PHYSICAL PROPERTIES OF ANORTHOCLASE FROM

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
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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
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January 11, 1966; accepted'
tr[anuscript recei,red.,
Jor publi,cation, April 1, 1966.