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Gedrite from the Yeltowknife

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Gedrite from the Yeltowknife C cna d inn M i.neralo gb t, Vol. 11, pp. 1012-1015(1973) GEDRITEFROM THE YELTOWKNIFE.BEAUTIEUREGION, DISTRICTOF MACKENZIE' N.W.T. D. C. KAMINENI aro A S. WONG Dept. of Geolagg, [Jniuersitg of Ottawa', Ottana IsrnonucrroN AND Moos or Occunnrncr The Yellovrknife-Beaulieu region is underlain by Precambrian rocls that forrn part of the Slave Province of the Canadian Shield. The litho- logical units of the area are mainly metavolcanics and metasediments (the Yellowknife Group), granites, granodiorites, pegmatites and diabases. The Precambrian geologic history of the region was described by Hen- (1943)' derson & Jollifie (1941), Folinsbee (1942), and Henderson Gedrite, which is an aluminous orthoamphibole, occurs in the meta- sedimentary rocks to the east of the Sparrow Lake granitic pluton, which lies about 40 km northeast of the town of Yellowknife. Lithologically, the gedrite-bearing rocks may be termed as metagreywacke and argillite. Pri- mary features, such as graded bedding and layering, are colnmonly pre- served in these rocks and gedrite is restricted to certain layers, indicating a compositional control in its paragenesis. The gedrite has a dark green to brownish needle-like habit and com- monly forms sheaf-like aggregates. The associated minerals are cordierite, biotite, almandine and oligoclase. Other phases, that may or may not be prur"ni, are ilmenite, magnetite and pyrrhotite. Gedrite-cummingtonite associationsare observed most frequently in contact zones between gedrite- bearing rocks and siliceous concretions in which cummingtonite is abun- dantly developed. The concretions occur as thin layers and ellipsoidal lenses within the metasedimentary rocks. PuYsrcer enP OPrrceL PRoPERTTES The gedrite was separated frorn a rock also containing garnet, biotite, cordieritg oligoclase, and ilmenite. The sample was handpicked to achieve a purity of 99/6 or better. The density determined by suspension in a mixture of methylene iodide and dimethylsulphoxide is approximately 3.27. The calculated densiry based on chemical composition and cell dimensions, is 3.32. GEDRITE FROM THE YELLOWKNIFE-BEAUI.IEU REGION t0l3 The optical propertiesare- as follows : f,: pale yellow, |: yellowish brown, Z: greyishgreen -l_ o: 1.673 0.00a 0: 1.684-+ 0.002,t: L692-r- 0.002 Optically +, 2V : 77ob 79o. Refractiveindices obtained on the basis of chemical compositionand regression equationsof Hey (i956) are o: 1,6208-f 0.0025,B: l.6Z8Z -r- 0.0015,Y: 1.6896-r- 0.0012. Deer, Howie & Zussman (1961) plotted y of anthophyllite-gedrite against100 Mg: (MC*F",**Feu* *Mn). From this diagram,a y value of 1.692is obtained,which is in agreementwith our measuredvalue. Cnrmrcar Couposrrrow The mineral was analysedby employtng a combination of atomic ab- sorption, wet chemical and electron microprobe techniques; the analysis is given in Table 1. Tasrr l. Cururcal Couposmow or. Gponrrn No. of ions on the basis of 24 (O, OH) si02 43.64 si 6.46 Ti02 0.31 AI 1.54 Alzos 16.42 AI t.32 Fe203 1.26 Ti 0.03 FeO 24.52 Fe8+ 0.14 MnO 0.77 Fe2+ 3.03 Mso 8.95 Mn 0.10 7.03 CaO 0.42 Mg 1.97 Na2O L,N Ca 0.06 Kzo 020 Na 0.35 Hzo+ 236 K O,M Hro- 020 OH 2.33 2.33 Total 10028 Talr.r 2. C.oupanrsowor Urrr Crn Drmwsrows Present Milton & Ito Papike & Ross work (1e61) (re70) a(A) 16.551 18.594 18.594 r8.601 b(A) 17.873 17.890 17.741 17.839 c(A) 5283 5.304 5.249 s2u 101,4 TIIE CANADIAN MINENALOGIST Tasm 3. X-Rat DrrrnecnoN Dara ron Gronrrr hkl d(calc.A) d(obser.A) l/l' 024 8.94 8.98 65 2t0 8.24 8.25 100 230 5.01 I11 4.89 - 400 4.M 4.M 040 4.47 4.47 l0 aq 4.r2 4.13 10 131 3.87 3.88 23i 3,64 3.64 10 331 c,qq 3.U 10 440 3.4 s.2, YU 610 en< 3.06 100 UL 2.988 521 2.876 2.878 n 260 2.836 2hI 2.82r 2.8% 40 630 \ 40 arl 2.745 2.750 OJI 2.706 2.67r 2.674 40 161 2.570 2.57r 30 202 2.ilL 2.543 10 .tor 2.496 2.502 4A 342 2.4n 2.433 bJU 2.339 2.341 551 2.315 2.3r4 o 271 2.231 2.230 8 542 2.152 2J52 10 56r zL27 2.rn 10 82r 2.067 2.067 8 480 2.0r3 661 1.9884 1.9870 o 750 I O?qA 702 | 1.8730 t.uzs ; 940 I 931 1.8281 1.8259 2A 0.10.0 1.7873 1.7876 10.3.0 r.7717 r.77% 10 961 1.6I50 1.6159 30 2.11..0 1.6007 1.6001 20 922 1 <OOl 1.5994 30 rr.2.l 1.5817 r.5822, 10 6.10.0 1.9?5 1.5472 30 12.0.0 10.6.1 1.5110 1.5109 l0 0.12.0 1.4894 1.4890 20 553 I.+CJO t.4524 l0 10.4.2 1.43U r.43Tl 30 6.11.0 GEDRITE FROM THE YELIOWKNIFE-BEAULIEU REGION IO15 The Fe/Mg ratios of coexistinggarnet, biotite and cordierite were also determined.Regarding their preferencefor iron relative to magnesium, the minerals may be placed in the order gamet > gedrite2 biotite 2 cordierite. X-nav Drrnecuow Dera The r-ray powder difrraction data of gedrite that were reported by Milton & Ito (1961) showed a Pnma spacegroup. This was confirmed in the single-crystalstudy by Papike& Ross (t970). Powder photographsof the Yellowknife gedrite were taken with man- ganese-filterediron radiation, and corrections for film shrinkage were applied. The results obtained by using a least-squarerefinement program are given in Tables 2 and 3. AcxlqowrrocrMENTs This study was supported by a grant from the National Research Council of Canada, awarded to Dr. Ralph Kretz. Thanls are due to Dr. D. B. Stewart of the U.S.G.S. for providing his unit cell refinement program. Rrrrnrrcns D"ry {.A, Howrg RA. & ZusmAN, J. (1961) : Rock-forming minerab. longmans, Iondon. Folnwrrr,3.E. (1942) : Metamorphism ani! ore ileposits of the yellowknife-Beaulieu Area. Plt-D. thesis, University of Minnesota, Minneapolis. Hrlornsory-J.F: (1943) : structure and metamorphism of Early precambrian rocks be- tween Gordon and Great Slave Lakes, N.W.t Arner. !. S;i. 24t, 490-416, HrNoF.frso\ ].F. & loryryrr, A.v/. (1941) : Beaulieu River, District of Mackenzie, North- west Territories. CeoI. Swo. Con, Map 5E1A. Hrv,- $.Fl- (1956) : on the correlation of physical properties- with chemical composition in multivariate systems.Mineral. Mag. 31, 6g-95. Mrr.rory,-P.I.. ! Iro, J. (1961): Gedrite from Oxford Coung, Maine. Arner. Mineral. 46,734740. Panry1, t,I. & Ros, M. (19?0) : Gedrites: crystal structures and inrracrystalline cation distributions. Arner. Mineral. 55, 1945-1972. Manwcript receiueil Decanber IWI..
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