Chemistry of biotites and muscovites in the Abas granite, … · 2007. 8. 29. · The Abas granite...

American Mineralogist, Volume 73, pages 754-765, 1988

Chemistry of biotites and muscovites in the Abas granite,northern Portugal

Ruov J. M. KoNTNGST* Jnr N. Bor-ANor** SrvroN P. VnrcNo, J. BrN H. JansnNDepartment of Geochemistry and Experimental Petrology, State University of Utrecht,

Budapestlaan 4, 3585 CD Utrecht, The Netherlands

ABSTRACT

The micas in the Abas granite (northern Portugal) have been examined by means ofelectron microprobe, scanning and transmission electron microscopy, and partial chemicalanalyses on mica separates. Four different types of postmagmatic muscovites are distin-guished in thin sections, and their chemical variations are described in terms of celadonite,paragonite, and pyrophyllite substitutions. The paragonite substitution indicates that thefour muscovite types formed at successively lower temperatures. Generally, the biotitesexhibit texturally magmatic features, whereas they chemically have a deficient octahedraloccupancy, suggesting considerable alteration to muscovite or zinnwaldite. Our observa-tions suggest that the chemical variations in the biotite composition can more likely beexplained by solid solution rather than by fine-scale intergrowths of the end-memberminerals. Deficient interlayer occupancy ofthe biotites is caused at least in part by fine-scale interlayering of retrograde chlorite.

INrnooucrroN

Petrologic and geochemical investigation of Hercyniangranitic rocks in central northern Portugal (Konings etal., 1988) indicated that large-scale metasomatic altera-tion of the Abas granite resulted in important changes inthe whole-rock chemistry. A zone of tin-tungsten depositsis located near the contact with the younger Viseu granite(278 Mq- H.N.A. Priem, pers. comm.). Ten Haven et al.(1986) related part ofthe alteration and the tin-tungstenmineralization in the Abas granite to the intrusion of theViseu granite.

The chemistry of the micas in the Abas granite is sig-nificant for the understanding of the whole-rock chem-istry and the alteration processes. Replacement of biotiteby muscovite has caused mobilization of refractory ele-ments from the octahedral sheets and from the enclosedaccessory minerals. In addition, the occurrence of sec-ondary muscovite seems closely related to the alterationprocesses that were induced by the intrusion of the Viseugranite and its derivatives.

In the present study, the results of electron-microprobeanalyses are interpreted in terms of either substitutionalreplacements or mica intergrowths. Supplementarychemical analyses of mica separates and scanning elec-tron microscope (serra) and transmission electron micro-scope (rnu) analyses were made to support these inter-pretations.

* Present address: Netherlands Energy Research FoundationECN, Department of Chemistry, P.O. Box 1, 1755 ZG Petien,The Netherlands.

** Present address: CSIRO, Division of Geomechanics, P.O.Box 54, Mount Waverley, Victoria 3149, Australia.

0003-004x/88/0708-o754$02.00 7 54

S.l.rvrpr,rNc AND TEcHNreuE

The samples examined in this study were selected from 54whole-rock samples collected from the Abas granite, both insideand outside the contact zone with the Viseu granite. The samplesrepresent the various stages of alteration, which gradually in-creases toward the contact zone. Samples 04,05,07,37,and79are heavily altered; 06, 25, and 80 are intermediate;20 and 77are less altered; and 29 has the most magmatic features. Thesample locations and details of the petrology are described byVriend, Konings, and Jansen (in prep.).

Most of the mineral analyses were made with a TPD electronmicroprobe at the University of Utrecht, operating at I 5 kV and4 nA, using energy-dispersive X-ray spectroscopy techniques.

The srrr,r and X-ray scanning images were made with a Cam-bridge Scientific Instruments Microscan M-9 at the Free Uni-versity of Amsterdam (VU), operating at 20 kV and 25 nA, usingwavelength-dispersive spectroscopy techniques. Some addition-al analyses that were made at the VU included fluorine. Naturaland synthetic minerals were used as standards, and ZAF onlinefull matrix corrections were applied according to the methodused by Microscan. The resolution of the microprobe and thesev analyses is 3-5 pm. The structural formulas were calculatedon the basis of 22 oxygens.

Specimens for rrrvr were prepared by Ar thinning of grainsselected from thin sections. The specimens were coated with C.Eleotron microscopy was performed with a rpr'r zooc microscopeoperated at 200 kV. Semiquantitative chemical analyses, with aspatial resolution of about 500 A (i.e., the approximate beamsize) were done with a Link X-ray analyzer using a C sampleholder.

Mica fractions were separated from a selection of whole-rocksamples by means of a Fault table, using the 30- to 50-pm frac-tion. The separation of biotite from muscovite was done with aFrantz electromagnetic separater, operated in steps of 0.2 mA.In general, several muscovite and biotite fractions were obtainedfrom each sample, and all fractions were analyzed if the yieldwas large enough. Further purification was performed by means

of heavy liquids. No feldspar or quartz impurities were observedafter this treatment, but contamination of especially the biotitefraction as a result of fine-scale intergrowths with muscovite orchlorite could not be avoided. The separates were analyzed byrcres (inductively coupled plasma-emission spectroscopy) aftersample decomposition with a HF solution.

Prrnor-ocrc oBSERvATToNS

The Abas granite is a two-mica granite that locallygrades into a muscovite granite. The bulk of the rockconsists of quartz, plagioclase that has been mostly re-placed by albite, and K-feldspar. Biotite is a primarymagmatic constituent, frequently replaced by muscovite,tourmaline, and chlorite. The accessory minerals zircon,xenotime, and monazite are of primary origin, whereassillimanite and rutile are secondary. Magmatic as well aspostmagmatic apatite is present in minor amounts.

The biotite is dark brown and encloses a large numberof accessory minerals, which are often obvious in thinsection as a result oftheir nearly black pleochroic halos.During the muscovitization process, the pleochroic colorsof biotite become pale brown, and the number of inclu-sions gradually decreases. During ubiquitous retrograda-tion, some biotite is altered to green biotite and chlorite.

Replacement textures suggest that muscovite is mainlyof secondary origin. Four types of muscovite can be dis-tinguished: (1) Large euhedral crystals apparently replacebiotite and tourmaline. Partially resorbed inclusions ofzircon, xenotime, and monazite, originally surrounded bybiotite, are occasionally observed in these muscovitegrains, and weakly colored pleochroic halos are rarely stillrecognizable in the muscovite matrix around these relicts.(2) Fine-grained sericite occurs as an alteration productof both magmatic and postmagmatic feldspar. (3) Fine-grained crystals occur along grain boundaries of type Imuscovites or as aggregates in cracks. (4) Small crystalsovergrow the other types of muscovite.

Muscovites of types I and 2 are ubiquitous and arerelated to the early and main alteration stages. Type 3muscovites do not occur in the least altered zones, butare common in the areas where the alteration is moreadvanced. Type 4 muscovites are restricted to some in-tensely altered areas. Types 3 and 4 are related to the latealteration stages. Type I muscovite forms the greater partof the muscovite fraction in the rocks, whereas theamounts of type 2 and especially type 3 and 4 muscovitesare relatively small. Consequently, the analyses of themuscovite separates mainly represent the type I com-posltron.

Rnsur,rs AND INTERPRETATToN

Muscovite

Partial chemical analyses of the mica fractions are list-ed in Table 1. Normally, two relatively pure muscovitefractions were obtained, the amagnetic > 1.5-mA fractionand the 1.3- to 1.5-mA fraction. The major differencebetween the two fractions is, of course, the Fe content,

755

but the concentrations of the trace elements Li, Rb, andCs also tend to be somewhat higher in the Fe-rich frac-tion, probably owing to minor biotite contamination. Theconcentrations of the trace elements Li, Rb, and Cs (Ta-bte 1) clearly increase with progressive alteration for mus-covite as well as biotite.

Representative microprobe analyses of the four mus-covite types are listed in Table 2. The variation diagramsof Si with respect to A1, Fe, and Mg as analyzed by mi-croprobe (Fig. l) show remarkable differences betweenthe chemical compositions of the four muscovite types.Type I muscovite has relatively low Si and Mg contentsand high Na and Al contents, whereas type 4 muscovitehas high Si and Mg contents and very low Na, (K + Na +2Ca), and Al contents. Types 2 and 3 have compositionsintermediate between those of types I and 4. The Ti con-tent of all muscovite types is extremely low and pointstoward postmagmatic origin (Speer, 1984), which is inagreement with our textural observations on the thin sec-trons.

The variation of the Na content of the muscovites isattributed to solid solution between the end membersmuscovite and paragonite (Yoder and Eugster, 1955a,1955b; Iijama, 1955). Recent studies by Chatterjee andFroese (1975) and Eugster eL al. (1972) have provideduseful information about the phase relations and theirtemperature dependence in this system. Discussing thepossibilities for the use of the muscovite-paragonite sys-tem as a petrogenetic indicator in pelitic schists, Guidottiand Sassi (1976) and Guidotti (1984) argued that its ap-plicability depends on the mineral assemblage. In the Abasg,ranite, all muscovite types coexist with albite, qtrartz,and K-feldspar, and sillimanite is locally present. The Nacontent of the whole-rock samples increases with pro-gressive alteration. This suggests that the altering fluidswere rich in Na and were constantly buffered by the gran-itic mineral assemblage. It is concluded that the decreas-ing Na contents of muscovite types I to 4 must suggesta decrease of formation temperature. The NarO concen-trations in the muscovite fractions (Table l), which aredominated by type I muscovite, are fairly constant with-in each sample set representing rocks with the same de-gree of alteration, and the analytical results are compa-rable to the type I microprobe analyses.

The variation of Fe, Mg, and Al with Si (Fig. l) can berelated to solid solution between the end members mus-covite and celadonite. The celadonite or Tschermak'ssubstitution can be defined as

R3+ + I4rAl : S i + Rr+ ( l )

in which R3+ : Al,Fe and R2+ : Mg,Mn. Velde (1965,1967) argued that in the system KrO-MgO-Al'O3-SiO,,substitution I is temperature and pressure dependent andthe Si content of the muscovites could operate as an in-dicator of the conditions of formation. The applicabilityof this geothermometer is dependent on the bufferingconditions during formation, i.e., the chemistry of thealterating fluids. The increasing (Fe,., * Mg) from the

KONINGS ET AL.: MICAS FROM THE ABAS GRANITE

756 KONINGS ET AL.: MICAS FROM THE ABAS GRANITE

TABLE 1. Some major- and trace-element analyses of the mica separates

Sam.:Type:Magn.: 'A t t :

04 04ms ms

> 1 . 5 1 5 - 1 3he he

25ms

> 1 . 5tn

05 06bi ms

0.5-0.3 > 1 .5he in

04 05m s + b i m s1.1-0 .9 >1 .5

he he

05 05ms bi

t .5-1.3 0.7-0.5he he

20 20 20ms bi bi

> 1 .5 0.7-0.5 0.5-0.3ls ls ls

25ms

r .5 -1 .3tn

NaroFeOFerO"MgoMnOLiRb

B A

5r

0.67 0.57 0.441 .59 2 .19 4 .071 .11 1 23 3 .860 80 0.96 1.540.06 0.09 0.27

1424 1922 40721526 1726 1713

127 219 49650 69 1814 5 8

0.51 0.281.60 2.251.O7 1 .550.78 0.800.05 0.06

1549 17651364 1456128 28666 744 4

0.05 0.05 0.3917 .74 20 .88 1 .181 .00 0.22 1.362.27 1.92 0.450.43 0.40 0.01

6207 4641 7152700 2107 19121352 710 244100 112 171

6 5 9

0.42 0.071.O7 14.501.10 2 .880.83 4.240.01 0.21

591 1890686 1231108 353123 75

4 6

0.04 0.46 0.3118.44 1.61 2.070.78 0.61 1.924.44 0.76 0.96o 23 0.03 0.04

1667 984 14021130 1016 1268269 158 26595 75 925 4 4

Note; NarO, FeO, FerOr, MgO, and MnO in weight percent; trace elements in ppm. Sam. refers to sample number; type refers to mineral type wherems is muscovite, bi is biotite, and chl is chlorite; and alt. refers to intensity of alteration where he means heavily, in means intermediate, ls means less,and mm means most magmatic.

- Magnetic interval in milliamperes on the Frantz magnetic separator.

type I to the type 4 muscovites is opposite to the trendin the whole-rock samples in which the MgO, FeO, andFerO. concentrations steadily decrease with progressingalteration (Vriend, Konings, and Jansen, in prep). Con-sequently, no evidence is present for buffering ofFe,o, andMg by a fluid during the alteration, and the muscovite-celadonite solid-solution series cannot be used as a pe-trogenetic indicator for the Abas granite. Nevertheless,the results would be consistent with the conclusions drawnfrom the paragonite-muscovite series. A Na{Fe + Mg)avoidance in muscovite has already been suggested byGuidotti (1984) and Katagas and Baltatzis (1980), and itmay be the cause of the observed consistency in this setof muscovites.

The negative correlation between (K + Na + 2Ca) andSi (Fig. 1) can be explained by a substitution involvinginterlayer vacancies,

K + r4rAl: Si * ve, Q)

in which Vo represents a vacancy on the interlayer or Asite, following the notation of Krdger (1985). As tetra-hedral Al and Si are involved in this substitution, it caus-es significant deviations from the ideal celadonitic sub-stitution. Because (Mg + Fe,",) do not balance the excessSi in several Si-rich muscovites, significant deviations inthe interlayer occupancy can be explained by substitution2 ralhet than by the incorporation of the Rb, Cs, Sr, andBa, which were analyzed in the bulk separates. The Srand Ba concentrations in the muscovite fractions are rel-atively low. The Cs and especially the Rb concentrationsare significantly higher, but the enrichment of these ele-ments is related to the main alteration stage in the Abasgranite and not to the late alteration stages during whichthe muscovites of types 3 and 4 were formed.

TneLe 2. Selection of 8 out of 70 chemical analyses of the four muscovite types (ms-1 to ms-4)

Type:Sam. :Att. :

ms-l

he

ms-179ne

ms-1 ms-2 ms-2 ms-304 80 07 04

ms-479ne

ms-379he

sio,Al2o3Tio,FeOMnOMgoCaONa"OK.o

Total

45.8334.84

U C J

1.47n.o.0.490.180.929.98

94.24

6.1 661.8343.6930.0540.166

0.0990.0270.2401.713

13.992

46.8033.740.821.53n.d.0.940.180.60

10.1294.73

6.2611.7393.5820 0830.171

0.1890.0270.1561.727

13.935

46.8631.860.723.96n.o0 7 3n.o.0 . 1 6

10.8395.12

6.3401.6603.4220.074o.445

0.1 48

0.o441.870

14.003

46.3734.440.142.35n.o.0.440.31o.62

10.3094.97

6.2191.781J .OOO

0.0140.265

0.0900.0460.1611.763

14.005

45.7930.790.404.630.300.58n.d.n.d.

10 .6193.10

6.3581.6423.3990.0420.5380 036o.122

1.87914.01 6

46.7331.630.313.830.140.82n.d.0 .17

10.7194.34

6.3681.6323.4500.0320.4370 0170.168

0.0461.826

13.976

48.4930.68

n.o.3.01n.o.1 .690.18n.o.

10 .5895.63

6.5351.4653.410

0.340

0.3400.026

1.82013.936

51.9629.62

n.d.1 .55n.d.2.210.27n.o.9.49

95.10

6.8341 .1663.428

0.171

o.4340.038

1.59413.665

5 lr4rAl16rAlTiFeMnMg

NaK

Total

Note; n.d : not detected. For exolanation of abbreviations. see Table 1.


37 37ms bi

1.5-1.3 0.7-0.5he he

757

TABLE 1-Continued

Substitution 2 leads to the theoretical end member py-rophyllite. Velde (1969) examined the phase relations inthe pseudobinary muscovite-pyrophyllite system at ele-vated temperatures and pressures. He suggested substan-tial solid solution in the muscovite-rich portion of the

system. Our data indicate an increasing pyrophyllite sub-stitution from type I to type 4 muscovite, which corre-lates with a decrease of the paragonite component and anincrease of the celadonite component.

Figure I also shows a discontinuity in the relationshipsof Al and Fe with Si for eight Si-rich muscovites. Theselate muscovites deviate from the linear trend in the bulkof the analyses in having relatively high Al and low Fe,",concentrations. Presumably a change occulTed in thechemistry of the late alterating fluids that drastically af-fected the Fe3+-vtAl exchange.

If we assume that (l) Ca substitutes in the interlayer,compensated by a Si-talAl exchange, and Ti substitutes inthe octahedral sheet and (2) all interlayer vacancies aredue to substitution 2, then the number of divalent cationsin the octahedral sheet can be approximated as Yr[(Si -

6) - V, - Ca - Til. Likewise, the substitution parameters,expressed in percentage par, cel, and pyr, can be calcu-lated. Figure 2 displays the distribution of the muscovitesin the muscovite corner of a pyrophyllite{muscovite +paragonite)<eladonite composition diagram. The type Imuscovites plot near the muscovite end member, whereastypes 2 and 3 and especially type 4 contain relatively high

25bi

0.7-0 5tn

37MS

> 1 . 5ne

29ms

> 1 . 5mm

29 29bi bi

0.7-0.5 0.5-0.3mm mm

0.05 0.4218 1 1 0.862.49 0.68252 0.760.28 0.01

2325 2291483 600

b J / 4 J

39 226I 1 0

0 07 0.0514.23 16.603 70 1.265.77 6.300.14 0 .17

1031 11341129 1226148 145121 129

5 4

0.44 0.30 0.05'l 74 n a. 10.661.27 n.a. 6.340.61 0.78 1.510.04 0.06 0.32

1231 1890 36771230 1582 2584117 272 109494 72 595 4 7

I

8

I

Fig. 1. Variation diagrams for A1, Fe, Mg,Na, and (K + Na + 2Ca) with Si in the fourmuscovite types. Circle : type l, triangle : type2, square : type 3, diamond : type 4.

n0D ^ D 0 " 0

' 6 - W - 1 - U U O a-ffi r&srP" 8Yd _"m.,o \o Da--u Ar D-A

- u

o _q l ^ uo"Slo'4 s o ^

oB'bof,do_oqo "

oF o

- A ^ ^ ^ - n ^ - nu&-q$'ofo" oJ o

". tr aD a o o o o

t

2

2 @

6 2 0

o o o

oIB - ^ o

o o .t r t r -O A o tr

a

trO o

0o a 0 g

' o . t - o o^onoa9a A"%€f "eP-

C

0

6 1 0 6 7 0 680


celadonite and pyrophyllite components, trending towardillite composition (Hower and Mowatt, 1966).

Biotite

Biotite microprobe analyses from the Abas granite de-viate from the ideal composition in having a low totalsum of cations (Table 3), which is caused by deficientoctahedral or deficient interlayer occupancy.

In Figure 3, the Si, Al, (Fe,", + Mg), and Ti contents ofthe biotites with a deficient octahedral occupancy andsome deviating muscovites are plotted against the totalsum ofcations. A total sum of l4 represents a mean typeI muscovite and a total sum of l6 represents a theoreticalmagmatic biotite (Albuquerque, 1973:- Neiva, 1977). Be-

Fig.2. Distribution of the muscovites on apyrophyllite-(muscovite * paragonite)-cela-donite composition diagram. Symbols as inFigure 1.

tween the Si, Ti, Al, (Fe,., * Mg) contents of the de-viating biotites and muscovites and the total sum of cat-ions there exist clear relations (dashed lines) that areparallel to the predicted relations (straight lines) for theAl and (Fe + Mg) contents. The observed and predictedtrends almost coincide for Si. After approximate conec-tions for the Fe3+-Al exchange (Fs:+ : AlpreaictedAl-***o), the predicted and observed lines are in goodagreement. Evidently, the Fe3+ content in the biotites isconstant and not affected by the processes causing thischemical variation. The FerO. concentrations in the min-eral separates (Table 1) support this conclusion, althougha clear difference in the FerO, concentrations exists be-tween the 0.7- to 0.5-mA and 0.5- to 0.3-mA fractions.

TneLE 3. Selection of I out of 62 chemical analvses of biotites and biotiteJike material

eno o.-d

i ^ o o o o o

77bits

29bi

mm

29bi

mm

Z J

bitn

zc zJ

bi + chl bi + chlrn tn

25chltn

07bine

Sam.:Type:A t t :

sio,Alro3Tio,FeOMnOMgoCaONaroK.O

Total

40.6623.' t2

| . JO

1 6 4 00.261 0 2n on o9.96

92.78

6.1231 8772 2 2 80 1 5 52.0660 0330.230

1 .91514.627

23.8421.86o.22

39.380.27230n.o.n.d.n.d-

87.87

25 bbzz-oon.o .

35.560.382.91n.d .0.210 . 1 8

87 56

34.9523.87n.d.

23.'t40.402.720.090232.68

87.08

35.6918.68

z-Jo

21.370.436.15n.o.0.58924

94.s0

2.4650.950o.2762.7710.0581.423

0.1 761.829

15.483

35.7718.033.26

21.67n.o.7.O1n.o.0.349 . 1 9

o E 1 7

5.4912.5090.7540.3772.782

1.604

0.1 031.806

15.426

35.4218.472.92

21.66o.237.40n.d.0.539.04

9s.67

5.4292.5710.7670.33727770 0311 691

0.1581.768

15.529

38.6221.211.82

20.450.681.86n.o.n.o.9 8 6

94.50

5.889' t . 1 1 12.7000.2092.6090.089o.425

1.92014.952

Sir4lAlr6tAlTiFeMnMg

NaK

Total

Notei n d : not detected. For exolanation of abbreviations, see Table 1 .

Fig. 3. The relationships between totalsum of cations and Si (squares), Al (trian-gles), Fe,., + Mg + Mn (circles), and Ti (dia-monds). Symbols refer to altered and unal-tered biotite, except those four with a totalsum close to 14. l, which refer to muscovite.

All the data suggest extensive solid solution between mus-covite and biotite that can be described by substitutionsinvolving octahedral vacancies,

't 59

in which V* represents a vacancy on the octahedral or Rsites, R2+ : (Mn,Mg,Fe), T3+ and i:+ : (Al,Fe). Exam-ination of data from natural and synthetic micas provid-ed no clear indication of the permissible ranges of thesesubstitutions (Crowley and Roy, 1964; Kwak, 1972;Rutherford, I 973; Robert , 197 6; Green, I 98 I ; Seifert and


3 R 2 + : 2 R 3 + + V R

2 T 3 + + R r + : 2 S i * V n ,

TneLe 4. Chemical analyses of biotites and muscovites

(4)(5)

Spot:. AA AB ACSam.: 25 25 25Type: bi bi msAlt : in in in

BE

M S

ne

BD05mshe

BC05bine

BB05bihe

BA05bihe

sio,Atr03Tio,FeOMnOMgoCaONaroK.oFO = F

Total

35 1019.903.01

24 000 3 13.05n.d0 0 89 4 40 9 3

-0.3995.43

5.4802.5201 .1400.3533.1400 0400 709

0.0231.880

15 2850 459

34.9020 302.47

23.90o.25323n.o.0 0 99 3 91 . 1 8

-0.5095 21

5.4602.5401.2100.2913.1200.0330.754

0 0291.880

15.31 70.584

45.0034.500.611.80n.d.0.84n.o.0.72

10.700.460 . 1 9

94 44

6.1001 9003.6100.0620.205

0.1 70

0.1901.860

14.O970.1 97

38.8023.501.56

17.000.351 . 1 7n.o.0.049.952.360.99

93.74E O e n

2.0702 1500.1802.1700.0450.266

0.0111 940

14.7621.140

37.6023.101 8 8

18.300.411 . 0 1n.o .0.049.694.19

- t . / o94.46

5.8102.1902.020o.2192.3700.0540.233

0 0 1 11.920

14.8272.050

37.2023.301.79

18 300.431.29n.d0 0 49.852.34

-0.99

5.7602.2402.0000.2082.3700.0560.298

0.0111.950

14.8931 .150

41.2022.601.06

14 200.321.25n.o.n.o.

10.303.40

-1 .4392.90

6.2701.7302.3100.1221 .8100.0420.284

2.01014.5781.640

44.3028.401.556210.071.03n d .0 . 1 1

1 1 . 0 01 0 4

-0.4493.27

6.2901.7103.0300.1 660.7380.0080.219

0.0301.990

14.1811.040

5 lrajAl

16lAl

TiFeMnMg

NaK

TotalF

Nofe.'n.d. : not detected. For explanation of abbreviations, see Table 1'Spots labeled by letters as indicated in Figure 4.


Fig. 4. serr,r images and characteristic X-ray scanning images of biotite and muscovite grains of (A) sample 25 and (B) sample

05. Analyses from the indicated spots are presented in Table 4. The counting time for Fe in sample 25 is a third of that for sample05. During the exposure, some of the C coating of sample 05 disappeared from the biotite grain in the left-bottom part of thephotograph. Scale: 30 pm between adjacent white squares indicated at the bottom ofeach photograph.

Schreyer, 1965, 197 l), but they certainly do not occur onsuch a large scale as suggested in Figure 3.

Figure 4 displays sEM images in combination with char-acteristic scanning maps for Fe, Mg, and Ti in muscoviteand biotite grains. In sample 25 (Fig. 4A), clear differencesexist in the concentrations of Fe, Mg, and Ti of bothminerals. In sample 05 (Fig. 4B), almost no difference inMg and Ti content can be observed, but there is still adistinct difference in Fe content of the minerals. Chemi-cally, the minerals are rather homogeneous. Microprobeanalyses of the same grains are listed in Table 4. Thegrains in sample 05 (Fig. 4B) have a remarkably high Fcontent, up to 4.19 wt0/0, and they display a very lowoctahedral occupancy.

Another explanation for the deficient octahedral oc-cupancy is the substitution of Li, which cannot be ana-lyzed for by electron-microprobe techniques. The prin-cipal Li substitutions can be expressed as

2R2+ : Li + r6rAl (6)R2+ + r4rAl: Li + Si. (7)

These substitutions are related to incorporation ofzinn-

waldite and lepidolite components. The pure mineralsoccur mainly in pegmatites and hydrothermal veins or asmetasomatic alteration products of biotite (Deer et al.,1962; Cemy and Burt, 1984). Both minerals may containa considerable amount of F. A hydrothermal replacementof OH by F in micas is reviewed by Munoz (1984). Postmagmatic processes have strongly affected the whole-rockchemistry of the Abas granite and enrichment of Li, Rb,Cs, and F, among others, has been established (Vriend'Konings, and Jansen, in prep). Oen (1958) proved thepresence of Li in micas of altered zones of the Abas gran-ite by means of qualitative spectroscopic determination.Our chemical analyses of the mineral separates (Table l)yield Li concentrations up to 6207 ppm in the biotitefractions. However, these amounts cannot totally accountfor the deficient octahedral occupancy as is evidenced byFigure 3. The biotite systematically contains more Li thanthe coexisting muscovite. The biotites in heavily alteredsamples contain 3677 Io 6207 ppm Li, whereas thosefrom less altered samples contain l03l to 2325 ppmLi.

Apart from the substitutions discussed above, thechemical variations can be caused by interlayering and


Fig. 4- Continued.

'7 6l

intergrowth of sheet silicates on a scale smaller than theresolution of the electron microprobe (spot size 3-5 pm).The muscovite-biotite interface of the grains in Figure 4,{(sample 25) was also examined in some detail by ruv.The interface itself was not clearly defined. It consistedof thin bands several hundred Angstrdms wide of retro-grade chlorite and a sheet silicate tentatively identified assmectite. The electron-diffraction pattern and the latticeimage from the muscovite side of the interface exhibitedsharp diffraction spots with a (001) repeat distance of20A, consistent with a perfect 2M, polytype viewed along[00](Fig. 5). The lattice image consisted of fringes witha dominant periodicity of 20 A. However, an intermediatel0-A spacing was observed in certain more favorably ori-ented parts of the sample. The biotite had greater vari-ability in its microstructure and diffraction pattern. Usingbrightfield revr imaging, the biotite consisted of variousirregularly spaced planar and lamellar structures super-imposed on a series of l0-A fringes (Figs. 6,4., 68). Thediffraction pattern (Fig. 6C) indicated that the basic repeatunit was 10 A with superlattice reflections correspondingto 30 A, indicating that IM and possibly -lZ polytypesare present. The streaking between the dominant spotsindicates that these polytypes are irregularly interlayeredor that they possess abundant stacking faults (Iijima and

Fig. 5. Electron-difraction pattem of muscovite from sam-ole 25.


Zhu, 1982; Iijima and Buseck, 1978; Bell and Wilson,1977). Extensive disordering and polytypism are observedin the biotite grains of Figure 48 (sample 05), in whichthe layer fringes in the biotite often appear to be discon-tinuous and distorted.

Using the semiquantitative X-ray analytical capabilityof the rru, it was established that in both samples, nosignificant chemical variations were associated with thestacking disorder in the biotite. This homogeneity indi-cates that the lamellae are chemically identical and thatno distinct intergrowths can be observed on a scale ofseveral hundred lngstrdms.

In the muscovite section, the interlayer deficiency of

Fig. 6. (A) Normal brieht-field image, (B) higher-resolutionimages, and (C) electron-diffraction pattern of biotite from sam-ple 25.

micas was discussed in terms of substitutions involvingvacancies. The incorporation of Sr on the interlayer siteof biotite is negligible, and that of Ba does not exceed129 ppm in the most magmatic sample (Table l). Al-though high Rb and Cs contents of2700 ppm and 1352ppm, respectively, were determined in biotite fractions ofthe heavily altered samples, for most biotites the inter-layer deficiency is not compensated by these amounts'Many biotite compositions with a calculated low inter-layer occupancy appeared to be green biotite, occasion-ally intergrown with chlorite (Table 3). We prefer to relatethese compositions to replacement by chlorite and ka-olinite, which form during retrograde low-temperature al-

KONINGS ET AL,: MICAS FROM THE ABAS GRANITE 763

K = K + N a + C a

F = F e + l v l n + M S + T i

Fig. 7. Distribution of the biotites with deficient interlayeroccupancy in a A' KF-diagram. The sftrr represents the compo-sition ofhypothetical biotite with 16 total cations as extrapolat-ed on the variation diagram ofFigure 3.

teration and weathering (Meunier and Velde, 1979 Crawet aI., 1982; Cathelineau, 1983; Ahn and Peacor, 1987:Konings et al., 1988). In Figure 7, these biotite analysesof the Abas granite are plotted on an AKF diagram. Mostanalyses with low interlayer occupancy fall near the chlo-rite-kaolinite join, and the intermediate compositionssuggest strong kaolinization of biotite.

However, only evidence for interlayering of chlorite inbiotite is registered in the rnrvr images. Fringes with 14-or 28-A spacings are observed in the biotites (Fig. 8) andcan be explained by the presence of brucite layers (Veblenand Buseck, l98l;Veblen and Ferry 1983; Olives et al.,1983; Olives and Amouric, 19841- Olives, 1985). Suchintercalations can account for the observed K depletion inseveral biotite analyses. The mechanism of chloritizationin natural micas can be described by the replacement ofTOT (tetrahedral-octahedral-tetrahedral) layers by bru-cite layers or by the replacement of the interlayer by abrucite layer (Veblen and Ferry, 1983; Olives andAmouric, 1984). From the present observations, it is notpossible to distinguish which mechanism occurred in oursamples.

CoNcr,usrous

I . The muscovites in the Abas granite are of postmag-matic origin and can be divided into four types on thebasis of petrologic observations.

2. Microprobe analyses indicate that three importantsubstitutions govern the muscovite composition:

R3+ + r4tAl: Rr+ + SiK + r4rAl: Si + v"

K : N a .

These substitutions relate muscovite compositions to theceladonite, pyrophyllite, and paragonite end members,respectively.

3. The paragonite substitutions suggest that the fourmuscovite types crystallized successively as the temper-ature decreased. The muscovite types that developed lat-

Fig. 8. Lattice image of biotite from sample 25. Arrow in-dicates a 28-A fringe, presumably the result ofintercalated chlo-rrte.

764

er show increased pyrophyllite and celadonite substitu-tions.

4. The deficient octahedral occupancy ofthe biotites isprobably due to postmagmatic hydrothermal alterationto form muscovite and zinnwaldite components.

5. The nature of the replacement mechanisms in biotitehas not been definitively established. Since no texturalintergrowths have been observed on a scale of severalhundred Angstriims with the methods employed in thisstudy, a substitution mechanism seems likely, but inter-layering on a smaller scale cannot be ruled out.

6. The deficient interlayer occupancy of the originallymagmatic biotites is caused at least in part by fine-scaleintercalation of retrograde chlorite and presumably ka-olinite.

7. The Li, Rb, and Cs contents of the mica separatesof the Abas granite increase with intensity of hydrother-mal alteration, whereas their Ba (and MgO) contents de-crease.

AcxNowr,BocMENTS

The electron-microprobe analyses and the transmission electron mi-croscopy were performed with financial and personnel support by ZWO-WACOM (research group for analytical chemistry of minerals and rocks,subsidized by the Netherlands Organisation for Advancement of PureResearch). M. van Bergen and W. Lustenhouwer are thanked for theirassistance with the analyses in Utrecht and Amsterdam, respectively. M.Amouric, M. van Bergen, J. T. Cheney, P. Hartman, and D. R. Veblencritically reviewed the manuscript and gave valuable comments for sub-stantial improvement.

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KONINGS ET AL.: MICAS FROM THE ABAS GRANITE 765

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Geochimica Cosmochimica Acta, 8, 225-280' MANUScRTPT AccEPTED Fesnunnv 29' 1988

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Transcript of Chemistry of biotites and muscovites in the Abas granite, … · 2007. 8. 29. · The Abas granite...