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AM44 1169.Pdf THE AMERICAN MINERALOGIST, VOL 44, NOVEMBER DECEMBER, 1959 THE ALKALI FELDSPARS V. THE NATURE OF ORTHOCLASE AND NIICROCLINE PERTHITES AND OBSERVATIONS CONCERNING THB POLYMORPHISM OF POTASSIUM FELDSPAR* Earth J. V. Surrn , Department oJ Miroeralogyand Petrology,Diaision oJ Sciences,The PennsylttaniaState Llniversity, Llniaersity Pork, Pennsyloaniaanil W. S. NIecKnNzrB, Department of Geology, Uniaersity of Manchester, ManchesterEngland'. ABSTRACT The observations recorded in paper 1 of this series have been extended to cover 37 specimens of orthoclase and microcline perthites. The exsolved sodium phase of the per- thitic specimens is usually a lorv temperature albite or oligoclase, although some specimens falling near the composition oreo(Ab*An)zo and with 2v between 50' and 70o contain an anorthoclase instead. The anorthoclase occurs as a pericline twin-type superstructule' Most of the albite-oligoclases are dominantiy albite twinned: four specimens show both pericline and aibite twinning, while seven specimens with compositions -Orzo and 2V" between 60' and 80' show mainly albite tr,r'in-type superstructure. The potassium phase is monoclinic in 14 specimens, triclinic in 4 and partly each in 15 specimens. The triclinic components give reflections that vary from sharp to difiuse, with Iattice angles a*, 7* varying from 90o, 90' to about 90"24" 92"20'' All the triclinic com- ponents are composed of two or more units whose angular relations vary between the ex- tremes of albite and pericline twinning, with the "diagonal" association fairly common. The optic axial angles fall either in or close to a triangle formed by the three extreme varieties: iow albite, Oro 2V" 100'; orthoclase, Ohes 2Vo 35"; and maximum microciine, Orroo 2Va 80'. There is a moderate, but not good, correlation between the position of the specimens in this triangle and the nature of the potassium-rich component Specimens which either contain anorthoclase or consist predominantly of the albite twin-type super- structure of low albite-oligoclase fall into two small areas in the 2V versus composition diagram. The preferential occurrence of the superstructure in the more potassium-rich compositions supports the hypothesis that strain favors its existence in orthoclase and mi- crocline perthites. The anorthoclase is thought to occur because the containing specimens have compositions that lie close to a phase boundary, a condition known to favor metasta- bility. INrnooucrtolr In paper 1 of this series(MacKenzie and Smith, 1955),the resultsof a mineralogicalstudy of some orthoclaseperthites were presented.Since then, data on additional orthoclaseperthites and on some microcline perthites have been obtained and form the basis of this paper. To con- serve space, familiarity with the literature reviews contained in the earlier papers of this serieswill be assumed.No papersthat bear solely * Contribution No. 58-22, College of Mineral Industries, The Pennsylvania State Universitv. 1169 I17O J V. SMITH I|ND W. S. MACKENZIE on the relations between the perthitic componentshave appearedsince these reviews,but a considerablenumber have appearedthat are con- cerned with the polymorphism of potassium feldspar (Bailey and Taylor (i955), GoldsmithandLaves (1954a,b),Hafner and Laves (1957), Heier (1957), Ferguson,Traill and Taylor (1953)). A detailed review will be given in a later paper of this serieswhich is to be devoted to the polymorphism of potassiumfeldspar, and it is sufficient for the present purposeto record that Hafner and Laves state that there is a continuous seriesof structuralstates between high-sanidine and maximum micrccline, that the conversionof a fully disorderedto a fully ordered potassium feldsparmay occurin innumerableways by variation of the distribution of Al ions between the four possiblesites, and that the distribution of Al in the intermediatestates depends on the previoustemperature-time relations. DrscnrpuoN oF THESpBcruBNs The origin, bulk chemicalcomposition and optic axial anglesof the 37 specimensstudied by singlecrystal r-ray methodsare recordedin Table 1, togetherwith the lattice anglesa* and 7* of the triclinic phases,where these have been measured.(All compositionsare expressedas weight per cent Or, Ab, An.) I{ost of the specimensare derivedfrom pegmatites, five are from charnockitic rocks and one each from a qvartz monzonite, a porphyroblastic orthoclasegneiss, a granite and from a vein in a low- grade schist.The specimenswere originally called orthoclase-or micro- cline-perthiteson the basis of the observedoptical symmetry. Fig. 1 showsthe relationshipbetween the optic axial angle and buik chemical composition,where it may be seenthat almost all specimensfall in the triangular area formed from the apiceslow-aibite, orthoclaseand maxi- mum microcline. Except for a few which are either very potassium-richor sodium-rich, the feldspars are perthitic, containing either two or three different phases.These can be most certainly characterizedby their o*, 7* angles as shown in Fig. 2. The measurementsfall into three groups. The first group composedof potassium-richcomponents, falls on an essentially continuousseries (the microcline series),with monoclinic orthoclaseat one end. The position of the other end is subject to some uncertainty becauSeone specimen,N, lies off the common trend: neglectingit the other specimensshow that the microcline seriesextends at least as far as a* 90"24',I* 92"20'-the marimum microcline defined by X{acKenzie (1954). The triclinic potassium phase of N occurs in an uncommon orientation(Smith and NIacKenzie,1954) which could not be interpreted in terms of the known twin laws, and while the anglesmay indicate a ORTHOCLASE AN D M ICROCLIN E PERTH ITES tt7 | Tasln 1. Pnoprprms or rrtn SrncruBNs Lher.ical composition Potassium phase Sodiun phase Specimen Number Or Ab AN Cs a A 96.1 3.9 0.0 3.1 8' B 91 2 79 06 03 68.4" c 908 o 2.5 43 6" u 859 t2 l4 76 2" 90'05' 90'46'M 3705 84 9 20 83-86' D 845 IJ 16 2" v 84.1 14 63.9" 88"29' 89'16'P E 83.3 149 1.8 57.6' 90'09' 91'09'M 87'58' 89'05',P LM3A 82 8 143 18 r0 80" 90'19', 9r"24',4 6436 82 .5 164 69" w 81 6 180 0.4 81 5' 86'19' 89"12'A 3708 80 7 179 t4 82-8L' 86"45' 89'36'A x 793 192 15 76 0' (86.s' 90.o)P F 78.8 189 2.3 52 9" z 782 202 70 6" 88'35', 89'31'P LM6 77.5 20.9 0 8 0.7 70' 90"21' 91"35',A 86"25' 90'18',4 Y 76 1 222 79 4' s347 i4 I 242 l4 62" 86'13', 89'19',A H 725 25.8 61 5" 1639 7t 3 258 29 62-78" 90"27' 92'O9'P I 694 289 69.1' J 66.6 32 .5 0s 75 0' K 644 338 1.8 61.4' M 573 403 24 87'00/ 90"32'A N 537 45.6 0.7 8:1.5' 90"i0' 92'35'D 86'48', 90"33'A o 48.5 173 82 8' I to"+s' eo'1s'A \so'+z' 90'1s'P oF4 16 4 .s2.5 1 1 86"32' 90'23'A Q 4s6 502 42 81.7" 86"52' 90'r7'A R 41.1 51 6 4 3 85'44' 90'00'A 86'4.5' 89'55',4 86"18' 89'58',A oF5 43 3 5,15 22 90" 86"29' 90"20'A (86.s' s0 s'P) oF2 43.0 53.7 3 3 86' 86'48' 90'28',4 oF3 1r.9 545 36 80" 86'38', 90'13'A oF1 390 58 8 2.2 90' 90'16' 9r"12'4, 86"23' 90'25'A 1 31.s 683 02 82 0" 86"39', 90'34'A 2 198 727 60 1.5 78 5' 86'13' 89"30'P 86"29' 89'.58'A s 26 962 12 97.8" T t4 982 04 102" Norrs SpecimensA-Z weredescribed by Spencer(1930, 1937): Specimens 3705, 6436, 3708 and 4639 bv Howie (1955); specimen5347 by Howie (Ph D Thesis, Cambridge);specimens LM3A and LM6 by Heald (1950);specimens OF4,5,2,3 and 1 by Oftedaht (1948);spe:inen 1 by MacKenzie and Smith (1955)and specinen 2 by Tilley ( 1954).The letters A, D, P, M, after the values for the anglesa* and denote, respective- ly, albite twinning, the diagonal association, pericline twinning and measurementsmade "* on powder patterns byMacKenzie(19.54)Weakpericlinetrvinningofasodium-richcomponentwasgivenbyspecimensXandOF5: the measured values of a* and "r* were good enough Ior identification of the component as low albite-oligoclase but not good enough for plotting on Fig. 2 1172 J. V. STIITH AND W. S. MACKENZIE genuine structural differencebetween the potassic phasesof N and of the other triclinic potassiumfeldspars, it is possiblethat the different nature of the associationshave led to a modification of the anglesby nothing more than lattice strains. The anglesfor the sodium componentsprovide the other two groups. Three fall with the anorthoclaseseries and twenty-one with the sodic o c o o o o o to 8o ol *,11, 0", ""t Frc. 1. The relation between the optic axial angle 2va and the bulk chemical composi- tion of the present suite of alkali feldspars. The diagram is a slight modification of the one presented by Tuttle (f952). The short vertical lines indicate a range of variation of optic axial angles. plagioclases.As describedin paper 1, the deviations of anglesfor the latter group from those of low albites are probably caused by solid solution of potassium and calcium feldspars,though for the latter the duality betweenstructural state and calcium content must be borne in mind. The triclinic phasesconsist of two or more components.fn addition to the simple albite and pericline twins, pairs of componentsmay occur whoserelative orientationslie betweenthose for the albite and periciine twins.
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