American Mineralogist, Volume 63, pages 1264-1273, 1978 Crystalstructures and compositions of sanidineand high albite in cryptoperthiticintergrowth Krtrs D. Krnrrn eNoGoRpoN E. BnowN Departmentof Geology,Stanford Uniuersity St anford, Cal ifu rnia 9430 5 Abstract The crystalstructures and compositionsof the phasesin a naturalcryptoperthite from the Rabb Canyon pegmatite,Grant County, New Mexico, have beendetermined using single- crystalX-ray diffraction,transmission electron microscopy, and electron microprobe analysis. The cryptoperthiteconsists of an untwinned,monoclinic sanidine[cell dimensions:d : 8.558(1),b -- 12.997(l),c:7.179(l)A,B: 116.07'(l)landapericline-twinned, triclinic high albite[celldimensions:a:8.AaQ),b:12.989(3),c:7.160(2)A,a:92.10"(2),0: I16.56(2)'and 7 : 90.21(2)"1.The similarity of theb andc celldimensions and precession X- ray photographsindicate that the phasesare partially, but not completely,coherent in the intergrowthplane, -(-601).The bulk compositionof the crystaldetermined by microprobe analysisis Oro.u,AbonrAno.or, and the mole fraction of the sanidinephase in the crystal determinedby X-ray scalefactor refinementis 0.68(l).TEM examinationshowed that the high-albitelamellae are -500,4'wide with -5OA-wide periclinetwin lamellae.The sanidine lamellaeare -1000,4.wide. The compositionof the sanidinephase, as determinedby direct crystallographicsite refinement, is Oro.56111Abo.su.The composition of the high-albitephase, as determinedby mass-balance,is Oro.2212,Abo.rr. Both phasesin this cryptoperthiteare strained, although the natureand amountof strain differ from predictionsof modelsof elasticstrain in perfectlycoherent feldspars. No pre- viously-reportedmethod of predictingthe compositionof the phasesin a cryptoperthitefrom the celldimensions yields correct results for thisspecimen. The observedcompositions lie on an experimentally-determinedcoherent solvus at a temperatureof 465 t 20". Introduction from the cell volumes (Stewart and Wright, 1974). Cell volumes are, however, affected by elastic strain Cryptoperthitesare submicroscopicintergrowths and thus do not yield a correct estimateof composi- of sodicand potassicalkali feldsparsresulting from tions for coherent cryptoperthite. Robin (1974) and exsolution of a homogeneousfeldspar of inter- Tullis (1975) have developedmethods for correcting mediatecomposition. The unit-celldimensions of the these compositional estimatesby calculating the elas- intergrownphases commonly differ significantly from tic strain of the unit cell caused by coherency. How- those of compositionally-similarnon-intergrown ever, their corrections apply only in the caseof com- phases(Stewart and Wright, 1974).This discrepancy plete coherency of monoclinic phases,and they have is attributedto elasticstrain inducedby preservation not been verified experimentally. Previous attempts of a partially to completelycoherent aluminosilicate to refine the crystal structures of the individual framework acrossthe phase boundary during ex- phasesof a cryptoperthite have not been wholly suc- solution(Owen and McConnell,1974;Yund, 1974). cessful becauseof the difficulties in resolving the dif- Effortsto determinethe amount of strain and how it fraction maxima of the intergrown phases, one or affectsexsolution have been hamperedby the diffi- both of which are usuallytwinned (Ribbe and Gibbs, cultyof determiningthe compositionof the exsolved t97s). lamellae,which are too smallto be chemicallyana- In this study,both intergrownphases of a crypto- lyzedwith an electronmicroprobe. The compositions perthite were examinedby single-crystalX-ray dif- of suchintergrown phases are commonlyestimated fraction and transmissionelectron microscopy with 0003-004x/78l| | | 2-1264$02.00 1264 KEEFER AND BROWN: CRYPTOPERTHITIC SANIDINE AND HIGH ALBITE t265 the following objectives:(l) to developtechniques by which the structuresof the phasesin such an inter- growth may be determined;(2) to determinethe com- position of the individual phasesusing X-ray diffrac- tion techniquesand data on the bulk composition;(3) to investigatethe structuralchanges caused by partial coherency;and (4) to use thesedata as a basisupon which to evaluatemethods of determining the com- position and amount of strain of the phasesin crypto- perthite from the cell dimensions. br Experimental The material used in this study came from the Rabb Canyon pegmatite,Grant County, New Mex- ico (Kuellmer, 1954).The pegmatite occurs in por- phyry that is the vent facies of a rhyolite dome (J. O'Brient, personal communication, 1976). The cryptoperthite occurs in a matrix of coarselycrystal- line quartz as clear,colorless single-crystal fragments about lcm in diameter, which display perfect cleav- ageparallel to (010)and (001). PrecessionX-ray photographsof a specimen(0.34 X 0.20 x 0.l4mm) taken from a crystal fragment (Molfs, :{E show that the cryptoperthite consists of an un- twinned, monoclinic sanidinephase and a pericline- twinned, triclinic high-albitephase'(Fig. l). The ma- jority of diffraction maxima from both phases are distinct and generally well resolved,although slight streakingbetween the maxima is observed.This crys- tal was later used for integrated intensity measure- tions using Luth and Querol-Sufr6's(1970) regression ments on the intergrown phases.A transmissionelec- equations. tron micrographof the materialis shown in Figure 2. Precessionphotographs show that the spacegroup All measurementsof the X-ray diffraction in- of the sanidine phase is C2/m (hkl systematically tensitieswere made on a Picker Fecs-l four-circle absentwhen ft + k : 2n 1- l). The high-albitephase diffractometer with graphite monochromatized was referredto the nonstandardspace group Cl (hkl : MoKa radiation (I 0.71069A). The unit-cell di- systematicallyabsent when i + k : 2n + l) which is mensionsat 23"C of all threelattices were determined conventionally used for triclinic feldsparsto facilitate by least-squaresfit to the angular coordinatesof re- comparisonof the monoclinicand triclinic structures. flectionscentered by the Fecs-1 programs. Eighteen Integrated intensitiesin the range 5-60o 20 wete reflections in the range 12-49" 20 were used for the measured for the sanidine phase with a 0-20 scanat a sanidinephase and l5 reflectionsin the range 12-49" 1"/min scan rate using a l.5o take-off angle and a 20 were used for each of the high-albite twins. The 2.6" 20 scan width compensatedfor ar-a2 splitting. cell dimensionsof the high-albitetwins differ by less Backgroundswere estimatedfrom 20-secondcounts than one standard deviation. The observedcell di- made at eachend of the scan.Intensities in the range mensions are given in Table l, together with cell 30-75o 20 were measuredfor one of the high-albite dimensions calculated from the observedcomposi- twins using a 0.5'/min scan rate, a 2.3" 20 scan width, and l00-second background counts. Other conditions were the same as those usedfor data col- 'The terms sanidineand high albite refer potassic to and sodic lection on the sanidinephase. feldspars, respectively,in which the Al-Si distribution among the from each lattice in the non-equivalent tetrahedral sites is nearly fully disordered. A peri- The diffraction intensities cline twin is a rotation twin with a twin axis oi [010] and a range 40-45' 20 (160 observations for the sanidine composition plane near (001), the so-calledrhombic section. and270 for each of the high-albite twins) were remea- KEEFER AND BROIyN: CRYPTOPERTHITIC SANIDINE AND HIGH ALBITE sured using identicalinstrument settings. These ob- servations were used to determine the relative amounts of each phasein the sample,as discussed below. The intensitiesfrom each high-albitetwin werethe sameto within 0. I percent,confirming that both twins havethe samestructure and arepresent in equalamounts. Observed intensities were corrected for back- ground, Lorenlz, and polarizationeffects, the latter assuminga 50 percentmosaic monochromator crys- tal. The observationswere scaled to normalizedstan- dard observationsto correct for instrument drif| maximum and minimum scalingfactors were 1.03 and 0.96, respectively.Corrections for absorption were made using an analyticalmethodz (p : 8.0 cm-1, calculatedfrom the bulk compositionof the crystal;minimum and maximumtransmissions were 0.91and 0.95, respectively). Standard deviations were estimatedusing the formula of Corfieldet al. (1967), whichemploys an instrumental instability constant of 0.04. The crystalused in the diffractionexperiment was analyzedwithan electronmicroprobe. The bulk com- position was: (weight percent)66.81 SiOr, 18.67 Al2Os,8.91 KrO, 5.39Na2O, 0.20 CaO, 0.13 FeO, Fig. 2. Bright-field transmission electron micrograph ofa section corresponding to Or0.51Abs.16Ano.or. from the fragment cut parallel to the (001) cleavage plane. The sanidine lamellae have a maximum width of 1000,4and alternate regularly with high-albite lamellae whose rnaximum width is 5004. '9Using the formulaof De Meulenaerand Tompa(1965) in the The intergrowth plane is approximately parallel to (601) and programAcNosr. Programsused for other cal0ulationswere A. normal to this section. Micrographs were made on a 100 kV Zalkin's Fonolp, a modifiedversion of Finger'sRrrrE, and lo- Philips 200 microscope in the D6partment of Materials Science cally-writtenprograms by K. Keeferfor reducingand correcting and Engineering at the University of California, Berkeley. thedata. Table l. Comparison of observed and calculated cell parameters for sanidine and high albite in cryptoperthitic intergrowth SANIDINE HIGH ALBITE 34 Observed Calculated ; Straln(Z) Obsened Calcutated A Stratp(Z) AOBS ACALC I a(E) 8.ss8(1) 8.452(5) 0.106 L.25 8.L44(2) 8.248(8) -0.1.04 -t.26 0.4L4 0.204 b(E) L2.997(!) 13.004(1) -O.OO7 _0.0s 12.989(3) L2.932(6) 0.057 o.44 0.008 0.072 (19) c 7.r79(L) 7.169(1) 0.010 0.14 7.L60<2) 7.L40(3) 0.020 0.28 0.019 0.029 c(') 90.00 90.00 0.00 92.LO(2) 92.05(18) 0,05 -2.LO -2.O5 B(') 116.07(1) 116.07(1) O.OO Lr6.s6(2) 116.37(L) 0.19 -0.48 -0,30 Y(') 9o.oo 90.00 0.00 90.2L(2) 90.20(2) 0.01 -o.2L -0.20 v(tr3) 7L7.2 708.0 9.2 1.30 676.9 681.7 -4.8 -0.70 40.3 26.3 t\wnber in patentheses ue e,s,d..,s (1;) in the Least signifi.eant digite.