American Mineralogist, Volume 76, pages 728-732, 1991

Energy calculationsbearing on biopyriboles

RrcH,lRD N. Annorr, Jn. Department of Geology, Appalachian State University, Boone, North Carolina 28608, U.S.A. Cnenr-ns W. BunNrunn Department of Earth and Planetary Sciences,Harvard University, Cambridge, Massachusetts02138, U.S.A.

Ansrn-tcr

Energy calculations were used to evaluate the stability relationships for low Ca, low Al biopyribole polysomesin the compositional rangedefined by (010) pyroxeneslabs (P slabs) with composition near (Mg,Fe)oSioO,,and (010) mica slabs (M slabs) with composition near that of , (Mg,Fe).SioO,o(OH)r.The calculations were performed using results of the structure refinementsof two orthopyroxenes(Opx), clinoenstatite(Cpx), twoampiboles (: An, : Cu), (Ch), jimthompsonite (Jt), clinojim- thompsonite, and talc (Tc). The energy calculations show the following. (l) For each in- termediate polysome, the energy is lower than the energyof a compositionally equivalent assemblageof Tc + (Opx or Cpx). (2) Assemblagesof Jt + Tc and Jt + (Opx or Cpx) have the lowest energies,all other assemblagesbeing metastable.Thus the enirgy calcu- lations indicate that Jt could be a stablephase. (3) The energeticallypermissible hydration reactions are generally consistent with reactions interpreted from textural observations. Only one kind of observed hydration reaction, (An or Cu) - 61r, could not be predicted from the energy calculations.

INrnooucrroN compositional range include the (MMPMp) polysome chesterite, and two (MMp) polysomes,jimthompsonite Biopyriboles form an important polysomatic series.In- and clinojimthompsonite lviUten and Burnham, 1977a, dividual structuresin the seriesconsist of (010) structure 1978b). Chesterite contains both triple chains of Si tet- modules,MandPslabs,whicharerespectivelylikethoserahedra (... PMMP...) and doubie chains(... pMp of 2:l mica [e.g.,talc, MgrSioO,o(OH)r]and pyroxene(e.g., . . .). Jimthompsonite and clinojimthompsonite have only enstatite, MgoSioo,r)(Thompson, 1970, 1978; veblen et triple chains of si tetrahedra. al., 1977; Veblen and Burnham, 1978a, 1978b). The Although it is well known that are stable polysomatic model is particularly useful for understand- under apfropriate conditions of p, T, and,a (H,O), it is ing the crystal chemistry of real and hypothetical biopy- not so clear ihat the relatively recently discovered ches- riboles becauseintermediate biopyriboles, containing both terite, jimthompsonite, and clinojimthompsonite (Veb- M and P slabs,have properties that relate to the physical len and Burnham, 1978a, l978bi necessarilycrystallize and chemical properties of the corresponding mica and as stableminerals. Certainly the relative oUscurityof nat- pyroxene end-members. Atomic sites in a P slab retain ural (MMPMP) and (MMP) polysomes, and occurrence nearly the samelocal environments that they would have as minute grains or lamellae in u hort of pyroxene, am- in the end-member pyroxene. Likewise, atomic sites in phibole, oimica, might suggestthat they are metastable. an M slab retain nearly the same local environments as Textural evidence(Veblen and Buseck,198 l) showssome the correspondingsites in the end-member2:1 phyllo- systematicrelationshipsinvolvingthesuccessionofpoly- silicate. Thus, cation site preferencein a given polysome somesduring the progressivehydration of either pvroi- can be readily anticipated by analogy with site prefer- ene or an , but the ieaction mechanisms are encesin minerals correspondingrespectively to the P and complex and remain poorly understood. M slab parts of the polysome. In an interesting analysis using the axial next-nearest- Perhapsthe widest variety of natural biopyriboles are neighbor ising (ANNNI) mode-I, price and yeomans defined by P and M slabs having compositions respec- (1986) show that jimthompsoniteor clinojimthompson- tively close to (Mg,Fe)oSioO,r,and (Mg,Fe)rSioO'o(OH)r. ite is very likely siable u.rd", uppropriateionditions (p, Enstatite and clinoenstatite would be examplesof the . . . ?1 composition), but chesterite1s probably metastable (P) IPP . . . , or polysome, where the parenthesesen- under any conditions. The qualitative relaiionships in_ close one cycle of the repeating sequenceof M and P dicate that the conditions foi the stability of jimthomp- slabs.Talc would be the (M) polysome. The amphiboles, sonite or clinojimthompsonite should be fairly restricted. anthophyllite and cummingtonite, are the (MP) poly- In the present investigation, structure energy calcula- somes. Other naturally occurring polysomes in the same tions weri used to gain some insight into the relative 0003-004x/9l/0506-0728$02.00 728 ABBOTT AND BURNHAM: BIOPYRIBOLE ENERGY CALCULATIONS 729 stabilities of the different polysomesin the pyroxene-talc Chesterite (MMP), jimthompsonite (MMPM), polysomatic series.Specifically we set out to determine clinojimthornpsonite(MMPM) whether or not any of the polysomes, chesterite, jim- For thesebiopyriboles we usedthe only available struc- thompsonite, and clinojimthompsonite could have a low- ture determinations (Veblen and Burnham, 1978b). er energythan compositionally oquivalent assemblagesof pyroxene + talc, or amphibole + talc. Talc (M) The following abbreviations are used in this paper: or- The Mg/(Fe + Mg) fraction is essentially I for the talc : : thopyroxene Opx, clinopyroxene Cpx, anthophyllite samples whose structures were refined by Rayner and : : : An, cummingtonite Cu, chesterite Ch, jimthomp- Brown (1973) and Perdakatsisand Burzlaff (1981). By : : : sonite Jt, clinojimthompsonite cJt, and talc Tc. analogy with our energy calculations on orthopyroxene and orthoenstatite. the lack of a structure refinement for Srnucrunns talc with a suitably comparable Mg/(Fe + Mg) fraction The energycalculations were limited by the availability should not afect our conclusions.We used the more re- of suitable structure determinations. Whereas structure cent structure refinement of talc by Perdakatsis and Burz- determinations are available for a wide range of amphi- latr ( 198I ) primarily becausethe refinement provided an boles (Hawthorne, 1983) and pyroxenes (Cameron and H site. Otherwise results of the refinement are not signif- Papike, 1980), structure determinations are available for icantly different from those ofRayner and Brown (1973). only three polysomes containing triple chains of Si tet- Talc crystallizesin spacegroup Cl, in such a way that rahedra, jimthompsonite, clinojimthompsonite, and the stacking of (001) unit layers (polytype 1Ic) is very chesterite(Veblen and Burnham,1978a,1978b),and for different from the way in which comparable units are only two specimens of talc (Rayner and Brown, 1973; stacked in the M slabs of an intermediate biopyribole. Perdakatsisand Burzlafl l98l). Further complicating the The M slabs in cummingtonite have C2/m symmelry analysis,the talc compositions are not strictly appropri- consistent with IM polytypic stacking, whereas the M ate for a comparison with the M slabsin jimthompsonite, slabsin anthophyllite have orthorhombic symmetry con- clinojimthompsonite, and chesterite.Like talc, the inter- sistent with 20 polytypic stacking.Therefore, in addition mediate polysomesCh, Jt, and cJt have only tracesof Al to calculating the energy of the observed talc-1lc struc- and Ca; unlike talc, the Mg/(Fe + Mg) fractions in Ch, ture, we calculated the energy for hypothetical C2/ m talc- Jt, and cJt are in the range 0.70-0.78. Within these lim- IM, which we fabricated from a unit layer of the ob- itations, we sought low Ca, low Al structureswith similar served talc-lTc structure. This was done in an effort to Mg/(Fe * Mg) ratios. gain a better appreciation for the actual contribution by the M slabs to the cohesive energy of each intermediate Pyroxene (P) biopyribole. No one pyroxene structure, of those we considered (Cameronand Papike, 1980;Ohashi, 1984),had a satis- Mnrrrol factory Mg/(Fe + Mg) ratio for comparison with Ch, Jt, Energy calculations were performed using the comput- and cJt. Consequentlywe used three structure determi- er program WMIN (Busing, 1981).For a given structure, nations spanning the range of Mg/(Fe + Mg) in Ch, Jt, the program calculated the cohesive energy,given by 'C, and cJt. The structures were of orthopyroxene [20 Mg/(Fe + MC) : 0.321determined by Smyth (1973), syn- tl thetic orthoenstatite [Mg/(Fe + Me) : l] and synthetic W-, : : ) f z,z,e,/r,+ i 2 ) t,,exp(-r,,/ p). oi+j j 'i+i clinoenstatite [Me/(Fe + MC) : l], both determined by i Ohashi(1984). The first sum is the Coulomb electrostaticcontribution, where z, and z, are formal valences on ions i and J, e is Amphiboles (MP) electrostatic charge, and r,, is the interionic separation. For amphibole structureswe used that of anthophyllite The summation is over all ions in the crystal, and the determined by Finger (1970, as reported in Hawthorne, factor t/zaccounts for the fact that each interaction is in- 1973) and,that of cummingtonite determined by Fisher cluded twice. The second term is the sum over short- (1966, as reportedin Hawthorne, 1983).The Mg/(Fe + range repulsive potentials, with coefficients tru and p, Mg) fractions in these minerals were 0.62 for the cum- charactoristic of each interacting ion pair; summations mingtonite and,0.79for the anthophyllite. The CaO con- are over all nearest neighbor cation-anion interactions tent of the cummingtonite was 2.2 wto/o,which translates and all nearestneiglrbor anion-anion interactions. to 0.35 Ca per 24 O atoms and is somewhat higher than For Mg-O, Si-O, and O-O interactions, we employed would be desiredfor our purposes.The chemical analyses short-rangecoefrcients determined by Post and Burnham for anthophyllite reported no Ca. Ratios of totAVSifor (1986) from modified electron-gastheory (MEG). Each both amphiboles were only slightly higher than the very O ion is surrounded with a spherical shell of charge *2 low tatAVSiratios in chesterite,jimthompsonite, and cli- in the MEG procedure. The potential of the shell com- nojimthompsonite (Veblen and Burnham, 1978a). pensatesfor crystal field effectson the O chargedensity. 730 ABBOTT AND BURNHAM: BIOPYRIBOLE ENERGY CALCULATIONS

TABLE2. Calculateddissociation energies

(kJ/mol Polysome of O) Structure determination Orthoenstatite -4635 Ohashi,1984 Clinoenstatite -4634 Ohashi,1984 MO M Orthopyroxene(20 "C) -4635 Smyth,1973 Anthophyllite -4631 Finger,1970 Cummingtonite -4632 Fisher,1966 Chesterite -4625 Velben and Burnham, 1978b -4634 P Jimthompsonite Veblenand Burnham,1978b Clinoiimthompsonite -4631 Veblenand Burnham,1978b Talc-1Tc -4616 Perdakatsisand Bruzlaff,1981 Fig. 1. Differentkinds of O atomsin M and P slabsof a Talc-lM -4610 This study biopyribole.Ob : bridgingO atom(Si-O-Si), Onb : nonbridg- ing O atom(Si-Or6rM), and Oh : hydroxylO atom.

Treatment of H in the various polysomes containing The shell potential, V"he +2/r"h), is adjusted by varying M modules posedtwo specialproblems. Short-rangeH-O the shell radius, r"n.The short-rangeenergy resulting from potentials cannot be obtained from MEG theory, and ob- an O atom interacting with other ions (including other O servedH positions are available only for talc (Perdakatsis ions), defined by tr and p, varies with shell potential. The and Burzlafl 1981). With regard to the first problem, for appropriate shell radius for O atom yields a shell poten- O-H interactions we used an empirical short-range po- tial that is equal to the Coulomb potential at the O site tential from Abbott et al. (1989). with regard to the sec- in the crystal (Post and Burnham, 1986; Muhlhausen and ond problem, it was necessry to calculate hypothetical Gordon, l98la, l98lb). H'* positions for all of the intermediate polysomes.The All octahedral sites were considered to contain Mg2*, procedure involved a search, using WMIN, for the H since no Fe2*-Oshort-range potentials are available. The position(s) of lowest energy. During the search, the po- presenceofFe2* in a site will affect the calculatedenergy sitions of all other atoms were fixed. Table I gives the only to the extent that the polyhedral geom€try is affect- calculatedH positions. We also calculateda hypothetical ed. H position for talc-lTc, which may be compared with Energy calculations by one of us (C.W.B.), by Ohashi the observed position (Perdakatsisand Burzlafi l98l). (1984),and by Smyth (1989) show that the O atoms of ln talc-lTc, the OH dipole orientation for the calculated low Ca, low Al biopyriboles can be categorizedconve- H site is nearly perpendicular to (001), consistent with niently by site potential as nonbridging P-module O at- the observedorientation. However, becausethe observed oms (Fig. l) with V : 1.83 eZA, bridging O atoms in H-O distancein talc seemsto be questionably short (0.84 either P or M moduleswith Z: 2.ll e/4, and nonbridg- A), and for the sakeofinternal consistency,in subsequent ing M-module O atoms or OH O atoms with Z: 1.98 energy calculations on talc-lTc we used the calculated, e/A. Appropriate shell radii for the three types are 1.09 hypothetical H position. In anthophyllite and cumming- A,0.95 A, and 1.01 A, respectively;values fortr and p tonite the calculatedO-H distancesand OH orientations, were selectedaccordingly from data given by Post and nearly perpendicular to (100), are consistent with ob- Burnham (1986). served relationships in tremolite (Hawthorne and Grun- dy,1976). The calculatedcohesive energies (Table 2) include self- Tler-e't. CalculatedH positions energyterms for O ions. Theseterms representthe energy Polysome H-o(A) differencebetween free ions and shell-stabilizedions. The Anthophyllite Ha 0.23345 0.25000 0.56023 0.9514 self-energyterms also include the energyfor the reaction Hb 0.01768 0.25000 0.22827 0.9599 02- - O1- + e, since 02- is unstable as a free ion (Post Cummingtonite H 0.21581 0.0 0.74267 0.9530 and Burnham, 1986). Chesterite Hta 0.23503 0.30035 0.31540 0.9598 Htb 0.51791 0.30106 0.01271 0.9609 Hda 0.26762 0.49956 0.81022 0.9511 Hdb 0.48524 0.50036 o.47464 0.9596 Rrsur,rs Jimthompsonite Ha 0.23467 0.33543 0.06311 0.9571 The calculated dissociation energies(Table 2) are rep- Hb 0.51700 0.33519 0.76679 0.9614 Clinoiimthompsonite H 0.21809 0.08539 0.22207 0.9570 resentedgraphically in Figure 2. Assuming the structures Talc-1Tc H 0.72532 0.66862 o.21525 0.9599 are correct, the dissociation energies would bo exact (observedvalues) (0.719) (0.66e) (0.203) (0.84) (within the limits of computing precision), except for un- Talc-lM H 0.69845 0.70238 0.21519 0.9593 certainties in the shell radii of the different kinds of O /Vote: H coordinates refer to structure determinationsas follows: an- atoms. If the shell radius for a particular kind of O atom thophyllite(Finger, 1970); cummingtonite (Fisher, 1966); chesterite, jim- thompsonite, clinoiimthompsite(Veblen and Burnham, 1978b); talc-tlc is changedby a small amount (e.g., *2o/o for an Onb-O (Perdakatsisand Burzlafi, 1981);talc- tM(this study; structureparameters atom in enstatite the effect on the calculateddissociation can be obtained from authors). energyis small ( + 3.5kJ/anion for enstatite;i.e., < a 0. I 7o), ABBOTT AND BURNHAM: BIOPYRIBOLE ENERGY CALCULATIONS 731

-4610 Tc'1M

Tc-1Tc -4620 n

aql An ch Jt (kJ) Cu cJt -4630 cpx *E O,, opx M/(P -4640 Fig.3. Schematicrepresentation of permissiblehydration re-

IMP] IIMMP] actionsfor polysomeson the pyroxene-talcjoin. IMMPMP] - ---> M/(P+M) l. Each of the polysomes,An, Cu, Ch, Jt, and cJt, has Fig. 2. Calculatedenergies (kJ/mol of O) of the diferent a lower energythan a compositionally equivalent mixture polysomes,plotted against fraction M/(P + M) of (010)M and of Opx * Tc or Cpx + Tc. During hydration, the replace- P slabs.Abbreviations: Opx : orthopyroxene,Cpx : clinopy- ment of Opx or Cpx by any one of the intermediate poly- roxene,An : anthophyllite,Cu : cummingtonite,Ch : ches- someswould be energeticallyfavorable inasmuch as the terite,Jt : jimthompsonite,cJt : clinojimthompsonite,and Tc energy ofthe product would be lower than the energy of : talc. a compositionally equivalent assemblageof Opx * Tc or Cpx + Tc. We will refer to such favorable replacement reactionsas permissible.With regardto Opx and Cpx the but more importantly, acrossthe polysomatic seriesthe permissible reactions include Opx - An or Cpx - An, effectis proportional to the M/(P + M) fraction. In Figure Opx - Cu or Cpx - Cu, Opx - Jt or Cpx - Jt, Opx - 2, this means that the slope of the line connecting the cJt or Cpx - cJt, and Opx - Ch or Cpx - Ch. dissociation energiesof enstatite and talc-lTc could be 2. The phasescJt and Jt each have a lower energythan slightly more or less steep and translated up or down a a mechanical mixture of An + Tc or Cu * Tc; thus the small amount, but the energiesfor the other polysomes permissiblehydration reactionsare An - cJt or Cu - cJt would be distributed in the same way relative to the en- and An - Jt or Cu - Jt. statite-talc line. Hence, the quantitative aspects of the 3. The phasescJt and Jt each have lower energiesthan results are relatively insensitive to small uncertainties in a mechanical mixture of Ch + Tc, thus two more per- the shell radii of the O atoms, whereasthe more impor- missible reactions are Ch - cJt and Ch - Jt. tant qualitative aspectsare for practical purposesinsen- 4. Any mechanical mixture of Jt + Tc has a lower sitive to the uncertainties. energy than a compositionally equivalent mixture of Tc Energiesfor the three varieties ofpyroxene are nearly and any other polysome; thus the final permissible reac- the same (Table 2, Fig. 2). Thus, at least within the lim- tion is Jt - Tc. Energiesfor mixtures of cJt + Tc are not itations of the present approach, Fe-Mg exchangewould much higher than those of compositionally equivalent seem to have little influence on the dissociation energy. mixtures of Jt + Tc. Except for assemblagesof Jt + Tc, As expected,the differencein the energiesof monoclinic mixtures of cJt + Tc have lower energiesthan composi- and orthorhombic polymorphs at specific polysomatic tionally equivalent mixtures of Tc and any other poly- M/P ratios (i.e., pairs Opx-Cpx, An-Cu, or Jt-cJt) is very some. Thus, except for the polymorphic transformation small, in generalsignificantly smaller than the energydif- cJt r Jt, the reaction cJt - Tc would be permissible. ferencesbetween polysomeswith different M/P ratios. Figure 3 summarizesthe permissible reactionsjust de- It is perhaps worth noting that our hypothetical talc- scribed. For simplicity we have omitted reactions to talc lM has a significantly higher energy than natural talc- that may be energetically favorable, but produce mixtures lTc. The IM configxation of the M slabs in clinoam- that have higher energies than chemically equivalent phiboles and clinojimthompsonite, and 20 configuration mixtures of Jt + Tc. The relationshipscompare favorably of the M slabsin orthoamphiboles,jimthompsonite, and with the petrographicobservations of Veblen and Buseck chesteritemust be stabilized at least in part by adjacent (1981), specifically their Figure 28. Only one important P slabs.As self-evident as this point may seem,the high replacement,(Cu or An) - 66, is not supported by the energy of the hypothetical talc- I M suggestsfurthermore calculatedenergies. The calculations suggestthat the sta- that there may be a natural limit to the width of chains ble assemblagesare Jt + Opx and Jt * Tc, all other of Si tetrahedra in intermediate polysomes-a limit (per- assemblagesbeing metastable. hapstriple chains)beyond which the structurewould oth- erwise tend to disrupt into discrete domains of lalc-lTc CoNcr,usroNs and a double or triple chain polysome. l. Using the method of Abbott et al. (1989),realistic Other important results are as follows: H positions can be calculated for biopyriboles. 732 ABBOTT AND BURNHAM: BIOPYRIBOLE ENERGY CALCULATIONS

2. For each intermediate polysome, the energyis lower Hawthorne, F.C. (1983) The crystal chemistry of the amphiboles. Cana- than the energy of a compositionally equivalent mixture dian Mineralogist, Zl, 173-480. Hasthorne, F.C., and Grundy, H.D. (1976) The crystal chemistry of am- ofpyroxene and talc. phiboles. IV. X-ray and neutron refinementsof the of 3. The energeticallypermissible hydration reactionsare tremolite. Canadian Mineralogist, 14, 334-345. generally consistent with reactions interpreted from tex- Muhlhausen, C., and Gordon, R.G. (198ta) Electron-gastheory of ionic tural observations.Only one kind ofobserved hydration crystals,including many-body effects.Physical Review, B23, 900-923. - (l Density-functional for the energyof crystals:Test reaction, amphibole - chesterite,could not be predicted 98 lb) theory ofthe ionic model. Physical Review, B24, 2147-2160 from the energycalculations. These relationships are con- Ohashi, Y. (1984) Polysynthetically-twinnedstructures of enstatite and sistent with the ANNNI model (Price and Yeomans, wollastonite. Physicsand Chemistry of Minerals, lO,217-229. 1986), which indicates that jimthompsonite or clinojim- Perdakatsis,B., and Burzlaff, H. (1981) Strukturverfeinerungungam Talk - thompsonite may be stable, but chesteriteis most likely Mg,[(OH),Si.O,o].Zeitschrift fiiLrKristollagraphie, | 56, 177 | 86. Post, J.8., and Burnham, C.W. (1986) Ionic modeling of mineral struc- metastable. tures and energiesin the electrongas approximation: TiO' polymorphs, Our results demonstrate a relatively simple and prac- quartz, forstente, diopside. American Mineralogist, 7 1, 142-1 50. tical application ofenergy calculations to the interpreta- Price, G.D., and Yeomans, J. (1986) A model for polysomatism. Miner- tion of phase relationships. We believe that the method alogicalMagazine, 50, 149-156 (1973) of talc. Clays may be equally useful in other systems. Rayner, J.H, and Brown, G. The crystal structure and Clay Minerals, 13,73-84 Sm].th, J.R. (1973) An orthopyroxene structure up to 850'C. American AcrNowr,nncMENTs Mineralogist,58, 636-648. - (1989) Electrostatic characterizationof oxygen sites in minerals. We wish to thank the reviewers, G.D. Price and R.F. Giese, Jr., and Geochimicaet CosmochimicaActa. 53. 1101-1110 specialeditor J B. Brady for their helpful comments. The work was sup- Thompson, J.8., Jr. (1970) Geometricalpossibilities for amphibole struc- ported by NSF grant EAR-8720666 to C.W B tures: Model biopyriboles (abs.).American Mineralogist, 55, 292-293. - (1978) Bipyriboles and polysomatic series.American Mineralogist, RnrnnrNcns crrED 63,239-249. Veblen, D.R., and Burnham, C.W. (1978a)New biopyriboles from Ches- Abbott, R.N., Jr., Post, J.E., and Burnham, C.W. (1989)Treatment of ter, Vermont: I. Descriptive mineralogy. American Mineralogist, 63, the hydroxyl in structure-energy calculations. American Mineralogist, 1000- l 009 74, 141-150. -(1978b) New biopyriboles from Chester,Vermont: II. The crystal Busing, W.R. (1981) WMIN, a computer program to model molecules chemistry ofjimthompsonite, clinojimthompsonite, and chesterite,and and crystalsin terms of potential energyfunctions. U.S. National Tech- the amphibole-mica reaction. American Mineralogist, 63, 1053-1073. nical Information Service,ORNL-5747. Veblen, D R., and Buseck,P.R. (198l) Hydrous p1'ribolesand sheetsili- Cameron,M., and Papike,J.J. (1980) Crystal chemistry of silicate pyrox- catesin pyroxenesand uralites: Intergowth microstructuresand reac- enes.Mineralogical Society of America Reviews in Mineralogy, 7, 5- tion mechanisms.American Mineralogist, 66, 1107-1134. 92. Veblen,D.R., Buseck,P.R., and Bumham, C.W. (1977)Asbestiform chain Finger, L.W. (1970) Refinement of the crystal structure of an anthophyl- silicates:New minerals and structural groups.Science, 198, 359-365. lite. CarnegreInstitution ofWashington Year Book, 68, 283-288. Fisher, K.F. (1966) A further refinement of the crystal structure of cum- Mer*uscnrrr REcETvEDJervum 31, 1990 mingtonite, (Mg,Fe),SirOrr(OH)r. American Mineralogist, 5 l, 8 I zt-8 I 8. Ma,wuscnrsr ACcEFTEDFnnnu.my 24. 1991