PHYSICAL PROPERTIES OF ANORTHOCLASE FROM
Transcription
PHYSICAL PROPERTIES OF ANORTHOCLASE FROM
THE AMERICAN PHYSICAL MINERALOGIST. VOL 51, SEPTEMBER OCTOBER, 1966 PROPERTIES OF ANORTHOCLASE FROM ANTARCTICA1 EucBNB L. BouoBrrE ANDAnrnun B. Fono, Surtey, Washington, D. C. and' U.S. Geological, Menlo Park, ColiJornio. Assrnnc't The lattice parameters of anorthoclase [OrrszAbza:Anror(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 leastsquares 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 responsible for the unusually high An content of the Cape Royds feldspar, although the latter i s n o t p r e d i c t e di n t h e n o r m . 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 l a t . 7 7 o 3 0 ' S .l ;o n g . 1 6 6 o 1 5 ' E .()F i g . 1 ) h a v e b e e nd e t e r m i n e da s p a r t o f a study of Quaternaryrhomb porphyry trach-vtesassociatedin 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 subsequentlyarousedconsiderableinterestfor their comparisonto kenyte of the African Rift. These phenocrystshave been classifiedvariousl.vas anorthoclaseor plagioclasein antarctic literature (seefor exampleDavid and Priestlv,l9l4; Jensen,1916;Woolnough, 1 9 1 6 ;S m i t h , 1 9 5 4 ;a n d T r e v e s ,1 9 6 2 ) . The present work was undertaken to clarify the nature of these feldsparsand to comparethem to the ternary feldsparsrecently describedby Carmichael and I,IacKen zie (.1964). Trachyte from the Crary ltlountains was coilectedin 1959by Boudette as a member of the 1950-60U.S. B1'rdlitndOversnowTraverse.The Cape Ro.vdstrachyte was collectedin 1961 by Ford and J. X'I. Aaron in conjunction with specialfield studiesof the occurrenceof the trachyte. 1 Publication authorized by the Director, U.S GeologicalSurvey. 1374 A NTA RCTI CA A NORTHOCLASD 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) describeanorthoclasefrom the summit of Mt. Erebus on Ross Island, Antarctica (Fig. 1, Table 1) that is comparable in occurrence.The occurrence,chemical analysis, texture and morphology of Mt. Erebus feldspar have been reported 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 were made 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 in the theoreticalmolecule(Table 1). Cn and Fs1s.y The chemicalanalyses,modes and norms of the rocks containing the feldsparsdescribedherein 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 Summit Mt. Erebus, Ross Island craryMts, ?0" T?d'i ^"Ti:i"" BrB_8 SiOs AlzOa Fero3 FeO Meo CaO BaO SrO Naro KzO HrO+ HzOTiOr PrOr MnO 6 5 .4 213 10 .10 10 20 071 82 2.6 .12 .00 05 07 00 Total No 8z 628 222 .18 65 23 20 68 20 .20 U5 37 .15* .15* 1a 2.8 28 00 l5 .06 00 996 .87 18 .22 845 378 26 06 99 93 62.79 22 1,2 36 ,41 00 376 62.49 21 86 .30 t3\ 16 374 60 83 23 92 11 214 07 339 735 298 .19 07 7 20 326 04 00 6.11, 296 07 t2 .36 10003 100 36 100 08 Number of iors on the basis of 32 (0)6 Si AI Fe+r Ti Mg Fe+r Na Ca Sr K Ba 11 .5804 + 4409 0213 0107 . 0 31 9 0106 2.8048 .3825 5950 6462 >z )w 16 0533 3 8248 74 2 10 1 or) eu!,uaZ, A"J p ^l 2Ya7 2YaB Ext. on {010J sp gr 1 5308 1 5364 1 5380 559 53 2" 6'-7" tr 2771 4 6961 021.5 0216 .0107 .o323 2 4988 .7109 11 6686 4 3583 . 0 2l 5 tt 257 0 048 rr 211 4 623 0 0,+1 0 061 2 554 722 0 043 0.196 2 504 .719 to 9289 5 0657 0.0215 0 0539 0 0216 0 32J3 2 1i40 .6575 681 .746 6682 16 0163 3 898q 2 9198 1718 0215 .8588 0107 16 048,1 3 9826 t5 98 402 t6 07 401 16 07 380 1 6 .8 648 184 218 73.3 49 170 635 195 186 62+ 190 193 61 7 190 1 5372 1 5416 1 5430 593 61 9 1 536 I 539 I 541 61 6" 62" 2 50** 2 620 1.536 1 539 1 541 61 6' 62" 2.6" 2 .620 r Chemical analysis by Paul Elmore, Sam Botts, Gillison Chloe, Lowell Artis, and H Smith by *-ray fluorescencemethods 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 universalstagetechniques. * 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 representapproximatemidpoints 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 0 05. t 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 BTB-8 159615 Field No Lab. No FR-1 163086 Chemical analysesr 57 6 183 38 28 77 19 73 .35 .81 .96 SiOz AlsOa FezOa l'eO Meo CaO Naro K:O HzO HOf TlOr Pror MnO COz .30 24 <05 559 198 19 3.5 l2 3.0 39 13 ..53 12 13 I4 < .05 Plagioclase* Clinopyroxene* Olivine* Ore minerals* "K-Ieldspal"*x Plagioclase*+ Orthopyroxene Clinopyroxene** Olivine** Amphibole** Apatite Calcite "Iddingsits"*x "Zeoli.tes'' Glass** Ore minerals** (A*Sp*Tr) Estimated /6 An in gloundmass plagioclase o7 13 04 <03 03 o2 tl 21. 145 30 104 Sp Sp Sp Sp Sp Tr Tr Tr 01 Tr Tr Sp 54i sp-A spA 126 (:2s) (2s) r00 Sum Powder density by air pycnometer -trace amount A : major component, Sp :5p21.", 1t * Reported as phenocryst, may also be in groundmass 269 if indicated. ** Groundmass constituent C.I P W Norms Semiquantitative Orthoclase AIbite Anorthite Nepheline Corundum |,*o Diooside{ En i- )-" lro Olivine j IFu Magnetite Ilmenite Hematite Apatite 21 1 .tn / 70 26 228 498 10 041 0 .2 1 07. 0 .r J 13l 'rr"\n ' 12.1 0 .8 J 28 23 :; 30 Sum Modes (volume per cent)2 Anorthoclase* 3 1. 0 333 B Ba Be Ce Co Cu spectrographic analyses3 0007 .15 .000.5 .o7 001 003 .02 0007 01 .02 000.5 05 .0007 t 0007 .o7 00.)7 0007 003 03 .001 01.5 03 .0005 15 002 005 .0005 La Mo Nb Nd Sc Sr V 00.5 Y 000.5 Yb 2t115 Looked for, but not found: Ag, As, Au, Bi, Cd, Cr, Eu, Ge, Hi, Hg, In, Li, Ni, Pb, Pd, Pt, Pr, Re, Sb, 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, Antarctica. 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 necessitates careful 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). Some individual crystals appear to be complexly twinned, in part on the Carlsbad law, but the most conspicuous twinning is close spaced (0.05 mm) lamellar albite twinning which is presentin all individuals, and commonly a second,finer reticulated polysynthetic pericline(?) twinning is developed nearly at right angleswhich produbesa fine cross-hatch(microcline-like)twinning pattern (Fig. 2 c, d). Textural evidencefor "flow cataclasis"in the trachyte ieaves the explanation of the secondpolysynthetic twinning open to reasonabledoubt; it may be strain twinning and not truly pericline,although we favor the interpretation that it is periclineon the basisof its orientation. Preliminary preparation of sampleswas done by gravity and magnetic purification of the 100 200 meshfraction of crushedanorthoclasecrystals 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 recognizable reflections were measured at the top of the peak, and the measurementswere adjusted by the amount indicated by the internal standard.The averageof the three measurementswere usedin the refinement, with Cu Ka1:1.5405 A. l|he feldspar patterns were refined using 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 o6 a: > '-.n o BF o5 H F Q tr @ € o d i o b 4 + i @ @ 4 O i N € 6 F O os o o ts z E tsl €i o € @ F o oi 2 o ba :o bo :o o z bi tso tso o ro E r o< 60 9i bo o :o oo F-j o on o6 6 <* 6- o o H s E a or)r)6 €(o(€ ; ooa O 3 a o6 --j dj- 6 $9 €$ I ti F] i ; oso F N ooo R3 \5 3 G M E - * F ! 93 :S B d N d : + i O N O E 6 O d O d O n € @ddi cpHTI FE: 6 @ 1381 ANTARCTICA ANORTHOCLASE Tasr-n 4. Isnnxrn Powoan Pe.trrnns ol Auontuocr-AsEsFRoMANtanc:rtce BTB-8 FR-1 hkl dcalcr 111 020 001 110 111 111 021 021 201 111 111 130 200 130 131 131 r12 221 221 r12 220 040 202 oo2 220 131 041 022 222 131 222 041 132 022 201 732 311 311 221 3t2 465 446 378 313 832 693 661 417 062 888 3 781 3 .7 7 5 3. 3 . 666 3 . 625 3 . 526 3 . 474 3 442 3 431 3 392 3 232\ 3 2%l 3 .22o) 3 18el 3 r a t) 3 008 2 941 2 .922 2 916 2 86s 2 847 2 816 2 807 2 . 799 2 707 2 .6 8 1 2 .676 2 . 667 2 s35 2 .5 3 1 dob"' 6 449 6 388 4 064 3.889 3.781 3 757 3 666 3 626 3 473 3 391 3.223 3.190 3 008 2 940 2 921 2 867 2.806 2 68r dcarcr 6.479 6 448 6.380 6 325 5. 832 s.698 4.660 4.420 4.069 3. 893 3.787 3. 760 3.686 3 669 3 625 3 529 3 472 3 446 3.436 3 392 3.240) I 3.2241 3.22r) 3. 190J 3.163) 3.011 2 941 2 922 2 916 2. 868 2.849 2 .8 1 8 2.805 2 801 2.713 2 682 2 682 2 673 2.540 2 533 dol'2 6 453 tI2 312 112 ,71 240 150 310 tot 4 068 3 894 3 787 3.760 3 667 3.627 3 . 4 7| 150 310 240 151 203 M2 113 242 331 113 242 r32 2.529 2 520 2 519 2.517 2 +70 2 458 2 457 2 454 2.421 2.4t9 2.4t2 2.396 2.391 2 369 2.348 2.330 2 322 2 313 2.3t3 2 297 2.281 2.2444 2.2399 2.525 2 531 2 . 5 2 7 2 523 2.522 2 520 2.473 2 463 2 461 2.456 2 427 2 419 2.41+ 2 402 2.394 2.37 r 2 348 2 330 2.321 2.317 t tt I 2 .3 0 0 2 281 2.246 2 242 3 391 3 223 BTB-8 dcalcr 3 191 3 010 2 940 2 -921 223 332 lDl o42 2 869 2 806 2 683 223 330 060 151 241 061 260 062 3.50 204 2 2363 2 2222 2.2193 2.2083 2.1941 2.1770 2.1549 2 1485 2.!228 2 1182 2.0038 1 8332 1 8280 1 8015 r.7802 dob"2 2 1498 2.1214 2 0035 1 8332 1 8280 r.8010 1 7804 FR-1 d"at"1 dobuz 2 2353 2 2234 2.2207 2.2lOO 2 . 1 9 71 2 1777 2.1.597 2 t494 2. t247 2-1212 2 0051 1 .8347 t.8279 1 8046 1 1799 2 1493 2.1214 r.8027 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 unambiguous reflections of the 25 or so present were used to calculate the Iattice parametersin the 3rd or4th cycle of the refinement.The standard deviation between observed and calculated 20 for each sample was 0.0151or less(Table 3). Observedand indexedlinesare 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 those of the anorthoclasefrom Antarctica describedby Carmichael and MacKenzie (1964) is given in Table 3. A graphical comparison of our Albite AbseAn,s Ab66An26 AbTsAnjs Ab5sAna6 Aq 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 anorthoclasedescribedby Carmichael and MacKenzie (1964,figs. 1,2, and 3) has beenmade on Figs. 4, 5, and 6 (Table 5). It is important to note that compositesampleswere used for the presentstudy in contrast to the selectedcrystalsusedby Carmichaeland MacKenzie, the Cn nnd Fs1s";componentsare not computed in the ternary compositioncalculationsof BTB-8 and FR-1 (Tables l and 5), the samples were not heated, and a different method of measurement was used in the determinationof the lattice parameters. 1383 A NTA RCTI CA A NORTHOCLASE 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 anorthoclasedescribedherein, as well as from a previous analysis of anorthoclasefrom Mt. Erebus (Table 1, Fig. 3). The "2" and "X" summations of both the old and the new tl I EXPLANATION a rf. i Anorthclase (this report) o Anorthoclase Plagioclase a I o I o 950 \.. i a o e a1 o ,""X a 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)aiz separationplotted against weight 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 possibly suspect; 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-x Albte r- { Po- Ab Abeoo. t.) 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 becauseof the good agreementwith and AnasAb66 line betweenOraoAboo the curve in Fig. 2, and it is anorthoclase of the highest An content for which the lattice parametersare known to us. Carmichael and 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 determinedparametersfor BTB-8 and FR-1 1385 A N TA RCTI CA A NORTTIOCLASE 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 (this report) Anorthoclase a Anorthoclase o-x- 5f' Albte Ab AbeoA.,a 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 possibledifferencesin the method of measurementit may not be possible to compare rigorously the results. It is obvious that duplicate samples should be run before a more quantitative comparison is made. Simple criteria are not available to determine with confidence the 1386 E. L. BOUDI],TTE AND 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 homogeneousanorthoclasesof Carmichael and VlacKenzie (1964) indicates, however, that sample FR-1 is highly disordered and sample BTB-8 may be somewhat more 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 generalcomparisonof the optics reported herein and the optics reported in the literature for all anorthoclasesindicatesmany similarities; refractive indices and 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 Ab BTB-8 FR-1 No. 83 B C (163) ( r 7. 4 ) (22 e) (18.0) (63 1) ( 1 8 e. ) An ( 7 3. 3 ) (63 4) (72.o) (re 7) (61.s) (188) ' 2o(20t)1 | (104) (1e 2) (s1 . ) (20.0) (60.3) (19.7) 21 8630' 2r.8257" 21 7550' 2o(20r)- VA3 20(1010)qt; 0.987 .950 .879 86.92" 86.97' 8 7. 3 9 ' 676.r 6 7 7. 9 6 8 3. 0 Lattice parameters not available Lattice parameters not available 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 significantly 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 hence the 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 believe that the sodium was partitioned into the sodaiite, resulting in correspondingdecreasein the relative Ab-An ratio. A NTA RCTI CA A NORTHOCLASE 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 single variation system available for our use in determiningrefractive indices.The work was done in conjunction 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 supplied by 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. (1963) The least squares reEv,llts, H. T., Jn , D E. Appr-nueN eNo P. S. Haxnwtmen finement 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 phosphate rocks, U.S Geol,.Suney Bull.ll44-A. (1958) The alka1i feldspars, IV, The cooling history of Surrn, J V. mm W. S. MlcKrNzrr 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), 169180. January 11, 1966; accepted' tr[anuscript recei,red., Jor publi,cation, April 1, 1966.