American Mineralogist, Volume61, pages1025-1026, 1976

A mineral phaseintermediate in compositionbetween sillimanite and mullite

We,RRrNcroNE. Ce.nrenoN Department of Mineralogy and Petrology Uniuersity of Cambridge, CB2,3EW, England

Abstract

A memberof the sillimanite-mullitesolid solutionseries with a chemicalcomposition midwaybetween that of sillimaniteand 3 : 2 mulliteoccurs in a metamorphosedlithomarge from Rathlin Island,Northern lreland.Its diffractionpattern displays weak but sharp/-odd maxima.characteristic of sillimanite.

Introduction 0.2 x 0.2 X 1.0 pm. No contrastattributable to structureor exsolutionwas apparent. Crystallographically,there is no reasonwhy solid domain solutionshould not be completebetween sillimanite ComPosition andrmullite(Burnham, 1964). The mostaluminous by XRF sillimanitereported so far, however,contains 48.6 Pure separatedmaterial was analyzed (method the mole percentSiO, and the most siliceousnatural of Norrish and Hutton, 1969)using has mullite43.0 mole percent SiO, (Aramaki, l96l;Cam- mostFe-rich stoichiometric sillimanite the writer eronand Ashworth,1972).The writer considersthat found(specimen 94191 from the BensonMine, New thesehave crystallized from a liquid formed by par- York), mullitefrom Rubh' a Chroman,Mull, Scot- land(specimen 76398), and a syntheticglass, FS45, as tial meltingof pelitic material,probably within the 'Accepted' temperaturerange 900-1100"C. Sillimanites occur- standards. valuesfor specimens94191 and ring in accidentalxenoliths or as xenocrystsin vol- 76398were obtainedby EPMA (methodof Sweat- canic rocks have been found to be stoichiometric man and Long, 1969),as wereanalyses of specimen p.27a) (Al,Fe)rSiOuwithin experimentalerror (Kwak, l97l; Sl800las(Bailey et al.,1924, nearthe Al-rich Cameron,in preparation),in accordancewith the limit for naturalmullites, and the oft-quoted Brandy- large activationenergy for mullitization(Wilson, wine Springssillimanite (Bell and Nord, 1974).Re- 1969).A third mode of formation,dehydration of producibleresults on finer-grainedmaterial could be powder gibbsite-halloysitemixtures resulting from the later- obtainedonly when the acid-treated hadbeen itic weatheringof basalts(Eyles, 1952) yielded mullite separatedfrom insolubleimpurities by heavyliquids at (Agrell and Langley,1958) but at and mountedin Araldite on a glassslide. Analyses Brockley,Rathlin Island, , a more werecarried out only on inclusion-freesingle crystals, siliceousphase is found. the estimatederror being t I percent (cl Kwak, t97r). Occurrence Structuralevidence The aluminosilicateforms the major component in posi- segregationsabout 5 mm in diameterin a contact- Cell dimensionswere computed from average powder metamorphosedbasaltic lithomarge, now a hercynite- tionsrelative to a Si standardof the I I strong radiation.The magnetite-cordierite(Ferr)-tridymite-'mullite' r ock linesbetween 56-77o 2d usingCuKa (specimens70846,708481). In thin sectionthe pale effectof substitutingFe for Al is seenby comparing 70846and yellow segregationslook like devitrifiedglass, but thefirst two resultsand those of specimens after acid treatment2they may be seen under the 70848(Table I ). Takinga setwith roughlyequivalent electronmicroscope to consistof tiny prismsup to Fe * Ti contents:specimens 94191, 100622,70846, 76398,and Sl800la, it may be shownthat a and V

1Catalog numbers in the Harker Collection,Cambridge. '20 percent HF followed by warming in l:l HCl. 3 Providedby the Instituteof GeologicalSciences, London 1025 t026 WARRINGTONE, CAMERON

TAsr-el. Chemicalcompositions and cell parametersof sillimanites, mullite.and the intermediateohases

SILI.Ir,IANITE MUI;LITE

Bradldlne Benson Asda Rathlin Mul I Springs 94I9I LOO522 70846 70848 76398 slSoola

A12O3 62.9 61.8 5r.5 54.A 54.1 67.6 68.9 S1O2 36.7 35.I 35.5 32.9 32.5 30.6 29.5 0.5 Fe20 3 2.6 2.O 2.e o.5 Ti02 o.02 o.o8 o.2s o.24 o.7 I.O

BotaI 99. I roo.2 99.8 1€.O 99.9 99.5 99.9 Molt SlO2 49.6 i O.5 49.6 r O.5 4S.6I O.5 45.4! O.5 45.6I o.5 43.Oi O.5 4r.6 ! O.5 a (El 7.4831(4) * 7.4883(4) 7 .4963(41 ?.5O8O(6) 7.5088(8) 7 .5260 13\ 7. s384(4) b (f) 7.6710(4) 7.575314) 7.685r (4) ?.688r (4) 7 .6934(61 7,5882(3) 7,5933141 c(f) 2.8850(2) + 2.Sg72Ql+ 2.8442 l2l 2.8880(2) 2.8893(3) 2.8552(2t 2.SAS4l2l v(!) 3 165.61(3) 166.ol ( 3) 156.39 (3) L66.70(41 165.9r (4) r57.rr (3) r57.52(3)

(esil a Puentheeited figwea tepreeilt thc eeti,rwted. stutd@d deviqtim in temg of, le@t mitq cited f,or the rnlre to thsi? imediate Laft' thw ?.4831(0 indiatee an eod of 0.ooo4. + ilalf orrr". Uae of, the peadenepeat faetlitataa aq9atieon uith tuLl.ite.

increasesmoothly with decreasingrpole percent SiOr. at possiblemetastability relative to a sillimanitewith The b and c dimensionsare rather more sensitiveto a small amount of excessAl, e.g. specimen100622, variationin minor elementcontents. coexistingwith a silica-richmullite, e.g.76398. Selectedafea electron diffraction patternsof the Rathlin phasesshow weak, sharp /-odd reflections References consistentwith the spacegroup Pbnm,that of silli- AcRELL,S. O. eNn J. M. LnNcl-ev(1958) The doleriteplug at manite.No diffusereflections are present. As a some' Tievebulliagh,near ,Co. Antrim. Proc. Roy. Irish what subjectivejudgment, the ratio of the average Acad.59.93-12'l . intensity of the /-odd to /-evenreflections appeared Annunxt,S. (1961)Sillimanite and cordierite from volcanic xeno- 154-l165. for stoichiometricsillimanite liths.Am. Mineral.46,l slightly lessthan that BArLEy.E. 8., C. T. Cloucn, W. B. WRlcHr,J. E. Rtcnrv nno and the Asama sample,100622. G. V. WILsoN (1926)Tertiary and post-Tertiarygeology of The infrarecl spectrum of specimen70848 was Mull, Loch Aline, and Oban.Mem. Geol-Suru. . found to be very similarto that of the Asamasilli- Brll, P. M. ANDC. Nono (1974)Microscopic and electrondif- Year manite (trace B, Fig. 2, Cameronand Ashworth fractionstudy of fibrolitic sillimanite.Carnegie Inst Wash. Book.13. 444-448. 1972),both in thepositions and shapes of absorption Bunuunv, C. W. (1964)Composition limits of mullite,and the maxima. sillimanite-mullitesolid solutionproblem. Carnegie Inst. Wash' Discussion Year&ook.63.227-228. The phasedescribed is besttermed an aluminous CmrenoN,W. E. aNo J. R. Asswonru (1972)Fibrolite and its to sillimanite.Nature Phys' Sci. 235' 134-136. with the implicationthat the changeto relationship sillimanite, Evt-Es,V. A. (1952)The compositionand originof the Antrim the mullite domain structureis at a greaterAllSi lateritesand bauxites.Mem. Geol.Suro. Northein lreland' 90 p. ratio. A simple hydrother,malreaction either of a Hnnrvl, Y., W. A. Dollrse nNn G. C. Klt'rnrov (1969)An highly specificgibbsite-halloysite bulk composition experimentalinvestigatiop of the relationshipof mullite to sil- or over a narrowly defined temperaturerange, or limanite.Am. Mineral. 54, 1419-1441. (1971) of naturalsillimanites from for the rarity of intermediate Kwnr, T. A. P. Compositions both, could accouht volcanicinclusions and metamorphicrocks. Am. Mineral. 56' phases.Alternatively, iron may providethe stabiliz- l 750-I 759. ing effect,since no intermediatecompositions in the MunN,A. (1957)Phase equilibria at liquidustemperatures in the binary systemAlrO3-SiO, have yet been described, systemiron oxide-AtOr-SiOr in air atmosphere.J. Am. Cetam. exceptperhaps those synthesized at high pressuresby Soc.40,l2l-133. rNo T. HurroN (1969)An accurateX-ray spectro- (1969)where chemical compositions Nonnrsu,K. J. Hariya et al. graphicmethod for analysisof a wide rangeof geologicalsam- were inferred from cell dimensions.Synthetic Fe- ples.Geochim. Cosmochim. Acta,33, 431-45J. mullites containingexcess silica have beenreported Sweerunx,T. R. ,ruoJ. V. P.LoNc (1969)Quantitative electron- (Muan, 1957).The writer'srepetition of his experi- probe micro-analysisof rock-formingminerals. J' Petrol. 10, ments,but followedby HF treatmentof the products, 332-379. (1969) formationfrom the sillimanite (by with (Al * Fe):Si WrLsoN,H. H. Mullite yieldedmullites E.M. andI.R.) group minerals.Ceramic Bull. 4t,'196-'197. ratios closeto 3 :2. The absenceof low Fe * Ti intermediatephases in sanidinite-faciesrocks which Manuscript receiued,August 19, 1975;accepted haveinvolved large-scale diffusion of Al and Si hints for publication,FebruarY 25' 1976.