The Anorthite Crystal Structure at 410 and 830"C

American Mineralogist, Volume 58,pages 665-675, 1973

The AnorthiteCrystal Structure at 410 and 830"C

FnnNxrlx F. Fort. Jn. Depqrtment of Geology, IVashingtonState Uniuersity, Pullman, Washington 99 I 63 DoNnrn R. Ppeconl Departmentol Geologyand Mineralo,gy,The Uniuersttyof Michigan, Ann Arbor, Michigan 48104

Abstract

The crystal structure of anorthite has been refined using data collected at 4lO'C and 830"C. Although at these temperatures the type 'c' reflections are nearly unobservable, the refinements yield results similar to those for primitive anorthite at room temperaure. The structure on the unit cell level is therefore still primitive with 16 symmetrically nonequivalent Si and Al atoms. Calculation of 264 'c' refl,ection intensities using the position and thermal parameters of the structure at25" (Wainwright and Starkey, l97l),41O', and 830"C indicate that (1) 'c' both the Ca and the alumino-silicate framework atoms contribute to the diminution of reflection intensity with increasing temperature, and (2) the overall intensity decrease (calculated) due solely to the changes in atom position and thermal motion, while following the general trend of the observed decreases,is too small to account for the total decreaseover the range 25410'C. This suggests that within this temperature range another process, possibly a variation in domain texture, may also be taking place.

fnfioduction tion and temperature.Smith andRibbe (1969) have comprehensivelyreviewed the experimental results The crystal structure of a primitive anorthite and have summarizedthe interpretations.Therefore, displayingsharp'a','b','c', and'd' type reflections only a brief resum6of the more significantrelations was determined using film methods by Kempster will be given in this paper. fn general,as the CaAl et al (1962) and subsequentlydescribed by Megaw content of anorthite decreases,the intensitiesof the et al (1962).They found that the anorthite unit cell, 'c' reflections decreasewhile they become increas- space group Pl, was composed of four subcells ingly diffuse.Fleet et al (1966) refinedthe structure related by three pseudosymmetryoperations; body- anorthite" of compositionAn66 centering, C-centering, and cf2 translation. The of a "body-centered no observable'c' or 'd' reflections. minor atomic coordinatedifferences between pseudo- which exhibited Appleman (1972) has refined the structure of a symmetricallyrelated subcellsgive rise to the weak 'c' 'b','c', plagioclaseof compositionAn6a having re- but sharp and't subsidiaryreflections, with lunar 'b' flectionswhich were too weak and diffuseto measure, the reflectionshaving an additional contribution 'd' due to topochemicaldifferences in the Si/Al distri- but lacking observable reflections.Both of these using 'a' and'b' bution between the subcells related both by c/2 refinementswere carried out only position translationand C-face-centering.This crystalstructure reflections and all show that each atom is has recentlybeen refined by Wainwright and Starkey best approximatedby a "split" atom pair, such that (1971) using over 7000 counter-measuredreflections. each "half atom" of the pair is related to the other (a The relative intensitiesand diffusenessof the 'c' by a body-centeringtranslation, * b + c) /2. reflections in primitive and transitional anorthite The generalinterpretation is that the crystal is com- have been shown to be a function of both composi- posedof a mosaicof primitive anorthite domainsre- lated to each other by a body-centeringtranslation ' Contribution No. 3 1l from the Mineralogical Laboratory, across domain boundarieswhich parallel Qil) Department of Geology and Mineralogy, The University of (Ribbe and Colville, 1968). The electron micro- Michigan. scopestudies of Miiller et al (1972) have shown 665 666 F. F, FOIT, AND D. R, PEACOR that the difiusenessof the 'c' reflectionsat room tem- such that above -400oC, they were no longer ob- peratureis a function of domain size;the more dif- servable.Upon cooling to room temperaturethe fuse the 'c' reflectionsthe smallerthe domains. domain boundariesreappeared in the same (pre- The variation of 'c' reflectionsin anorthiteas a heating) position.While the "fading" of the domain function of temperatureis more complex.Laves and boundariesis correlatedwith the marked increasein Goldsmith(1954) demonstratedthat the diffuseness the diffusenessof the'c'reflections,the reappearance of the 'c' reflectionscould be increasedby quench- of the original texture on cooling has been interpre- ing from high temperatures(950-1350"C) in much ted to indicatethat the domain texture has under- the sameway that the diffuse'c' reflectionsof a vol- gone no actual changeas a function of temperature canic transitionalanorthite are producedby natural at least up to 600oC (Nord, personalcommunica- quenching.The structure of a transitional anorthite tion). wasrefined by Ribbe and Megaw (1962). Although The interpretation of the nature of the changein weak and diftuse 'c' reflectionswere present,they the anorthite structure with increasingtemperature were omitted from considerationsince their intensi- is further complicatedby the 2?Alnuclear quadrupole ties could not be accuratelydetermined. Except for resonance(n.q.r.) data of Brinkman and Staehli slightdifferences in temperaturefactors, the structure ( 1968a,b) . They observedspectra indicative of eight was indistinguishablefrom that of primitive anor- symmetricallynon-equivalent Al sites at low tem- thite. Brown et al (1963) and Bruno and Gazzomi peratures, as consistent with the X-ray diffraction (1967) using film techniquesat high temperatures, results for primitive anorthite. In the temperature observeda continuousand reversiblechange in the range 250-350"C (temperature depending upon diftusenessof the 'c' reflectionsas a functionof tem- samplecomposition) there occurs a reversibletran- perature, with the temperatureof disappearance sition to a more diffuse four-fold overlapping spec- varyingfrom 125to 350'C dependingupon the com- trum which they interpret as a change to a body- positionand thermalhistory of the sample.Foit and centeredcell with only four non-equivalentAl sites. Peacor(1967b), usinga single-crystaldifiractometer This temperaturerange is in accord with the X-ray technique,confirmed the previouslyobserved changes resultswhich show the near absenceof the 'C re- in diffusenessand intensity and also showed that flections,and thus the apparentpresence of a body- they occurred simultaneouslywith temperature centeredcell. Smith and Ribbe (1969) gave an al- change.However, theseresults were ambiguousas ternativeexplanation of the n.q.r. resultsbased on to whetherthe 'c' reflectionswere truly absentabove a shifting of the Ca atom betweenbody-centered re- 350"C or weremerely too weakand diffuseto be ob- lated sites.They noted that if the electrostaticfield served.Czank et al (1970) found that the intensities gradientsat the two pseudo-body-centeredAl atoms of the 'c' reflectionsdiminish rapidly between25 and is reversingmore rapidly than 10-' second(above 230"C, but that for some reflectionsa very small 250'C) due to a possible"bouncing" of the Ca residualintensity is observableto temperaturesabove atoms within the framework cavities, then only four 1500'c. overlappingn.q.r. spectrawould be observable.With These observationsat high temperatures,like referenceto changestaking place in the structure as thosemade with varyingcomposition, have been in- a whole they state that "increasingthermal vibration 'appear terpretedusing a domainmodel. An essentialaspect makesthe Ca cation larger' thus promoting of this model is the increasein the number of do- larger holes in the framework with a tendency to- mains relatedby the body-centeringvector (corre- wards the I2/m requirementsof the celsian struc- spondingto a decreasein domainsize) as an equili- ture," and further that "Ultimately the vibrations brium function of temperature(Czank et aI, I97O; becomeso large that the alumino-silicateframework Laves et al, l97O). However, recent electronmicro- beginsto move to a body-centeredconfiguration with scopestudies of anorthiteconducted at high temper- displacementsof the order of 0.1-0.3 A." The ulti- atures (Nord, personalcommunication; Wenk, per- mate result is alteration of the domain texture. sonal communication)revealed no change in the Therefore,there is doubt not only concerningthe numberor the positionof the domainboundaries as nature of the changesin the domain texture as an a function of temperature.The domain boundaries equilibrium function of temperature,but also con- which were imaged using the diffuse 'c' reflections cerning specific changeswithin a given domain on merely "faded" away with increasingtemperature the unit cell level, especiallywith regard to the rela- THE ANORTHITE CRYSTAL STRUCTARE 657 tive contributions of the Ca atoms and the aluminosilicate framework. The purpose of this study is to elucidatethe generalnature of the changesin the anorthitestructure as a function of temperatureand thereby resolve some of the ambiguitiespresent in 300 previous interpretations.The refinementat 410"C was undertakento determinethe structuralchanges F

Experimental Ftc. l. A plot of integrated intensity (I) os temperature ('C) for class 'c' reflectionsof Miyake anorthite. The sample used in this study was a transitional anorthite from Miyake, Japan. A composition of Ann.,, was obtainedfrom refractiveindex measure- reflectionswere monitoredthroughout the 6-10 day ments of anorthite glass. At room temperature the data collection periods to insure that any deteriora- sampleused in this study displayedsharp 'a' and'b' tion of the crystal or in the performance of the and 'c' equiprnent would not go unnoticed. No significant diffuse reflections.Although the'c' reflections 'b' rapidly become weaker and more diffuse with in- changein the intensitiesof these reflectionswas creasing temperature(Fig. l), their presenceat observedduring data collection.Also, a microscopic temperaturesin excessof 830"C requires that the examinationof the crystal at the termination of the spacegroup experimentsrevealed no crystal-capillaryreaction. be Pi with 8 CaAl,SirO, formula units 'a' per cell. Approximately 22OOand 2400 non-equivalent A cleavagefragment of dimensions0.49 x 0.11 x and'b' reflectionswere gatheredat 410 and 830oC, 0.24 mm was mounted along the a axis in a silica respectively,using a single-crystalscintillation coun- capillary for use with a high temperaturediffractom- ter diffractometer employing Weissenbergflat-cone eter furnace (Foit and Peacor,1967a). The lattice diffractiongeometry and pulse-height-analyzed MoK" constantsat the two temperaturesunder study (410 radiation.Due to a changein instrumentationbe- and 830"C) were approximatedusing room-temper- tweenthe two setsof data. the methodsof data col- ature least-squaresrefined parametersand the ther- lection differed. The 410'C data were collected mal expansiondata for calcic labradoritegiven by graphicallyusing a chart recorderwith the diffracto- Stewartet aI (t966). The changesin three of the meter in the manual mode. The reflectionswere six lattice constantsof Miyake transitionalanorthite integratedthree times using a polar planimeterand wereobserved over the temperaturerange 25-830"C and were found to be comparableto those for the Tlnre 1. Miyake Anorthite Lattice Parameters calciclabradorite. In addition,the exactpositioning of the diffractionpeaks at the calculatedvalues for 25oc* 4tooc 83ooc all levels of data confirms the appropriatenessof theselattice parameters.The a s.L7s(2)fi a.re4(8) [ a.zztgl $ lattice constantsused b 12.873(3){ 12.8s2(a){ 12.e15(4)4 for this anorthite at 4lO and 830'C are listed in r4.r72(3)x r+.reo(7)x t+.zOtr])X Table 1. or tclt\o qr qnls)o s2.7sQ): As a precaution,the crystal was allowedto equili- B 115.86(r): ris.82(5): 115.80(2): or ral1\- 91.15(5)- 91.02(2)- brate for a 24-hour period at both 410 and 830.C Y precedingthe measurementof intensities,even though *Values for primitive anorthite (An1gg) fron VaI Pasmeda, Tyro1, Austria, a 8.173(1), b 12.869(1), c 14.165(1)' all previous temperature-inducedchanges were ob- o 93.113(6)", B 115.913(6)", and Y 91.261(6)-, Wainwright servedto be instantaneous.The intensitiesof five 'b' and Starkey (1971). F, F. FOIT, AND D, R, PEACOR

the valuesaveraged. The 830"C data were collected using the diffractometerin the automatic mode (Supper-PaceSystem) and a flat graphite crystal monochromator.This systememploys a background count on both sidesof a peak scan. Temperature measurementwas by meansof. aPt/Pt-I3 percentRh thermocoupleplaced within one millimeter of the crystal.The estimatedaccuracy of the measurements is :t10"C. The data were correctedfor Lorentz- polarizationfactors and crystal absorption.No corrections were made for absorption by the silica capillary. The distribution of reflection intensities accordingto diffraction type is presentedin Table 2.

Refinement Frc. 2. Schematic representation of rings of silicon-aluminum tetrahedra in two subcells showing generation of "half- In their refinement of the structure of "body- atom" pairs by the addition of a body-centering translation to a primitive ring configuration. centeredanorthite" (Anro), FLeetet al (1966) found that the atom configurationwas still primitive despite the body-centering symmetry indicated by the 'e' 'b' prominent and reflection pattern. It was then during the course of the refinementsthe "half suspected,therefore, that the anorthite structure atom" pairs should coalesceto form single atoms (Ann.) might still be primitive above 350oC. The and the atom arrangementsin subcell pairs l-4 starting parametersfor the 410"C refinement were and 2-3 (Fig. 2) should becomeequivalent. those of room temperature transitional anorthite The two data setswere refined using a full-matrix (Ribbe, personal communication), while those for least-squaresmethod. The form factor tables were the 830oC refinement were the final parametersof formulated assumingcomplete Si/Al ordering and the 4l0oC refinement.The useof the primitiveatomic half-ionization of all atoms, from data in Vol. III configuration of transitional anorthite (space group of the Internotional Tables for X-Ray Crystallog- Pl) in conjunction with the high temperattte 'a' raphy. Isotropic approximations to atom thermal and'b' reflectionsresulted in an averagestructural motion were used in all stagesof both refinements. model composedof "half-atom" pairs. In Figure 2 Only structure factors greater than zero were em- the atoms connected by solid lines represent two ployed and these were weighted according to sev- pseudo-body-centeredZ-O rings in the primitive eral trial schemes including ones proposed by anorthitestructure projected on (010). The addition Cruickshank(1965) and Hanson (1965). In the of body-centering symmetry, corresponding to re- last analysis,unit weightingyielded the most rea- finement in spacegroup ('a' 'b' /i and data), results sonable bond distances.In general the refinements in a superpositionof subcellsl-4 and 2-3. Sincethe followed a pattern of cyclesof coordinateand scale rings are not identical, "half atom" pairs are gen- factor refinementalternating with isotropic tempera- erated with the distancebetween them representing ture factor refinement. A problem encounteredin the deviation from body-centeredsymmetry. If the anorthite cell were truly body-centeredabove 350"C, the refinement of both data sets was the marked oscillation of approximatelyten oxygen atoms about relatively fixed positions. During refinement of the TlsrE 2. Distribution of Observed and Unobserved Data gradual with ces- According to Diffraction Type for Miyake Anorthite 410"C data, there was a damping sation at R : 0.13, while in the 830'C refinement the oscillation, though greatly diminished,continued 4rooc 83ooc to convergence.This is undoubtedlyresponsible for the relatively high standard errors in the positional Observed 980 452 L432 I0t3 627 t640 Unobserved 145 624 769 198 597 795 parameters.The final cyclesfor both data sets were Tota 1s f.125 1076 2201 L2II 1224 2435 carried out varying coordinates,scale factor, and THE ANORTHITE CRYSTAL STRUCTURE 669

temperature factors simultaneously.The final resi- Tlrle 5, Continued dualsare: aEon 410"c 830'c atoE 4I0'c 830"c r r Tr (m000) o.6742(18) 0.6763(16) O.(0000)- 0,0283(26) 0.0365(29) 410'c 9300c I 0.8806(7) 0.8820(6) ! 0.2754(13) 0.27s7(r4) z o.1864(6) 0.1864(s) e 0.1491(11) 0.1sls(1"3) B 1.2(t) 1.2(r) B 1.7(2) 3.0(3)

T, (n0i0) o 0.1805 (17) 0.1814(16) 0.(0200) 0.0384(24) 0.0108(32) From observedreflections: 0.076 0.104 ' r a 0.3822(7) 0.3815(7) a 0,2922<13) 0.2952(16) Observedp/asunobserved: 0.099 0.137 a 0.6753<6) 0.673s(6) ; 0.64i1(13) 0.6342(1r) B 0.9(1) 1.3(r) B 1.6(3) 2.1(3) factors at 11(0200) r 0.0059(19) 0.0053(16) 0.(00i0) r 0.4994(23) 0.501,7<22) The observed and calculated structure I o. 1567(6) 0.1586(5) a 0.7846(11) 0.7873(11) z 0.6I11(9) 0.6099(8) ,0,6253<9) 0.6226(10) 410'C and 830oC are given in Tables 3 and 4, B 1.1(1) 1.5(I) B 1,1(2) L5(2)

respectively,zwhile the final structural parameters (02i0) r 0.4994(18) o.(02i0) Tl' 0,5001(13) r 0.5002(24) 0.5210(33) ! 0.6704(6) 0.673s(5) a 0.7974(L2) 0. 7967(1s) are listed in Table 5. z 0.1r26(8) 0.1150(6) z 0.1511(13) o.r572(12) Figure 3 is a plot of the isotropic temperature B 0.8(1) 0.8(1) B r.3(2) 2.2<3) Tl (mOo(r) r 0.9897(16) 0.9958 (20) 0"(m000)o 0.9987(33) 0.0021(31) ' a 0.8110(8) 0.8088 (7) u 0.6808(rs) 0.5780(13) s copy Tables 3 listing the valuesof F(o), F(u), z o.r19z(9) 0.1190(10) ; 0.11s2(14) 0.1187(13) A of and 4 , r.r(1) 1.6 ( r.) B r.3(2) 1.8(3) and F(c) may be obtained by ordering NAPS Document T, (m0i0) r 0.511s(16) 0.5051(17) o.(m200) c 0.0042(40) 0.0114(26) - Number 02122 frorn Microfiche Publications. 305 East 46th a 0.3187(7) 0.3217(s) ! 0.6888(17) 0.6944(11) Street, New York, N. Y. l00l7. Please remit in advance a 0.6204(8) 0.6199(8) e 0.6098(15) 0.6146(10) B 0.8(1) 1.0(1) I 1.8(3) r.3(2) $1.50 for microfiche or $6.50 for photocopies. Please check T,(0000) f 0.6896(28) 0.6940(20) o.(m0i0) r 0.5193(34) O.5222(35) the most recent issue of this journal for the current address ' a 0.1146(13) 0.1173(9) ! 0.1812(1s) 0.186r(rs) and prices. (40 pages). z 0.1s6s(9) 0.L543(7) a o.6023(14) 0.6020(15) B r.6(2) 2.2<2) B i.6(3) 2.4(3)

rr(00i0) r 0.1899(26) 0.19r1(14) o.(nzi0) o 0.5114(39) 0.5060(31) ! 0,6126<12) 0.6130(6) " !1 0.1e26(17) 0.1881(14) Teere 5. Atom Coordinates and Isotropic Temperature z 0.6645(9) 0,6674(5) z 0.0946(1s) 0.0918(12) Factors for Miyake Anorthite at 410'C and 830'C B o.9(2) 0.7(r) B 1,5(3) 2.3(3)

Tr(n200) r 0.6835(20) 0.6881(20) oD(0000) t 0.1821(41) 0.1771(38) a 0.8ill(7) 0.8725(8) 0.1131(r7) 0.1053(17) 4tooc E3ooc 41ooc 83ooc z o.6721(8) 0.6726(7) z 0.r941(r8) 0.1930(17) B 1.1(1) r.2(r) B 2,6<3) 2.5<3) ce(000) t o.2672(LO) 0.2704(.11)0a(20i0) ' 0.0783(18) 0,0822(19) I 0.9878(5) 0.9831(5) " ! 0.4874(8) 0.4886(8) r^(mzi0) i 0. 1666(18) 0.1857(23) o.(0200) s 0.2126(31) 0.2004(3r) . a 0.0822(5) 0.0843(s) z 0.6308(8) 0.6282(8) 0.3799<7) 0.3790(9) l/ 0.1087 (13) 0.1226(r2) a a B 3,8(r) 4.4(1) E o.7(2) r.2(2) z o. r813(7) 0,1819(9) o.6879(r4) 0.6962(r5) B 0.8(1) r.8 (2) B r,3(2) 2.9<3) Qa(zOO) t 0,2740(7) 0.2756(9) o^(7zi,o) r 0.0793(17) 0.0789(18) ^ (34) ,0.0333(3) o.o32s(4) ! 0.4905(8) 0.491e(8) o. (1000) 3 0.01s6(39) 0.0102(21) 0n(00i0) s 0,69a4(32) 0, 7009 y 0.6029(13) 0.6093(15) z 0,5452(3) 0.5468(4) z 0.1313(8) 0.1276(8) " 0.1335(13) 0.1413(7) ! B r.65(7) 2.73(8') b 0.7(2) 1.0(2) 2 O.9943(19) 0,9929\9) z 0.6822(13) 0.6812(14) B \.7<3) r.0(2) B r.1(2) 1.8(3) ca(0i0) t 0,7735(6) 0.7737(8) 0R(0000)t 0.8r03(32) 0.8046(32) - O^(1200) t 0.9785<22) o.9678(24) on(02i0) c 0.6885(31) 0,6996(23) a 0,5326(3) 0.5294(3) A 0.1086(14) 0.1144(13) ' (13) 0.1276(r4) 0.6001(14) 0.59s4(8) z 0.5465(3) 0.5488(3) ? 0.0865(14) 0.0866(Is) a 0.129s I E 0.4869(13) 0.4843(14) z o.1973(14) 0. r91E(I1) B r.27(6) 2.68(8) B r,9(3) 2.6(3) B r,2(3) 2.4 <3) B 1.7(3) r.2(2) c f, Ca(zi0) 0.7623(8) 0.7640(9) o-(0200) 0.8II7(25) 0,8I33(33) o"(m000) o.2043(29) 0.198r(28) D g o^ (1OiO) r 0.4986(39) o.4993(27) t v 0.50ee(4) o.sos3(4) o.ror3(r3) 0.1045(16) ^ (I4) (10) o.8773(12) 0.8832(12) z E a 0.6191 o. 6122 u 0,0655(4) 0.0689(4) 0.6035(10) 0.6110(12) (12) 0.2136(r3' 0.2076(13) B B ? 0.4870(19) 0.48 92 ; 2.2L(7) 2.73(8) r.4(2) 2,8(3) B 1.s(3) 1.8 (2) B r.7 (2) 2.6<3) r1(0000)e 0.0132(r5) 0,0144(14) O"(00i0) c 0.3343(31) 0,3403(27) ' o^(1ziO) t 0.5199(25) 0.5247 (2r) oD(n200) , 0.r6s7(33) 0.r6s5(35) a 0.1548(5) 0.15s6(6) " !0.5947(r4) 0.s95s(r2) jJ O.6262(r5) 0,6269(rr) 0'8s92(15) 0.6639(17) a z 4 0.1069(8) 0,1087(6) 0.s985(15) 0.5994(13) z o,9947(L5) 0.9986(r1) ; 0.7160(16) 0.7176(17) B B t.o(r) 1.3(1) 1.3(2) 2.o(2) B 1.5(3) L.2 (2) B 2,8(3) 3.9(4) Tr (00i0) s c o.2943(27) - 0.5009(7) 0.5011(14) 0a(02t0)- 0.2825(30) o^(2000) s 0.5826(20) 0,58i7(2r) On(nOi0) s 0.6857(30) 0.6880(26) 0.6644(7) 0.6672(6) 0.6068(16) 0,6064(I4) ! ! " a 0.993e(9) 0.9948 (9) q o.3597(12) 0.3604(11) z 0.6026(8) 0,6002(6) z 0.08r9(r2) 0.0851(10) z 0.1s16(9) 0,1ss6(9) ; 0.725r(r3) o.124o

T1(m200)c 0.9998(6) 0.9994(18) O.(m000)' 0.8226(36) 0.8008(38) O^(2200) x 0.5730(23) 0.5775(23) 0^(mzi0) t 0.70211(25) 0,7034(26) y 0.8r15(5) 0.8104(6) " 0.85E5(19) 0.8460(14) ^ 0.3621(11) U a 0.9931(10) 0,9898(10) 4 0.3629(r1) z 0.6106(8) 0.6119(6) z o.r374(r4) 0.1255(20) a o.6473(Io) 0.6s03(11) ; 0.204r(r2) 0.2023(r1) B 0,9(r) r,7(1) B 2.6(3) 3.5(4) B r.7<3) 1.8 (3) B 1.3(2) r.7 (2) Tr (n210) r 0.5066(17) 0.5063(15) o.(n200) 4 0.8129(3I) 0.8142(23) a 0.3238(5) 0.3234(s) " a 0.8546(16) 0.8556(I1) factors for Ca, Si/Al, and O as a function of tem- z 0.1r29(8) o,r1o5(7) a 0.6066(n) 0,6015(9) B 1.1(1) 1.0(r) I r.6(3) r.6(2) perature.The data of Quareni and Taylor (1.971) T2(0200) t 0.6723(20) 0.6742<16) 0-(noi0) r 0.2960(31) 0.3166(38) on albite are plotted for comparison.The tempera- y 0, Ios9(9) O.IOE7(6) " A 0.3s46(16) 0.3706(14) z 0.662s(8) 0,6658(6) z 0.6161(12) 0,6270(20) ture factors of all atoms extrapolateto zeto in the 8 r.4(r) 1,3(1) B 2.1(3) 3.6(3) vicinity of 0oK. The similarity of the Si/Al and O 1"(02i0) r 0.1834(i9) 0.I842(I5) o-(mzi0) t 0.3603(37) 0.3402(33) o indicatesthat, a 0.6063(8) 0.6037(6) A 0.3592(20) 0.3569(17) behavior for both albite and anorthite z O.1545(7) 0,1535(6) a o.r262(rs) 0.1319(13) B O.7(r) 1.2(r) B 3,2(4) 3.7(4) for anorthite, the refinementusing the "half atom" 670 F. F. FOIT, AND D. R, PEACOR

Tesrp 6. Tetrahedral Interatomic Distances for Primitive (An,*) Miyake Anorthite at a 6lbrte (Qurreni & TayLor, l97l) Anorthite at 25"C and 410"C and 830"C

prinitive anorthiEe Miyake anorthite 800 25" C 4100C 8300c

(0000)- (1000) rr- oA r.645<2) 1.62(3) 1. 63(2) 0;(0000) 1.61e(2) 1. 65(3) 1. 69(3) oc(0000) r.582(2) | .62(2) 1.62(2) 0t (0000) 1,616(2) r.53(3) r,54(2)

Mean: 1.616 1.62

r, (00i0) - 04(10i0) 1.632(2) r.71(3) t .69(2) 0B(00i0) r . 606(2) 1. 60(3) r. 60(2) 0c (00i0) 1.588(2) r.s] <2) r.57(2) oD(00i0) r.626(2) r.75(2) 1.75 <2)

Mean: 1.613 t.66 1.65

0 r1(mz0c) - 04(1200) r.647(2) r.69(2) 1.73(3) 0s (mz0c) r.617<2) 1.62(3) 1.59(2) oc(mz0c) r.6t 1(2) r-.58 ( 2) r.s0(2) Flc. 3. A plot of isotropic temperature factors (B) os 0p (nz0c) r.51r(2) 1. 60(2) r.64<2) temperature (K) for albite (Quareni and Taylor, 1971), for Mean: 1.613 1.62 primitiveanorthite at25"C (Wainwrightand Starkey,1971), and for Miyake anorthite at 410 and 830'C. r1 (mzic) - oA(12i0) 1.62(3) r.62(2) oB(nzic) 1 583(2) 1.53(3) 1.s9(3) oC(mzic) 1.599(2) r.7o(2) r.16(2) oD (mzi c ) 1.626(2) r. 61(2) 1.64(2) model was realistic and accountsfor all apparent Mean: I.613 1.62 r,65 positionaldisorder. r2(0200) - 0A(2200) 1.635(2) 1.61(2) r.68(2) oB(0200) 1.624(2) 1. 6e(3) 1.64(3) Description and Discussion of the Strucfure at oc (n0i0) I 606(2) 1.56(3) 1.59(2) Elevated Temperatures op(n00c) 1.605(2) 1. 60(2) 1.64(2) llean: t.617 1.62 L.64 In general, the average structure obtained using 'a' the afid'b' reflections measured at high tempera- T2(02i0) - oA(22i0) r .611(2) r.65(2) r.61(1) oB(02i0) | 628(2) r.57<3) r.s9(3) tures is similiar to the structures for primitive anor- 0c (m000) r .614<2) 1,70 (3) 1.69(2) 0e (m0ic) r.514(2) 1.60(2) L.63 (2) thite obtained by Kempster et al (1962), Megaw Mean: I.608 t.63 t 63 et aI (1962), and Wainwright and Starkey (1971)

(for structural details refer to the excellent diagrams T2(mooc) - oA(200c) 1.644(2) )..66(2) r. 65(2) cB (m00c) 1.581(2) 1.67(3) r.66(4) in their papers). Therefore, as is indicated by the oc (ozic ) 1.647<2) r.64(2) 1.57(3) (2) r 61(2) r.53(2) near absenceof the 'c' and'd' reflections, the anor- oD(0200) r.62e 1.60 thite unit cell does not beco ne nearly body-centered Mean: 1.615 1,65

above 350'C, but retains its primitive character at r2(mOic) - oA(20ic) r. 641(2) | .62(2) 1.63 (2) o- (moic) 1. 618(2) 1.55(3) 1.53(4) least to 830'C and probably to the melting point. o; (ozoo) 1.586(2) 1.54(2) 1.65(3) It is of interest, therefore, to examine the ways in oD(02i0) 1.616(2) l.6s (2) 1.74(2) which the structure within the unit cell changes as Mean: 1.615 1.59 1.64 a function of temperature. To accomplish this we r1(0200) - 06(I200) 1.760(2) r. t 0(2, 1.70(2) made a comparison of the geometry of the coordina- os (0200) 1.743(2) 1.70(2) L.72<3) o6 (0200) 1.709(2) 1.15 (2) 1. 78(2) tion polyhedra and then analyzed the changes in op (0200) r .776 (2) r.13(2) r.63(2) pseudosymmetry of the atomic configuration in the Mean: r,147 r .71 anorthite unit cell. rt (02i0) - oA(rzi0) 1.112(2) 1.82( 3) 1.83( 2) The interatomic distances and angles were calcu- oB(02i0) 1.155 (2) 1.80(3) |.76(2) os(0ziO) r.727(2) 1,10(2) 1.65 (2) lated using the function and error program oRFFE oD(02i0) L.t 67(2) 1.7e(2) r.87 (2)

(Busing et al, 1964) along with the estimated errors Mean: t.755 1.78 1.78 in cell parameters and the correlation matrix of the - r.111(2) 1. 78(3) r.13 (2) T1(m00c)- 0o(1u00) least-squares refinements. An examination of the o;(m00c) | .705<2) 1.63(3) 1..7 2(4) os(m00c) r.738 (2) i.68(2) 1.6e (2) data in Tables 6, 7, and 8 reveals a greatervariation 0D(m00c) t.77e(2) 1.84(2) 1.81 (2)

in individuaL T-O distances and O-T-O angles Mean: 1.750 r,73 within a given tetrahedron at high temperaturesthan at low, while for the Ca coordination polyhedra wainwrishE €nd Starkey (1971) THE ANORTHITE CRYSTAL STRUCTURE 671

Terre 6, Continued large decreasein difference between the maximum and minimum angles(Table 9). primitive anorthite Miyake 0C anorEhite The next step in the examinationof the effects 25 4lo"c 83ooc of Tr (noic) - (10i0) 0^ r,777(2) 1.73(3) L.7 (2) temperatureon the anorthite structure was to make (moic) s oB r.74 t- (2) 1.81(3) r.73(4) oc (moic) 1.752(2) L.79(2) r.77(2) a detailedanalysis of the shifts in atomic coordinates 09 (mOic) r.702 (2) 1.60(2) r.63(2) with regard to the pseudosymmetryof the structure. Mean: t.745 r,73 L.73 The primitive anorthite structure is highly pseudo-

12(0000) - symmetric with the atomic configurations in the oa(2000) r.760 (2) 1.75<2) r.82<2) (0000) 0d 1.769 (2) 1.68 (4) 1.59 (3) four subcellsrelated to one anotherby three pseudo- 0C(n210) r.740(2) r,72(3) r.72(2' OD(mz0c) 1.698 (2) r.69(2) 1.68(2) symmetry operations: body-centering,C-centering, Mean: r.71 1.70 and c/2 translationalsymmetry. If one of a pseudo-

- symmetrically related pair of atoms is transformed T2(00i0) oA(20i0) r.769 (2) 1.78 (2) r.77 (r) ou (00i0) r.?5r(2) 1.81(4) 1.87 (3) by the correspondingtrue symnetry operation and o; (m200) r.754(2) r.73(3) t,14(2) oi (nzi c) r.727<2) r,69(2) 1.68(2) the distancebetween the two is calculated.this dis-

Mean: 1.750 L.75 r.77 tance (in A) representsthe deviation from true symmetryfor that pair of atoms.If the value is zero, - T2(mzoc) 0A(220c) r .15s (2) 1.81(2) r.75(2) Q(nzoc) r.748(2) r.70(3) r. 74(3) then the pair would be truly symmetry-relatedand oC(00ic) |.7t 6(2) r.12 {2) 1.73 (2) oD(0000) r.7s7 (2) r.72<2) r.75 (2) not just pseudo-related.This analysiswas carried

Mean: 7.744 t.74 7,74 TAsrE, 7. Bond Angles at the Tetrahedral Sites for Primitive - T2(nzic) 04(2zic) r.759<2) 1.71(t) r.75(2) Anorthite (Anrm) at 25"C and for Miyake Anorthite at oB (nzic) r.713(2) r,77(4) r,73(4) 410"Cand 830"C 0c(000c) r.73s (2) r.75 <2) 1,70 (3) oD (00i0) 1.774(2) r.7s (2) r.75(2) Mean: r.75 r.73 oA-oS ol-oC oe-oo og-oc og-oo oc-oo

r1 (0000) r00.90 r18.07 101.40 111.73 113.80 110.44 crand Mean Si-o: 1.6t4 1-63 410 r03.5 r16.4 110.6 106.5 119.4 101.1 830 I03;0 112.8 r12.4 106.2 118.7 103.8 Grand Mean A1-O: L.747 t.74 r1 (ooio) 25 103.09 116.38 102.05 110.92 rr3.s6 110.51 410 99,0 119.1 95.I r17.r 106.4 I16.7 830 96.tl t23.6 93.3 119.0 105.8 rr4,4 T1(m200) 25 100.78 rr3.70 108.85 111.38 113.45 108.61 410 103,3 118.3 to4.9 112.8 108.8 108.2 thesevariations are not as marked. Two factors con- 830 101.9 119.1 100.7 116.r 109.0 108.7 tribute greatly T, (mzi0) 25 106.15 rr2,27 102.08 113.14 t12.06 110.54 to thesevariations: (1) the greater 410 102.5 tO7.6 108.6 113.9 116.8 106.8 error in the positional parametersof the high tem- 830 105.6 107.8 109.8 r11.1 1I4.6 tO7.1 perature refinements; (2) the distance between T2 (0 200) 105.34 10r.24 1r0.17 112.51 111.79 114.83 410 109,0 106,2 104.2 107.5 107.8 L2r.7 mean positionsof the atomsmay not be realisticin 830 107.8 101. 6 100.6 tr9.4 r05.9 119.2 I2Qzi9) 25 109.I9 102.50 r10.99 r12.86 ro7.14 114.10 view of the increasedanisotropy of atomic thermal 410 105.4 99.0 116. 1 tr4,2 r13.3 108.1 830 106.8 97.9 118 .0 rL7.7 11r.6 I04 8 motion at high temperatures.If, rather than com- T, (m000) 25 11 r.89 r04.43 108.95 1r2.98 to7.26 113.33 paring 410 1I0. 1 104.6 t-06.3 117.8 107.0 110.6 individual distances,we compare mean 830 115.4 r07.8 1r3.1 101.4 105.8 107.0 values,the effects T2(m0i0) 108.79 106.37 107.89 112 50 I08. 75 t1.2.27 of these factors are minimized. 410 rc8.0 109.1 110.2 106.1 110.3 113.0 This is reflectedin both the reasonablemean bond 830 103.7 103.1 106.4 I 18.r r08.6 115.4 r (0200) 99.75 ll7.5L 98.98 rL2,77 115.60 111.25 distancesfor individual polyhedra and also in the r 410 99.8 rr3.3 103.4 115.9 119.8 r04,2 grand means for all coordination polyhedra of the 830 101.0 rr1.1 tl3.2 109.1 117.8 r04,7 11(02i0) 25 97.25 121.03 97.t6 113.20 1t6.44 I 10.66 same type. The following generalizationsregarding 4r0 98.2 t24,1 93.8 110.0 r13.5 115.6 changes in the anorthite 830 105.8 r24.4 86.8 t17.6 r05.5 114.4 structure with increasing T1 (n000) t07,57 r1.2.19 99.12 t14,34 111.15 111.40 temperature 4r0 106.3 113.7 96.4 116.3 109.3 r12,9 can be made on the basisof our data: 830 105.9 r13.9 94.9 108.9 114.6 t1.7.6 ( 1) there is a significantincrease T1(m010) 25 99.2a 113.08 108.52 113. 59 rr3.26 108.83 in size of the Ca 410 99.1 111.6 111.2 111.4 114.8 108.7 coordination polyhedra with no apparent changein 830 100.0 106.3 I08.5 t20.7 I 10.5 105.8 the regularity (as deduced from the variations in r2 (0000) 25 108.87 r04.rr I07.84 tr2.63 108.08 115 .02 410 111.5 103.4 r05.2 ro7,7 108.4 120.5 Ca-O distancesand O-Ca-O angles); (2) except 830 115.4 98.8 99.8 108.4 110.5 t23.5 r2 (0010) 100.96 99.94 107.64 1t2.74 115.95 116,74 for a possibleincrease in distortion,the mean dim- 410 97.8 99.8 ttt.2 1r8.9 116.0 110.8 ensions 830 95.7 rO2,3 I14.1 I18.1 tL6.2 I08.9 of the Si and Al tetrahedra remain un- T2(n200) r08.59 105. 69 103.15 110.45 r1r.95 r16,30 changed; (3) while the 410 111.5 r00.9 I01.5 1tl.7 t I0.8 rr9 .4 mean and grand mean 830 rr2.6 99.2 98.1 110.0 ttz.9 122.3 T-O-T anglesremain essentiallyunchanged, T2(nzio) 25 110.32 rO5.24 107.3s 111.99 108.71 1r3.09 two of 410 106.1 1i0.3 I09.8 r11.7 I10.8 108.2 the four rings becomemore regular as reflectedin a 830 105-9 tr2.l r13.0 110,7 L09.2 105.9 672 F. F. FOIT, AND D. R, PEACOR

TlrLB 8. Calcium-Oxygen Interatomic Distances Less Than Tenre 9. T4-T Angles in the Four Rings of Primitive 3 A for Primitive Anorthite (Anrm) at 25"C and Miyake Anorthite (An'*) at 25'C and Miyake Anorthite at 410"C Anorthite at4lO"C and 830"C and 830'C

primitive anorthite Miyake anorthite Drimltive' anorthite Mivake anorchi!e 25oc 4ro"c- 83ooc z5oc 4tooc 8300C (1) (r) ca(000)- 0o(2000) 2.292(2) 2.33(2) 2.3r(2) oB(0000) 129.41 I 34. 134. oi;(000c) 2.378<2) 2.45(2) 2.so(2) on(0000) 136.45 r42. (2) 136.(2) on(oooo) 2.39o(2) 2.52(3) 2.51(3) Oi (m200) r44.07 146.(r) 141,(1) oi (r00c) 2.5r5<2) 2.5s(3) 2.s7<2) Oi (mz0O) 165.21 162.(2) 158.(1) o;(.ooo) 2.538{2) 2.62(2) 2.49(2) oA(looo) 2. 608(2) 2.74<2) 2.90(1) Mean: 143.78 r46. r42.

Mean: 2,454 2,54 2,55 Ou(0200) r38.19 141.(i) 146.(1) oD(0200) 723.67 r28. (2' r45. (2) - 2,333<2) 2.33<2) 2.36(2) ca(zoc) o|(zzoc) (m000) (2) (2) 09 (020c) 2.372<2) 2,t5 (2) 2.6s(3) o[ t69.67 162. 155. 0B (0200) 2.443(2) 2.50(2) 2.62(2) o9(m000) 139.70 r4o. (2) r36.(r) os (n200) 2.494(2\ 2.5O (2' 2.46 (r) 0a (120c) 2.496(2) 2.s6<2) 2.64(2) Mean: 143,03 L43. t46. o; (moic) 2.559(2) 2.51(2) 2.64(2) 06 (1200) 2,733(2) 2.79(2) 2.13<2) oR(m0i0) t45.76 149.(1) r54. (2) 2.53 2.59 Mean: 2.490 oi (m0i0) 165.08 162.(2) 1s8,(2) o; (ozio) 125.88 126.(1) 128.(1) (2) 0n (02i0) 135.0r 131.(r) 120.(1) ca(0ic) - oA(20ic) 2.336(2) 2.3s(r) 2,37 oB (00i0) 2 .426(2) 2,39(2) 2.4r<2) (3) oD (00ic) 2.434(2) 2.4r (2) 2.40 Mean: 143,18 t42. 140. (2) 01 (10ic) 2.448(2) 2.36(3) 2,34 o; (molc) 2.494(2) 2.65<2) 2,68(3) oc (nz0c) 2.563<2) 2.70(2) Oo(m210) 165.67 L67.(2) 167. (1.) oA ( 10io) 2.8r7(2) 2.80(3) 2.72(2) oi (mzi0) r35.14 r4r.(2) r4o. (2) oE(00i0) 131.(1) 131.(1) 2.52 Mean: 2.503 o; (ooio) r27.r5 125.(1) r28. (2)

(2) Mean: 14L.64 140. r42. ca(zi0) - 0s(22i0) 2.300(2) 2.37(r) 2.36 oR(0zic) 2,405(2) 2,40(2) 2.42<2) o; (ozio) 2,440 <2) 2,45(3) 2.3r (2) Grand Mean: t42.90 r43. 143. 0o (12i0) 2.454(2) 2.40(2) 2.43(2) o; (1zic) 2 ,6t6 (2) 2,67(2) 2.68(2) Oo(nzi0) 2,717(Z) 2,98(2) 2.90(2' 0c (n000) 2.834(2) 2.75(2) 2.79(2) of temperature(25,410 and 830'C) and compar- Mean: 2.525 2.57 2.56 ing thesewith the observedchanges in the intensities 'c' Grand Mean: 2.493 2.56 of the reflections.In addition, this type of analysis allows separateassessment of the relative contributions of the Ca and the framework atoms to the out for all atoms and all pseudo-operationsfor the variation in intensity of both individual'C reflections structureat 25oC, 410'C, and 830"C. The results and the 'c'reflectionsas a whole. are presentedin Table 10a and show that with an 'c' with increase in temperature,the separation between A random sampling of 264 reflections atoms related by the pseudo-bdy-centeringand the Tnsrs 10a. Deviation (in A) from "True" Symmetry for pseudo-C-centeringincreases. fn contrast, no dis- Pseudo-Symmetrically Related Atom Pairs tinct trend is evident from coordinate shifts of the atoms related by the c/2 pseudo-translation. pseudo- 25oc walnwrighi c starkey (1971) 4looc 83ooc Our investigationsas well as those of others in- :;;;;." dicate that variations in 'c' reflection intensity and a+b+c 0.268 o.29 0.34 in diftusenesswith temperatureare a function of at 2 leastone of the followingparameters: (1) the small a+b 0.170 o.2L O.25 shifts in atomic coordinatesand the marked changes 2 in thermal motion observedin the high temperature c refinements,and (2) an increasein domain bound- 2 0.314 0.26 0.29 ary frequency with a concomitant increase in the relative volume of the structure represented by Tesre l0b. Deviation (in A) from True Body-Centering and Oxygen Atoms domain boundarieswithin which the structure must for the Calcium, Silicon/Aluminum, - be very nearly body-centered(Czank et al, 1970; 25oc 41ooc 83ooc Laves et el, 1970). A comparisonof thesetwo fac- 'c' Ca 0.770 0.63 0.56 tors may be obtained by computing reflection si/Al 0.152 0.17 0.20 intensities for the anorthite structure as a function o 0.263 0.30 0.37 THE ANORTHITE CRYSTAL STRUCTURE 673 sin d values within the range of the measured'a' The following generalizationscan be made on the 'b' and reflections were chosen for computation. basis of the data in Table 11. First. the Ca atoms 'c'reflection Structure factors were then calculated separately have a greatercontribution to the total for the Ca atoms and for the Si, Al and O atoms intensity and its diminution with temperaturethan using the positional and thermal parametersfrom the framework atoms (refer to the mean values of each of the three refinements[25' (Wainwright and F6aand Fn.,-").Secondly, the calculated(Table 11) Starkey,l97l); and 410' and 830"C1.The separate as well as the observedintensity reductions (Fig. 1) contributionsof the Ca atoms(F.") and the Si, Al were for the most part quantitatively similar regard- and O atoms (Fr"**") for a number of the most less of the relative contributions of the Ca atoms 'c' intense reflectionsare listedin Table 11, with the and the framework atoms. This further emphasizes ayeragesfor all 264 appearingat the bottom. These the "cooperativebehavior" of the Ca and the frame- reflections are some for which (l) either the Ca work atoms in bringing about an overall reduction 'c' atoms(025, 045,065, 445,441 and 625)or the frame- in reflection intensity. Third, overall changesin work atoms(221,227,267,42i,623, and 663) have atomic positions and thermal motion account for the greatestcontribution, and (2) intensity has been approximately47 percent of the total intensity de- 'c'reflection previously monitored as a function of temperature creasefor the over the range25-4l0oC (025and 027;Foit and Peacor,1967b), (227 and245, and for approximately 2 percent over the range Fig. l), (045, Czank et al, 1970) and (065, Laves 410-830'C. et al, 1970).The valuespresented in the third column While these decreasesconform to the observed (I"o-oo"i,")are equal to (F", * F,".^")' which is generaltrend of the'c' reflectionintensity decreases proportional to the calculated intensity for a par- with temperature(Fig. 1, Brown et al, 1963; Foit ticular reflection. Since no corrections for Lorentz, and Peacor, 1967b; Czank et al, l97O; and Laves polarization, or absorption effectshave been made, et al, 1970), the observeddecreases in the range Icomposireis only an approximaterepresentation of 25-41,0"C are greater than can be accounted for 'c' the observed reflection intensity relationships. solely by the changesin atomic positions and ther- However, these corrections are not significant here mal motion. Assumingthat both the calculatedand since this analysis involves only comparison of observed 'c' reflection intensities at elevated tem- intensity as a function of temperature. peratures are accurate (i.e., the reflectionshave

TAels 11. CalculatedCa and Framework Contributions to Structure Factors and Resultant Intensitiesfor'c'Reflections

Reference 25"c 4100c 8300c 'c' to high temperature reflection Fc. Ffr.^. rcomposite Fc" Ffr*. rcomposite Fc. Ff.*. rcomposite observation

-11.5 -13.9 025 645 6.1 5.8 L66 -6.1 -8 .4 2L0 Foit, Peacor I967b ntR -28.0 L7.6 108 -zL.z a. I z4u -21.0 1.8 369 -15.4 -11.8 02'7 740 -11.6 -10.3 479 -9.5 -13.7 538 Folt, Peacor L967b 045 -30.2 -4,2 1183 23.3 47r Czank et -al. 1970-

065 -27.7 -8.r L282 -zz.v -J. -5. o tbz 2L.3 I 697 Laves et -aL. L970- 22L -4.6 -20.1 610 -4.5 -t7.2 47L -4.9 -10.5 237 227 -0. s -2L.5 484 -2.r -r3.3 237 -1. 3 -1r.6 156 22't 7.5 32.8 L624 4.5 16.5 44I ?o 13.6 272 This study

245 30.7 7 .9 1490 24.6 s.8 924 24.7 9.4 LL62 This study 267 L4.2 -31. 7 306 tt.o -16.9 35 7.5 -r6.0 72 -u.c Lt.z zt9 -1.9 9.0 50 -1.0 1.3 0 445 24.6 r. I 552 -I9.0 0.7 335 18.7 3.6 228

447 -25.I l. 5 557 -20.7 4.7 257 18.6 r.5 292 623 4.6 L9.4 576 4.7 8.8 r82 2.8 2-2 25 L8.2 1.6 393 L3.2 0.5 r88 rl.7 -3.2 72 -0 E 17 ? 242 -1.3 L2.2 119 -0.7 6.0 28 ean ror 264 re- 8,8 6.4 205 4.5 109 6.0 4.7 105 flections 674 F. F, FOIT, AND D. R, PEACOR been fully integrated),it is difficult to escapethe in domain texture in the temperature range 25- conclusionbased on this data that reversiblechanges 400'C is in part contradictory.The X-ray diffrac- in the domain texture are taking place over the ap- tion data suggeststhat the frequency of domain proximate temperaturerange 25-4O0oC,concomi- boundaries increases as a reversible, equilibrium tant with the observedchanges in atomic positions function of temperature. On the other hand, elec- and thermal motion within the domains. tron diffraction data as earlier reviewed shows that Megaw (1962) proposedthat disorderof Si and the original textures are unaltered with heating, Al provides the principal control over the forma- providedthat temperatureson the order of 1000'C tion of out-of-stepdomains in quenchedspecimens. are not exceeded(Nord, personalcommunication). Smith and Ribbe (1969) have reviewedthe perti- One explanation consistentwith these data is that nent data and note that "If Megaw'sidea is correct, although the frequencyof domain boundariesvaries the domain structure should be relatively unaffected in the temperature range 25-4O0oC,the occurrence by heating and cooling cycles below the tempera- of specificboundaries is reversiblydetermined by the tures at which the Si and Al atoms can diffuse. . . . existing Al-Si distribution. We emphasizethat this If, however, the domain structure is controlled by is highty speculative,and appearsto be inconsistent uncorrelatedvibrations of the aluminosilicate frame- with the electron diffraction data of Nord. who work, such temperature cycling would drastically notes that the image of the domains simply "fades" 'c' changethe domain structure." The intensitiesand with increasing temperature, as the reflections diffusenessof the 'c' reflections provide a measure becomeincreasingly diffuse. It is also interestingto of the nature of the domain structure. The crystal note with regard to the "bouncing" Ca atom mech- usedfor our intensitymeasurements has undergone anism suggestedby Smith and Ribbe (1969) that a number of heating cycles. For example,intensity there is a significant increase in the pseudo'body- data have been collectedat elevatedtemperatures centerednessof the Ca atoms over the temperature on threeoccasions (410"C for 10 days,830oC for range 25-4I0oC, while above this range there is 6 days,and againat 830oCfor 6 days),the crystal little or no change (Table 10b). While our data being returned to room temperature after each set does not by itself provide conclusive evidence of of measurements.The samecrystal was alsoused to either the absenceor the presenceof domain tex- study the 'c' reflectionsas a function of temperature ture changesin the temperaturerange 25-400oC, (up to a maximum of 1003"C for a brief period) they do suggestthat atomic position and thermal both before and after measurementof these data changeswithin the existingdomain structure are not sets,with the resultsof the first study being reported adequateto explainthe observedchanges in the'c' by Foit and Peacor (1967b). Figure 1 showsthe reflectionintensities. variationsin integratedintensity for some selected 'c' reflections as determined after the crystal had Acknowledgments undergonethe heating and cooling cycles.These re- Science Foundation sults are qualitatively similar to those obtained with This study was supported by National GA-994 and GA-17402 with the senior author re- the first heatingexperiments, except Grants that they were ceiving additional financial support through an Owens- obtained using monochromated radiation, which Illinois Fellowship in Crystallography and a National Science permitted observation of residual intensity at high Foundation Postdoctoral Fellowship. temperatures.In addition,electron microscope stud- 'c' ies of anorthite have shown that the domain tex- References ture is altered only by heating to high temperatures. (1972) structure of a lunar One sample of anorthite examined by G. Nord AIrLEMAN, D. E. Crystal Ans' Amer. Crystallgr' Ass. (personal plagioclase of composition communication) exhibited a change in (abstr.) Amer. Mineral. 57, 324' domain texture only after heatingto 975oC. These BnrNrIrtenr, D., eNo J. L. Srlnurr (1958a) Magnetische resultsare entirely consistentwith Megaw'sproposal Kernresonanz von aAl im Anorthit, CaAl^Si,Or. Ilelu. that the domain texture of quenchedsamples as ob- P hys. Acta, 41, 27 4-28 l. - ( magneticresonance of served at room temperaturemay be altered only by AND 1968b) Nuclear oAl in anorthite tCaALSLO"l. Geol. Soc. Amer. Annu, heatingto the very high temperaturesrequired for M eet., Abstr. Programs, 37-38. Si-Al interchange. BnowN, W. L., W. Horrulu, rNn F. Lrvrs (1963) Uber The evidenceregarding the possibilityof changes kontinuierliche und reversible Transformationen des THE ANORTHITE CRYSTAL STRUCTURE 675

Anorthits (CaAl"Si"O') zwischen 25 und 350"C. Naturwis- -, M. Czlpr, lNo H. Scnulrz (1970) The tempera- senschaften,50,221, ture dependence of the reflection intensities of anorthite BnuNo, E., eNo G. GezzoNr (1967) Ricerche roentgenogra- CaAlSi,Os and the corresponding formation of domains. fiche su plagioclasi bytownitico-anorthitici tra 15'C. e Schweiz. Mineral. Petrogr. Mitt. 50, 519-525. 1300'C. Period. Mineral. 35, 683-69'8. MEclw, H. D. (1962) Order and disorder in feldspars. BusrNc, W. R., K. O. MenrrN, eNo H. A. Lpw (1964) A Norsk Geol. Tidsskr. 42/2, 104-137. Fortran crystallographic function and error program. U.S. C. J. E, KrMpsrnn, eNo E. W. Reoosrovlcn (1962) N at. Tec h, I nf orm..Seru. ORI\L.TM-306. The structure of anorthite, CaAl^SLOs.II. Description and CnurcxsneNx, D. W. J. ( 1965) Errors in least squaresmeth- discussion.Acta Crystallogr. 15, 1017-1035. ods. In J. S. Rollet, Ed., Computing Methods in Crystal- Miiurn, W. F., H. R. WpNx, eNo G. Tttoues (1972) lography. Pergamon Press,Oxford. Structural variations in anorthites. Contrib, Mineral, CzeNx, M., F. Levns, lNo H. Scsulz (1970) On the Pe trology, 34, 304-3 14. domains of anorthite. N at urw issenschal t en, 57, 128. QuenrNr, S., aNo W. H. Tlvlor (1971) Anisotropy of the Frenr, S. G., S. CneNonAsEKHAR,AND H. D. MEcAw (1966) sodium atom in low albite. Acta Crystallogr. 27,281--285. The structure of bytownite ("body-centered anorthite"). Rlsnr, P. H., rNp A. A. Corvu-re (1968) Orientation of the A cta C ry st al lo gr. 21, 7 82-8O1. boundaries of out of step domains in anorthite. Mineral. Fort, Jn., F. F., ,rNo D. R. PEAcoR(1967a) A high tem- Mag. 36, 814-819. perature furnace for a single crystal X-ray diffractometer. eNo H. D. Meclw (1962) The structure of transi- J. Sci.Instru.44, 183-185. tional anorthite. Norsk Geol. Tidsskr. 42/2, 158-167. AND _ (1967b) High temperature diftraction Surru, I. V., eNo P. H. RrsrB (1969) Atomic movements data on selected reflections of an andesine and anorthite. in plagioclase feldspar: kinetic interpretation. Contrib. Z. Kristallogr. 125, 147-156. ' Mineral. Petrology, 31, 157-202. HrNsoN, A. W. (1965) The crystal structure of agulene, Srrwlnr, D. B., G. W. WerxBn, T. L. Wnrcur, lNo J. I. S-trinitrobenzene complex. Acta Crystallogr. 19, 19'-26. FnHBv (1966) Physical properties of calcic labradorite Kerursrnn, C. J. E., H. D. Mscew, a,NoE. W. Rloosrovrcn from Lake County, Oregon. Amer. Mineral.51, 177-197. (1962) The structure of anorthite. CaAlfi"O". L Struc- Welxwnrcur, I. E,.,eNo J. Stenxev (1971) A refinementof ture analysis.Acta Crystallogr. 15, 1005-1017. the structure of anorthite. Z. Kristallogr. 133,75-84. Levts, F., lNo J. R. GorosMrrn (1954) Long-range-short- range order in calcic plagioclases as a continuous and reversible function of temperature. Acta Crystallogr. 7, Manuscript receiued, October 26, 1972; 465-472. acceptedfor publication, March 30, 1973.