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The Anorthoclase Structures: the Effects of Temperature And American Mineralogist, Volume 67, pages 975-996, 1982 The anorthoclasestructures: the effectsof temperature and composition GBoncn, E. Henlow Department of Mineral Sciences American Museum of Natural History New York, New York 10024 Abstract Cell-parameterand structurerefinements using single-crystal X-ray difraction havebeen carried out on three natural low-Ca anorthoclases(K-analbites)' (Or:z.sAboouAno s, Or223Ab7s.3An6 e, Or133Abs3 7An2 5), at room temperatureand at temperaturesabove the triclinic/monoclinic symmetry inversion (400', 510", and 750"C, respectively). Room temperaturecell parametersare consistentwith monoclinicSi,Al topochemistryand a high degreeof disorder (tr : 0.50 to 0.52 from the b-c plot). A room-temperatureinversion compositionof 0136is extrapolatedfrom cos2a*and T-O-T angleplots. Mean M-O bond lengths and T-O-T angles increasewith potassium content, whereas their variances decrease,indicating a trend to more regular coordination, as in sanidine. A similar relationshipdevelops with thermal expansionand the structuralchanges associated with inversion to monoclinic symmetry, but the individual (M-O) values suggest a non- isotropic, ellipsoidal expansionof Na-rich alkali cavities. Examination of dimensional changein the structuresdue to changein temperatureand alkali compositionindicates that most of the effectis relatedto changein M-O bonding,with the aluminosilicateframework respondingpassively. Anisotropic thermal modelsfor singleM sitesproduce non-positive definiteellipsoids. Modeling with isotropic split-sitesyields four parts for the more sodic structures,essentially identical to the models for high albite or analbite. For the more potassiccrystals, a separateisotropic K site and an anisotropicNa(Ca) site is indicated. Introduction Wainwright, unpublished,see Smith, 1974:'Ptewitt et al..1976;Winter andGhose, 1977;Winter et al., Thereis much interestin characterizingthe varia- 1979).which are all triclinic (Cl) at room tempera- tion in the feldsparstructures because of the abun- ture, have shownthat they only becomemonoclinic dance and importance of feldspars in petrologic (monalbite, CLlm) upon rapid heating through a processesand becauseof their generalsignificance displacive transformation when the Na-feldspar is in mineralogicalstudies of exsolutionand polymor- very highly to fully disorderedand has monoclinic phism,especially order-disorder. The structuresof Si,Al topochemistry.Disordered Na-feldsparwith disorderedalkali feldsparswith near end-member triclinic Si,Al topochemistryis termed high albite compositionshave been widely studied.Sanidine, a whereasNa-feldspar that has monoclinic Si,Al to- monoclinic (CZlm) hiph-temperature modification pochemistry but a triclinic morphology (i.e., at of K-feldspar, has been examined at 23'C with temperaturesbelow the displacivetransformation) neutrondiffraction (Brown et al., 19'14)and at23", is termed analbite.These studieshave also indicat- 400'and 800' C with X-ray diffraction (Ribbe, 1963; ed the possibilityof positionaldisorder of the alkali Weitz, 1972;Phillips and Ribbe, 1973a;Ohashi and cation(M) site in the collapsedand elongatedcavity Finger, 1974,1975:'Dal Negro et a|.,1978).These at room temperature and have characterized the studieshave shown that sanidinemay have varying nature of the expansionor inflation of the M-site degreesof Si,Al disorderconsistent with the mono- cavity upon rapid heating. clinic symmetry(monoclinic Si,Al topochemistries) Less is known about the structuresof disordered and have elucidated the nature of the irregular alkali feldsparswith intermediatecomposition' An- coordinationof the alkali cation. Similarly, studies orthoclases,one of the most abundant naturally- of disordered Na feldspars (Ribbe et al., 1969; occurring alkali feldspars of this type, are recog- 0003-m4)v82l09I 0-0975$02. 00 975 HARLOW : ANORTHOCLASE STRUCTT]RES nized by petrographersby their volcanic associa- A structural study has been completedon three tion, rhombic shape and fine polysynthetic, often natural low-Ca (less than Anls) anorthoclase(K- cross-hatched("tartan") twinning. In terms of analbite) samplesat room temperature and at tem- Si,Al order they may occur as topochemicallytri- peraturesat which the crystals are monoclinic (from clinic K,Ca-high albite, topochemicallymonoclinic 400'to 750"C. The resultsof this study are reported K,Ca-analbite or a cryptoperthitic intergrowth of in two parts. This first part treats the features of the Na-sanidine and K,Ca-analbite or K,Ca-high al- crystal structure and cell geometry related to varia- bite. Thoughthere are someproblems with compo- tions in alkali chemistry and temperature. The sitionalcriteria (De Pieri et al.,1977), the composi- secondpart dealswith Si,Al order and the specific tional limits of anorthoclasesare as indicated in nature of apparent thermal motion. Figure I (adaptedfrom Kroll and Bambauer,lggl). Included are cryptoperthites in which the heated Materials and methods phase assemblagesconsist of monoclinic feldspars Specimensof anorthoclasewere provided by W. or a singlehomogenized monoclinic feldspar. Only S. MacKenzieand the National Museumof Natural one published refinement of an anorthoclase is History of the SmithsonianInstitution. Specimens available(De Pieri and Quareni,1973),but as it was from three localities were selected: Grande Cal- refinedfrom film data, the precision is not compara- deira, Azores, Or32.5Ab66.7Anq.s(fragments from ble to that of other refinementson disorderedalkali lavas, #15 of Carmichaeland MacKenzie, 1964); feldspars. Thus we do not have the information Mt. Gibele, Pantelleria Islands, Italy, Orzz-t about disordered Na-rich alkali feldspar structures Ab7s.sAn6.e(loose phenocrysts , #12 of Carmichael neededto make comparisonswith the near end- and MacKenzie,1964); and Kakanui, New Zealand,, member composition structures, or to allow the Or13.sAb33.7An2.5(asxenocrysts in Deborahvolcan- desirablethermodynamic calculations of their equa- ic sequence,NMNH #133868,see Dickey, 1968). tions of state(c/. Thompsonet al., 1974). The most K-rich anorthoclasewas a cryptoperthite Ab59Or59 Sonidines Ab5OOr OO A bTgOr3p Two feldspors Abgg0r2t Anorthoclose-s Ab99Or19 logioclos e s An Ab AbggAn;o AbggAnzo AbToAn3o AbsoAn4o AbroAn5g Fig. l. The phase diagram in the Na-feldspar corner of the feldspar composition triangle: solid lines with temperatures("C) indicate the approximatepoint at which K,Ca-analbites become monoclinic (after Fig. 8, Kroll and Bambauer, lgEl). Samplesexamined in this study are shown by circles. Compositions are in mole percent. HARLOW : ANORTHOCLASE STRUCTURES 977 consistingof a monoclinic K-rich phase (Na-sani- Table 2. Electron microprobe analyses phase (K- dine) and a twinned triclinic Na-rich Grande analbite). Fragmentsof the cryptoperthitic material |,/'r.% Caldeira Mt. Gibele Kakanu i were placedin a platinum capsuleand heatedin an Cao 0.17 1q 0.53 0.79 Ktl 5.9 2.5 2.05 electric furnace at 700"C, within the stability field na20 7,9 8.2 9.9 9.44 of single alkali feldspar (cf. Luth et al., 1974),for Ti02 0.09 Fe203 0.43 0.17 minor approximately24 hours to homogenizethe sample. 51U2 00.O 65.7 65.4 67.4 SubsequentX-ray reexaminationconfirmed the ho- Al2b3 18.6 19.5 20.4 -+20.1 e8.9 99.88' mogenization,and suitable crystals were then se- Total 99.6 98.8 Cationsper lectedfor structureanalysis. 8 oxygen atoms The samplesselected for intensity data collection Ca 0.008 0.069 0.026 0.04 K 0.314 0.224 0.141 0.12 were cleavage fragments, generally bounded by Na 0.686 0.710 0.85n 0.81 (001),(010), and (201) or (100).Approximate dimen- Subtotal 1.028 I .003 7.O230.97 sions of the fragmentsused are given in Table 1. Ti 0.003 (Table Fe 0.014 0.006 The chemicalcomposition of the specimens Si 2.994 t aq2 2.9272.97 2) was determinedwith an ARL-EMXelectron micro- AI 0.987 1.036 7.077 1.05 qq probeusing the datareduction method of Benceand Total 5.023 4.994 ( n?? 4 Albee (1968)with the factors of Albee and Ray Fel dspar component(mol.%) (1970).No inhomogeneitiesor Sr or Ba were detect- 0r r2.5 13.8 72.7 ed in the samples. Ab 66.7 70.8 83.7 84.0 An 0.8 6.9 tq io The furnacesused for the high-temperatureX-ray sj:R* 3.02 3.05 2.77 2.94 1.04 examination(one for the precessioncamera and one RZ0g 1.01 7.O7 1.02 for the diffractometer) are a modification of the r Dickey ( 1968) * n is ii,. totit of Fe+3and Al+3; R203is normalized designof Brown et al. (1973)that allow for easier to the anorthite content to examinepossible non- stoichiometryof the feldsPar Table l. Structure refinement information and sample sizes furnacealignment. For a completedescription see Grande Caldeira Mt. Gibele Kakanui Harlow (1977). Crystals selected for high-temperature study RoomTemperature: Reflections col lected 2026 1846 3002 were cementedto silica-glassfibers. Encapsulation Initial unrejected 2003 1822 2460 in silica-glasscapillaries was not deemednecessary Final unrejected I 834 1697 2141 Rwisotropic 0.061 0.090 0.107 becausethere was no iron in the samplesto cause Ru isotropic 0.080 0.097 0.1t8 problems, and becausethe highest tem- Rwanisotropic 0.035 0.041 0.052 oxidation Ru anisotropic 0.046 0.047 0.058 peratures were to be below 800'C, judged low enough not to cause alkali sublimation. For tem- High Temperature(CI): 4000 5t00 7500 peraturesbelow 400"C, Dupont NR-15082polyim- Reflections collected 2030 I 403 2039 ide binder solution(supplied courtesy E. I. Dupont Initial unrejected .167t2016 2002 Final unrejected 1752 de Nemours & Co.) was used. For temperaturesof Rw isotropic 0.1l1 0.096 Ru isotropic 0.138 0.1l9 400' C or greater,the clear cementin Ceramabond Rwanisotropic 0.062 0.063 503 High-TemperatureAdhesive (Aremco Prod- Ru anisotropic 0.074 0.074 ucts. Inc.) was used. No diffraction contribution from the cementswas noticed. High Temperature (C2/n): 4ooo 5100 7500 Initial unrejected I 050 I 044 I 043 The temperature of the displacive monoclinic/ Final unrejected 855 931 899 triclinic inversion for the feldsparswas estimated Rwisotropic 0.077 0.062 0.067 Ru isotropic 0.127 0.089 0.085 by examiningX-ray precessionphotographs taken Rwanisotropic 0.043 0.043 0.022 at temperaturesup to about 50oabove the estimated Ru anisotropic 0.073 0.047 0.'l05 inversion.
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