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PHYSICAL PROPERTIES of ANORTHOCLASE from ANTARCTICA1 Eucbnb L. Bouobrre and Anrnun B. Fono, U.S. Geological, Surtey, Washington

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PHYSICAL PROPERTIES of ANORTHOCLASE from ANTARCTICA1 Eucbnb L. Bouobrre and Anrnun B. Fono, U.S. Geological, Surtey, Washington THE AMERICAN MINERALOGIST. VOL 51, SEPTEMBER OCTOBER, 1966 PHYSICAL PROPERTIES OF ANORTHOCLASE FROM ANTARCTICA1 EucBNB L. BouoBrrE ANDAnrnun B. Fono, U.S. Geological,Surtey, Washington,D. C. and' Menlo Park, ColiJornio. Assrnnc't The lattice parameters of anorthoclase [OrrszAbza:Anro r(mol per cent)] from the Crary Mountains, and anorthoclase lOrrosAbonsAnraa(molpercent)] from Cape Royds,Ross Island, Antarctica, have been determined from r-ray diffractometer patterns by a least- squares cell refinement program. Chemical and spectrographic analyses and optical data are also furnished from the feldspar and Quaternary alkaline trachytes in which they occur. The Cape Royds feldspar is markedly higher in An content than any anorthoclase in which lattice parameters have been determined Both suites of new data compare favorably with data from other modern rn'ork, considering that difierent methods of measurement are involved. The antarctic anorthoclase is believed to be highly disordered and it difiers optically from average anorthoclase by higher indices of refraction and 2V. The Cape Royds rock which bears the anorthoclase contains 7 4 per cent normative nepheline which is manifested by sodalite(?) in the mode. The partitioning of sodium to the sodalite is believed to be re- sponsible for the unusually high An content of the Cape Royds feldspar, although the latter is not predictedin the norm. INrnonucrror.t The composition, optical properties, and lattice parameters of an- orthoclasefrom the Crary Ilountains, West Antarctica (lat. 76005'S.; long. 118o15'W.)and from Cape Royds, Ross Island, East Antarctica lat. 77o30'S.;long. 166o15'E.)(Fig. 1) havebeen determined as part of a study of Quaternaryrhomb porphyry trach-vtesassociated in an antarctic alkaline basalt-trachyte province. The crystals occur as conspicuous phenocrystsin trachyte flows that were first describedin detail on Ross Island by Prior (1907)and subsequentlyaroused considerable interest for their comparisonto kenyte of the African Rift. Thesephenocrysts have beenclassified variousl.v as anorthoclaseor plagioclasein antarctic litera- ture (seefor exampleDavid and Priestlv,l9l4; Jensen,1916; Woolnough, 1916;Smith, 1954; and Treves,1962). The present work was undertaken to clarify the nature of thesefeld- sparsand to comparethem to the ternary feldsparsrecently described by Carmichael and I,IacKen zie (.1964). Trachyte from the Crary ltlountains was coilectedin 1959by Boudette as a memberof the 1950-60U.S. B1'rdlitndOversnow Traverse. The Cape Ro.vdstrachyte was collectedin 1961by Ford and J. X'I. Aaron in con- junction with specialfield studiesof the occurrenceof the trachyte. 1 Publication authorized by the Director, U.S GeologicalSurvey. 1374 A NTA RCTI CA A NORTHOCLAS D 1375 Frc. 1. Index map of Antarctica showing extent of Quaternary alkaline basalt-trachyte province and anorthoclase trachyte localities (l). Anorthoclase from locality "S" (at Mt. Sidley, Executive Committee Range) is not specifically described. Mnrnoos AND SAMPLES Neither of the samples has been previously aualyzed chemically or studied structurally. However, Carmichael and MacKenzie (1964, p. 950-957, sample No. 8) describeanorthoclase from the summit of Mt. Erebus on RossIsland, Antarctica (Fig. 1, Table 1) that is comparable in occurrence.The occurrence,chemical analysis, texture and mor- phology of Mt. Erebusfeldspar have beenreported by Mountain (1925) and Jensen (1916)I Carmichael and MacKenzie do not specify which sample was used in their work. Calculations of the theoretical feldspar molecules from the analyses weremade on the basisof 32 oxygens(Deer, et al,.1963, p. 106-107).We assumedthat Ba substitutesfor K, and Sr for Ca, but have not computed Cn and Fs1s.yin the theoreticalmolecule (Table 1). The chemicalanalyses, modes and norms of the rocks containing the feldsparsdescribed herein are given in Table 2. Crary X,Iountainssample 'fanr,r 1. Cnnurclr, AN.rr,vsrs rN Worcnt Pon CrNr ,tNo Ca.r,cur,areu Couposrrrolt or ANontnocr,ASE lRoM ANrancucA Mt. craryMts, ?0" T?d'i Summit Erebus, Ross Island BrB_8 ^"Ti:i"" No 8z SiOs 65.4 628 65 23 62.79 62.49 60 83 AlzOa 21 3 222 20 68 22 1,2 21 86 23 92 Fero3 10 .18 20 36 .30 11 FeO .10 .20 ,41 t3\ 214 Meo 10 U5 00 16 07 CaO 20 37 .87 376 374 339 BaO .15* 18 SrO 071 .15* .22 Naro 82 1a 845 735 7 20 6.11, KzO 2.6 2.8 378 298 326 296 HrO+ .12 28 26 .19 04 07 HzO- .00 00 06 07 00 t2 TiOr 05 l5 .36 PrOr 07 .06 MnO 00 00 Total 996 99 93 100 03 100 36 100 08 Number of iors on the basis of 32 (0)6 Si 11 .5804 tr 2771 11 6686 tt 257 rr 211 to 9289 AI + 4409 4 6961 4 3583 4 623 5 0657 Fe+r 0213 021.5 .02l5 0 048 0 0,+1 0.0215 Ti 0107 0216 0 0539 Mg .0319 .0107 0 043 0 0216 Fe+r 0106 .o323 0 061 0.196 0 32J3 Na 2.8048 2 4988 2 9198 2 554 2 504 2 1i40 Ca .3825 .7109 1718 722 .719 .6575 Sr 0215 K 5950 6462 .8588 681 .746 6682 Ba 0107 >z 16 0533 16 0163 16 048,1 t5 98 t6 07 16 07 )w 3 8248 3 898q 3 9826 402 401 380 or) 16.8 21 8 170 186 193 eu!,uaZ, 74 2 648 73.3 635 62+ 61 7 A"J 10 1 184 49 195 190 190 1 5308 1 5372 1 536 1.536 p 1 5364 1 5416 I 539 1 539 ^l 1 5380 1 5430 I 541 1 541 2Ya7 559 593 61 6" 61 6' 2YaB 53 2" 61 9 62" 62" Ext. on {010J 6'-7" 2.6" sp gr 2 50** 2 620 2 .620 r Chemical analysis by Paul Elmore, Sam Botts, Gillison Chloe, Lowell Artis, and H Smith by *-ray fluo- rescencemethods supplemented by methods describedby Shapiro and Brannock (1962), 2 SpecimenNo 8 of Carmichaeland MacKenzie (196,1,p. 950 Table 1). s "Potash-oligoclase,Type 1" of Mountain (1925,p 336); Analyst: E D Mountain Theoreticalmolecule fuomDeer el al,-(1963, p 42) I "Potash-oligoclase,Type 1" of Mountain (1925,p 336); Analyst; E. D. Mountain. Theoreticalmolecule fuomDeer et al (1963 p a2) s "Anorthoclase"of Jensen(1916, p. 122);Analysts: G E Burrous and A B Walkom 6 Af ter the method ol Deer,et al,. (1963, p 106-107). 7 CalculatedJrom refractiveindices. 3 Direct optical measurementmethods BTB-8 and FR 1 neasuredby universalstage techniques. * Semiquantitative spectrographic analysis by J L Harris Results are reported in percent to the nearest number in the series1, 0,7, 0 .5,0 3, O 2, O 1.5,and 0 1, etc ; which representapproximate midpoints of group data on a geometric scale The assigned group for semiquantitative results will include the quantitative value about 30ol of the time N ol included in analysis summation *t Determined with specific gravity bottle and toluene by Blanche Ingram Precision t 0 05. A N T A RCT I CA A NORT IIOC L.4 S E Tegt-n 2. Cnrurcar, AN.a.r,vsrs, Nonus, Moors, ,qlu SnlrrQuaNTrrATrvE SpBcteocnlpntc ANalvsn's op ANtanctrc ANonruocle.ss Tn.qcnvtns Field No BTB-8 FR-1 Plagioclase* o7 Lab. No 159615 163086 Clinopyroxene* <03 13 Olivine* 03 04 Chemical analysesr Ore minerals* o2 tl "K-Ieldspal"*x 30 21. SiOz 57 6 559 Plagioclase*+ 104 14 5 AlsOa 18 3 198 Orthopyroxene Sp FezOa 38 19 Clinopyroxene** Sp l'eO 28 3.5 Olivine** Sp Sp Meo 77 l2 Amphibole** Sp CaO 19 3.0 Apatite Tr Tr Naro 73 Calcite Tr K:O .35 39 "Iddingsits"*x Tr Tr HzO .81 13 "Zeoli.tes'' 01 HOf .96 ..53 Glass** sp-A TlOr 12 Ore minerals** Sp spA Pror .30 13 (A*Sp*Tr) 54i 126 MnO 24 I4 Estimated /6 An in COz <05 < .05 gloundmass plagioclase (:2s) (2s) Sum r00 -trace A : major component, Sp :5p21.", 1t amount Powder density by air * Reported as phenocryst, may also be in groundmass pycnometer 269 if indicated. ** Groundmass constituent C.I P W Norms Semiquantitative spectrographic analyses3 Orthoclase 21 1 228 AIbite .tn / 498 B 0007 .0007 Anorthite 70 Ba .15 t Nepheline 26 Be .000.5 0007 Corundum 10 Ce .o7 .o7 00.)7 |,*o 041 Co Diooside{ En 0.210. 7 Cu 001 0007 i- 0.rJ 003 003 )-" 03 lro 1 3l 'rr"\n La .02 Olivine j 12.1 ' Mo 0007 .001 01.5 IFu 0.8J Nb 01 Magnetite 28 Nd .02 03 Ilmenite 23 Sc 000.5 .0005 Hematite Sr 05 15 Apatite :; 30 V 002 Y 00.5 005 Sum Yb 000.5 .0005 2t115 Modes (volume per cent)2 Looked for, but not found: Ag, As, Au, Bi, Cd, Cr, Eu, Ge, Hi, Hg, In, Li, Ni, Pb, Pd, Pt, Pr, Re, Sb, Anorthoclase* 31.0 333 Sm, Sn, Ta, Te, Th, Tl, U, W, Zn, Pr I BTB-8, Crary Mountains, West Antarctica; FR-1, Cape Royds, Ross Island, Ross Archipellago, Antarc- tica. Analysts: Paul Dlmore, Samuel Botts, Gillison Chloe Analyzed by methods similar to those described by Shapiro and Brannock (1962) r The ratios of phenocrysts to groundmass in the trachyte were made by a macroscopic grid counting of sawed slabs; groundmass modes were point counted microscopically by the Chayes nethod and then combined with the macroscopiccount and recalculated as the reported rock modes. a Analyst: J. L. Harris. Resultsare reported,in per cent, to the nearestnumber in the series1,0 7,0 5, 0 3, 0.2, 0.15, and 0 1, etc ; which represent approximate midpoints of group data on a geometric scale The assigned group for semiquantitative results will include the quantitative value about 3016 of the time.
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