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 anorthoclasefrom 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 literature (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. 1378 Ii- L BOUDETTE ANI) A. B. FORD

(BTB-8) is from a moraine,but the location of the sourcecan be inferred from the radial flow of ice from the mountain slopesand the abundance of the lithology in the moraine. The sample from Cape Royds, Ross Island, (Fr-1) is from the upper part of a flow about 50 feet thick. The similarit.v of the texture of the Crary l,Iountains specimento that of the FR-1 trachyte (Fig. 2 a, b) Ieaveslittle doubt that the former is alsofrom a similar flow. The anorthoclaseis optically homogenousbut extremelypoikilitic; the Iatter feature necessitatescareful purification. The inclusionsare olivine, pyroxene, apatite, ore minerals, and groundmassmicrolites and glass. BTB-8 is noticeablv zoned in osciilatorl' habit; the zoning in FR-1 is not conspicuous(Fig. 2). The anorthoclaseis partly glomeroporphyryticand partly in mosaic aggregateswhich appear in hand specimenas single morphologicaleuhedra (Fig.2 a, b). Someindividual crystalsappear to be complexly twinned, in part on the Carlsbadlaw, but the most conspicuous twinning is close spaced (0.05 mm) lamellar albite twinning which is presentin all individuals, and commonly a second,finer reticu- lated polysynthetic pericline(?) twinning is developednearly at right angleswhich produbesa fine cross-hatch(microcline-like) twinning pat- tern (Fig. 2 c, d). Textural evidencefor "flow cataclasis"in the trachyte ieavesthe explanation of the secondpolysynthetic twinning open to rea- sonabledoubt; it may be strain twinning and not truly pericline,although we favor the interpretation that it is periclineon the basisof its orienta- tion. Preliminary preparationof sampleswas doneby gravity and magnetic purificationof the 100 200mesh fraction of crushedanorthoclase crystals handpicked from broken large sawed slabs of trachyte. We therefore consider the samples to be representive of the bulk specimens.Purifi.ed anorthoclase was first checked on the :u-ray diffractometer for possible contamination and homogeneity by scanning between 70 and 6O" 20; neither sample required further purification and both were reasonably homogeneous.Individual samples,after being ground in an agate mortar with fluorite (a:5.4620 A) tor 5-10 minutes, were made into a smear mount on a (0001)quartz plate and were traversed3 times between 130 and 56o 20 on a diffractometer using Ni filtered Cu radiation at 45KV and 20 ma. The diffractometer settings were: scale factor 2; multiplier 1; time constant 4, divergenceand scatter slits 10; receiving slit 0.006; scanning speed]o 20 per minute; and chart speedI inch per minute. AII recognizablereflections were measured at the top of the peak, and the measurementswere adjusted by the amount indicated by the internal standard.The averageof the threemeasurements were used in the refinement, with Cu Ka1:1.5405 A. l|he feldsparpatterns were refinedusing A N TA RCTI CA A NORTIIOCLA SE 1379

Frc.2. Photomicrographs showing texture of anorthoclase trachyte BTB-8 (a) and FR-l (b), and twinning in anorthoclase phenocrysts in BTB-8 (c), and FR-1 (d)' crossed polars. 1380 E. L. BOUDETTE AND A, B. FORD

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Tasr-n 4. Isnnxrn Powoan Pe.trrnns ol Auontuocr-AsEsFRoM ANtanc:rtce

BTB-8 FR-1 2.529 2.525 2 531 2.527 hkl 2 520 2 523 dcalcr dob"' dcarcr dol'2 tI2 2 519 2.522 312 2.517 2 520 111 465 6.479 112 2 +70 2.473 020 446 6 449 6 448 6 453 ,71 2 458 2 463 001 378 6 388 6.380 240 2 457 2 461 110 313 6 325 150 2 454 2.456 111 832 5. 832 310 2.421 2 427 111 693 s.698 tot 2 .4t9 2 419 021 661 4.660 150 2.4t2 2.41+ 021 417 4.420 310 2.396 2 402 201 062 4 064 4.069 4 068 240 2.391 2.394 111 888 3.889 3. 893 3 894 151 2 369 2.37r 111 3 781 3.781 3.787 3 787 203 2.348 2 348 130 3.775 3 757 3. 760 3.760 M2 2.330 2 330 200 3. 3.686 113 2 322 2.321 130 3.666 3 666 3 669 3 667 242 2 313 2.317 131 3.625 3 626 3 625 3 .627 2.3t3 t tt I 131 3.526 3 529 331 2 297 2.300 r12 3.474 3 473 3 472 3.47| 113 2.281 2 281 221 3 442 3 446 242 2.2444 2.246 221 3 431 3.436 r32 2.2399 2 242 r12 3 392 3 391 3 392 3 391 220 3 232\ 3.240) I 040 3 2%l 3.223 3.2241 3 223 BTB-8 FR-1 202 3.22o) 3.22r) oo2 3 18el 3. 190J dcalcr dob"2 d"at"1 dobuz 3. 190 3 191 220 3 rat) 3.163) 131 3 008 3 008 3.011 3 010 223 2 2363 2 2353 041 2 941 2 940 2 941 2 940 332 2 2222 2 2234

022 2.922 2 921 2 922 2 -921 lDl 2.2193 2.2207 222 2 916 2 916 o42 2.2083 2.2lOO 131 2 86s 2 867 2. 868 2 869 2.1941 2 .1971 222 2 847 2.849 223 2.1770 2 1777 041 2 816 2.818 330 2.1549 2.1.597 132 2 807 2.806 2 .805 2 806 060 2 1485 2 1498 2 t494 2 1493 022 2.799 2 801 151 2.!228 2.1214 2. t247 201 2 707 2.713 241 2 1182 2-1212 2.1214 732 2.681 2 68r 2 682 2 683 061 2.0038 2 0035 2 0051 311 2.676 2 682 260 1 8332 1 8332 1 .8347 311 2.667 2 673 062 1 8280 1 8280 t.8279 221 2 s35 2.540 3.50 1 8015 r.8010 1 8046 r.8027 3t2 2.531 2 533 204 r.7802 1 7804 1 1799 1.7800

r All calculated spacingsare given Ior d ) 2.150 A; calculated spacings less than 2 160A are given only when they correspond to an indexed observed reflection. z Average of three observations with annealed CaF, as internal standard, o :5 462A at 25" C Ni-filtered CuKarradiation(I:1.54054) Lowerlimitof 2dmeasured:13'CuKd(6.8094).Patternobtainedat26'C.

the variable index option of the least-squaresrefinement program devised by Evans, Appleman, and Handwerker (1963). Between 18 and 22 un- ambiguous reflections of the 25 or so present were used to calculate the Iatticeparameters in the 3rd or4th cycleof the refinement.The standard deviation between observed and calculated 20 for each sample was 0.0151or less(Table 3). Observedand indexedlines are given in Table 4. t382 E. L. BOUDETTE AND A. B. FORD

Rtsulrs In the nomenclatureof Smith and MacKenzie (1958)our samplesare anorthoclase(Fig. 3). Comparison of the measuredlattice parameters with thoseof the anorthoclasefrom Antarctica describedby Carmichael and MacKenzie (1964) is given in Table 3. A graphical comparison of our

Aq Albite AbseAn,s Ab66An26 AbTsAnjs Ab5sAna6

Frc. 3. Ternary diagram (after Smith and MacKenzie, 1958, p. 874) showing plots (weight percent) of analyzed anorthoclase from Antarctica. BTB-8 and FR-1, this report; No. 8, Carmichael and MacKenzie (196a); A and B, Mountain (7925, p.336);and C., Jensen (1916, p. t22). Anorthoclase of this report and sample No. 8 of Carmichael and MacKenzie shown as circlesl other Ross fsland anorthoclases (Table 1) shown as triangles (lattice parameters zol available) anorthoclasewith the anorthoclasedescribed by Carmichael and Mac- Kenzie (1964,figs. 1,2, and 3) has beenmade on Figs.4, 5, and 6 (Table 5). It is important to note that compositesamples were used for the presentstudy in contrastto the selectedcrystals used by Carmichaeland MacKenzie, the Cn nnd Fs1s";components are not computed in the ternary compositioncalculations of BTB-8 and FR-1 (Tables l and 5), the samples were not heated, and a different method of measurement was usedin the determinationof the lattice parameters. A NTA RCTI CA A NORTHOC LAS E 1383

The chemical analysis of the IrIt. Erebus summit sample given by Carmichael and MacKenzie (1964) differs markedly in CaO and KzO content from the comparably occurring anorthoclasedescribed herein, as well as from a previous analysis of anorthoclasefrom Mt. Erebus (Table 1, Fig. 3). The "2" and "X" summationsof both the old and the new

tl I

EXPLANATION rf. a i Anorthclase (this report) o Anorthoclase

Plagioclase a I

o I o 950 i \.. a e a1 o

o ,""X

a \

Ite. 4.20((20r)-2a(1010) qua.t,of anorth .:;::^plagioctase plotted against or content found by chemical analysis (after Carmichael and MacKenzie; 1964; p. 953, Fig. 1) showing comparison of anorthoclase of this report (Table 5).

analyses are good. Carmichael and I,IacKenzie (1964), however, worked on a selectedcrystal which when comparedto the compositesample reported by Deer et al. (1963)may accountfor the differencein composition. The 20(201)p.-20(1010)aizseparation plotted againstweight per cent Or for BTB-8 shows the most divergence from the curve of Carmichael and McKenzie (1964,p. 953, Fig. 1), and its analysisis possiblysuspect; however, it was made at the same time by the same analysts as FR-1 Ii. L. BOUDETTE AND A. B. FORT)

(Table 1) which is apparently a good analysis by the criteria of. the 20 separationused and )X and 2Z (Table l). The determinedlattice parametersa* and cell volume are reasonably compatible with the findings of Carmichael and MacKenzie (Figs. 5 and 6). FR-1 plots outside the anorthoclasefield erectedby Carmichaeland MacKenzie (1964,Figs. 2 and 3), but it may furnish valid data for ex-

EXPLANATION a Anorth@lase(this report) a Anorthoclase

Plagioclase

z'' 2eQor) -./ o.' l.A 'i { i,o-xr- Po-

Albte Abeoo. t.) Ab Or

Wt%

Frc. 5. Analyzed anorthoclase and plagioclase plotted in the Ab-rich portion of the ternary feldspar diagram (after Carmichael and MacKenzie;1964, p. 954, Fig. 2) showing comparison of theoretical to measured 20(2Ol) and a* of anorthoclase from this report (Table 5). tending the 20(201), a*, and cell volume curves in the direction of the tie line betweenOraoAboo and AnasAb66because of the good agreementwith the curve in Fig. 2, and it is anorthoclaseof the highest An content for which the lattice parametersare known to us. Carmichaeland NlacKenzie (1964,p. 960) believe that the curves of variation of a* and 20(201)F"-20(10I0)qt,when used in combination provide an f-ra)' method of determining ternarv composition of feldspars more potassicthan Orrr.Our determinedparameters for BTB-8 and FR-1 A N TA RCTI CA A NORTTIOCLA S E 1385 would not, however, provide an accurate ternary composition (compared to the chemical-analyses)using the curves (Figs.4 and 5). FR-l plots outside the data field, and although its Or content can be predicted quite accurately, the Ab-An ratio cannot be determined from the a* curve intersection.If the a* curves are extrapolated for FR-1, the feldspar would be estimated to be more sodic than its analysis shows. BTB-8

EXPLANATION a Anorthoclase(this report) a Anorthoclase

o-x-

5f'

Albte AbeoA.,a Ab O.

wt%

Frc.6. Analyzed anorthoclase plotted in the Ab-rich corner of the ternary feldspar diagram showing variation in the cell volume (after Carmichael and MacKenzie; 1964, p. 955, Fig. 3). The comparison of anorthoclase of this report is shown (Table 5). would be initially estimated to contain less than 016 (Fig. 4), and hence the above curves would not be applicable. The danger of rigidly comparing our lattice parameter determinations to the curves erectedfrom the data of Carmichael and MacKenzie (1964) has been indicated above. Becauseof possibledifferences in the method of measurementit may not be possibleto compare rigorously the results. It is obvious that duplicate samplesshould be run before a more quantitative comparison is made. Simple criteria are not available to determine with confidence the 1386 E. L. BOUDI],TTEAND A. B, FORD structural state of each of the antarctic anorthoclases(c.,f. Carmichael and nfacKenzie, 1964,p. 961). Our graphic comparison(Figs. 4, 5, 6) with the homogeneousanorthoclases of Carmichael and VlacKenzie (1964) indicates,however, that sample FR-1 is highly disordered and sample BTB-8 may be somewhatmore ordered.It is possiblethat the divergenceof the plot of BTB-8 from the 20(20I)F,-2d(1010)q1,curve (Fig. a) is attributable to structural state. A generalcomparison of the optics reported herein and the optics reported in the literature for all anorthoclasesindicates many similarities;refractive indicesand 2V's of antarctic anorthoclasestend to be hisher than usual.That the refractive

T,rero 5. SouB CouplnrsroNs oF Pa,relmtnns ror AmontsocLASE FRoM ANtaxcrrca

[Ternary composition is given for each sample as calculated from KzO, Na:O, and CaO by direct weight per cent molecu-larproportion. These values are given for comparison in Figs. 3, 4, 5 and 6, and difier somewhat from ternary composition (mol per cent) determined by the 32 oxygen method (see Table 1)l

Weight per cent - ' 2o(20t)1 2o(20r) VA3 Ab An 20(1010)qt;

BTB-8 (16 3) (73.3) (10 4) 21 8630' 0.987 86.92" 676.r FR-1 (r7.4) (634) (1e 2) 2r.8257" .950 86.97' 677.9 No. 83 (22 e) (72.o) (s.1) 21 7550' .879 87.39' 683.0 (18.0) (re 7) (20.0) Lattice parameters not available B (63 1) (61.s) (60.3) Lattice parameters not available C (18.e) (18 8) (19.7) Lattice parameters not available | I Cu radiation,Ni filtered. 2 20:20.876'. 3 Specimen8 of Carmichaeland MacKenzie(1964, p.950);includes Qn and Fs1s,1. indices and optic angle of the Cape Royds anorthoclase(FR-l) is sig- nificantly higher than the rest is attributable to its notably higher An content. The FR-1 rock is distinctly more silica undersaturated than rock BTB-8 to the extent thatT.4/6 normative nephelineis present(Table 2). The Or-Ab-An ratio in the normative feldsparin rocks FR-1 and BTB-8 are closelycomparable (Table 2), and hencethe partitioning of feidspar components,in particular Ab, is not accuratelypredicted in the norm as compared to the analyses.We have made a tentative identification of sodalitein rock FR-1. We believethat the sodium was partitioned into the sodaiite,resulting in correspondingdecrease in the relative Ab-An ratio. A NTA RCTI CA A NORTHOCLA S E t387

AcrNowlBpGMENTS Thanks are particularly due to our colleagueD. B. Stewart of the GeologicalSurvey for guiding as through the preparationof data for computer least-squaresrefinement and for frequent consultation. We also acknowledgethe kindnessof John B. Lyons and Richard E. Stoiber of Dartmouth Collegein making the singlevariation system availablefor our usein determiningrefractive indices. The work was done in conjunc- tion with a continuing study of alkali feldspars by Stewart and T. L. Wright, also of the GeologicalSurvey. D. B. Stewart, D. R. Wones and D. W. Rankin read the manuscript and made many helpful suggestions which are gratefully acknowledged. The research on the minerals was supported by the U. S. Antarctic ResearchProgram, National Science Foundation. Logistical support in Antarctica was suppliedby the U. S. Navy.

RrlrruNcns

Cerrrncnlrr, L S. E. nNo W. S. M,c,cKnNzre (1964) The lattice parameters of high-temperature triclinic sodic feldspars. Mineral. Mag. 33,949-962. Devrn, T. W. E. exr R. E. Pnrnsrr.nv (1914) Gloci'ology, physiography, stroligraphy, and' tectonic geol,ogyof South Victoria Land., Britdsh Antarcti.e Erpedi'tion 1907-9. t:. l, Geol'ogy. Dnrr, W. A., R. A. Howre aNn J. Zussu.lNu, (1963), Roch-Jorm'ing Mi'neral's v. 4, Framquorh Silicates: John Wiley & Sons, Inc., New York. Ev,llts, H. T., Jn , D E. Appr-nueN eNo P. S. Haxnwtmen (1963) The least squares refinement of crystal unit cells with powder diffraction data by an automatic computer indexing method labs.l Am. Cryst. Assoc.,Ann Mtg., Carnbridge,Mass., Mor. 1963, P r ogr am and,A bs. E-lO, 42-+3. JENSEN,H. I. (1916) Report on the petrology of the alkaline rocks of Mount Erebus, Antarctica. IL British Antarct'ic Expediti,on 1907 9, Rep. Sci.. Imestigations, GeoI. v.2, (7),93-128. MouNr,rlr, E. D. (1925) Potash-oligoclase from Mt Erebus, Antarctica and anorthoclase from Mt. Kenya, East Africa. Mineral'. Mag 29,331-345. Prron, G. T. (1907) Report on the rock specimens collected during the Discotery Antarctic Expedition l90l-4. NatI. Antarctic E*ped. N at. Hist., tt. l, Geol'ogy,101-140. Sn,rprno, L. lxo W. W. BreNxocr (1962) Rapid analysis of silicate, carbonate and phos- phate rocks, U.S Geol,.Suney Bull.ll44-A. Surrn, J V. mm W. S. MlcKrNzrr (1958) The alka1i feldspars, IV, The cooling history of high temperature sodium-rich feldspars. Am. M'ineral'. 43, 877-89. Srrrrr, W. C. (1954) Thevolcanic rocks oJ the RossArchipelago, British Antarctic Expediti'on 1910, Natwal History, Geology.r. 2. Trr:vrs, S B (1962) The Geology of Cape Evans and Cape Royds, Ross Island, Antarctica. In Antarctic Researclt Am Geoplrys. LInion, Geoph.ys.Mon 7r 4046. Wootwoucn, W. G. (1916) Petrological notes on some of the erratics collected at Cape Royds. In British Antarcitc Expedition 1907-9, Rept Sci' In",testGeol,. tt' 2, (11), 169- 180. tr[anuscript recei,red.,January 11, 1966; accepted'Jor publi,cation, April 1, 1966.