American Mineralogist, Volume 58, pages 733-735, 1973 NickelMinerals from Barberton,South Africa: Vl. Liebenbergite,A NickelOlivine SvsneNoA. nn Wnar Nationnl Institute lor Metallurgy, I Yale Road.,Milner Park, I ohannesburg,South Alrica Lnwrs C. ClI.x Il. S. GeologicalSuruey, 345 Middlefield Road, Menlo Park, California 94025 Abstract Liebenbergite, a nickel olivine from the mineral assemblage trevorite-liebenbergite-nickel serpentine-nickel ludwigite-bunsenite-violarite-millerite-gaspeite-nimite, is described mineral- ogically.Ithasa-1.820,P-1.854,7-1.888,ZVa-88",specificgravity-4'60,Mohs hardness- 6 to 6.5, a - 4.727, b - 10.191,c = 5.955A, and Z - 4. X-ray powder data (48 lines) were indexed according to the space grottp Pbnm. The mean chemical composi- tion, calculated from electron microprobe analyses of eight separate liebenbergite grains, gives the mineral formula: (NL eMgoaCoo,sFeo o)Sio "nOn The name is for W. R. Liebenberg, Deputy Director-General of the National Institute for Metallurgy, South Africa. Introduction mission on New Minerals and Mineral Names (rMA). A re-investigationof the trevorite deposit at Bon Accord in the Barberton Mountain Land, South Experimental Methods Africa, led to the discovery of two peculiar but distinct nickel mineral assemblages.Minerals from The refractive indices were determined by the the assemblagewillemseite-nimite-feroan trevorite- conventional liquid immersion method using a reevesite-millerite-violarite-goethitehave been de- sodium lamp as light source.The optical and crystal scribed in earlier papers in this series (de Waal, morphologicalparameters were studiedwith the aid 1969, l97oa, 1970b;de Waal and Viljoen, I97L). of a universal stage. The macrohardnesswas esti- From the secondassemblage trevorite-nickel olivine- mated from the microhardness,which was deter- nickel serpentine-nickel ludwigite-bunsenite-viola- mined with a Leitz Miniload hardnesstester. The rite-millerite-gaspeite-nimite,only the trevorite (de curve of Young and Millman (1964) relating micro- Waat,1.972)has been described to date. hardnesswith Mohs hardnesswas usedfor this pur- The present paper deals with the nickel olivine, pose. here named liebenbergite.A paper on the nickel The X-ray data were obtained by means of a serpentine(a nickel-rich lizardite?) and nickel lud- Debye-Scherrercamera (diameter 114.6 mm) with wigite is in preparation. CuKa radiation. Corrections were made for film The name liebenbergiteis proposedfor the pure shrinkage,and the relative line intensitieswere esti- nickel end member of the olivine gloup and is ap mated visually. By analogyto forsterite and fayalite, plicable to all such minerals that contain nickel as the space group Pbnm was assumedfor the lieben- tho major cation in octahedral coordination. The bergite and the powder pattern was indexed accord- name is for W. R. Liebenberg, Deputy Director- ingly. Cell parameterswere refined using the com- General of the National Institute for Metallurgy, puter programcELFIT (Bracher, 1,967). South Africa, who has made a major contribution Microprobe analyseswere made with an Applied to the advancementof mineralogicalscience in that ResearchLaboratories model sMx electronmicro- country. The name has been approvedby the Com- probe, using a 15 kV excitation potential and a 733 734 S, A, DE WAAL, AND L. C, CALK 0.0250 ,,oaspecimen current. Wet-chemically ana- The liebenbergiteforms part of the assemblage lyzednatural silicateswere usedas standardsfor Si, trevorite-nickel serpentine-nickelludwigite-bunsen- Mg, and Fe. Syntheticsulfide standardsprepared by ite-violarite-millerite-gaspeite-nimite.The original Gerald K. Czamanskewere used for Ni and Co. liebenbergitegrains filled the intersticesbetween the Correctionswere made for background,instrumental trevorite grains and medsuredup to 1 mm or more drift, matrix absorption, characteristicfluorescence, in diameter. They are, however, almost completely and atomic number effects (Beeson, 1967; Beaman replacedby the secondarynickel serpentine,with the and Isasi, l97O). resultthat only small irregulargrains, few exceeding 150 micronsin diameter,are presentin the serpen- Occurrence and General Mineralog5r tine matrix. The fine intergrowth between the liebenbergite Tho trevorite-bearingores at Bon Accord formed the nickel serpentinethat replacesit prevented a small tabular body approximately20 feet long and and of the two phasesfor bulk chemical or about 2 feet thick at the contact between the separation physical studies.Fortunately, as was proved by the Moodies quartzite and serpentinizedultramafite of microprobe analysis,the liebenbergiteis homogene- the JamestownIgneous Suite. The width of the body ous composition within hand is uncertain, but judged from the prospecting in a specimen.This imparts greaterconfidence in the optical and physical trenches it was probably not much more than 10 properties that were not determined on the same feet. Unfortunately, most of the ores have been grains removedduring the prospectingand mining activities as were analyzed. so that in situ investigationof the different assem- Optical, Physicat and Chemical Properties of blages is not possible.This makes the study o,f the the Liebenbergite origin and mutual relationship of the two assem- The optical and physicalparameters of the lieben- blages extremely difficult. The present work was bergite are listed in Table 1. The crystallographyis done on samplesobtained from the waste dumps. similar to that of the olivines,but the specificgravity, refractive indices, and optical axial angle of lieben- Teers l. Optical and Physical Properties of Liebenbergite bergite are distinctly different from either forsterite or fayalite. Crystal System: 0rthorhonbic The indexed X-ray powder pattern of lieben- Unlt Cell Parameters: a = 4,727 ! O.OOri, b = 10.191 t O.OOrE bergite (Table 2) correspondsclosely to that of c = 5.955 t O.OOZi synthetic NizSiOr (Pistorius, 1963). Liebenbergite volue = zAS.e (L)3 has a d(130)-spacing diagnosticallysmaller than Space group Pbmn (asswed) those for forsterite, fayalite, tephroite, and monticel- 0ptical Properties lite. Moreover, d(130), in conjunctionwith either Refractive Indlces: o=1.82010.003 refractive index or specific gravity, can be used to B = 1.854 (calc) estimate the composition of an olivine within the Y=1.88810.003 (y-o)=0.068 liebenbergite-forsterite-fayalite compositional tri- optical Orlentation: a=brB=cs"(=d angle. o.A.P. (001) The chemicalcomposition and mineral formula of Optlcal Axial Angle: 2V =88"!2" d liebenbergiteare comparedin Table 3 with those of Dispersion: T> 7) Ni2SiO4. The results indicate that liebenbergite is Pleochrolsm: X = I = colorless to pale green homogeneousand is close to ideal NizSiOain com- (in thin sectlon) Z = greenish yellow position. Assuming that the deviation from stoichi- Physical Properties ometry reflects analytical error, the formula may p (cal-c) : 4.60 be written: Ilardness (Mohs): 6 to 6.5 (derlved fron nicrohardness) (Ni, Mg, Fe, Co)zSiOa Cleavage: {010} weak ro moderate {t0O} weak Conclusions Twinnlng: None observed Color: Yellowlsh green ln grain The data presentedleave little doubt that lieben- concentrate bergite belongsto the olivine group of minerals and LIEBENBERGITE, A NTCKELOL7VTNE 735 Ttsrn 2. X-Ray PowderData of Liebenbergite origin is enigmatic. Neither of the two commonly acceptedpr@esses of nickel enrichment' i'e., mag- hk1 tl hkr r/to -calc or enrichment to matic segregationof sulfide superficial under tropical weathering conditions, prduces as- 020 ,.o9 5.10 Ljz r.583 1.t85 semblagesanything like those found at Bon Accord. f10 l+.29 \.29 ol+3 10 r.16r r 166 020 3.87 3.87 ?tt 5 r.5oT r.ra6 )+98 lor 10 3.70 3.?0 , r. )497 1. Acknowledgrnents 11f 3. 1+7 3. \8 2A r.\86 r.48? of the r27 10* 2.986 2.996 o62 30 1.r+73 r.\75 The authors are indebted to the Director-General 130 90 2,759 2.759 330 10 r.\29 r.\29 National Institute for Metallurgy and to the Director of the a22 20x 2.56\ 2.57o 20 1.390 r.390 paper' 8o 2.503 2.rO3 10 1.380 r.380 U.S. Geological Survey for permission to publish this TI2 r00 2.\)t2 2.\\5 T' 1.343 1 343 Thanks are also due to Dr. S. A. Hiemstra for his unfail- 200 10 2.361 2.36t+ 1 308 1. 3oT ing interest and assistanceduring the investigation. 10 2.337 2.3\2 5 r.289 r 289 210 5 2.303 2.302 2o\ r.2j9 r-.260 30x 2.252 2.259 I.25A :1.250 References 2. j.\6 2.1+1 23\ r.r83 1.181 BEAMAN,D. R., eNo J. A. Isesr (1970) A critical examina' r.160 2.022 2.O2\ 025 1.r50 programs used in quantitative electron 10 I .935 1.935 1.150 f.151, tion of computer f0 r.8?o L.872 244 L.r29 I.r29 microprobe analysis.Anal. Chem. 42, 1540-1568. r.a92 \-092 113 , r.799 r.801 (1967) A computerprogram for processing 5 1.?83 1.785 10 r. o?)+ t. o?lr BEEsoN,M. H. electron microprobe data' U'.i. Geol. Suru. Open-File 90 1.?38 r. 7\0 f0 1.03r 1.031 15 r.65\ r.663 10" 1. 015 1.0f5 Rep. 051 IO 1. 53r 1. 531+ 38)+ 1o* o . B2j+5 o. B2L? Bnr.cnr,n, B. H. (1967) CErrlr. A computer program to 133 r.609 1.611 )' o.7753 a.7712 refine crystal unit cell dimensions. United Kingdom Atomic Energy Auth. Res. Group Rep. lnnn'n'5412t 26 pp- on Weer, S. A. (1969) Nickel minerals from Barberton, South Africa: I. Ferroan Trevorite. Amer. Mineral. 54, that it approachesin composition the ideal nickel t2a4-1208. end-memberof this mineral series. (197}il Nickel minerals from Barberton, South nickel-rich chlorite. Amer. Mineral. The liebenbergite-and willemsite-bearingassem- Africa: II. Nimite, a s5. 18-30. blages are unique in mineral composition. Their (1970b) Nickel minerals frorn Barberton, South Africa: III. Willemseite, a nickel-rich talc. Amer' Min- 3' chemicar-:::::::.:f Liebenbersite era\.55,3142.