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Uraniun{ and Thorium in the Accessory Allanite of Igneous Rocks1

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Uraniun{ and Thorium in the Accessory Allanite of Igneous Rocks1 URANIUN{ AND THORIUM IN THE ACCESSORY ALLANITE OF IGNEOUS ROCKS1 Wrrrreu Lre Slrttn,2 MoNa L. Fna'Ncr,3 ,q.NpAr-BxeNnrn M. Suonwooo, Ll. S. GeologicalSuraey, Washington25, D'C ' ABsrRAcr u'eight in the rocks studied. The mineral is confined largely to the more siiiceous phanerites. The uranium content is highest in the ailanite from the granites sampled, ranging from 0.004 to 0.066 per cent, whereas the thorium content is high or low regionally, ranging from 0.35 to 2.33 per cent. Allanite was found to be otherwise of exceptionally uniform composi- tion. IwrnooucrroN Allanite, (RE,*Ca)z(Fe,Al)sSirOrz(OH),may occur as an accessory mineral in siliceousand intermediate igneous rocks, in limestone contact skarns, pegmatites, crystalline metamorphic rocks, and as a component of magmatic iron ores. The mineral is monoclinic and varies in color from light brown to black. Its hardnessranges from 5 to 6 dependingupon its degreeof alteration, and similarly its specificgravity varies from 3.4 to 4.2. Allanite is a member of the epidote group with rare earths sub- stituting for calcium. Allanite is often found with epidote; some of it is intergrown with epidote (Hobbs, 1889).The metamictizationof allanite produces an amorphous alteration product, and some allanite from pegmatite is completely isotropic. The alteration is inferred to be the result of the destruction of the crystalline structure of allanite by the radioactive decay of its uranium and thorium. N{rNBnarocv Separat'ionmethod.s In the granitic rocks that were studied, allanite was found to average 0.1 mm. in diameter. The rocks were pulverized on a roll-type crusher which liberated the allanite cleanly. The allanite was generally found to concentrate in the 100- to 200-meshsize fraction. Allanite concentrates + Rare earths. 1 Publication authorized by the Director, U. S. Geological Survey. 2 Present address:Minerals Benefication Div., Battelle Memorial Institute, columbus, Ohio. 3 Present address: Ferro Corp., Technical Service Lab., Cleveland, Ohio' 367 368 w. L. SMITH, M. L. FRANCRAND A. M. SIIERWOOD with the other accessoryminerals in the sink of a methylene iodide sep- aration (sp.g..3.33). It is easilyseparable from the other heavy minerals in a Frantz rsodynamic magnetic separator, allanite becoming magnetic between0.4 and 0.6 amp. at crossand logitudinal settingsof 10o.when epidote occurs with allanite, the separation becomesmore difficult. The epidote floated and the allanite sank in methylene iodide saturated with iodoform (sp.gr.3.45). Thus a minimum of hand-pickingwas required.to obtain clean separatesof the minerals. Opti,calproperties All of the ailanite studied was optically negative. The indices of re- fraction of the allanite were generally higher than those described in literature. This may be becausethe minerals generally described are the more metamict varieties from pegmatites.rt is likely that the allanite described here is more abundant and more typical of the fresh mineral than the larger specimensobtained from pegmatites. The optical data in Table 1 show that the a indicesrange from 1.690 to 1.775,B from 1.70to 7.789,and 7 from 1.706to 1.291with a possible error of +0.002. Allanite is reported to have indices as low as n 1.60 Teslr 1.Imrcrs or.RprrlcrroNol All,qxrrelnou IcxuousRocrs Birefrin- q R o I l_ ____l_-i:_ Southern California batholith Coarse phase Granite from Rubidoux Mountain, Riverside t.ltJ 1.789 1.791 0.016 Fine Phase Granite from Rubidoux Mountain, Riverside r. /.1.) I . /.)t, l-lJz 0.017 Woodson Mountain granodiorite, Descanso 1.745 1.7601.763 0.018 Woodson Mountain granodiorite, Temecula 1,.735 1.750 1.753 0.018 Woodson Mountain granodiorite, Rainbow 1 740 1.755I.IJ9 0.019 Woodson Mountain granodiorite, Elsinore r.740 I . /.).) 1.760 o.o20 Granodiorite, Stonewall formation, Cuyamaca 1.7051.7r7 |.720 0.015 Mount Hole granodiorite, Mount Hole 1.695 1.710t.tl+ 0.019 Tonalite, Aguanga 1.7431.7601.763 0.020 Sierra Nevada batholith Quartz monzonite, Basin Mountain 1.750 l. /oo 1.770 0.020 Idaho batholith Porphyritic granodiorlte, Cascade 1.740r. /.)l I. /JJ 0.015 Granodiorite, Stanley r.76r 1.776 1 780 0.019 Granodiorite, Atlanta 1.752 1.768 1.771 0.019 White Mountains, New Hampshire Fresh Conway granite, Conway 1.72r r.7381 1A1 0.021 Weathered Conway granite, Conway 1.720|.7371.740 o.020 Albany porphyritic quartz syenite, Passaconway 1.690 1.700 1.706 0.016 TJ AND TE IN ACCESSORY ALLANITE 369 among its isotropic varieties. The birefringence' 7-a, is shown to vary from 0.015 to 0.021 in the specimensstudied. The indices of refraction of allanite from a single rock were found to be variable. The indices listed are representative.fn the fresh Conway granite the allanite varies in a index from 1.695to I.739, and in the Cascadegranodiorite the allan- ite variesin a index from 1.740to 1.760. There seemsto be no clear relationship between the indices of refrac- tion and the rock type. The Idaho minerals have consistently higher in- dices, whereasthose from New Hampshire have lower indices. AII of the allanite studied is from rocks of Late Jurassic or Cretaceousage (Larsen, 1948; Chapman, 1955; Hinds, 1934) with the exception of the New Hampshire sampleswhich are of Mississippian age (Billings, 1945). Cneursrnv Chernical analysis Chemical analyses (Table 2) of allanite from different areas and of different optical properties were made by Glen Edgington of the Geolog- ical Survey. Tltntn2. Cgrurclr AxAtvsns,ru Pnn Cnwr,or Ar,laNrrnlnou ClscAor, Io.lno; Cotwev, Nnw HeursurnnleNn Rrvrnspr, CeluonxtAl Granite from Cascade Constituent Conway granite Rubidoux Mountain, granodiorite (fine), Calif. Sioz 30.35 26.05 29.75 AlzOs / .50 7.54 8.42 Fe:Oa 18.14 t7 .ol 20.68 CaO 12.90 10.45 8.S5 Mgo 7.43 0.81 0.91 MnO 0.38 0.58 0.31 HrO (total) 2.00 5.60 2.40 CezOa 11.06 12.45 11.38 Re2OB (other) 15.69 18.69 15.81 (Total RE incl. ThO) (26.7s) (31.14) (27.re) Total 99.51 99.18 98.61 Determined on separate samoles bv Sherwoodz Th02 1.2 o.92 0.76 U 0.0036 0.0540 0.0400 I Analyses made by methods outlined in Hillebrand and Lundell (1929) 2 Uranium determined fluorimetricallv. Thoria determined colorimetrically by thoron method. 370 w. L. SMITH, M. L. FRANCK AND A. .4[. SHERWOOD Spectroscopy Semiquantitative spectrographic analyses of nine allanites from igne- ous rocks are compared in Table 3. The minerals were separated from rocks of the white Mountains batholith of New Hampshire, the southern california batholith, the SierraNevada batholith, and the rdaho batho- Aside from the radioactive components,which are discussedseparately, most of the variations in compositionare within the limits of precision of the method. one immediately obvious fact is that each alianite con- tains the same 37 elements,with the exception of the absenceof thulium from the Basin I'rountain minerar and the absenceof Iutetium from the cascade granodiorite mineral. The variations, which seemto be regional, are the higher content of Nb, Be, and sn in the New Hampshire minerals, and the higher content of the elementsMn, Ti, Ni, and cu in the Sierra Nevada specimen. Among the rare earths, in all samples the order of abundance is Ce,(La,Nd,Pr), (Sm,Gd), variable tracesof Lu, Ho, and Eu. Tm is the least abundant. Of the cerium earths the order is Ce,(La,Nd,pr), Sm, and Eu. Of the yttrium earths the order is (Gd,Dy), (Er,yb,Lu), (Ho,Tm.) Radi oocl iae com ponen !s U AND TH IN ACCESSORYALLANITE 371 +@ F._ ,,x-3 ds 0a-rn>r:lu - qd '" ri I d,- -! ,q < ! ai NlE Fz e '662>Ea>Ezo6t4rc uq O 46 .9o a2 (Ji iE '1 FP' - !3 v QZ O ,9<Z> -'r Flii o gii .X ^i ; it s *a -43 zc-Ez N.: >H' dN Aa-,;t,\6 Fr ,e e S ii y r\ S; ;f 3 z E o I g F EE ,i; fii 6 fi |l 8"1.=E .Y B! !9F .q d E ; ? Tc X S :x .', i s ; s i !-: E z O 7, ,v 2 a tr i ia d ,:,8 & z <,y F.8.: E-3.r8 EI: {:€ 8d nE I;3 i g s* z- 6 F.l ;g f, n 1 f t d'i 4 &-; a 3 2 > ir8 A ;fi d dF d p.a ds,c q.1 tF .h :: E Z-FLrot i5 .=rJ>3i FX {:€ JFI_>r,1; a az ;s g F f, o T" E ;-i 2,F' H1 a3?#irBs#E-3aAA ts z Ei.q ar 9rc; .22) '5= gE F t- E 0 .,i <d Z n c*a 6I 14 a 4. d ri 7, > F >r ,?, zF EC=G a s s F sE A aE il o; a |l tr zza?_o g F ,i F c Ao .g a ai > >E # ,y 8d s i: fi dj i. t 3X F E a afi i ia A aA H c,i E -Sa3.-E F., z Aq UE 2 g d fi dfi q 6 F s cI S ; 1 a 2a; fi_ il c < e gE A ai I o€ :o E- !?Q999 .90 0 ' v: = b H o YY;YYY ;A ; ] ; J P:A=P: \dc; exx= oood Et5 oo 372 W. L. SMITH, M. L. FRANCR AND A M. SHERII/OOD OH O\4O\ Nb€()oO NNi:iiiiiN Oc] Ohi NOooO $$ oobN QN4\ON \\ \\\ \\\:\ -HHHiiiHHd N€ \Oi@ ONOO\O so Nb\o roi4\o\o \\ \\\ \\\\": HiHHHiiiii o oi ioN bh440 NN \O<t4 OOO<rr- \\ \\\ \\\\\ i_HiH d (q o^ O\\c) \O@S N:\c)N\O .+4 o|.\Oo OO\r+OO\ d;-fi €N NNN NiHHO d'd ;J O -i-l :-i; ,i-i-i-:.{ a) a o ON \O<rO\ NOdNcO do oOO oO\O<t \OaO<tO\cO 4N NNN :iiOO -o -i.i ^,{-i :-:^-:.{ h rad N@ N\o ;; d*'<i 'c;<,J*<.
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