American Mineralogist, Volume62, pages999-1002, 1977

Otwayite,a newnickel mineral from Western Australia

ERNpsrH. NrcrEr-,Bnucn W. RosrNsoN,CHenlss E. S.D,cvrs Diuisionof Mineralogy,CSIRO P.O. Wembley,W.A., Australia 6014

ANDRoBERT D. MlcDoNnr-o Diuision of Applied Organic Chemistry, CSIRO Fishermen'sBend, Vic., Australia 3207

Abstract

Otwayite, found in the Nullagine regionof WesternAustralia, occursas fibrous veinlets in a -rich assemblage.The composition of otwayite corresponds to (Ni,Mg)r(OH), COa.HrO. Fiber-rotation and powder X-ray diffraction patterns can be indexed on an orthorhombicunitcellwitha:10. l8b=27.4,andc:3.22A.MeasuredGis3.4l;calculated G, assumingZ : 8, is 3.346.Otwayite is green in hand specimen,pale green and weakly pleochroic in transmitted light, has parallel extinction,is length-fast,and has j : 1.72,a = 1.65.

Introduction t.ransectnickeloan serpentine,, polydymite, and apatite.Associated with the otwayite in the vein- The mineral describedin this paper was found in lets are ,gaspeite, and pecoraite. threesamples from the Otway prospect(lat. 2l'39'S; The otwayite has a pronouncedfibrous habit, with long. 120"06')in the Nullagine region of Western the fibers usually arranged in rosette-likeaggregates Australia,23 km north of the town of Nullagine.An (Fig. l) more or less normal to the veinlet walls. ultramafic body is cut by numerous shearsthat con- Electron transmissionmicrographs (Fig. 2) indicate tain an unusualmineral assemblage including apatite, that individual fibers are about 0.05rrmin width. chlorite, magnesite, gaspeite, pecoraite, millerite, Fiber bundles,as observedunder electron-scanning polydymite,and anothernew basicnickel carbonate and optical microscopes,are seento be severalhun- that will be describedelsewhere. A brief description dred micronsin length. of the gaspeite and pecoraite has been published (Nickel, 1973),and a more completedescription of Chemicalcomposition the entire occurrenceis currently in preparation. A sampleweighing 23 mg was hand-pickedfrom The mineral is named after Mr. Charles Otway, one of the veinletsin sample 10027. A chemicalanal- Gosnells,Western Australia, who hasbeen very help- ysis, in which NiO, MgO, and SiOzwere determined ful in providing accessto, and samples from, his by atomic absorption and colorimetric procedures, mineral lease. Both mineral and name have been and CO, and H2O were determinedas C and H by a approvedby the IMA Commissionon New Minerals microanalyticalprocedure (MacDon ald, 1974), gave and Mineral Names.Type specimensare preservedin the resultsshown in Table 1, col. 2. No elements the mineral collection of the Division of Mineralogy, other than Ni, Mg, and Si were detectedby electron- CSIRO. Perth. Western Australia under catalosue microprobe analyses.Assuming that the 0.28 percent numbers 10016,10027, and 10740. SiOz is due to admixed pecoraite,the analysiswas recalculatedas shown in column 3. The atomic pro- Occurrence portions (col. 6) are close to whole numbers if their Otwayite has so far been found only in narrow sum is taken as 6. Recalculationof the analysison veinletsbetween 0.5 and I mm in width. The veinlets this basisgives Ni, ,onMgo.our(OH)'.nro(COr)'006.1.102 appear to represent late-stagefracture filling, and HrO, or nearlyNir(OH)rCO'.HrO. 999 1000 NICKEL ET AL OTWAYITE, A NEW NICKEL MINERAL

Fig. I (top left). Electron scanningmicrograph of a broken surface,showing typical otwayite morphology Fig. 2 (top right) Electron transmissionmicrograph, showing dispersedotwayite fibers. Fig. 3 (bottom). Fiber-rotation X-ray diffraction pattern of otwayite.

Table l Chemical composition of otwayite

Weight Percent Atonic Proportions

Ideal Ideal Analysis Recalc. Otwayite Ni2 (oH) . H2o Ni2(oH)2c03 . H2o 2co3 (1) (2) (3) (4) (s) (6) (7)

Ni0 62.20 62.87 65.12 Ni 1.904 2 Mgo L.12 1.14 Mg 0. 064 sio2 0,28 OH L.924 2 co2* 19.24 19.57 19.18 co- I .006 I J "2-H n** 16.t7 L6.42 15.70 Hzo 1.102 1

99.01 100.00 100.00 6.000

* Deteynined as C. x* Determined as H, NICKEL ET AL.: OTWAYITE. A NEW NICKEL MINERAL l00l

Optical and physical properties Table 2. X-ray powderdiffraction pattern of otwayite Otwayite is bright greenwith a silky luster in hand hkl d*dtt*hkl n+T*-calc _est specimen.In transmitted light it is pale green and oos cal c esf weakly pleochroic, with the deepestcolor normal to 040 6.84 6 .850 l0 440 2,370 2.386 the fiber axis. Extinction is parallel. Refractive in- 140 5,67 5 .680 8 32t 2.297 2,302 <1 zoo 2.I1.0 2.242 2.237 I )'uq fs.osol 2 5 dices are 1.65 parallel to the fiber length, and 1.72 zroJ Ls.ooaJ 191 2.166 2,t62 2 060 .588 1 normal to it; the mineral is thereforelength-fast. The 4 4 .567 0.10.I 2.097 2.087 2 mean refractiveindex n is .72) + 1.65]/3: 240 4.0a7 4.086 1 510 2.03t 2.030 3 [2(l 1.70. 170 3.651 3.653 I 2.I3.0 L940 1.947 I o8o l.t zs1 3-12.O 1,898 1.894 2 Calculatedi, using the Gladstone-Daterelationship 1 1 t.j^ I 2 260 l L :.:gg J 391 1 85r r.853 I and the valuesfor specificrefractive energies recently 001 3.227 3.220 2 570 I .8I3 I .806

MacDonald,R.D.(1974)DevelopmentsinquantitativeelementalOswald,H R.(1961)UbernatrirlichenuridkrinstlichenGerhard- microanalysis.Proc, R Aust. Chem Inst , 4l, 3-l-40. tit Z Kristallosr.,l16, 210-219. Mandarino,J A (1976)The Cladstone-Dale relationship-Part I: Derivationof new constahts.Can. Mineral., 14,498-502. Nickel,E. H. ( I 973)An occurrenceof gasp6iteand pecoraite in the Nullagine region of Western Australia. Mineral Mag., 39, Manuscriptreceiued, March 14, 1977;accepted I l3-l15. for publication,May 19, 1977.