Canadtan Mineralogist Vol. 16, pp. t7-22 (L978)

HVDRATED$ULFATES IN THE SVDNEVCOALFIELD, CAPE BRETON, NOVA SCOTIA*

ERWIN L. ZODROW Research Associate of the Nova Scotia Museurn, Ealifax, and Department of Mathematics and Natural Sciences, College of Cape Breton

KEITH McCANDLISH Department of Mathematics and Nataral Sciences, College ol Cape Breton, Sydney, Nova Scotia BlP 611

Arsrnecr visoirement dans la mine Evans, situ6e dans un bas- sin houiller carboniflre plus ancien que celui de Syd- Hydrated sulfates associatedwith decomposition ney. Ces sulfates sont des produits de la d6composi- of and copper-iron sulfides occur in coal in tion de sulfuresde Cu-Fe et peut-Otreaussi de pyrite. contact with the atmosphere., rozenite, (Iraduit par la R6daction) , pickeringite, halotrichite, aluminocopia- pite and sideronatrite were identified from coal seams and mines in the Sydney coalfield. This is PYRITE IN COAL apparentlythe first reported ocquren@ of siderona- trito and alumino-copiapite in North America, and investigation of occurrences and patterns for the remaining with the possible ex- An ception of melanterite, first occurrencesfrom Cape of distribution of hydrated sulfate minerals con- Breton. Crystallization of melanterite, and possibly stitutes the first phase of a proposed environ- of o&er hydrated sulfates, contributed substantially mental study program. The overall purpose of to the mechanical instabili8 of coal faces in old the investigation is to identify .and define water underground workings by erosion of supporting pollutants associated with oxidation of pyrite in pillars. In addition, alunite, , natrojarosite exposed coal seams, producing mines, and in and barite (?) were tentatively identified from the coal piles stored in the open air. Pyrite is a ubi- Evans coal mine situated in a Carboniferous basin quitous in the coal seams of the Sydney somewhat older than the Sydney basin. These sul- fates are associated with the decomposition of coalfield. copper-iron sulfides and possibly pyrite. AII minerals The Sydney coalfield, situated on the north- were identified by X-ray powder diffraction. eastern part of Cape Breton, fotms the soutb" eastern border of a deformed basin assumed to be of Carboniferous age @ell 1938). The basin Sorrtrvreng trends northeast into the Atlantic Ocean. Coal measures in the basin outorotrr along the Atlan- On trouve dessulfates hydrat6s r6sultant de la d€- tic coast in an east-west belt, but synclinal-anti- pyrite compositionde et de sulfuresde Cu-Fe dansle clinal structures on-shore modify this trend, charbon expos6 i l'air. M6lant6rite, roz6nite, epso- quite Bell (1938) divided the mite, pickeringite, halotrichite, aluminocopiapite et locally substantially. sid6ronatrite ont 6t6 identifi6es (par clich6s de pou- Sydney coalfield into three distinguishable zones dre) dans les couchesde charbon des mines du bas- using assemblages of fossil flora, i.e.r fern-like sin houiller de Sydney.C'est la premidre d6couverte and fern-type fossils. All coal mines in this basin de sid6ronatriteet d'alumino-copiapiteen Am6rique exploit coal measures that belong to the young- du Nord; quant aux autres min6raux, sauf peut-Otre est zone, which may be equated with the Upper Ia m6lant6rite, c'est la premidre fois qu'on les trouve Pennsylvanian. The Lingan mine and the 1-A au cap Breton. La formation de la m6lant6rite, et mine in Colliery exploit the Harbour seam, peut-Ctre #26 aussi celle des autres sulfates hydrat6s, a and the Prince mins 41 Point Aconi, the Stubbart grandement contribu6 i l'instabilit6 des fronts de sampling has not been taille, par 6rosion des piliers de soutbnemen!dans les seana@ig. 1). However, aucienschantiers souterrains.De plus, alunite, jaro- restricted to these seatns. sito, natrojarosite et barite(?) ont 6t6 identifides pro- Fine-grained pyrite in the coal measures of tle Sydney coalfield originated in peats of Penn- sylvanian age; however, structural studies indi- *Contribution from the Nova Scotia Museum, cate that coalifisation of the peat took place Halifax. N.S. much later. Pyrite mineraTization in coal is pos- L7 18 THE CANADIAN VIINERALOGIST

Pennian

olace Bey, Novescotla

q €6 SelectedCoal Measures of SydneyCoalfjeld o -€ e PoJntAconi o $ Lloyd Cove I Stubbart Y+ Harbour I I I Bouthil1 ier

Backpit to Sydney, N. S. Alr photoA 21536-42,t970 Phal en I Emery Frc. 2. Location map for the Phalen and Emery s€amsnear Glace Bay, Cape Breton, N.S. Gardiner

Tracy the original has apparently been trans- o formed to pyrite. Shoemaker ! o The Stubbart seam exploited in the Prince McAulay u J mine contains two conspicuousfine-grained py- J rite bands; thesemay be traced over a wide area in the mine. They are irregular in thickness Modified after Zodrowand (pyrite specimen 977GF-622 is approximately Mc0andlish(1978). I 4 cm thick), I locally nodular and discontinuous. Specimensof hydrated sulfates collected under- r ground in the Prince mine occur in these two bands. In addition pyrite may occur in radiating ppl Itlissi ssi an rosette-type structures less than 6 cm across (977cF-243), and in isolated nodules of 3 to 5 Frc. l. Coal meazuresof Sydney coaltield in ap- cm (977GF-205). As a replacement of plant proximate correlation to the Pennsylvanianseries tissue, pyrite plays only a minor role, but in of Nova Scotia (Cumberlandto Morien), Penn- specimen 977GF-244 it replaces a stigmarian sylvania @ottsvilleto Conemaugh),and Europe structure (root system of either Lepidodendron (Namurian to Stephanian). or Sigillaria species). Locally replacement of foliage of arborescent plants of the type Lyco- sibly derived from ester sulfates, hydrocarbon podites meekii Lesquereux (977GF-97b) or Le- sulfates that originate from biogenic sources pidodendron dawsoni Bell also occurs. (Casagrande& Siefert 1977). Concentrations of In the Phalen searn at the 1-B mine of #26 ester sulfates are much greater in marine than Colliery in Glace Bay, pyrite is disseminatedin in fresh-water peat. the coal, and hence hydrated sulfate minerals Selectedpyrite sampleswere examined by op- are distributed througbout this seam. In out- tical methods arrd by X-ray powder diffraction crops of the seam (Fig. 2) pyrite may be con- analysis (XRD). Preliminary results show traces centrated in bands or nodules; specimen977GF- only of marcasitein sample 977GF-58 (accession 177 furnishes an example of nodular pyrite. The and catalogue number of the Nova Scotia Mu- presenceof pyrite in coal seams,however, is not seum) from the McAulay seam. In sa,mples a guaranteethat hydrated sulfates have formed; 977GF-I76, F-177" and F-178 from the Phalen certain pyrite specimens show no evidence of seam (Fig. 2), polished sections (Fig. 3) show sulfate forrnation. that pyrite is present in the coal a) as nodules with apparently unstable cores, and b) between ANer-vrrcer. TscHNreurs veins of nodules as elongate crystals aligned perpendicular to tle veins. In tle second case XRD techniques were used in all identifica- SULFATES IN THE SYDNEY COALFIBLD t9

(977GF-653-3) were exposed to Fe-filtered CoKa radiation in the same camera, except that a 114.6 mm film of sideronatrite was also made. Epscmite was identified by comparison to a standard film, whereas sideronatrite was identi- fied by comparison with PDF 17-1,56,and with a standard pattern at the Geological Survey of Canada (pers. comm. L. J. Cabi L977). Samples 977GF-691 and 977GF-704 from the Evans Mine at St. Rose, Cape Breton, were exposed to Ni-filtered CuKa radiation (pers. com.m.M. Zentilli 1977), samples977GF-682-2 and 977GF-682-1-1 were exposed to CoKa radiation with an Fe-filter (pers. comm. J. K. Sutherland L977).

Hvoneruo Sur.rerrs AT 1-B MrNB. GLecB Bev

Co-existing in the Phalen seam (Fig. 4) are melanterite, sideronatrite, and epsomite. Me- lanteritq FeSOa.THrO,is the name given to the species wherever Fe ) Cu. Melanterite occurs throughout the coal face and crystallized per- pendicular to it as talcate fibres up to 3 cm long, and within the coal as oval-shaped, fibrous masses generally less than 5 cm long. Crystal growth of melanterite and other hydrated sul- fates is directly responsible for fragmentation of coal faces, which leads to progressive mechan- ical erosion of supporting pillars. Indeed, the stress necessaryfor the mechanical erosion of coal is supplied by the oxidation of pyrite and

Frc. 3. Infrared reflection image of elongatecrystals of pyrite perpendicularto the veins (x8); 977GF- 17? Negative.

tions of sulfates; these were supplemented by qualitative chemical analysesand optical exam- ination in specific cases.Melanterite and roze- nite (977GF-653-2) were identified by compari- son (PDF) with Powder Diffraction File 1-225 Frc. 4. Part plan of the 1-B mine, #26 Colliery, and 76-699, respectively; pickeringite (977GF- sample loca- 'rvas Glace Bay, N.S. showing numbered 682-l) compared to PDF 12-299. T\ese tions. Elevations (in feet) below sea level. Arrows minerals were exposed to Ni-filtered CuKa indicate return air flow (to the surface). Roof- radiation n a 57.3 mm Debye-Scherrercalnera. sulport pillars from mining activity (1913 to Sideronatrite (977GF - 653-1) and epsomite 1916) are shown as stippled areas. 20 THB CANADIAN MINERALOGIST attendant volume increases that occur to pro- to :t 90o; this compa.resto EOo in Mason & duce hydrated sulfates. Crosscuts driven sixty Berry (1968), and 85"276r synthetic melan- years ago now have a V-shaped appearance due terite in Na light @alache et al. L951.). Yari- to accumulation of melanterite-bearing coal de- ability in the optic sign implies variability in the tritus. Roofs consist of shalesor shaly sandstone composition of melanterite. This conjecture may in which the fossil plant content is high; in addi- be supported by difference in color of specimens tion, the growth of sideronatrite, epsomite and roughly equal in thickness, from greenish-blue other unidentified non-metallic minerals may to straw-yellow to white; impurities, of course, lead to weakening then roof collapse.The fossil may also cause color changes. Variation in the plants most detrimental in this context are large cornposition of melanterite may reflect samFle sigillarian trees, now compressed, and their location; this aspect has not been investigated. abundant reed-like foliage. The carbon remains Sideronatritq Na"Fe(SOa)e(OH).3H2O, is of this fossil material created planes of weak- bright lernon yellow and occurs as a fibrous ness in the sandstone; similar cornments apply crust on coal; it is stable in surface conditions. to selectedroof zones in the Prince mine, where Becauseof their similar colors, sideronatrite and tho most detri.mental fossil to roof stability is jarosite may be confused. Qualitative wet chem- CoJamites carinatus Sternberg (977GF-245), ical analysisindicates the presenceof phosphate, herbaceousreed-like plants several meters long. chromate, and possibly Ni, Al, and Mg in our However, as these old mine workings show,,'fie sideronatrite samples(977GF-653-1). Under the size of the fallen slabs is largely dependent on polarizing microscope, parallel extinction can enlargement of rooms by erosion of pillars. be easily observed dlre to the excellent cleavage Table I summarizes values of relative hurnid- on {10O}. Since the discovery of this mineral in ity and temperature of return air (ventilation) at the Sydney coalfield, we suspect that many min- which melanterite is found in the 1-B mine; no eral specimens tagged as jarosite are actually inference oan be made from this tabulation con- sideronatrite. This applies especially to the roofs cerning the lower limits of melanterite stability. of various working areas of the Prince mine and Tho temperature and humidity of the air re- the l-B mine. turning to the surface are constant and inde- The only location of sideronatrite known to pendent of surface atmospheric conditions; air the authors is in tle 1-B mine. At samples sites pressure in the mine is directly under the influ- I and 7 , Figure 4, sideronatrite "mushrooms" out ence of surface air pressure. of the coal face in patches parallel to the bed- Experiments in our laboratory indicate that ding planes. Flecks of a yellon mineral are melanterite is stable in an atmosphere having visible throughout the coal faces along the travel a relative humidity as low as 60Vo at 25-28"C. road (escape drift to surface); the mineral is as- The tetrahydrate rozenite, FeSOa.4HaO, was sumed to be sideronatrite. Yellow material. also obtained from melanterite specimens by dehy- coats fossil plants, especially Sisillcria, that are dration in atmospheric conditions. However, it preserved in the syringodendron condition is not entirely clear whether the tetrahydrate can (977GF-625,F-627). be formed without passing.first through the hep- Epscmite, MgSO4'7H,O, is white and fibrous; tahydrate stage, melanterite. Deternination of the fibres attan34 crn. Epsomite cannot be con- optic sips of melanterite from various sarnple fused with rozenite because of their different sites (Fig. 4) shows that the angle 2V lies close modes of origin and crystal habits. Epsomite is highty localized in tle roof of the Phalen seam and occurs with quartz; an inverse correlation is TABLEt. ArRTEMpERATURE (oc) nno nsurrvr HuMIDtry(s) FRoM observed between the occurrence of epsomite HYCROMETRICDATAAI.OIIG THE TRAVEL ROAD IN I-B l.itt{E,GLACE and melanterite. Epsornite was not observed on BAY,N. S. coal faces. Therefore, we conclude that this seam

sanple slte Toc R.H.g samplesite loc R.H., did not carry suf{icient quantities of mape- provides 'I tq o q sium to form tle mineral. The 1-B mine 90 6l '10 the only occurrence of this mineral known so I6 A 90 8l far. The collection sites include 5, 6 and 10 '15.4 'll 't 90 5.8 A] (Fig. 4), where tthe mineral was collected from 81 12 lE a 8l slabs of sandstone saved in from the roof. 15.8 8l l3 r5.3 8l Hoffmann (1890) was the first to report me- 8l l4 on lanterite and epsomite in Nova Scotia (Iraill l97O); he found melanterite on shale of mined 7 8l lE I 90 coal in Glace Bay, and epsomite in a A 81 quarry in Hants County. SULFATES IN THE SYDNEY COALFIELD 2l

HypnerEp Surnetss AT THE PnrNcr MrNr, 2), where it is visibly associated with fine- Pornr AcoNr grained pyrite (977GF - 177). Pickeringite, MgAL(SOa)a'22HzO (977GF-682-1), alumino- Melanterite and rozenite are the hydrated copiapite, MeAlFe(SOa)OH.H:O (977 GF-682), sulfates that have been identified from this mine. and halotrichite. FeAlz(sos)q.22H"o (977GF- Rozenite, FeSO,a.4H:O,is white and powdery, 682-l-l) coexisted in surface outcrops of the and readily soluble in water as is melanterite. Emery seam; heavy rains have since obliterated Some optical and chemical data relating to ro- these water-soluble minerals. zenite are given by Jambor & Traill (1963). This mineral is stable over a large range of fluctuat- The Evans coal mine, situated in an Upper ing atmospheric conditions. Our investigation Carboniferous basin near the town of Inverness, into sulfate mineralization was confined to areas exploits the #5 coal seam. Typical fresh-water about 450 rn from surface entrances of adits favna, Anthraconaia cf. modiolaris, a pelecypod, #4 and #5. In this area humidity and temper- and Carbonita fabulina, an ostracod (977GF- afure conditions are dependent in part on at- 696, F-697, respectively) occur in the roof of mospheric conditions at the surface. Relative this coal seam. The faunal association can be humidity recorded on 28th September 1977 was directly correlated with that of the I-ower Coal between 90 and 98Vo and the fresh-air temper- Measures of Great Britain: Westphalian A, ature between I2.l and 16.8oC. As a result of early B, i.e., older than the strata of Sydney coal- variation of humidity and temperature in this field (Copeland 1977). Preliminary interpreta- area, melanterite growth is not spectacular,aud tion of XRD data suggestedthe presenceof ba- is restricted to short fibres in small quantities; rite and alunite, KAL(SO")(OH)., in a dis- tlerefore, mechanical erosion of the coal faces cordant 2-5 mm vein cutting the upper contact is hardly noticeable. between coal and overlying sediments (977GF- 691); however, further analytical work is neces- Melanterite and rozenite are believed to co- sary on this specimen to confirm the presence exist in these two adits; however, a phase equili- of theseminerals. Sanple 977GF-7O4 and others brium study would be necessaryto check this originating from the top of the p.roducing coal preliminary hypothesis. Rozenite is also ob- seam, f5, show evidence of gypsum and mem- served on surface outcrops of the Stubbart seam bers of the jarosite-natrojarosite series in which (977GF - 66'0). Rain water that percolated Na substitutesfor K: K,Na)Feg(SOn)r(OH)u. through coal storagepiles has a low pH value of about 2.0; bacterial activity is believed to aid in The distribution and occurrence of hydrated the breakdown of pyrite. sulfates and sulfides in the Evans coal mine are fundamentally different from those in the Syd- Pyrite specimen 977GF-655-L continues to ney coalfield. In this mine all hydrated sulfates grow melanterite crystals (f rozenite?) in a and sulfides are restricted to a 3-5 mm vein that hermetically sealed glass container stored in contains , , pyrite and other our laboratory. The relative humidity in the con- unidentified minerals. The vein, concordant with tainer is kept at about 90% . Howeveroadditional the coal seam and directly on top of it, is a XRD data are needed to confirrn that rozenite tracer horizon in the mine and may indeed be is growing directly on tle pyrite without passing an indicator of the #5 seam. However, hydrated through the heptahydrate stage. A 28 by 15 sulfates occur only in those places where the cm pyrite specimen (977GF-622) collected sulfide vein has been exposedto the atmosphere from a coal storage pile and now stored in at- for a number of years, and thus never along mospheric conditions in tle laboratory is devel- working faces. Indeed, no sulfateshave yet been oping fracture planes; we believe that these are collected from these faces; this is also true for due to the growth of a fine-grained whitish, un- Prince mine, Lingan mine, and 1-A mine of identified material #26 Colliery in the Sydney coalfield. The #5 coal seamhas prominently developedjoint planes perpendicular to the seam and trending along planes are occupied by thin sheets Hvnnlrso StrLFATrs, Surrerss, Sur,nnes, eND its strike; the (977GF-699); similar situation exists CensoNerns er OrHpn LoceuoNs oN of calcite a (977GF-7L9). Calcite Cepp BnBroN Isr-er.rp in the Sydney coalfield (977GF-7O0) was also identified from a very small stalactitic-type of structure on a coal face. Rozenite occurs also in minor quantities in Rozenite is also suspectedat this mins fsf ps1 outcrops of the Phalen seam in Glace Bay (Fig. vet identifid. 22 THB CANADIAN MINERALOGIST

DrscussroN the safe travel road in this mine. Similarly, D. Merner, manager of the Prince mine, supplied Apart from having world-famous fossils of mine technologist G. Ellerbrok for our safety; the type Lonchopteris eschweileriana Andrae we express our thanks. (977GF463), the coal rnines of the Sydney Identifications of minerals by XRD were coalfield show renewable hydrated sulfate min- carried out by J. M. Stewart (1977) and L. J. erals in their natural environments.As manv of Cabri (L977) of the Canada Centre for Mineral these salts are readily soluble in water, the oc- and Energy Technology, Ottawa, by M. Zentilli, currence of sorne of them is dependent on hot, Department of Geology, Dalhousie Univenity humid weather conditions. Simple experiments and by J. K. Sutherland, Research and Produc- in the laboratory show that fine-grained pyrite tivity Council, Fredericton. The authors are also started to "growo' sulfates within three days after grateful to referees of the journal for pointed collection at ambient temperatures of about critique and suggestionscontributing to improve- 25-28'C. However, the chemical process in the ment of the presentation. The authors gratefully Sydney specimensdid not take place if copper acknowledge financial assistancefrom the Col- sulfides were present in addition to pyrite (977- Iege of Cape Breton (Grant CCB10O58). GF-308, F-382). We have collected only the Note added in proof: Fibroferrite, Fe(OH)SO4. obvious, but are convinced that many esbteric 5H:O has been identified in coal specimen977GF- sulfates await discovery. The research needed 655-13,from the Stubbartseam at the Prince mine, for this investigation includes microscopy, Point Aconi. to investigate the relationship between m.arcasite and pyrite, not only vis-d-vis oxidation to pro- duce hydrated sLrlfates,but also in search of geo- RSTBRB!.Icss chernical factors that played a temporal role in the coalification of the Pennsylvanian swamps. BnLL, W. A. (1938): Fossil flora of Sydney Coal- Thus elucidation of the origin of pyrite, and field, Nova Scotia. Geol. Surv. Can. Mem, 2L5. hence its distribution in the coal, would form a Ceslcnero4 D. & Snrr.nr, K. (1977): Origins of firm basis for environmental studies and the in coal: Importance of the ester sulfate study of hydrated sulfates; geostatistical meth- content in peat. Science Lgi, 675-676, ods, such as oblique factor analysis (Zodrow CoPELAND,M. J. (1977): Upper Carboniferous 1976) are appropriate. Detailed optical and fresh-water fauna from the roof of #5 coal seam, chemical data from collected samples may aid St. Rose, Inverness. Geol. Surv. Can., East, in fitting the empirical data to theoretically pos- Paleont. Sect. Rep. MP-9-1977-MJC. sible solid-solution series of the type alunite- HorrMaNN, G. C. (1890): Annotated list of the iarosite-natrojarosite(M.ason & Berry 1968) and minerals occurring in Oanada. Ann, Rep. Geol. to substitution schemesin sideronatrite and pick- Nat. Hist. Sum. Can. 4, Tl-67. eringite as well. J,eu:ron, I. L. & Tleu,l, R. J. (1963): On rozenite Questions of environmental nature are also and siderotil. Can. Mineral. 7, 751-763, broached by this investigation: the role of bac- Mesor, B. & Brrnl L. G. (1968): Elements ol teria in the formation of sulfuric acid in stqage Mineralogy. W. H. Freeman & Co., San Francisco. piles yis-d- of coal, and spontaneouscombustion Par.ecnr, C., BrnMAN, H. & Fnor.rDrr,, C, (1957): vi^r hydrated sulfate-mineral formation. The Systems of Mineralogy 2, 7th ed. J, Wiley & Sons, New York. (1970): AcxNowI,sDGpMENTS T\.uLL, R. J. A catalogue of Canadian min- erals. Geol. Surv. Can, Pap. 6945. Zoonow, E. (1976): Minres factor analysis of hy- The Cape Breton Development Corporation persthene data, Strathcona Mine (Ontario, Cao- (Devco) gave permission to publish part of the ada). Math. Geol. 8, 395-412, plan of the 1-B mine, and to collect at various & McCarvnrrsn, K. (1978): Annotated cata- underground mines. J. Maclellan, manager of logue of Carboniferous fossil plants of Nova Devco's #26 Colliery provided us with under- Scotia in the collection of the Nova Scotia Mu- ground "guideso' B. Morrison, who suggested seum, Halifax, Nova Scotia Museun (in press). investigating the travel road in 1-B mine for fossils, J. Nicholson'and G. Crosby who watched Manuscript received September 1977, emended Nov- us like hawks so that we would not strav frorn ember 1977, Canadian Mineralogist Vol. 16, pp.23-30 (1978)

TA.LNAKHITEAND :THE AGCESSIBILITYOF ORDERED STRUCTURESIN THE METAL-RICHREGIONAROUND GHALGOPVRITE

A. PUTNIS Depaftment of Mineralogy and Petrology, University of Cambridge, Cambriilge CB2 sEW, England

Assrr.Acr ,amongtle interstitial sites, giving rise to super- structures with a face-centred cubic subcell. Of Phase transformations in natural talnakhite, and particular interest here are talnakhite, wrth a 2a natural and synthetic mooihoekite have been studied space gtoup 143m, dynamic,ally by in situ experiments in a transmission X2aX2a cubic supercell, electron microscope. The talnakhite structure is a and mooihoekite with a ZaX.2aXLa tetragonal metastable transformation product formed under supercell, spac€ group P42rn (where a is the cotrditions in which the transformation to the more cell parameter of the subcell). Low-temper- stable mooihoekite structure is kinetically impeded. ature phase relations are generally inte"rpreted Tho relationship between tatnakhite and mooihoekite by considering these minerals as separate en- is described in terms of Time-Temperature-Trans- tities. formation diagrams for the two phases. The nature of the transformations can be described theoreticallv This paper is concerned with the route by using the symmetry-point ordering theory. which the high-temperature solid solution orders to form these phases, the constraints Sorvruene on the ordering processesand hence the rela- tionship between partly ordered and ordered L'6tude dlmamique des transformations de phase phases.Putnis & McConnell (1976) have shown dans la talnakhite et la mooihoekite (y compris la that for a metal-enriched chalco,pyrite with mooihoekite synth6tique) reposesur des exp6riences composition of tle form Cur+"Fer+-Sz, the faites in situ au microscope6lectronique i transmis- equilibrium behavior involves the formation of sion. La structure de Ia talnakhite est le produit m6- a tetragonal P42m stracttte directly from the tastable d'une transformation qui, cin6tiquement,en- trave le passagei la structure plus stable de la mooi- F43m disordered solid solution. Under condi- hoekite. La relation entre talnakhite et mooihoekite tions of supercoolingtwo metastablephases were est illustr6e par des diagrammesTemps-Temp6ra- formed. At temperatures slightly below the sta- turo-Tiansforrration pour les deux phases. Les ble transformation temperature a P43m phase transformations sont trait6es selon la th6orie de la with a cell edge equal to that of the solid solu- mise en ordre des points de symm6trie. tion was observed, and at a slightly lower tem- (Traduit par la R6daction) perature a modulated i43m phase of cell dimen- sion 2a was formed. These results lrere largely INTRODUcTIoN in agteement with Hiller & Probsthain (1956) who first suggested that the two phases equiva- phases be At elevated temperatures, phase relations in lent to the i43m and P42m could these the central part of the Cu-Fe-S system are dom- formed with the srune composition. As phases been equated with the natural- inated by the existence of an extensive solid two have solution in which the cations are disordered over ly occurring minerals talnakhite and mooihoek- the tetrahedral interstices of a cubic close-packed ite, respectively (Cabri L973), the question of sulfur structure, space group F43m (Cabri 1973). the relationship between them arises,specifically At low temperatures a number of ordered or in the liebt of the kinetically controlled behavior parfly ordered phases exists on the metal-ric.h observedby Putnis & McConnell. side of stoichiometric CuFeSr. These phases, Natural assemblagesof talnakhite and mooi- lalnakhite (CueFeeSro),mooihoekite (CueFeoSrr) hoekite and synthetic mooihoekite were gener- and haycockite (CunFesSa)are considered to be ously provided by Dr. L. J. Cabri; the ordering stable phases with distinct stoichiometric com- behavior of these materials sould thus be com- positions and well-defined stmctures (Ilall & pared with that of metal-enrished chalcopyrite. Gabe 1972; Cabi 1973l.Hall & Rowland 1974; The processes relating to the formation of hay- Rowland & Hall 1975; Hall 1975). Their struc- cockite, also present in the material, will be dis- tures are based on ordering of the extra cations cussed in a later paper. 23 24 THE CANADIAN MINERALOGIST

ExpEnilvleNter, TEcHNTQUE microstructural inhomogeneities in the sample or the possibility that the phases have a range The talnakhite used in this study is frorn of composition. The microstructures and the Noril'sk, western Siberia; the material and its transformation behavior of the phases,described associatedintergrowths were describedby Cabri later in this paper, suggestthat both may apply. (1967), The mooihoekite, from Mooihoek, Trans- Samplesof the rninerals were crushed and the vaal, occurs as an intergrowth with haycockite. finest grains collected from suspensionin abso- This material has been described by Cabri & lute alcohol and mounted on a standard carbon- Hall (1972). The synthetic mooihoekite, speci- coated grid for observations in an AEI EM6G men number 41.4 of. Cabri (1973), is described transmission electron microscope operating at as single-phasemooihoekite with bulk composi- 100 kV. After crushing, all material was kept in tion corresponding to CusFesSre.Further details a vacuum to prevent tarnishing. More than 40 of the annealing treatrnent during synthesisare such mounts were prepared with material from provided by Cabri (1973). diflerent parts of the polished sections, and The natural minerals were analyzed with an within each mount more than 100 grains were energydispersive electron rnicroprobe compris- observed before and after heating, so that the ing a Si(Li) solid-statedetector and Harwell pulse results described here probably reflect an accu- processor (Kandiah et al. 1975) interfaced to a rate sampling of the material. Data General Nova 1220 minicomputer with The heating effect of the electron beam was 20K core storage.The results for talnakhite and used to induce the transformations in the grain mooihoekite, both apparently single-phase in under observation. A combination of focusing, reflected light, are shown in Figure 1. The re- defocusing and lateral shifts of the beam enables sults were calculated using a computer program the temperature in the grain to be controlled employing the iterative spectrum stripping tech- precisely, although the inability to determine nique (Statham L976). Relative accuracy of the temperaturesdirectly in these iz .rl7zexperiments results is estimated at !2Vo, and the precision constitutes a limitation of the technique. The of the analysesestimated at tLVo (relative). It transformations were observed in a large num- would appear therefore that the scatter shown ber of grains in different orientations, providing in Figure 1 is not spurious but reflects either detailed information on the crystallographic changesin the sample as a function of temper- ature. Any indication of bulk compositional change due to sulfur loss is immediately obvious as a surface degradation of the grain, and the re- sul.tspresented here describe the transformation behavior of the grains at constant composition. ,11. o/o The reversibility of the tlansformation is a fur- ther indication of an isochemical process. The phases present were identified by selected-area cpr electron diffraction, enabling diffraction patterns \ to be recorded from regions of 10O0A in size.

tu t t t't*,._.,:. o o^o RBSULTS mht $ oS hc' The translormation behavior ol synthetic mooiho e kit e, CusFessro Frc. 1. Electron microprobe compositions of appar- The observationsdessribed belorr were nade ently single-phase (in reflected light) talnakhite on sample414 synthesizedby Cabri (1973). Elec- from Noril'sk (closed circles) and mooihoekite tron-microscope observations made on the ma- from Mooihoek (open circles). The Ni contents terial before heating showed that, although it (average A3 wt.Vo in mooihoekite and 0.7 wt.Vo appeared to be single-phase in both reflected in talnakhite) have been added to the Fe in this light and X-ray powder photogtaphs (Cabri plot. The filled triaqgles are the 5 talnakhite anal- consist- yses from Cabri & Harris (1971) and the open 1973'),the material is in fact two-phase, triangle is the average of 13 mooihoekite analyses ing of a coherent intergrowth of lamellae of a from Cabri & Hall (1972). Electron microscope P42m phasein a matrix of. the i43n'r phase.For observations, however, show that apparently convenience,the phasespresent will be referred single-phase regions may be multiphase. to by their space groups. Observations over a TALNAKIIITE AND MOOIHOEKITE 2s

Frc. 2. (a) Transmission electron micrograph of a typical sample of specimen 414. The lamellar phase has tle P42m mooihoekite structure whereas the matrix has the I43m talnakhite structure. (b) After dis- ordering and tben cooling, the whole grain has the I43m structure. (c) After a second disordering- reordering cycle, but with a slower cooling rate, the grain contains both phases. Dfferent proportions of the two ,phases can be fonned by varying tle cooling rate. Scale bar represents 0.2 u,m. large number of grains indicate that the sample behavior of this disordered phase on cooling is contains about 6OVo of the P42m phase. Elec- the same as that desqribed for metal-enriched troq-diffraction patterns taken of such two-phase chalcopyrite @utnis & McConnell 1976). With regions show no splitting of reflections, and as very rapid cooling, the F43m phase can be the tetragonal phase exists in all possible twin- quenchedto room temperature. The appearance related orientations, such a coherent intergrowth of partly or fully ordered phases is dependent would be impossible to detect by X-ray diffrac- on the cooling rate. tion. Figure 2a shows a Epical sa.mple of this The most convenient way of illustrating these material prior to the heating experi.ments.Note transformations is with referense to the schema- the perturbed nature of the /43m structure. tic Time-Temperature-Transformation (I-rT) This two-phase material disorders in stages diagram shown in Figure 3a. In this diagram on heating. T\e i43m phase transforms to a the onset of a transformation is displayed as a P43m phase(see below) prior to completely dis- function of temperature and time, and it is ordering Io the F43m phase. The P42m lamellae therefore important to rcahze that it describes disorder, at'a slightly higher temperature, direct- thermodynamically irreversible behavior. It can ly to the F43m phase. Complete disorder to the be considsred as a non-equilibrium phase dia- single-phaso F4jm sttuct:sre over the whole graxn rElresenting phase changes as a funstion grain is achieved at around 200"-250oC. The of thennal historv.

E ,o

Frc. 3. (a) Schematic TTT plot illustrating the transformation behavior for natural and syntheic mooihoekite. (b) A similar diagram for natural ralnakhite. The dotted curves in both d.iagrams show the onset of modulation n the i43rn stnrcture. 26 THB cANADTAN

In a thermally activated process the TTT very similar composition in this case. The prob- curve has the characteristic C-curve shape, as lem of converting the entire grain to the P42m the transformation rate depends on the thermo- phase is discussed in the next section. dynamic driving force AG and the diffusion co- Transformation from the I43m phase to tle ef.ficient D. At small degreesof undercooling AG P42m phase occurr by annealing below the dis- is small and D is la,rge, whereas at low temper- ordering temperature. The nature of ttris traDs- atures AG is l.arge and D is small. The optimum formation, which cau proceed by two different degree of undercooling for which the transforma- mechanisms, provides a fundamental insigbt into tion rate is a maximum corresponds to the the relationship between tle two phases. Thc 'nose' of tle Gcurve. TTT diagrams are de- P42rn phase can either nucleate and grow co- scribed by Chadwick (1972), and their applica- herently within the l43rn phase, producing a tion to mineralogical systems has been outlined microetructure as shown in Figure 2c, in which by McConnell (1975) and Champness & Lorirner the P42m lamellae form in twin-related orienta- (re16). tions, or the modulated structure sharacteristic To a first approximation such diagrams can of the i43m phase can progressively coarsen be used to predict the transformation sequence until it exceeds a critisal value and proceed to at a given cooling rate, as a superimposed cool- convert tle entire grain to twinned P42tn phase, ing curve will indicate whether or not a partic- Figure 4 a-d illustrates the second transforma- ular phase transformation will orcur. Strictly tion sequence in synthetic mooihoekite. Further speaking, however, they describe the t;me and details of these two mechanisms are in Putnis & temperature required for an isothermal trans- McConnell (1,976). The significance of this se- formation. quence is that it characterizes the modulated na- Experimentalln TTT curves are derived by ture of the i43m structure in terms of an at- quenching to a particular temperature followed tompted trandormation to the stable P42m by an isothermal anneal. In the preselt \rork, phase, confirming the metastability of the l43rn however, owing to the uncertainty of the temper- phase in relation to the P42m structure. atures of the grains in the iz ,vla experiments, A large number of such heating experiments Figures 3a, b were derived from heating and has been performed and many disordering-re- cooling experiments nhich defined the relative ordering cycles carried out under differing cool- positions of the C-curves for the phases observed. ing rates in order to sonstruct the TTT diagram In that sense the diagrams are schematic, but in Figure 3a. The ease of fmmation or accessi- they are an accurate description of the relation- bility of. the phases in terms of tle relative kine- ship between the phases and are consistent with tics of the transformations can be described with all the experinental results. reference to this diagram. Figure 3a shows that the P-42m phase wl/ll form directly from the disordered phase on The translormation behavior ol natural slow cooling but that on more rapid cooling, the mooihoekite hansformation involves tle formation of two metastable phases: firstly, the P43m phase (with Electron microscope observations of unheated a cell edge equal to that of the F43m phase), and natural mooihoekite show tlat again' both the secondly, the I43m phase. Over a fairly wide I43m pbase and the P42m phase are present in range of cooling rates the A3m pthaseis the end the sample, .although the P-42m phase predomin- result of the transformation sequence and at ates. Fine intergrowths of "haycockite-type min- room temperahlre no further transformations erals" (with superstructures involving a factor occur, effectively stranding tlis .metastablephase. of 3) a,re also present in the apparentb single- Figure 2b shows the grain .after disordering phase mooihoekite. Single-pbase grains can be and cooling. This microstructure is characteris- found, however, and their transformation beha- tic of the i43m phase which exists in this case vior is the same as tlat described for the syn- over the whole grain. Figure 2c shows the result thetic sample, although one additional observa- of a further hating and cooling cycle on this tion is worthy of mention. This concerns the grain, in which the cooling n'as crarried out qver nucleation and growth of the P42rn phase orr a longer period to enable the nucleation of the cooling from the disordered form. P42rn phase. Alnost the whole of tle grain has When a singleghase grain of. the P42nt transformed to the P42rn structure, coherently strustwe is heated it transforms directly to the intergrown with the stranded I43n phase. Dif- disordered form. On cooling at a rate appro- ferent proportions of the two phases can be ob- priate to the formation of the F42m phase, nu- tained by varying the experimental cooling rate, cleation and gronth occur in much the sarne way illustrating that both phases have the same or as shown in Figure 2c. T\e result is an inter- TNLNAKHITE AND MOOIHOEKITE 27

:i Fra. 4. The processof transformation from (a) the I43n structure to (d) tha P42m structure by progressive coarsening. Tbe specimen is syn- thetic (414), and the total annealingtime from a-d is 50 secondsat a temperature just below the disordering temperature. Scale bar'repre- sents 0.2 unr.

growth of. the P42m arid i43tn phases.However, The transforrnation behavior ol talnakhite it is generally not possible to transform the whole grain back to the F42m structure, as the Electron microscope observationsof unheated volume of the phase formed is controlled by the talnakhite from Norilsk show that it has the coherency strains associated with the inter- modulated microstructure characteristic of the growth. As the coherent lamellae grow, the mag- A3m phase, Figure 5 shows the typical micro- nitude of the elastic energy is proportional to structure of natural talnakhite. This microstruc- their volume; thus ultimately growth will stop ture is hiehly significant in the interpretation unlessdislocations form at the interface, reducing of the nature of this phase, a point which will be the elastic energy and replacing it with a surface taken up later. Commonly, small coherent la- energy. The introduction of these interface dislo- mellae (approximately 0.t-0.2 g,m long) of cations, however, requires an activation energy, chalcopyrite can be seen to have nucleated with- and therefore a barrier exists between the grornth in the talaakhite, and any interpretation of the of tle coherent phase up to a certain critical probe analysesin Figure I must be made bear- voh:mg and the completion of the transforma- ing this in mind. tion which involves loss of coherency. In these On heating the mdulation is lost, and there in situ experiments, loss of coherency was not is a temperature interval over which the i43nt observed; indeed, considering that the free- structure exists without the modulation. At a ensrgy difference between the partly ordered slightly higher temperature a transformation to and ordered phases is likely to be small, thc the P4lm phase occurs, just prior to complete kinslies associated with the loss of coherency disordering and tle formation of. the F43rn may be such tlat the complete transformation phase. The ternperature for disordering is simi- to the P42m structure is not achieved even over lar to that for the P42m natural mooihoekite. long periods. The presence of the I43m struc- On cooling, this transformation sequence is re- ture in apparently single-phasesynthetic mooi- versed, but tle rates at which the ordering pro- hoekite that had been annealed at 100'C for 25 cesses occur are many tirnes slower than the days (Cabri 1973) supports this conclusion. equivalent transformations in natural and syn- 28 THB CANADIAN MINERALOGIST

DrscussloN

The transformation behavior described here confirms the earlier results of Putnis & McCon- nell (1976): the formation of partly ordered and ordered phases in this part of the Cu-Fe-S system is governed by the kinetics of the pro- cesses involved, and suggests that the l43rn structure of talnakhite should not be tied to a strict stoichiometry, nor should it be considered without reference to its attempted transforma- tion to the P42rn structure of mooihoekite. The relative rates of the processes as a firnction of semFosition establish that there will be composi- tional restrictions on tle accessibilityof the trro phases. This can be illustrated by changes in their relative phase fields on a TTT diagram. The problem of dealing with a system of this kind, in which alternative metastable behavior (McConnell 1975; Putnis 1976) occurs, is first- ly, to recognize the metastable and stable beha- vior, and secondly, to explain why certain states are easier to nucleate, i.e., more accessiblethan others. In sulfides, where dynarnic observations of phase trans,formations are possible, a study of the transfor,mation behavior,as a funstion of temperature and time can provide, under favor- Frc. 5. Electron micrograph of talnakhite from able conditions, an answer to the first problem. Noril'sk showing the typical modulated micro- The importance of the microstrusture in this phase. structure characteristic of tbe I43m Scale context makes transmission electron microscopy represents 0.2 bar u,m. an ideal tool for this purpose. The second prob- lem requires a fheoretical consideration of the symmetry changes associated with possible transformations, and hence of the constraints on thetic mooihoekite. This can be seen by com- the processes involved. Such a theory has been parison of the schematic TTT diagram for tal- developed by McConnell '(1977), based on the nakhite.(Fig. 3b) with that of mooihoekite (Fig. group theoretical alryroach used originally by 3a). After the initial formation of the P43m Landau & Lifshitz (1958), and can be directly phase on cooling the i43m phase developsrela- applied to this system. Both the theoretical a,nd tively sluggishly, and exists initially without any the exporimental aspects must be considered to- modulation. The modulation develops on slower gether to provide a more complete description cooling or on annealing. The development of of the transformations which have produced the this modulation is a clear indication of a fur- phases observed in natlrral and experimental ther attempt at transformation from the I43m systems. structure, and the transfor'mation behavior ob- In the theoretical treatment of the problem, served in mooihiekite shows that the P42m the key issue determining the route taken by a structure is the end result of this process. In transforming species is the accessibility crf the natural talnakhite, however, the kinetics of tle possible states of order relative to the free- transformations are such that the P42m phase energy reduction associated with the transforma- could not be obtained by the in situ experiments tion. In general terms, the accessibility depends and the I43m phase remains stranded. The per- on how great are the differences between the two sistence of the modulated sfucture in naturat statesin an ordering sequence,i.e., oD.the num- material suggeststlat the P42m phase is kins- ber of order parameters which must be speci- tically unattainable at low temperatures over fied in the transformation. In a transformation long periods of time at these compositions.This from a disordered to a fully ordered state leads to the apparent separation of the composi- a hrgh degree of specification is required and tional phase ,fields for the talnakhite and mooi- this will be associated with a greater struc- hoekite structures. tural change than in a transformation to a TALNAKTIITB AND MOOII{OEKTTE 29 partly ordered state (e.9., one in which the na- situation has been found in a number of otlier cancies are ordered, but cations disordered). sulfide systems (Putnis 1976). Thus the change in space-group symmetry can Thus we have on the one hand the equilibrium be used as an indicator of the arnount of struc- behavior which involves the formation of a pos- tural shange, the type of transforrration and sibly fully ordered, and hence compositionally hence the accessibilityof the process.In his sym- restricted structure in a single step,, requiring metry-point ordering theory, McConnell (1977) the specification of parameters associated with relates the changes in space-group symmetry to the ordering of vacancies, Cu and Fe atoms. one of three possible tnres of transformations: This substantial change in structure involves a (A) a second-order transformation involving no kinetically inaccessibleprocess over a range of nusleation incident, (B) a first-order transforma- compositions and cooling rates. On the other tion involving a minor change in structure and hand, more accessible transformations which symmetry, which is therefore fairly readily ac- increasethe degree of order in a stepwise fash- cessible .(general symmetry-point ordering), and ion will occur under metastable conditions and (C) a.first-o.rder transformation involving a sub- produce a series of partially ordered phases over stantial drange in structure and symmetry in a range of compositions. The behavior of the whish nucleation will generally be difficult. This iss should be considered in these terms. provides a theoretical basis on which alternative This approach emphasizes the importance of transformation behavior can be discussed. TTT diagrams in the interpretation of solid- McConnell (1977) has discussedthe applica- state phase transformations in systems where tion of symmetry - point orderi.g theory to stable equilibriu,m may not be reached at low transformations in metal-enriched chalcopyrite temperatures. Considering that high-temperature and tlis is directly relevant to the behavior of phase equilibria in a number of important zul- talnakhite and mooihoekite. The equilibrium fide systems are characterized by the existense transformation from the F43rn structure to the of disordered solid solutions, a reappraisal of P42nt structure is of the type C above, which the methods used to study their behavior on implies a substantial degree of ordering" but a cooling may be required. In many cases,the tra- kinetically difficult process. Under conditions of ditional sealed silica-tube experiments will not supercooling, tlis transformation is not 'asces- be satisfactory, and in casessuch as the iss, witl sible and alternative metastable behavior .mav tho possibility of complex coherent intergrowths, operate. The transformation from the F43m X-ray diffractornetry is not sufficient in the structure to the P43m stnrcture is, however, of identification of the phasespresent. The electron typo B, and c.anbe associatedwith a single aspect microprobe also may have severe limitations in of ordering, in this case with the allocation of describing the cornpositions of apparenfly single- occupied interstitial sites @utnis & McConnell phase materials in a systern where the balance 1976). This transformation, as well as being between thermodynamically ideal behavior and more accessible, does not require a specific stoi- kinetic accessibility leads to a multiphase, me- chiometry, and thus the P43m sffucture would tastable assemblage.In such a system the trans- be expected to be the first-for,med structnre over mission electron microscope should be regarded a wide compositional range in the intermediate as a routine instrument in the identification of solid solution (is9. This is found to be the case, phases in experimental runs, and in sira studies both in this work and in tle results of Cabri of transforrnation behavior may provide some (1973), who found a primitive cubic phase on insights into the nature of the processes which quenching a range of compositions in the iss lead to such complex assemblages. field. The transformation f,rom the F43m strusture to the I43m strucfirre is again of type B, imply- CoNcr,usroNs ing a further stage in tle ordering sequence, associated with a further restriction in composi- 1. Talnakhite has a metastable structure in rela- tion. The fact that at this stage the structure is tion to a transformation to the mooihoekite still not completely orde,red is shown both by its structure. subsequent behavior and by the structural data 2. T\a talnakhite structure has a broad lange of for tlis phase (Hall & Gabe L972). By following possible compositions including both CusFesSro the sequenceof more ascessibletransformations, and CueFeeSro. the transforming species is led ultimately to a 3. The relatively restricted compositions found state of pa,rtial order from which nucleation of for natural talnalhite are tfie result of a change the stable product becomes progressivd less in kinetics of the ordering transfor,mations witl likely as the temperature is desreased. This composition. 30 THE CANADIAN MINERALOGIST

4. The ordering transformations can be dessribed Her.r,, S. R. (1975): C\ystal structures in the chal- in terms of fre symmetry-point ordering theory copyrite series. Caa. Mineral. tg, t68-172. which provides a theoretical basis for the ob. & GasE,E. l. (t972): Tbe crystal structure served kinetics. of talnakhite, CureFeroSgg.Atner. Mineral. 57, 5. Ordering transformations in sulfide solid solu- 368-380. tions should be considered in terms of the ac- & RowrlNo, I. F. (1973): The crystalstnrc- cessibility of both stable and metastable phases. ture of synthetic mooihoekite, CueFesSre.lcta Cryst. 829,579-585. HILLTR,f. E. & Pnonsrnanv,K. (1956): Thermische AcrNowr,BrcrMENTS und rdntgenographischeUntersuchungen am Kup- ferkies.Z. Krist.108, 108-129. I would like to thank Dr. L. J. Cabri for his KANDTAIT,K., Srflrn, A. J. & Wrure, G. (1975): cooperation and provision of tle samples, \trith- A pulse processor for X-ray spectrometry witl out which this study would not have been pos- Si(Li) detectors. Paper 2.9 2nd ISPRA Nuclear Electronics Symposium, Stresa, Italy. sible. I am grateful to Dr. J. D. C. McConnell for m.any discussions during the course of tlis LaNoeu,L. D. & I-rFsrilrz, E. M. (1958): Statisti- cal work and to the Natural Environment Research Physics.Addison Wesley,Reading, Mass. Council for the provision of a Research Fellow- McCoNNBrr,,J. D. C. (1975)z Microstructureof ship. minerals as petrogenicindicators. Ann. Rev. Earth Planetary sci, 3, t29-I55. (1977): K-slmce symmetry rules anal their ap,plicationto ordering behavior in non-stoicbio- REFERBNCES metric (metal-enriched) chalcopyrite. Phys. Chem. Minerals (In press). CABRT,L. J, (1967): A new copper-iron sulfide. Ptmvs, A. (1976): Alternative transformation be- Econ. Geol. 62, 9t0-925. havior in sulfides: direct observationsby trans- (1973): New data on phaserelations in the mission electron microscopy. Science L93, 4t7- Cu-Fe.Ssystem. Econ. Geol.68, M3454. 418. & Hnr,r-, S. R. (1972): Mooihoekite and & McCoNNrrr,,I. D. C. (L976): The trans- haycockite, two ne\p copper-iron sulfides, and formation behaviour of metal-enricbedchalcopy- their relationship to chalcopyrite and talnakhite. rite. Contr. Mineral. Petrology 58, 127-136. Amen Mlneral, 57, 689-708. Rowr,er.ro,J. F. & Her.r.,S. R. (1975): Haycockite, & Henrrs, D. C. (1971): New compositional CunleoSs:a superstructurein the chalcopyrite se- data on talnakhite.Econ. Geol. 66, 673-675. ies. Acta Cryst. BBL,2105-21L2. CneDwrcx, G. A. (1972): Metallography of Phase Srarnarvr,P. J. (1976): A comparativestudy of Transformatio,ns.Butterworths, London. techniquesfor quantitative analysis of the X-ray spectra obtained with a Si(Li) detector. X-roy CHAMpNnss,P. E. & LoRrMaR,G. W. (1976): Ex- Spectrometry 5, 16-28. solution in Silicates. /z Electron Microscopy in Mineralogy Gf.-R. IVenk, ed.). Springer-Verlag, Received iane 1977: revised manuscript accepted Berlin lleidelberg New York. September1977.