Limitations on the Interpretation of Biotite Substitutions from Chemical Analysesof Natural Samples Davro A

American Mineralogist, Volume 71, pages I 126-1 128, 1986

Limitations on the interpretation of biotite substitutions from chemical analysesof natural samples Davro A. Hrcwrrr Department of Geological Sciences,Virginia Polytechnic Institute and State University, Blacksburg,Virginia 24061, U.S.A. JiincnN Arnncnr Mineralogisch-PetrographischesInstitut, Universitiit Basel,Basel, Switzerland

Ansrru,cr Biotites, as well as other phases,are sufficiently complex chemically that there are numerous sets of linearly independent exchangecomponents with which to describe the composition of a naturally occurring crystal. Even after complete chemical analysis, this leads to ambiguities in the correlation of exchangecomponents with valid substitution mechanismsin complex phases.The best means of identifuing valid substitutions are by the observation of simple endmember compositions, the investigation of compositional changesin zoned crystalsor petrogeneticsequences, and by direct experimental synthesis.

INrnonucrrou of the phase in this structural arrangement.As pointed out by Thompson (1982) and Labotka (1983),there are Studiesof the crystal chemistry of biotite, particularly numerous ways of expressingthese restrictions. The stan- those investigating the substitution of Ti, frequently try dard structural formula givesthe number of moles of each to define individual substitutions as important in pro- of the l8 components in such a way that chargebalance ducingthe chemicalcomposition of panicular phases.This and the crystal-chemicalintegrity of the biotite are main- has led to disagreementamong various researchersas to tained. A more useful approach,because we are trying to which substitutions are important and how the compo- simplify our expressionof the biotite composition, is to sitions of biotite should be expressedin terms of end- define one set of equations basic to the crystal chemistry members. of biotite and then a smaller set of exchangecomponents The purpose of this paper is to present an analysis of with which to describe the composition of the phase biotite compositions using the methods presented by (Thompson,1982; Labotka, 1983). Thompson (1982) applied along the lines of a similar The first group ofequations defining the biotite struc- treatment of biotites by Labotka (1983). We will show ture for these 18 components is againthat any common biotite can be representedin terms of an additive componentand a limited number of linearly K+Na*Ca*tr(XID:2 (l) independent exchangecomponents. Furthermore, there Mg * Fe2** Mn2* + Fe*r(VI) are numerous sets of viable exchangecomponents that + AI(VI) + Ti + tr(VI) : 6 (2) can be used, and the substitutions touted will depend on si+A(I\o+Fe3+(IV):3 (3) the particular choice of exchange vectors. Finally, al- o+oH+F+cl:24 (4) though it has been widely understood that incomplete 2 positive charges: ) negativecharges (5) analyses-especially those neglecting FerO, and HrO- Labotka (1983) shows that using a single additive com- will lead to ambiguities regardingbiotite substitutions, it ponent, suchas phlogopite, is equivalent to thesefirst five can also be demonstratedthat even with complete biotite equationsas long as the additional exchangecomponents analyseswe are unable to make unique substitution models. are charge-balanced.Therefore, we need to define only l3 exchangecomponents: The choiceof these l3 is arbitrary, Brorrrn coMPoNENTS Table1. Commoncomponents in biotite Based on crystallographic information, summarized most recently by Hazen and Burnham (1973) and Bailey Two inter- Six octa- Eight tetra- Twenty-four (1984), the structural formula for biotite, normalized to laver sites hedral sites hedral sites anions 24 anions and including the commonly discussedsubsti- KMgSiO tutions, can be expressedas shown in Table 1. Na Fe2* A(lV) OH Ca Fe3.(Vl) Fe$(lV) F Altogether theseamount to l8 components.By analogy !(xil) A(vr) cl with the phaserule, the variance of the systemequals the Ti Mn number of variables minus the number of equations re- nryt) lating those variables: Therefore, we must place l8 restrictions on the systemin order to definethe composition Note: !n reters to a vacancy of coordinationnumber n. 0003-o04x/86/09l 0-l l 26$02.00 tt26 HEWITT AND ABRECHT: INTERPRETATION OF BIOTITE SUBSTITUTIONS tr27 but all of the original 18 components must be included Table2. Calculatedbiotite formulae and the exchangesmust be linearly independent and FD-12- Au Sable-t charge-balanced.The first group ofthese can be expressed Si 5.42 5.587 as simple two-element exchanges: A(rv) 2.58 2.413 NaK-, (6) A(vr) 0.67 0.243 Ti 0.35 0.636 FeMg-, (7) Fe'3* 0.44 0.000 Fee+ 2.78 2.786 MnMg-, (8) Mg 1.22 2.321 Fe3*AI(IV)-, (e) Mn 0.14 0.015 qvt) 0.40 -0.001 FOH_, (10) 0.00 0.005 cloH_, (l l) Na 0.15 0.040 K 1.86 1.971 -0.01 -0.016 The choice of the remaining components is not so well !(xil) 0.02 0.275 defined. There are many possibilities, and the particular F 0.39 0.165 choice that is made will influence any interpretation on OH 3.17 2.435 ot 20.42 21.125 substitutionsthat might be attempted. In our comparison . we will use two very similar setsof exchanges,both con- Dodgeet al.(1969). '- Bohten (1980). taining only commonly quoted mica (given et al. substitutions f Calculatedby differencebased on 24 anions. in parenthesesin the following lists). Even though more divergent sets can easily be constructed, the minor dif- ferencesbetween these two setsare sufficient to yield sig- nificantly ditrerentresults when applied to natural biotites. We will comparethese sets of equationsfor two natural biotites that have been analyzed for all the elements in ExchangeSet I our original list except oxygen.Table 2 lists the structural formulae for both biotites calculated on the basis of 24 CaAI(IV)IL,Si_, (Clintonite) (r2) anions. The first is an analysis(FD-12) of a biotite from the Taft Granite in the Sierra Nevada reported by Dodge sitr(xrr)K (Talc) (l 3) ,A(rv)_, et al. ( I 969). The secondis an analysisofa granulite-facies A(IV)A(VDFe_,Si_, (Al-Tschermak's) (14) metamorphic biotite from the Adirondacks (Au Sable)for Fe3*(VI)OFe-,OH, (Fe oxy-annite) (15) which Bohlen et al. (1980) have reported both a complete Titr(VI)Fe-, (Ti-vacancy) (16) chemical analysisand structural determination. A(IV),TiFe-,Si-, (Ti-Tschermak's) (17) Table 3 compares the vector coefficients necessaryto describe the biotites with the two different sets of ex- TiOrFe-,OH-, (Ti oxy-annite) (18) changesusing phlogopite (24 anions) as the additive com- ponent. The first eight exchangesneed not be compared ExchangeSet II becausethey are the same and yield identical results for CaAI(IV)IL,Si-, (Clintonite) (12) both sets.Even though there are only two differencesin sitr(xll)rL,A(rv)_, (Talc) (13) the last five exchangecomponents, there are significant diferences in the apparentsubstitutions. Looking at sam- Al(IV),TiFe_,Si (Al-Tschermak's) (14) , ple FD-12, just three exchangesout ofthe last five in Set Fe3*(VI)OFe-,OH-, (Fe oxy-annite) (15) I account for most of the necessarychanges in composi- Al(v!,tr(vr)Fe, (Muscovite) (16) tion. Those substitutions are the Al-Tschermak's, Ti-va- Al(IV),TiFe_,Si_, (Ti-Tschermak's) (17) cancy, and Fe oxy-annite exchanges,all of which have AI(VI)OFe ,OH_, (Al oxy-annite) (18) beenshown to be important substitutionsby experirnental synthesisand/or have beencommonly proposedas major substitutions in biotite (Rutherford, 1973; Forbes and Applrca,tloN To NATURALBToTITES Flower, 1974;Wones, 1963).Using ExchangeSet II we How do these different sets of equations describe the see that the same material is equally well expressedby compositions of natural biotites? Both work equally well major amounts of Ti-Tschermak's, muscovite, and Fe and are accurate to the degree that the original analyses oxy-annite exchangeswith a small negativeamount of Al- are accurate.If they are applied to incomplete analyses, Tschermak's substitution. Thesetoo are well-acceptedmica particularly those neglectingFerOr, H2O, F, and Cl, some substitutions and give an equally good desoription ofthe coefficientsof the exchangevectors must be falsely set to biotite; however, we get a very different view of the im- zero, and the results obtained are questionable at best. portance of particular substitutions than we got with Ex- Clever treatment of the data and choicesof the exchange changeSet I. componentscan alleviate some of the problems, but the A similar result is obtained when we look at the data coefficientsofthe vectors chosencan only be reliable for for the Au Sablebiotite. This has been cited as a biotite the part of the biotite analyzed.and not for the whole. demonstrating the importance of the Ti oxy-annite sub- l r28 HEWITT AND ABRECHT:INTERPRETATION OF BIOTITE SUBSTITUTIONS

Table3. Vectorcoefficients from phlogopite

Set I FD-12 Au Sable Set ll FD-12 Au Sable

NaK-l 0.15 0.040 NaK_r 0.15 0.040 FeMg_1 4.64 3.664 FeMg-, 4.64 3.664 MnMg-, 0.14 0.015 MnMg_1 0.14 0.015 Fe,*Al(lV)_l 0.00 0.000 Fe3tAl(lV)-1 0.00 0.000 FOH_1 0.39 0.165 FOH_, 0.39 0.165 cloH_1 o.02 o.275 ctoH, 0.02 0.275 Clintonite 0.00 0.005 Clintonite 0.00 0.005 Talc -0.01 -0.016 Talc -0.01 -0.016 Al-Tschermak's 0.67 0.243 Al-Tschermak's -0.11 -0.880 Fe oxy-annite 0.44 0.000 Fe oxy-annite 0.44 0.000 TLvacancy 0.40 -0.001 Muscovite 0.40 -0.001 Ti-Tschermak's -0.04 0.074 Ti-Tschermak's 0.35 0.636 Ti oxy-annite -0.01 0.563 Al oxy-annite -0.02 1.125

stitution, although Bohlen et al. (1980) only refer to it as position of a panicular substitution are compelling evi- one of the substitutions that might be considered. The dencefor its validity. With lessthan 1000/osynthesis, ac- vector coefficients in Table 3 using the Set I exchange curate microanalysis is a necessityand often shows the componentsgrve very similar results to those in Table 7 phase composition located off the planned substitution ofthe Bohlen et al. (1980) paper. However, an equally vector. It has been our experiencethat this is especially good representationof the Au Sablebiotite is given with- true for biotites and that often there is no unique set of out any Ti oxy-annite substitution by using the exchange simple vectors leading to the measured composition in componentsin Set IL Here the Al oxy-annite substitution these multiphase experiments (Abrecht and Hewitt, in replacesthe Ti oxy-annite substitution; we seeno reason prep.). why the latter substitution should be any more or less In summary it must be remembered that there is no valid than the former. No experimental or observational generallyvalid correlation betweenexchange components data are currently available to give preferenceto either and substitution mechanisms.For complex phases,mul- substitution. tiple equivalent setsofexchange components are available to describe the phase compositions. Additional infor- DrscussroN mation must be obtained in order to choosea set or sets Thompson (1982) has demonstrated the vector ap- of exchangecomponents that are also valid substitutions proach to compositional analysis of minerals and has made for the phasein question. it clear that there are many combinations of exchange vectors that lead to the same composition. Whether or R-nrnnnNcns not particular vectors correspond to crystal-chemically Bailey,S.W. (1984) Classification and structuresof the micas' valid substitutions may be an important question, but it MineralogicalSociety of AmericaReviews in Mineralogy,13, is usually not answerableby analysisof complex natural t-12. phases.Other criteria must be used to determine an in- Bohlen,S.R., Peacor, D.R., and Essene, E.J. (1980) Crystal chemistry of a metamorphicbiotite and its significancein water dependentset of important substitutions. Three kinds of barometry.Arnerican Mineralogist, 65, 55-62. data seemmost likely to yield this information. First, the Dodge,F.C.W., Smith, V.C., and Mays, R.E. (1969) Biotites from natural occurrenceof compositions related to a principal graniticrocks ofthe centralSierra Nevada batholith, Califor- additive componentby a single"simple" exchangevector. nia. Journalof Petrology,10, 25U27l. Flower,M.F.J. (1974) Phase relations of ti- The occurrencesof these endmember phases provide Forbes,W.C., and ran-phlogopiteKrM&TiAl,Oro(OH)o: A refractoryphase in the valuable information on the validity of particular vectors uppermantle? Earth and Planetary Science Irtters, 22,6046. as substitutions, although any single substitution may be Hazen,R.M., and Burnham,C.W. (1973)The crystalstructure a combination oftwo or more other substitutions.It seems of oneJayerphlogopit0 and annite.American Mineralogist, reasonable,however, that we should choose the set of 58.889-900. Labotka,T.C. (1983)Analysis of the compositionalvariations substitutions that representsthe least common denomi- of biotite in pelitic hornfelsesfrom northeasternMinnesota. nators. Second, changesin compositions of phasesthat AmericanMineralogist, 68, 900-914. occur as zoned crystals or in petrogenetic sequencecan Rutherford,M.J. (1973)The phaserelations of the aluminous be directly related to appropriate substitutions. In these iron biotitesin thesystem KAlSiOo-AlrOr-Fe-O-H. Journal of 169-179. samples there is the added advantage of being able to Petrology,14, Thompson,J.B., Jr. (1982)Composition space: An algebraicand define initial and final compositions and therefore the geometricapproach. Mineralogical Society of AmericaRe- vectors that relate them. This allows us to examine more viewsin Mineralogy,10, I-32. complex phase compositions, but we should still search Wones,D.R. (1963)Physical properties of syntheticbiotites on thejoin phlogopite-annite.American Mineralogist, 48, 1300- for simple changesin composition if we want to identify r32r. the most fundamentalsubstitutions. Finally, experimental methods can be useful in establishingsubstitutions. The MaNuscnrrr REcETvEDOcronrn 2, 1985 l00o/osynthesis and persistenceof a phase on the com- M,o'NuscnrprAccEPTED Mav 16, 1986