23.437(5), b : 3.3137(3), c : l 6.6l 8(6)A, B : l l l. l 5(2)" oxide-faciesore contain the assemblagehausmannite + V : 1203.7(q 4,, Z: 4. The powder X-ray diffraction barite + calcite + pyrochroite * manganosite,but hold- pattern was obtained using a I14.6-mm-diameterGan- awayitewas not noted in immediate associationwith these dolfi camera,FeKa (Mn filter) radiation, Si as an internal phases. standard, and a multicrystal mount. The data are listed The holdawayite veins are composedof monomineral- in Table 1. ic granular aggregatesof the mineral, with grains I to 5 Defernite was originally describedby Sarp et al. (1980) mm in size and usually showing marked elongation and ashaving the compositionCau(COr)r(OH,Cl)r.n HrO and subparallelalignment ofcleavage surfaces.Rocks broken as being orthorhombic with spacegroup Pna2, or Pnam along the veins expose faces up to 30 cm square com- and with latticeparameters a: 17.860(5),b: 22.775(6), posed of virtually pure holdawayite that, when freshly and c : 3.653(l) A. fns unit cell has featuresthat are exposed,is of a deep red-pink color and transparent to very similar to those of holdawayite, especially in the translucent in thick cleavagefragments. equivalenceof the small value of c as compared to D of Many holdawayitesamples appear to have beenleached holdawayite, and the large values of the other two trans- by hydrothermal solutions with the resultant formation lations; i.e., both cells have unusual, tabular shapes.Lie- of a pink, fibrous mineral with a silky luster that appears bich and Sarp (1985) determined the crystal structure of to be in parallel growth with, and to grade into, massive defernite, but their determination appearsto be unsatis- holdawayite. This material was found by single-crystal factory in that the apparent formula derived from the X-ray diffraction methods to be holdawayite elongated structureis Cau(COr)r(OH,Cl)r.nHrO,which is not charge- parallel to [0 I 0]. Electron-microprobeanalysis confirmed balanced. Nevertheless,it is closely related to that of this identification and showed that the fibrous material holdawayite, Mn.(COr),(OH)?(CI,OH). The relationship 634 PEACORET AL.: HOLDAWAYITE

TneLe1. PowderX-ray diffractiondata for holdawayite

0o" 10.99 2o1 5 2.117 2.117 715 100 10.93 2.084 116 10.93 200 2 2.079 202 2.077 408 40 7.77 7.78 7.75 002 2.043 608 5.465 400 2.041 712 80 5.459 2.040 5.460 202 2.039 913 4 693 403 2.038 208 4.680 2.027 605 4.654 401 20 2 019 3.880 602 2.018 911 70 3.879 3.875 004 2 1.959 1.959 910 10 3.640 3.643 600 1 1.890 1.890 12.0.1 10 3.302 3.297 204 5 1.847 1.847 717 ,l 3.077 a n7e 112 1_823 515 2.929 602 10 1 820 1.822 12.0.0 40 2.926 2.917 602 1.820 606 1.792 i i.1.3 2.883 803 70 1.789 30 2.879 2.881 311 1.790 117 1.754 11.1.1 2.874 801 1.752 2.838 2.837 113 1.750 12.0.7 2.793 313 1.740 917 40 2.792 5 1.739 2.750 406 1.738 12.O.'l 1.713 718 30 2.749 2.748 804 2b 1.710 z.t 4c 206 1.709 i 18 2.703 511 1.682 11.1.6 1.677 913 2.702 113 1.678 2 690 2.685 F;12 1.673 516 2.679 312 1.670 317 z.o5I 2.641 510 40 1.657 1.657 020 z.cYc buo 1.618 2.0.10 2.591 513 20 1.616 1.613 11.1.7 50b 2.589 1.612 519 2.583 006 '1.602 2.574 314 10 1 602 319 2.450 313 10 1.588 2.443 2.444 514 c 1.573 2.344 315 2 1.540 2.343 2.339 10.0.3 1 1.526 2.295 2.295 607 1.473 2.281 10.0.1 1.460 277 'lb 2 2.276 604 1.446 2.193 115 5 1.419 2.190 2.186 10.0.0 1 1.408 30 2.166 2.168 711 2 1.396 Note:Thed"","valuesarebasedontheunit-cellparameters a:23.437(5),b:3.3137(3),c:16.618(6)A,P:111.15(2)'Reflectionswereindexed with the aid of the structure-tactordata (Peacor and Rouse, 1988). The symbol "b" indicatesa broadenedline

between defernite and holdawayite has been confirmed uscn Camebax electron microprobe with a 15-kV oper- by the solution of the holdawayite structure and compar- ating voltage and a 0.01-pA beam current. Tests for F, ison of the structure with that of defernite. Each structure Al, Ti, Zn, andAs yielded none at the detection limits of has atoms in layers with coordinates in adjacent layers 0.1 wto/0. differing by 0.5 in the narrow slab direction, with units Water was determined using the Penfield method. In consisting of edge-sharingCa or Mn octahedra knit to- this procedure,the sample,mixed with flux to aid decom- gether by CO. groups, and separatedby large zeolitelike position, is placed in a bulb at the end of a borosilicate voids. glasstube. The tube itself is wrapped with a piece of wet cloth, and the water condensesas the bulb is heated. At Cnntlrrc,lr, coMPosITIoN the end ofthe heating period, the bulb is cut offusing a Holdawayite was chemically analyzed using a combi- torch while at the sametime the tube is sealedat the end. nation of procedures.The elementsMn, Mg, Ca, Fe, and The total water collected is determined by weighing the Cl were obtained by microprobe analysis, employing an tube before and after drying. ARL-sEMeelectron microprobe utilizing a 15-kV operat- The C was analyzed with a Leco low-C analyzer. In ing voltage and a 0.025-pA sample current measuredon this determination, the sample is heated under a stream brass. The standards used were manganite (Mn), horn- of oxygen in a induction furnace. The CO. is adsorbed blende (Ca, Fe, Mg), and chlorapatite (Cl). The data were on a molecular sieve material in a trap while the oxygen corrected using a modified version of the uectc-4 pro- passesthrough. At the end ofthe burning cycle, the trap gram. The coexisting minerals were analyzed with a ce- is heated,and the CO, flushed out with He. The conduc- PEACORET AL.: HOLDAWAYITE 635

tivity of the helium-CO, mixture is measuredagainst pure the thin section reveals that it is sussexite,and micro- He; a calibrated digital voltmeter servesto indicate the probe analyses are consistent with the formula total micrograms of C produced. (Mno,, Mgo ,r)BOr(OH) with tracesof Ca and no detectable For the B analysis, the sample was fused with potas- F, Na, Al, Si, Cl, Ti, Fe,Zn, or As. This phasecommonly sium carbonate,the B extracted and separatedfrom in- coatsand replacesthe holdawayite and may be the source terfering elements and COr. After careful neutralization of the B reported in the bulk analysisof holdawayite. The of the solutions, the B was titrated with NaOH in the sussexiteand holdawayiteveins also contain small amounts presenceof mannitol. The B content of holdawayite may of an opaque,anisotropic Mn mineral. Microprobe anal- be due to contamination by the borate mineral sussexite ysis yields 7l wto/oMnO and 6 wto/oMgO. The opacity of (seebelow). this mineral suggeststhat it has Mn3+ andlor Mna+' the Theseanalyses yielded the valuesMgO 4.4, MnO 64.6, low totals of the chemical analysis imply that it contains CaO 0.5, FeO 0.2, B2O31.2, CO2 14.2,HrO I 1.47,Cl Mna* and/or HrO. Unfortunately, the small amount of 4.4, lessO = Cl 1.0,total 100.I wt0/0.The empirical for- this phaseprecludes further characteization by X-ray dif- mula, basedon (O + OH + Cl) : 8 is [Mnrurlr4go:,C?oor-fraction. Fsoo,],rr,[(CO3)or]@Or)o,ol'0, [(OH)ru, Clorr]"ooo. The crystal-structure analysis shows that Cl is ordered in a single site, giving rise to the idealized formula DrscussroN Mnu(COr)r(OH)?(Cl,OH),for which Z : 4. If ideal holdawayite is consideredto be MnrCOr(OH)o, several simple reactions relate it to other Mn minerals. PBrnocn-q.pHy Holdawayite may form from rhodochrositeor pyrochroite The sample containing holotype holdawayite and its by the reactions matrix, from level I I in the Kombat mine, was selected 3MnCO. + 2H2O: MnrCOr(OH)4+ 2COr; for detailed petrologic study. A polished thin section was 3Mn(OH), + CO,: Mn.CO.(OH)4+ HrO. examined optically, with backscattered-electronimaging and with quantitative electron-microprobe analysis. A Some evidence for the first reaction is provided by ob- matrix of fine-grainedkutnahorite hosts polysyntheticalty servations of holdawayite veins and patchesin the rock twinned porphyroblastsof a humite-like silicate mineral. with a matrix of manganesecarbonates. If ideal holda- Microprobe analysisof the silicategives (Mno rlvl1,s&r,ooo wayite has a stability field in the system MnO-COr-HrO, Caoor)SirOr(OH,rrFoor),which correspondsto the com- topological constraints require that its stability field will position of ribbeite or alleghanyite. The observation of intervene between those ofpyrochroite and rhodochros- macroscopictwinning suggeststhat this mineral is mon- ite. Although pyrochroite has been reported from the oclinic alleghanyiterather than orthorhombic ribbeite. In- Kombat mine (Peacoret al., 1987a),its textural relations deed, powder X-ray diffraction ofa different spot on this to holdawayite have not yet been observed. sample confirms the presenceof alleghanyite.The rock is Holdawayite may also become oxidized to higher ox- studded by small euhedral manganian jacobsite ides of Mn, for instance, hausmannite (MnrOo) or man- [(Mnfr{rMgo,r)(Fefj.Mnfll0Alor?)O4] and fine subhedral ganite (MnOOH), by reactions such as galena with rare specks of native copper and a copper 2MnrCO.(OH)o* 2MnrOo sulfide. Small euhedral grains of hydroxylapatite Or: + 4HrO + 2COr; 4Mn,COr(OH)o+ 30,: l2MnOOH + 2H,O + 4CO,. [(CaonrMno or)s(POo)j(OH)0 ruFo oo] are scattered through the matrix carbonate. Larger anhedral kutnahorite Oxidation of holdawayite to form higher oxides of Mn is [(Cao'MnoouMgo or)COr] occasionally contains tablets of supported by direct observations of its alteration to the calcianrhodochrosite [(Cao ,,Mno rn)COr],and the two may opaque oxide in the rock. More complex reactions in- collectively representthe two limbs of a solvus (Peacoret volving various Mn silicatesmay also be written but re- al., I 987b).Energy-dispersive analysis ofclusters ofa phase main speculative without direct observation or through having a high atomic number revealsmajor Ce and minor calculation with an adequatethermodynamic data base Ca, Pb, and Th; it is presumedtentatively to be bastniisite. involving ideal holdawayite.Nevertheless, if ideal Cl-free Severalgenerations ofveins are evident in the sample. holdawayite has a stability field, it would dominate other Early veins of coarseholdawayite irregularly traverse the manganeseoxides, carbonates,and oxyhydroxides at low sample apparently accompanied by replacement of the temperatures, at relatively reducing conditions, and at matrix by patchy holdawayite. Kutnahorite, apatite, leu- moderate pressuresof CO, and HrO. The presenceof Cl cophoenicite,and jacobsite are preservedwithin and im- in holdawayite will affect the idealized reactions by the mediately adjacent to the holdawayite vein without evi- productionof HCI at the expenseof someHrO, e.g., denceof reaction. Veins and patchesof barite and, more 3Mn,(COr)r(OH)?Cl+ 9Or: 7MnrO4 + HCI rarely, witherite cut the host rock. Late veins ofa trans- + 3H,O + 6CO,. parent, light-brown, fibrous Mn mineral commonly tra- verse the matrix and replacethe holdawayite veins along The resultant stability of Cl-bearing holdawayite is thus grain boundaries. Debye-ScherrerX-ray diffraction pho- controlled by the relative fugacitiesof four fluid species. tographs of a small amount of this phaseextracted from If Cl is indeed essentialto the structure of holdawavite. 636 PEACORET AL.: HOLDAWAYITE then its stability field will be extendedby the increasein Peacor,D.R., and Rouse, R.C. (1988) Holdawayite, Mnu(COr)r- (OH),(CI,OH), containing anions in zeolitelike channels H,O and CO,. a structure fugacity of HCI relative to American Mineralogist, 73, 637-642. Peacor,D.R., Dunn, P.J.,Su, S.-C, and Innes,John. (1987a) Ribbeite' a AcrNowr-nocMENTS polymorph of alleghanyiteand member of the leucophoenicitegroup Mineralogist, 72' 213- We are grateful to the managementof the Tsumeb Corporation Ltd from the Kombat mine, Namibia. American and Mr R C. Chaplin, the geologicalsuperintendent, for facilitating the 2t6 A.M' (1987b)Petrological and collection and transport of samplesfrom the Kombat mine. Peacor,D.R., Essene,E.J., and Gaines, crystal-chemicalimplications of order-disorderin kutnahorite RrrBnr,Ncns crrBo CaMn(CO)' American Mineralogist, 72' 3 19-328. Sarp, Halil, Taner, M.F., Deferne, J., Bizouard, H, and Liebich, B'W' Innes,John, and Chaplin,R.C. (1986)Ore bodiesof the Kombat mine, (1980)La defernite,Cau(COr)r(OH,Cl)r'nH'O, un nouveaucarbonate South West Africa/Namibia. In C.R. Anhaeusserand S. Maske, Eds', de calciumchloro-hydroxyl€. Bulletin de Min6ralogie,103, 185-189' Mineral depositsofsouthem Africa, vol. 2, p. 1789-1805.Geological Societyof South Africa, Johannesburg. Liebich, B.W , and Sarp, Halil. (1985) La structure cristalline de la defer- nite, SchweizerischeMineralogische und PetrographischeMitteilungen, MlNuscnrpr REcEIVEDJulv l, 1987 65.153-158. MaNuscnrpr AccEprEDJexuenv 20, 1988