American Mineralogist, Volume 71, pages 955-965, 1986

Miissbauer spectral study of ferruginousone-layer trioctahedral

M. DAnsv DYAn,* Rocnn G. BunNs Department of Earth, Atmospheric, and Planetary Sciences,Massachusetts Institute of Technology, Cambridge, Massachusetts02 I 39

Ansrn-r.cr A Mdssbauerspectral study is describedof I 5 potassicfemrginous I M micas representing a wide rangeof chemical compositions and parageneses.Considerable precaution was taken to alleviate problems due to oxidation and preferred orientation of platelets during sample preparation. Mdssbauer spectrawere resolved into two Fe2+and two Fe3+ doublets assignedto cations in the two cis-M2 and one trans-Ml positions. Deviations of peak areasfrom the site multiplicity 2:l ratio demonstratedthat Fe2+ions are relatively enriched in cis-M2 positions and that this is accentuatedin from metamorphic rocks. Tetrahedral Fe3+ions were confirmed in ferriphlogopite, ferriannites from banded iron formations, and Someannites. Annite from the type locality in Massachusettscontains high concentrations of octahedral Fe2+and Fe3+cations in comparable amounts, in ac- cordancewith Dana's (1868, ,4 System of Mineralogy, 5th edition) original definition of this annite. Of the samplesstudied, the annite from Pikes Peak, Colorado, approximates most closely Winchell's (1925, American Journal of Science,5th ser., 9, 309-327) classi- fication of annite as the Fe2+endmember analogueof .

IurnooucrroN when the bulk compositions contain adequate(Si + Al) Iron-bearing micas play a prominent role in the earth to fill the tetrahedral sites. sciences(Bailey, 1984a).There have been num€rous an- Preferredorientation. The perfect basalcleavage ofmi- alytical, structural, and spectroscopicmeasurements of cas renders them highly vulnerable to preferred orienta- femrginousmicas aimed at identifying coexistingFe2+ and tion ofthe platelets when specimensare mounted for X-ray Fe3+ions and their distributions in the crystal structures diffraction and Mdssbauerspectral measrrements ofpow- of several mica polymorphs. However, the literature on dered specimens.The adverseeffects of preferred orien- iron-rich micas contains inconsistenciesconcerning the tation on relative intensitiesofpeaks in spectrogramsneed oxidation states, coordination numbers, and site occu- to be eliminated. pancies of iron cations in the mica structure. Many of Fe3+/Fe2+ratios. Mdssbauerspectroscopy has high po- these ambiguities stem from incorrect interpretations of tential for determining proportions of Fe3+and Fe2+rap- the Mdssbauer spectra.This paper aims to clarify some idly and nondestructively. However, accuraciesmay be ofthe confusionover the crystal chemistry of iron-bearing affectedby preferredorientation, oxidation during sample mlcas. preparation, different recoil-free fractions for Fe3+ and Some of the problems that surfaceupon perusal of the Fe2+ions, and problems of peak resolution and assign- literature on iron-bearing micas include the following: ment. Nomenclature. The namesferruginous biotite, annite, Atomic substitution. The influence of compositional induced lepidomelane, siderophyllite, ferriannite, and monrepite variations, vacancies,and local chargeimbalance have all been applied to iron-rich micas, often with little by the substitution of Fe3+,Ti4+, and Al3* for Fe2* and regard to historical sequence.The connotations and lim- Mg2* in the octahedral sites needsto be evaluated.Also, itations of eachof thesenames need to be clearly defined. efects of replacement of OH- by F-, Cl , and 02- and Cation-site terminology. Most mica structures contain influencesof nonstoichiometry of K*, Na*, Ca2*,etc., in two octahedral and one or two tetrahedral positions, as interlayer sites need to be assessed. well as interlayer positions. However, the nomenclature Many of these problems are addressedhere and in a of the two octahedral sites, in particular, is inconsistent companion paper (Dyar, ms. in prep.), where the focus is and needsto be unified. on lM trioctahedral micas. Following a brief review of Fe3+site occupancy.A fundamental question is whether the nomenclatureof iron.rich micas, essentialfeatures of tetrahedrally coordinated Fe3+ions exist in these micas the mica are described, and the correct designationof the cation sitesis specified.New Miissbauer for a suite of *Present address:Division of Geological and Planetary Sci- spectral measurementsare then described ences,I 70-25, California Institute ofTechnology, Pasadena,Cal- iron-bearing micas representing diverse chemical com- ifornia91125. positions, crystal chemistries,and parageneses.Results of 0003404X/86/0708-0955$02.00 955 9s6 DYAR AND BURNS: FERRUGINOUS ONE.LAYER TRIOCTAHEDRAL MICAS

Table l. History of nomenclatureof iron-rich micas r'bl I t 'or'q o context/0rl gl n o

cryophyll ite Cooke, 1867 Fe3+-.tch blot{te f.@ Cape Ann, l{assachusetts

annlte Dana, 1868 Fe3+-.ich blotite fron Cape Ann, l{assachusetts

ann'lte' Yllnchell , 1925 hypothetlcal Fe21 analogue of phl ogopite, KFe3z+[Sl3Al 010](0H)2

I epidorel lne Dlna, 1892 bl lck blottte rlth subst.ntlal FeJ+ -o o o I epldmel aner Foste., 1960 K(Fex2+Fev3+)x+vIsi3At 016](0H)2 3.00tx)1.50, 0x>1.50, 0

non.eplte {.hl, 1925 non-alumlnous, fearo-fear'l nlca r Si

ferrtannlter lones,1963 KFe32+[sr 3Fe3+olo](0H)z Ox Donnay et !1., 1964 Oo *Nonenclrture ox.r used ln the pfesent study O Fig. l. Schematic diagram of the mica crystal structure. (left) Projected onto (l 00) showing the locations ofthe cation sites and OH- anions. (right) Projected onto (001), showing the configu- iron site occupanciesare subsequentlycorrelated with rations of the cls-M2 and trans-Ml coordination polyhedra. structuraland bondingfeatures of the mica crystalstruc- ture. domelane series and had proposed the name monrepite. Stbse- NovrnNcr,lrunn quently, iron-rich, Al-poor micas from West Australian banded The name phlogopite has a precise connotation for the com- iron formations havebeen described (Miyano and Miyano, 1982; position KMgr[SirAlO,o](OH)r, whereas biotite implies atomic M. J. Gole, unpub. data) and named ferriannites. Tetrahedrally substitution prirnarily of Fe2*for Mg2+in this trioctahedral mica. coordinated Fe3* is acknowledged in the species names tetrafer- However, some confusion exists over the nomenclatureof iron- riphlogopite and tetraferribiotite (Embrey and Fuller, 1980). rich micas, particularly those containing Fe3* ions, and names In the present study, we adopt the names annite, ferriannite, that have been proposed are listed in Table l. Winchell (1925) lepidomelane, and siderophyllile as defined sensu stricto in Table applied the name anniteto ahypothetical Fe2*analogue ofphlog- l. Phlogopite ard biotite refer to Mg-rich and intermediate Mg- opite, and this was used by him as one of four components- Fe2* compositions, respectively, and feniphlogopite artd ferribi- phlogopite, annite, ferriphlogopite, and ferriannite-to define oljle are used to designatecorresponding tetrahedral Fe3*-bearing biotite compositions. However, the name annite utasoriginally compositions. given by Dana (1868) to a Fe3*-richbiotite from the type locality ClrroN-srrn DESIGNATToN at Cape Ann, Massachusetts,which had earlier been described, analyzed,and named cryophyllite by Cooke (1867). Therefore, Two different octahedral sites are distinguished by the config- the name that Dana gave to a Fe3*-rich mica was rnisapplied by uration of the OH- ions shown in Figure l. One site, designated Winchell to the hypothetical Fe'z*mica, which was assumed to cr's,has the two hydroxyls on adjacent corners ofthe octahedra; contain no Fe3* (Foster, 1960, p. 29). Dana (1892) listed lepi- the other site,desig\atedtrans, has the two hydroxyls on opposite domelane as a variety of black biotite characterized by large sides of the octahedron.The ratio of cls-octahedrato trans-oc- amounts of Fe3*.Foster (1960, p. 3l) pointed out other ambi- tahedra is 2:1. In dioctahedral micas (e.g., ), cations guities, but recommended applyrng the name lepidomelane are located primarily in the cls-octahedra,whereas both cls- and to micashaving compositionsof K(Fel+Fel*),*,[Si,AtO,o](OH),, trans-octahedra are fllled in trioctahedral micas. Thus, in the where 3.00 > x > 1.50 and 0 < y < 1.00. She also rec- muscovite crystal structure, only one octahedral Al position- ommended (Foster, 1960, p. 30) that the name siderophyllitebe the cls-octahedron-is defined. for which the mean Al3*-O dis- applied to aluminous biotites having compositions tanceis about 1.95 A (Radoslovich and Norrish, 1962;Richard- K(Fe?+Al)"*,[Si3AlO,0](OH),,where 3.00 > x > 1.50 and 0 < son and Richardson, 19 8 2). The v acafi tr a ns -octahedron is much z < 1.00. larger in muscovite, having dimensions corresponding to a mean Note that each of these speciesnames-lepidomelane, sider- cation-oxygen distance of about 2.20 A. Following the termi- ophyllite, and Dana's original annite-imply that Si and Al fill nology of Pabst( I 9 55), adoptedby Bailey ( I 975, I 984b), the two tetrahedral sites and that Fe3+ions occur in octahedral coordi- octahedral sites in both dioctahedral 2Mr and trioctahedral lM nation. The existenceoftetrahedrally coordinated Fe3*ions was polytypes are now designated as the cls-M2 and trans-I&{l oc- recognizedby Wones (1963) when he describedthe synthesisof tahedra. This standardization of octahedral-site nomenclature is ICr4+[Si3Fe3+O,o](OH)r.However, Wones (1963), and later important because one ambiguity in mica Mdssbauer spectros- Donnay et d. (1964) in their structure determination of this copy has arisen from switching of the Ml and M2 site designa- synthetic iron-rich rnica, designated the phase as "ferriannite" tions. and utilized quotation marks because the composition had not Numerous structurerefinements have beenmade of lM trioc- yet been found in nature. Wahl (1925), however, had previously tahedral micas (Bailey, 1984a),and data relevant to the present described a ferro-ferri-mica from rapakivi granite in Finland with study are summarized in Table 2. In , biotites, and a (nonaluminous) composition paralleling the phlogopite-lepi- annites, the average metal-oxygen distances of the cls-M2 and DYAR AND BURNS: FERRUGINOUS ONE.LAYER TRIOCTAHEDRAL MICAS 957 trans-Ml octahedra are comparable, and the dimensions of both Table 2. Structural data for selected trioctahedral lM micas sites increasewith rising Fe2+content. The larger dimensions of the tetrahedral sites of ferriphlogopite and ferriannite correlate l'li neral Aver.ge (Ranges)l'letal-oxygrn 0istances (A) Reference with Fe3*ions replacingA13* ions in these sites. The schematic structural diagram illustrated in Figure I also c i s -1,12 trans-lll Tetrahedral shows that each trans-Ml octahedron shares edgeswith six cls- octahedra oiltatredron site M2 octahedra,whereas each cls-M2 octahedron is adjacent to phlogopi te 2,064 2.016 1.554 three crs-M2 and three trans-Ml octahedra.Next-nearest-neigh- (2 .063 -2.075 ) (2.063-2.076) (1.527-1.654) bor distancesbetween cations located in the center of these oc- biotite 2.068 2.086 r.659 (2.004-?.l18) (2.053-2.102) (r ,655-l.667 ) tahedraare 3. l0-3. I I A in annite, for example(Hazen and Burn- annite 2.101 ?,r2r 1.659 (2.083-2.112 ) 12.119-2.r23')(r.653-r.666) ham, 1973).Chemical formulae calculated from bulk compositions oxJbiotite 2.059 2.O71 1.655 oftrioctahedral micas invariably show a deficiency of cations, lr.e41 -2.r421 (2,03r-2.100) (l .650-1.568) (in ferr'lphlogoplte 2.096 2.101 I.681 and vacanciesare assumedto occur in the octahedral sites (2.094-2.097) l. 975-2.1 r9 ) (1.673-1.594) addition to interlayer positions). Recent Nun studies of alumi- ferllblotlte 2,098 2.111 1.676 flrrlannite 2.106 2.t07 1.685 nous biotites (Sanzet af., 1984) have demonstrated that cation (2.015-2.r32) (2.O1 4-2 .r23) (1.670-1.59i) vacanciesoccur in trans-Ml sitesin trioctahedral micas,by anal- ogy with dioctahedral in which the trans-Ml posi- Hazenand Burnham,1973; Rayner, 1974. T.keda and Ross. 1975. tions are unoccupied(Radoslovich and Norrish, 1962). Hazenand Burnham,1973. In iron-rich micas approaching annite and ferriannite end- ohta et al., 1982. Stelnfl nk, 1962. member compositions, the site multiplicity ratio of 2'.1 for cis- Senenovaet al., 1983. at., M2 to trans-Ml octahedra imposes a constraint for assigrring Donnayet 1964. Fe2* and Fe3t doublets in the Mtissbauer spectra. Significant de- partures from this 2:I ratio for peak area data of intermediate biotite compositions are indicative ofcation ordering in the cls- In paper, Mineeya (1978) has drawn at- M2 and trans-Ml positions. a thought-provoking tention to several factors that may complicate the fitting and interpretation of the Mdssbauer spectra of biotites. Her calcu- PnBvrous MosssA,unn spEcrRAL MEASUREMENTS lations oflattice electric-field-gradient(ero) components for an quadrupole Numerous studieshave been made of natural, synthetic, oxi ideal mica structure indicate that the splittings of dized, heat-treated, and irradiated iron-bearing micas. Refer- Fe3*ions tn cis-M2 and trans-Ml sites fall close together, sug- gesting encesto most of the earlier Mdssbauerspectral measurements of that only one Fe3*doublet should be resolved. She also probably phyllosilicatesare cited elsewhere(Heller-Kallai and Rozenson, suggestedthat Fe'z* cations occupy two distinct sites, 1981; Dyar, ms. in prep.). The Mdssbauerspectra of micas usu- producing an outer doublet representing Fe2t in an idealized oc- ally contain contributions from coexistingFe2* and Fe3*ions. In tahedral site and an inner doublet due to Fe2+in sites atrected by most specimens,two Fe2+doublets were resolved, the outer doub- defectssuch as vacanciesofH*, clustersincluding Fe3*,substi- let with larger quadrupole splitting generally being assigned to tution of OH- by F-, etc. Fe2*in cls-M2 sitesand the inner doublet with the smaller quad- Expnnrrvru,{TAl DETAILS rupole splitting to Fe2+in trans-ML sites for both dioctahedral and trioctahedral micas. However, in some reports (Goodman Given the inconsistenciesof the published Mtissbauerspectral and Wilson, 1973;Goodman, 1976;'Finch et a1.,1982), these data on micas, this study was conceived to examine a range of Fe2*-peakassignments were incorrectly reversed.Wide rangesof specimensin a consistentmanner, using the same spectrometer Fe2t isomer shift and quadrupole splitting parameters are re- and the same fitting program for all samples. Mica selection was ported in the literature (see reviews by Heller-Kallai and Roz- accomplishedwith two goals:(l) to study well-characterizedsam- enson, l98l; Dyar, ms. in prep.), which hint at misfitting of the ples used in earlier spectral and structural measurements so as Mdssbauer spectra. An even wider range of parameters has been to establishconsistent ranges of Mdssbauerparameters for iron reported for Fe3*peaks, which often have beenresolved into two cations in eachsite and (2) to measurea suite ofnatural annites doublets, the outer Qargerquadrupole splitting) doublet usually and ferriannites in order to determine the crystal chemistry of being assignedto Fe3*in the trans-Ml site and the inner doublet Fe3*. to Fe3+in the crs-M2 sites (Bancroft and Brown, 1975). Some The chemical compositions and sourcesof the 15 femrginous data fall beyond the range found for octahedrally coordinated micas used in the present study are summarized in Table 3. Many Fe3+ions in other silicateand oxide @yar, ms. in prep.). of the specimens were obtained from samples that had been Again these inconsistencies suggestincorrect fitting of the Miiss- ana|yzed previously by wet-chemical or electron-microprobe bauer spectra. techniques(specimens l-4, 9-I4). New analysesof specimens Peaks attributed to tetrahedrally coordinated Fe3* ions have 5-8 and 14 were performed at MIT using a uec-5 electron mi- beensuggested for some micas (Annersten,1974,1975). Such an croprobe with alpha corrections applied to the data. assignment is unambiguous for ferriphlogopites and ferriannites Four igneous biotites from granitic rocks of the central Sierra becausetheir compositionsnecessitate substitution ofSia* by Fe3* Nevada batholith, California (specimensl-4), were provided by in the tetrahedral sites. However, in some Al-rich micas con- G. M. Bancroft. These biotites are among those described by taining a surplus ofAl3* and Si4*to fill the tetrahedral sites, the Dodge et al. (1969), who provided X-ray powder-diftaction and parametersobtained from Mdssbauerspectra fitted to Fe3*dou- wet-chemicalanalytical data on the samples.Subsequent Miiss- blets lie beyond the range ofisomer shifts for tetrahedrally co- bauer analysesofthese biotites by Bancroft and Brown (1975) ordinated Fe3+ions in other minerals (Annersten and Olesch, indicated that they contain Fe2* and Fe3* in two octahedral sites, 1978).Again the fitting of such Mtissbauerspectra is suspectand with no evidence of tetrahedral Fe3*. All four biotites studied warrants further investigation. here have similar compositions (see specimens l-4, Table 3), 958 DYAR AND BURNS: FERRUGINOUS ONE-LAYER TRIOCTAHEDRAL MICAS

Table 3. Chemical compositions of the femrginous micas*

1l l3a I3b 14a t4b 15

Si02 36.29 36.34 37.55 36.45 33.92 34.95 35.76 40.88 38.99 33.75 38.44 36.7i 33.96 33.92 39.91 33.90 Al203 16.53 15.37 15.49 15.17 19.07 19.91 19.53 9.25 0.90 5.r3 5.84 19.82 13,10 11.87 16.73 1t ,a Tl 02 1,54 2.16 1.70 2.50 2.49 1.39 r.4l 0.12 0,00 0.15 0.02 0.55 3.61 3.50 Fe203 4.28 3.64 3.49 4.23 12.60 8.1r 6.65 3.06 12.O1 liS0 11.16 9.43 12.23 u.68 9.52 9.53 9.61 23.58 6.07 2.98 11.50 0.04 0.97 0.89 0.62 0.31 Fe0 15.5r 18.37 15.10 15,91 20.61 2l . l8 20.98 10.25 29.60 34.88 25.15 22.46 32.04 36.4? 1i.48 34.65 f,tn0 0.84 0.33 0.43 0.41 0.15 0.00 0.06 0.04 0.00 0.03 1.60 0.65 0.64 0.45 Ca0 0.57 0.54 0.60 0.57 0.01 0.00 0.00 0.37 0.62 0.01 0.01 o.42 0.03 Na20 0.30 0.25 o.24 0.21 0,40 0.34 o.27 0.10 0.00 0.15 0.16 0.49 0.10 0.08 Kz0 8,28 9.98 8.53 8.44 8.33 8.1I 8.85 t0.21 6.56 9.33 8.30 9.66 8.41 8.94 10.66 8.02 Hr0' 3.50 3.l0 3.90 3.70 2.50 1.50 Hzo- 0.30 0.18 o.32 0.30 0.40 0.28 0.38 0.26 . 3,79 0.82 0the.s 0.04 0.04 0.07 0.05 0.01 0.34 - 1.19 0=F,Cl 0.18 0.13 0.18 0.12 --

Total 99.42 100.56 99.89 99.70 94.50 95.42 96.41 94.49 91.99 94.95 96.09 94.81 100.37 96.42 100.16 92,t4

si 5.488 5.5t2 5.637 5.509 ( ,22 F 217 5.388 6.048 6.490 5.884 6.111 5.946 5.532 5.633 5.040 5.827 6.000 Al iv^ 2.512 2.488 2.363 2.491 2.767 2.683 2.612 1,6r3 o.177 1.054 1.094 2.054 2.468 2.324 1.960 2.173 2.000 Feinr' 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1.333 1.062 0.795 0.000 0.000 0.000 0.000 0.000 0.000 SumTet 8.000 8.000 8.000 8.000 8.000 8.000 8.000 7.661 8.000 8.000 8.000 8.000 8.000 7.951 8.000 8.000 8.000

Alvi 0.435 0.260 0.378 0.2t2 0.701 0.887 0.857 0,000 0.000 0.000 0.000 1,731 0.048 0.000 1.025 0.103 0.000 Ti - 0.177 0.318 0.194 0.287 0.292 0.161 0,161 0.013 0.000 0.020 0.002 0.068 0.439 0.455 0.000 0.457 0.000 Feni" 0.487 0.41.5 0.394 0.481 0.000 0.000 0.000 0.000 o.245 0,002 0.001 0.000 0.375 0.000 r.375 0.000 0.000 l4g^ 2.515 2.132 2.136 2.63r 2.189 2.160 2.158 5.199 1.506 0.714 2.725 0.010 0.235 0.220 0.140 0.079 0.000 Fe.' 1.962 2.330 r..896 2.01I 2.659 2.694 2.643 1,268 4.120 5.086 3.344 3.042 4.365 5.058 2.2t2 4.981 6.000 lln 0.108 0.042 0.055 0.052 0.020 0.000 0.008 0.005 0.000 0.000 0.004 0.220 0.090 0.090 0.000 0.066 0.000 Sumoct 5.684 5.497 5.653 5.674 5.861 5.902 5.821 6.485 5.871 5.882 6.076 5.071 5.552 5.780 4.752 5.686 6.000

Ca 0.092 0.088 0.097 0.092 0.002 0.000 0.000 0.000 0.066 0.116 0.002 0.002 0,073 0.005 0.000 0.000 0.000 Na 0.088 0.074 0.070 0.062 0.120 0.100 0.079 0.029 0.000 0.000 0.046 0.050 0.0155 0.032 0.000 0.027 0.000 K 1.597 1.931 i,634 t.621 1.639 1.574 1.701 1.939 1.393 2.015 1.683 1.996 1.760 1.894 2.058 1.759 2.000 SumA 1.777 2.093 1.801 1.,781 1.761 1.674 1.780 1.968 1.459 2.t91 1,731 2.048 1,988 1.931 2.058 1.786 2.000

* The fomulae of all specimenshave been recalculated on an anhydrousbasis (oxygens= 22) to facilitate comparisonsbetween nicroprobe and ,et chem'icaldata. I Biotite llT-1, in fheeler Crest Qrartz lbnzonite, Central Sierra NevadaBathollth (Dodgeet al., 1969; BancrofLand Brown,1975). Analysis includes 0.04XCl . 2 B'iotite SL-1.8,in granodiorite of DinkeyCreek Type, Central Sierra ilevadaBatholith (Dodgeet al., 1969; Bancroft and 8rown,1975). Analysis inc'ludes0.0/X Cl. 3 Biotite BP-1, in TinenahaGranodiorite, Central Sierra ilevadaEatholith (Dodqeet al., 1969; Bancroft and 8rown, 1975). Anatysis includes 0.07x cl . Eiotjte 16-1. in LanarkGranodiorite, Central Sierra ilevadaBatholith (Dodgeet al., 1969;Bancroft and Brorn, 1975). Analysis includc5 0.051cl, 5-1 Biotltes 8F9A,BF9E, and 8F9Gfrm the arphibolite facies, Range'leyFn., Eetlds Falls, iler Hanpshire(analyzed and donatedby D. Hickmtt). c Ferrlphlogopite, locality unknorn(R. I. lhzen, personal cmnicrtion). 9 Ferriannite SA-2,banded iron fornation, Hame.sleyRange, lestenn Arstralia (analysedand donatedby li. J. Gole). Contains1-21 riebeckite and very fine grained hematite. 10 Fer|iannite BllD2l448,banded iron fomation, HamersleyRange, Iestern Australia (analysedand donatedby fi. J. Gole). l1 Ferriannite GE,banded iron formation, Dales Gorge,Hamersley Range, llestern AJstralia (l{iyanoand f,liyano,1982). 12 Annite (siderophyllite), Lost Creek, ibntana (R.-A. Robie, personil comunication). 13 Annite, B65DS,Devil's 5l ide, Pikes Peak, Colorado, (Barker et al., 1975; Hazenand Burnham,1973) (a) let chen.; (b) nicroprobe (Analysis includes 0.341 Ct). 14 Aonite (lepidorelane), CapeAnn, l.lassachusetts,Analysis includes 0.59t Li20 and 0.601 t4n203. (a) ret chen (Foster, 1960); (b) microprobe. 15 Synthetic fluorannite (synthesizedby J. t'tunoz;donated by 0.R. Iones).

suggestingthat their M6ssbauer spectra might also be similar. finement by Hazen and Burnham (1973). The Cape Ann speci- This group of igneous biotites was studied to establish on our men (sample 14) was collected by us at the type locality near spectrometer the range of Miissbauer parameters that might be Rockport, Massachusetts.The chemical analysis of this annite expectedin subsequentanalyses of more-complicated micas. Tfuee indicates a high Fer* (lepidomelane) content. Sample 15 is a metamorphic biotites (specirnens5-7) from amphibolite-facies synthetic fluorannite prepared by J. Munoz and lent to us by the rocks ofthe RangeleyFormation, BellowsFalls, New Hampshire, late D. R. Wones. were lent by D. Hickmott. Samples 1-7 show a surplus of (Si + For the Miissbauer spectral measruements, sampleswere finely Al) over that required to completely fill tetrahedral sites (Table ground under acetone (to minimize possible oxidation of iron) 3). The composition of specimen8 obtained from R. M. Hazen and mixed with sugarand acetone.This method of sampleprep- is deficient in tetrahedral (Si + Al), suggestingthat Fe,3Jis present aration coats the individual biotite grains with sugar so that each in this ferriphlogopite. Three ferriannites from banded iron for- particle is well rounded, thereby alleviating preferred orientation mations (specimens 9-1 l) with compositions indicative of of the mica flakes in the plexiglass sample holder. However, Felf were provided by M. J. Gole and S. Miyano. Three annite because past studies suggestedthat preferred orientation afects specimenswere acquired from granitic rocks at Lost Creek, Mon- in Mijssbauer spectral profiles are difrcult to overcome (Bowen tana; Pikes Peak, Colorado; and Cape Ann, Massachusetts.The et al., 1969; Annersten, 1975), the experimental technique de- Lost Creek specimen (sample 12) contributed by D. A. Robie scribed above was also tested. Biotite MT-l was gound dry in was used by him in calorimetric measurements and has hrgh Al the mortar and pestle for 30 min; other aliquots were ground wet (siderophyllite) and F contents. The Pikes Peak annite (sample (under acetone) for 30 min and for I min, and also mounted as 13) donated by F. Barker was used in the crystal-structurere- the small flakes supplied to us as the mica separate from the DYAR AND BURNS:FERRUGINOUS ONE-LAYER TRIOCTAHEDRAL MICAS 959

Table 4. Miissbauer parametersfor iron-bearing micas

Fetd3+ Fect3+ (cis{z) Feat3+ (m-tl) Featz+ (tr.ns-it) Fectz) (cts{2)

tlll I urccr- Soei*n' 6 AAreaf 6 a Araaf 6 a lre.r 6 a |l€f 6 a lr.. r ltsfi I t.l nty

o.tl 0.55 l5 0.39 0.42 l.za 1l 0.39 1.07 2.22 15 0.39 r.13 2.50 5t 0.39 -0.3t -0.05 0.14 0.47 t2 0.3!t 0.39 1.15 9 0.39 1.10 2.12 n 0.39 1.13 2.57 5l 0.39 0.07 0.02 0.39 0.52 l5 0.3E o.al l.16 12 0.38 1.07 ?.15 la 0,38 l.l3 2.6 59 0.38 0.0t 0.01 0,/l5 0.52 l5 0.40 0.t5 1.17 ll 0.{0 1.09 2.15 20 0,{0 1.12 2.9 5l 0.r0 -0.03 -0,01 0.13 0.66 0.39 0.49 1.36 | 0.39 1.r5 1.87 9 0.39 l.ltt 2,55 79 0.39 -0.s7 -0,(n o.trl o.za 0.3t 0.a3 1.19 8 0.37 l.r3 2.L9 14 0,37 t.la 2.53 n 0.37 -0.04 -0.(I2 0.4,1 0.37 0.38 0.r3 r.20 5 0.38 l.lf 2.15 15 0.3t l.1l 2.6r 0.38 0.19 0.03 0.19 0.56 Q O.a5 - t.lz 2.63 38 0.3E. -0.33 -0.0r 0.2r 0./g) 20 0-29 0.36 0.s6 o -29 r.it z.it ri 0,29 l.l3 2.50 t3 0.29 0.51 0.05 t0 0.20 0.t5 l8 0.29 - 0.a2 l.0l I o.29 1.16 2.15 t9 0.28 l.l0 2.55 59 0.?E 0.09 0.o2 ll 0.19 0.39 l5 0.28 0.12 0.a5 o.2a 0.39 1.94 5 0.28 1.12 2.p 19 0.3t l.l3 2-51 52 0.37 l.0l 0.09 L2 0.11 0.15 r0 0.36 0.3| 0.71 l0 0.36 r.10 2.35 27 0.35 l.lt 2.64 0.36 -0.17 -0,15 l3 0.39 0.31 4 0.37 0.'13 o.qt 1 0.31 1.10 2.6 n o.31 l.ra 2.s3 60 0.37 0.25 0.02 I4 0.20 0.37 3 0.r0 0.37 0.6r 30 o.fo o.12 0.9a a o.ao l.l5 l.9o la 0.40 l.l2 2.$ I o..{} 0.05 0,01 l5 l.r5 l.L 3l 0./r3 l.r5 2.11 I 0.r3 0.06 0,01

rI SiotitefT-l;2 8iotite9.-I8;3 EiotitegP-l;l EiotitetG-l;5 Biotit.SfgA;6 BiotiteDfgE;7 8totit.8F9C;8 Ferr{9hlo9o0it.;9 Ff.{lnnit.S-2; l0 Fe.riannite EtllP/418; U Ferrl.nnite 6f; l2 Anrite Lost Crek; 13 Arnite Ptt€i Feak; ll Annlte C.p! lrn; 15 Fluor.nnlte synthelic

original rock. Some sampleswere measuredwith the plexiglass as good fits that adequately explain the data set but that may not holder orientated at 54.7"(the "magic angle") to the gamma ray be unique. flux to confirm the absenceofpreferred orientation effects. The practical application ofthese statisticalparameters is dis- Mdssbauerspectra were recorded in 5 I 2 channelsofa constant- cussedelsewhere (Dyar, 1984).At best,the precisionofthe Miiss- accelerationAustin ScienceAssociates spectrometer, using a lOf bauer spectrometerand the fitting program is approximately + 0.02 50 mCi 57Coin a Rh matrix source. Results were calibrated mm/s for isomer shift (6)and quadrupolesplitting (A) and t 1.50/o againsta-Fe foil (6 pm thick, 99.990/opurity). Spectrawere trans- for area data for spectra urith well resolved, distinct peaks (Dyar, ferredto a unc rsr I 1/73 minicomputer, whereinteractive curve- 1984). However, the mica spectrain this study contain several fitting was executed.Spectra were fit using a Gaussiannonlinear overlappingpeaks; the parameterslisted in Table 4 are estimated regression procedure with a facility for constraining any set of to be +0.03 mm/s for isomer shift and quadrupole splitting, and parameters or linear combination of parameters (Stone et al., at least + 30/ofo r area data. It is important to keep the magnitude 1984).Lorentzian line shapeswere used for resolving peaks, as of these errors in mind when evaluating the data. Note that these tlere wasno statisticaljustification for the addition ofa Gaussian errors are realistic limits not only for this spectrometer,but also component to the curve shapeused. for other Mdssbauerspectrometers (see Dyar, 1984). Each M

ly*:-fly--r,h[**,,h fi'iliN'tr .o,ll,n". r \ rildr

i j I ,,11'tltrFfu*t; (spec- unground i ,/',11il1.il / Fig. 3. Mtissbauerspectrum of igneousbiotite SL-18 ' r't I i' imen 2); peaksl-1-FeS in trans-Ml; peaks2-2-Feffi in cls- lsmail '\ Frakesl ''i M2; peaks3-3-FeS in cls-M2;peaks 4-4-Fe1;j in traw-Ml. rcr rf,n,i,nT tl l\ti I, llI it and trans-Ml sitesand to tetrahedrally coordinated Fe3* l,/ ,l substituting for Si and Al in the sheetsof corner-shared tetrahedra.Note that for most micas the outermost Fe2+ doublet (largest quadrupole splitting) originates from cls- M2 Fe2*ions, whereasthe innermost Fe3* doublet with smaller quadrupole splitting, when resolved, is assigned to Fe3*in cis-M2 sites. As noted later, the Fe2*doublets may be reversedin some ferriannites. The choice of two Fe3* doublets is vindicated becausethe mica structure containstwo potential octahedralsites (cl's-M2 and trans- Ml) available to Fe3*ions. We also invoked the inverse relationship of quadrupole splitting parametersbetween Fe2*and Fe3*peaks to assignthe octahedralFe3* doublets. The spectraof the four igneousbiotites (specimensl- 4) are remarkably similar and resemblethat of specimen in Figure -5-4-3-2-112345 SL-18 shown 3. Note that the area ratio of Fe2* cis-M2/trans-Ml in thesesamples approaches 3:1, which mm/sec is significantlydiferent from the site multiplicity 2: I ratio, Fig.2. Mtissbauerspectra of biotiteMT-l (specimenl) pre- indicating Fe2*enrichment in the cis-M2 positions. The paredin differentways. Spectra (a), (b), and (c) are remarkably Fe3*crs-M2 doublet is only slightly more intense than the similar, but spectrum(d) showsan enlargedFe3* peak around I Fe3*trans-Ml doublet, suggestingthat Fe3*ions are rel- mm/s resultingfrom oxidationduring dry grindingof the biotite atively enrichedin the trans-Ml position. The three meta- speclmen. morphic biotites (specimens5-7) also have similar M0ss- bauer spectra(e.g., Fig. 4). The Fe2*ions again are very strongly enriched in the cls-M2 sites. perhapscoupled with the conversion of OH- to 02- ions, The expectedocclurence oftetrahedral Fe3*ions in fer- thereby altering the oxygen ligand characteristicsof the riphlogopite (Steinfink, 1962) suggestedby the chemical cis-M2 and /rans-Ml sites. formula of specimen 8 is confirmed by its Mdssbauer spectrum illustrated in Figure 5. The spectrum is domi- The suite ferruginous of micas nated by the Felf doublet, with only one other Fe2*doub- The results of extensivecurve-fitting of the Mdssbauer let being resolved.The parametersof this doublet corre- spectraof the l5 femrginous micas are presentedin Table late with those of Fe2*ions in cis-M2 sitesof the biotites 4. Representativespectra are illustrated in Figures 3-9. (Table 4, specimens l-7), suggestingthat Fe2* ions are In accordwith earlier studies(e.g., Annersten, 1974;Ban- located predominantly in the cis-M2 positions of ferri- croft and Brown, I 975), resolved doublets are assignedto phlogopite.Alternatively, the presenceof tetrahedralFe3* octahedrallycoordinated Fe2*and Fe3*ions in the cli-M2 ions in the structure may cause the Fe2* trans-Ml and DYAR AND BURNS: FERRUGINOUS ONE-LAYER TRIOCTAHEDRAL MICAS 961

F

2 z E z I *F

ilulaEc HTl8EC Fig. 4. Mdssbauerspectrum of metamorphicbiotite BF9E Fig. 5. Mtissbauerspectrum of ferriphlogopite(specimen 8). (specimen6); peaks1-1-Fe|l in trans-Ml; peaks2-2-Fefi in The spectrumis dominatedby peaksdue to tetrahedralFe3* ions; cis-M2;peaks 3-3-Fe|l in cls-M2;peaks 4-4-Fe3$ irt trans- peaksA-A-Feil; peaksT-T-Fe'3J. M1.

a Fe3*-richbiotite. The annite from Lost Creek (Fig. 7), cis-M2 doubletsto be superimposedin the ferriphlogopite although containing high concentrationsofAl and F, has Miissbauer spectrum. Mdssbauerparameters for the two Fe2t doublets that are The three ferriannites from banded iron formations comparable to the ranges obtained for the other natural (specimens gave 9-l l) also M6ssbauer spectrathat showed biotite and annite specimens(Table 4). The approximate the majority of the Fer* ions to be located in tetrahedral 2:l ratio of peak areas for the Lost Creek annite also sites. The spectrum of specimen l0 shown in Figure 6 indicates that Fe2*and Al3* ions are both randomly dis- showsa small FeS doublet, too. Two Fe'?*doublets with tributed over cli-M2 and trans-Ml sites.The fact that F parameters consistentwith Fe2*ions in cis-M2 and trans- in the Lost Creekannite, and to a lesserextent in the Pikes Ml sites are also displayed in the spectrum of specimen Peak annite, does not significantly affect their Mdssbauer (specimens 10. The two magnesianferriannites 9 and I l) spectrais surprising,since the two well-resolvedFe2* dou- yielded also two Fe2*doublets, but the outermost doublet blets in the synthetic fluorannite spectrum (Fig. 9) have quadrupole is lessintense and has an unusually large split- the lowest quadrupole splittings of all femrginous micas (Table (specimen ting 4). By analogywith ferriphlogopite measured(Table 4). 8) discussedearlier, the presenceof Feli and Mg2* may influence the parameters of Fe2* in trans-Ml positions. Specimens9 and I I also have significantly high deficien- cies of interlayer cations (Table 3), suggestingthat the absenceof K* ions in A sitescould be drastically affecting the M0ssbauerspectra of ferriannites. This aspectis dis- cussedfurther in a companion paper (Dyar, ms. in prep.). The Mtissbauer spectra of the three natural annites (specimens 12-14) and synthetic fluorannite (specimen 15) each contain two Fe2* doublets with area ratios ap- F proximating the 2:l multiplicity of the cis-M2 and z trans-Ml sites,in agreementwith site occupanciesin the a crystal-structurerefinement of specimen 13 (Hazen and Burnham, 1973).The three annites also contain variable proportions of Fe3* iron, a portion of which occurs in tetrahedral coordination in the Lost Creek (Fig. 7) and CapeAnn (Fig. 8) specimens.Of the I 5 specimensstudied, the Pikes Peak annite with -ll0lo Fe|l and negligible Fefufcomes closestto Winchell's (1925) definition of an- lrIlsEc nite as the Fe2* analogueof phlogopite. The Mdssbauer Fig. 6. Mdssbauerspectrum of ferriannite BHD2/448 (spec- spectrum of annite from the type locality at Cape Ann imen 10) from a banded iron formation. The majority of Fe3* (Fig. 8) showscomparable amounts of Fe2*and Fe3'ions, ions are in tetrahedralsites; peaks 1-l -Fe|l in lrans-Ml; peaks confirming Dana's (1868) description of this specimenas 2-2-Fe'zj in cis-M2; peaksT-T-Fell; peaks0-0-Fe*. 962 DYAR AND BURNS: FERRUGINOUS ONE-LAYER TRIOCTAHEDRAL MICAS

z z e * a

MM./8EC Fig. 9. Mdssbauerspectrum of syntheticfluorannite (speci- Fig. 7. Mdssbauerspectrum ofannite from Lost Creek,Mon- men l5); peaks1-l-Fe2j in trans-Ml;peaks 2-2-Fell in cls- tana (specimen l2); peaks peaks l-l-Feful ir trans-Ml;, 2-2- M2. Fefi in cis-M2; peaksT-T-Feli; peaks 0-0-Felj.

DrscussroN confirms the assignmentof the outermost and innermost This study was conceivedto addressrecurring problems Fe2* doublets to Fe2* ions in the cls-M2 and trans-Ml concerning the crystal chemistry of iron in femrginous sites, respectively.Note that similar peak resolution and micas.Some of the uncertaintiesraised in the introduction assignmentsapply to Mdssbauerspectra of alkali amphi- may now be discussedin the light of results obtained in boles (Bancroft and Burns, 1969),in which the amphibole the present study. Ml and M3 octahedrahave similar hydroxyl anion con- figurations as the mica cis-M2 and trans-Ml octahedra, Distinguishable iron-cation species respectively.We conclude, therefore, that so far as ocla- The easewith which the two Fe2*doublets in the Mtiss- hedral Fe2*ions are concerned,one of Mineeva's (1978) bauer spectraofbiotites and annites were resolved, cou- assertionsis incorrect; Miissbauer spectra of biotites do pled with the consistencyof their isomer shift and quad- distinguish between cis-M2 and /rans-Ml sites. rupole splitting parameters,proves that Fe2*ions in the However, in ferriphlogopite and ferriannites, the cis-M2 and, trans-Ml positions are readily distinguish- presenceof Fe3*ions in tetrahedral coordination appears able. Furthermore, the approximate 2:l peak-arearatios to shift peaks,lowering resolution and possibly reversing of specimensclose to endmember annite compositions assignmentsof the Fe2*doublets.This effect may be aug- mented by the presenceof octahedral Mg'* cations and by severe deficienciesof alkali-metal ions in interlayer positions. Peak resolution and assignmentof the Fe3*doublets is not clear-cut. On the one hand, tetrahedral Fe3*ions, which are readily identified in the Mdssbauer spectra of ferri- phlogopite and ferriannite specimens,yield just one Fe3+ F doublet that is unambiguously assignedto Fe3*ions sub- z stituting for Si and Al in the silicate layersof the lM mica E F structure.On the other hand, the assignmentofoctahedral I Fe3* doublets remains controversial. In this study, two doubletswith isomer shifts consistentwith Felj ions could be resolved in all biotites and in some annite and fer- riannite specimens.Although such two-Fe3*-doubletfits for thesespecimens are statisticallysuperior to a one-Fe3*- doublet fil, the quadrupole splitting parametersdisplay a range of values owing to acute problems of resolving two Felg doublets in the heavily overlapping region of the Fig. 8. Mdssbauerspectrum of annite from the type locality spectrum near zero velocity where the more intense low- at Cape Ann, Massachusetts(specimen l4); peaks 1-1-Fe|l in velocity Fe2*peaks occur. Mineeva (1978) suggestedthat trans-Ml; peaks 2-2-Fe|{ in cls-M2; peaks 3-3-Fe3J in cls- Fe3*ions in mica cis-M2 and trans-Ml octahedracannot M2; peaks4-4-Felj in trans-Ml peaks T-T -Fe|{. be distinguished and implied that two-Fe3*-doubletfits DYAR AND BURNS: FERRUGINOUS ONE-LAYER TRIOCTAHEDRAL MICAS 963

Table 5. Formula proportions for femrginous micas, based on analytical and Mossbauer data

I1 t2 IJ L4

Sr 5.466 5.492 5.608 5.489 5.201 5.27r 5.3s6 5.942 6.507 5.845 6.060 5.856 5.508 5.49I Aliv 2.5J4 2.5'18 2.392 2.511 2.199 2.729 2.644 t.286 0.177 1.048 19407 1.844 2.492 2.L45 Fe,*.i C.000 0.000 0.000 0.000 0.000 0.000 0.000 0.772 1.316 1.106 0.854 0.300 0.000 0.364 sun Tet" 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000

Al vr 0.402 0.23i 0.335 0.182 0.648 0.810 0.804 0.299 0.000 0.000 0.000 r.884 0.012 0.000 Ti J.176 c.317 0.193 0.286 0.290 0.160 0.150 0.013 0.000 0.0r9 0.002 0.067 0.437 0.431 F"3*rj 0.659 0.574 0.615 0.646 0.264 0.374 0.263 0.000 0.167 0.238 0.2q5 0.300 0.566 ?-.253 rg^ 2.505 2.124 2.722 2.52t 2.).76 2.141 2.145 5.108 1.509 0.769 2.702 0.010 0.235 0.074 Fet+ I .78I 2.t50 1.663 1.838 2.379 2.297 2.364 0.473 4.22t 4.766 2.956 2.396 4.t52 2.159 Hn 0.107 0.042 c.054 0.052 0.020 0.000 0.008 0.005 0.000 0.000 0.004 0.2r7 0.089 0.062 SLinOct 5.630 5.448 5.582 5.625 5.n1 5.182 5.144 5.898 5.891 5.792 5.960 4.874 5.491 4.979

Ca 0.092 0.088 C.096 0.092 0.002 0.000 0,000 0.000 0.067 0.1l 5 0.002 0.002 0.073 0.000 Na 0.c88 c.073 c.069 0.061 0.119 0.099 0.079 0.028 0.000 0.000 0.046 0.049 0.ls4 0.025 K i.591 1.924 r.625 1.62L r.530 1.s60 1.691 r.905 1.396 2.062 r.669 r.966 L.752 1.658 Sum A )..111 2.085 1.790 1.174 1.75I 1.659 1..110 1.933 1.463 2.717 t.7t7 ?.IO1 r .979 I .583

Fe3+/tFeo.z7 D.zl o.zl 0.26 0.10 0.14 c.I0 c.62 0.26 0.22 0.28 C.20 o.tz 0.52

ci s ll2 E:* c.3eo 0.328 0.342 ,1.373 0. 159 0.160 0.131 0.000 0.055 0.000 0.173 0.200 0.206 r,284 Ferr 1.391 1.558 1.344 1.34r 2,136 r.923 1,970 0.299 2.453 3.605 2.165 1.587 2.168 L.252 t.ans l{l trd3- 0.268 0.246 0.273 0.273 0.106 0.2t4 0. i3l 0.000 0.111 0.238 c.123 0.100 0.360 0.969 FeZ+ 0.390 0.601 0.3r9 0.491 0.243 0.374 0.394 0.000 r.768 1.161 0.791 0.809 r.384 0.907

Fe3+ci s P/tnans Hl I .455 I .333 i .253 1.366 1.500 0,748 1.000 0.504 1.406 2.C00 0.572 r.325

Fe2+cis t€/trans ill 3.561 2.592 4.213 2.698 8.790 5.14? 5.000 i.387 3.105 ?.t37 L.962 2.000 1.380

could represent differencesof next-nearest-neighboren- Cation ordering vironments induced by vacancies,cation clustering, etc., The structural data for a variety of trioctahedral lM in adjacent cis-M2 afld trans-Ml sites. Therefore it re- micas summarized in Table 2 indicate that the cis-M2 mains unclearwhether the two doubletsactually represent andtrans-Ml octahedra have comparable dimensions and two sites or whether they represent different defect dis- distortions. The average metal-oxygen distances of the tributions independent of site occupancies. cis-M2 octahedra are perhaps marginally smaller than those of the trans-Ml octahedra. As noted earlier, the Proportions of Fe3* most noticeable differencesbetween the two octahedral Although Mdssbauerspectroscopy provides a nondes- sites are the configurations of the two hydroxyl anions tructive techniquefor identifying coexistingFe2+ and Fe3+ constituting the coordination polyhedra and the arrange- ions in minerals, the task of estimating accurately the ments of next-nearest-neighborcation sites. Therefore, proportions ofFe2* and Fe3+still presentformidable prob- structural characteristicsalone provide little insight into lems so far as micas are concerned.Effects of oxidation predicting preferred cation site occupancies.The Mdss- during sample preparation, preferred orientation of the bauer spectraldata, however, provide direct evidence of mica flakes, and different recoil-free fractions ofthe cat- Fe2rand Fe3+site occupanciesand, in somecases, indirect ions may all bias the peak-areadata toward spuriously evidenceof Mg2*, A13*,and Ti4+ site enrichments. high percentagesof Fe3+ions. This study has confirmed Table 5 summarizesthe chemical frrrmulae of the nat- that reproducible Fe3*/Fe2*ratios for micas can be ob- urally occurringmica specimensstudied here, recalculated tained provided adequate precautions are taken when from the data in Table 3 by using the Fe3*and Fe2*cation mounting pulverized mica crystals prior to a M6ssbauer proportions determined from the Mdssbauer spectralpeak- spectral measurement.However, it is worth noting that areadata. The recalculated formula proportions show that samplesare usually supplied to the spectroscopist the cis-M2/trars-Ml ratio data for Fe2*in Table 5 gen- as sieved mineral separatesof uniform grain size. Oxi- erally exceedthe ideal 2: I site multiplicity ratio, indicating dation ofFe2* may have occurredbeforehand during com- that Fe2* ions are relatively enriched in the crs-M2 po- minution of a bulk rock sampleto particle sizesamenable sitions. This observationconfirms resultsobtained earlier to mineral beneficiation by hand-picking, heavy liquid, @ancroftand Brown, 1975)for biotites from igrreousrocks. and magnetic separationtechniques. Our results for metamorphic biotites indicate that Fe2* 964 DYAR AND BURNS: FERRUGINOUS ONE-LAYER TRIOCTAHEDRAL MICAS ions are even more highly enriched in cls-M2 octahedra the Fe2* analogue of phlogopite. Ferriannite, hitherto than they are in igneousbiotites. Whether the surplus of known as a synthetic phase (Wones, 1963), is confirmed Fe2*ions in cls-M2 positions is induced by vacanciesin to be a naturally occurring mineral in metamorphosed trans-Ml positions or is caused by cation ordering of iron formations (e.g., Miyano and Miyano, 1982; M. J. Mg2+,Al3+, and Ti4* ions in trans-Ml sites is impossible Gole, pers. comm.) by the positive identification of tet- to evaluatefrom the Mdssbauerspectral data for biotites. rahedral Fe3*ions by M6ssbauerspectroscopy. The closenessof the cls-M2/trans-Ml ratio to 2: I for the Cation-site terminology. Since the two octahedral sites Lone Creek annite (specimen 12 containing hieh AISJ) are so clearly distinguished by the configuration of the and the Pikes Peak annite (specimen 13 containing high hydroxyl anions, there is no longer cause for confusing Ti4*) may suggestthat Al3* and Ti4* have no strong site them with the designatedcrs-M2 and trans-Ml positions preferencesin these Fe2*-rich specimens.Therefore, it is having a site multiplicity ratio of 2:1. probably Mg2* ions that are relatively enriched in the Cations distinguishablein a Miissbauer spectrurn.The trans-Ml positions of biotites, and this factor contributes annite and fluorannite spectra demonstrate that octahe- to Fe2t enrichments in cis-M2 positions of these speci- dral Fe2* in cli-M2 and trans-Ml positions may be re- mens.Strong Mg2* ion enrichmentsin trans-Ml positions solved as the outermost and innermost Fe2+doublets, are also suggestedby the Mtissbauerspectral data of ferri- respectively.The doublet representingtetrahedral Fe3*is phlogopite and two ferriannites (specimens8, 9, and 1l). also characterizedin ferriphlogopite and ferriannite Miiss- bauer spectra.However, the poor resolution of doublets Tehahedral Fe3* attributed to octahedralFe3* ions in micas containing low This study demonstratesfor the first time by Mdssbauer concentrations of Fe3* may not belay the ambiguity of spectroscopythat Fe3+ions occur in tetrahedral coordi- peak assignmentsin biotite Miissbauer spectra(Mineeva, nation in naturally occurring ferriannites. Wones (1963) I 978). had earlier synthesizedand establishedthe stability field Oxidation and preferredorientation. Having confirmed of ferriannite. He predicted that similar mica composi- that Mdssbauerspectra are adverselyaffected by oxidation tions might occur naturally in rocks devoid of Al, such as during grinding and by alignment of mica crystallites in iron formations and evaporite deposits. Subsequently, holders, this study demonstratesthat theseproblems can Miyano and Miyano (1982)and M. J. Gole (pers.comm.) be reduced or eliminated through use of adequate pre- described such ferriannites in metamorphosed iron for- cautionsduring samplepreparation. Mixing the mica sep- mations. These authors deduced from microprobe ana- arate with sugar,grinding wet under acetone>and sprin- ll"tical data that tetrahedrally coordinated Fe3+is present kling the sugar-coatedpowder into holders is the procedure in thesemetamorphic ferriannites becausethere is insuf- recommendedfor Mdssbauerspectral studies of micas. ficient (Si + Al) to completely fill the tetrahedral sites in Cation-site occupancy.The clear distinction between the phyllosilicate layers.The Mdssbauerspectral data for Fe2*ions in cis-M2 and trans-Ml positions enablescon- specimens9-l I confirm the presenceoftetrahedral Fe3* firmation to be made of the relative enrichment of Fe2* ions in naturally occurring ferriannites and correlate with rn cis-M2positions ofbiotites (Bancroftand Brown, 1975), previousmeasurements of feniphlogopite (Steinfink, I 962; particularly in specimensin metamorphic rocks. If the Annersten,1975). assignmentofthe Fe3*doublets in biotite Mdssbauerspec- The occurrenceof small proportions of tetrahedralFe3* tra is correct, Fe3*ions are slightly enriched in trans-Ml ions in the Lone Creek and Cape Ann annites is unex- positions. Mg, and not necessarilyAl or Ti, appearsto be pected, since both of these specimenscontain a surplus enrichedin trans-Ml positions, particularly in ferriphlog- of(Si + Al) to fill the tetrahedral sites.The occurrenceof opite and ferriannite. This may be induced by tetrahedral someFe3* ions in the tetrahedrallayer may be rationalized Fe3*and deficienciesof K* in interlayer positions. by polyhedral distortion criteria (Bailey, 1984a; Hazen and Wones, 1972).The high Fe2+content ofannite induces AcxNowr,nncMENTs expansionofthe octahedral(hydroxyl) layer that may be We areextremely grateful to Martin J. Gole,cslno, for pro- compensatedfor by enlargementof the tetrahedral (sili- viding specimensof natural ferriannitesand unpublishedana- cate) layer. This can be achieved by accommodating the lyticaldata that first provokedand inspiredthis study.We also larger (i.e., relative to Si4* and Al3*) Fe3* cations in the thankG. M. Bancroft,F. Barker,D. Hickmott,R. M. Hazen,S. tetrahedral sites (Hazen and Wones, 1972). Miyano,D. A. Robie,and the late D. R. Wonesfor donating specimens.We thank F. Spearfor helpfuldiscussions and the Coxcr.usroNs useof his mineral-formularecalculation program. H. Annersten and an anonymousreviewer are thanked for providing construc- Nornenclature.Annite from the type locality at Cape tive criticismsof themanuscript. M. D. Dyaracknowledges sup- Ann, Massachusetts,is a Fe3*-richmica as originally de- port from the MineralogicalSociety of America Biennial Crys- fined by Dana ( 1868). The Fe3*ions, however,occur most- tallographicResearch Fund. Funding for this researchhas been Iy in octahedral sites with small amounts in tetrahedral provided by a grant from the National Aeronauticsand Space coordination. The annite from Pikes Peak, Colorado, Administration(NSG-7604). Finally, we thank Francis Doughty comes closest to Winchell's (1925) concept of annite as for his patientassistance with thepreparation ofthis manuscript. DYAR AND BURNS: FERRUGINOUS ONE-LAYER TRIOCTAHEDRAL MICAS 965

RnrBnnncns Goodman, B.A., and Wilson, M.J. (1973) A study of the weath- ering of biotite using the M6ssbauereffect. Mineralogical Mag- Annersten, (1974) H. Miissbauer studies of natural biotites. azine, 39, 448-454. American Mineralogist, 59, 143-15 1. Hazen, R.M., and Burnham, C.W. (1973) The crystal structure -(1975) Mossbauer study of iron in natural and synthetic of oneJayer phlogopite and annite. American Mineralogist, biotites. Fortschritte der Mineralogie, 52, 583-590. 58, 889-900. Annersten, (1978) H., and Olesch,M. Distribution of ferrousand Hazen, R.M., and Wones, D.R. (1972) The effectof cation sub- ferric iron in clintonite and the Mdssbauer characteristics of stitutions on the physical properties of trioctahedral micas. ferric iron in tetrahedralcoordination. CanadianMineralogist, American Mineralogist, 57, 103-129. 16, 199-203. Heller-Kallai, L., and Rozenson,I. (1981) The use of Mdssbauer Bailey, S.W. 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