Gahnite compositions compared

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1 MINERALOGICAL MAGAZINE, SEPTEMBER 1984, VOL. 48, PP Gahnite cpsitins cpared RICHARD A. BATCHELOR AND JUDITH A. KINNAIRD Departent f Gelgy, University f St Andrews, St Andrews, Fife, Sctland AI~STRACT. Blue-clured ge-quality spinel fr Nigeria was analysed by wet cheical ethds (using atic absrptin spectrphtetry) and investigated by X-ray diffractin. The results shwed it t be gahnite (unit cell diensin a= a) cntaining 36.7 ~ ZnO, 3.58 ~ FeO, and 0.12 ~ MgO. The spinel has an RI f 1.79 and density between 4.4 and Bradening f the n[111] XRD reflectins indicates a easure f cpsitinal hetergeneity. The gahnite analyses were cpared with cpsitins f zinc spinels fr ther parts f the wrld. The analyses duster int tw distinct grups, Mg-rich spinels f etarphic rigin and Mg-pr spinels (including the Nigerian gahnite) with igneus affinities. Diadchy sees t perate within the zinc spinel structure between (Zn + Mn) and (Fe + Mg). GEM-QUALtTY gahnite (zinc spinel) ccurs as an accessry phase disseinated in pegatites and quartz-sillianite veins f late Pan-African age ( Ma) in central Nigeria. The veins crsscut etasedientary sequences f ica schists and aphiblites. The pegatites are in a zne extending fr near Ire, in SW Nigeria, 400 k NE twards Js in central Nigeria. Within the ineralized pegatites, cassiterite is the dinant re ineral, fllwed by inerals f the clubitetantalite series. Pegatites fund within granitic bdies are invariably barren f re inerals. The Nigerian pegatites have been crrelated with siilar Sn-Nb-Ta ineralized pegatites f the Aapa Territry situated alng the eastern argin f the Guyana cratn in Suth Aerica (Klsteran, 1969). Sn- and Zn-bearing varieties f spinel have als been identified in these pegatites (Klsteran, 1970). Jacbsn and Webb (1946) subdivided the Nigerian pegatites int three grups accrding t their ineralgy: (1) icrdine-quartz pegatites, which cnly ccur within the ealc-alkaline granitids and are rarely ineralized; (2) icrdine-quartz-ica pegatites, fund within etasedientary sequences; (3) quartz-ica veins, which ccur in schists and gneisses r arginal t grup 2 pegatites. Gahnite ccurs in grups 2 and 3. The pegatite bdy fr which the analysed gahnites were cllected frs a heavily frested t~ Cpyright the Mineralgical Sciety ridge, 3.2 k nrth f Jeaa (9 ~ 29' N 8 ~ 24' E), with a strike length f apprxiately 100. The pegatite, which is 18 wide, cuts dark streaky hrnblende-bitite gneiss. Quartz and uscvite are the ajr inerals, with gahnite as the st cn accessry, fring apprxiately f the rck. Traces f albite and tantalite ccur, and schrl turaline is abundant at the cntact with the cuntry rck. Mineral descriptin. The ge-quality gahnite varies in clur fr greenish blue t deep blue. It ccurs as euhedral ctahedra fr 2 t 5 in size, rarely reaching 1 c. Ddecahedral faces ccur ccasinally as a slight bevelling between the ctahedral faces. Twins are rare. The refractive index, deterined n a Rayner Dialdex refracteter using sdiu light, was , and the specific gravities, easured n a Beran density trsin balance, lay in the range t (Jacksn, 1982). Speciens with a greener hue tended twards the lwer specific gravity values. The blue clur in gahnites has been ascribed t the Fe 2+ catin by Andersn and Payne (1937). Jacksn (1982) nted that n heating t 1000 ~ the blue Nigerian gahnite changed t a peranent blue-green clur, and at 1400~ the clur becae live green. He pstulated that the clur change was due t irn xidatin. If ferrus irn causes the blue clur and ferric irn a yellw clur, a ixture f ferrus and ferric irn ay give a green clur. Thus the clur f gahnite ay be an indicatr f crystallizatin teperatures and/r xygen fugacity at the tie f fratin. Analytical prcedure. A nuber f unbleished transparent blue crystals were pwdered t less than 150:< 10-6 and fur aliquts f 0.05 g were analysed. The analytical ethd invlved fusin with lithiu etabrate fllwed by atic absrptin spectrphtetry, based n Ingaells (1970), van Ln and Parissis (1969), and Bar and Ingrain (1970). The slutin was aspirated directly int a Varian Techtrn AA4 spectrphteter and the readings cpared with artificial aqueus standards spiked with lanthanu, lithiu etabrate, and nitric acid t atch the saple slutin atrix. Fr the deterinatin f zinc a 1 : 50 dilutin was prepared and spiked accbrdingly.

2 426 R. A. BATCHELOR AND J. A. KINNAIRD X-ray diffractin studies were perfred using Cu-K~ radiatin (36 kv, 18 A) n a Philips PW1540 gnieter syste which incrprated a curved crystal nchratr. Results. An X-ray pwder diffractgra prduced a subset f peaks which culd nt be attributed t gahnite. They cincided with ~-A120 3 (crundu) and a trace f lw quartz. Fr the area under the 11 0 peaks fr gahnite and crundu (333 and 6.5 units respectively) an estiate f 5 wt. ~ A120 3 was btained which was used t crrect the cheical analysis (Table I). The crrected psitin clse t the zinc end-eber f the spinel grup (8.08 A; Deer et al., 1962). The bradening f the n[111] reflectins suggests cpsitinal znatin within the ineral (Whittaker, 1981). An electrn icrprbe study f ferran gahnite in granite fr New Zealand (Tullch, 1981) shwed cpsitinal zning in irn and zinc fr cre t ri. Discussin. In additin t the new Nigerian analyses, previusly reprted zinc-spinel cpsitins were pltted n a triangular variatin diagra displaying the three ajr catins, Zn, Mg, Fe, expressed as lecular ratis (fig. 1). A bidal TABLE I. Cheical analysis f Nigerian 9ahnite Actual Crr. Std. Frula wt. ~ wt. ~ dev. based n 32 O Zn GAHNITE ASSOCIATIONS 9 Igneus SiO Si 0.18 A Al FeO Fe 0.77 MnO Mn 0.08 MgO Mg 0.05 ZnO Zn 7.00 Ttal Metarphic 21 The values and standard deviatins fr A1, Fe, and Zn are based n fur deterinatins. The crrected values were btained after reval f an estiated excess 5 A120~, ascertained fr XRD data. analysis is siilar t that f a gahnite described by Sipsn (1931) which ccurred in an alkali feldspar granite fr New Zealand (Table II, cl. 4). The lw MgO cntent f bth (0.13 and 0.27 ~ respectively) and the igneus assciatin are ntable. Althugh an ptical exainatin f this gahnite saple (Jacksn, 1982) revealed n free crundu, the presence f crundu peaks n the X-ray diffractgra shws that the excess aluina in the structural frula ust be ascribed, at least in part, t crundu, prbably in cryptcrystauine fr, thugh catin deficiency in the spinel lattice ight accunt fr se f the excess A120 a. Artificial Mg-spinels have been prduced cntaining excess A120 a withut changing the spinel structure (Deer et al., 1962), and crundu can ccur as a partial r cplete pseudrph after hercynite (Palache et al., 1944). The unit cell average fr all the peaks is t-0.011/~. The large errr results fr peak bradening in the n[111] reflectins. Discunting these brad peaks gives a unit cell diensin a = 8.091_+0.003A. This crrespnds t a crn- Mg FIG. 1. Mlecular ratis fr Mg, Zn, and Fe. Saple pints fall int tw ain fields, ne an igneus assciatin and the ther a etarphic assciatin. Saple nubers as in Table II. distributin is evident, with Mg acting as the discriinatr. An investigatin f the gelgical envirnent f each saple led t the cnclusin that lw-mg gahnites are assciated ainly with igneus pegatites r highly differentiated granites, whereas high-mg gahnites are invariably assciated with schists r anatectic pegatites. Exceptins t this ce fr ld data (Shannn, 1923; Brush, 1871). The behaviur f Mg in igneus and etarphic envirnents differs. Igneus pegatite envirnents reflect the final stages f a crystal fractinatin sequence, in which Mg, being assciated with early crystallizing afic phases, wuld be greatly depleted in the elt. During etarphis, hwever, Mg culd be re-bilized int the fluid phase by the breakdwn f bitite r aphible and bece available t a crystallizing spinel. Staurlite dehydratin has been identified as Fe

3 GAHN1TE COMPOS!TIONS COMPARED ~ % ~ ! ~ N # -,t ~! 9, ~. u 9 ~ ~! 9!..... ~ I.... ~ ~ 9 + d E

4 428 R. A. BATCHELOR AND J. A. KINNAIRD the echanis fr the prductin f zinc-rich hercynite in etapelitic gneisses in the Adirndacks (Stddard, 1979). These etarphic spinels cntain 1.59 t 9.9 % MgO. T investigate the behaviur f Zn, Fe, and Mg within the spinel structure, lecular ratis f these ajr catins were pltted against a cn structural eleent, aluiniu. This diagra, prpsed by Pearce (1968), highlights the extent f diadchic substitutins within a structure. Perfect diadchy prduces a straight line plt with slpe -1. The relatinship between zinc and irn was exained. The plt gave a gd linear crrelatin but the slpe eant that these tw eleents alne d nt explain the eleent variatins in gahnite. A plt f tl Zn/A1 versus tl (Fe + Mg)/A1 gave a slpe f which leaves zinc sites deficient. Ml (Zn+Mn)/A1 versus l (Fe + Mg)/A1 were then pltted (fig. 2) and yielded a tl Zn+Mn AI 5"" (21) ~ "'. Igneus assciatin "...122,, 1~7".. t. $. Metarphic 19 20,15.. assciatin <t:< tl Fe+Mg AI 0"5 FIG. 2. Bivariate plt f the lecular rati (Zn + Mn)/A1 against the lecular rati (Fe + Mg)/AI. (Diagra after Pearce, 1968.) The slpe f the best-fit line is Saple nubers as in Table II. gd straight line f slpe This suggests that diadchy exists between (Zn + Mn) and (Fe + Mg) within the gahnite structure. An arbitrary divisin drawn between the igneus and etarphic gahnites ccurs at l Zn/Fe = 3.8. Zinc tends t cncentrate (within a fractinating regie) in later irn inerals and therefre Zn/Fe ratis will tend t increase with differentiatin (Taylr, 1965). This bservatin is brne ut in zned gahnites (Tullch, 1981) in which the cre (saple 12) has a l Zn/Fe rati f 3.9, cpared with the ri (saple 13) with a l Zn/Fe rati f 6.9. The saples fred in highly fractinated agas will have the highest rati, e.g. saple 5 fr alkali feldspar granite has a tl Zn/Fe rati f Saple 10 riginates fr a skarn depsit in which alteratin f feldspar under xidizing cnditins in the presence f zinc-rich slutins is prpsed fr its fratin (Lisitsyn and Yurkina, 1974). It plts in the transitin zne between inferred znes f igneus and etarphic rigin. Acknwledgeents. JAK wuld like t thank Mining Cnsultants f Nigeria Ltd., fr financial supprt, and particularly G. Staley and A. Chilctt wh helped t lcate the gahnite. The encurageent f Prfessr E. K. Waltn is acknwledged. The authrs wuld als like t thank Brian Jacksn f the Ryal Scttish Museu fr his interest and fr the physical deterinatins n the blue gahnite. Alistair Reid helped with X-ray diffractin wrk. REFERENCES Alvarez-Perez, A., Capa-Vineta, J. A., and Mntril- Pus, J. (1974) Acta Gel. Hispanica, 9, Andersn, D. W., and Payne, C. J. (1937) Mineral. Ma9. 24, Bar, P. L., and Ingra, L. K. (1970) Analyst, 95, Brush, G. J. (1871) A. J. Sci. (3rd ser.), 1, Deer, W. A., Hwie, R. A., and Zussan, J. (1962) Rckfring inerals, ft. Lngans, Lndn. Eskla, P. (1914) Gel. Fren. Frhandl. 36, Frst, B. R. (1973) A. Mineral. 58, Gandhi, S. M. (1971) Mineral. Mag. 38, Ingaells, C. O. (1970) Anal. Chi. Acta, 52, Jacksn, B. (1982) J. Gelgy, 18, Jacbsn, R. R. E., and Webb, J. S. (1946) Bull. Gel. Surv. Nigeria, N. t7. Klsteran, J. B. (1969) Prc. 2rid. Tech. Cnf. Tin, Bangkk, 1, Int. Tin Cuncil, Lndn, (1970) Prc. VIII Guyanas Gel. Cnf. Gergetwn, 18 pp. Lisitsyn, A. E., and Yurkina, K. B. (1974) Zap. Vses. Mineral. Obshch. 103, Palache, C., Beran, H., and Frndel, C. (1944) Dana's Syste f Mineralgy, 7th edn. 1. Jhn Wiley. Pearce, T. H. (1968) Cntrib. Mineral. Petrl. 19, Pehran, B. (1948) Bull. Gel. Inst. Uppsala 32, Shannn, E. V. (1923) A. Mineral. 8, Sipsn, E. S. (1930) J.R. Sc. West. Australia 16, (1931) Ibid. 17, (1937) Ibid. 23, Stddard, E. F. (I979) A. Mineral. 64, Taylr, S. R. (1965) Phys. Che. Earth, 6, 178.

5 GAHNITE COMPOSITIONS COMPARED 429 Tullch, A. J. (1981) Mineral. Mag. 44, van Ln, J. C., and Parissis, C. (1969) Analyst, 94, vn Knrring, O., and Dearnley, R. (1960) Mineral. Mag. 32, Whittaker, E. J. W. (1981) Crystallgraphy, Pergan. [Manuscript received 9 Septeber 1983; revised 25 Nveber 1983]