Mössbauer spectroscopic and X-ray powder ... - Springer Link

Phys Chem Minerals (1995) 22:282-294

PHYSlCS CHEMISIRY NMINEIWS 9 Springer-Verlag 1995

Miissbauer Spectroscopic and X-ray Powder Diffraction Studies of Synthetic Micas on the Join Annite KFe3AISi3Olo(OH)2-Phlogopite KMg3AISi3Olo(OH)2 G.J. Redhammer, E. Dachs, G. Amthauer Institute of Mineralogy, University of Salzburg, Hellbrunnerstrasse 34, A-5020 Salzburg, Austria Received June 15, 1994/Revised, accepted February 21, 1995

Abstract. Micas of the composition K(Fe3_xMg~)A1Si3 Olo(OH)2 (x=0.6, 1.2, 1.8, 2.4 and 3.0, corresponding to anns0phl20, ann6ophl4o, ann4oPhl6o, ann20phls0 and annophl~00) were synthesized hydrothermally under controlled oxygen fugacity conditions. Lattice parameters a o and b0 show a distinct linear decrease with increasing Mg content. With increasing ferric iron content a deviation from this linear trend is observed especially within iron rich samples. The tetrahedral rotation angle c~ increases smoothly from 0 ~ in annite to 9.1 ~ in phlogopite. M6ssbauer spectra show Fe a+ and Fe 3+ on the octahedral M1 and M 2 sites and partially also Fe 3+ on the tetrahedral site. There is a smooth increase of the quadrupole splitting on both the M1 and the M 2 site going from annite to phlogopite, probably due to changes in the lattice contribution to the electric field gradient, assuming a positive correlation between quadrupole splitting and distortion. Fe 3+ contents, as determined by M6ssbauer spectroscopy, versus oxygen fugacity shows that, depending on the composition of the micas, minimum amounts of Fe 3+ are present. For annsophl20 this minimum amount of Fe 3+ is about 8% decreasing to about 1-2% Fe 3+ for ann20phlso. The molar volume of each solid solution member has been estimated from the determined relations of the molar volume versus % Fe 3+ contents, extrapolated back to 0% Fe 3+. Plotting these volumes as a function of Xph1 shows that negative excess volume occur in the annitephlogopite join, with the maximum deviation from ideality around Xph1= 0.3. Margules volume parameters have been constrained as: Wv, AnnPhl = 0.018 __0.016 J/(bar.mol) and Wv, PhlAnn = - 0.391 + 0.025 J(bar.mol) (three site basis).

Correspondence to: G.J. Redammer

Introduction Biotite micas on the join annite KFe3A1SiaOlo(OH)2phlogopite KMg3A1Si3Olo(OH)2 are favourite minerals for investigations in experimental petrology, crystallography and spectroscopy as they are very common throughout all petrologic systems. They have been one of the first minerals synthesized hydrothermally under exactly defined P - - T - f o 2 conditions using solid state oxygen buffers. The stabilities of phlogopite and annite were first determined by Yoder and Eugster (1954) and Eugster and Wones (1962), respectively. Investigations of Wones (1963) and Wones and Eugster (1965) determined the stability and the physical properties of biotites on the annite - phlogopite join. Hazen and Wones (1972) studied the effect of various cation substitutions in the inter-, tetrahedral and octahedral layer on the stability of trioctahedral A1SI 3 micas. The stability field of annite was recently revised by Dachs (1994), using the H 2 sensor technique to relocate the upper log fo~-T stability limit given by the reaction annite= sanidine+magnetite + H2. It has been suggested by the above authors that iron in the octahedral layer not only occures in the divalent but also in the trivalent state in this solid solution series. Different approaches have been undertaken to determine the amount of the ferric iron content including wet chemical analysis and indirect methods like variations in cell dimensions and refractive indices. Tikhomirova et al. (1989) studied the amount of ferric iron in synthetic iron magnesium biotites using chemical and pyrolytical methods. They observed ferric iron ranging from 16% to 27%. The most precise Fe2+/Fe 3+ ratios however can be obtained from M6ssbauer spectroscopic studies. In a previous paper (Redhammer et al. 1993) we investigated annite, synthesized at different oxygen fugacities by M6ssbauer spectroscopy and found values of Fe3+/Fe ranging from 0.09 to 0.19. It was found that the Fe 3 § content decreases linearly with decreasing oxygen fugacity. Below - 2 6 . 5 log fO2 a kinck of the linear relation is observed and no more reduction of the Fe 3 +

283 c o n t e n t c o u l d be achieved. T h e a b o v e results are in g o o d a g r e e m e n t w i t h H a z e n a n d W o n e s (1972), w h o stated on the basis o f crystal g e o m e t r i c a r g u m e n t s t h a t a n n i t e with F e 2+ o n l y is n o t stable b e c a u s e o f a d i m e n s i o n a l misfit b e t w e e n the o c t o h e d r a l a n d the t e t r a h e d r a l layer. Thus, m o s t r e d u c e d a n n i t e s w o u l d still c o n t a i n a b o u t 12% Fe 3 +. S i m u l a r results were o b t a i n e d b y F e r r o w a n d A n n e r s t e n (1984) using M 6 s s b a u e r s p e c t r o s c o p y a n d P a r t i n et al. (1983) a n d P a r t i n (1984) using wet c h e m i c a l a n d M 6 s s a u e r analysis. H o w e v e r , n o w o r k so far has been d o n e to p r e d i c t the F e 2 + / F e 3+ r a t i o s o f synthetic m i c a s o n the j o i n a n n i t e - p h l o g o p i t e as a f u n c t i o n o f F e / M g r a t i o s a n d o x y g e n f u g a c i t y using M 6 s s b a u e r spectroscopy. M 6 s s b a u e r s p e c t r a o f m i c a s have been r e c o r d e d since the e a r l y times o f the a p p l i c a t i o n o f this m e t h o d to m i n eralogy. M o s t studies, h o w e v e r , h a v e b e e n c a r r i e d o u t on n a t u r a l s a m p l e s w i t h p a r t l y very c o m p l e x o c t a h e d r a l a n d t e t r a h e d r a l c h e m i s t r y (e.g. D y a r a n d B u r n s 1986; D y a r 1987, 1990; R a n c o u r t et al. 1992). M o r e r e c e n t l y a series o f p a p e r s was p u b l i s h e d d e a l i n g in a very critical view with the e v a l u a t i o n o f the M 6 s s b a u e r s p e c t r a o f micas with r e s p e c t to L o r e n z i a n line w i d t h , d e t e r m i n a tion o f a c c u r a t e M 2 / M 1 r a t i o s a n d the q u e s t i o n w h e t h e r one c a n resolve cis a n d t r a n s o c t a h e d r a l sites in 2:1 layer silicates o r n o t ( R a n c o u r t et al. 1994; R a n c o u r t 1994 a, 1994 b). It is c o n c l u d e d , t h a t L o r e n z i a n s are i n a d e q u a t e because o f several r e a s o n s a n d t h a t Voigt lines b e t t e r fit the o b s e r v e d s p e c t r u m . F u r t h e r it is s h o w n t h a t M 6 s s b a u e r s p e c t r o s c o p y o f 2:1 l a y e r silicates cann o t resolve the o c t a h e d r a l cis (M1) a n d t r a n s (M2) sites t h a t it is i m p o s s i b l e to e x t r a c t exact cis/trans p o p u l a t i o n r a t i o s f r o m two F e 2 + d o u b l e t analysis. I n s t e a d o f this, one observes a single q u a d r u p o l e splitting d i s t r i b u t i o n ( Q S D ) . This Q S D is the result o f several local d i s t o r t i o n e n v i r o n m e n t s ( L D E ) a n d shows t w o m a i n c o m p o n e n t s t h a t c o u l d be cis a n d t r a n s like. I n general there is a l a c k o f M 6 s s b a u e r d a t a on synthetic micas. H o w e v e r , the effect o f a specific c a t i o n o n the M 6 s s b a u e r p a r a m e t e r s a n d o n the lattice in general c a n o n l y be s t u d i e d on s y n t h e t i c m a t e r i a l w i t h exactly d e f i n e d c o m p o s i t i o n . T h e a i m o f the p r e s e n t p a p e r t h e r e f o r e w a s (i) to synthesize b i o t i t e m i c a s o n the j o i n a n n i t e p h l o g o p i t e a t t e m p e r a t u r e s b e t w e e n 450 ~ C a n d 650 ~ u s i n g p r e d o m i n a t e l y r e d u c i n g solid state state buffers like I Q F (iron - q a r t z - fayalite), I M (iron m a g n e t i t e ) , I W (iron - w/istite) a n d the Q M F buffer ( q u a r t z - m a g n e t i t e - fayalite) in o r d e r to o b t a i n the m i n i m u m c o n t e n t o f F e 3+ a n d to c o r r e l a t e the F e 3 + / F e values o b t a i n e d f r o m M 6 s s b a u e r s p e c t o s c o p y with oxygen f u g a c i t y a n d m a g n e s i u m c o n t e n t ; (ii) to o b t a i n a set o f M 6 s s b a u e r p a r a m e t e r s o f s y n t h e t i c b i o t i t e s with v a r y i n g F e / M g r a t i o in o r d e r to c o r r e l a t e p o s s i b l e v a r i a tions o f M 6 s s b a u e r p a r a m e t e r w i t h crystal c h e m i c a l features, a n d (iii) to r e d e t e r m i n a t e v o l u m e - c o m p o s i t i o n r e l a t i o n s h i p s in the a n n i t e - p h l o g o p i t e series t a k i n g into a c c o u n t the m e a s u r e F e 3 + c o n t e n t s , neglected so far.

Experimental All syntheses were done with the convential hydrothermal technique (cold seal pressure vessels). Temperature regulation and control was performed using Ni/NiCr thermocouples connected with electronic regulators communicating with a computer which served as online control unit during the run. Temperatures given in Table 1 are thought accurate to +_3~ C. Water mixed with highly dispersed oil was used as pressure medium. For further details of the experimental design see Dachs (1994). Starting materials for syntheses were gels with a composition of K(Fe3-xMgx)A1Si3010(OH)z with x=0.6, 1.2, 1.8, 2.4 and 3.0 (serie A80, A60, A40, A20 and phlogopite respectively). The gels were prepared by using K2CO3, MgCO3, Fe(NO3)z.9 H20, Al(NO3)2. H20 and CsH2oO4Si (all Merck, at least 99%) as source for K, Mg, Fe, A1 and Si. All ferric iron present in the gels was transformed to the divalent state by reducing it in a Hz stream at 600~ C for at least 4 hours. In a first step of each synthesis large amounts (up to 500 rag) of one composition were synthesized in gold tubes at 650~ C, 4 or 5 Kbars and the redox conditions of the pressure vessel (ca. MW (magnetite - wiistite) as determined by the hydrogen sensor technique at the early stage of investigation). In a second step these micas were annealed at different T fO/ conditions using the solid state buffers IQF (iron q u a r t z - fayalite), IM (iron - magnetite), IW (iron - w/istite) and QFM (quartz - fayalite magnetite). AgToPd3o was used as material for the inner capsule contianing the mica gel, Au for the outer capsules, containing the buffer. Chemical composition of selected grains were determined with an electron microprobe (Jeol JXA 8600, automated by LEMAS system, acceleration voltage 15 KV, initial beam current 40.0 nA, beam focused to 1 ~tm). For calculation of lattice parameters X-ray powder diffi'action patterns were measured with an automatic diffractometer (Siemens D 500) with Cu Ks radiation (1.5405 A_, 40 KV, 30 mA) between 3~ and 86~ 20 (stepsize 0.02 ~ measuring time 7 seconds per step). Pure synthetic silicon (ao = 5.43088 ~) was used as internal standard. The cell dimensions were calculated with a modified version of the least square refinement program of Appleman and Evans (1973). M6ssbauer spectra were recorded at 293K using a conventional M6ssbauer spectrometer (source 50 mCi SVCo/Rh, multichannel analyser with 1024 channels, electromechanical drive system, symmetric triangular velocity shape). The spectrometer velocity was calibrated to an e-iron foil. The absorber density was 2.5 mg Fe/ cm 2, thus, thickness effect can be excluded and the absorbers can be denoted as thin. Some spectra were recorded with the absorber orientated at an angle of 54~ (magic angle, Ericson and Wgppling 1976) to the incident gamma - rays in order to test preferent orientation, but no differences to spectra recorded without this angle were observed. The two symmetric spectra obtained (512 channels each) were folded and evaluated assuming Lorentzian lineshape using the least squares program MOESALZ (Lottermoser et al. 1992). The spectra were evaluated with up to five doublets dedicated to Fez+ and Fe 3+ on the octahedral M1 and M2 sites and to Fe 3+ on the tetrahedral site. The Fez+ doublet with the smaller quadrupole splitting (QS) was assigned to the centrosymmetric octahedral M1 site with (OH)- in trans position, the one with the larger QS to the smaller M2 site with (OH)- in cis position. The Fe 3+ doublet with the smaller QS was attributed to the M2 site, the one with the larger QS to the M1 site. This attribution is in agreement with several other authors (e.g. Annersten 1974). In annsophl2o it was possible to resolve Fe 3+ on the tetrahedral site and on both Fe 3+ octahedral sites. Attempts to resolve all three sites of Fe 3+ in the other solid solution members were only partly sucessful for ann4ophl6o. In all other spectra it was imposible to resolve the broad Fe3+oct absorption feature into the two components because of low concentrations and overlap and the parame-

284 Table 1. Run conditions and experimental results of a n n i t e - p h l o g o p i t syntheses

Run

Start

Temp [~

Pressure [bar]

Duration [h]

Buffer"

- l o g fO2

Product b

PM + 1 Phl ~ 2 Phl ~ 3 Phi # 4 Phi ~ 5

gel gel gel gel gel

500 600 600 700 680

4000 3000 4000 4000 4000

621 336 621

-

-

1717

A201 A202 A203 A204 A205 A206 A207 A208

gel gel 201 201 201 202 202 202

650 650 400 500 500 450 600 600

5000 5000 4000 4000 4000 4000 4000 4000

768 768 862 481 838 838 838 268

IM IM IQF QMF QMF IQF

-34.09 -28.56 -30.36 -25.57 - 19.64 -26.05

bio,(hy) bio,(hy) bio,(hy) bio,(hy) bio,(hy) bio,(hy) bio,(hy) bio,(sa,hy)

A401 A402 A403 A404 A405 A406 A407 A408 A409 A410 A411 A412

gel gel 401 401 401 402 402 gel gel 409 409 402

650 650 500 400 500 600 450 780 780 700 600 600

5000 5000 4000 4000 4000 4000 4000 1800 1800 4000 4000 4000

745 745 482 308 308 546 546 308 308 190 311 168

IQF IM IM QMF QMF -

-30.36 -34.09 -28.56 - 19.64 -25.57 -

IW IW IQF

-21.33 -24.55 -26.05

bio bio bio bio bio bio bio bio bio bio bio bio

A601 A602 A603 A604 A605 A606 A607 A608 A609 A610 A611 A612

g~ gel gel gel 604 604 602 gel 601 601 601 601

680 680 680 680 600 500 500 780 700 600 650 600

5000 5000 4000 4000 4000 4000 4000 2000 4000 4000 4000 4000

886 886 886 886

-

-

QMF IQF IM

- 19.64 -30.36 -28.56

380 190 314 190 141

IW IW IQF IQF

-21.33 -24.55 -24.23 -26.05

bio,(hy) bio,(hy) bio,(hy) bio,(hy) bio,(hy) bio,(hy) bio,(hy) bio,(hy) bio,(hy) bio,(hy) bio,(hy) bio,(hy)

A801 A802 A803 A804 A805 A806 A807 A808 A809 A810 A811 A812

gel gel 801 801 801 801 802 gel 801 802 808 808

650 650 400 500 450 600 500 780 600 700 600 650

5000 5000 4000 4000 4000 4000 4000 4000 4000 4000 4000 4000

767 767 886 885 748 792 546 308 170 170 196 196

IM IM QMF QMF IQF IW IW IQF IQF

-34.09 -28.56 -25.57 -19.64 -30.36 -21.33 -24.55 -26.05 -24.23

bio bio bio bio bio bio bio bio,(sa,fa) bio bio bio bio

phl,(fo) phl,(fo,sa) phl,(fo) phl,(sa) phl,(sa)

" Q M F = q u a r t z - m a g n e t i t e - fayalite buffer, IM = i r o n - magnetite buffer, IW = iron wuestie buffer, IQF = i r o n - q u a r t z - fayalite buffer; b = bio = biotite, sa = sanidine, fa = fayalite, fo = forsterite, qz = quartz, hy = hypersten (...) mineral names in parentheses represent amounts < 5% o f the whole run product

ters for such doublets should be intermediate between M1 and M2. The linewidths of the two Fe 2 + doublets wer kept equal but allowed to vary free during the fitting process. The same procedure has been applied to doublets of octahedral Fe 3 + and was used by most authors before. The linewidth were constrained because this is the only way to obtain M2/MI ratios near the value of 2, demanded by the crystal structure.

Results

Synthesis and Mineral Chemistry Results of the synthesis together with the run conditions a r e g i v e n i n T a b l e 1. O x y g e n f u g a c i t i e s w e r e s u c c e s s f u l l y f i x e d i n n e a r l y all r u n s e x c e p t o f t h o s e p e r f o r m e d a t 450~ with the QFM buffer. At this temperature no reaction of quratz + magnetite to fayalite was observed.

285 Table 2. Chemical composition and structural formula of selected synthetic micas anlong the join annite-phlogopite, derived from microbeam analysis and M6ssbauer spectroscopy, n =number of analyses, numbers given in parentheses refer to the last digest and are one standard deviation. Fe 3+ contents as determined by M6ssbauer spectrocopy Sample n Fe 3+

Phl~ 5 20 -

A205 20 1.6

A405 45 6.2

A408 30 9.0

A410 28 6.2

A611 26 6.7

A802 26 14.2

A803 46 7.0

SiO2 A1203 FeO FezO3 MgO K20

43.77(34) 12.76(17) 29.47(35) 11.14(28) 97.14

42.02(31) 12,08(24) 9.97(46) 0.18(45) 22.70(37) 10.75(25) 97.62

38.64(21) 11.15(09) 1.7.02(21.) 1.26(21) 15.48(16) 9.93(11) 93.48

40.44(17) 11.28(19) 17.02(21) 1.89(21) 16.31(29) 10.27(22) 97.21

40.03(27) 11.61(11) 17.60(15) 1.30(15) 16.11(15) 10.26(10) 96.91

38.04(32) 11.18(8) 25.29(32) 2.03(32) 9.76(18) 9.90(14) 96.20

36.10(17) 10.68(6) 28.36(18) 5.32(18) 4.72(5) 9.43(9) 94.70

35.60(30) 10.58(14) 31.77(42) 2.68(42) 4.49(9) 9.46(11) 94.58

K+ Fe 2+ Fe3+oct Mg 2+ A13+oct ~oot A13+tet Si4+ Fe3+tet

0.97(3) 3.00(4) 0.02(2) 3.02 1.01(1) 2.99(2) -

0.98(3) 0.59(3) 2.41(3) 0.01(2) 3.01 1.00(2) 2.99(2) 0.01(1)

0.98(1) 1.11(2) 0.05(1) 1.79(2) 0.03(1) 2.97 0.98(1) 2.99(1) 0.03(1)

0.97(2) 1.07(2) 0.09(1) 1.80(3) 0.01(1) 2.97 0.98(1) 3.00(1) 0.02(1)

0.98(1) 1.11(1) 0.04(1) 1.79(2) 0.04(1) 2.98 0.98(1) 2.99(2) 0.03(1)

0.99(2) 1.67(3) 0.12(2) 1.14(2) 0.05(1) 2.98 0.98(1) 2.98(2) 0.04(2)

0.99(1) 1.98(3) 0.26(2) 0.58(1) 0.06(1) 2.88 0.97(1) 2.96(1) 0.07(I)

1.00(1) 2.23(4) 0.10(2) 0.57(1) 0.06(1) 2.96 0.98(1) 2.96(2) 0.08(2)

0.59(1) 0.35(1) 0.05(1) 0.001 0.001 0.001

0.59(1) 0.36(1) 0.03(1) 0.003 0.005 0.011

0.38(1) 0.54(1) 0.05(1) 0.009 0.006 0.012

0.19(1) 0.63(1) 0.15(1) 0.000 0.000 0.031

0.18(1) 0.71(1) 0.07(1) 0.003 0.000 0.031

classical molar fraction calculated on anion base X(phl) 0.985(16) 0.794(10) 0.589(8) X(ann) 0.190(2) 0.361(7) X(vac) 0.001(5) 0.033(7) X(sid) 0.001 0.001 X(east) 0.015 0.005 0.001 X(f-sid) 0.008 0.001

T h e micas a p p e a r e d as small flakes with grain sizes up to 10 ~tm. T h e color varies with b o t h M g and Fe 3+ content. With increasing phlogopite c o m p o n e n t run p r o d u c t s b e c o m e m o r e and m o r e grey to pale bluegreen and turn to white at phlogopite. A higher Fe 3 + content causes the color to b e c o m e m o r e intense. In X - r a y p o w d e r diffraction patterns no impurities were detected in runs o f the series A80 and A40, with the exception o f run A808/780 ~ C were olivine and sanidine indicate a beginning b r e a k d o w n o f the mica. Sanidine and hypersthene were f o u n d in small a m o u n t s in the runs o f the series A60 and A20. Phlogopite syntheses were p e r f o r m e d at 4-5 k b a r s and t e m p e r a t u r e s ranging f r o m 500 ~ to 700 ~ C. Syntheses gave up to 9 5 % phlogopite, additional phases were sanidine and an unidentfied p r o b a b l y A 1 - M g - Si phase. Chemical c o m p o s i t i o n s and structural f o r m u l a s derived f r o m m i c r o p r o b e analyses a n d M 6 s s b a u e r spectroscopy are given in Table 2. As it can be seen micas in the desired stoichiometry have been formed. Octahedral A13+ is f o u n d in all p r o d u c t s in concentrations up to 0.06 f o r m u l a units, while the M g content p r o b a b l y is lower in the micas than in the starting materials. The t e t r a h e d r a are Si saturated with the exeption o f series A80. It is interesting to note that in the case of series A80 Fe 3+ fills the remaining Si sites and f u r t h e r m o r e displaces some A13 + into the octahedral site. Additionally Fe 3 +tet was calculated to the tetrahedral site in series

A40 and A60 if evident M 6 s s b a u e r spectra.

proofs

were

present

in

Lattice Parameters

Representative cell dimensions and distortion angles (tetrahedral r o t a t i o n and octahedral flattening angles e and q~, D o n n a y et al. 1964) are given in Table 3. Ionic radii used for calculation ,of c~ a n d q~ were r 0 2 - o c t = 1.34 ~ , r MgZ+oct=0.72, r FeZ+oct=0.780 ~ , Fe3+oct=0.645 (Hazen and B u r n h a m 1973; S h a n n o n 1976). All micas show sharp and well resolved reflections and C u K c q / Ke2 splitting can be observed above 45 ~ 2 0 . F o r calculations o f lattice p a r a m e t e r s m o r e than 40 reflections can be used in a 2 0 range f r o m 2 ~ to 70 ~. In Fig. l a - c the lattice p a r a m e t e r s ao, bo and co are plotted versus the ferric iron content as determined by M 6 s s b a u e r spectroscopy. D a t a o f annites f r o m R e d h a m m e r et al. (1993) are also included. In Fig. l d - f the cell p a r a m e t e r s are plotted against the phlogopite c o m p o n e n t . Lattice parameters of Hewitt and Wones (1975) are added and show g o o d a g r e e m e n t with our data. Q u a d r a t i c regressions in Fig. 1 d - f were p e r f o r m e d to the data o f micas with low Fe 3 + contents synthesized with the I M buffer at 400 ~ C and are given below (x = phlogopite c o m p o nent).

286 Table 3. Representative lattice parameters of micas on the join annite-phlogopite synthesized at different oxygen fugacities and their

Fe3+ contents, determined by M6ssbauer spectroscopy. Numbers in parantheses are one standard deviation and refer to the last digit run

Fe3+ [%]

a [~]

b~ [A]

ca [A]

fl [o]

V [J/bar]

~" [~

cpb [~

Phl#4 Phl~5 A203/IM400 A204/IM/500 A205/IQF/500 A206/QMF/450 A207/QMF/600 A208/IQF/600 A403/IQF/500 A404/IM/400 A405/IM/500 A406/QMF/600 A407/QMF/450 A408/780 A409/600 A410/IW/700

2.0 !.9 1.6 4.6 4.8 1.5 4.8 5.8 6.2 9.5 16.9 9.0 5.3 6.2 5.6 3.4 7.1 6.8 6.9 9.7 n.d. 7.8 8.4 6.7 14.2 7.0 7.7 21.6 12.7 10.3 nd 7.8 8.6 7.7 7.4 10.0

5.3154(5) 5.3153(5) 5.3349(4) 5.3349(9) 5.3345(3) 5.3335(5) 5.3341(4) 5.3333(6) 5.3506(7) 5.3507(7) 5.3516(6) 5.3489(4) 5.3469(5) 5.3487(13) 5.3503(2) 5.3501(2) 5.3500(2) 5.3502(3) 5.3673(3) 5.3670(2) 5.3674(2) 5.3647(5) 5.3679(2) 5.3679(4) 5.3672(5) 5.3675(2) 5.3808(3) 5.3852(4) 5.3850(7) 5.3750(5) 5.3808(3) 5.3831(3) 5.3801(3) 5.3833(2) 5.3845(5) 5.3844(5) 5.3810(6) 5.3995(5)

9.2067(9) 9.2108(12) 9.2343(13) 9.2418(14) 9.2391(13) 9.2398(28) 9.2399(11) 9.24t8(13) 9.2675(17) 9.2660(18) 9.2659(14) 9.2674(9) 9.2599(12) 9.2592(56) 9.2650(7) 9.2646(5) 9.2719(7) 9.2679(15) 9.2986(10) 9.3044(8) 9.2972(5) 9.3015(11) 9.2968(7) 9.3021(8) 9.2959(6) 9.2946(7) 9.3228(9) 9.3289(15) 9.3252(21) 9.3135(12) 9.3228(9) 9.3256(9) 9.3183(7) 9.3264(5) 9.3244(12) 9.3255(12) 9.3250(10) 9.3535(16)

10.3102(9) 10.3096(10) 10.3094(9) 10.3094(12) 10.3123(8) 10.3060(15) 10.3038(12) 10.3090(11) 10.3159(15) 10.3125(13) 10.3135(16) 10.3082(9) 10.2979(13) 10.3089(31) 10.3106(4) 10.3085(4) 10.3083(4) 10.3163(10) 10.3124(8) 10.3140(6) 10.3152(5) 10.3082(8) 10.3156(4) 10.3141(9) 10.3145(7) 10.3165(5) 10.3124(7) 10.3164(10) 10.3143(17) 10.2967(10) 10.3124(7) 10.3217(9) 10.3202(6) 10.3171(5) 10.3192(9) 10.3218(9) 10.3198(9) 10.3254(13)

99.908(10) 99.929(11) 99.938(10) 99.938(16) 99.905(8) 99.961(15) 99,926(10) 99.930(9) 99.997(15) 99.998(16) 99.998(13) 99.970(9) 99.979(11) 99.951(26) 99.962(4) 99.975(3) 99.970(3) 99.984(9) 99.987(7) 100.000(6) 99.998(4) 100.010(11) 99.995(4) 100.055(8) 99.960(8) 99.990(6) 100.045(7) 100.022(10) 100.033(13) 100.040(11) 100.045(7) 100.022(8) 100.017(7) 99.994(5) 100.062(11) 100.056(11) 100.051(11) 100.070(13)

14.987(2) 14.972(3) 15.064(3) 15.077(4) 15.077(3) 15.063(5) 15.064(3) 15.072(3) 15.170(4) 15.163(4) 15.167(4) 15.155(2) 15.121(3) 15.148(11) 15.159(1) 15.154(3) 15.165(1) 15.171(3) 15.164(2) 15.274(2) 15.265(1) 15.254(3) 15.267(1) 15.273(2) 15.263(2) 15.264(2) 15.339(2) 15.369(3) 15.358(5) 15.284(3) 15.339(2) 15.356(2) 14.343(2) 15.362(1) 15.362(3) 15.367(3) 15.354(3) n.d.

9.2 9.0 8.1 7.7 7.9 7.9 7.9 7.7 6.5 6.5 6.5 6.5 6.9 6.9 6.6 6.6 6.2 6.4 4.4 3.9 4.6 4.2 4.6 4.2 4.7 4.8 1.7 0.0 1.0 3.1 1.7 0.0 2.4 0.5 1.3 0.9 1.1 0.0

59.23 59.37 59.08 59.16 59.15 59.20 59.17 59.15 58.97 58.98 58.99 59.05 59.19 58.95 58.03 58.96 58.97 59.00 58.88 58.93 58.86 59.01 n.d. 58.95 58.91 58.83 59.02 58.73 58.52 59.29 58.95 58.86 nd 58.97 58.77 58.45 58.51 58.71

A411/IW/600

A412/IQF/600 A602/650 A603/650 A605/IQF/500 A606/QMF/600 A608/800 A609/IW/700 A610/IW/600 A611/IQF/600 A802/650 A803/IM/400 A804/IM/500 A805/QMF/450 AS06/QMF/600 AS07/IQF/500 A808/780 AS09/IW/600 A810/IW/700 ASI 1/IQF/600 A812/IQF/650 A39/IM/400 c

" e = tetrahedral rotation angle, b q)= octahedral flattening angle, ~taken from Redhammer et al. (1993)

ao [A] = 5.39979(.0048604)- 0.0741 (6.86759E-006)x0.010048(2.23903E-006)x z bo [A] -- 9.35514(.015428)- 0.144209(1.78918E-008)x0,00555791(1.64557E-005)x z Co []k] = 10.3244(0.000170541)- 0.0350712(2.72692E005)x + 0.0210318(6.99168E-006)x z In samples with low ferric iron contents the lattice parameters ao and bo show a distinct linear decrease with increasing M g substitution in annite. However, a deviation from this trend can be observed in biotites with higher ferric iron contents (e.g. samples synthesized with Q M F at 450 ~ C). Taking annite A39 (Table 3) with the m i n i m u m ferric iron content of 10% and phlogopite P h l ~ 5 as an example ao decreases by 0.084 ~ and bo by 1.43 A. For Co a somewhat different picture can be observed. The substitution o f Fe 3 § strongly influences this cell parameter. Micas with low Fe 3+ contents display a small decrease of 0.016 A (A39 to Phi ~ 5), where-

as in micas with high Fe 3 + concentrations an increase is observed (Fig. I f). It is interesting to note that the orthohexagonal cell p a r a m e t e r relation b--- a~3 applies to all samples. Deviations f r o m this relation are only in a few cases up to 0.004 ~ in bo, but in most cases only about 0.002 A. Deviations f r o m ideal mica geometry can thus be described by octahedral flattening and tetrahedral rotation angles only. The same was pointed out by Hazen and B u r n h a m (1973) in investigating the crystal structure of both an annite and a fluorphlogopite sample. The tetrahedral rotation angle e increases f r o m 0 ~ in annite to 9 ~ in phlogopite and the octahedral layer is flattened from ~ 58.6 ~ to ~ 59.3 ~ showing a clear dependence from both the M g and the Fe 3 + content.

287 5.42.

5"42 L

,4~

"'"tk

5.40

o< E

5.38

. . . . . . . . lilt-.

Q.

5.36,!

"-'"--

[3

5.34

5.36 -',~l"tt~'---t~ ...............

,~--

9

pure annite

O

""'~1~,~

5.321

9 serie A80 [] sefie A60 a serieA40 0 serie A20

5.34 ~'

"" " ' < : ; .... ' - ~

9 H.W. ('75)

[] IM 400 A QMF 600 0 QMF 450

5.30 5.28

5.32

9.40 i

9.37

9.35

o

U

..0.08 9-0, 10

0

0:2

0:4

Xpht

annito- phlogopite 0:6

018

Fig. 4. Excess molar volumes (J/bar) along the annite-phlogopit join. For further explanations see text

M6ssbauer Parameters. The M6ssbauer parameters of the synthetic Fe - M g biotites are given in Table 4. L o o k ing at the parameters of Fe 2 + first, it can be seen that the isomer shift (IS) shows no variation across the solid solution series having values o f 1.11 /.14 m m / s on both M1 and M2. In contrast the QS shows a distinct increase f r o m iron rich to magnesium rich micas on both M t and M2 sites. However, within a series the parameter display only little variations. The increase in QS on the M1 site (0.25 ram/s) is larger than on the M2 site (0.15 mm/s) when going f r o m annite to biotites with x =2.4, and the mean difference in QS between the two octahedral sites also decreases from 0.33 to 0.23 mm/s. The parameters o f Fe 3 + are affected by larger errors due to small concentrations and a high a m o u n t of

289 Table 4. M6ssbauer parameters of micas on the join annite-phlogopite synthesized at different oxygen fugacities Fe 2+ (M1)

Fe z+ (M2)

IS

QS

IS

QS

A802 AS03/IM/400 A804/IM/500 A805/QMF/450 A806/QMF/600 AS07/IQF/500 A809/IW/600 A810/IW/700 A811/IQF/600 A812/IQF/650

1.11 1.12 1.11 1.10 1.11 1.11 1.13 1.12 1.12 1.13

2.31 2.33 2.34 2.34 2.35 2.35 2.32 2.35 2.34 2.33

1.11 1.12 1.12 1.12 1.12 1.12 1.14 1.13 1.13 1.14

2.60 2.66 2.63 2.64 2.65 2.65 2.63 2.66 2.65 2.66

A602 A603 A605/IQF/500 A606/QMF/600 A607/IW/600 A610/IW/700

1.11 1.11 1.11 1.10 1.11 1.12

2.38 2.38 2.37 2.34 2.37 2.38

1.12 1.12 1.12 1.12 1.12 1.13

A403/IQF/500 A404/IM/400 A405/IM/500 A406/QMF/600 A407/QMF/450 A408 A409 A410/IW/700 A411/IW/600 A412/IQF/650 A412/IQF/600

1.12 1.12 1.12 1.11 1.11 1.13 1.13 1.12 1.12

2,43 2.46 2.45 2.43 2.43 2.43 2.43 2.43 2.43

1.14

AZ03/IM/400 A204/IM/500 A205/IQF/500 A206/QMF/450 A207/QMF/600 A208/IQF/600

1.12 1.11 1.12 1.12 1.12 1.14

Run

FWHM

Fe 3+

(M2)

Fe 3+

(M1)

IS

QS

IS

QS

0.25 0.26 0.27 0.25 0.24 0.23 0.26 0.29 0.28 0.27

0.46 0.50 0.48 0.46 0.41 0.43 0.52 0.53 0.43 0.53

0.65 0.50 0.74 0.68 0.65 0.82 0.48 0.50 0.60 0.64

0.56 0.60 0.53 0.49

1.02 1.00

2.68 2.67 2.68 2.68 2.68 2.67

0.26 0.26 0.26 0.26 0.25 0.26

0.43 0.48 . . 0.46 0.57 0.44

0.72 0.81 . 0.83 0.75 0.89

1.12 1.12 1.12 1.12 1.12 1.13 1.13 1.12 1.12

2.69 2.70 2.70 2.70 2.70 2.68 2.68 2.70 2.70

0.25 0.24 0.24 0.25 0.25 0.26 0.25 0.26 0.27

0.46 0.45 0.47 0.46 0.44 0.45 0.44 0.49 0.57

0.44 0.47 0.75 0.50 0.65 0.74 0.73 0.80 0.79

2.43

1.14

2.69

0.27

0.54

0.57

2.51 2.50 2.49 2.48 2.50 2.49

1.12 1.12 1.11 1.12 1.12 1.14

2.72 2.72 2.71 2.72 2.72 2.71

0.26 0.24 0.24 0.25 0.26 0.26

0.55

0.34 -

. 0.46 0.46 0.46

.

0.56 0.61 0.50 0.56 . -

1.03 0.98 1.00 1.06 1.05 1.08

0.22 0.27 0.23 0.21 0.24 0.24 0.26 0.29 0.27 0.28

0.40 0.32 0.34 0.40 0.29 0.32 0.35 0.34 0.28 0.38

0.26 0.24 0.29 0.33 0.27 0.37 0.22 0.22 0.26 0.36

-

0.32 0.32

-

0.52 0.47 0.53

0.23 0.22 0.27 0.24 0.28 0.25

0.37 0.38 0.36 0.33 0.37 0.32

0.24 0.24 0.31 0.29 0.34 0.26

-

-

-

0.25

0.41

0.23

-

0.38 0.27 0.42 0.40 0.38 0.50 0.59 0.49 0.53

0.27 0.23 0.28 0.20

0.44 0.38 0.36 0.32

0.20 0.26 0.24 0.28

-

0.54

-

-

-

0.30 -

0.27 0.27

0.46 0.52

-

-

0.98 0.99 1.05 -

.

. -

9QS

0.36 0.33 0.47 0.33 0.34 0.31 0.34 0.41 0.33 0.39

-

-

FWHM

Fe3+(tet)

IS

.

0.53 0.58 0.58

0.67 0.31 0.01

FWHM

-

0.38 0.62 0.44

0.36 0.40 -

QS= Quadrupol splitting, IS = Isomer shift relative to ~-Fe, FWHM= full width at half maxinmm; [S, QS and FWHM in mm/s +_0.02 mm/s

s t r o n g l y o v e r l a p p i n g lines. W h e r e a s t h e h i g h e n e r g y p a r t o f t h e f e r r i c i r o n d o u b l e t s c o r r e s p o n d s to a visible s h o u l d e r , t h e l o w e n e r g y p a r t c o r r e s p o n d s to a n a b s o r b t i o n f e a t u r e w h i c h is h i d d e n u n d e r t h e s t r o n g F e 2 § abs o r p t i o n lines. W h e r e p o s s i b l e t h e b r o a d a b s o r b t i o n feat u r e o f F e 3§ in o c t a h e d r a l c o o r d i n a t i o n was r e s o l v e d i n t o t w o c o m p o n e n t s , b u t t h e a s s i g n m e n t to M 1 a n d M 2 is u n c e r t a i n . D o u b l e t s w i t h a n I S o f ~ 0 . 2 ~ 0 . 2 8 m m / s a n d a QS o f ~ 0 . 3 0 - 0 . 4 5 m m / s w e r e d e d i c a t e d to F e 3§ o n t h e tet r a h e d r a l site. E s p e c i a l l y w i t h i n t h e s p e c t r a o f series A 8 0 t h e d i s t i n c t s h o u l d e r at t h e h i g h v e l o c i t y side o f t h e l o w energy absorption peak indicates the necessity of a supplementary doublet.

Site Occupations. T h e site o c c u p a t i o n s w e r e c a l c u l a t e d u n d e r t h e a s s u m p t i o n t h a t t h e r e c o i l free f r a c t i o n s o f F e z+ a n d F e 3+ o n d i f f e r e n t sites are t h e s a m e . T h e y a r e g i v e n in T a b l e 5 t o g e t h e r w i t h M 2 / M 1 ratios. M 2 / M 1 r a t i o s f o r F e z+ a r e in all cases l a r g e r t h a n t h e i d e a l v a l u e o f 2. M e a n v a l u e s a r e 2.32(9), 2.43(15), 2.34(10) a n d 2.20(15) f o r t h e series A 8 0 , A 6 0 , A 4 0 a n d A 2 0 re-

spectively. H o w e v e r in t h e l i g h t o f t h e p a p e r s o f R a n c o u r t et al. (1994) a n d R a n c o u r t ( 1 9 9 4 a , 1 9 9 4 b ) it s h o u l d be n o t e d t h a t e x t r a c t i o n o f M 2 / M 1 v a l u e s is v e r y problematic and the derived valus given above are the r e s u l t o f e v a l u a t i n g the s p e c t r a u s i n g e q u a l w i d t h f o r b o t h d o u b l e t s . T h a t is w h y o n e c a n n o t c o n c l u d e a d h o c t h a t v a l u e s l a r g e r t h a t the i d e a l v a l u e o f 2 d e m a n d s ord e r i n g o f F e 2 + in M2.

Discussion

Molar Volumes The linear correlation between composition and volume a p p e a r i n g in t h e r e f i n e m e n t o f H e w i t t a n d W o n e s (1975) h a s b e e n u s e d to a r g u e f o r i d e a l F e - M g m i x i n g in b i o tite (e.g. B e r m a n 1990). H e w i t t a n d W o n e s (1975) p e r f o r m e d t h e i r e x p e r i m e n t s at 1000 b a r p r e s s u r e , 100 b a r h y d r o g e n p r e s s u r e a n d ~ 750 ~ C in S h a w h y d r o g e n diff u s i o n b o m b s w i t h h y d r o g e n a r g o n m i x t u r e s as p r e s s u r e m e d i u m , a l o g fo2 o f ~ - 2 6 . 9 c a n b e e s t i m a t e d for t h e i r

290 Fe 3+ (tet) I

t

I

I

I

I

Fe 3+ (M1) Fe 3+ (tet)

Fe3+ (M2)

|

O-

I Fe3+ (oct)

0-

0.02-

0.02

~ 0.04-

~10.o4

tO

0.06 -

I

0.06

I

I Fe 2+ (M2)

annite 80 A 8 0 2 / 6 5 0 ~ t

1

I Fe 2+ ( a 2 )

I Fe2+ (M1)

[

O. 8

I Fe2+ ( U l )

I

I

annite 80 A811 / IQF 600 ~

Kbar i

i .....

I

I

i

h

O-

0.08

I

I

Kbar !

I

I

I

I

! 2.50

I

I

0-

0.02-

o 0 . 1 2 -_

0.04-

.~ 10.24

t-

O

0.06-

0.36-

annite 40 A410/700 ~ 0.08 -2.00 C

1

""1 -0.50

I annite 20 A 2 0 6 / Q M F 450 ~

Kbar I

l J 1.00 Velocity [ m m / s ]

I 2.50

I

I 4.00

0.48

I -2.00

! -0.50

d

Kbar I

I I 1.00 Velocity [ m m / s ]

I 4.00

Fig. 5 a-d. Typical M6ssbauer spectra of micas on the join a n n i t e phlogopite, synthesized at different oxygen fugazities and tempera-

tures, a A802/650 ~ C/4 kbar, b A811/IQF/600 ~ C/4 kbar, e A410/ 700 ~ C/4 kbar, d A206/QMF/450 ~ C/4 kbar

experiments roughly centered around the range of the present experiments ( - 1 9 . 6 4 - - 3 4 . 0 5 ) . It is thus very likely that the Hewitt and Wones (I 975) F e - M g biotites contain at least the minimum Fe 3+ amount, as determined for the present F e - - M g solid solution members (Fig. 3). The linear correlation observed by Hewitt and Wones (1975) thus probably represents biotites with increasing Fe 3 + content towards the annite - rich compositions [ ~ 11% Fe 3 + in annite, compatible with a molar volume of 15.478 J/bar, Hewitt and Wones (1975), Table 2]. In fact, if Fe 3 + is taken into account, the annite-

phlogopite join turns out to deviate significantly from ideality (Fig. 4). Current experiments, designed to study activity - composition relationships o f the annite-phlogopite join, are in progress to clarify this point.

Lattice Parameters

As it was shown by Hazen and Wones (1972) and in addition by Redhammer et al. (1993) for annite, the ao and bo cell parameters of micas are sensitive to changes

291 Table 5. Site occupations and area ratios of Fe in synthetic micas on the join annite-phlogopite derived from M6ssbauer spectroscopy

Run

Fe 2+

Fe 3+

M1

M2

M2

M1

A802/ A803/ A804/IM/500 A805/ A806/QMF/600 A807/IQF/500 A809/ A810 A811/ A812/

25.9 29.4 27.3 22.1 26.1 27.0 27.6 28.1 27.6 28.9

59.9 63.6 65.0 56.3 61.2 62.7 64.6 63.3 64.7 63.7

6.4 2.2 3.9 8.1 3.3 3.7 3.1 3.2 2.2 3.0

4.7 1.9

A602/650 A603/650 A605/IQF/500 A606/QMF/600 A609/IW/600 A610/ A611/

27.3 29.2 26.4 26.1 28.4 27.8 26.1

65.5 63.9 66.7 64.2 63.8 63.7 65.2

3.7 3.2 5.3 3.8 4.1 4.6

A4/IQF/500 A4/IM/400 A4/IQF/600 A4/IM/500 A4/QMF/600 A4/QMF/450 A409/ A408/ A4/IW/600 A4/IW/700

26.4 26.8 27.9 26.3 24.9 23.7 28.0 28.0 29.0 29.5

68.8 67.4 68.0 67.5 65.5 59.4 66.7 63.0 65.4 64.4

4.8 2.3 4.1 3.9 4.4 8.4 3.3 7.3 3.3 3.3

A2/QMF/450 A2/IM/400 A2/QMF/600 A2/IM/500 A2/IQF/500 A2/IQF/600

29.3 31.3 30.9 31.4 28.1 32.2

66.4 67.4 64.6 66.8 70.2 66.6

4.3 1.4 4.5 1.3

6.2 4.8 2.9 3.5 1.8 2.4 -

1.8 3.3 4.9 -

Fe 3+ tet

Fe 3+ ~

Fe2+

Fe 3+

M2/MI

M2/M1

3.1 2.9 3.8 7.3 4.5 6.6 1.8 1.9 3.8 2.0

14.2 7.0 7.7 21.6 12.6 10.3 7.8 8.6 7.7 7.4

2.32 2.16 2.38 2.54 2.35 2.33 2.34 2.26 2.35 2.21

1.16 1.29 0.69

3.4 3.6 6.9 4.4 4.0 4.3 2.1

7.1 6.8 6.9 9.7 7.8 8.4 6.7

2.40 2.19 2.52 2.43 2.24 2.88 2.32

-

1.7 2.3 1.8 3.7 2.0 1.7 2.3 2.8

4.8 5.8 4.1 6.2 9.5 16.9 5.3 9.0 5.6 6.1

2.61 2.52 2.43 2.56 2.63 2.51 2.38 2.25 2.25 2.18

1.29 1.30 1.72

4.3 1.4 4.5 1.8 1.6 1.3

2.27 2.15 2.09 2.13 2.50 2.07

-

-

1.8 1.6 -

1.04 0.95 1.12 1.26

-

(1) for evaluations with only one Fe3+loct) doublet no differentiation between M1 and M2 site was made

in o c t h a e d r a l (and t e t r a h e d r a l ) chemistry. T h e o b s e r v e d decrease o f lattice size in t h e ~ b p la n e thus c a n be interp r e t e d in t er m s o f M g a n d Fe 3+ s u b s t i t u t i o n for Fe 2+ in annite. T h e decrease o f bo d u e to a c o m p l e t e Fe 2 § - M g subs t i t u t i o n was c a l c u l a t e d t h e o r e t i c a l l y using ideal o ct ah edral c o m p o s i t i o n s (Fe 2 § 2.7Fe 3 + o.3 f o r annite, R e d h a m m e r et al. 1993), i o n ic radii o f S h a n n o n (1976) a n d octah e d r a l f l a t t e n i n g angles o f H a z e n a n d B u r n h a m (1973). T h e c a l c u l a t e d s h o r t e n i n g o f 0.1524 ~ is in g o o d agreem e n t with the o b s e r v e d one. Synthetic biotites with l o w ferric i r o n c o n t e n t s s h o w only a small decrease o f bo w i t h increasing M g c o n t e n t s w h i c h a m o u n t s to ~ 0.010 ~ g o i n g f r o m a n n i t e to p h l o g opite (cf. Table 3). U s i n g the g e o m e t r i c a l m o d e l o f D o n n a y et al. (1964), w h e r e the thickness o f the o c t a h e d r a l sheet toot is given by toot = 2 do cos ~o, the o c t a h e d r a l sheet s h o u l d c o i n c i d e a b o u t 0 . 0 8 0 / k w h e n substituting Fe 2 + by M g , a s s u m i n g a m e a n q0 angles o f 58.45 ~ a n d 58.95 ~ for a n n i t e a n d p h l o g o p i t e respectively ( H a z e n a n d Bu r n h a m 1973). F r a n z i n i (1964) also suggested, t h a t a F e 2 + M g s u b s t i t u t i o n p r o d u c e s a c o n t r a c t i o n o f the o ct ah edral layer. This is also s u p p o r t e d by d a t a f o u n d in the

literature f o r n a t u r a l bioties ( ~ 0.08 ~ s h o r t e n i n g f r o m annite to p h l o g o p i t e ) . As the decrease in c ' = c sin [1 i s smaller t h a n the expected o n e due to the c o n t r a c t i o n o f the o c t a h e d r a l layer, it is a s s u m e d t h a t the i n t e r l a y e r r e g i o n also alters its thickness. U s i n g the f o r m u l a Ti,t = c sin [3--2 do cos (p-(8/3) dt ( D o n n a y e t a l . 1964), o n e o b t a i n s large values o f h,t f o r p h l o g o p i t e a n d smaller o n e f or annites. It m i g h t be possible t h a t the d e s t r u c t i o n o f the ideal t w e l v e f o l d h e x a g o n a l p r i s m a t i c c o o r d i n a t i o n o f the K + ion as a c o n s e q u e n c e o f increasing c~ r o t a t i o n o f t e t r a h e dras n o t only leads to t w o different < K - O > distances, b u t also gives rise to an increase o f tint. In synthetic biotites c o n t a i n i n g h i g h e r a m o u n t s o f ferric i r o n the decrease o f Co is m a i n l y d u e to the Fe z + Fe 3 + s u b s t i t u t i o n w h i c h d e m a n d s d e p r o t o n i s a t i o n a n d c r e a t i o n o f o c t a h e d r a l Fe-vacancies. T h e s e t w o m e c h a nisms r e d u c e the cell size in Co ( R e d h a m m e r et al. 1993).

Isomer Shifts T h e i s o m e r shift o f F e 2 + o f all m i cas is c o n s t a n t t h r o u g h o u t the c o m p l e t e solid s o l u t i o n series i n d i c a t i n g

292 that the Fe 2 +-Mg substitution has no effect on the nature of chemical bonding (covalency) of the octahedral cation. However there seems to be the tendency that the IS on the M2 site is somewhat larger. Similar results apply for synthetic annites (Redhammer et al. 1993) and in general for a large number o f trioctahedral micas (Heller-Kallai and Rozenson 1981) displaying the fact that the M2 site may possess a slighty more covalent bonding.

Ferrous Quadrupole Splitting Whereas the isomer shift shows no changes with Mg-Fe substitution, the QS markedly does. In Fig. 6 the dependence of the ferrous QS on MI and M2 from octahedral composition is shown. As illustrated by open squares and triangles, the QS on both M1 and M2 sites increases with increasing Mg. Annersten (1974) noted in his study on the M6ssbauer spectra of biotite micas, that the sample richest in magnesium displays the largest QS, but he also found comparatively large QS for samples rich in iron thus suggesting factors other than Fe/mg ratios. The incrase of QS o f the Fe-Mg micas of this study shows a good correlation with the Mg content. The QS of the M1 sites displays a somewhat larger increase with increasing Mg content as the M2 site. Thus the mean difference in AQS=QS(M2)-QS(M1) decreases from micas rich in iron to micas rich in Mg. There are two terms contributing to the electric field gradient, a valence term and a lattice term, which are opposite in sign. Although difficult to decide, it is assumed that the changes o f ferrous QS are mainly due to changes in the lattice term. Substituting Fe 2-- by Mg should not introduce changes within valence configurations inside the Fe-octahedra. Thus the change in ferrous AQS is interpreted in terms of differences in size and distortion between M1 and M2 with changing Mg substitution. It is known from structure refinements (Hazen and Burnham 1973), that the differences in size and flattening of the M1 and M2 octahedras are smaller in phlogopite than in annite. Because of this AQS is believed to be mainly due to the different (OH) arrangements (cis and trans) in samples richest in Mg. A larger diversity is found in micas rich in iron. The diversity may be due to different ferric iron concentrations, and thus to different amounts o f octahedral vacancies. Especially the QS of annites (Redhammer et al. 1993) shows an ill defined tendency of a decrease with increasing Fe 3 + content, which is also present within micas of series A80. With increasing Mg substitution, the hexagonal arrangement of tetrahedra is destroyed by e rotation. This also has effects on the local geometry of octahedras as the apical oxygens of the tetrahedral (03) are shared with the octahedra. Ferrow (1987) also observed a marked increase in ferrous QS within micas of the general form K(Mg2Fe)[A1Si3_~Ge0Oao(OH)2] when substituting Si by Ge. Within these micas, the octahedral composition was kept constant and the changes in QS are induced by changes inside the tetrahedral layer. Making only geometric reasons responsible for the QS changes,

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Tetrahedral Ferric Iron M6ssbauer parameters of tetrahedral coordinated ferric iron are of similar dimensions as those reported in the literature for ferriannite and ferriphlogopite (Annersten et al. 1971 ; Annersten and Olesch 1978; Dyar and Burns 1986). However data of tetrahedral coordinated ferric iron in biotites are rare in the literature, thus direct comparison with our parameters is difficult. Dyar (1990) investigated the Mtssbauer spectra of pelitic biotites and

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stated the Fe3+tet is p r e s e n t in n e a r l y all spectra. T h e M 6 s s b a u e r p a r a m e t e r s in the cited s t u d y r a n g e f r o m 0.05 to 0.23 m m / s ( ~ 0.15 m m / s ) a n d f r o m 0.13 to 0.56 m m / s ( ~ 0,25 m m / s ) for IS a n d QS respectively. In o u r study, it has to be a s s u m e d t h a t ferric i r o n occupies to s o m e a m o u n t the t e t r a h e d r a l p o s i t i o n , b e c a u s e o f the p a r t l y visible s h o u l d e r , c h a r a c t e r i s t i c for Fe3+tet ( R a n c o u r t et al. 1992). F e r r i c i r o n in t e t r a h e d r a l c o o r d i n a t i o n is e v i d e n t l y p r e s e n t in series A80. L o o k i n g o n the c h e m i c a l c o m p o s i tions o f the micas given in Table 2, it c a n be c o n c l u d e d t h a t the m i c a s o f this series h a v e s o m e Si deficit, w h i c h is c o m p e n s a t e d b y a s u b s t i t u t i o n o f Fe 3 § into the t e t r a h e d r a . T h e micas o f the series A 6 0 a n d A 4 0 also s h o w F e 3§ on the t e t r a h e d r a l site. This m i g h t be the result o f a n u n d e r s a t u r a t e d c o m p o s i t i o n with r e g a r d to Si, as it is k n o w n t h a t Si disolves in the v a p o r p h a s e in h y d r o t h e r m a l runs leaving a solid with Si u n d e r s a t u r a t i o n . H o w e v e r , it m a y be also suggested t h a t Fe 3 + shows s o m e preference for the t e t r a h e d r a l site a n d replaces s o m e A1 into the o c t a h e d r a l site. I n Fig. 7 a p l o t o f ferric i r o n c o n t e n t in b i o t i t e versus o x y g e n f u g a c i t y is shown. S i m i l a r to a n n i t e the m i c a s o f a specific series s h o w no decrease o f ferric i r o n b e y o n d a value o f ~ - 2 8 log fO2, t h o u g h m o r e r e d u c i n g r e d o x c o n d i t i o n s were used, w h e r e a s the F e 3§ c o n t e n t increases t o w a r d h i g h e r o x y g e n fugacities. This suggests t h a t a m i n i m u m a m o u n t o f F e 3 + a l w a y s is p r e s e n t in b i o t i t e micas. C o n t r a r y to annite, w h e r e a m i n i m u m ferric i r o n c o n t e n t o f ~ 10% is d e m a n d e d b y g e o m e t r i c a l features, F e 3 § is n o t necessary in the M g - - F e micas.

Acknowledgements. This study was supported by the Austrian "Fonds zur F6rderung der wissenschaftlichen Forschung" grant P 9382 CHE which is greatfully acknowledged. We thank A. Benisek for help during microprobe and K. Forcher and W. Lottermoser for technical assistance and very useful discussions during M6ssbauer work.

Annersten H (1974) M6ssbauer studies of natural biotites. Am Mineral 59 : 143-151 Annersten H, Olesch M (1978) Distribution of ferrous and feric iron in clintonite and the M6ssbauer characteristics of ferric iron in tetrahedral coordination. Can Mineral 16:199 203 Annersten H, Devavarayan S, H/iggstr6m L, Wfippling R (1971) M6ssbauer study of synthetic ferriphlogopite KMg3FeSi3010(OH)2. phys status solidi (b) K137-K138 Appleman DE, Evans HT (1973) Indexing and least-square refinements of powder diffraction data. US Geological Survey, Computer Contributions 20, US National Information Service, Document PB2-16188 Berman RG (1988) Internally-consistent thermodynamic data for minerals in the system N a e O - K 2 0 - M g O - F e O - F % O 3 A1203 - SiO2 - TiO2 - I-I20- CO2. J Petrol 29(2) :445 522 Brigatti MF, Davoli P (1990) Crystal structure refinements of 1M plutonic biotites. Am Mineral 75:305-313 Brigatti MG, Galli E, Poppi L (1991) Effect of Ti substitution in biotite-1M crystal chemistry. AM Mineral 76 : 1174-1183 Bancroft GM, Maddock AG, Burns RG (1967) Application of the M6ssbauer effect to silicate mineralog. I. Iron silicates of known structure. Geochim Cosmochim Acta 31 : 2219-2242 Dachs (1994) Annite stability revised: 1. Hydrogen sensor data for the reaction annite = sanidin + magnetite + H2. submitted to Contrib Mineral Petrol Donnay G, Donnay JDH, Takeda H (1964) Trioctahedral onelayer micas: II. Prediction of the structure from composition and cell dimension. Acta Crystallogr 17:1374-1381 Dyar MD (1987) A review of M6ssbauer data of trioctahedral micas. Evidence for tetrahedral Fe3 + and cation ordering. AM Mineral 72:102-112 Dyar MD (1990) M6ssbauer spectra of biotites from metapelites. Am Mineral 75:656-666 Dyar MD, Burns R (1986) MSssbauer spectral study of ferruginous one-layer trioctahedral micas. Am Mineral 71:955-965 Ericson T, W/ippling R (1976) Texture effects in 3/2~1/2 M6ssbauer spectra. J Phys (Paris) 12-C6:719-723 Eugster H, Wones DR (1962) Stability relations of the ferruginous biotite, annite. J Petrol 3:82-125 Ferrow E (1987) M6ssbaner Effect and X-ray diffractionstudies of synthetic iron bearing trioctahedral micas. Phys Chem Mira erals 14:276-280 Ferrow E, Annersten H (1984) Ferric iron in trioetahedral micas, University of Upsala UUDMP repost no 39 Franzini M, Schiaffino L (1963) On the crystal stuctur of biotites, Z Kirst 119:197-209 Grevel KD, Chatterjee ND (1992) A modified Redlich-Kwong equation of the state of H 2 - H20 fluid mixtures at high pressures and temperatures above 400 ~ C. Eur J Mineral 4:13031310 Hazen RM, Burnham CW (1973) The crystal structure one-layer phlogopite and annite. AM Mineral 58:88%900 Hazen RM, Wones DR (1972) The effect of cation substitution on the physical properties of trioetahedral micas. Am Mineral 57:103-129 Heller-Kallai I, Rozenson I (1981) The use of M6ssbauer spectroscopy of iron in clay mineralogy. Phy Chem Minerals 7:223238 Hewitt DA, Wones DR (1975) Physical properties of some synthetic F e - M g - A 1 trioctahedral biotites. Am Mineral 60:854-862 Holland TJB, Powell R (1990) An enlarged and updated internally consistent dataset with uncertainties and correlations: The system K 2 0 - Na20-- CaO-- MgO-- M n O - FeO-- Fe20 3 A l z O 3 - - T i O 2 - S i O 2 - - C - - H a - - 0 2. J Metamorph Geol 8 : 89124 Ingalls R (1964) Electric field gradient in ferrous compounds. Phys Rev 133(3A) :A787-A795

294 Lottermoser W, Kaliba P, Forcher K (1992) MOESALZ - A computer program for M6ssbauer data evaluation. University of Salzburg, Austria, unpublished Partin E (1984) Ferric/Ferrous determination in synthetic biotites. MS thesis, Virginia Polytechnical Institute and State University, Blacksburg Partin E, Hewitt DA, Wones DR (1983) Quantification of ferric iron in biotites. Geol Soc Am Abstr with Programs 15, 659 Rancourt DG (1994a) M6ssbauer Spectroscopy of Minerals I. Inadequacy of Lorencian-line doublets in fitting spectra arising from quadrupole splitting distributions. Phys Chem Mineals 21 : 244-249 Rancourt DG (1994b) M6ssbauer Spectra of Minerals II. Problem of resolving cis and trans octahedral Fe2 + sites. Phys Chem Minerals 21 : 250-257 Rancourt DG, Dang MZ, Lalonde AE (1992) M6ssbauer spectroscopy of tetrahedral Fe 3+ in trioctahedral micas. AM Mineral 77: 34--43

Rancourt DG, Christie IAD, Royer M, Kodama H, Robert JL, Lalonde AE, Murad E (1994) Determination of accurate ~4j Fe 3+, ~6~Fe2+ and L6JFe2+ site populations in synthetic annite by M6ssbauer specroscopy. Am Mineral 79:51-62 Redhammer GJR, Beran A, Dachs E, Amthauer G (1993) A M6ssbauer and X-ray diffraction study of annites synthesized at different oxygen fugacities and crystal chemical implications. Phys Chem Minerals 20(6) : 382-394 Shannon RD (1976) Revised effectiv ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr A32:751-767 Tikhomirova VI, Konilov AN, Koshemchuk SK (1989) The degree of oxidation of iron in synthetic iron-magnesianbiotites. Mineral Petrology 41:41 52 Yodes HS, Eugster HP (1954) Phlogopite synthesis and stability range. Geochim Cosmochim Acta 6 : 157-185 Wones DR (1963) Physical properties of synthetic biotites on the join phlogopite-annite. Am Mineral 48:1300-1331 Wones DR, Eugster HP (1965) Stability of biotite: Experimental theory and application. Am Mineral 50:1228-1272