American Mineralogist, Volume 76, pages 1905-1909, 1991

Miissbauer spectra of minnesotaite and ferroan

J.M.D. Corv, T. Blxls* Department of Pure and Applied Physics,Trinity College,Dublin 2, Ireland S. GuccnNnprnr Department of Geological Sciences,University of Illinois at Chicago, Chicago,Illinois 60680, U.S.A,

Arsrru.cr Mdssbauer spectra of five samples of minnesotaite and one of ferroan talc have been recordedat 296 K and 4.2 K. A singleferrous doublet with isomer shift 6 : 1.13 mm/s relative to aFe and quadrupolesplitting L:2J4 mm/s is observedin all minnesotaite samples at room temperature. The ferroan talc has a smaller quadrupole splitting, A : 2.55 mm/s. All minnesotaite samplesorder antiferromagnetically,with N6el points in the tange 25-37 K, but ferroan talc does not order magnetically down to 4.2 K. Two distinct ferrous populations in minnesotaite are inferred from the resolved magneticallysplit spec- tra at 4.2 K. The minority sites may be those coordinated by the excessOH in the min- nesotaitestructure. The P-cell and C-cell variants have distinctly different hyperfine fields (10.4 and 14.9T for the two sitesin the P cell but 8.9 and 14.4T in the C cell),although their N6el points are similar.

INrnooucrroN taite, ferroan talc, and talc and found a lack of compo- Minnesotaite was originally considered by Gruner sitions having the octahedralFe fraction in the range0.45- (1944) to be an Fe analogueof talc having ideal formula 0.60, perhapsindicating the chemical limits of the struc- tures. {Fel*}[Sio]O,o(OH),,where { } and [ ] denotethe oc- tahedral and tetrahedral sites, respectively.More recent- Both minnesotaite and talc have a continuous octahe- ly, X-ray and electron diffraction studies (Guggenheim dral sheet(Fe in minnesotaite, Fe,Mg in ferroan talc) but and Bailey, 1982; Guggenheim and Eggleton, 1986) differ in the nature ofthe tetrahedral sheets.It is therefore showed that the structure is considerablymore complex. of interest to determine whether the different structures The structure differs from that of talc by its periodic mod- produce distinctive Mdssbauer spectra. Minnesotaite is ulation alonga as a result ofinverted tetrahedrathat form one of the few sheetsilicates that exhibit long-rangemag- chains along b in the interlayer region. Thesechains cross neticorder (Ballet al., 1985a,1985b; Coey, 1987, 1988). link adjacent 2:1 layers and tetrahedral strips. Three ma- A disordered P-cell sample investigated by Coey et al. jor structural varieties exist; two possessunit cells form- (1990) showedplanar antiferromagneticorder below about ing primitive (P) or end-centered(C) Bravais lattices. A 30 K with moments in the ab planes, but both the inter- third form shows extensive streaking in the diffraction layer magnetic order and the long-range ferromagnetic pattern, indicating disorder in the displacements and correlations in the octahedral sheetscan be destroyedby widths of the strips. Intergrowths of the three forms as applying a magnetic field. Miissbauer spectraof the same well as less common forms that have greater variations sample in the magnetically ordered state show a charac- in strip widths have been reported by Guggenheim and teristic ferrous hyperfine pattern (Coey, 1984; Townsend Eggleton(1987) and Ahn and Buseck(1989). et al., 1985), which was earlier fitted with a single Fe2* Although the ideal formula of the P form of minneso- magnetic component. Here we analyzeMdssbauer spec- taite is (Fe,Mg).oSiooOr6(OH)r8and that of the C form is tra of five samples of minnesotaite and one of ferroan (Fe,Mg)rrSiruOru(OH)ru,the analysesmay be recastto give talc in order to deflne the range of hyperfine parameters a formula analogousto talc, with a tetrahedral: octahe- for these minerals and to attempt to correlate variations dral cation ratio of 4:3. Furthermore,it is noted that when in the spectrawith structural and magnetic properties. the structural modulation is ignored, minnesotaite has a ExpnnrunNTAL REsuLTs subcell that closely approximates the unit cell of talc (Guggenheim and Eggleton, 1986). Evans and Guggen- Details of the six Fe-rich samplesexamined are given heim (1988) reviewed published analysesof minneso- in Table l. The cell type (P or C) was determined by selected area electron diffraction using a transmission electron microscope (TEM). The Bluebell minnesotaite * Present address: Physics Department, University of Ioan- (MCl) showed ftkO patterns with maxima within diffuse nina, P.O. Box 1186,GR 54110 Ioannina,Greece. streaking along a*, indicating structural organization ap- 0003-o04x/91/l 1I 2-l 905$02.00 I 905 1906 COEY ET AL.: MOSSBAUER SPECTRA OF MINNESOTAITE AND TALC

TABLE1. Sampledata

Minnesotaite' Minnesotaite Minnesotaite*' Minnesotaitet Minnesotaite+ tatc$

Sample 1 BM 131 2 Brit. Mus. 3 4DC-21 5 MC1 6 1975,420 Locality Sagamore pit, Cu- near Empire mine, Auburn mine, Min- near Ardua Lake, Bluebellmine, Sterling mine, Ant- yuna district, Marquette Range, nesota Quebec Riondel, British werp, N.Y. Minnesota Marquette Co., Columbia Michigan Structure C unknown P P disorderedP lTc-disordered type r" (K) 37(21 33(2) 31(1) 28(21 Composition sio, 51.79 52.26 53.85 51.47 54.5 At,o3 1.46 0.36 0.15 1.57 0.2 Fe,Os 0.61 31.4511 30.5311 3.23 4.7 FeO 35.65 30.50 12.6 MnO 0.78 0.02 0.53 1.83 MgO 3.21 8.40 9.68 5.10 17.2 Na,O 0.06 b.d. 0.15 K,O O.44 0.36 O.17 H,O* 5.03 5.88 4.5 H,O- 0.03 0.16 0.5

Note-'The abbreviationb.d. : below detection. . See Blake (1965) and Guggenheimand Eggleton(1986). t'Analysis represents an average from Guggenheimand Eggleton(1986) f See Lesher (1978) and Guggenheimand Eggleton(1986). + Analysis from Perraultand Hebert (1968); see also Guggenheimand Eggleton(1986). This sample shows severe disorder along )c, but maxima within streaks approximate a Pcell. $ Analysis from Robinsonand Chamberlain(1984). The sample shows complete disorder along Z. ll Fe,",as FeO.

proaching the P cell but with considerable strip-width are shown in Table 2, with parameters for magnesium disorder. Talc, in contrast to minnesotaite,does not show talc included for comparison. Ferrous line widths (full- disorder along a* but may show stacking disorder along width at half maximum) are approximately 0.4 mm/s, as c* due to 2: I layer displacements.The Sterlingtalc is fully is usual for clay minerals. As in chlorite (Coey, 1984), disordered. The Empire mine minnesotaite was not ex- there is no evidenceof two separatedoublets in the min- amined by TEM or microprobe. nesotaite spectra that could be attributed to Fe on Ml Mdssbauer spectra were obtained using a constant ac- and M2 sites, nor is the goodness-of-fitparameter 12 sig- celeration spectrometer with a source of 5tCo in a Rh nificantly reduced when two components are allowed. The matrix. Experimental data were fitted using a nonlinear parameters for P- and C-cell minnesotaite are indistin- least-squaresprogram that adjusts the parameters ap- guishablefrom eachother [D: l.l3(l) mm/s, A : 2.74(2) pearing in the hyperfine Hamiltonians for Fe2+and Fe3+. The Hamiltonian for the nuclear excited state for Fe2*is not diagonal for a generalorientation ofthe electric field gradient relative to the direction of the magnetic hyper- 20 fine field, so the ferrous magneticpatterns normally com- --oFoo- .d prise eight lines. Isomer shifts are quoted relative to dFe. 'od- 15 Sampleswere dispersedin icing sugar to avoid preferred ol trN onentatron. " The magnetic ordering temperature was taken as the . lTp 10 point of inflection in thermomagnetic scansin an applied field of 50 mT (Fig. l). This definition of the magnetic

ordering temperature was establishedby neutron diffrac- a 5 tion and thermomagnetic analysis of sample 5 (Ballet et al., 1985a). No sign of magnetic order was observed in the ferroan talc. l0 20 30 40 Figure 2 shows typical room-temperature spectra,the Temperature (K) main which is ferrous there is component of the doublet; Fig. l. Magnetic susceptibility on heating zero-field cooled also a small amount of Fe3* present (5-20o/oof the Fe,., samples in an applied field of 50 mT. Open squares: P-cell in the silicate). Spectra of associatedFe oxides are de- minnesotaite (Auburn mine); open circles : C-cell minnesotaite tected in some cases-magrretitefor the Ardua Lake min- (Sagamorepit); crosses: ferroan talc (Sterling mine). The N6el nesotaite (650/oof Fe,.,) and hematite for the ferrous talc temperaturesof the minnesotaite samplesare indicated by the (lessthan 50/oof Fe.",).The least-squaresfitted parameters AITOWS. COEY ET AL.: MOSSBAUER SPECTRA OF MINNESOTAITE AND TALC t907

(a)

5 z 100.0 U) q

q z z (b)

q F q r.o q! z

F

r/L\Me.n!!9w!r?b.'Sed.r4

98 -6 -4 -2 0 2 4 6 96 VELOCITY ( mm/s ) 94 Fig. 2. M

-10 -5 0 s 10 mm/s for bothl, but the quadrupole splitting for ferroan VELOCITY( mm/s) talc is significantly different : l.l2(l) mm/s, A : 2.53(4) [6 Fig. (a) mm/sl. 3. Miissbauerspectra at 4.2K for P-cellminnesotaite (Auburnmine); (b) C-cellminnesotaite (Sagamore pit); (c) fer- Figure 3 shows typical spectraat 4.2K for three sam- roan talc (Sterlingmine). Solid lines representleast-squares fit ples. The spectra for P- and C-cell minnesotaite exhibit to theexperimental points. clear ferrous magnetic hyperfine structure, with a ferric hyperfine pattem appearing also for the P-cell samples. The ferroan talc, which showedno sign of magneticorder spectrum that may reflect a broad range of magnetic re- in the susceptibility measurements, exhibits a mixed laxation times centered around 5 x lO-n s (Rao et al., 1979). The magnetically ordered spectra of all the minneso- Trsle 2. Hyperfineparameters at 296 K taite samplesrequire a minimum of two ferrous subspec- Sample 1 3 4 tra for an adequatefit that reproduces,for example, the asymmetry in the line at -5 mm/s. The x2 parameter is 6 (mm/s) 1.13(2) 1.1 3(2) 1.13{2) 1.13(2) 1.12(2) A (mm/s) 2.74(2) 2.74(2) 2.7s(21 2.72(21 2.53(2) reducedby approximately 30o/oon passingfrom a fit with A(o/") 100 82 34 85 83 one eight-line ferrous pattern to one with two eight-line 6 (mm/s) 0.32(2) 0.41(21 0.27(2) patterns. Fitted parametersare included in Table 3. Both (mm/s) a 0.47(21 0.78(2) 0.7s(2) x A (o/o) 18 15 I ferrous subspectrahave similar isomer shifts 6 1.28(2) 6 (mm/s) 0.25(21 0.30(2) mm/s, a negative quadrupole splitting of A = -3.0 mm/ -0.20(21 A (mm/s) 0.02(1) s, and an asymmetry parameter7, = 0. The negativesym- A"'(T) 49.5(2) 51.7(3) A (y"',, 208 metric quadrupole interaction is familiar from other 6 (mm/s) 0.64(2) trioctahedral silicates such as and (Coey, A (mmis) 0.02(1) 8", (T) 46.1(3) 1988), where it is associatedwith the hard c-axis mag- A(%) 46 netic anisotropy resulting from the orbital singlet ground x" 1.1 1.3 1.4 1.2 1.5 state of Fe'zt(Varret 1976). Furthermore the angle 0 be- /Vote.'Thesymbols are defined as follows: 6 : isomer shift relative to tween the c axis and the direction of the magnetichyper- aFe; A : quadrupolesplitting or quadrupoleshift; BH: magnetichyperfine fine field is 90', as expected for easy-planeanisotropy. fiefd; A : relative absorption arcai per degree of freedom in the fit. x2: The two subspectra have distinctly different hyperfine 1908 COEY ET AL.: MOSSBAUER SPECTRA OF MINNESOTAITE AND TALC

TasLe 3. Hyperfine parameters al 4.2 K two OH groups in the minnesotaite subcell.However, the averageferrous hyperfine field in biotite, which has both Sample M I and M2 ferrous occupancy,is I 5 T (Coey, I 988). The 't.27 6 (mm/s) 1.26(2) 1.30(2) 1.28(2\ are likely -2.96(2) -3.01 -3.03(2) -3.06(2) partial tetrahedral inversions in minnesotaite A (mm/s) field be- 4,O) 14.4(21 15.1. 14.9(2) 14.8(21 to create greater perturbations in the hyperfine 4 0 0 00 causethey increasethe number of OH groups in the oc- df) 90 90 90 90 tahedron. The excessOH is related to substitution of OH A (o/") 72 39 53 50 d (mm/s) 1.28(2) 1.28(2) 1.30(2) 1.30(2) for tetrahedralapical O where the tetrahedraare inverted. A (mm/s) -2.94(21 -2.s6(2) -2.95(2) -2.e4(2) Furthermore, these OH groups are not linked to further 8", (T) 10.4(2) 10.q2) 8.s(2) 10.6(2) In view of the similarity in quadrupole inter- ,l 0 0 00 Si atoms. dc) 90 90 90 90 action, the orbital contribution to the ferrous hyperfine A(%) 28 53 42 40 field should be similar at the two sites, so the main dif- d (mm/s) 0.49(2) 0.46(2) 0.s5(2) a (mm/s) -0.15(2) -0.02(2) 0.03(2) ference should be in the contact term. It is known from 4",(T) 51.0(2) 52.5(21 45.5(2) the Fe3* hyperfine fields in ferric oxides and hydroxides A(%l 8 5 10 of OH for O'? reducesthe contact field; 1.0 1.6 1.8 1.9 that substitution hence, we suggestthat the Fe with the smaller hyperfine : Note.'The symbols are defined as follows: 6 isomer shift relative to field is in sitesthat are coordinated by excessOH, where- cFe; A : quadrupolesplitting or quadrupoleshift; 4, : magnetichyperfine field; ,{ : relative absorption area; 4 : asymmetry parameter; 0 : angle as the Fe with the greaterhyperfine field is located in the between the principalcomponent of the electric field gradient and B^;72 unperturbed segmentsof the 2:l layers. A quantitative : per degree of freedom in the fit. explanation of the diferences between these fields in P- and C-cell minnesotaite, in terms of the contact, orbital, fields: 14.9 and 10.4 T for P-cell minnesotaiteand 14.4 and dipolar contributions, is not feasible without much and 8.9 T for C-cell minnesotaite. The smaller hyperfine more detailed structural information. field for the C-cell structure is an intrinsic effect,unrelat- In contrast to the ferrous absorption, the ferric hyper- ed to ordering temperature, since it may be seen from fine field at 4.2 K in P-cell minnesotaite appearsto be Table I that Z* is higher in the C-cell material. The Fe3* quite sample dependent, being greatest in the samples in P-cell minnesotaite gives a separatemagnetic hyper- with the greateststructural order. fine pattern, with a hyperfine field that is quite sample dependent,ranging from 45.5 to 52.5 T (seeTable 2). CoNcr.usroNs l. Minnesotaite shows a characteristicferrous doublet DrscussroN at room temperature,with 6: l.l3(l) mm/s and A: A great deal has been written about the interpretation 2.74(2)mm/s, that is insensitiveto the cell type. of the paramagneticMiissbauer spectra of 2: I layer sili- 2. Ferroan talc has a diferent quadrupole splitting, A cates in terms of Fe site preferences(cls vs. trans; prox- : 2.55(2) mm/s, and can thereforebe distinguishedfrom imity of trivalent cations on tetrahedral sites; octahedral minnesotaiteon this basis. site environment). Some reviews are given by Heller-Kal- 3. The subspectraof two Fe site populations are re- lai and Rozenson(1981), Coey (1984), and Dyar (1987). solved in magnetically split spectrataken in the liquid He However, there is little to be learned from Mdssbauer temperature range but not in the paramagneticspectra at spectraif the paramagneticdoublets from Fe in different room temperature. The minority sites may be those co- sites overlap and are not at least partially resolved. In ordinated by the excessOH in the minnesotaitestructure. minnesotaiteand chlorite (Balletet al., 1985b),the Fe is 4. The hyperfine fields at 4.2Kfor C-cell minnesotaite presentin such large amounts that it must occupy differ- are distinctly lower than for P-cell minnesotaite.Relative ent sites, yet the ferrous absorption is well fitted by a site occupanciesalso differ, with a smaller minority site single quadrupole doublet with somewhat broadened lines. population in C-cell minnesotaite. Different site populations may neverthelessbe resolvedat low temperatures if the Mdssbauer spectrum of minne- AcxNowr,nocMENTS is magnetically split. sotaite This work was completed when T. Bakas was on sabbatical from the It is apparent from the 4.2-Kdata that there are at least Physics Department, University of Ioannina, Greece,in the summer of two distinct types of Fe sites, which have different hy- 1989. S. Guggenheimacknowledges the support ofthe National Science perfine fields but are similar in other respects.The Fe- Foundation (gant EAR-8704681) and the Petroleum ResearchFund of We site populations are roughly 3:l in the C-cell and 3:2 in the American Chemical Society (grant ACS-PRF 21974-AC8-C). gratefully acknowledge the use ofthe electron microscopy facilities ofthe the P-cell samples,with the larger amount of Fe in the Research Resources Center of the University of Illinois at Chicago. site with the greater hyperfine field. Minnesotaite has no net layer charge,and there is little substitution oftriva- RrrnnnNcns crrED lent cations for Si, so one possibility might be to associate Ho, Buseck,P.R. (1989) Microstructures and tetrahedral (- Fe Ahn, Jung and M2 Fe with the larger hyperfine field I 5 T) and Ml strip-width order and disorder in Fe-rich minnesotaites American with the smaller (-10 T), where Ml and M2 refer to Mineralogist, 74, 384-393. octahedral sites with cls and trans confrgurationsof the Ballet, O., Coey, J.M.D., Mangin, P., and Townsend, M.G. (I985a) Fer- COEY ET AL.: MOSSBAUER SPECTRA OF MINNESOTAITE AND TALC 1909

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