Iron.Rich Amesite from the Lake Asbestos Mine. Black

Canodian Mineralogist Yol.22, pp. 43742 (1984)

IRON.RICHAMESITE FROM THE LAKE ASBESTOS MINE. BLACKLAKE. OUEBEC

MEHMET YEYZT TANER,* AND ROGER LAURENT DAporternentde Gdologie,Universitd Loval, Qudbec,Qudbec GIK 7P4

ABSTRACT o 90.02(1l)', P W.42(12)',1 89.96(8)'.A notreconnais- sance,c'est la premibrefois qu'on ddcritune am6site riche Iron-rich amesite is found in a metasomatically altered enfer. Elles'ct form€ependant l'altdration hydrothermale granite sheet20 to 40 cm thick emplacedin serpentinite of du granitedans la serpentinite,dans les m€mes conditions the Thetford Mi[es ophiolite complex at the Lake Asbestos debasses pression et temperaturequi ont prdsid6d la for- mine (z16o01'N,11"22' W) ntheQuebec Appalachians.The mation de la rodingite dansle granite et de I'amiante- amesiteis associatedsdth 4lodingife 6semblage(grossu- chrysotiledans la serpentinite. lar + calcite t diopside t clinozoisite) that has replaced the primary minerals of the granite. The Quebec amesite Mots-clds:am6site, rodingite, granite, complexeophio- occurs as subhedral grains 2@ to 6@ pm.in diameter that litique, Thetford Mines, Qu6bec. have a tabular habit. It is optically positive with a small 2V, a 1.612,1 1.630,(t -'o = 0.018).Its structuralfor- INTRoDUc"iloN mula, calculated from electron-microprobe data, is: (Mg1.1Fe6.eA1s.e)(Alo.esil.df Os(OH)r.2. X-ray powder- Amesite is a raxehydrated aluminosilicate of mag- diffraction yield data dvalues that are systematicallygreater nesium in which some ferrous iron usually is found than those of amesitefrom Chester, Massachusetts,prob- replacingmapesium. The extent of this replacement ably becauseof the partial replacement of Mg by Fe. The calculatedunit-ceu parameters are: a 5.385(Q,b9.2gl(5), can be appreciable in some cases,so that the ideal composition of arnesiteis given in terms of AlrO3, c 14.124(16)A and o 90.02(ll)", B %.AQ2). afi 7 89.96(8)'. To our knowledge, this is the fhst specimenof (MgO, FeO), SiO2and HrO. Amesite, with chamo- iron-rich amesite to be described. It formed during site, greenalite and cronstedtite, is related chemical- hydrotiermal alteration of granite in serpentinite at the ly to the chlorite goup aud structurally to the samelow P-T conditions that prevailed during the forma- serpentineand kandite gfoup of minerals. The name tion of the rodingite suite and the chrysotile asbestosin the septechlorite has been proposed for these minerals serpentinite. (Nelson & Roy 1958)as they are structurally charap- terized by serpentine-like layers with d@r - 7 L. Keywords: amesite, lgdingrte, ophiolitic complex, Thet- Largely on the basisof thermal ford Mines, Quebec. studies, Orcel(1927) and Orcel et al. (1950)were the first to suggestthat Souuarnn amesitemust be regarded as distinct from the main group of chlorites. According to the AIPEA (Associ- On trouve une nm6site riche en fer dans une dcaille de ation Internationale Pour I'Etude des Argiles) 20 i 40 cm d'dpaisseurde granite mdtasomatiquementalt€r6 Nomenclature Committee, ,tmesitebelongs to the d la mine du lac d'Amiante (46'01'N, 71o22'W). Ce gra- kaolinite-serpentine group (Bailey 1980a);this com- place nite a 6td mis en dans la serpentinite du complexe mittee recommendedthat the nameskandite andsep ophiolitique de Thetford Mines. L'am€site est associ€ei techlorite not be used for kaolinite- and un assemblagersdingitigue Grossulaire + calcite * diopside t clinozoisite) qui remplace les min&1ux primaires serpentine-group minerals. du granite. L'am6site qudbdcoiseapparait en grains sub- We found an iron-rich variety of amesiteassociat- idiomorphes i facies tabulaire de 200 d 600 m de diam&- ed with gxossular,calcite and, more rarely, diopside tre. EIle est optiquement positive av*, faible2V et a 1.612, and clinozoisite in a rodingitized body of granite en- - 1 1.630 (r o : 0.018). Sa formule structurale esl closedwithin the serpentinizedperidotite of the Lake partir calcul6e i des donndes de la microsonde: Asbestos mine, at Black Lake in the Ap- (Mg1.1Fe0.eAb.t(Ab.eSi1.dO5(OH)r.u. par Quebec Les donndes palachians. We report here the description of this diffraction des rayons X (mdthode des poudres) indiquenr que les espacementsd sont systdmatiquementplus grands occurTence. pour l'am€site du lac d'Amiante que pour celle de Ches- ter, Massachusetts,ce qui est probablement dOau rempla- GEoLocIcAL SETrING cementpartiel de Mg par Fe. La maille 6l6mentairea pogr The Lake Asbestosmine is located at 46'01 'N, param&trescalcut6s: a 5.385(6),b9.291(5), c 14.12-4lAL, 71"22'W rnthe Black Lake areanear Thetford Mine (Frg. l). The serpentinized peridotite bearing the *Present address:Mineral Exploration ResearchInstitute, chrysotile-asbestosore forms the lower strustural Casepostale @79, SuccursaleA, Montrdal, eudbec H3C unit of the Thetford Mines ophiolite complex (Lau- 3N rent l95a). This ophiolite was tectonically emplaced 437 438 THE CANADIAN MINERALOGIST

:=-:-:€loct t =_:-:_:-_-,------=l ./I EXPLANATION ffi cotcrellGroup f] Porldoilirlcclonllc @ Grcnlrr nr Thrult toult C? Opcnplt

lo.l8ea.> *n-/:+ l63ellc-rich drkc

['"{W/ o I -.\ :-tlo.l898l -l- \gl I LAKEAsBEsTos MNEI /

oLi-t' LocATroNtvrAp

Ftc. l. Geological map of the Lake Asbestos mine at Black Lake and location of the amesite-bearingrodingrte.

in the internal tectonic domain of the Quebec Ap- palachians during late Early or Early Middle Ordo- vician time (St-Julien & Hubert 1975).The ophiolite has been thrust over regionally metamorphosed Cambrian rocks known as the Caldwell Formation. The thrust fault dips at a high angleto the soutleast. The asbestosmines are located near this tectonic contact in the peridotitic lower unit of the ophiolite. The serpentinizedperidotite of the Lake Asbestos mine @ig. l) is a tectonite harzburgite grading locally to dunite. The rock is strongly serpentinized along shear zones, which trend in a northeasterly direction parallel to the basaltectonic contact. Sheet- like and lenticular bodiesofgranite' from lessthan I m to more than 100 m thick, occur especiallywi- thin the shear zones. They are deformed and hydrothermallyaltered (Cooke 1937,Riordon 1953' De 1972,Laurent 1975b,Laurent & H|bett 1979, Laurent 1982, Laurent et al. 1984). is found 11 a l6dingife lo- Ftc.2. View ef tfus thin and lenticular body of amesite- The iron_rich amesite bearing rodingrte within the highly serpentinizedharz- catedinthe southeasterncorner of the LakeAsbestos burgite of the Lake Asbestosmine (ocation of outcrop mine at 3500N and 9250E (Fig. 1). This ro.lingite given in Figure l). oscupiesa northwest-striking fault plane (oblique to AMESITE FROM TIIE LAKE ASBESTOS MINE, QUEBEC 439

Fro.3. Samplela,l898l of iron-rich amesitein thin sectionunder crossednicols. a. Single crystal showing a perfect cleavageparallel to (@l) and sector twinning on (@l), surroundedby isotropic grossular.b. Group of amesitecrystals (A) with different optical orientations, showing cleavageand twinning, in associationwith calcite (C) and grossular (G), Bar represents0.1 mm.

the main shear-zone)that dips subvertically within 2 m, hydrothermal alteration is limited to their mar- hiebly serpentinizedharzburgite. The body of roding- gins. This alteration id thought to be associatedwith ite is thin and lenticular (Fig. 2), between 20 and the episode of serpentinization that led to the de- 40 cm thick and at least l0 to 20 m long. The pro- velopment of chrysotile-asbestosin the enclosing tolith of the redingite was a calcalkatinebiotite-rich peridotite (Laurent & Hdbert 1979,Laurent 1980, eranite sirnilar in composition to the thick and well- Laurent et al. 1984). preservedsheets of granite tlat crop out in the mine; they have been describedrecently by Laurent et al. PBTROGRAPHY (1984).In thosegranite sheetsthat are thicker than Rodingite sample la. 18981containing the iron- 440 TIIE CANADIAN MINERALOGIST

TABLE I. CHE}'IICAL @}TPOSITION OF A'1F.9ITE-BEARINC NODINGITE HABIT AND OPucel PnoPsnuEs AND OF CROSSI'LAR Samle la.l898l crosaular 10 contact Gruner (1944) described the macroscopic and samle zonei rlth [email protected] physical properties of amesite. Becauseof the small core @r91tr ;ize of the crystals, our observations of the sior 37.35 33.18 | 2 Quebec rro; o.o? 0.09 sto2 40.20 40.27 iron-rich amesite are limited to their microscopic Al2d1 23.3E 24.71 AIZ03 22.04 20.65 O.44 features. fer-oe- 0.33 0.14 Feo 0.28 re6 l.6I o.a7 Mno o.o9 o.l7 The iron-rich amesiteoccurs as groups of euhedral MgO l.2l 0.97 cao 37.A4 38.45 600 pm lino 0.19 O.ll to anhedral crystals oftabular habit, 200 to cao 34.24 34.5I T0TAL 100.45 99.97 wide and 50 to 100 1r.mthick, occupying the inter- Naro - O.3l No. of toc o ttE ba81sof 24(0) xr6 o.o9 o. 19 si 6.ot 6.07 sticesbetween clusters of grossular grains. The ane- H;o r.r5 4.lo Al 3.88 1.67 site crystalshave a perfect cleavageparallel to {001} c6r o.57 0.43 Fe 0.03 0.05 roiAl 16:ZZ ld6;ZT lh o.ol o.o2 6pical of phyllosilicats, and display sectortwinning Ca 6.07 6.21 on (01) as well as hourglasstexture @ig. 3a, b). The sattole la.1898l: Frossulat (-902)' dlopslde' aneslte' calclte' amesite is biaxial positive, with a 2V of about Z)o ctLnozoldtte, hlottte (pri@ry), chlortte' sPhene"!rcon (prtury), opaque phaae. and positive elongation. It has weak dispersion, r < Gro66u1ar amlyzed by electron idcroplobe' rodloglte aoal)€eg v. XRF AA. by and Indices of refraction were measlued with immer- sion oils calibrated with a refractometer:a 1.612, "y 1.630(t 0.002), cu- y 0.018. The small crystal- rich amesiteis a white, homogeneous,fine-grained size of our sampleshas allowed us to measureonly rock with a ganoblastic textue. A daxk margin ttrat the maximum and the minimum indices. Becauseof is a few millimetres thick, composedof chlorite and the very small 2Z angle observed and the pseudo- vermiculite, is presentat the contact with the serpen- hexagonalsymmetry of the crystals, B has been con- tinite. The rodingite con$istsof about 9090euhedral sidered equal to o. Its ideal space-group is P63 to anhedral grains of grossular 50 to 100 pm in di- (Steinfink & Brunton 1956). The indices given for ameter, replacingthe much larger grains of feldspar. the Chester amesite (Gruner 1944) arc: a = I = The grossular is closely associatedwith amesiteand L.597,t : 1.6L2.The inferredrelationship between calcite, which occupy spacesbetween cluster$ of gax- the indices of refraction of amesite and the ratio of net gpins (Fre. 3) and constitute less than 1090 of Fd+ and Mg is summarized in Figure 4. the material. Small amounts of diopside, titanite and clinozoisite are locally associatedwith this rodingitic assemblage.Rare relics of primary biotite and zir- con of the granite protolith can be observed.Locally, TABLE 2, CHEX'TICALCOMPOSITIO}I OF AHESITE amesite appears to have grown at the expenseof primary biotite and may be regarded as its replace- (fa.l898l) c:hester' Mreaachuaetta ment product. Table 1 gives the chemical composi- lake Asbestos tion of the amesite-bearing rodingite and of the I grossulax in contact with amesite. SiO2 20.32 20.lr 20.95 Tro2 0.03 Ar2o3 31.68 31.14 35.21 FeO'* 21.74 22.48 8.28 Un0 0.41 0.34 frac€ CaO 0.43 0.58 22.88 ilgo ^ 14.88 14.59 l12O z* 10.54 10.84 13.02 TOTAl. i60m r00.@ 100.92 or5 (o) *u ll*'*'i.;;r "'*,l3ii" 1.[31". atlv 0.943 0.953 0.994 Alvt 0.999 0.960 0.999 Fe 0.945 o.979 0.330 Mn 0.017 0.0t4 Cs 0.023 0.025 MS t. r53 l.132 1.637 OH 3.656 3.765 4.00

llg" Fe2' l. (Ms1.15Fes.e4 Aro.ss)!!o.sa !fr.oslls !9H13'6s z. 1ngi.i! ree.!7 n16.e6)(Aro.l5st1.s4)o5 !0Hi3.76 Frc.4. Graph of the inferred relationship between the in- 3. (ldi.;; F"o.;i ero.ii)(ero.gg st1.6e)o5(oH)4.00 dices of refraction and the Mg:Fe ratio of amesite. 1. deposit, Northern Urals, USSR lr A1l- lron @n8ldered 6 feoi Saranovskoyechromite 2. HrO deternlned bY dlfference; (Anderson & Bailey l98l). 2. Chester, Massachusetts 3* c;lculated by Deer et aL. (1962) (Gruner l9M). 3. Lake Asbestos mine (this study). (lnalyet: J.-P. TrenblaY) AMESITE FROM THE LAKE ASBESTOSMINE, QUEBEC 441

Csgpttstny TABLE 3, X-RAY POI.IDER-DIFFRACTIONDATA I'OR AIISSITE Two single Lake Aabeatos l{lne Cheater, l'laaaachusetts crystalsof amesitewere analyzed with do6s(A") Iobs hk1 d(A') I/to hkl an ARL electronmicroprobe utilizing an operating voltageof 15kV and a beam currentof0.15 nA. The 7.06 100 002 100 ao2 standardsused are orthoclase for Al and Si, and 4.63 20 020 4.54 20 o2* kaersutite(Kakanui hornblende,Smithsonian Insti- 3.53 90 004 100 004 2.640 60 20I 20 201 tution) for Fe and Mg. Integration time was ten se- 2.5t2 70 202 2.48 60 202 conds,and four points wereexamined per analysis. 2.332 30 203 2.32 50 006,203 2.t32 l0 244 2.ll 30 204 The data were corrected using a ZAF program. I .938R 30 205 t.925 70 205 H2O was calculated by difference. Repplicate anal- I .76lR 5 008 t.749 50 206,OO8 yses of standards indicate that 1.609 5 207 r. )ro 60 207 the determinations I .549 20 060 l .531 60 060 have a relative accuracyof x. I to 2t/o for major ele- l.5t5R 5 062 t.495 20 o62 ments. Samplesare consideredto be relatively pure 1.472 5 208 1.461 60 208 1.4r8 5 064 r.401 60 064,0010 and homogeneous.No visible inclusionsof opaque I .336 5 307 1.342 50 209 minerals were observed.Only trace amounts of gros- l.3I7R l0 262 1.304 l0 262 t.290 5 263 t.276 l0 263 sular and calcite@ig. 3a, b) are present,as suggest- plus 22 l-lnes to 0.0788 ed by the Ca content recorded(Table 2). However, Data oo Quebec anealte obtal.ned on a polycryatalllne it should be noted that the Chesteramesite has the aanple Gi.ng caodolfi 114.6 mn qrera utllizing nlckel- same concentrationof calcium. flLtered CuKo X-radlatlotr. Iotensttlea estl.nated z* pDF The oxidation state of iron vteually. PDF 9-493. rsee reoark. R! dlffrac- was not determined; ti.oo l{nes showlng slgntficant differences betoeeo the for the calculationof the structural formula, all the calculated and observed 20 valuea. The structural Lnter- iron was consideredto be Fd+ pretatlon of the obaerved dlfferenceB requires new rork and to substitute for ns lo progreas. Mg2+ in the amesitestructure. Thus, the ideal structural formula of amesite becomes [(Mg,Fe)Al] (Si,Al)Oj(OH)n. The amesite from the Lake As- bestosmine containsabout 22t/oF{ and l59o MgO. It is an iron-rich amesite. Anderson& Bailey(1981) to be more in keepingwith Our electron-microprobe data give structural for- the biaxial characterof amesiteand the intensity dis- mulae (Table 2) that are consequently different in tribution of its diffraction lines. conlposition from that of typical magnesianamesite. The iron-rich amesitewas collecedfrom athin sec- For example, the structural formula of the tion under the microscope.The microsample,which amesite-2H, from the Saranovskoye chromite representsa polycrystalline aryregateof severalcrys- deposit, northern Urals, USSR, analyzedwith the tals of apparentlypure amesite,was analyzedwith electronmicroprobe by Anderson & Bailey (1981), a ll4.Gmrn Gandolfi camerausing Ni-filtered CuKcu gives: (Mgs.e36Alo.ea3Fe2+o.625Cre.qr.fl6.ozJ(Sir.o,X radiation at room temperature.The X-ray-diffrac- Ah.ql)(OH)o on the basis of sevenoxygen atoms. tion data are reported in Table 3 together with the For comparison,we give (Table 2) the composition original data of Brindley et al. (1951).The compar- of the Chesteramesite (Gruner 1944),whose struc- ison showsthat the dvalues of the iron-rich amesite tural formula has been calculated by Deer et al. are systematically greater than the d values of the (1962). In the calculation of the structural formula magnesianamesite. This reflectsthe substitution of of the Black Lake amesite,we have followed the Mg2* by Fd+, the ionic radius of {d+ (0.71A) be- procedureof Deer et al, (1962). ing larger than that of Mg2* (0.66 A) in six-fold co- ordination (Whittaker & Muntus 1970). It should also be noted that the lines at.2,733A and 1.995A CRYSTALLoGRAPHY in the pattern of ttre magnesianamesite were assigned by Brindley et al. (1951)^toimpuritigs: thesecor- Gruner (1944) showed that the amesite structure respondto the lines2.7414 and 1.988Ain the Black is basedon a 1:l type la5rcr.Presently, amesite is clas- Lake amesite.They probably reflect impurities also. sified by AIPEA Nomenclature Committee as a spe- The refined values of the unit-cell dimensions de- cies of the subgroup serpentine,type l:l (Bailey termined by Anderson & Bailey (1981) are: a 1980a).The unit cell of amesitewas consideredto 5.307(l), b 9.195Q),c 14.068(3)A, o 90.09(2)',B be hexagonal and to contain two kaolin-type layers 90.25Q)', 7 89.96Q)". The unit-cell parametersof @rindley et al. l95l). Steinfink & Brunton (1956) the Black Lake amesitewere calculatedusing the cell- made the first detailed structural study of amesite; refinementprogr?m of Appleman & Evans (1973): by assuming an ideal o, hexagonal symmetry of P6r, a 5.385(Q,b 9.291(5),c 14.r24(lQA, cu90.02(l I ) they determined the cation distribution to be ran- P 90.42(12)'and.y 89.96(8)";V706.7r(L43) A3. It dom. Bailey (1980b) has confirmed this finding. is probable that the differences are due to the par- N.Iorerecently, a triclinic symmetry was shown by tial replacement of Mg by Fe. M2 THE CANADIAN MINERALOGIST

DlscusstoN CoorE, H.C. (1937):Thetford, Disraeli qqd "ealtern hali of Warwick map-areas,Quebec. Geol. Sum. The iron-rich amesite is closely associated with Can.Mem,2l7. grossular and calcite. This mineral assemblagede- Dn, A. (1972):Petrology of dikes emplacedin the veloped through metasomatism at the expenseof ulta;afic'rocks of southeasternQuebec and ori- quartz, feldsparsand biotite of the granite protolith. gin of the rodingite.Geol, Soc.Amer- Mem. 132, In this occlurence, amesite may be regarded as a 489-501. replacement product of the primary biotite. DBen, W.A., Hown, R.A. & ZussueN,J. (1962): The transformation of the granite into a rodingite Rock-forming Minerals. 3, SheetSilicates, Long- through calcium metasomatismis a byproduct of the mans,London. serpentinization of the enclosingperidotite (Laurent Evess,B.W. (19?7):Metamorphism of alpineperido- et ql. 1984). Thus the episode of hydrothermal al- tite and serpentinite.Ann. Rev.Earth Planet. Sci. teration responsiblefor rodingitization of the granite 5, 397447. is also responsible for the development of the Gnuurn,J.W. (1944):The kaolinitestructure of ame- chrysotile-asbestosore in the enclosingperidotite. It site,'(oH)6(Mg,Fe)aAt2(Si2Al)ors?9d -{dil^i9nal is therefore probable that the P-T conditions con- data on chlorites.Amer. Mineral. 29, 422430. llslling the amesiteand rodingitic paragenesisare Leunrrr, R. (1975a):Occurrences and origin of the similar to those that promoted the concomitant ser- ophiolites of southern Quebec, northern Ap- pentinization. Retrograde serpentinization usually palachians.Can. J. Earth Sci. 12' 43-455. occurs at temperatures in the 200-400oC range at I (1975b):Petrology of the alpine-typeserpen- to 5 kbar water pressure@vans 1977).This range tinites of Asbestosand Thetford Mines, Quebec. in temperature and pressureallows the development Schweiz.Mineral. Petrog, Mitt. 55, 431-455. of both lizardite and chrysotile that make up more (1980):Regimes of serpentinizationand rodin- than 5090 of the country rock at the contact with gitization in QuebecAppalachian ophiolites. ,4rci. the rodingite. The amesiteis thought to have formed Sci.Gendve 33, 311-320. under similar P-T conditions. (1982): Appalachian ophioliies ?9d actualism: a discussion.ofioliti 7, 213, 351-368. AcrNowt socsil{sNTs & HEnnnr,Y. (199): Paragenesisof serpentine *rcrnUtug.t in trariUurgitetectonite and dunite cu- We thank Gilles Bonin of the Lake Asbestosmine mulatefr6m the QuebecAppalachians.Can' Miner' for his support of our field work and R. B€landand al.17.857-869. for reviewingand T. Feiningerof Laval University M.F. & Brntn,lNo,J. (1984):Mise en grateful +,-place TANEI, editing our manuscript. We are also to the et p6trologiedu graniteassoci6 au complexe refereesfor their helpful commentsand to R. F. Mar- bphiolitiquede Thetford Mines, Qudbec'Can' J' tin, McGill University, for the calculation of the unit- Earth Sci.21 (in Press). parameters. by cell This study has been supported Nmsox,B.W. & Rov, R. (1958):Synthesis of the chlo- erant A8293 from the Nafural Sciencesand Engineer- rites and their structural and chemicalconstitution. ing ResearchCouncil of Canada to R. Laurent and Amer, Mineral. 43' 7W-714. by a grant from the "Direction Gdn6ralede l'En- Oncer, J. (1927):Recherches sur la composrtion-chi- seignementSup6rieur du Qu6bec". mi