American Mineralogist, Volume 72, pages213-216, 1987

Ribbeite, a polymorph of alleghanyiteand member of the leucophoenicitegroup from the Kombat mine, Namibia

DoN.q,Lo R. Pucon Department of Geological Sciences,University of Michigan, Ann Arbor, Michigan 48109, U.S.A. PBrn J. DUNN Department of Mineral Sciences,Smithsonian Institution, Washington, D.C. 20560, U.S.A. Srru-CnuN Su* Departmentof GeologicalSciences, Virginia Polytechnic Institute and StateUniversity, Blacksburg,Virginia 2406 l, U.S.A. JoHN INNBs MineralResearch Laboratory, Tsumeb Corporation, P.O. Box 40, Tsumeb,Namibia 9000

Ansrnlcr Ribbeite, Mn,(OH)r(SiOo)r, is a new mineral from the Kombat mine in Namibia. It is orthorhombic with spacegroup Pbnm or Pbn2.,,and with lattice parametersa: 4.799(l), b : 10.742(6),c : 15.70(D A, Z: 4. Chemicalanalytical data are 5iO224.3, FeO 0.3, MgO 5.2, MnO 65.1, CaO 0.2,H2O 4.9 (by difference),sum: 100.000/0.Optical data included : 1.780(3),P : 1.792(3),r : 1.808(3),2vz(calc.) : 82.5;X : bo Y : a, Z : c; X, Y: colorless,Z:light pink; absorptionZ > X: Y. It is inferredto be a member of the leucophoenicite group having octahedron sequence(1,22)2 and is the unit-cell-twinned equivalent of alleghanyite. It occurs in Mn-rich silicate-carbonateunits within lensesof manganeseoxide ores as equigranular grains associatedwith alleghanyite, manganoan calcite, pyrochroite, chlorite, galaxite,jacobsite, and a mcgoverniteJike mineral.

INrnonucrroN ternal standard,FeKa (Mn-filtered) radiation, and a pow- During the course of mining at the Kombat mine in dered sample. The pattern has the generalappearance of Namibia, several rare and interesting mineral specieswere those of the Mn-humites and other members of the leu- encountered.At least four of these apparently are new cophoenicite group in the most intense lines, and espe- minerals, and one was found to be related to the humite cially in the grouping of intense reflectionswith d values and leucophoenicite groups. Detailed investigation has near 2.8 A. Althougtr mixtures of such phases would confirmed that it is a new mineral, the unit-cell-twinned thereforebe difficult to characterize,patterns ofpure rib- equivalent of alleghanyite. We have named this new beite are easily identified as can be seenby comparison speciesribbeite in honor of Dr. Paul H. Ribbe in recog- of the data of Table I with the powder-diffraction data nition of his many contributions to mineralogy, both to for the Mn-humites as given by Winter et al. (1983). the sciencethrough his research,and to the society of Single-crystalWeissenberg and precessionphotographs mineralogists,in part through his editorship of the Re- showed that ribbeite is orthorhombic with spacegroup : viewsin Mineralogy seriesof the Mineralogical Society of Pbnm or Pbn2,. The unit-cell parametersla 4.799(l), America. It is particularly appropriate that ribbeite is b: 10.742(6),c: 15.70(DA, V: 809.2(5)A'1 were closelyrelated to the humite group, to which he has made obtained by least-squaresrefinement of the powder-dif- major contributions. The new speciesand the name have fraction data: Z: 4 for this cell. The lattice parameters been approved by the Commission on New Minerals and of ribbeite (r) are related to those of monoclinic allegha- Mineral Names, I.M.A. Type material is depositedin the nyite (al), a member of the humite group, by the relations : : : Smithsonian Institution under catalogue no. NMNH e, a,t, b, b", c, 2c sin 8.,. Becausealleghanyite and l 63208. ribbeite have the samecomposition, they are polymorph- ically related. X-ru.v cRysrALLocRApHy White and Hyde (1983) have shown that the structures powder X-ray diffraction data (Table l) were recorded of members of the leucophoenicitegroup are related to with a I14.6-mm-diameter Gandolfi camera,Si as an in- those of the humites. Each group has a characteristicpat- -* tern of sequencesof edge-sharingoctahedra, which they p.r-urr*t address:Institute of Geology,Chinese Academy designatedas (3, 2') for humites and (1, 2') for leuco- of Sciences,Beijing, People's Republic of China. phoenicite group minerals. Yau and Peacor (1986) have 0003404x/87l0102-o213$02.00 2t3 2t4 PEACOR ET AL.: RIBBEITE

Table 1. Powder X-ray diffraction data for ribbeite Table2. Opticalproperties for ribbeite

ilh Pleochroism X Y colorless Z lightpink 5 5.08 5.08 o21 10 1.769 AbsorptionZ> X: Y 30 4 41 4.43 o22 20 1.744 Optical orientation X:b,Y:a,Z:c 4.38 110 10 1.720 Optic axial angle2vz fl486 nm): 80.1'(2) 2 4.23 4.22 111 10 1.695 q589 ,l nm): 81.3'(2) 3.93 3.93 004 20 1.661 q656 nm):81.8"(3) 30 3.83 3.83 112 5 1618 r > v moderate J 3.53 3.54 103 2 1.587 Refractiveindices (at 589 nm) a : 1.780(3) 3.49 3.49 121 5 1.569 B :1,792(3) 10 3.362 3.359 113 30 1.554 r: 1.808(3) J 3.256 3.256 122 10 1.490 zvz@alc.\:82.5' 70 2.925 2.923 114 20 1.460 80 2.873 2.870 130 1 1.441 Nofe: Standard errors are given in parenthesesand refer to the 70 2.821 2.823 131 5 1.415 last digit. bU 2.69s 2.695 132 20 1.378 80 2.552 2.552 115 20 1.304 40 2.515 2.516 133 1 1.277 2.398 2400 200 2 1.2262 40 2.3s6 2.360 125 5 1.2034 tical properties, which were measured on a single grain, z.5az 026 5 1.1894 are listed in Table 2. 10 1.1810 2.315 2.31I 141 10 1.1728 2.317 134 1 1.1599 Crrnrvrrcl'r- coMPosITroN 2 316 211 1 1.1529 the associatedminerals were chemically 10 2.246 2.246 116 2 1 1437 Ribbeite and 2.246 142 2 1.1210 analyzed using an ARL-sEMQelectron microprobe with 2.244 212 5 1.0996 operating voltage 15 kV and sample cunent 0.025 pA, 2 2216 2.216 o44 20 1.0861 5 2117 2.118 135 10 1.0695 measuredon brass.A wavelength-dispersivemicroprobe 'I 1.996 2,011 214 scanfailed to detectelements with atomic number greater 1 996 117 than 9 except those reported here. Special care was used 1.993 230 1 1 834 1.836 153 for F, but none was detected.Standards for the analysis 100 1 796 1.797 225 of ribbeite and alleghanyitewere synthetic tephroite (Mn, 1 7Q1 118 Kakanui hornblende (Fe, Mg, Ca). Standardsfor 1.790 060 Si) and 1.789 240 the analysisof associatedspinels were magnetite(Fe), spi- (Mg, manganite (Mn), and zincite (Zn). The as- Note. lntensities visually estimated from Gandolfi photo- nel Al), graph: d valuescalculated with latticeparameters a: 4.799, sociated chlorite was analyzed with the standardshorn- b : 10.742.and c : 15.70A. blende (Si, Al, Fe, Mg) and manganite (Mn). The data were correctedusing standard Bence-Albeefactors. The analyticaldata for ribbeiteare SiO, 24.3,FeO 0.3, further shown that the sequencesfor any two poly- MgO 5.2, MnO 65.1, CaO 0.2, with HrO 4.9 (by differ- morphs, one of which is monoclinic and one orthorhom- ence). sum : 100.00 wt0/0.Calculation of the chemical bic, differ simply by addition of a unit-cell-twinning formula on the basis of l0 oxygen atoms gives mechanism (this term is used in the sensedefined by Ito, (MnoroMgo uoFeo orCao or)o nosir noHr roO,o. The ideal for- 1950) to the monoclinic phase;where the sequenceof a mula is inferred to be Mn'(SiOo)r(OH)r, identical to that humite is (3, 2'), the sequencein the equivalent leuco- of alleghanyite. These and other data given above con- phoenicitegroup mineral is (1, 2'*'). The lattice param- firm that ribbeite and alleghanyiteare polymorphs. eters of alleghanyiteand ribbeite as compared above are consistentwith such a relation. Ribbeite is thus inferred OccunnBNcE to have the octahedronsequence (1,2')'and is the unit- Ribbeite has been found only at the Kombat mine, 49 cell-twinned equivalent of alleghanyite,which has the se- km south of Tsumeb, Namibia. The ore consistslargely quence (3, 2). The second superscript in the ribbeite se- of copper and lead sulfides deposited in essentiallyun- quence shows that the basic repeat must be doubled in metamorphoseddolomite (Innes and Chaplin, 1985).The order to achieve translation periodicity. dolomite contains lensesof iron and manganeseoxides, the former dominated by magnetiteand hematite and the PHvsrcl'l, AND oPTrcAL PRoPERTTES latter by hausmannite, barite, alleghanyite, manganoan Ribbeite is transparent and pink and has a light pink calcite, and pyrochroite. The type specimen of ribbeite streak. The luster is vitreous; cleavagewas not observed; was found in one of the manganese-richlenses in August, the hardness is approximately 5 (Mohs'). The density, 1982,in the El5-ll Southstope, ll-level, l24l-m ele- measuredusing heavy liquid techniques,is 3.90 (5) g/cm3; vation of the Asis West sector of the Kombat mine. Our D,n :3.84 g/cm3. Ribbeite forms fine-grained,anhedral description is restricted to the type sample. granular aggregatesof crystals approximately 0.5 mm in Ribbeite occursas pinkish lensesup to 5 cm thick and diameter with a typical granulitic texture. Optically, rib- 20 cm long within a 20-m-wide zone of tectonically in- beite is biaxial positive and weakly pleochroic. The op- tercalated manganeseand iron ores. Locally within the PEACORET AL.: RIBBEITE 215 stope environs, the complexly deformed manganeseand quently twinned, some grains having several twin do- iron ores assumethe attitude of a vertically plunging cy- mains; it is then readily identified. The alleghanyitehas lindroidal fold in which ribbeite has a preferential asso- a composition identical within error to that of ribbeite, ciation with carbonate-silicate facies within the man- exceptthat it contains 0.30/oF; we thereforedo not report ganese ores. The lenses are derived by tectonic it separately. transposition and boudinaging of the original sedimen- Although it was not possible to identify all grains as tary protore layering. The area containing ribbeite mea- ribbeite in one areaofthe specimen,and all asalleghanyite sured only ca. 40 cm acrossas exposedon the stopeback. in the other, no exceptionswere found to their occurring Ribbeite can be regardedas a scarcemineral. The trans- in separatedomains in which identifications were made. posedlayering is transgressedby an abundanceofveinlets The textural relations thus imply that large numbers of of a proposed new manganesehydroxy-carbonate that ribbeite grains separately formed in a prograde meta- clearly postdate the major tectonism but are themselves morphic environmentonly millimeters from the unit where slightly deformed. Of interest also is the occurrence of alleghanyiteof the same composition formed. manganositeas a constituent (to 5oloby volume) of some Yau and Peacor(1986) showed that the orthorhombic hausmannite-pyrochroite-barite-calcitelayers within the polymorph of either the leucophoeniciteor humite group Mn ores but not immediately associatedwith the rib- could be derived by unit-cell twinning of the monoclinic beite-alleghanyitelayers. The manganositeis altering to form of the other group. Becausethe twin operation is pyrochroite and hausmannite and occurs as ragged em- associated with the (OH, D ligands of octahedrally erald-greenrelicts up to 2 cm in diameter. The textural coordinated Mn, and becausethe Mn-bearing humites evidence strongly suggeststhat manganositewas a syn- invariably contain F and the leucophoenicitegroup min- genetic precursor of the hausmannite ores. erals do not, Yau and Peacor suggestedthat leucophoe- All of the minerals intergrown with ribbeite occur as nicite-group minerals formed in the absenceof F. There approximately 0.5-mm-diameter grains. There are four is indeed no detectableF (<0. l0l0)in ribbeite. However, distinct bands in the type specimen that seem to have there is an indication of only a minor amount of F (0.30lo) been inherited from a sedimentary protolith, each dom- in the associatedalleghanyite, implying that the relative inated by different minerals. The first is a 3-mm-wide activities of OH and F may not be the determining factors band composed primarily of ribbeite and also having in stabilizing members of one group relative to another. subhedral pyrochroite (containing 7.80/oMgO), which is Dunn (1985) has also pointed out that membersof the preferentially oriented with layers approximately parallel leucophoenicitegroup are limited in their Mn contents, to the principal zoning, and a mcgovernite-like mineral. especiallyin that they usually contain measurableamounts This grades moderately sharply into a 2-cm-wide band of Ca, Zn, or Mg substituting for Mn. We have recently composedlargely ofribbeite and chlorite, with minor spi- observed that for jerrygibbsite from the Kombat mine. nels and calcite. The chlorite, which is not visible in hand Whether or not membersof the leucophoenicitegroup are specimen,consists of subhedralcolorless laths in thin sec- stabilized relative to humite-group minerals by the ab- tion. Thecompositionis(Mg roMno oo Fe. 0s Alr 3o)5ee(Si2 67- senceof F or the presenceof cations other than Mn cannot Alr 34)4.,O,.(OH)'. The lathshave a preferredorientation, now be determined. with (001) parallel to the mineral zoning. Two spinels Winter et al. (1983) showedthat the occurrencesof the occur, both as separatesubhedral to euhedralgrains or as various Mn-bearing humites and tephroite are principally irregular intergrowths. One is opaque, black, aluminian, controlled by the activities of SiO, and HrO. They pre- manganianjacobsite and the other is transparent,orange, dicted that the Mn analogueof norbergite might be found zincian, magnesiangalaxite. The compositions of two ad- in environments with relatively high a'ro and in equilib- jacent grains having the highest Zn content are (Mnonu- rium with alleghanyiteor pyrochroite. Becauseribbeite is Mg oo[Fe,,rMno rrAlo rr)Oo and (MnoouZno rrM& ,rXAl, so- polymorphic with alleghanyite,the Mn analogueof nor- FeoouMnooo)O.,respectively. There is no evidencefor bergite might occur with ribbeite and pyrochroite. The exsolution at optical-microscope levels of resolution. leucophoenicite-groupmineral grains occurring adjacent Manganoancalcite of composition (Ca"rnMno,nM& or)CO, to the pyrochroite and within millimeters of ribbeite were is intimately intergrown with ribbeite. The mcgovernite- thereforecarefully studied using optical, powder xno, and like mineral occurs sparingly. electron-microprobeanalyses, but they were confirmed to The band containing ribbeite gradescontinuously into be ribbeite. The absencein this paragenesisof the Mn a 2-mm-wide band of manganoancalcite with composi- analogueof norbergiteor of a leucophoenicite-grouppoly- tion (Cao,rMno,rMgoo,)CO.,nearly identical to that of the morph of sucha phaseimplies that they may not be stable calciteassociated with ribbeite. Thesedata imply that this phases,at least at the Mn/(Mn * Fe + Mg) ratios ob- band is of primary metamorphic origin rather than being served. a late-stagevein. The calcite unit in turn grades into a band consisting almost entirely of equigranular allegh- anyite, but with some galaxite,jacobsite, and calcite. The AcxNowr-nncMENTS alleghanyitemay be differentiatedfrom ribbeite only with We thank E. J. Essenefor his reviewof the manuscript.We difrculty in thin section.However, the alleghanyiteis fre- aregrateful to themanagement of the TsumebCorporation Ltd. 216 PEACORET AL.: RIBBEITE and R. C. Chaplin, the geologicalsuperintendent, for facilitating Ito, T. (1950) X-ray studieson polymorphism. Maruzen, Tokyo. the collection and transport of samplesfrom the Kombat mine. White, T.J., and Hyde, B.G. (1983)A descriptionof the leuco- phoenicite family of structuresand its relation to the humite family. Acta Crystallographica,B39, 10-17. RnrnnnNcns Winter,G.A., Essene,E.J., and Peacor,D.R. (1983)Mn-humites from Bald Knob, North Carolina: Mineralogy and phaseequi- Dunn, P.J.(1985) Managanese humites and leucophoenicitesfrom libria. AmericanMineralogist, 68, 951-959. Franklin and Sterling Hill, New Jersey:Parageneses, compo- Yau, Y.C., and Peacor,D.R. (1986) Jerrygibbsiteleucophoeni- sitions, and implications for solid solution limits. American cite mixed layering and generalrelations betweenthe humite Mineralogist, 7O, 379-387 . and leucophoenicitefamilies. American Mineralogist, 71, 985- (1985) Innes,J., and Chaplin, R.C. Ore bodiesof the Kombat 988. mine, South West Africa/Namibia. In C.R. Anhaeusser,Ed. Mineral deposits of southern Africa. Geological Society ofSouth Mnwuscnrpr REcETVEDApnn 2, 1986 Africa SpecialPublication, in press. Mem;scnrrr AccEprEDSerrsvnnR. 2, 1986