Ameican Mineralogist, Volume 79, pages 63-72, 1994

Crystal chemistry and microstructures of plutonic biotite

Srvroua Brcr, M.tnr,t Fnc.NrcA, BnrclrrrIstituto di Mineralogia e Petrografia, Universitd di Modena, Via S. Eufemia,19,41100 Modena, Italy

Ansrnlcr

Transmission electron microscope and single-crystal X-ray diffraction studies were car-ried out on biotite from the Valle del Cervo plutonic body (northweslern Italy) with theaim of characteizing their structures and microstructures and also of establishing possiblerelationships between polytypism and crystal chemistry. The crystals, from the same lith-ologic complex, do not present significant chemical variations. On the other hand, theycontain many diflerent polytypic sequences, such as lM and 2Mr, which represent themost common polytypes, 3T and disordered and long-period stacking sequences, whichoften coexist in the same crystal. Four-, five-, six-, seven-, eight-, nine-, 13-, l4-, and 2l-layer polytypes, often interrupted by stacking faults, were also observed.

No relationship was found between polytypic sequences and chemistry, whereas theobserved microstructures seem to be related to the petrological environment. The mi-crostructural study facilitated the interpretation of difference-Fourier maps obtained fromstructure refinements of biotite from the same rock samples. The crystal-chemical studyof two biotite samples, IM (space group C2/m) and 2M, (space group C2/c), from thesame rock sample and with very similar chemical composition did not reveal any sub-stantial differences in the mean bond lengths, whereas the octahedral and tetrahedraldistortion parameters are higher in 2M, than in lM. Both polytypes exhibit Ml meanbond lengths greater than M2 mean bond lengths, suggesting partial cation ordering, whereasno tetrahedral cation ordering can be detected in the 2M, polytype.

INrnooucrroN

Biotite is widespread in igneous rocks, and its chemicaland structural properties influence many important ig-neous reactions. Although much attention has been de-voted to chemical studies of biotite, until recently therehas been a paucity of crystal structure refinement databecause biotite is commonly affected by stacking faults,chemical disorder, superstructures, twins, and inter-growths with other phases (Bailey, 1984). Information onmicrostructures can be obtained by transmission electronmicroscope (TEM) studies. Nevertheless, TEM studies onplutonic biotite have been mostly concerned with alter-ation processes (Veblen and Ferry, 1983; Eggleton andBanfield, 1985); only a few studies on metamorphic bi-otite, pegmatitic muscovite, and lepidolite samples havedealt with the coexistence ofdifferent polytypic sequencesin the same crystal (Bell and Wilson, 1981, 1986; Ruleet a l . . 1987).

Accordingly, the aim ofthis study was to characterizethe crystal chemistry and the microstructures of plutonicbiotite using TEM techniques [selected-area electron dif-fraction (SAED) and bright- and dark-field imagingl andsingle-crystal X-ray structure refinements of LM and 2M,polytypes. Crystal structure refinements on biotite-l-4-1from this plutonic complex have been previously pub-lished (Brigatti and Davoli, 1990). In the present studytwo new structure refinements of coexisting biotite-lM

0003404x/94l0102-0063$02.00 63

and -2M, from the same rock sample and with similarchemical composition were carried out with the aim ofcomparing crystal-chemical variations induced only bythe different polytypic arrangements.

S.llrlpr-n DESCRIpTIoN

The biotite samples studied came from the Valle delCervo plutonic complex (Vercelli, northwestern Italy).Geochemical and petrological features of the Valle delCervo area have been extensively studied, and a reviewis presented in Bigioggero and Tunesi (1988). The pluton,of orogenic character and shoshonitic affinity, is intrudedinto the pre-Alpine basement rocks of the Sesia-LanzoZone at the boundary of the south-Alpine Ivrea-VerbanoZone. lt comprises three main lithologic complexes in aroughly concentric arrangement. The core consists main-ly of granite and the rim of quartz syenite and quartzmonzonite; they are named the granitic, syenitic, andmonzonitic complexes, respectively.

In the granitic complex, biotite figures as a major min-eral in addition to quartz, potassium feldspar, plagio-clase, and sometimes hornblende; apatite, sphene, andzircon are accessory minerals. Biotite appears as coarsebrown flakes and prevails over hornblende; alteration tochlorite at the rim of the grains and along cleavages issometimes observed. In the syenitic complex, smallamounts of augite also appear, although hornblende is

64 BIGI AND BRIGATTI: CRYSTAL CHEMISTRY OF PLUTONIC BIOTITE

TABLE 1. Averaged chemical compositions and structural formulae of Valle del Cervo biotite determined by EPMA except wherenoted

Sample: M13 M105 M7. M32 M16 M14 M62

Polytype: 1 M 2M,

Biotite chemical compositionsio,Tio,AlrosFerO"FeO-'MgoMnOLi,otNa"OK.oCaOH,O+cl

Total

AIFg3+

Fg2+

MgMnTiLi

TotalCaNaK

SiAIFe3 *

Total

TotalOHclo

Total

37.113.99

13.34s.86

14.1411.820.510.050.08

10.120.022.830 . 1 2

qo oq

5.6912.309

8.0000.1030.6761.8132.7020.0660.4600.0315.8520.003o.0241.9802.0072.8950.031

21 07424.000

35.703.53

13.996.88

14.6711.780.480.020 .149.290.003.410 .13

100.02

5.4562.5210.o238.000

0.7681.8752 6830.0620.4060.0125.806

0.0411.81 11.8523.4760.034

20.49024-OOO

36,843.49

13.748.77

10.9013.510.33

no0 . 1 19.800,002.500.00

99.99

36.443.90

13.518.99

10.9013.57o.27

no0.219.700.002.500.00

99.99

0.0621.8911.9532.548

21.45224.000

37.122.70

13.658.06

1 1 .3113.380.430.040.089.590.063.440.14

100.00

35.944.25

13.559.26

13.939.810.210 0 00 .109.250.013 .140.55

100.00

5.5272.4560.0178.000

1.0541 .7912.2480.0270.491

5 .6110.0020.0301 .8151.8472.4560.143

21.40124.OOO

36.504.03

13.208.39

12.3412.620.190.030.179.240.003.000.30

100.01

5.5612.3700.0698.000

0.8931.5722.8650.0250.4620.0185.835

0.0501.7961.8463.049o.077

20.87424.OOO

36.253.39

13.906.80

14.8111.800.490.030.109.570.002.800.06

100.00

5.5812.419

8.0000.1040.7881.9072.7070.0640.3920.0195.981

0.0301.8801.9102.8750.016

21 .10924.000

Biotite structural formulas (O + OH + Cl:2415.6192.381

8.0000.0901.0071.3903.0710.0430.400

6.001

0.0331.9071.9402.544

2't.45624.000

5.5682.432

8.000

1.0341.3933.0900.035o.448

6.000

5.5952.405

8.0000.0200.9141.4263.0060.0550.3060.0245.7510.0100.0231.8441.8773.4590.036

20.50524.000

Note.'biotite chemical compositions are given in weight percentages.'Analyses on crystals used for structure refinements.

-- The Fe oxidation state was determined by semi-microvolumetric method (Meyrowitz, 1970).i Determined by atomic absorption spectrophotometry.+ Determined by thermogravimetric analysis.

the prevailing ferromagnesian mineral. Biotite can be iso-lated, aggregated, or included in hornblende. In the mon-zonitic complex, augite becomes the major ferromagne-sian mineral.

Two samples from each lithologic complex were se-lected for TEM studies and single-crystal X-ray structurerefinements: Ml3 and Ml05 from the granitic complex,M7 and M32 from the syenitic complex, Ml4 and Ml6from the monzonitic complex, and M62 from the tran-sition zone between the granitic and monzonitic complexes.

ExppnrlrnnrAr- TEcHNIeUES

Chemical analyses

Electron probe microanalyses (EPMA) were carried outusing an ARL-SEMQ electron microprobe. Operatingconditions were a 15-keV accelerating voltage, a 20-nAsample current, and a 3-rrm beam diameter. Data werereduced by the method of Ziebold and Ogilvie (1964),using the correction factors of Albee and Ray (1970). Thebiotite samples analyzed were homogeneous, and their

averaged compositions are reported in Table l. Since therewas little variation in the biotite composition of any giv-en rock sample, OH, Fe'?*, and Li were determined overa wide range of selected crystals: HrO was determined bythermogravimetric analysis (Du Pont 990 thermal ana-lyzer, heating rate l0 "C/min) in Ar gas to prevent Feoxidation (flow rate 30 mllmin); Fe2* was determinedby a semi-microvolumetric method (Meyrowitz, 1970);and Li was determined by atomic absorption spectropho-tometry. Structural formulae were calculated on the basisof (o + oH + c l ) : 24.

TEM study

The biotite crystals used in the TEM study were hand-picked from thin sections. In order to investigate poly-typism and stacking disorder, crystals having c* parallelto the plane ofthe section were selected, prepared by Arion thinning techniques, mounted on Cu grids, and lightlycoated with C. The TEM specimens were examined witha Philips 400T transmission electron microscope oper-

BIGI AND BRIGATTI: CRYSTAL CHEMISTRY OF PLUTONIC BIOTITE

TABLE 2. Unit-cell parameters and details of X-ray data collection and refinement

65

DimensionsSample (mm)

R"r. R*" a bN.* (x100) (x100) (A) (A) (A) (4")f)

lMpolytype O.27 x O.24 x O.022M.polytyp 0.15 x 0.12 x 0.02

s49 2.31 3.33724 2.63 2.72

5.335(2) 9.244(2)5.339(1) 9.249(1)

10.206(3)20.1 96(1 )

100.08(2) 49s.69s.06(1) 993.4

Nore; F"y. : (; i 'r" -'^)lhz,-)

ating at 100 keV and equipped with an EDAX energy-dispersive SiLi detector. SAED patterns were used tocharacterize the main periodicity and ordering of thepolytypic sequences and intergrowths with other phases.In mica SAED patterns obtained with the beam parallelto [100], I l0], or [Tl0], the rows wirh k: 3n exhibit al0-A periodicity for every kind ofsequence; the true pe-riodicity and ordering are indicated by the distributionand shape of the spots in the rows with k + 3n. Weakspots, corresponding to forbidden reflections, sometimesappear in the [00/]* rows and are generated by multiplediffraction effects in the thickest areas. The main peri-odicity and ordering were also studied by brighf and dark-field imaging and high-resolution (HRTEM) imaging.HRTEM images were obtained by selecting the 00/ and02/ reflections with the objective aperture (70-pm diam-eter); one-dimensional bright-field images were obtainedusing the 00/ reflections and centering the objectiveaperture (30-pm diameter) on the transmitted beam; dark-field images were obtained using the same aperture cen-tered about lhe l02ll* reciprocal lattice row. Superstruc-tures were detected in thick areas of the specimens, wheredynamic diffraction effects were present (Iijima and Bu-seck, 1978).

SAED patterns were interpreted by comparing ob-served and calculated diffraction patterns for [02l]*, Il l/]*,and [1]/]* reciprocal lattice rows. A periodic intensitydistribution function for each complex polytype wascomputed following the formulae proposed by Take-da (1967).

X-ray data collection and structure refinement

Biotite crystals, selected from crushed samples, weremounted along the b direction on a CAD4 (Enraf-Non-ius) four-circle diffractometer with graphite-monochro-matized MoKa radiation, operating at 52kY and 40 mA.The unit-cell parameters reported in Table 2 were cal-culated by means of least-squares refinement using 25reflections (15" < d - 25); technical details of data mea-surement and reduction have already been extensivelydescribed (Brigatti and Davoli, 1990; Brigatti et al., 1991.,Bigi et al., 1993). Two equivalent sets of reflections weremeasured in the d ranges 1.5-35.0" (biotite-lM, 2400measured reflections) and 2.0-33.0" (biotite-2Mr, 3070measured reflections) and were merged after correctionfor absorption (North et al., 1968) and Lorentz-polaiza-tion effects. The crystal-structure refinements were per-formed without chemical constraints using the least-

squares program ORFLS (Busing eI al., 1962). Scatteringfactors for fully ionized atoms were used for M I and M2octahedral and K interlayer sites, whereas scattering fac-tors for neutral atoms were used for the tetrahedral andO sites. The interpretation ofstructural data is based onaveraged C2/m and C2/c structures for lM and, 2M,polytypes, respectively. Crystallographic coordinates andisotropic and anisotropic temperature factors are report-ed in Table 3; relevant bond lengths are reported in Table4. Observed and calculated structure factors are listed inTable 5.'

Cuntrrslny

Electron probe microanalyses of Valle del Cervo biotitesamples are reported in Table l. The main chemical vari-ation relates to the Fe'?*-Mg ratio, which ranges between0.47 and 0.70. The Fe3* content is always less than thatof Fe2t. No significant chemical variations occur in dif-ferent crystals from the same rock sample, and zoning isunlikely to be present. No well-defined relationships be-tween chemistry and microstructures were observed inTEM-EDS microanalyses, since different stacking se-quences coexisting in the same crystal have very similarchemical compositions. Only in sample M7 is there aweak relationship between the Fe content and poly-typism, the lM polytype being enriched in Fe with re-spect to 2M, and 37"; but that can be ascribed to localenvironmental chemical variations occurring during crys-tal growth.

MrcnosrnucruREs

Weissenberg and precession photographs emphasizedthat Valle del Cervo biotite is mostly characterized bylM,2Mt, and disordered polytypes. TEM study showedthat ordered lM,2M,, and 3Zsequences can extend overlarge areas of the crystal, whereas disorder is character-ized by the coexistence of different microstructures,namely, ordered, simple, and complex polytypes, twin-nings, completely disordered stacking sequences, and in-tergrowths of l4-A phases. These sequences can occur inareas comprising a few nanometers to a few micrometers,producing a patchwork of microstructures. Also, the sim-ple polytypes are often interrupted by isolated stacking

I A copy of Table 5 may be ordered as Document AM-94-543from the Business Ofrce, Mineralogical Society of America, I I 30Seventeenth Street NW, Suite 330, Washington, DC 20036,U.S.A. Please remit $5.00 in advance for the microfiche.

66 BIGI AND BRIGATTI: CRYSTAL CHEMISTRY OF PLUTONIC BIOTITE

TABLe 3. Crystallographic coordinates and equivalent isotropic (A") and anisotropic temperature factors (43 x 104)

ylb zlc Bq 9u 0", 4," A." Fr"

0-6(3)

4(3)0000

1(1)

21(7)34(4)29(4)27(6136(3)28(2)42(4)21(11

4(3)0(3)7(2)0(2)4(2)5(2)3(1)4(1)7(1)2('tl2(1)

o102o3o4M 1M2KT

0 1 1o21o22031o32o4M1M2KT1T2

o.0247(8)0.3219(5)0.1334(4)0.1301(7)0000.o752(21

0.7385(6)0.2368(6)0.441 8(5)0.4324(510,93s9(5)0 93s1(s){c0.2473(2)00.4615(2)0.9636(2)

00.2336(3)0.1 675(3)th

00.3343(1)Y20.1 669(1 )

0.31 76(4)0.3478(4)0.0844(4)0.2376(3)0.4047(3)0.0695(3)t/1

0.0825(1 )0.0852(3)0.2498(1 )0.4166(2)

0.1 688(4)0.1683(3)0.3905(3)0.3952(4)'/2

'/2

00.2258(1)

0.16sq2)0.1 665(2)0.1 662(1 )0.05500)0.0550(1 )0.0s1 8(1 )00.0000(1 )th

0.1 370(1 )0.1 371 (1 )

47(4)88(4)44(215q4)38(2)44(1)87(3)38(1)

51(4)44(4)38(3)6(3)

22(3)19(3)31 (1 )3q1)81(2)23(1 )24(1)

71(5)66(3)61(3)64(4)67(2160(1)

107(3)52(1)

1 1 ( 1 )1 1 ( 1 )1 1 ( 1 )5(1 )6(1)s(1)8(1)8(1)

21(117(117(1)

0-22(5)

0(4)0000

-2(2)

- 15(6)1 0(6)7(7)

-2(4)2(5)

10(s)-8(4)-3(5)

03(3)5(4)

-1(2)-1(2)-3(2)- 1 ( 1 )-2(1)-s(1)-4(1)-3(1)

01(1)1(1)

1}'polytype2.60) 306(16)2.U(71 212(912.01(6) 197(8)2.17(9) 198(13)1.92(41 172(5)1.86(2) 160(4)3.65(6) 339(9)1.71(2) 161(3)

2ntl polytype1.64(8) 129(12)1.62(8) 143(1211.66(6) 165(10)0.63(6) 82(e)0.95(6) 98(e)1.08(6) 103(9)1.04(3) 69(4)1 .22(21 1 12(3)2.e3(4) 238(5)0.89(2) 74(3)0.91(2) 76(3)

Note . ' an i so t rop i c t empe ra tu re fac to r s ,B r i , a reo f t he fo rmexp l - ( h ' ?B l , * . . . * 2hkB i2+ . . . 1 ] .

faults, giving rise to weakly disordered sequences. Thedistribution ofthe stacking sequences in biotite from thedifferent rock samples is reported in Table 6.

Simple polytypes and twinnings

In biotite from the granitic and syenitic complexes, thelM polytype prevails, whereas in biotite from the mon-zonitic complex, the 2M, polytype is the most commonlayer sequence. Both polytypes commonly contain stack-ing faults, as indicated by weak streaking along c* ofthespots with k + 3n (Fig. l). It is unusual in biotite fromthe syenitic complex that lM,2M,, and 3Ipolytypes thatare almost ordered and exhibit crystallographically con-trolled orientations coexist in the same crystal (sampleM7). Twinned sequences were found only in graniticcomplex biotite (Fie. 2) and consist of lamellae of rhe lM

polytype approximately 100 A wide alternating with twoorientations, which follow the most common twin lawsreported by Smith and Yoder (1956). Twin lamellae co-exist with disordered sequences and a dominant lM poly-type. Figure 2a illustrates two distinct lM polylype pat-terns, depending on the orientation of the lamellae. Thoseviewed along the [T0] axis are wider and are representedby the stronger spots in rows witlr. k + 3n. Those viewedalong the [100] axis are less abundant and are representedby the weaker spots in the same rows. The correspondingtwinning laws may be U l0l (or IT0]) and [310] (or [3T0]),produced by *60 and + 120' layer rotations around c*.Figure 2b shows twinning around the [ll0] (or [110])axis, or the [00] axis, or both, corresponding to +60 and+180" layer rotations around c*, which is uncommonin biotite.

TABLE 4. Selected bond lengths (A) from structure refinements of biotite-1 Mand -2M1 samples from Valle del Cervo syenitic complex*

Ml-O3 (x 4)Ml-O4 (x 2)(M1 -O)M2-O3 ( x 2)M2-O3'(x 2)M2-O4 (x 2)(M2-O)

M1-O31 (x 2)M1-O32 (x 2)M1-O4 (x 2)(M1 -O)

M2-O31M2-O31',M2-O32M2-O32'M2-O4M2-O4'(M2-O)

2.105(212.074(4)2.0952.100(3)2.O76(2)2.O57(3)2.O78

2.1 1 1(3)2.018(3)2.163(3)2.097

2.019(3)2.177(3)2.1 08(3)2.181(3)1.960(3)2.048(3)2082

T-O1r-o2T-O2'T-O3(T-O)

T1-O11T1-O21T1-O22T1-O31(T1-O)

T2-O11r2-021r2-022T2-032(T2-O)

1 it polytype1.654(2)1.6s2(3)1.6s5(3)1.655(3)1.654

2nt, polytype1.662(4)1.6s6(4)1.646(4)1.652(3)1.654

1.654(4)1.653(4)1.668(4)1.6sq3)1.657

K-O1 (x 2)K-O2 (x 4)(K-O),,*,

K-O1' ( x 2)K-O2' ( x 4)(K-O).,8

K-O11 (x 2)K-O21 (x 2lK-O22lx 2l(K-O),""-

K-O11' ( x 2)K-O21' ( x 2)K-O22'(x 2 l(K-O).,,",

3.01e(4)3.015(3)3.016

3.305(4)3.306(3)3.306

3.010(4)3.mq4)3.02213)3.023

3.322(413.274(4)3.295(3)3.297

- Sample M7

BIGI AND BRIGATTI: CRYSTAL CHEMISTRY OF PLUTONIC BIOTITE 67

Fig. L HRTEM image and electron diffraction pattern of a 2M, sequence affected only by a few stacking faults, as indicated byarrows. Sample Ml6 from the monzonitic complex.

Long-period stacking sequences

Long-period stacking sequences are less common thanthe simple polytypes described above. They occur in bi-otite from the granitic and monzonitic complexes and inbiotite from the transition zone between them, whereasthey are absent in biotite from the syenitic complex. Theselong-period stacking sequences are easily identified bymeans of SAED patterns, i.e., they repeat quite regularlyover an area of about 0.5 pm in diameter, which is thesize of the selected-area aperture used in this work. Thebright- and dark-field one-dimensional images showedthat the stacking sequences are sometimes periodic for agreal number of unit cells or can be intemrpted by fre-quent stacking faults, giving rise to complex sequenceswith a dominant, but inconsistent, periodicity. Only incertain cases was it possible to determine the exact stack-ing sequence by interpreting the intensity distribution ofthe spots in the SAED patterns and the HRTEM images.This enabled us to describe new complex polytypes, whichhitherto had only been described in theoretical terms. Ingranitic complex biotite, long-period stacking sequencesare widely ordered and coexist with disordered sequencesand simple lM and 2M, polytypes. Unusual superstruc-

tures were observed in sample M 13. Sequences with two-,three-, nine-, and l4Jayer periodicities follow one anoth-er (Fig. 3); the nature of the lamellar interfaces is some-times characterized by disordered sequences or cleavageplanes, which are often difficult to recognize because theyhave been damaged by ion thinning. Another area of thesame crystal shows f,ve- (Fig. a) and 13-layer polytypes.The HRTEM image and the intensity distribution of thespots in the SAED pattern indicate that the five-layer

Trau 6. Distribution of simple and complex polytypes in biotitefrom Valle del Cervo oluton

Sam-ple Rock type

ComplexSimple polytypes

polytypes (no. of layers)

M13 Granitic comolexM105 Granitic comolexM14 Monzonitic comolexM16 Monzonitic complexM7 Syenitic complexM32 Syenitic complexM62 Monzonitic-syeniticcomplex

transition zone

1 M , 2 M , , 3 T1 M2 M . , 1 M2 M , , 1 M1 M , 2 M . , 3 71 M1M,2M1

3 , s , 9 , 1 3 , 1 4

218

3 , 4 , 6 , 7

Note: polytypes are indicated in decreasing order of importance.

68 BIGI AND BRIGATTI: CRYSTAL CHEMISTRY OF PLUTONIC BIOTITE

Fig.2. Electron diffraction patterns of twinned lMpolytype.Twin laws: [ 1 l0] (or [l 10]) or [3 10] (or [310]) axes (a) and [1 l0](or [1T0]) or [100] axes (b). Sample M105 from the graniticcomplex.

sequence can be attributed to the complex poll'type 00022,synthetically written (0)122, using the RTW notation ofRoss et al. (1966).

In the monzonitic complex biotite, eight- and 2l-layersequences were observed near disordered sequences; theytoo were often interrupted by stacking faults (Fig. 5). Thecomplex sequences can be ascribed to the complex poly-types 00202022 and (22)4Q2)5222, respectively. Com-plex sequences ofthree, four, six, and seven layers werealso found in biotite from the transition zone between thegranitic and monzonitic complexes (Fig. 6); they coexist

Fig. 3. Bright-field image and electron diffraction pattern ofthree-layer polytype (a), nine-layer polytype (b), and l4-layerpolytype (c). The viewing direction is parallel to [100] or [10]or I l0]. Sample Ml3 from the granitic complex.

in the same crystal with dominant lM, 2M,, and, lMdpolytypes.

Cnvsur, cHEMrsrRY

Prearnble

Brigatti and Davoli (1990), in their biotite-1,4y' struc-ture refinements on five crystals from the three main lith-ologic complexes of Valle del Cervo pluton (sample M13,Ml4, M32, M62, Nd73), showed that (1) differences inbond lengths and distortion parameters exist between Mland M2 sites because of partial cation ordering, the great-est differences being found in monzonitic complex bio-tite; (2) bond lengths and distortion ofthe octahedral sheetare influenced not only by the local chemical composition

BIGI AND BRIGATTI: CRYSTAL CHEMISTRY OF PLUTONIC BIOTITE

Fig. 4. Bright-field image and electron diffraction pattern offive-layer polytype. Sample M I 3 from the granitic complex.

but also by confinement of the octahedra between twoopposing tetrahedral sheets; (3) almost regular tetrahedraare linked in pseudohexagonal rings, whose distortionfrom hexagonality (a angle, 5.3' < a < 7.4\ seems to beindependent of octahedral features; and (4) difference-Fourier map analysis reveals significant peaks, suggestingtwo different orientations of OH groups in samples fromdifferent complexes.

X-ray analyses and structure refinements ofM7 biotite (lM and 2M, polytypes)

The crystal-chemical relationships of biotite-lM and-2M, from the syenitic complex are discussed and com-pared with those of other biotite samples from the sameplutonic body. Bond lengths are given in Table 4, andadditional structural parameters compared with those re-ported by Brigatti and Davoli (1990) are given in Table 7.

The two studied biotite samoles are of verv similar

Fig. 5. Dark-field image and electron diffraction pattern ofeightJayer polytype (a) and bright-field image of 2 I Jayer poly-type (b). The viewing direction is parallel to [100] or [l10] or[T10]. Sample M16 from the monzonitic complex.

chemical composition (Table l). The greatest variability(little more than 100/o) was observed for the TiO, content.

Octahedral sheet. Mean bond lengths are greater forthe Ml than for the M2 site, which indicates that large

"i\

240 A' ,!.!,

TABLE 7. Octahedral, tetrahedral, and interlayer parameters derived from structure refinement

Sample: M13 M7 M7 M32Pofytype: 1M 1M 2M, 1 M

M 1 41 M

M621 M

8,0"", f)B*" f)(M1-o) (A)(M2-o) (A)oQE",ooE",p", f)v".(l

v*,v*,(r1-o) (A)(r2-o) (A)TQETlTQET'r-, (')re f)a (')(K-O),..., (A)(K-o)",,- (A)

100.03100.15(3)

2.0962.0811 . 01 101.0097

58.958.7

1 .1 0551.0988

12.071 1 . 8 41.663

1.0006

1 1 0 . 6

100.041 00.08(2)

2 0952.0781 .01061 .0091

58.858.s

1 .10321.0958

12 .O711 . 801 654

1 0003

1 1 0 . 4

6 43 .0163.306

95.0695.06(1 )2.0972.0821 . 01211 .0133

58.858.6

1.09941.0936

12.091 1 . 8 31.6541.6571 .00161 .0016

110.71 10 .6

6.03.0233.297

100.02100.02(3)

2.O922.0801 .01071.0098

58.958.71 .10411.0992't2.02

1 1 . 8 31.657

1.0003

110.4

100.031 00.26(2)

2.0952.0771 . 01 141.0099

59.058.71 .1 0731.0996

12.0511.771.657

1.0003

1 10 .4

5.73.0373.294

100.031 00.1 s(2)

2.0892.O791.01031.0096

58.858.6110241.0982

1 1 .9611.821.654

1.0002

1 10 .3

o.J

3.0203.306

6.43 0263.316

3.0293.308

Note:T1 : tetrahedron T1 in2Ml setting and tetrahedron T in lMsetting. Sample M7: this study; samples M13, M14, M32, M62: from Brigatti andDavoli (1 990).

BIGI AND BRIGATTI: CRYSTAL CHEMISTRY OF PLUTONIC BIOTITE

TABLE 8. Mean atomic number of octahedral and interlayer sitesand determination of Al in tetrahedral sites

IM 2M.

M1 XrefM2 Xref( M 1 + 2 x M 2 ) X r e f( M 1 + 2 x M 2 ) E P M A 'K XrefK EPMAAlr'.'Altr**Alr*.AIT EPMA

19.041 8 . 1 055.2455 .1117.9718 29

31 .8929.76

18.7518.2655.2755.4518 8118.3133.4934.3333.9130.40

Note: Xrel : X-ray refinement; EPMA : electron probe microanalyses;K : interlayer site.

'Sum of octahedral cation electrons..- Determined by Alberti and Gottardi's (1988) method.

cations and vacancies prefer the Ml site. Both polytypeshave similar mean bond lengths and mean atomic num-bers (Table 8), whereas the polyhedra are more distortedin the 2M, than in the lM polytype (see OQE values,where OQE is the octahedral quadratic elongation, cf.Robinson eI al., l97l; Table 7). At constant polyhedralvolume, the distortion is caused by an increase in Ml-04, M2-O3l', and M2-O32' and a decrease in Ml-O32,M2-O31, and M2-O4 individual bond lengths (Fig. 7).As already observed by Takeda and Ross (1975), thisbehavior produces a shift along b ofthe triad ofthe upperO atoms with respect to the triad of the lower O atomsin the 2M, polytype. In Figure 7, the M-O3 and M-O4bond lengths of sample M7 are compared with those ofother biotite- lM and, -2M, structure refinements from theliterature. In the lM polytype the mean bond length (M 1-03) increases as Ml-O4 increases, whereas in Ihe 2M,polytype the (Ml-O3) mean bond length increases as(M1-O4) decreases. No well-defined correlation exists forthe M2 site. Thus, the octahedral-sheet geometrical vari-ations seem mostly due to an increase in the Ml sitedimension in the lM polytype and to an increase in thedistortion, mostly of the M I site, in Ihe 2M, polytype.

Tetrahedral and interlayer sheets. The mean bondlengths (T1-O) and (T2-O) for the biotite-2M, are verysimilar, suggesting no tarAl ordering. Tetrahedral and in-terlayer parameters (i.e., a, (K-O)-*, and (K-O).,,".) oflM and 2M, polytypes are comparable. The only, albeitlimited, variation concerns the TQE parameter (tetrahe-dral quadratic elongation: Robinson et al., l97l), whichis greater in the 2M, than in the lM polytype. The tet-rahedral deformation is caused by a decrease in the O22-T1-O3 I angle (107.01') and an increase in the O22-T2-O32 angle (114.4) with respect to the ideal value of

ts-

Fig. 6. Electron diffraction patterns of fourJayer polytype(a), six-layer polytype (b), and seven-layer polytype (c). Theviewing direction is parallel to [00] or [10] or [10]. SampleM62 from transition zone between the granitic and monzoniticcomplexes.

BIGI AND BRIGATTI: CRYSTAL CHEMISTRY OF PLUTONIC BIOTITE 7 l

109.41". Similarly Tl-O22 is significantly shorter, andTl-Oi I is significantly longer.

Difference-Fourier maps and microstructures. A differ-ence-Fourier map calculated at the end ofanisotropic re-finement affords useful information. However, care mustbe exercised in its interpretation, since some peaks, es-pecially in micas, can be ascribed to artifacts linked tosystematic errors or to poorly described thermal ellipsoids.

The standard deviations for the estimation of electrondensity using the equation ofLipson and Cochran (1953)were 0.06 (for the 2M, polytype) and between 0.04 and0.08 e/A: (for lMpolytypes: samples from this study andBrigatti and Davoli, 1990).

In sample M7 the 2Mrpolytype exhibits l8 peaks abovethe background (3o). Of these, only one peak (0.26 e/A3)was found in a position suitable for H, though the dis-tance H-O4 is somewhat short (0.86 A). On the otherhand, the lM polytype exhibits peaks (about 5o) in po-sitions corresponding to a shift of +b/3 for T atoms and+b/3 for K. Similar peaks were observed also in differ-ence-Fourier maps of biotite from the granitic complex(M13) and from the transition zone between the syeniticand monzonitic complexes (M62). These features may beexplained by twinning about [310] (or [310]), or by 2M,domains in a lM sequence. TEM analyses on crystalsfrom the same rock samples indicated that the b/3 shiftcan be attributed mainly to twinning in the granitic bio-tite and prevalently to 2M, coexisting domains in biotitefrom the other complexes.

CoNcLusroNrs

Comparison between biotite-lM and -2M, from theValle del Cervo syenitic complex reveals similar chemicalcomposition and similar crystal-chemical features. Thedifferences mostly concern the distortion parameters ofboth tetrahedral and octahedral polyhedra. This behaviorextends the conclusions of Takeda and Ross (1975) re-garding the volcanic biotite to the plutonic environment.

The microstructural study shows the existence in thesame crystal of extensive areas characterized by orderedsimple sequences, mostly belonging to the lM polytype,variably ordered complex sequences, and disorder. Thedistribution oflayer sequences in the biotite can be roughlyrelated to different lithotypes: the 2M, polytype is morediffuse in crystals from the monzonitic complex andtwinned sequences are present only in crystals from thegranitic complex, whereas long-period stacking sequencesare found in biotite from both granitic and monzoniticcomplexes and in the transition zone between them. Sy-enitic complex biotite is characterized by simple orderedpolytypes, sometimes coexisting in the same crystal, orby completely disordered stacking sequences (lMd). Fur-thermore, no evident correlations were found betweendifferent polytypic arrangements and chemical composi-tion, which supports the claims in the literature for poorchemical control over polytypism; small differences be-tween the Fe-Mg ratios of the lM and 2M, sequences canbe ascribed to the local chemical environment.

2 0 4 2 . 0 6 2 0 8 2 1 0 2 L 2

<M1-03>

*a)

2 2 2

2 1,6

2 . r o

2 0 4

1 9 8

2 0 2

<-,

1 2 1 0c\?

2 2 2

2 1 6

2 . O 4

1 9 8

2 . O 2 2 0 4 2 . 0 6 2 0 B 2 L O 2 1 2

<M2-03>Fig.7. Plots of (Ml-O3) (A) vs. M1-O4 (A) (a) and (M2-

03) (A) vs. (M2-O4) (A); (b) mean bond lengths for biotite-lM(open circles) and -2M, (solid circles). The samples from litera-ture are from Brigatti and Davoli (1990); Brieatti et al. (1991);Takeda and Ross (1975); Bohlen et al. (1980).

Long-period stacking sequences in the Valle del Cervobiotite have high statistical significance (Veblen and Bu-seck. 1979) and are therefore useful indicators ofmech-anisms acting during crystal growth. Theories explainingthe origin of polytypism in layered compounds falls intotwo main categories: kinetic and structural. In micas, onthe basis of experimental studies, the kinetic theory seemsto prevail; ordered long-period sequences can, in fact, begenerated in an ordered simple sequence by screw dislo-cation, followed by spiral growth (Baronnet, 1980). How-ever, this mechanism cannot explain the origin of par-tially disordered long-period sequences, and other

b

o ao

o M 7 0

o o

72 BIGI AND BRIGATTI: CRYSTAL CHEMISTRY OF PLUTONIC BIOTITE

mechanisms involving long-range interactions must beinvoked accordingly.

AcxNowr,nlcMENTS

We would like to thank S. Merlino for his critical reading of the manu-script; A. Gregnanin and B. Biggioggero, who supplied samples, are alsoacknowledged.

Financial support was provided by the Centro di Calcolo and CentroStrumenti of Modena University, the Ministero della Ricerca Scientificae Tecnologica, and the Consiglio Nazionale delle Ricerche of Italy. TheConsiglio Nazionale delle Ricerche is also acknowledged for financing theelectron microprobe laboratory in Modena University.

Rnrnnrucns crrEDAlbee, A.L., and Ray, L. (1970) Correction factors for electron micro-

analysis of silicates, oxides, carbonates, phosphates and sulphates. An-al1'tical Chemistry, 42, l 4O8- 1 4 1 4.

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Bailey, S.W (1984) Crystal chemistry of the true micas. In MineralogicalSociety of America Reviews in Mineralogy, 13, l3-60.

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Bell, I.A., and Wilson, C.J L. (1981) Deformation of biotite and musco-vite: TEM microstructures and deformation model. Tectonophysics,78,20r-228

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M,rm-rscnrm RECEIvED Decevsep 22, 1992MeNuscnrm AccEprED Sesreruesn 3, 1993