NEPTUNITE FROM SEAL TAKE, LABRADOR1

Transcription

1 NEPTUNITE FROM SEAL TAKE, LABRADOR1 E. WM. HEINRICH AND SHI H. QUON The Unfuers,ity of Mi,chigan, Ann Arbor, M.i,ch,igan INrnooucrroN In their report on the occurrence of barylite at Seal Lake, Labrador, Heinrich & Deane (1962) noted a bright red, unidentified mineral in the beryllium-bearing fenites. This species has now been identified as the rare titanium silicate, neptunite (NazFeTiSi.Or:). The mineralogy and petrography of the rocks have been described in detail (Heinrich & Deane, 1962; Nickel, 1962), so that it suffices to report that the host rocks are metasomatic syenitic gneisses (fenites), representing amphibolites altered by alkalic hydrothermal solutions in the exomorphic aureole of an intrusive body of alkalic syenite. Table 1 lists the minerals of the svenite and the fenites. T""* 1. Mr**"* "" l"r A Syenite Fenites Veinlets in fenites Anorthoclase Aegirine Riebeckite Sphene Apatite Eudialyte Orthoclase Aegirine Arfvedsonite Biotite Muscovite Barylite Eudidymite Pyrochlore Triplite Apatite Epidote Sphene Calcite Sphalerite Hematite Neptunite NBpruNrrB Quartz Barylite Neptunite occurs as anhedral grains that range in size from 0.10 to 2 mm. Many of smaller grains are skeletal (Fig. 1) with numerous inclusions of other minerals, particularly albite. Much neptunite, particularly the larger grains, is closely associated with pyrochlore (Fig. 2) and barylite. In some rocks, minute anhedral specks are included in clusters of sodic amphibole. Most of the radioactivity of the fenites stems from pyrochlore, and where pyrochlore grains are embedded in neptunite, a pleochroic halo has been developed in the titanium silicate. lcontribution No. 245 The 1\4ineralogical Laboratory, Department of Geology and Mineralogy, The University of Michigan, Ann Arbor, Michigan. 650

2 F i o F - y:a og{?;: FoU!)v c.i 5 H: B o h o F b0 ro i3 c) 0,!) o H,i.:

3 652 THE CANADIAN MINERALOGIST The optical properties o{ neptunite are listed in Table 2, and Table 3 presents the :c-ray powder diffraction data, which agrees substantially with that given by Sabina & Traill (1960). TasLs 2. Orrrcer- PnopBntlps or Nrprurrro a p 7-a a B t Absorption Sign,27 Dispersion Orientation USSR (Fersman & Bohnstedt, 1937) L.7t7-r H.033 light yellow lemon yellow orange, orange yellow bright orange, yellow red t)9)a (*) 35'(Na) r < r, strong Optic plane : 010 b:9 :"Ac:20" California (?) (Larsen & Berman, 1,934) oM pale yellow yellowish red deep ocherous vellow to irrownish red t)9)q (+) 49' r < t, extreme b-9 1lc:160 (110) cleavages Ireland (Nockolds, 1S50) Labrador r pale yellow yellow-brovn orange-red 'r)fl2cu (*) ca.40' strong L.7LI T.72L t.7m golden orange orange-red deep red t)9)a (+) 62' r ( a, extreme two cleavages at 80o Teslo 3. X-nev PowpnR DTFFRAcTIoN DATA ron NBpmrrrts Seal Lake, Labrador camefi : 57'3 mm cu radiation, ffifjit al- r ax I da I 4.68A ro 2.85 N t '.5t 80 2.t s.is 1oo z.gg t.gt go r.ao ao PenecBNBsrs Table 4 lists information on the ocgurrences of neptunite. For all but the California occurrence, neptunite is known to be genetically related to alkalic or alkalic-subsilicic rocks, forming as a deuteric accessory species' (1) in endo- or exo-morphic contact zones, (2) in recrystallized xenoliths or'(3) in consangineous pegmatites. Stewart (1959) states that narsarsukite, NarTiSiaOrr, forms where there is a silica excess (from siliceous wall rocks), high partial pressure of oxygen and abundant Na and Ti' whereas neptunite, Nar(FeMn)TiSiaOrz, forms directly or replaces preexisting naisarsukite under conditions of lower partial pressure of oxygen. In addition to replacing narsarsukite (Flink, 1901), neptunite also replaces sphene (Nockolds, 1950) and eudialyte (Bondam & sdrensen,

4 NEPTUNITE FROM LABRADOR ). Neptunite from the Kola peninsula also is regarded as a lowtemperature mineral, having crystallized probably at the same time as associated analcite (Fersman& Bohnstedt, lgbt). The san Benito assemblage also indicates deposition at low temperatures. Tenr,p 4. Goor-ocv or OtuBn Nprrurnr OccunnBrcrs Loelity Type of deposit Associated Minerals Reference Tutop- Agdlerkofia, Grmland Nasarsuk, Grenland Kvaefjeld, Julimehaab district. Greenland Foidal syenite pegmatite in sodallte syenite Sodic quartz syenite (?) pegmatite in augite syenite Augite syenite and malcite-richlujavrlte Kvmefjeld, Recrystallized inclusions Julianebaab ofpegmatoidalnaujaite district, in lujawite Greenland Khibina Endocontact zone-altered Tundm, USSR nepheline syenite (decrease in nepheline ad transforma_ tion to homblerde syenite) Lovoao Syenite pegmatite (aegirlne- Tundra, USSR nicrocline type) in lujawite Barmvave, Quartz-bering syenite Carlingford, Ireland Near New Idria Zeolite veiu in greenstones San Boito Co., (altered diabrc and Na_ Califonia amphibole schist) that form a xenollth enclored in ser- IEntinite Analcite zimwaldite Aegfuine microclhe, quartz, elpidite, epididymite, synchysite, polyuthlonite, calcite IVllcrocline, soda mphlbole, analcite, nepbeline, schtzoltte, natrolite, monulte, acmite, epistolite, sphalerite Eudialyte, ss'dalite, annr- Do. cite, and natrolite md Dseudomorphs of analcig * kataplelte f acmite * niobian perovskite + neptunite aiter eudlalyte Aegirine, natrolite, Fersnan, 1926i adalcite Fssmm& Bohnstedr,1937 aegirine, natrolite, eucolite, ramsayite Orthoclase, quartz regirine-augite, sphene Natrolite, benitoite, ioaquinite, actinollte, crossite, glaucophane, albite, aegirine, calclte, aragonite, chalocite Bogsild, 1905 Baggild, 1905 Bondm & Sorensen, lg69 Do. Nockolds, 1960 Loudsback & Btasdale, RBrBnpNcBs Bficorut, o.,8..(1905): Mineralogia Groenlandica. Medd.et. Grrnr.and (Lists s2, 502_.506. all older original references), BoNnaur J..& SOnensoN, H. (1959): Uraniferous nepheline syenites and related in the Ilimaussaq rocks area, J_urianehaab district, souihwest ci*"r""a. u".-fuat. peace ful Uses Atom. Energy, proc.?nd, Intt. Conf.2, SSS_BE}. I'-ERSMAN' A. E. (1926): Minerals of the Kola peninsura. Am, Minerar,. rr,2gg-2gg. & BosNsrpor, E._M. (ed.)(1992): Minerals of the KhibinulriJmoo"".o Tundras. Lornonossozt rnst. Acad.. sai. I/ssR, 1g0-1at. oirg- *rgrrir references.) Russian Ft tu5,-q.- (19_01):_Underslgelll at minerallen fra Julianehaab, pt. I, On the Medd.el. minerals. Gri;nl,and, 24,

5 654 THE CANADIAN MINERALOGIST GOSSNER, B., & MUSSGNUG, F. (1928): Uber die systematische Stellung von Neptunit und Babingtonit. Cent. Mineral. Abt. A, 1928, HEINRICH, E. WM., & DEANE, R. W. (1962): An occurrence of barylite near Seal Lake, Labrador. Am. Mineral. 47, LARSEN, E. 5., & BERMAN, HARRY. (1934): The microscopic determination of the nonopaque minerals. U.S. Ceol. Survey Bull. 848, 127. LOUDERBACK,G. D., & BLASDALE,W. C. (1909): Benitoite, its paragenesis and mode of occurrence. Univ. Calif. Dept. Ceol. Bull. 5, NICKEL, E. H., & CHARETTE, D. J. (1962): Additional data on barylite from Seal Lake, Labrador. Am. Mineral. 47, NOCKOLDS, S. R. (1950): On the occurrence of neptunite and eudialyte in quartzbearing syenites from Barnavave, Carlingford, Ireland. Mineral. Mag. 29, SABINA, ANN P., & TRAILL, R. ]. (1960): Catalogue of x-ray diffraction patterns and specimen mounts on file at the Geological Survey of Canada. Ceol. Survey, Canada, Pap STEWART, D. B. (1959): Narsarsukite from Sage Creek, Sweetgrass Hills, Montana. A m. Mineral. 44, Manuscript received APril 23, 1962