Tetrahedral-site occupancies in reedmergnerite and synthetic boron albite

American Mineralogist, Volume 77, pages 76-84, 1992

Tetrahedral-site occupancies in reedmergnerite and synthetic boron albite(NaBSirO')

Mrcrrlnr- E. Fr.nnrDepartment of Geology, University of Western Ontario, London, Ontario N6A 587, Canada

Ansrucr

The structures of three B-substituted albite samples (NaBSirOr), including reedmerg-nerite from Utah and low and high synthetic boron albite, have been refined at roomtemperature with single-crystal X-ray intensities. For reedmergnerite, R : 0.019, the struc-ture is ideally ordored with B in the T,0 site, and the electron density distribution at theNa position is anisotropic as in low albite and low gallium albite. Average T-O distancesare T,0 11.472(l)1, T,m [l.6] l( l)], T,0 [l.615(1)], T,m [l.619(l) A]. The structure of lowboron albite refined to R : 0.027 . Average T-O distances and B site occupancies are T,0

l r .497(2) ,0.805( l )1, T,m [ l .606( l ) , 0 .014( l ) ] , T,0 [ l .593(1) , 0.135( l ) ] , T,m [ l .610( l ) A,0.046(l)1. The structure of high boron albite refined similarly to R : 0.037, Tr0 ll.57l(2),0.289(3)1, T,m u.598(2) ,0.058(3)1, T,0 [ l .553(2) ,0.384(3) ] , T,mlr .574( l A,0.269(3)1.

Average T-O distances for reedmergnerite and boron albite exhibit an almost linearcorrelation with site occupancies ofB (t,B). The tetrahedral-site ordering sequences oft'0>> trO >> t2m > trm in low boron albite and t20 > 1,0 = tzm > ttm in high boron albiteare very unusual compared with albite but do correlate with stereochemical features. Highboron albite is antiordered with (1,0 + t,m) < (trO + trm). The order-disorder transfor-mation in boron albite appears to occur within the temperature range 500-550 oC at P"ro= I kbar.

INrnooucrroN

The triclinic alkali feldspars (space group Cl) have fournonequivalent tetrahedral sites for Si and Al, designatedT,0, T,m, Tr0, and Trm, with occupancies of Al (or tri-valent atoms) designated 1,0, t,m, tr0, and t2m, respec-tively. As reviewed by Fleet (l99la), tetrahedral-site or-dering closely approximates to t,0 > trm - t20 = t2m <0.25 in high albite, t,0 - l, t,m = t20 = tzrn - 0 in lowalbite, and t10 - l, t,m > trO r tzrn - 0 in triclinicK-rich feldspars.

Recent studies (Burns and Fleet, 1990; Fleet, 1990,l99la, l99lb; Kroll et al., l99l) have used isostructuralanalogues of albite with Ga substitution for Al or Gesubstitution for Si as an additional (chemical) dimensionin the study oforder-disorder in alkali feldspar. The Riet-veld study of Burns and Fleet (1990) determined the or-der-disorder transformation in Ga-substituted albite(NaGaSi.O., hereafter referred to as gallium albite) to becontinuous with a well-defined region of intermediatestructure and an ordering path oft,O - l, trm r t20 =t2m - 0. Single-crystal X-ray studies revealed a largelyordered structure for low gallium albite (Swanson, 1986;Fleet, l99la). Also, within the stability field for low gal-lium albite, Ga remains largely ordered in a matrix ofdisordered aluminum-silicon feldspar (Fleet, I 99 I b). Cu-riously, the preferred sequence oftetrahedral sites in gal-lium albite and aluminum-gallium albite is t,0 >> trO =t2m > t,m - 0.0 (Fleet, l99la,l99lb). The Ge-substi-

0003 -00 4x/ 9 2/0 l 02-0076 $02.00

tuted albite (NaAlGerOr, hereafter referred to as germa-nium albite) presently synthesized has disordered (inter-mediate and high) structures with the unusual sitepreference to trm > t,0 (Fleet, l99la; Kroll et al., l99l).Both Ga- and Ge-substituted albites provide examples ofantiordered feldspars (cf. Thompson et al., 1974). Fleet(1990, l99la) suggested that the tetrahedral-site orderingin alkali feldspar is promoted by the distinctive stereo-chemistries of the T,0 and T,m sites, which are imposedby spatial accommodation of the Na cation within thefeldspar framework. Site preference is determined by fa-vorable charge or valence electron distribution, minimi-zation of T-Na repulsion, and spatial accommodation. Itwas noted that the preference for the T'0 site increases inthe sequence Fe3t ) Ga > B > Al, which correlates withthe absolute difference in size with Si.

Reedmergnerite (NaBSirOr) occurs as an authigenicmineral in black oil shale and brown dolomitic rock ofthe Green River Formation (Milton et al., 1954, 1960)and also in peralkaline pegmatites from Tadzhikistan(Dusmatov eI al., 1967). The crystal structure, isostruc-tural with low albite, was confirmed and refined usingfilm intensities by Appleman and Clark (1965), who re-ported an ordered structure w'ith no apparent anisotropyof the electron density distribution at the Na atom posi-tion (cf. Fleet, l99la). Synthetic reedmergnerite (hereaf-ter referred to as boron albite) has been studied by Eug-ster and Mclver (1959), Bruno and Pentinghaus (1974),Kimata (1977'), Ostertag (1980), and Mason (1980a,

76

FLEET: TETRAHEDRAL OCCUPANCIES IN REEDMERGNERITE AND ALBITE

TABLE 1. Experimental details

Reedmergnerite Utah Low boron albite Gbl00 High boron albit€ Gb87

Crystal size (mm)Unit-cell parameters

a (A)b

o (')

B"Y

d range (') of reflectionsIntensity data collection

MethodReflections, type

Number2d limit (J

Transmission factorsFinal data list

Total reflectionsNumber unobserved

Final refinementFl (unweighted, o/4

R, (weighted, %)Extinction 19, x 10)

0 . 2 9 x 0 . 2 0 x 0 . 1 5

7.8388(9)12.3730(10)6.8082(7)

93.324(8)116.381(9)92.014(8)17.7-31.6

0-2e+ h ! k + l

51 8070

0.87--0.90

2590219

1 . 92.50.91(5)

0 . 1 2 x 0 . 1 9 x 0 . 0 7

7.8s2(3)12.350(4)6.806(1 )

93.37(2)116.3q3)91.58(3)17.6-21.8

@

+ h + k + l3414

600.8H.94

1707416

0 . 1 6 x 0 . 0 7 x 0 . 1 9

7.910(2)12.336(2)6.8200)

93.54(2)1 1 6.1 3(2)90.60(2)17.5-31.4

+ h ! k + l

345860

0.87-0.94

1729277

3.73.00.28(6)

2.72.0

1980b). Eugster and Mclver (1959) synthesized low bo-ron albite at 300-500 "C, 2.0 kbar, and Bruno and Pen-tinghaus (1974) reported unit-cell parameters for high bo-ron albite synthesized at 700 oC, 4 bars. Kimata (1977)suggested that the order-disorder transformation oc-curred in the range 500-700 oC, but this was not wellconstrained. Mason (1980a, 1980b) reported that themorphology of boron albite grown by hydrothermal syn-thesis changes systematically with time and rate of changeof B-Si order. Also, in contrast to the behavior of albite,ordering in boron albite is insensitive to composition ofthe coexisting fluid and may occur through recrystalliza-tion rather than by solid state diffusion.

This paper describes the tetrahedral-site ordering andelectron density distribution at the Na atom position inreedmergnerite from the Green River Formation and lowand high structure boron albite, based on structures re-fined using single-crystal X-ray diffraction data.

ExpnnrvrnxrAl, pRocEDLJREs

Materials

Reedmergnerite from the type locality (DuchesneCounty, Utah, Milton et al., 1960) was obtained from theNational Museum of Natural History, Smithsonian In-stitution (catalogue number NMNH 140710). Crystals oflow and high boron albite (NaBSirOr) were grown hydro-thermally, using a standard cold-seal reaction vessel, andquenched in air and HrO. Starting material was a me-chanical mixture of purified SiO, and analytical gradeNarCO, and H,BO, in the stoichiometric proportion forNaBSirOr. Charges consisted of 0.05 g boron albite mix-ture and 0.02 cm3 of deionized HrO in a sealed Au cap-sule about 4 cm in length. For low boron albite, the cap-sule was heated at 462 t and 1.8 kbar for 6 d. Theproducts consisted of low boron albite and minor quartz.For high boron albite, the capsule was heated at 850 "C

and 1.6 kbar for I h (to achieve complete melting), cooledand maintained overnight at 580 'C and 0.8 kbar, andthen cooled stepwise and maintained at 478 "tC and 0.5kbar for 3 d. The products consisted ofhigh boron albiteand minor quartz.

Structure refinements

Single-crystal measurements were made with an Enraf-Nonius CAD-4F diffractometer, using graphite-mono-chromatized MoKa X-radiation. Structure refinementsfollowed Fleet (l99la), and experimental details are giv-en in Table l. The ideal formula (NaBSirOr) was assumedfor all three crystals, and scattering factors for neutralatomic species and values of f' and f" were taken, re-spectively, from Tables 2.28 and 2.3.1 of the Interna-tional Tables for X-ray Crystallography 097D. Final pa-rameters are given in Tables 2 and 3, and observed andcalculated structure factors are given in Table 4.'

DrscussroN

Selected bond distances and bond angles for reedmerg-nerite and low and high boron albite (synthetic reed-mergnerite) are given in Tables 5 and 6, tetrahedral-siteoccupancies are given in Table 7, and the stereochemicalenvironments of Na are represented in Figures I and 2.The low and high boron albite were not chemically ana-Iyzed to confirm stoichiometric proportions. However,with allowance for the effects of order-disorder, their unit-cell parameters are closely comparable to the parametersfor synthetic reedmergnerite from other studies employ-ing diferent methods of sample preparation (cf. Kimata,

I A copy of Table 4 may be ordered as Document AM-92-486from the Business Office, Mineralogical Society of America, 1130Seventeenth Street NW, Suite 330, Washington, DC 20036,U.S.A. Please remit $5.00 in advance for the microfiche.

78 FLEET: TETRAHEDRAL OCCUPANCIES IN REEDMERGNERITE AND ALBITE

TrEu 2. Positional and isotropic thermal parameters (4'?) Tlele 3. Anisotropic thermal parameters ( x 1O'? A'?)

B* Bf B- B$ Br" 8." 4"

Reedmergnerite, UtahNa 0.2s82(1) 0.007s(1) 0.1330(1)r,0 0.0121(21 0.1616(1) 0.2215(2)r,ffi 0.00585(4) 0.80969(3) 0.20966(5)T,0 0.70278(41 0.10177(21 0.32033(5)r.h 0.68471(4) 0.864s0(2) 0.3s438(5)oo1 0.0077(1) 0.1363(1) 0.0037(1)oo2 0.s931(1) 0.9813(1) 0.2757(1)o"0 0.84490) 0.0989(1) 0.2116(1)o"nr 0.8161(1) 0.8349(1) 0.2336(1)o.0 0.0068(1) 0.276s(1) 0.2733(1)o.n 0.0290(1) 0.67980) 0.2060(1)oo0 0.19040) 0.1186(1) 0.3829(1)Oom 0.1921(1) 0.8680(1) 0.4171(1)

Low boron albite, Gb1@ (NaBSi.Oo)Na 0.2640(1) 0.0079(1) 0.1335(2)r,0 0.0083(2) 0.1640(1) 0.221312)T,h 0.0075(1) 0.8097(1) 0.2158(1)T,0 0.7021(1) 0.1030(1) 0.3199(1)T,n 0.6862(1) 0.8674(1) 0.3546(1)oo1 0.0061(2) 0.1370(1) 0.9990(2)oo2 0.5961(2) 0.9839(1) 0.2765(2)o"0 0.8392(2) 0.1022(1) O.2O92(21o"n 0.8177(2\ 0.8363(1) 0.2365(2)o"0 0.0098(2) 0.28070) 0.2749(21O"m 0.0298(2) 0.6801(1) 0.2110(21oo0 0.1901(2) 0.1174(1) 0.3834(2)Oom 0.1917(2) 0.8680(1) 0.4209(21

High bolon albite, Gb87 (NaBSi3O.)0.0075(2)0.1681(1)0.8100(1)0.1056(1)0.8758(1)0.1390(1)0.9890(1)0.1 108(1)0.8392(1)o.2922(110.6810(1)0.1 134(1)0.86810)

0.1356(3)0.2203(210.2305(1 )0.3219(2)0.3s56(2)0.9876(3)0.278q3)0.2042(31o.24s2(3)0.2810(3)0.2215(3)0.3866(3)0.4281(3)

Na 93(2)T,0 59(4)T,m 47(11T.0 43(1)T,fi 42(1\oo1 98(3)oo2 51(3)o"0 83(3)O,fr 76(3)o"0 68(3)O"n 71(3)oo0 81(3)Oom 80(3)

Na 218(6)T,0 96{4)T,h 88(3)T,0 77(51T,n 63(4)oo1 217(81oo2 101(7)o"0 189(8)O"n 138(7)o"0 127(71O.n 132(7)oo0 152(71Oom 151(7)

- 61(2)s(3)8(1 )5(1)8(1)

12(2)14(2)

-12(2110(3)-4(2)

e(2)1 0(2)-5(2)

-141(4)7(4)

11(2)3(2)

17(2117(4121(41

-20(5)1 8(5)1 s(s)e(4)7(4)0(s)

-568(9)5(3)

1 9(2)-2(3)1s(3)1 1(5)21(5)

-21(6)8(6)

26(6)1(5)

1s(5)-1(6)

1.s4(1)0.51(2)0.39(1)0.40(1)0.38(1)0.65(2)0.s4(2)0.78(210.82(210.66(2)0.68(2)0.77(2)0.78(21

2.8s(2)0.68(2)0.55(1 )0.50(1)0.56(1)1.12(4)0.s0(3)1.36(4)1.21(4)1.13(4)1.09(4)1.12(411.18(4)

7.08(6)0.78(210.82(210.80(3)0.7q2)1.49(5)1.21(4)1.78(5)1.72(s)1.43(411.39(4)1.42(4)1.54(4)

Na 180(4)T,0 84(5)T,n 62(21T,0 59(2)T"trt 58(2)oo1 162(6)oo2 89(5)o"0 162(6)O"n 114(6)o"0 115(6)O"m 122(6)ooo 123(6)Oom 122(6)

Reedmergnerite, Utah190(2) 1s4\21 21(21 41(2)s1(4) 48(4) 1(3) 27(3)39(1) 34(1) 14(1) 20(1)33(1) 43(1) 70) 18(1)34(1) 42(1) 11(1) 21(1)67(3) 40(3) 1s(2) 40(2135(3) 72(31 e(2) 23(2171(3) 104(3) -19(2) 6q3)s9(3) e6(3) 25(2) 61(3)41(3) 96(3) 1(2) 46(3)45(3) 73(3) 24(21 16(2)s3(3) 43(3) 39(2) 12(2)85(3) 49(3) -1(2) 10(2)

Low boron albate, Gbl00 (NaBSi"O")364(5) 262(51 37(4) 62(4173(s) 4e(5) 1q4) 31(4)54(21 61(2) 12(2) 36(2)40(2) 50(2) 2(2\ 23(2172(2) 4e(21 21(2) 32(2)

10e(6) 89(5) 22(5) 76(5)79(5) 101(5) 11(4) 39(4)

116(6) 144(6) -34(5) 89(5)138(6) 136(6) 34(s) 76(5)11s(6) 123(6) 18(s) 68(s)82(s) 107(6) 20(41 36(s)

121(6) 84(6) 34(5) 37(5)113(6) 97(6) -4(5) 31(5)

High boron albite, GbET (NaBSi"O.)1072(131 613(10) 35(7) 62(6)

75(41 59(4) 3(3) 32(3)75(3) 84(3) 14(2) 36(2)74(41 71(41 -1(3) 18(3)ss(4) 7o(4) 23(3) 31(3)

127171 116(7) 10(6) 85(6)12't(7) 125(7) 11(5) 34(5)164(8) 182(8) -s1(6) 90(6)181(8) 20s(8) 33(6) 88(6)155(7) 15e(7) 25(6) 73(6)105(7) 144(71 16(5) 30(6)14q4 f7(7) 25(6) 42(61132(71 145(7) -7(6) 37(6)

Na 0.2823(2)T,0 0.0093(1)T,m 0.0093(1)T.0 0.6995(2)T,m 0.6910(1)oo1 0.0048(2)oo2 0.6050(2)o"0 0.82u(2)O"ln 0.8197(2)o"0 0.0189(2)O"n O.O2S4(21oo0 0.1921(2)Oom 0.1895(2)

Note: 84: %2,21 Aaa,.a,

1977; Bruno and Pentinghaus, 1974). The parametersquoted by Mason (1980b) are all for samples more or-dered than the present low boron albite.

Refined structures

Refinement of the structures of reedmergnerite and bo-ron albite was unexpectedly complicated. Following theprocedure of Fleet (l99la), with the occupancies con-strained by )t,B : 1.0, all three refinements appeared toconverge normally (albeit slowly). However, some dis-order was computed for reedmergnerite [trO : 0.896, trm: 0.039(3), t,0 : 0.028(3), t,m : 0.037(3), and the ther-mal ellipsoids for the T,0 site were anomalously large (Bq: 1.47, 1.54, Ll9 e.A3 for reedmergnerite, low boronalbite, and high boron albite, respectively) compared withvalues for B in tetrahedral coordination in borates (e.g.,Ghose and Wan, 1979). The thermal parameters for T,0were not strongly correlated with the occupancies, but theleast-squares procedure clearly was placing too muchelectron density (in the form of Si atoms) at the Tr0 site.This aberration was attributed to the contribution ofva-

Note.'Anisotropic temperature factors have the form expf-V4(q1tfd2+ . .. + 2Baklrfdcos a')1.

lence electrons to the electron density distribution. Forthe ideally ordered structure, the electron density at thepositions whose occupancies are being refined (T,m, Tr0,Trm) is too high relative to the density at the O atompositions, and the least-squares procedure attempts to re-duce it by replacing Si atoms by B. Although there is goodX-ray scattering contrast between Si and B, the X-raysite-occupancy reflnements of the present structures arelimited by the lack of X-ray scattering factors for thepartially charged atoms. Unfortunately, the quantitativeestimation of atomic charge and deformation electrondensity remain controversial (e.9., Sasaki et al., 1980),and their contributions to the reedmergnerite structurewould be appropriately considered in a future study.

Reedmergnerite, in fact, has an ordered structure towithin resolution of the present methods, and this con-clusion is demonstrated quite readily. The residual index(R) is minimized for the ordered structure; R decreasesfrom 0.030 for the above refinement with occupancies to0.019 for the ordered structure (t,0B : I .0) and increases

FLEET: TETRAHEDRAL OCCUPANCIES IN REEDMERGNERITE AND ALBITE

TABLE 5. Selected interatomic distances (A) and bond angles (.)

79

Low boron High boronReedmergnerite albite Gb100 albite Gb87

Low boron High boronReedmergnerite albite Gb100 albite Gb87

Na-Oo1Na-OA1Na-OA2Na-O"0Na-OoOT,o-oa1T,0-oB0T,0-o"0T10-oD0AverageT1m-O^1T1m-OBmT,m-O"mT1m-OomAverage

2.4s4(112.490(1)2.402(1)2.403(112.377(111.483(1)1.472(1)1.452(111.482(111.4721 .ss6(1 )1.606(1 )1.624(1)1.619(1)1 . 6 1 1

2.487(2)2.533(2)2.37s(1)2.416(2)2.389(2)1.522(1)1.481(2)1.464(2)1.520(2)1.4971.600(1)1.599(1 )1 .61s(2)1 .61 0(1)1.606

2.607(2)2.679(212.316(2)2.457(212.443(211.591(2)1 .560(1 )1.552(211.582(1)1 .5711 .610(2)1 .s90(1 )1 .601(2)1.590(1 )1.598

Na-O"mNa-OcoNa-O"mNa-Oom

Tr0-o^2Tr0-oB0T"0-O"mT"0-OomAverageT2m-O^2T"m-O"mTrm-O"0T"m-Oo0Average

3.11q l )

2.8141112.864(1)

1 .636(1 )1.588(1 )1.622(111.61 4(1)1 .6151.647(1)1 .617(1)1.605(1)1.608(1)1 .619

3.108(2)

2.7s4(2)2.e09(2)

1.615(1)1.563(1 )1.s98(1 )1 .594(1 )1.5931.633(1)1.606(1)1.595(1)1.605(1)1 .610

3.124(3)3.215(212.737(2)3.028{(2)

1.563(2)1.540(2)1.s62(2)1.546(2)1.5531.577(211.56s(2)1.572(',t)1.582(21't.574

if as little as 0.lolo disorder is introduced. Therefore, anordered structure was assumed for reedmergnerite, andsite occupancies were not included in the present refine-ment.

For low and high boron albite, atomic charge effectswere compensated during refinement by lifting the con-straint of Zt,B : 1.0 but retaining the total occupancy(Si + B) ofeach site at 1.0. For low and high boron albite,total Si cations pfu [>(Si)] refined to 2.79 and, 2.75, re-spectively; they were less than 3.00 because the total re-duction in electron density due to atomic charge is greaterfor Si than B. Compared with the aberrant refinementsfor low and high boron albite, R was reduced from 0.032to 0.027 and from 0.042 to 0.037, respectively. The re-sulting tetrahedral-site preferences for boron albite, nor-malized to 2t,Si : 3.0 (Table 7), are qualitatively similarto those derived from the aberrant refinements [whichwere t,0 : 0.705, t,m : 0.053(3), trO : 0.158(3), trm :0.084(3), for low boron albite and t,0 : 0.219, t,m :0.091(4), t,0 : 0.397(4), tzm : 0.294(4) for high boronalbitel.

It is emphasized that all of the presently discussed re-finements for reedmergnerite and boron albite converged.The aberrant and accepted refinements differ in the meth-

od for estimating site occupancies. Site occupancies re-fined with the constraint 2t,B : 1.0 are reported in thetext but are limited by lack of appropriate X-ray scatter-ing factors. The presently accepted site occupancies resultin significant reduction in R (cf. Hamilton, 1965), thecommonly recognized criterion for acceptability of re-finement.

The present refinement of the structure of reedmerg-nerite is very similar to that of Appleman and Clark(1965). Both studies show a fully ordered structure. Thepositional parameters agree to within I or 2 sd, and theisotropic thermal parameters are similar. The only sig-nificant difference is in the electron density distributionat the Na atom position, which the present study showsto be anisotropic (Figs. I and 2) as in low albite (e.g.,Harlow and Brown, 1980) and low gallium albite (Fleet,I 99 la).

Fleet (l99la) noted that, as a result of its accommo-dation within the feldspar framework, the Na atom inalbite structures is relatively unconstrained transverse tothe plane of the strongest Na-O bonds (Na-On2, Na-O"0,and Na-OoO; cf. Table 5, Figs. I and2). Therefore, evenin an ideally ordered albite structure, the thermal vibra-tion is exaggerated in this transverse direction. Addition-

TABLE 6. O (T-O-T) bond angles in reedmergnerite, boron albite, and other alkali feldspars

Low boron High boronReedmergnerite albite Gb100 albite Gb87

Reed-mergnerite Low albite

1 2

Inter-Germanium mediate

albite albite3 4

Galliumalbite

3

T,0-O^1-T,mT,0-oa2-TrmTr0-oB0-Tr0T,m-O"m-TrmT,0-o"O-T,mTlm-Ocm-TroT,0-OoO-T.mT,m-Oom-Tro(T,0-o-T)(T,m-O-T)(T,0-o-T)(Trm-O-T)

142.s(11128.9(1)139.8(1)158.2(1)125 .1 (1 )135.7(1)134.9(1)146.4(1)135.7145.8137.7136.8

142.8{1)129.50)140.1(1)157.e(1)127.O(11135.2(1)1U.2(11146.7(1)136.0145.7137.9137.2

143.8(1)131.3(1)142.2(1)1s7.4(1)128.8(1)133.8(1)133.7(1)147.6(1)137.1145.7138.7137.8

143.1128.7140.5158.1124.9135.9135.4146.3136.0145.9137.9136.8

141 .5129.7139.8161 .5129.8135.81St.9151 .8136.3147.7139.3138.7

137.9130.7134.9161 .9125.2138.1131.8154.7132.5148.2139.6137.4

139.5125.2132.8157.0124.4131.3130.8150.0131.9144.6134.813r'..4

142.7129.8140.4159.4130.3134.7134.9150.5137.1146.8138.9138.6

Note :1 :App lemanandC la r k (1965 ) ;2 :Ha r l owandBrown(1980 ) ; 3 :F l ee t (1991a ) ;4 :Ph i l l i p se ta l . ( 1989 ) .

80 FLEET: TETRAHEDRAL OCCUPANCIES IN REEDMERGNERITE AND ALBITE

reedmergnerite

Tr m

Fig. l. Stereochemical environment of Na to 3.6 A in reed-mergnerite(cf. Fig. I ofFleet, 1991a).

ally, with disorder of the tetrahedral-site atoms, the Naatom will experience static displacement along the majoraxis of the vibrational ellipsoid, stretching the strongbonds only marginally and forming shorter bonds withother proximal O atoms. The anisotropy of the "thermal"motion in albite structures should increase systematicallywith disorder (from ideally ordered to incipiently disor-dered to largely disordered). This prediction is illustratedin the present study, with the progression from reed-mergnerite to low boron albite to high boron albite (Fig.2).

The refined structure of low boron albite is comparableto that of reedmergnerite, with allowance for the smallamount of tetrahedral-site disorder in the former (Table7), which modifies T-O distances (Table 5) and increases

TleLE 7. Tetrahedral-site occupancies for reedmergnerite, boron

low boron olbite

ooo

high boron o lb i te

Fig.2. Progressive increase in anisotropy ofcomputed ther-mal ellipsoid of Na normal to plane of strongest Na-O bonds inreedmergnerite, low boron albite, and high boron albite (see Fig.I for complete stereochemical environment).

all isotropic thermal parameters (Table 2). The refinedstructure of high boron albite (Tables 2 and 3) is com-parable to that of high albite at room temperature (Prew-itt et al., 1976) and. that of disordered germanium albite(Fleet, l99la), with allowance for the differences in idealB-O, Al-O, Si-O, and Ge-O bond distances. O (T-O-T)

albite, and other alkali feldspars

Relerence T atom t 0t t0 ttfi Lm

BBBAIGaGaGaGaAIAIAIAIAI

Nofe. 'References;1 : presentstudy;2: Har lowand Brown (1980);3: Fleet(1991a);4: Swanson (1986);5: Fleet(1991b);6: Phi l l ipsetal .(1989); 7 : Ribbe et al. (1969); I : Kroll et al. (1991); 9 : Colville and Ribbe (1968).

'Total Ga cations 0.1 and 0.3, respectively; o@upancies normalized to > t,: 1.6.

Reedmergnerite 1Low boron albite, Gb100 1High boron albite, Gb87 1Low albite 2Gallium albite 3Gallium albite 4Alscaro albite 5Al?ocas albite 5Intermediate albite 6High albite 7Germanium albite 4Germanium albite ILow sanidine 9

1.00.805(1)0.289(3)0.970.9350.887o.7120.5580.5100.280.274o.212

0.00.014(1)0.058(3)0.040.0160.0310.0040.0550.1550.250.3750.436

0.0 0.00.135(1) 0.046(1)0.384(3) 0.269(3)0.0 0.00.025 0.0250.044 0.0380.139 0.145'0j77 0.211'0.164 0.1710.22 0.250.185 0.1660.150 0.202

0.1250.345

t2mtp t20high boron albite

low boron olbite

reedmergneri te

FLEET: TETRAHEDRAL OCCUPANCIES IN REEDMERGNERITE AND ALBITE

t . 4 5o.o o.2

8 l

oQ t 'eo

o , '55I

F-

r .50

t .no-8u'oo-4t t t l l

o.o o-2 0.4 0'6 0'8 t.o

t i (€)Fig. 3. Tetrahedral-site occupancies of B (t,B) in reedmerg-

nerite, low boron albite, and high boron albite. The lines con-necting points in Figures 3 and 4 emphasize trends but do notimply that a continuous ordering path is necessarily present.

bond angles for these feldspar structures are compared inTable 6 (cf. Fleet, l99la).

Tetrahedral-site occupancies

Tetrahedral-site occupancies for B (t,B) in reedmerg-nerite and low and high boron albite are given in Table7 and Figure 3. Whereas reedmergnerite is effectively ide-ally ordered, the low boron albite is incipiently disor-dered with a significant amount of B in Tr0 and the site-preference sequence trO >> t20 ) tzrn ) t,m. This sitepreference is maintained with increase in B-Si disorder(Fig. 3). In high boron albite, B is enriched in Tr0 anddepleted in T,m. Thus, the tetrahedral-site ordering inboron albite is very unusual compared with albite(NaAlSi3Os). The site preference in high boron albite, withB favoring Tr0 and Si strongly ordered in T,m, is essen-tially opposite to that of germanium albite (Table 7), butdepletion of the trivalent cation (and enrichment of Si)in T,m is similar to the site ordering in Ga-substitutedalbite (Fleet, l99lb). The depletion of B in T,m is sostrong that high boron albite is antiordered in equivalentmonoclinic symmetry [t,B : 0.17 < t2B : 0.33; Z neg-ative (Thompson et al., 1974)1.

The approximately linear correlations between averagetetrahedral bond distance and Al-site occupancy in alkalifeldspars are well established (e.g., Kroll and Ribbe, 1987).The present limited structural data for reedmergnerite andboron albite exhibit a good correlation between (T-O)and t,B (Fig. a). The values for T,0, Tr0, and Trm forma single curr.ilinear distribution, whereas the values forT,m, which Fleet (l99la) noted has the smallest averageT-O distance in low albite, low gallium albite, and reed-mergnerite, appear to represent a separate distribution.

t i (B)Fig. 4. Variation ofaverage tetrahedral bond distance ((T-

O)) with site occupancy of B (tB) in reedmergnerite and boronalbite.

The excellent correlation between (T-O) and t,B servesto confirm the present values for the refined tetrahedral-site occupancies.

The unit-cell parameters also correlate with change insite occupancies in boron albite. However, the shifts inparameters with order-disorder are each opposite to thosein albite feldspars (see also Mason, 1980b). This is be-cause Si is larger than B but smaller than Al. Thus, (T'0-

O) is appreciably smaller than (T(Si)-O) in low boronalbite but greater than (T(Si)-O) in low albite. Whereasthe disordered structures of NaBSirOr and NaAlSirO. al-bites are dimensionally analogous, the dimensional dis-tortions to transform to low boron albite and low albite,respectively, are ofopposite sign. The angle 7 ofthe pres-

ent high boron albite is somewhat larger than that re-ported by Bruno and Pentinghaus (1974), 90.60 com-pared with 90.32, indicating greater disorder in theirmaterial (see below).

Stability, synthesis, and order-disorder

Reedmergnerite and alkali feldspars from the GreenRiver Formation are generally considered to be authigen-ic phases (Milton et al., 1954, 1960; Desborough, 1975).Whereas the present reedmergnerite is essentially ideallyordered, authigenic albite from the Green River Forma-tion has only intermediate states of order (Desborough,1975). In comparison, authigenic albite from marinelimestones and dolostones is more ordered but seeminglynot as ordered as low albite from low-gmde (and very lowgrade) metamorphic rocks (Kastner, l97l). Although thecontrols on ordering in Na-Al albite remain controver-sial, the association of fully ordered reedmergnerite withintermediate ordered albite is qualitatively consistent withthe preference sequence of trivalent substituents for theT,0 site of alkali feldspars (Fe'* t Ga > B > Al Fleet,1991a). On the basis of the crystal-chemical behavior ofI4lB, t41Al, and I4lSi and the rate of ordering under labo-

82 FLEET: TETRAHEDRAL OCCUPANCIES IN REEDMERGNERITE AND ALBITE

ratory conditions (e.g., ready synthesis of low boron al-bite at low P"ro), reedmergnerite would be expected to bemore ordered than albite under authigenic conditions,where equilibrium may not be achieved. However, in viewof the probable lower temperature for the order-disordertransformation in boron albite (see below), the relativestate of ordering of reedmergnerite and albite in pegma-tites is debatable.

The presence of quartz in the experimental products isconsistent with disproportionation and the incongruentmelting reported by Eugster and Mclver (1959); theamount of quartz was proportional to the amount of HrOadded. The present experiments synthesized both low andhigh forms of boron albite and more closely defined thetemperature of the order-disorder transformation. On thebasis ofthe synthesis oflow boron albite at 500 "C and2.0 kbar by Eugster and Mclver (1959), the synthesis ofhigh boron albite at 700 "C, 4 bars by Bruno and Penting-haus (1974), and his transformation of low to high boronalbite at 830'C, Kimata (1977) suggested that the trans-formation temperature is between 500 and 700'C. In thepresent study, all single-crystal feldspar products grownon a cooling gradient from above 500'C were high boronalbite. Eugster and Mclver (1959) reported incongruentmelting to ql:a;rtz + liquid (melt) at 567 "C, P"ro : 1.0kbar and at 516 oC, P.ro:2.0 kbar. In numerous exper-iments of the present study, glass was a common productfor quench temperatures at or slightly above 500 .rC. Thus,high boron albite may have nucleated at or below 550'Cin some of these experiments. Correspondingly, low bo-ron albite was formed only in experiments maintained ator below 500'C. Therefore, the order-disorder transfor-mation in boron albite may be within the range 500-550"c.

Further exporimentation to bracket more closely thetransformation temperature was beyond the scope of thepresent study. Although the high boron albite materialpresently studied was annealed aI 478 "C, in retrospect itis considered unlikely that significant ordering was intro-duced after crystal growth. Under laboratory conditions,the rate of order-disorder of all feldspar materials in thesolid state at low pressure is extremely low (cf. Martin,1974; Goldsmith and Jenkins, 1985; Burns and Fleet,1990). Solid-state diffirsion is promoted by very fine grainsize (Burns and Fleet, 1990), mineralizers (Martin,1974),and high pressure in the presence of sodium carbonate(Goldsmith and Jenkins, 1985). This evidence suggeststhat under laboratory conditions even solid state pro-cesses in alkali feldspars are initiated at grain boundaries.

In this study the estimated temperature range for theorder-disorder transformation in boron albite is lower thanthat for gallium albite (89G-970'C; Burns and Fleet, 1990)and albite (660-700 "C; Raase, l97l; Goldsmith and Jen-kins, 1985). Interestingly, the sequence for T,0 site pref-erence in these materials (Fe3* > Ga > B > Al) does notcorrelate on a one-to-one basis with transformation tem-perature. The transformation in boron albite is ratherabrupt and could be offirst order under laboratory con-

ditions; for example, Mason's (1980a, 1980b) observa-tions point strongly to a recrystallization mechanism.However, because the order-disorder transformation iscontinuous in gallium albite (Burns and Fleet, 1990) andalbite (at least at high pressure; Goldsmith and Jenkins,1985), it may be continuous in reedmergnerite (naturalNaBSirOr) as well.

Crystal-chemical controls on ordering

The additional tetrahedral-site preferences revealed bystudy of analogue and substituted feldspars (Fleet, I 99 I a,l99lb; Kroll et al., l99l:. present study) allow a morecomplete understanding of the crystal-chemical controlson ordering in alkali feldspars. The incompletely orderedand disordered materials are particularly important inthis respect. Fleet (l99la) emphasized the role of spatialaccommodation of the Na cation within the feldsparframework in defining the distinctive stereochemistriesof the tetrahedral sites, particularly of T,0 and T,m. Itwas tentatively concluded that the ordering scheme is de-termined by the favorable charge or valence-electron dis-tribution resulting when the trivalent cation occupies T,0and that this site preference is proportional to absolutedifference in size with the quadrivalent cation.

As reviewed by Smith (1974), site ordering in mono-clinic alkali feldspar is readily attributable to the favor-able charge balancing of Al by Na when the former isordered into the T, site. Fleet (l99la) noted that thisfavorable stereochemical relationship is maintained forT,0 in triclinic albite but not for T,m. In fact, the distor-tion required to accommodate Na in low albite structurescauses T,m to be less favorable for the trivalent cation,in respect to proximity to Na, than Tr0 and Trm (e.g.,Fig. l). Crimping of the framework in the plane of strong-est Na-O bonds (particularly at O atoms, O"0 and O"0)forces T,m away from Na. This relative isolation of T,mwould explain the marked proference of Si for that site inGa-substituted albite and boron albite. Fleet ( I 99 I b) con-cluded that the tetrahedral-site preference of Ga in so-dium aluminum gallium feldspars is determined largelyby the stereochemical association with nearest-neighbor(O) atoms. If next-nearest-neighbor tetrahedral atoms werea critical factor in determining site preference in alkalifeldspars, in ordered albite structures the quadrivalentcation should exhibit a stronger preference for Trm, whichhas two next-nearest-neighbor trivalent cations, than Tr0.Nevertheless, favorable charge balancing through next-nearest-neighbors is a possible explanation for the pref-erence of Si for Trm and B for Tr0 in boron albite (Table7\.

Several studies have suggested the importance of Obond angles (T-O-T) in controlling site preference in al-kali feldspars (e.g., Brown and Gibbs, I 970; Smith, 1974;Geisinger et al., 1985; Kroll et al., l99l); these studiesfound Si favoring tetrahedra with wide O bond anglesand B and Ge favoring tetrahedra with small angles. Krollet al. (1991) pointed out that this correlation suggests anexplanation for the antiordered site preference in ger-

FLEET: TETRAHEDRAL OCCUPANCIES IN REEDMERGNERITE AND ALBITE 83

manium albite, Ge favoring Tr0 ((T-O-T) : 131.6") andAI T,m ((T-O-T) : 144.0). This suggests that in ger-manium albite the stabilization associated with optimi-zation ofO bond angles far outweighs that from favorablecharge balancing. As noted above, the contribution ofthelatter when Al is in the T,m site is minimal.

The correlation between site preference and average Obond angle may be extended to gallium albite and boronalbite (Tables 6 and 7) because Ga and B both favortetrahedra with small O bond angles (cf. Klaska, 1974;Fleet, 1987; Geisinger et al., 1985), and (T-O-T) : 132.5"and 136.0'for T,0 in low gallium albite and low boronalbite, respectively. Also, Si shows the highest preferencefor T,m, for which (T-O-T) : 148.2 and 145.7" in lowgallium albite and low boron albite, respectively. How-ever, the above explanations related to balancing valencerequirements, proximity to Na, and spatial accommo-dation appear to be more appropriate for gallium albiteand boron albite.

O bond angles in albite structures are largely definedby the accommodation of Na (e.g., Fleet, l99la) and cat-ion repulsion rather than the nature ofthe T-O bondingprocesses. The correlation between average bond angleand site preference can be rationalized on electrostaticgrounds as well as from covalent bond theory, for ex-ample, strong repulsion of proximal Si cations, longerT-O bond distances for Ga and Ge, and screening of theresidual charge on the small B cation by the surroundingO atoms. However, tetrahedral-site ordering patterns arestrongly influenced by the stabilization afforded when thetrivalent and quadrivalent cations are placed in the mostappropriate stereochemical environments, and the O bondangle correlation, regardless of its physical basis, is cer-tainly useful for qualitative understanding ofsite prefer-ence.

In germanium albite, the small difference in size be-tween Ge and Al diminishes the tendency for ordering ofthe tetrahedral cations (Fleet, l99la). Charge balancingthrough Na is apparently not a critical factor, and thestabilization related (directly or indirectly) to O bond an-gles determines the weak preference of Ge for T,0. Insilicate albites, on the other hand, the competing effectsof charge balancing requirements, atomic size difference,and T-Na and T-T repulsion appear to predominate.

AcxNowr,nocMENTs

I thank G.E. Harlow and M.W. Phillips for constructive reviews of themanuscript, J.M. Hughes for editorial assistance, P.J. Dunn for provision

of the specimen of reedmergnerite, F.J. Longstaffe for helpful discussion,and the Natural Sciences and Engineering Res€arch Council of Canadafor financial support.

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MaNuscnrpr RECETVED Mlv 3, l99lMxvuscnrpr AccEprED Ser-reuuen 9, l99l