INERALOGIST. VOL. 51. NOIIEMBER_DECEMBER. 1966

THE CRYSTAL STRUCTURE OF FLUELLITE.AlzPO4Fr(OH).7HrO

BnraN B. Guvr aNl G. A. Jolrnov, The CrystallographyLaboratory, The Uniaers,ity of Pittsburgh, Pittsburgh, Pa. 15213.

ABSTRAcT

The crystal structure of the mineral fluellite has been determined with the use of three-dimensional r-ray photographic data. The composition, previously reported as that of analuminum hydroxy-fluoride monohydrate, was found to be AlzPOrFz(OH).7H:O. Thepresence of phosphorus in the structure was confirmed by electron microprobe analysis.The space group is Fdd.d. with unit cell dimensions a:8.546, b:11.222 and c:21.158 A,and Z:8. The phase problem was solved by the interpretation of a multiple minimumPatterson function and of its convolution with the original Patterson synthesis. The finalR-factor for 468 observed reflections is 0 12. The structure is that of infinite chains ofcorner-sharing [AIF2.O4.HB 5] octahedra linked through [POr] tetrahedra, forming chan-nels which contain water molecules The water molecules and hydroxyl ions are hydrogen-bonded in a way which permits proton configurational alternatives. Of the four oxygenatoms in the alurrinum octahedra, two are shared with the [PO+] tetrahedra and the othertwo are statistically one quarter that of a hydroxyl ion and three quarters that of a watermolecule.

INrnooucrroN

The mineral fluellite has been described from only a limited number oflocalities, chiefly in southern England and Germany, where it occurs insmall quantities in granitic regions as minute dipyramidal crystals liningcavities or veins. It was originally named and described in 1824by L6vy,who observed it on qlrartz associated with fluorite, wavellite, arsenopy-rite and torberite. No quantitative chemical tests were attempted on thesmall amounts of material available, and L6vy only reported the presenceof aluminum and fluorine. No further chemical data were presented until1882 when Groth recorded the following percentage composition; F56.25, Al 27.62,Na 0.58, H2O by difference 15.5, and proposed the for-mula AIFa.HzO.In1920, Laubmann and Steinmetz described a mineral,kreuzbergite, from the Oberpfalz, Bavaria, occurring with phospho-siderite and strengite. It was described as an aluminum phosphate chieflyon the basis of crystal morphology and its association with other hv-drated aluminum phosphates. Several further reports of kreuzbergite fol-lowed, and Larsen and Berman (1934) noted the similarity of this min-eral with fluellite. In 1940, Scholz and Strunz carried out qualitativechemical tests on kreuzbergite and concluded that is was the same asfluellite. They obtained only 1 to 2 per cent of phosphorus by precipita-tion as the ammonium phosphomolybdate. X-ray powder data confirmed

1 Present address: Dept. of Geology and Geophysics, University of Sydney, NSW,Australia.

1579

1580 B. B . GUY AND G. A . JEFFREY

the structural similarity of the two minerals and small compositional dis-crepancies were attributed to variation in the (OH) and F ionic ratio.The name kreuzbergite was then discredited and a composition ofAl(F*.OHt-*)r.H2O, where x(1.0, was accepted for f luell i te. Scholz andStrunz postulated that f luell i te is a secondary product formed whenpyrite breaks down with acid solutions attacking fluoro-phosphates (e g.apatite and triplite) and alumino sil icates.The only additional data wasthat of Palache, et al. (1951) who report a personal communication fromC. W. Wolfe on the cell dimensions and space group.

The present crystal structure determination was undertaken in the be-lief that the mineral was a hydrate of aluminum fluoride, since severalsuch hydrates have been described, but the l iterature is somewhat con-tradictory (cf Simons, 1950) and the monohydrate is unknown as alaboratory product. We found that fluellite is a hydrated aluminumfluoro-phosphate and believe that both minerals described as kreuzberg-ite and fluellite are in fact one species, with minor compositional vari-ations. The difficulty in obtaining sufficient quantities of fluellite for de-tailed chemical analysis was presumably the principal factor causing theconfusion in composition.

Cnvsr.qr Der,q

The specimens of fluellitel that were selected for structure determina-tion were small ((0.15 mm), colorless, dipyramidal crystals Iining thesurf ace of a somewhat altered granitic rock. Onl.v the { 111 } form was ob-served. The crystals have orthorhombic symmetry with o:8.546+0 .008 , b :11 .222+0 .005 , and c :21 .158+0 .005 A , and V :2029 43 .The measured density was D-:2.18 g. cm-3, in agreement with the cal-culated value of D*:2.16 g. co-3, with the molecular formula, AI2PO4F2(OH) .7HrO and Z:8. The space group is uniquely determined as Fdd,dfrom systematic absences, (hkl) with h+k, k,+1, l+h+2n; (0kl) withk*ll4n; (hjl) with kll l4n; (hkj) with hfkl4n; and (/200), 0ft0),(001) with hl4n, kl4n and l+4n respectiveiy. The refractive indicesare or : l.a7 8 ; B : 1.490 ; r : 1.5 10, with a : a, ̂ y : c.

ExpBntltpNt,q,t

The unit cell dimensions were determined by diffractometer methods,and the intensity data were recorded photographically with equi-inclina-lion Weissenberg techniques using CuKa radiation. The intensities wereeye-estimated, by comparison with a calibrated scale, from multiple-filmphotographs for layers zero to eight about the b-axis and zero to fifteen

1 From Cornwall, England. Specimen (Catalogue No C1028) on loan from Smithsonianlnstitution, Washington, D. C.

STRUCTURE OF FLUELLII-E 1581

about the c-axis. The 855 measured reflections were assigned values from1 to 20,000 and the conversion of intensities to structural amplitudes wascarried out with a series of IBM 7070 programs (l,IcMullan, 1964) withleast squares correlation of the equivalent reflections occurring alongboth the D and c-axes. Due to the space group extinctions, only reflectionof the type ggg and uuu ate present, so that it was impossible to obtaindirect correlation between these two sets of data without informationfrom an oblique axis. However, because "even" and "oddt' layers were re-corded alternately along both the 6 and c axes, a scale factor of one couldbe assumed between these sets of data without giving rise to errors sig-nificant enough to hinder the solution to the phase determination. Thisscale factor was subsequently redetermined more precisely in the refine-ment stage of the structure deterrnination. The number of observed in-dependent reflections was 468, representing approximately 83 per centof the limiting sphere for CuKa radiation. An additional 72 reflectionswere recorded as unobserved and given intensity values of one half theIowest observable intensity. Approximately 75 per cent of the unobservedreflections were in the sin d range of 0.7 to 1.0. The Wilson statisticalmethod was used to place the observed structure factors on an absolutebasis and to obtain approximate isotropic temperature factors. No cor-rection was made for absorption since the value of trr.r was about 1.0. Thecrystal density was determined by flotation methods in a mixture ofbromoform and alcohol.

Tnr SrnucruRE DETERMTNATToN

Initially it was assumed that the molecular formula was approximatelyAlF3'H2O (with Z:24 or 32), although it was evident that this exactstoichiometry was incompatible with the cell dimensions, density andspace group requirements. The presence of phosphorus in the structurewas not suspected until the bond distances and electron density distribu-tions indicated that not ail the cations could possibly be aluminum. Thetrue composition of the mineral was therefore unknown until after thesolution of the phase problem, when it was revealed in the process of therefinement calculations.

An examination of the Harker sections of a three-dimensional Patter-son synthesis suggested two alternatives for a pair of independent atoms:these were the combinations oI the equivalent positions of 8(a) and 16(c)or of 8(b) and 16(c). Assuming that the atoms were aluminums, com-bination (1) was selected on the basis of a better agreement between ob-served and calculated structure factors, but further interpretation by in-spection of the Patterson function was not possible. Difference Fouriersynthesis based on this partial structure revealed two lighter atoms, (4)

1582 B. B. GUY AND G. A. JEFFREY

and (5), in the 16(g) and 32(h) positions. The former corresponded to atetrahedral arrangement about the aluminum atoms at the eight-fold(8a) position. The inclusion of these atoms resulted in little improvementin the disagreement index R, which was discouraging and further inter-pretation of the Fourier synthesis was not possible.

The solution of the Patterson function was then approached more sys-tematically using the method of multiple superposition of the vectorshifted function to calculate a minimum function (Buerger, 1951).Vector shifts corresponding to the aluminum sites originally located weremade and a three-dimensional multiple minimum function (MMF) wascalculated, using the programs of Corfield (1965a) for the IBI{ 1620.This function de.frnitely confirmed the position of the two sets of lighteratoms obtained previously, i.e. (4) and (5). Further vector shifts involv-ing these two atoms revealed a fifth set of atoms, (3) in the 32(h) posi-tions. The combinations of the positions (3), (4) and (5) now completedoctahedral and tetrahedral arrangements of anions about the heavier(supposedly aluminum) atoms. Despite the fact that five out of six of thesymmetry independent atoms, (excluding hydrogens), had been located,the R-factor was sti l l high at 0.46 and further vector shifts failed to re-veal addit-ional atomic positions. The original Patterson function wasthen convoluted with the I{NIF using the IBM 1620 program of Corfield(1965b), which computes the value ). 'p(r-r ').o(r') at various values ofr' over the unit cell, where p(r) and o(r) represent the two three-dimen-sional periodic functions (ai,z. the Patterson and MMF). The peaks for thetwo heavier atoms and the three lighter atoms were considerably sharp-ened and a new peak (6) appeared above the background. Structurefactor calculations with aluminum for the heavier atoms and oxygen orfluorine for the lighter atoms reduced the R value to 0.35. The differenceFourier syntheses then indicated that all the atomic sites had probablybeen located. The formula unit at this stage was thought to be eitherAlaF1o' (H) '4HrO or AlaFsOz.(3H).4HrO, so as to approximate asclosely as possible a hydrated aluminum fluoride, with the appropriatenumber of hydrogen atoms necessary to achieve neutrality at undeter-mined positions.

Tur Srnucruno RBlrNBlroNr

Full matrix least squares refinements of the structure were carried outon the IBX{ 7090 using a modification by Shiono (1965) of the programby Busing et al. (1962). Four cycles of refinement on the positional param-eters and a scale factor reduced R to 0.20, and a further four cycles onthe positional parameters with'isotropic temperature factors and a dif-ferent scale factor for the ggg and uuu reflections lowered R to 0.15. The

STRUCTURE OF FLUELLITE

temperature f actor for the aluminum atoms in the 8(a) positions then be-came negative. fnteratomic distance calculations showed that theseatoms had four neighbors at 1.55 A, which was too short for Al-F or Al-Odistances. Difference Fourier syntheses also suggested that there weremore electrons at these sites than those assumed for aluminum. Alternatecations were therefore either silica or phosphorus, either of which could betetrahedrally coordinated to the four surrounding oxygen or fluorineatoms. An electron microprobe analysis of the specimen for aluminum,sil icon and phosphorus showed that Si was entirely absent and AI and Pwere estimated at 17 and 10 weight per cent respectively. These values

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1 583

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Fro. 1. Electron density distribution of anions in fluellite. The distribution for fluorine,

F(5), is significantly different from that of the oxygens.

agreed reasonably well with the theoretic al 16.4 per cent Al, by occupyingthe (16c) sites, and 9.4 per cent P, by occupying the (8a) sites in thestructure.

With the introduction of phosphorus into the tetrahedral sites, therewas a further improvement in the structure factor agreement and theP-O or P-F distances of the surrounded tetrahedron were 1.52 A. Sincethe [PFa]- ion is a very unlikely mineral ion (Wells, 1962, p.643), it wasassumed thzrt a phosphate group was present. The electron density dis-tribution at the anion sites was then carefully examined in the three-dimensional Fourier syntheses. A very similar distribution was observedfor atoms (3), (4) and (6), but atom (5) had a significantly sharper, andgreater electron density, as shown in Fig. 1. On this basis atoms (3), (4)

1584 B. B. GUY AND G. A. JEFFREY

and (6) were identified as oxygens and (5) as the fluoride ion. These as-signments resulted in an improved electron density on the difference syn-thesis as well as more appropriate temperature factors for the cations andthe fluoride ion. A sensible formula and hydrogen-bonding scheme couldthen be postulated for the structure consistent with the composition ofAl2PO4Fr(OH)'7HrO. One hydrogen atom was located directly on a finaldifference Fourier synthesis in a 32(h) position between O(a) and O(6)and this corresponded to four of the fifteen hydrogens in the formula unitgiven above.

For the terminal cycles of least squares refinement, a correction foranomalous dispersion for the AI and P atoms was included. Althoughsmall, the fact that these atoms are in special positions appears to en-hance the importance of this correction, which significantly improved theelectron density distributions and prevented some of the anisotropic tem-perature factors from becoming negative. The atomic parameters andtheir standard deviations are l isted in Table 1. The corresponding ob-served and calculated structure factors are l isted in Table 2.1 Only thehydrogen atom H(7) on O(6), which was directly observed, is includedin these calculations. Eight reflections having a high observed structurefactor and poor agreement with calculated values are marked**; theywere omitted from the final cycles of refinement. The final R-farctor was0.12 for the observed reflections and 0.13 per cent including the unob-served values.

DBscntptroN oF THE Srnucrunr

Framework. The fluell i te structure consists of octahedraliv and tetra-hedrally coordinated cations in a rather open framework arrangement,within which there are distinct channels containing hydrogen-bondedwater molecules. The aluminum atoms are situated at centers oI sym-metry and are bonded octahedrally to two centro-svmmetric pairs ofoxygen atoms, and one pair of f luoride ions, O(3), O(3)', O(4), O(4)'andF(5), F(5)'. These octahedra are connected by corner-sharing at thefluorides to form infinite chains along the [110] and [110] directions. Thechains with aluminum atoms atz:0 and I extend along [110] and alter-nate with those at 2:l and f which are in the [110] direction. The equer-tor ia l p lanes of the octahedra which conta in O(3) , O(3) 'and F(5) , F(5) 'are nearly perpendicular to the c-axis, with a small tilt so that the atomsO(4), O(4)'at the apices alternately approach and withdraw from each

l Table 2 has been deposited as Document No. 9117 with the American Documenta-tion Institute, Auxiliary Publications Project, Photoduplication Service, Library of Con-gress, Washington 25, D. C. Copies may be secured by citing document number, and re-mitting $2.50 for photoprints or $1.75 for 35 mm microfilm. Advance payment is required.

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1586 B. B. GUY AND G. A. JEFFREY

other along the direction of the chain, as i l lustrated in Fig. 2. The chainsof octahedra at adjacent levels are l inked through a tetrahedral arrange-ment of oxygens O(4), O(4)", O(4)"', O(+;" about a central phosphorusatom (see Table 3). The shorter distances between the apices of adjacent

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octahedra which result from the ti l t of the O+-O+ axes form the edses ofthe tetrahedra.

The octahedra and tetrahedra of the structure are regular within 3o asshown by the interatomic distances and angles given in Table 3. Themean Al-O, AI-F distance is 1.855 A, and it is interesting to note thatthere is no distinction between the oxygens and the fluoride ions with re-spect to these bond lengths.

STRUCTURE OF FLUN,LLITE 1587

Tlerr 3. INrnn,lrourc Drsr,tNcrs ar.rn Ancms rN FLUELLTTE

Estimated standard deviations in parentheses refer to the Iast decimal nosition reDorted.

Octahetlra Tetraheilra

A1-O(3) 1.868(s) A p_o(4) 1.s30(3) AA1-O(4) 1.847(s)Al-F(s) 1.8s1(s) o(4)-p_o(4),, 109.8(2).

o(4)-P-O(4)" ' 108 .9(2)o(3)-41-O(4) 92.0(2)' O(4)-p-o(4)/" 109.s(2)o(3)-A]-F(s) e2.s(2)o(4)-Al-F(s) 88.7(2)

Channels Hyilrogens0(6)-0(6), 2.688(8) A o(4)_H(7) 1.80(16) A0(6)-0(3), 2.660(8) o(6)_H(7) 1.11(16)0(6)-0(3)" 2.708(8)0(6)-0(4), 2.802(8) o(4)_H(7)_O(6) r47.o(r2.0)"o(6)-0(3) ", 3 .410(9)o(6)-0(3)'" 3.sel(e)

o(4)-0(6)-o(6), 110.8(3)"o(3)'-o(6)-0(6), 100.s(3)o(3)",o(6)-0(6), 124.6(3)o(3)'-0(6)-o(4) 13s.7(3)o(3)"-0(6)-0(4) e8.8(3)o(3)'-0(6)-0(3),, 87.7(3)

Symmetry relations:

(3 ) r , ! , 2 ( 4 ) \ ! , 2 ( 6 ) r , y , z(3) ' i - r , i ly , t ] -z (4) ' i , , , z (6) , * - r , y , I -z(3)" * , t -y , 1-z (4) ' , r , i -y , i -z (6) , , I - * , I -y , i l ,(3)" ' 1 l r , -y , * lz (4)" ' f , - r , y , f , -z (6) , , , L** , i *y , t -z(3) ' " i - r , y , l -z (4) , " ! - r , ! -y , z

Channels. The framework of corner-linked [AIO4F2] octahedra and [POa]tet rahedra form channels in the [110] and l1 l0] d i rect ions which conta inthe water molecules (or hydroxyl ions) corresponding to the O(6) atomsin 32-fold (h) positions. These oxygens occur in pairs, linked by one of theshorter hydrogen bonds in the structure of 2.688 A, in the upper andIower regions of these channels, as shown in Fig. 3. Their hydrogen-bondcoordination is that of a distorted tetrahedron (Table 3). The O(6)-O(6)bonds lie in the (010) plane, being nearly perpendicular to the c-axis anddirected towards the o-axis, as shown in Fig. 4. The remaining threebonds from O(6) are to the framework structure, one to the O(4) and twoto the O(3). Each O(3) atom is thereby involved in hydrogen bonding to

1 588 B, B . GUY AND G. A . JEFFREY

both pairs of channel oxygens through the iower oxygen in the upper re-gion and the upper oxygen in the lower region. There are four channeloxvgens per formula unit, and they participate in all fourteen hydrogenbonds, either with each other or to the anionic framework. These four-teen distances are between 2.688 and,2.802 A, as shown in Table 3. Sincefifteen protons are required to produce an electrically neutral structure,there is presumably one "free" hydrogen present which is not involvedin hydrogen-bonding. The shortest F-O distances are those which form

.----------------.--- [,io]

Frc. 3. [110] view of framework polyhedra and channel oxygen pairs. The bonding of the

0(6) atoms in indicated.

the edges of the AlOaFr octahedra which are 2.57,2.59,2.65 and 2.69 L.These are normal non-bonding distances and there is no evidence thatthe fluoride ions are involved as acceptors in the hydrogen bonding in thestructure.

Hyd,rogen Bondi.ng. With the reasonable assumption that the mineral is atertiar.v orthophosphate, O(4) wil l be a hydrogen-bond acceptor onlyand the four O(4) . H(7)-O(6) bonds per formula unit wil l have asingle proton location closer to 0(6) than O(a). This corresponds to thelongest O . . . . O hydrogen bond separation of 2.802 L. There are then

srRUCTURElor ntuarrtrn 1's89

two possible sets of hydrogen atom configurations, depending upon

whether the hydroxyl ions are associated with the framework octahedra

or the channel oxygens. The proton configuration corresponding to the

formula {A12PO4Fz'3HrO'OH}4HrO, with the framework enclosed in the

brackets, is shown in Fig.5(a). All the other ten h1'drogen bonds can

Frc 4. Projection on to (001) of oxygen pairs in the upper and lower regions of a [1f0]channel (outlined). Adjacent oxygens, 0(6), at z:007 (small solid circle) andatz:O-25

(large open circle) share O(3) oxygens of the framework. Oxygens at z:0 (small open circle)

and at z:0.32 (large solid circle) share O(3) oxygens with 0(6) atoms in [110] channels.

Broken line: Hydrogen bond between lower oxygen pair; Solid line: Hydrogen bond be-

tween upper oxygen pair.

then have a double potential minimum with a synchronized distribution

of the hydrogen atoms. The non-bonding hydrogen atom H(13)' would

be attached to one fourth of the O(3) framework oxygens in a statistical

random arrangement, being also synchronized locally with the donor/

acceptor distribution of the hydrogen bonds. There are reasonable posi-

tions for this atom in the directions of 0(6)" or 0(6)"' at 3.5 A away' as

shown in Fie. 6a which i l lustrates the O(3) environment' The angles be-

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1590 B. B, GUY AND G. A. JEFFREY

tweentheO(3)-H(13) bondandtheO(3) . . . O(6)hydrogen-bonddirec-tions for the coordinates given in Table 1 and shown in Fig. 6a are 99"and 103".

The alternative configuration, {AlrpO4Fr.4H2O}3HrO.OH, in whichthe hydroxyl ions are located in the channels, permits the double mini-mum proton position for the 0(6) . . . 0(6) hydrogen bonds only, asshown in Fig. 5b. This would therefore be a configuration of less residualentropy. The non-bonding hydrogen atom for this configuration is then

Fro' 5. schematic diagram for possible hydrogen bonding configuration in fluellite.The non-bonding proton (not shown) is attached to the hydroxyl ions. (a) Hydroxyl ionsIocated on framework o(3) oxygens statistically, o(3) atoms are 3H2o.oH and 0(6)atoms 4H2o per formula unit. (b) Hydroxyl ions located in channels. statistically, o(3)atorns are 4HzO and 0(6) atoms 3HeO'OH per formula unit. Large H: full proton position;small H: fractional proton position; Alternative locations for hydrogens along a bond areshown by solid circles.

Iocated on one of the four 0(6) atoms. A possible position is along the0 (6 ) . . . 'O (3 ) " d i r ec t i on , such tha t t he O(6 ) -H bond wou ld be a tangles of about 65o to three of the four hydrogen bonds from O(6). This isstereochemically less acceptable than that for the configuration 5a, asshown in Fig. 6b. The observation that the only hydrogen atom directlylocated on the Fourier difference synthesis was that on O(6), which formsthe donor bond to the phosphate O(4), suggests that this is the only sitewith unit weight for the hydrogen atoms and thus also favors the con-figuration 5a with only water molecules in the channels and the maximumnumber of double minimum hydrogen bonds.

( b )

ST'RUCTURE O F FLUELLITE 1591

( " )

(b)

Frc. 6. Stereographic projection showing location of the non-bonding hydrogen instructure. (a) For configuration illustrated in Fig. 5a. An O(3) oxygen is at center of sphereof projection. (b) For configuration in Fig. 5b The 0(6) oxygen is at center of sphere ofprojection. The environment of the atom at center of the sphere of projection is illustratedadjacent to the stereogram. Solid line: Hydrogen bonding linkages; Broken lines: Van derWaal's contacts.

While it is possible that both sets of configurations represented by 5aand 5b can exist in the structure with a tautomeric exchange such that itis impossible to distinguish whether the (OH)- ions are on the framework

.o' 06

t592 B. B . GUY AND G. A . JEFFREY

or in the channels, we believe that this is less l ikely than a structure withonly the water molecules in the channels, on the basis of the evidencepresently available.

Acrwowr-BoGMENTS

This research has been supported by the Office of Saline Water, U. S.Department of the Interior through Grant No. 14-01-0001-394. We aregrateful to Dr. Vassamillet of the l,Iellon Institute, Pittsburgh, for theelectron microprobe analysis which was diff icult to obtain because of thenature of the small crystals.

RnlnnpNcos

Bunnctn, M. J. (1951) A new approach to crystal structure analysis. Acta Cryst. 4,531-544.

BusrNc, W. R., K. O. ManrrN .rNl H. A. Lrw (1962) ORFLS-A Fortran least squaresprogram. Oak Rid,ge Nationol Loboratory, Tenn., Rept. ORNL-TM-30S.

Conlrnr.n, P. W. R. (1965a) Solution of the Patterson function by superposition methods.Cr y s tdlo gr a p hy Lab or ator y, U nit. Pi.tt sbur gh, T ec h. Re pl.

- (1965b) Programs for 2 or 3-dimensional convolutions. Crystal,Iography Laboratory,Unit:. Pittsburgh. Tech. Rept.

Gnout, Paul (1882) Beitrage zur Kenntnis der natijrlichen Fluorverbindrngen Zeit.Krist.7 , 482-487 .

LarseN, E. S eNo H. Bnnuan (1934) The microscopic determination of the non-opaqueminerals (2nd ed.). U . S . GeoI Sura Bull 848.

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Manu,scr'ipt receited, M arch 14, 1966; accepted lor publi.cation, June 10, 1966.