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.