American Mineralogist, Volume 64, pages I0S2_1055, Ig79 Acentricity in the micas: an optical secondharmonic study Devn L. BrsH Department of GeologicalSciences, Harvard University Cambridge, M assachusetts 02I 3g RrcHnno S. Honspy eNo Ronnnr E. NnwNneu Materials ResearchLaboratory, pennsylvaniastate (Iniversity University Park, pennsylvania16g02 Abstract Optical secondharmonic analysishas been conductedon samplesof muscovite,phlogo- pite, biotite, margarite,ephesite, lepidolite, and zinnwaldite. Ephesite,margarites, uia.io- waldites gave appreciablesecond harmonic signals,indicating that theseminerals are non- centrosymmetric.These results provide the first physicalevidence for the absenceof a center of symmetryin the mineralsand supportpublished refnements of margariteand zinnwaldite. A very weak and questionablesignal in a2M.rlepidolite with a minor amount of admixed lM lepidolite suggeststhat the lM form may be acentric.The absenceof optical secondharmon- ics for the remaining samplesis fairly convincingevidence that they are all centrosymmetric. Theseresults are important in providing constraintson possiblecation ordering schlmes,and they illustrate the usefulnessof secondharmonic generationin detecting small deviations from centric symmetrywhich are difficult to observeby other techniques. group Cc instead of gave Introduction A/c an improved refine- ment and an ordered Al-Si distribution. An earlier Although the crystal structuresof micas have been study of margarite (Tak6uchi, 1965),based on the studied for many years, very few refined structures centrosymmetricspace grottp A/c, gave a structure exist in which the absenceof a symmetry center has with a completelydisordered arrangement of Al and been confirmed by physical tests.This is becauseAl_ Si, rather unusual for a structure having a I : I tet- Si ordering has a minor effect on the statistical tests rahedral Al-Si ratio. Thus, knowledgeof whether or for centrosymmetry (Bailey, 1966),and becausetra- not a crystal is centrosymmetriccan sometimesbe of ditional tests for acentricity such as piezoelectricity crucial importance,but suchinformation often is dif- are either not sensitiveenough or require fairly large ficult to obtain. crystals.As a result, when refining the structuresof Shortly after the discoveryof lasers,Franken et al. micas, crystallographerscommonly have employed (1961) found that optical harmonics were generated the ideal centrosymmetricspace-group symmetry by noncentrosymmetriccrystals when exposed to (Bailey, 1974).Assuming an ideal spacegroup im_ powerful laserbeams. This efect, called SecondHar- posesrestrictions on the number of symmetry-inde_ monic Generation (SHG), involves the emission of pendent tetrahedraand octahedra,precluding certain light from an acentricmaterial at twice the frequency types of cation ordering. For example, C2/m micas ofthe lasersource. have only one symmetry-distincttetrahedral site, and Although SHG has been shown to be a highly re- most common micas have only one independentin_ liable and sensitivephysical test for acentricity, it is terlayer cation site. only beginning to be used as a tool for the structural ln a few casessuccessful refinements have been characteirzation of minerals. Recent studies have ap- carried out in acentric subgroupsof the ideal space plied SHG to fresnoite(Bechthold et al.,l97g),bary- group, the work of Guggenheim and Bailey (1975, lite (Robinson and Fang, 1977), and several clay 1977) being an excellentexample. They showedthat minerals (Newnham et al., 1977).In addition to for margarite, CaAlr(Alrsir)O,o(OH)r,the space being appreciablymore sensitivethan the piezoelect- 00n3-{o4x/79/09 l0- I 052$02.00 to52 BISH ET AL.: ACENTRICITY IN MICAS 1053 FUNDAMENTAL SECOND HARMONIC ric effect,the SHG test can be applied easily to both FILTERS SAMPLE ON FILTERS GLASS SLIDE powders and single crystals,provided that acentric BEAM SPLITTER as quartz are not present.This paper i impurities such PHOTOMULTIPLIER presentsSHG data collectedon a number of micas to provide physical evidencefor the absenceofa center of ry--"try in the minerals and to demonstrate the usefulnessof the SHG experimentin detectingsmall deviationsfrom centrosymmetricsymmetry. Experimentalresults No complicated sample preparation is necessary for the SHG experiment,and all micas were exam- ined in both powder and single-crystalform, using the apparatusillustrated in Figure 1. The instrument is similar to that usedby Newnham et al- (1977),but the additional use of a signal integration technique has increasedthe sensitivity of the experiment over the previous [mits. A neodymium glass laser pro- vides coherent radiation at 1.06pm containing 50- 200 individual pulsesin about 10-' seconds.Optical filters block out unwanted backgroundradiation, al- lowing only the fundamentalwavelength to impinge Fig. l. Schematic diagram of the SHG instrument' upon the sample.Additional filters positioned in the exit beam transmit only the secondharmonic to the (001) plane, whereasa 3I photomultiplier. Signalsfrom the two photodetectors zinnwaldite lies within the its polarization direction per- are displayed on two oscilloscopes.A digital os- polytype must have (Horsey, in preparation)' There- cilloscope records the individual laser pulses from pendicular to (001) listed as a lM polytype in Table both the fundamentalreference beam and the second iore, zinnwaldite is lepidolite (sample B4l of harmonic. This allows comparisonof pulse to pulse l. The Czechoslovakian identified as the 2M, polytype intensities as well as time synchronizationbetween Cerny et al., 1970)was of the University of Wis- the secondharmonic and the fundamental. The sig- by ProfessorS. W. Bailey lepidolite is pre- nals are integrated and displayed on a second os- consin-Madison. The Swedish but powder diffraction cilloscope to give quantitative data on the signal dominantly tine2Mrpolytype, presence of a very minor magnitudes. patterns indicate the Table I lists the resultsobtained from a number of amount of the lM PolYtYPe. The margarites,zinnwaldites, and mica samples. Discussionand conclusions ephesite gave second harmonic intensities com- measurement parableto the signalsobtained for kaolin mineralsby The SHG results representthe first that margarite, Newnham et al. (1977).The Swedishlepidolite gave of a physical property demonstrating non-centrosymmetric, an extremely weak and questionablesignal at least zinnwaldite, and ephesiteare spacegroups for an order of magnitude abovethe noise level, and the and support the choice of acentric (Guggenheimand Bailey, remaining samplesgave no signalsdown to the limits -atg"tit" and zinnwaldite ephesitehas not yet of detection. 1975, 1977).The structure of (1956)found The mica polytypes were identified using X-ray been determined,but Smith and Yoder Africa, was powder and single-crystalinformation. Since X-ray that material from Postmasburg,South indications of powder diffraction patterns of the lM and' 3T tri- predominantly ttre 2M, polytype with polytype' Schaller et a/' octahedral micas are virtually identical, zinnwaldites a minor amount of a l2M sample from Post- which gave signals raised the question whether the (1967) reported that another group A/c or Cc' Our signal originated from the 3Z acentric form or an masburg had the space compositions of ephe- acentric subgroupof the lM form. A polarized laser results and the similarity in and margarite, beam experiment indicated that the electric vector in site, NaLiAlr(Al'Sir)O,o(OH)r, 1054 BISH ET AL: ACENTRICITY IN MICAS Table l. Results of the second harmonic analysis of selected micas and Bailey (1977) suggestedthat the octahedral or- Locality pattern and Sanple No. Polytype Signal dering observedrn C2znnwaldites should be found in all F-rich zinnwalditesand Huscovi te Chlqultos, lepidolites, but Bolivia, /198713 --1 No we find Muscovite Methuen, no convincingevidence from the SHG exper- O^Lario, llIl279I Mt No Phlogopire Edwards, york, iments that the 2M, and2Mrlepidohtes New i/96988 IM No which we ex- Phlogopite Lanark Co., ontario, i190776 IM No amined are acentric.Dr. StephenGuggenheim of the University of Illinois-Chicago (personal Phlogopite Llano, Circle com- Texas, S. Bailey 1M No munication, I'-PhlogopIte Synthetic, 1978)has reported that a structure re- R. Nemhan IM No Biotite Bloomingdale, finement of a 2M, N, J., il1i280g IM No lepidolite shows it to be centric Ep hesi te Hotazel, S. Africa, //l092l8 Nt Yes while the lM lepidolite is non-centrosymmetric.Ad_ mixture of a small amount Margarite Chester, of an acentric lM poly- t'lass., ll57I22 Nt Yes Margarlte Uolonville, pa., type with the 2M, Swedishlepidolite may /189086 --1 Yes account Lep idol i te Varutresk, Sweden, /192850 for the very weak and questionableSHG 2M2>>rM signal ob_ Lepldollte Biskupice, Czech., S. Bailey 2Mt No tained for this sample.Although theseresults suggest that lM lepidolite is Z innwald lte Zinnwald, acentric and2Mris centric,veri_ Gernany, /199553 1M Yes Zinnwald I te Zinnwald, fication by SHG must await examination Germany, /138912 IM Yes of a rela_ tively pure lM lepidolite. A11 numbered sanples vere obtalned from the llarvard Unaver_ sity Mineralogical The resultsfor muscovite particularly Museum. Other samples were obtained from are notewor_ the deslgnated sources. thy and suggestthat there is no ordering to an acent- ric subgroup.This agreeswith Guggenheimand Bai- ley (1975),who found that the atomic coordinatesof CaAlr(AlrSi,)O,o(OH)r, lead us to predict that eph_ an acentric model convergedto