Canadian Mineralogist Vol. 23, pp. 491494 (1985) PYROPHANITEMnTiO. FROM STERLINGHILL, NEW JERSEY JAMES R. CRAIG, DANIEL J. SAND}IAUS* ENo RUSSELL E. GUY Departmentof GeologicalSciences, Wrginio Polytechnic Institute and Stote University, Blacksbwg, Virginia 24061, U.S.A. ABSTRACT MooB or OccURRENCEAND PHYSIcAL PRoPERTIES Pyrophanite of composition (Mng.e3sFee.ossZno.oro)The pyrophanite was found in samplestaken from Ti0.es0O3has beenidentified in the zinc- and manganese- the footwall contact of the eastlimb of the orebody, rich ores of the Sterling Hill deposit, New Jersey.It occurs l0 m abovethe 500level of the Ster- as anhedral grains up to 2 mm in length in associationwith approximately gahnite, manganiferous augite, hendricksitic biotite and ling Hill mine. The sampleswere collected at about fluorescentMn-calcite. Unit-cell dimensions:a 5.161(l), c the 1000N co-ordinatein October 1980.Megascop- l4.3l7(8) A; microhardnessin the rangeVHNlen 567-607; ically, the samplesconsist of coarse-grained(l-2 cm) reflectanceR Qn t/o\ 18-22(4.ffi nm), 16-19(546 nm), lzl--18 brown to greenish grey manganiferous augite with (600 nm). The pyrophanite is a product of high-grade scatteredrounded to euhedral grains of dark green metamorphismand is formed in Mn- and Ti-rich areaswith gahnite and irregular patchesofbiotite and while ca]- lower-than-averaee f (O). cite, The original, rougtrly lGcm-diameter specimens were cut in order to preparepolished sectionsofgah- pytophanite, gahnite, Keywords: hendricksite,Sterling Hill, part of that mineral (Sandhaus New Jersey. nite as of a study l98l). The cut and polishedsections reveal that the gahnitegxains are commonly sheathedby a narrow SoMMAIRE irregular rim of zincian biotite that appearsto be a reaction product. The pyrophanite occursdissemi- On a identifi6 la pyrophanite, de composition nated within the pyroxene and within the white (Mn6.e3sFq.6s5Zne.mdTis.esqO3, dans le minerai zincifbre fluorescent Mn-calcite as anhedral to subhedral et manganifEre du eite de Sterling Hill (New Jerse9. Elle grains or aggregatesup to 2 mm across;most gxains' sepr€sente en grains allotriomorphes, longs de 2 mm, asso- however, are less than 0.5 mm in their maximum ci6sd gahnite, augite mpnganifbre, biotite hendricksitique dimension. Subsequent to our discovery of et calcitemanganifbre fluorescente. Dimensions de la maille: (1983) pyrophanite a 5.161(l),c 14.317(8)A; microduret6VHNr66 567-607; pyrophanite, Valentino found rdflectanceR (en 9o) 18-22(460 nm), lG19 (546nm), 14-18 lamellaeoriented on the (11l) plane of magnetitethat (600 nm). La pyrophanite rdsulte d'un m6tamorphisme defiiles an exsolution texture vrith franklinite at Ster- intenseet caractdriseles rdgionsriches en Mn et Ti et d fuga- ling Hill. cit6 d'oxygbneanormalement basse. The reflectanceof severalgrains was measured using a McCrone pholometer; measurementswere (Traduit par la Rddaction) calibrated against pyrite, galena and magnetite that had previously been calibrated against Zeiss WTiC Mots-cMs: pyrophanite, gahnite, hendricksite,Sterling Hill, number 474253and against a silicon stan- New Jersey. standard dard provided by Dr. Hans Pauly of the University of Copenhagenas part of an I.M.A. Commission INTRODUCTIoN on Ore Microscopy test of interlaboratory accluacy. The rangesof reflectancevalues R (in 9o) observed Pyrophanite MnTiO., the manganese analogue of at three arbitrary wavelengthsare:. 18-22 at 460 nm, ilmenite, has been observed in ore s€rmplesfrom the 16-19at 546nm and 14-18at 600nm. Thesevalues Sterling Hill mine near Franklin, New Jersey. This are comparableto those reported by hek 0971). rare mineral had not previously been reported in the The microhardness was determined on a Tukon zinc- and manganese-rich Franklin-Sterling Hill ores MicrohardnessTester using a 1@ g load for approx- and has only been found in a few localities in the imately 20 s. Vickers HardnessNumbers were found world since its original description in the Pajsberg to lie in the range 567-607. deposit in Sweden by Hamberg (1890). The present Examination of the pyrophanite under the ore brief report provides additional information on its microscopereveals that it is grey, and resembles chemistry, characteristics and mode of occurrence. ilmenite; it exhibits a weak bireflectance and a moderate anisotropism. On broken surfacesand *Presentaddress: NUS Corporation, 19 Crosby Drive, Bed- locally within polishedgrains, the pyrophanite dis- ford, Massachusetts01730, U.S.A. plays reddish internal reflections. The grains are 491 Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/23/3/491/3420178/491.pdf by guest on 23 September 2021 492 THE CANADIAN MINERALOGIST anhedraland appearto have formed at the sametime (Mne.e3sFee.s55Zno.osd(Tio.e8o)O3. No evidence of as the enclosingminerals. compositionalzoning was observed.The coexisting An X-ray-diffraction pattern (Table l) was gener- manganiferouspyroxene (Can.aaMgs.2TMno.rsFd*0., ated from a 0.5- to l-mm fragment in a Gandolfi Zn6.e5Nq.s3Ks.0J(Si0.esAl0.0)O3 approaches busta- cnmera.The unit-cell dimensionscalculated in space mite in composition; the Zn-biotite [(Ko.ssNao.os) + group R3 on the basisof six of the sevenlines are (Mg1.66Mn6.3nZno.' oF e2 o.azTio.r:) (Si3.07.4'11.35) a 5.161(l), c 14.317(8)A. Thesevalues compare well O2(OH)21apparently is hendricksite. The gahnite is with those giveF in the ASTM file (PDF 12-435:a compositionally similar to that found in many local- 5.137, c 14.29A). The causeof the slightly larger- ities (Sandhausl98l) except for a slightly higher- than-expectedcell dimensionsof the Sterling Hill than-normal manganese content (galaxite com- pyrophanite is not known. ponent). Electron-microprobeanalyses were performed on It is apparentfrom Figure I that the pyrophanite an ARL SEMQ usingan operatingvoltage of 15kV from the Sterling Hill mine is similar composition- and a beam current of 15 nanoamperes.Four typical ally to that from eight other localities and that natur- compositionsof pyrophanite are given in Table 2 ally occurring compositions span the ilrnenite- along with the composition of coexistinggahnite, pyrophanite join. pyroxeneand Zn-biotite. The averagecomposition of pyrophanite is equivalent to a formula of OnIcTn oF THEPYROPHANITE Pyrophanite has heretofore been reported from TABLE I. X-RAY POI{DER-DIFFRACTION DATA FOR only a few types of geologicalsetting: peralkaline PYRoPHANITE FROII THE STERLING tltLL tllNE rhyolites from Mont-Dore, France@rousse & Maury pegmatitesfrom Baikalia, Reflectlon dmeas dcalc Intenslty 1976),nepheline syenite USSR (Portnov 1965)and the Oslo area, Norway ro4 z.zgs B 2.794 A stnon€l (Neumann & Bergstdl 1964), and from Mn-rich 110 ?.576 2.581 Fodefat€ hydrothermalveins in hornfels from Bohemia 113 2."74 2.270 roak @6k o24 1.903 1.A96 Eod€rate l97l). The host rocks in the SterlingHill deposithave I 16 I .750 1.752 Dodsratq 2L4 1.A?O 1.5?A oodorate been interpreted in recent years as a Zn-Fe-Mn- 300 t,42|J 1.490 aodetato bearing dolomite bed that was decarbonatedwhen metamorphosedto sillimanite grade (Callahan 1966, Cell dtbenglons calculat€d using le€st-squaros unl,t- cell refltrqoqnt of Appl€q€n & Evsns (1973)r Metsgeret al. 1969,Frondel & Baum 1974).Squiller a 5.1.61(l)' c 14.3I7(8) A. & Sclar(1980) reached a similar conclusionand pro- posed, in light of their own findings and those of previous workers, that the peak temperature of TA8I.E 2. CHEI.IICAI Ct'I'IPOSITIONTOF PYROPTANTTE AID COATSTINC OAI{IIITE, PYROXEIIEAT{t' BIOTITE metamorphismwas in the range700-800oC at pres- FRori TFE STEit lflo lttlt i|lIE, ilEt{ JERSEY suresof at least3 kbar. They further noted that the composition ofthe oxide phaseswould be dependent Ordde proportion Ft. t PwroEhentta Psoxehe OEhntta Zn-btottta upon variations in the of metallic ele- ments and local differences in the fugacity of oxy- It02 5O.7 5O,5 54.1 4E.t t.2 8.2 0.t 0.I 2.0 1.9 gen. Haggerty(1976) reported that at high valuesof tifno 42.a 41,7 44.4 43,9 9.4 9.t 0.6 6.7 5.5 5.3 and at high temperatures,Mn3+ can be stabi- Fso 2,7 2.7 2.4 2.4 4.5 4.o 5.9 5.5 5.7 5.9 /(O) Zdg 1.6 !.7 t.3 l.? 3.2 5.9 41.0 40.E tt.6 to.4 lized in the spinelstructure, whereas at lower values 41203 0.1 0.5 0.0 0.0 l. O r. t 50.6 51.6 t!.0 13.7 nso 0.1 0.0 0.0 0.0 9.2 9,q o.z 0.4 t2.6 t2.6 of /(O) and with a dominanceof Mn2+, the rhom- st02 O,2 0.I 0.0 0.t 51.6 5O,7 0.t 0.1 J5.t 56.4 bohedral pyrophanite] structure is decidedly izO 0.0 0.0 0.0 0.2 0.t O.t 0.0 0.0 6.6 8.5 [i.e., cqo 0,I O.I 0.! O.? 21.5 21.I 0.0 0.0 0.0 0,0 preferred. The pyrophanite observedin this study !la:0 0.1 0.0 O.l 0.t 0.9 0.6 r..l t.t 0.5 0.5 occurswithin a fresh coarse-grainedmatrix of pyrox- Total 98.4 97.1 102.E 96.7 L0t.2 100.E 99.6 \OO.S 9q,6 .gS.z ene, calcite, gahnite and biotite, and certainly appearsto be a product of the high-grademetamor- Cation fo6uls-unlt5 bas6d on S orygan sto6 phism. The traces of pyrophanite may well have formed in areas of slightly lower-than-average t{ .979 .965 ,99E .958 avaEss foroule I{n .931 .916 ,919 .985 (inO.9ggFoO.05SZng.j39)Tlg.9gp0g "f(O), ap suggestedby HaggertyOnq, but there is F6 .059 .05E .049 .055 insufficient information to permit further specu- Z^ .030 .042 .02! .033 AI .004 .010 .000 .oot lation.