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The Anorthite Crystal Structure at 410 and 830"C

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The Anorthite Crystal Structure at 410 and 830 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 refine- ments 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 (cal- culated) 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 alumino- silicate 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 <n 200 taking place over the temperaturerange where the z 'c' tJl intensitiesof the reflectionsare subjectto major F change,while the refinementat 830oC z was carried roo out both to revealany additionalchanges and to pro- vide a further basisfor comparisonof the structures at 25oC (Wainwright and Starkey, l97I) and at o 410'C. o roo zoo 300 400 500 TEMPERATURE
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