American Mineralogist, Volume 65, pages 8I-95, 1980

The average structure of An 62-6labradorite

HeNs-RuooLF WENK

Department of Geology and Geophysics, University of CaliforniaBerkeley, Caffi rnia 947 20

WnnNrn JoswIG, TorunEr TAGAT, MASAAKI KonBraw^e

Institut fiir Kristallographie und Mineralo gieUniversitcit Frankfurt a.M., Frankfurt, F.R. Germany

AND BRADLEY K. SUITH

Department of Geology and Geophysics, University of CalifurniaB erkeley, Caffi rnia 9 47 20

Abstract

The paper reports results of X-ray and neutron structure refnements of four labradoritesAn 62-An 66 with di-fferent geological histories and di-fferent superstructures. A rapidlyquenched crystal collected during the eruption of Surtsey (Iceland) is essentially disordered.Labradorite from massive lava flows of Lake County (Oregon) shows D reflections, and twometamorphic labradorites from the Central Alps display an intermediate plagioclase struc-ture with e reflections. The average structure (c : 7 A) of all four samples converging at Rfactors of 3-6Vo are virtually identical with well-defined Si/Al and smeared-out oxygen posi-tions. We interpret this as evidence that the di-fferent superstructures consist of periodic stack-ings of similar basic units probably lssepfling those in 1l anorthite but with diflerent stack-ing orders. The main intensity contribution to superstructure reflections comes fromdisplacements of oxygens in the superstructures, probably caused by Al-Si ordering.

Phase relations are illustrated in a hypothetical TTT diagram in which we summarize re-sults from structure refinements, microstructural observations and changes during experimen-tal heating. At high temperature Qittle undercoqling below the critical temperature) orderingtakes place by a nucleation and growth mechanism which produced 1l domains separated bycurved APB's (antiphase boundaries) (Lake County). At larger undercooling continuous or-dering prevails, giving rise to periodic APB's and satellites in the diffraction pattern.

Introduction

Calcic plagioclase shows considerable structuralvariation which is expressed in superstructures anddocumented by reflections which appear in additionto the basic reflections present in X-ray di-ffractionpatterns of albite (for a review and references seeSmith, 1974). While the low and high albite struc-tures are fairly well established (e.g. Harlow et al.,1973; Prewitt et a1.,1976;Winter et al.,1977), all su-perstructures of plagioclase, including anorthite, arestill open to debate. Structural changes in plagioclaseare on one hand caused by variations in chenicalcomposition, while on the other hand they are also afunction of the history of formation. In this paper we0003-004x/80/0102-0081$02.00 8l

compare average structures derived from X-ray andneutron diffraction data of four specimens with al-most the same chemical composition, An 62-An 66,but of different geological origin. We discuss struc-tural features and relate them to microstructuralcharacteristics as observed with X-ray and neutronfitm and diffractometer methods and electron micros-copy, hoping to gain further information about thegeom€try and stability of intermediate plagioclase.

Material

We chose four specimens of labradorite: two ofvolcanic origin but a different cooling history andtwo from regionally metamorphic marbles and am-

82 WENK ET AL.: STRUCTURE OF L.IIBRADORITE

K z 0F e 2 0 3 . 3 6

F o r m u l a b a s e d o n 8 o x y g e n s

A l z O s 3 0 . 9 0C a o l 3 . l lN a o 3 . 5 1

2 . 3 81 . 6 3

C a . 6 3N a . 3 0 4

L i e i g h t p e r c e n t o x i d e s

s i 0 , 5 2 . 13 0 2r 3 . 0 81r .08

. 6 702

2 . 3 71 . 6 2

. 6 \36

. 0 0 4

. 0 0 1

8 . l 5 t ( 3 ) 3t2 .829 $)7 . 1 0 3 ( 4 )

93 .62 (3 )l r 5 . 2 r ( 3 )89 .70 (2 )

i n d i c a t r i x

4 0 . 5 533.0

- 1 4 . 08 8 . o

52.03 1 . 2r 3 . 83 . 3 9

. 0 3l l

2 . 3 41 6 6

. 3 0

. 0 0 2

.0011

8 . 1 5 2 ( 4 ) 312.83 \ G l1 . 0 7 9 $ )

93.\9 12)i l 6 . l ] ( 2 )9 0 . 4 0 ( 2 )

Table l. Chemical and physical properties oflabradorites

S u r t s e y Lake County V e r z a s c a S i s s o n e

eruption. Steinthorsson (1972) concluded from thecomposition of coexisting titanomagnetite and hemo-ilmenite that the temperature of equilibration was1020'C and the oxygen fugacity 1og /o, : -11.5. Butsince opaque minerals are relatively late crystalliza-tion products, 1020"C is a lower limit for the temper-ature at which the labradorite megacrysts formed.Direct temperature measurements in the extrudinglava in falT 1964 with thermocouples gave valuesranging from ll25 to ll80oC (Sigurgeirsson, 1965).We estimate that labradorite crystallized betweenI100 and l250oc, close to the liquidus t€mperature(-1350'C), and was quenched from a temperatureabove l000oC within a few minutes.

The megacrysts are chemically very homogeneouswith no observable 2oning. Table I gives the resultsof new microprobe analyses which document devia-tions from stoichiometry. In H. R. Wenk and Wilde's(1973) vector representation, the analysis plots at alocation typical of the substitution of Na by Ca plusvacancies, and we assume that the non-stoichiomet-ric composition was attained during crystallization inthe magma chamber. Optical properties are consis-tent with the high plagioclase series and an An con-tent of 63 percent (E. Wenk et al., 1965).

The preliminary X-ray experiments included a lat-tice-constant determination by Dr. Kroll, Miinster,using a Jago&inski-type Guinier camera (Table l)and a photographic investigation s5ing both focusingand non-focusing techniques. Figures la and b showstrongly exposed X-ray and neutron Okl Weissenbergphotographs on which diffuse e ard c reflections (firstmentioned by H. R. Wenk, 1966) are observed to-gether with strong and sharp a reflections. e reflec-tions are streaked normal to (013) in Okl photographsand have a spacing of l/30A-'. Note the similarity ofX-ray and neutron diffractograms.

Electron diffraction shows similar features (Fig. 2a,inset). The material is very homogeneous on sub-microscopic scale also. Locally a low-contrast lamel-lar structure can be imaged with a reflections whichdocuments weak structural or chemical hetero-geneities (Fig. 2a).

Lake County. The second sample is from the "sun-stone" north of Plush, Lake County, Oregon. Lab-radorite occurs as large phenocrysts in porphyriticbasalt with augite, olivine, labradorite (An 60), andmagnetite. Physical properties of the labradoritephenocrysts, which compose about lOVo of the rock,have been extensively studied by Stewart et al.(1966). Optical properties are in better agreementwith the low-plagioclase series (Table l). On the

. 0 0 5

. 0 r 4

5 2 . 1 63 0 . 8 81 3 . 7 1J . 9 6

' 1 2.4 ' r

t . 6 \. 5 6

.007

. 0 t 7KFe

" ( A ) 8 . 1 7 J 6 ( 5 \ 2 8 . t 7 \ 7 O l 2

r (A) 12 .8736 (s \ 12 .8706 (6 )c ( A ) 7 . r o 2 2 ( 2 \ 7 . r o r ! ( 3 )o (deg) 93 .462 (6 ) 93 .461 (5 )B ( d e g ) l 1 5 . 0 5 4 ( 5 ) l 1 6 . 0 8 5 ( 5 )y ( d e g ) 9 0 . 4 7 5 ( 5 ) 9 0 . 5 r l r ( 6 )

E u l e r a n g l e s d e f i n i n g o r i e n t a t i o n o f o p t i c a l

o ( d e g ) 5 1 . 8 qo ( d e g ) 1 5 . 2Y ( d e g ) - 2 5 . 8

2 V y ( d e s ) 8 2 . 0

q 4 . o s3 \ . 5

- 1 7 . 58 5 . 0

' l ' l e a s u r e d w i t h 8 - c h a n n e l A R L m i c r o p r o b e .

: D e t e r m i n e d w i t h J a g o d z i n s k i t y p e G u i n i e r c a m e r a b y D r , K r o l l , } { u n s t e r .' D e t e r m i n e d b y a u t o @ t i c c e n t e r i n g o f 2 0 r e f l e c t i o n s o n s i n g l e c r y s t a l

d i f f r a c t o m e t e r ." F r @ E . l r e n k e t a l , ( 1 9 6 5 ) .- F r o m E . V e n k ( u n p u b l i s h e d ) ," F r o m E . l , l e n k e t a l . ( 1 9 7 5 ) , S p e c i r e n V z 6 2 4 f r o m t h e s a m e l o c a l i t y .

phibolites. All samples are chemically homogeneouson a microscopic scale. Their composition is close toAn 65 (Table l). Individual chemical formulas de-rived from microprobe analyses were used as con-straints in the crystal structure refinements. Latticeconstants and Euler angles defining the orientation ofthe optical indicatrix are listed in Table l.

Since the geological history has a crucial influenceon the development of the superstructures, a brief de-scription of each specimen seems pertinent.

Surtsey. Large phenocrysts of labradorite wereejected during the submarine eruption of Surtsey,south of Iceland, in November 1963. Loose crystalsup to 4 cm were collected by Dr. G. Sigvaldson in thebeach sand of the newly-formed island. First theeruption was explosive, and during this phase a tufconsisting of translucent sideromelane glass (73Vo'Sand abundant olivine (7.7Vo, Fo 80) and labradoritecrystals (7.3Vo, An 66), including the phenocrystsused in this study, was deposited (Steinthorsson,1965). In May 1964 megacrysts of calcic plagioclasebegan to disappear. Steinthorsson interpreted theirdisappearance as resorption in a magma in which amore sodic plagioclase (groundmass plagioclase isAn 53-56) is stable. We were unable to find much in-formation in the geological literature about the tem-perature evolution of the Surtsey magma prior to

W.ENK ET AL.: STRUCTURE OF LABRADORITE 83

basis of lack of zoning Stewart et al. conclude thatthe phenocrysts formed in a magma chamber atdepth under uniform conditions. They estimate acrystallization temperature of I100'C. [t seems thatthe primary crystallization may have been similar inSurtsey and Lake County labradorite, but that cool-ing in extensive lava flows was considerably slower inLake County.

Single-crystal photographs show, in addition to areflections, sharp b reflections as reported by Stewartet al. and diffuse e and c reflections. Diffuse e reflec-tions can be distinguished from b reflections on thebasis of intensity differences (Rainey and Wenk,1978) and streaking. Di-ffuse streaks in c reflectionsare similar to those in the Surtsey specimen. With theelectron microscope it is possible to image a domainstructure with b reflections (Fig. 2b). We interpret thecurved boundaries as APB's corresponding to thosewhich have been observed in lunar anorthite.

Verzasca. This specimen is a regionally metamor-phic labradorite of the same chemical compositionfrom Gordemo, Verzasca Valley (8. Wenk et al.,1975). While the crystals from Surtsey and LakeCounty grew in a liquid magma, the metamorphicone crystallized in the solid state in rocks of sedimen-tary origin during regional metamorphism whichreached peak temperatures of 620-650"C at thesample locality in late Miocene. Cooling was slowand extended over many million years. The crystalanalyzed is extraordinary because it constitutes thecalcic portion of an andesine (An 38)-labradorite(An 63) intergrowth, documenting simultaneouscrystallization of the two phases. Optical propertiesand lattice constants are those of low plagioclase.Single-crystal photographs display sharp e satelliteswith a satellite vector Ah : 0.080(9), Ak : 0.031(8),Al : -0.261(10), T : 26.7(.3)4, which is typical ofcalcic labradorites of medium grade (Wenk, 1979).With electron microscopy e reflections can be used toimage regular e fringes and curved domain wallsalong which the periodic APB structure is distorted(Fig. 2c). c reflections are weaker and more diffusethan those of volcanic labradorites.

Sissone. A second metamorphic sample Sci 974 isfrom amphibolites in contact with tonalite in thehigh-grade metamorphic Bergell Alps (H. R. Wenket al.,1977). Wollastonite, spinel, corundum, andalu-site, and sillimanite in associated rocks suggest crys-tallization temperatures between 700 and 900oC. Inthis amphibolite from the border zone of mobiliza-tion, labradorite composition is bimodal. The majorphase is An 66 with local domains of An 50. X-ray

. , , . . 6 . i i , . . i . ' . . , . ' . , b ' ' ' '

Fig. 1 b*c+ Weissenberg photographs ofSurtsey labradorite' (a)

X-ray CuKa radiation using graphite monochromator (0001). 657.3 mm. (b) Neutron radiation I : 1.607A, pyrolytic graphitemonochromator (0001). O 57 3 x 2 mm (Grenoble reactor).

photographs show sharp e reflections with a satellite

vector Ah : 0.015(5), Ak : 0.080(7), Al : -0.109(l l),

T: 51.4(1.1)A, which is close to that reported for ig-neous plagioclase of similar composition (Gay,

1956). Electron microscopy reveals a fringe pattern

which is, however, fairly irregular. / fringes, imagedin darkfield with / and a reflections (Fig. 2d) repre-sent more or less periodic APB's which decomposelocally by pairwise recombination into b boundaries(compare Wenk, 1978b). The periodic structure is

best preserved in areas which contain defects, reduc-

ing the mobility of APB's.

Experimental techniques

The refinement of the average structure u5ing only

a reflections was done with X-ray and neutron dif-fraction data. (For a general appraisal of advantagesand disadvantages of neutron diffraction, comparee.g. Will, 1969.) Neutron diffraction was used for twomain reasons. Firstly, we were hoping to be able to

distinguish between Al and Si, whose neutron scat-

tering lengths show a greater difference than X-ray

scattering factors (Table 2). Secondly, we anticipatedgetting better resolution for positional and particu-

.*b

: l

84 WENK ET AL.: STRUCTURE OF LABRADORITE

Fig. 2. Microstructural characterization oflabradorites, using a JEMI0OC electron microscope at l00kv accelerating voltage. Attached0/</ SAD's are taken on the same area but not in the same orientation as images. c* is horizontal. Bars in micrographs are 10004. (a)Surtsey. Fine structures are probably artifacts due to carbon coating and astigrnatism. A possible lamellar structur€ is indicated by inkline. Darkfield with a reflection. (b) Lake County. Curved 6 APB's. Darkfield imaged with D reflection. (c) Verzasca. Periodic APB'simaged with e reflections (c/E. Wenk et al., 1975). The periodic structure is intemrpted by curved boundaries. (d) Sissone. Periodicantiphase boundaries decomposing into curved b APB's imaged with c and / reflections. The diffraction pattern with e and / satellites istaken in a region with periodic APB's.

larly for thermal parameters because of the angle in-dependence of neutron scattering lengths. Accuratethermal parameters are particularly important in av-erage structures, where they do not only express ther-

mal motion but also periodic or nearly periodic dis-placements of the atoms. Neither expectation wasfulfilled. However, in the Surtsey and Lake Countysamples the agreements between neutron and X-ray

C a 2 0 . 0 0 - 1 9 . 0 0 1 2 . 9 4 5 - 1 2 , 9 7 3

r { a ] 1 . 0 0 - 1 0 . 5 0 7 . 6 9 3 - 7 . 6 3 1

s i 1 4 . 0 0 - 1 2 . 0 0 8 . 8 5 2 - 8 . 8 2 1

A l r 3 . c c - l r . 5 o o 8 . 4 5 0 - 3 . 4 9 0

o 8 . 0 0 - 9 . 0 0 4 . 8 0 5 - 4 . 8 3 9

Table 2. Comparison of X-ray and neutron scattering factors

WENK ET AL: STRUCTURE OF LABRADORITE

8 . 2 5 2 - 8 . 2 \ 1 . 4 9 0 ( r o )

\ 289 - \ .237 .35r (8 )

6 . 2 3 2 - 6 2 \ 5 . 4 r 5 ( r o )

5 . 6 8 6 _ 5 6 \ t t . 3 4 5 ( B )

2 . 3 3 5 - 2 . 3 t \ . 5 7 8 ( r )

according to individual standard deviations. Weightsw: l/d were assigned to avoid overweighting of in-tense reflections, where e : A + d + CFI and o. isthe standard deviation based on counting statistics.For neutron data: A : 20, C: 0.001; for Surtsey X-ray data: A : 3, C : 0.001; for Lake County, Ver-zlsca, and Sissone X-ray data: A : 0, C : 0.0025.Atomic scattering factors were interpolated from thetable of Cromer and Mann (1968). Program cRYLSQ(from X-nav-sYSTEM, Stewart et al., 1970) was usedin Frankfurt, NUcLS6, a modified version of onrus(Busing et al., 1962), at Berkeley. Surtsey X-ray datawere refined with both programs, and the resultswere identical. Refinements converged with R factorsbetween 3 and 6Vo (Table 3).'

In the asymmetric unit of the average structure,there are 4 tetrahedral atoms, 8 oxygens, and a largecation site occupied by Ca and Na which was splitinto two positions of unequal occupancy. We at-tempted to split oxygen sites following the procedureof Toman and Frueh (1973), but the refiaement didnot converge. Effective electrical charges were foundto be close to one-half of the full ionic charge andscattering factors were adjusted accordingly in the re-finement.

Results

We notice a Ereat similarity of most parameters inthese refinements of the average structure of plagio-clase (Tables 4, 5, and 6). Surtsey and Lake Countylabradorites are almost identical, even though theformer has very diffuse and weak e satellites and thelatter sharp b reflections in the diffraction pattern.Neutron and X-ray refinements of the same samplesgave identical results, differences fging within onestandard deviation. Despite advantages of angle-in-

rTo receive a copy of Fo and d tables for all crystals, order

Document AM-79-ll8 from the Business Office, Mineralogical

Society of America, 2000 Florida Avenue, N.W., Washington,

D.C. 20009. Please remit $1.00 in advance for the microfiche.

' ! R e f i n e d f r o m S u r t s e y d i f f r a c t i o n d a t a . S t a n d a r d d e v i a t i o n o f l e a s t

s ; g n i f i c a n t d i g i t s i n p a r e n t h e s e s ,

refinements are excellent, which makes us confidentin the significance of the derived parameters (com-pare Tables 4 and 5) and the reliability of both dif-fraction techniques.

For the X-ray investigation full data sets of in-tensities were collected using MoKa radiation from agraphite monochromator by the o-scan (Surtsey) and0-20 scat technique (Lake County, Verzasca, andSissone). An absorption correction (p : 13.4 cm-')was applied only to Surtsey (computer program AB-sons of X-ney-sysrEM, J. M. Stewart et al., l97O).

Neutron di-ffraction data were collected on a four-circle diffractometer at the research reactor FR2 ofthe Kernforschungszentrum Karlsruhe, Germany.The approximate flux at the sample position was 6 xl0' n cm-' sec-'. With a neutron wavelength of A :l.O327A from (3ll) planes of a copper mono-chromator, intensities were measured in a or-scanmode with variable step sizes according to the resolu-tion curve.

For Lake County labradorite we also measured in-tensities of b reflections and for the Verzasca and Sis-sone samples pairs of e reflections, but the refinementof the superstructures is not part of this paper.

Structure refinements were done with full-matrixleast-squares programs, varying 134 positional, ther-mal, and occupational parameters with all reflectionsof more than 3o intensity. Reflections were weighted

Table 3. Parameters for data collection and structure refinement

C r y s t a l s i z eD i f f r ac to me te r

u sed

N u m b e r i n d e p e n d e n tr e f l e c t i o n s w i t h

l F l > 3 0

R - f a c t o r R - f a c t o ru n w e i g h t e d w e i g h t e d

(z) (z)

su r t sey x - r ay

Su r t sey neu t ron

Lake Coun ty X - ray

Lake Coun ty neu t ron

Ve rzasca X - ray

S i s s o n e X - r a y

0 . 2 7 x 0 . 3 \ x 0 . 2 m m

2 . 7 x 2 . 0 x 2 . 0 m m

sphe re 0 .2 mm

2 . 0 x 2 . 0 x 3 . 0 m m

0 . 3 x 0 . 2 x 0 . 0 7 m m

0 . 2 x 0 . 1 x 0 . 0 4 m m

Syn tex P2 l

P32 - FRz

N o n i u s C A D

P32 - FR2

Non i us CAD

Non i us CAD

33

2610

9\323067\ \

r 6601612

3 . 84 . 1

2 . 7

4 . 1

3 . 2q n

4 . 7

5 . 1

4 . ro . )

86 WENK ET AL.: STRUCTURE OF LABRADORITE

Table 4. Atomic coordinates of labradorites. Estimat€d error of the least significant digits in parentheses. Refined in space group Cl, c :

7 . L A

Sur t seyX- ray

Su r t seyNeut ron

Lake CountyX- ray

Lake Coun tyNeut ron

Ve rzascaX- r ay

S i s s o n eX- ray

Ca-Na ( l ) xvz

Ca-Na (2 )

r l (o)

r , (m)

12 (o)

T , (m)

0 ^ ( l )

o^ (2)

oB (o)

ou (m)

oc (o)

0, (m)

oD (o)

oo (m)

. zoo>

.9862

. 1 6 4 9

. 2 7 2 3

.0306

.0952

.00632

. l 5304

. 2 r 5 r 8

.00264

.81626

.23099

.58589

. l 0882

. 3 t 7 7 5

.68 r 00

.87870

. 5 > I t >

.oo\2'1290. 9 8 1 I

.5798

. 9 9 1 4

.2789

. 8 r 3 1

.1037

. 1 8 8 6

. 8 r 5 6

.8532

. 2 \ 3 5n l ? q

.2895

. 2 8 r 8

. 0 t 2 5

.6869

.2133I 0 7 q

. t073

.3822

. 1 9 0 0

. 8 6 5 I

. \292

.2666 (9 )

.987 (2 )

. t 6 2 ( 3 )

.2733 (9 )

.0309 (9 )

. 0 9 5 \ 2 )

.0062 (3 )

. 1 6 3 2 ( r )

. 2 1 5 3 ( 3 )

.0025 (3 )

. 8 1 6 3 ( l )

.2306 (3 )

.5848 (3 )

. r 0 8 8 ( l )

.3177 (3 )

.6805 (21

.8790 ( l )

.357t (3 )

. 0 0 3 9 \ 2 )

. l z 9 o ( r )

. 9 8 0 8 ( 2 )

.5799 Q)

. 9 9 t 2 ( l )

.2786 (2 )

. 8 1 3 0 ( 2 )

. 1 0 3 6 ( l )

. t892 (3 )

. 8 1 5 8 ( 2 )

. 8 5 3 0 ( l )

. 2 \ \ 4 ( 3 )

. 0 1 4 3 Q )

.2895 ( l )

. z 8 v Q )

.6866 ( l )

.2130 (2 )

. 1 9 7 2 ( 2 )

. 1 0 7 4 ( l )

. ja r s (3 )

. t9o2 (2 )

.8660 ( I )

. \293 (3 )

.2563 Q)

.9834 ( I )

.1687 Q)

. 2 7 0 9 ( l )

. o 3 o o ( r )

.0970 ( I )

.00588 (5 )

. 1 6 2 7 1 ( 3 )

. 2 1 4 8 1 ( 6 )

.00275 $)

. 8 1 5 4 5 ( 3 )

.z l tog 6)

.68425 G)

. 1 0 8 7 8 ( 3 )

.31709 n

.68085 (5 )

.87873 0)

.35765 rc)

.0027 Q)

. r 2 8 1 ( r )

.9796 (2 )

.5787 ( I )

. 9 9 r 3 ( r )

. 2 7 7 7 ( 2 )

. 8 1 2 r ( 2 )

. 1 0 3 3 ( l )

. 1 8 7 9 Q )

.8152 (2 )

. 8 5 4 0 ( l )

.?435 Q)

. 0 r 4 r ( 2 )

. 2 8 9 2 ( l )

.2830 (2 )

. 0 1 l 5 Q \

. 6 8 6 6 ( l )

. 2 1 l 8 ( 2 )

. 1 9 7 6 Q )

. 1 0 6 5 ( l )

. 3 8 1 5 ( 2 )

. r 9 0 6 ( z )

.8657 ( l )

.4307 Q)

. 2 6 9 ( l )

. 9 7 5 ( l )

. t 7 6 ( l )

. 2 7 3 Q )

. 0 3 2 ( r )

. 0 9 8 ( l )

.0053 (5 )

. 1 6 3 2 ( 3 )

.2152 (3 )

.0034 (6 )

. 8 1 5 3 ( 4 )

. 2 3 t \ 3 )

. 6 8 3 1 ( 5 )

. 1 0 8 7 ( 3 )

. 3 1 5 8 ( 3 )

.6796 (6 )

.8786 (3 )

. 3 5 8 4 ( 3 )

.0029 (4 )

. 1 2 7 9 Q )

.979\ Q)

.5794 ( \ )

.99 t3 (2 )

.278\ (2 )

.8123 $)

. 1 0 3 0 ( 3 )

. 1 8 8 2 ( 3 )

. 8 1 5 0 ( 5 )

.8537 3)

. 2 \ \ 4 ( 3 )

. 0 1 4 5 ( 4 )

.2869 3)

.2823 3)

. o l I 7 ( 5 )

. 6 8 5 8 ( 3 )

. 2 t 2 6 ( 3 )

. rgz4 (4 )

. 1 0 6 5 ( 2 )

.3822 (3)

. I 905 (5 )

. 8 6 5 5 ( 3 )

. \299 3)

. 2 6 7 9 Q )

.9787 Q)

. 1 6 5 9 ( 3 )

. 2 7 t 3 ( 2 )

.0285 ( l )

. t023 Q)

.00665 0)

. r 6503 (4 )

.21328 (9 )

. 0 0 3 3 8 ( 7 )

. 8 1 7 5 8 ( 4 )

.23205 e)

.68674 (7 )

. I 0935 (4 )

. 3 1 7 1 I ( 9 )

.6820\ (7)

.87952 ( \ )

.35720 (9 )

.0033 e ' i

. 1 3 0 3 ( r )

.9779 Q)

.5833 QJ

. 9 9 3 2 ( l )

.2788 (2 )

. 8 1 2 7 ( 2 )

. 1 0 5 8 ( l )

. t 9 0 2 ( 2 )

. 8 1 7 0 ( 2 )

.8527 (r )

.2466 (3 )

. 0 1 4 5 Q )

. 2 9 3 1 ( l )

.2778 (2 )

. o r 6 3 e )

.6890 ( r )

. 2 t 7 \ Q )

. l 9 9 o ( z )

. 1 0 8 2 ( r )? A q n ( ) l

. I 889 (2 )

. 8 5 7 1 ( l )

. 4 3 I 4 ( 2 )

.2665 (4 )

. 9 8 3 5 ( 6 )

.1692 (8 )

.2701 (3 )

. 0 3 0 4 ( z )

.0956 (5 )

. 0 0 5 4 6 ( 1 4 )

.16279 (8 )

. 2 1 4 8 4 ( 1 5 )

.00267 (t4)

. 8 I 6 9 5 ( 8 )

.23t07 05)

. 6 8 3 7 0 ( r 3 )

. r 0883 (8 )

. 3 r 5 0 9 ( r 5 )

. 6 8 0 3 5 ( 1 4 )

.87876 (8 )

. ' t o t 5 \ t J 1

. 0 0 1 2 ( 4 )

. 1 2 7 4 ( 2 )

.9783 (4 )

. 5 7 7 7 ( 3 )

. 9 9 1 8 ( 2 )

. 2 7 6 5 Q )

. 8 l 0 0 ( 4 )

. 1 0 3 1 ( 2 )

. I 8 5 1 ( 4 )

. 8 1 4 0 ( 4 )

.8539 (z)

. 2 4 1 0 ( 5 )

. o l 4 l ( 4 )

.2893 Q)

.2821 (4 )

. o r l o ( 4 )

. 6 8 6 4 ( z )

. 2 r 0 8 ( 4 )

.1967 (4 )

. 1 0 6 4 ( r )

.3798 (4 )

. r 903 (4 )

.8562 (2)

. 4 3 I 5 ( 4 )

X

z

X

vz

xvz

x

z

X

vz

xvz

x

z

X

Yz

X

z

X

z

X

vz

x

vz

X

vz

( 3 )( 5 )( 7 )

( 2 )( 2 )( 4 )

( l o )( 5 )

( l o )( 1 0 )( 5 )

( l 0 )( l o )( 5 )

( l l )

( 1 0 )( 5 )

( l o )( 3 )Q )(3)

(3 )( l )( 3 )

( 3 )(2)( 4 )

( 3 )( 2 )( 4 )

( 3 )( 2 )( 3 )

( 3 )( 2 )( 3 )

( 3 )( 2 )( 3 )

( 3 )( 2 )( 3 )

dependent scattering amplitudes and lack of absorp-tion uncertainties, standard deviations for all param-eters are higher in the case of neutrons, which weattribute to the fact that a rather large crystal had tobe used and the number of measured intensities wasmuch smaller.

Atomic positions

From atomic positions selected interatomic dis-tances and angles (Table 7) were calculated. Of theseaverage (T-O) distances are most informative, be-

cause they have been used to derive the Al-Si distri-bution, assuming a linear relationship between bond-lengths and Al-Si occupancy (Ribbe and Gibbs,1969; Ribbe et al.,1974).In all four samples we findthat T,(m) : Tdm) : T,(0) and T'(0) is larger, in ac-cordance with previous determinations of sodicplagioclase (Fig. 3a). There is a larger difference insize in the metamorphic crystals, implying that Alhas a site preference for T,(0), but average bond-lengths are nowhere close to those of fully-orderedstructures (e.g. for low albite: (A1-O) : 1.7434, (Si-

WENK ET AL.: STRUCTURE OF I-/IBRADORITE

Table 5. Anisotropic thermal parameters of labradorite 0Sxl00). Estimated errors of the least significant digits in parentheses.Temperature factors are of the form exp l-(pr1h2 + Brrt? + h3p + Zpnhk + 2BBhl + 2lztkl)l

87

S u r t s e yX- ray

S u r t s e yNeut ron

Lake Coun tyX- ray

Lake Coun tyNeu t ron

Ve rzascaX- ray

S i ssoneX- ray

c a - N a ( l ) B l l9 2 29 3 3g t zB 1 39 2 3

c a - N a ( 2 ) B l l9 2 2s 3 3B t 2g 1 39 2 3

. 4 7 ( 3 )

. 9 5 ( 4 )t . 5 0 ( 8 )- . 2 3 ( 2 )

. 4 8 ( 4 )- ' 7 \ ( 5 )

. 3 \ Q )

. 2 7 6 ( t 0 )

. 8 7 ( 4 )

. r 2 7 ( r o )

. i l ( 2 )- . r 5 7 ( 1 4 )

. 3 3 ( 3 )

. 1 2 0 ( 8 )

. \ 2 ( 3 )- . 0 4 3 ( 5 )

. 1 5 3 ( 1 5 )

.022 (5 )

. 3 4 2 ( t \ )

. 1 2 6 ( 4 )

. 3 8 e ( 1 7 )

. o9o (5 )

. 1 4 5 ( l r )

. o 2 o ( 5 )

. 3 0 ( 3 )

. 0 9 l ( B )

. 6 2 ( 3 )

.024 (5 )

. 1 4 4 ( 1 5 )

. 0 1 3 ( 5 )

. 2 5 ( 3 )

. 0 7 0 ( 9 )

. 4 e ( 4 )

. 0 1 7 ( 5 )

. 1 2 8 ( 1 5 )

. 0 4 3 ( 5 )

. 6 1 ( r 2 )

. 9 9 ( 1 6 )r . 8 0 ( 3 3 )- . 2 \ ( 7 )

. 5 8 ( 1 3 )- . 7 3 ( 2 a )

. 2 1 ( 1 0 )

. 3 1 ( 5 )r . 0 2 ( r 8 ). l 8 ( 4 ). 1 2 ( B )

- . 0 4 ( 7 )

. 3 7 ( 5 )

. l 8 ( 2 )

. 7 7 ( 6 )- . o o ( r )

. 2 5 ( 4 )

. 0 4 ( 2 )

. 4 2 ( 5 )

. 2 0 ( 2 )

. 7 5 ( 5 )

. t 2 ( 2 )

. 3 2 ( 4 )

. 0 3 Q )

. 3 \ ( 5 )

. 1 5 ( t )

. 8 e ( 6 )

. 0 5 Q )

. 2 e ( 4 )

. 0 2 \ 2 )

. 3 2 ( 5 )

. r 3 Q )

. e 5 ( 7 )

. 0 3 Q )

. 3 \ ( 4 )

. 0 5 ( 2 )

. 4 7 9 ( 1 8 )

. 6 5 1 ( 1 2 )r . o r ( 3 )- . 1 9 7 ( l l )

. 3 7 2 ( t 8 )- . \ \ 7 ( t 5 )

. 4 3 4 ( I o )

. 3 3 8 ( 4 )

. 9 6 0 ( 1 5 )

. I l 5 ( 5 )

. 0 8 5 ( 1 0 )- . 2 2 1 ( 6 )

. 4 3 1 ( l l )

. 1 5 4 ( 4 )

. 4 9 8 ( 1 5 )- . 0 4 3 ( 3 )

. 1 7 \ 0 )

. 0 3 5 ( 3 )

. 3 6 5 0 )

. 1 3 9 ( 2 )

. 3 2 9 ( 9 )

. 0 7 r ( 3 )

. 1 2 1 ( 5 )

. 0 0 3 ( 3 )

. 3 3 1 ( i l )

. l o 7 ( 4 )

. 4 9 1 ( r 5 )

. 0 1 5 ( 3 )

. l 2 0 ( 7 )

. o o o ( 3 )

. 3 3 1 ( l l )

. l l 3 ( 4 )

. 5 o B ( 1 5 1

. 0 0 5 ( 3 )

. t 2 6 ( 7 )

.049 (3 )

. 5 1 ( l l )

. 5 2 ( 2 0 )l . 8 7 ( 4 1 )- . r o ( 8 )

. 0 8 ( t z 1- . 3 \ ( r 4 )

. 5 0 ( r 7 )

. 3 3 ( l l )l . oo (35). 3 5 ( 1 7 )

- . 3 1 ( 1 5 )- . \ 7 ( I 9 )

. \ 9 ( 9 )

. 1 5 ( 3 )

. 6 8 ( l l )- . 0 4 ( 4 )- . 0 0 ( 8 )

. 0 3 ( 5 )

. 4 8 ( e )

. 2 0 ( 3 )

. 6 2 ( i l )

. 0 5 ( 4 )

. 0 3 ( 8 )

. 0 3 ( 5 )

. 5 1 ( 9 )1 . 8 9 ( 3 ). 4 5 ( l l ). 0 2 ( 4 ). 0 2 ( 8 )

- . 0 2 ( 4 )

. 4 7 ( e )

. 1 2 ( 3 )

. 5 9 ( t z )- . 0 5 ( 4 )- . 0 3 ( 8 )- . 0 I ( 4 )

. 5 \ ( 3 )

.76\ 1171l . 53 (5 )- . r 8 4 ( r 5 ). 4 e ( 3 )

- . 5 7 Q ). 3 5 a 1 1 9 1.293 (9)

1 . 0 0 ( q ). l l 7 ( 1 0 ). 0 8 9 ( 1 9 )

- . 1 7 2 ( t 3 )

. 3 \ 2 ( t 5 )

. 1 7 7 ( 5 )

. \ \ ( 2 )- . 0 4 8 ( 4 )

. 1 4 0 ( 1 0 )

. 0 4 5 ( 5 )

. 3 3 5 ( 1 0 )

. 1 7 2 ( 4 )

. 3 4 7 ( 1 6 )

. l 2 l ( 4 )

. l 1 9 ( 9 )

. 0 2 2 ( 5 )

. 2 2 6 ( 1 4 )

. 0 8 2 ( 5 )

. 4 0 ( 2 )

. 0 3 3 ( 4 )

. 0 8 2 ( 1 0 )

. 0 0 8 ( 5 )

. 2 6 3 ( t \ )

. l o 8 ( 5 )

. \ 6 ( 2 )

. 0 1 9 ( 4 )

. l 4 l ( 1 0 )

. 0 5 1 ( 5 )

. 43 (4 )

. 7 e ( 5 )r . r 3 ( r o )- . 2 4 ( 3 ). \ 5 (5 )

- ' 62 (6 )

. 3 5 ( 3 )

.267 05)

.70 (5 )

. 1 0 4 ( l l r )

. 1 3 ( 3 )- . i l ( 2 )

. 5 2 ( 3 )

. 2 \ 9 ( 9 )

. 4 0 ( 3 )- . 0 3 9 ( 9 )

. 1 9 0 ( 1 9 )

. 0 6 5 ( 9 )

. 5 2 ( 3 )

. 2 3 6 ( 9 )

. 3 5 ( 3 )

. 1 2 6 ( 8 )

. 1 8 4 ( 1 8 )

.032 (9 )

. 2 \ ( 3 )

. 1 2 3 ( 1 0 )

. 2 \ ( 4 )

.055 (8 )

. 0 7 ( 2 )- . o r o ( 9 )

. 3 e ( 3 )

. r 9 r ( 9 )

. \ 5 ( 4 )

. 0 3 4 ( 8 )

. r 3 5 ( r 9 )

. 0 9 5 ( l o )

T l ( o )

T , ( m )

12 (o )

r, (m)

g l l9 2 2p 5 5B t 2B 1 3

B l l9 2 2

g t zB 1 39 2 3

B l lg z z9 3 3g l zB 1 39 2 3g l l

9 2 2s 3 3g t 2B 1 3

O) : l.6124, Harlow et al., 1973: or for anorthite:(Al-O) :1.746A and 1.756,4., (Si-O) : 1.613 and1.623A, Czank,1973). This is not due to partial orderbut to the fact that in the average structure (c : 7 A)ofordered anorthite Al and Si are superposed, result-ing in an equal distribution of Al and Si over all fourT sites ((T-O) - 1.685A) (Fig. 3c). Figure 4 is a plotof (T-O) distances of average structures of plagio-clase, which shows that the size of the tetrahedradoes not vary linearly with the An content, implyinga different Al-Si ordering pattern in chemically in-termediate plagioclase from end members or in-dicating that the size-occupancy relation is not lin-ear. In fact, the size of the Al site in sodic plagioclaseseems much more sensitive to minor Si substitutionthan does the size of the Si site to Al substitution.which could be explained by a regular solutionmodel with Al close to the radius-ratio limits fromtetrahedral coordination Seing more unstable. Surt-

sey, Lake County, and Sissone follow closely the gen-eral trend, while for Verzasca the di-fference betweenT,(0) and the other three sites is slightly larger.

We tried to refine the AllSi distribution directly,based on diferences in scattering power. We are atthe limit of resolution but in most refinements theT,(0) site is slightly lighter and therefore preferen-tially occupied by Al with fewer electrons than Si.

Thermal parameters

Anisotropic temperature factors (in Table 5 we listf,r) for T atoms are surprisingly small, almost of thesame magnitude as in refinements of anorthite(Wainwright and Starkey, 1969). This indicates thatin the superstructures of the plagioclases we studiedatomic shifts of T atoms from average positions areminimal. Oxygen atoms, on the other hand, havelarger and strongly anisotropic temperature factors incontrast to e.g. low albite. This is illustrated in oRrnp

Table 5. (continued)

u r t s e y S u r t s e yNeu t ron

La ke

88 WENK ET AL.: STRUCTURE OF LABRADORITE

0 ^ ( 2 )

0 ^ ( 0 )

0 u ( m )

oc (o )

0 , (m)

oD (o)

0o (m)

B l l8 2 2s33g 1 2B l 3R ? ?

B l lp z zA ? 2

g t 2B l 3g 2 3

B l r9 2 29 3 39 1 2g r 38 2 3

B.r r9 2 29 3 39 1 2B l 3p L 5

B r l922

B t 2B l 39 2 3

B l lQ t 2

p t tg t z3 1 39 2 3g l lg z z$ l l9 t zB 1 3g 2 3

B l l9 2 2R ? ?

8 1 2B 1 39 2 3

I . r 4 ( 4 ). 2 8 7 ( i l ). 8 0 ( 4 ). 1 o l ( 1 8 ). 5 8 ( 4 ). 0 8 6 ( r 7 )

. \ 3 ( 3 )

. 1 4 5 ( 9 )

. e 9 ( 4 )

. 0 1 7 ( 1 3 )

. 2 6 ( 3 )

. 0 7 9 ( r 5 )

. 8 2 ( 4 )

. r 9 2 ( 1 0 )1 . 4 7 ( 5 )- . 0 4 1 ( 1 5 )

. 6 7 ( 4 )- . 0 3 1 ( 1 7 )

. 8 2 ( 4 )

.269 \12)2 . t 2 ( 6 )

. 0 8 l ( I 7 )

. 7 e ( 4 )- . 0 7 ( 2 )

. 7 2 ( 4 )

. 2 \ 2 ( 1 1 )1 . 2 2 ( 5 )- . 0 5 5 ( 1 6 )

. \ 5 ( 4 )

. o 4 B ( r 7 )

. 7 \ ( 4 )

. 2 4 4 ( l l )

. 9 7 ( 4 )

. l B l ( 1 5 )

. z t ( 3 )- . 0 2 ( 2 )

. 7 o ( 4 )

. 2 2 6 ( 1 0 )

. 8 6 ( 4 )

. 0 6 0 ( 1 5 )

. 1 5 ( 3 )

. 0 5 5 ( I 6 )

. 7 5 ( 4 )

. z 5 l ( l l )r . l B ( 5 ). 0 8 5 ( I 7 )

- . o l ( 4 )- . 0 6 2 ( r B )

I . 0 8 ( 4 ). 3 4 ( l )

l . 1 4 ( 5 ). 1 0 ( t ). 7 t ( 3 ). l r \ 2 ). 4 7 ( 3 ). 1 9 ( l )

t . 2 5 ( 4 ). 0 4 ( r ). 3 5 ( 3 ). 0 9 ( 1 )

. 8 0 ( 3 )

. 2 6 ( l )1 . 7 2 ( 5 )- . 0 3 0 )

. 7 6 ( 3 )- . 0 1 ( 2 )

. 8 0 ( 3 )

. 3 \ ( l )2 . 3 6 ( 5 )

. l l ( l )

. 9 7 ( 4 )- . 0 4 ( z )

. 6 9 ( l )

. 2 8 ( r )l .46 (5 )- . 0 5 ( l )

. 5 1 ( 3 )

. 0 6 ( 2 )

. 6 9 ( 3 )

. 2 9 ( l )L l 5 ( 4 )

. 1 6 ( l )

. 2 7 ( 3 )- . 0 3 ( l )

. 6 e ( 3 )

. 2 7 ( l )l . l 3 ( 4 ). 0 5 ( l ). 2 5 ( 3 ). 0 5 ( l )

. 7 2 ( 3 )

" 3 2 0 )I . 4 8 ( 5 ). 0 7 ( l ). 1 3 ( 3 )

- . 0 5 Q )

1 . 1 t 9 \ 2 2 ).289 (5 ). 6 4 5 Q 3 ).087 (9 ). 5 5 1 ( 1 9 ). 0 8 8 ( 1 0 )

. 4 9 5 ( 1 6 )

. 1 5 8 ( 5 )

. 7 8 9 Q 2 )

. 0 1 0 ( 7 )

. 1 8 4 ( 1 5 )

. 0 5 0 ( B )

. 9 5 3 ( 2 0 )

. 2 1 3 ( 6 )1 . 3 2 1 ( 2 9 )- . 0 6 4 ( 8 )

.56 \ (20)- . 0 4 7 ( I 0 )

.868 (22)

. 2 9 \ ( 7 )2 . r 6 8 ( 3 8 )

. 0 5 4 ( I o )

. 7 \ 3 Q \ )- . 1 0 7 ( r 3 )

. 6 8 1 ( 7 )

. 2 8 0 ( 7 )t . 1 7 9 ( 2 8 )- . 0 7 5 ( 9 )

. 3 7 0 ( 1 8 )

. o 8 o ( i l )

. 6 5 7 ( l B )

.269 (6 )

. 8 8 0 ( 2 5 )

. 1 1 7 ( 8 )

. 1 5 9 ( r 7 )- . 0 7 7 i l 0 )

. 7 2 9 ( t B )

. 2 3 1 ( 6 )

.75 \ Q4)

.027 (8 )

. 0 5 8 ( r 7 )

. 0 4 3 ( 9 )

. 8 1 0 2 0 )

. 3 0 1 ( 7 )

.gg9 (z t )

. 0 7 0 ( 9 )- . 0 5 5 0 8 )- . 0 7 9 ( i l )

l . 1 9 ( 7 ). 2 9 Q ). 5 0 ( 9 ). 0 5 ( 3 ). 3 e ( 5 ). l o ( 3 )

. \ 9 ( 5 )

. 1 9 Q )1 . 0 4 ( 9 )- . 0 1 ( 3 )

. t 7 ( 5 )

. 0 2 ( 3 )

. 9 0 ( 7 )

. 2 6 ( 2 )I . 4 5 ( 9 )- . 0 5 ( 3 )

. 3 9 0 )- . 1 2 ( 4 )

. 86 (8 )

. 3 t ( 3 )2 . 6 8 ( l r )

. 0 5 ( 3 )

. 5 e ( 8 )- . l o ( 4 )

. 7 1 0 )

. 2 8 ( 2 )I . 2 8 ( 9 )- . 0 8 ( 3 )

. 2 0 ( 6 )

. 1 5 ( 3 )

. 8 6 ( 7 )

. 3 e Q )l . l 7 ( 9 ). 1 7 ( 3 ). 2 2 ( 6 )

- . o o ( 3 )

. 7 \ ( 7 )

. 2 \ ( 2 )I . 0 0 ( 9 ). 0 5 ( 3 )

- . 1 3 ( 5 ). l o ( 3 )

. 8 2 ( 7 )

. 3 1 Q )r . l o ( 1 0 ). 0 7 ( 3 )

- . 7 e ( 7 )- . 0 9 ( 4 )

1 . 0 0 ( 3 ). 3 5 4 ( l o ). 8 5 ( 4 ). 0 6 8 ( 1 3 ). 5 5 ( 3 ). 1 3 2 ( 1 5 )

. 5 0 Q )

. 1 8 7 ( 8 )

. 7 e ( 4 )- . 0 0 3 ( l o )

. 1 7 Q )

.079 (12)

. 7 5 ( 3 )

.233 (9 )t . 2 o ( 4 )- . 0 4 7 ( l l )

. 5 8 ( 3 )- . 0 0 8 ( 1 4 )

. 8 4 ( 3 )

. 3 6 9 ( 1 0 )l . 9 l ( 5 )

. o 7 o ( l 3 )

. 8 1 ( 3 )- . 0 3 r ( 1 7 )

. 6 7 ( 3 )

. 3 0 6 ( 9 )I . 0 8 ( 4 )- . 0 8 8 ( r I )

. 3 2 ( 3 )

. 0 7 3 ( I 5 )

. 7 5 ( 3 )

. 3 2 5 ( r o )

. e 3 ( 4 )

.238 (12)

. 1 4 ( 3 )- . s 9 5 ( 1 4 )

. 7 1 ( 3 )

. 2 6 5 ( 9 )

. 7 1 ( 4 )- . 0 0 4 ( l l )

. 0 9 Q )

. 0 4 4 ( 1 3 )

. 7 5 ( 3 )

.290 (9 )

. 8 6 ( 4 )

. 0 4 4 ( r 2 )

. o B ( 2 )- . 0 4 5 ( 1 4 )

l . 3 l ( 6 ). 4 0 8 ( 1 9 ). 5 0 ( 6 ). 0 7 ( 3 ). 5 6 ( 5 ). 0 5 ( 3 )

. 6 8 ( 5 )

.275 ( t6 )

. 6 3 ( 6 )

. 0 2 ( 2 )

. 2 5 ( 4 )

. 0 6 ( 2 )

. 8 9 ( 5 )

. 3 6 0 ( r 9 )r . 4 3 ( 8 )- . 0 7 ( 3 )

. 5 8 ( 5 )- . 0 2 ( 3 )

. 8 7 ( 5 )

. 3 9 Q )2 . 3 4 ( 1 0 )

. 0 8 ( 3 )

. 6 5 ( 6 )- . l o ( 4 )

. 7 6 ( 5 )

. 4 5 ( 2 )r . 0 2 ( 7 )- . 0 4 ( 3 )

. 3 4 ( 5 )

. 1 5 ( 3 )

. 8 4 ( 5 )

. 4 0 3 ( r 9 )

. 7 0 ( 6 )

. 1 9 Q )

. 0 4 ( 5 )- . 1 4 ( 3 )

. 8 e ( 5 )

. l z \ ( 1 9 )

. 5 0 ( 6 )

. o o ( 2 )

. 0 4 ( 4 )

. 0 1 ( 3 )

. 9 5 ( 5 )

. 4 1 ( 2 )

. 9 0 ( 7 )

. 1 5 ( 3 )- . 0 4 ( 5 )- . 0 7 ( 3 )

plots in Figure 3, where 98Vo probability contours aredrawn. Assuming a real mean amplitude of thermalvibration of 0.12A (B : l.l) for oxygen (e.9. low al-bite, Winter et al., 1977) then the high apparent tem-perature factors for O" and OD atoms (B" - 2.5) hasto be explained by actual atomic displacements of atleast 0.1A. Thermal parameters of all four specimensare similar.

Splitttng of Ca/Na

We have followed the procedure applied by Ribbeet al. (1969) to albite and by Czank (1973) to anor-thite to split the CalNa atoms, distributing CalNaunequally over two positions. Positional, thermal,and occupational parameters were refined independ-ently and the R factor improved' As Czank (1973)

I'VENK ET AL.: STRUCTURE OF L,,IBRADORITE 89

showed for anorthite, splitting persists up to hightemperatures but occupation of the split positions be-comes more and more similar with increasing tem-perature. He interprets it as a statistical distributionof the large cation over two positions of diferent en-ergy, with preferences determined by the environ-ment (such as the A|lSi distribution in impure anor-thite), rather than a thermal vibration between twopositions (Quareni and Taylor, 1971).

Splitting of the CalNa site is visible in Fouriermaps but only if low-order components, which in-clude several very strong reflections, are omittedfrom the summation. Figure 5 shows Fourier mapswith doo, < 1.25A for all four samples and documentsa great similarity of this structural detail. We deter-mine from Fourier maps and direct refinement of oc-cupancies (Table 6) that the CalNa (2) peak is pref-erentially occupied over Ca,/Na (l), which is inagreement with Czank's findings. The separation ofsplit CalNa atoms is about 0.8A in all four structures(Table 7) and is very close to that of anorthite, in-dicating that this aspect of the plagioclase structure issimilar over a wide composition range (e.g. Quareniand Taylor, 1971, and Winter et al., 1977, find it evenin low albite; and Prewitt et al.,1976, document it inhigh albite), and in crystals with different thermalhistories.

Extinction cofficient

At the last stages of the refinements, relativelylarge extinctions of strong low-angle reflections wereencountered in the volcanic samples, and an isotropiccorrection for secondary extinction was carried out(Zachariasen, 1968). Values for extinction coeffi-cients of Surtsey t.156(3) x 10-51 and Lake Counry10.80(4) x l0-'l are rather large, indicating a highperfection of the volcanic crystals, while metamor-phic crystals showed low extinction (<10-7).

Discussion

Average structure

In this paper we will discuss average structures andatomic parameters that have been derived from mainreflections only (type a). All information is containedin a c :7A unit cell with space-group symmetry CT.The presence of superstructure reflections in the dif-fraction pattern (b and e type) indicates that this isnot the true structure. Physically the average struc-ture is best visualized as a projection of the larger

Table 6. Occupancies of Ca-Na positions

R e f i n e d o c c u o a n c i e s

E l e c t r o n d e n s i t yo f C a - N a ( 2 ) -p e a k i n e A - J

c a - N a ( l ) C a - N a ( 2 ) ( c o m p a r e F i q u r e 5 )

Lake County

S i s s o n e

. 4 \ 7 O l

. 3 6 6 1 2 )

. 4 8 5 ( 2 )

. 4 3 8 ( | 0 )

( ( 1

. 6 3 4

q l !

. ) o l

unit cell of the true structure into the unit cell of al-bite. This means that corresponding atoms withslight atomic shifts in the true structure appear in theaverage structure as partially occupied sites, veryclose to each other. Ifthey are far enough separated,their positions can be refined as split atoms such asCa/Na in our refinement. Alternatively an averagesite may be calculated and displacements are in-tegrated in an apparent temperature factor which isgenerally larger than that of the true structure. In or-der to deconvolute the average structure, one has touse the intensities of superreflections.

The superstructure of plagioclase with b reflections(body-centered anorthite) is well established (e.9.Fleet et al., 1966; Czank, 1973; Berking, 1976; Tagaiet al., 1978). For intermediate plagioclase with e re-flections, several more or less quantitative modelshave been proposed, which all differ in important re-spects and emphasize either CalNa or Al,/Si order,or wavelike or discrete periodic atomic displacements(Kitamura and Morimoto, 1977; Korekawa et al.,1978; Toman and Frueh, 1976).In this paper we donot discuss detailed atomic arrangements in the su-perstructures, but wish to see how results from aver-age structure determinations constrain models for thetrue structure of labradorites.

The principal conclusion is that all four averagestructures are very sirrrilar, even though Verzasca andSissone have sharp e satellites, Lake County has D re-flections, and Surtsey is highly disordered. Thisseems to imply that all crystals are composed of simi-lar structural units, and differences are mainly due tothe stacking of these units. In intermediate plagio-clase (e plagioclase) they are stacked in a long-rangeperiodic pattern with antiphase relationships whichare expressed in extinctions in the satellite di-ffractionpattern (Korekawa, 1967). In b plagioclase units al-ternate regularly along the z axis, causing a deufling

38

\ l

43

4 l

90 TryENK ET AL,: STRUCTURE OF LABRADORITE

Lobrodorite AnortlileAlbite

Fig. 3. Comparison of the average structure of Surtsey labradorite with the structur€s of low albite and anorthite. ORTEP plots with987o probability contours of the vicinity of the rzo plane are shown. (a) Albite from Wainwright h Smith (1974, p. 86). (b) Suruey. (c)Anorthite from Wainwright and Starkey (1971).

os i@ A l

of c. The ielative displacement between units is l/2[001] (which is equal to l/2 [10] in space group .Il;compare Miiller et al., 1973). In disordered plagio-clase distribution of the units is random and neither enor 6 reflections are observed. (Very weak and dif-fuse c and e reflections are present on strongly-ex-posed photographs of Surtsey, indicating that eventhis rapidly-quenched volcanic crystal shows at leastshort-range order.)

A second feature of all refinements is that temper-ature factors for tetrahedral sites are of the samemagnitude (Fig. 3b) as those observed in albite (Fig.3a) and anorthite (Fig. 3c). Averaging does not smearout atomic positions of Al and Si and displacementsin the true structure from average atomic positionsare minimal. This is different for oxygens, particu-larly O"(m), O"(0), Oo(m), and Oo(0), which havemuch larger and more strongly anisotropic temper-ature factors than equivalent atoms in the resolvedalbite and anorthite structures. and must therefore bedue to displacements and not anisotropic thermal vi-bration. The oxygen displacements in the averagestructures are plausible by comparison with the anor-thite structure (Fig. 3c, fiorr Wainwright andStarkey, l97l), with distorted chains of tetrahedraextending parallel to x. Anisotropic temperature fac-tors for Surtsey are identical to those of the other

samples which show a well-developed superstructure.This implies that this rapidly-quenched crystal withessentially only a reflections has not a low or high al-bite structure. Rather Surtsey is composed of thesame structural units as the others, except that stack-ing is random. The sharp split of the CalNa positionin all four samples, including e plagioclase, is evi-dence for stacking of units with discrete dis-placements rather than a sinusoidal variation (e.9.Fig. 4 in Kitamura and Morirtoto,1977).

A third striking observation is that intensities of band e reflections are similar in X-ray and neutrondiffraction photographs (Fig. l), even though scatter-ing powers of atoms are diferent (Table 2). This sup-ports a model with atomic displacements rather thanoccupational di-fferences.

Another structure with periodic stacking is ferroe-lectric NaNOr. Bohm (1977) has used this example toextend the satellite theory of Korekawa (1967),which is mainly concerned with transverse and longi-tudinal displacement and density waves, to includeperiodic stacking, and was able to explain intensityvariations with glancing angle. In general, several or-ders of satellites are expected at medium and highglancing angles if a modulated structure contains pe-riodic atomic displacements. Higher orders have notbeen observed in NaNOr, nor in intermediate plagio-

WENK ET AL: STRUCTURE OF I,/IBRADORITE

c a - N a ( l ) - c a - N a ( 2 )

r , ( 0 ) - 0 A ( r ), o B ( o )0c (o )0 D ( o )

Average

T . ( O ) A T B' atc

ATDBTCB T DC T D

T , ( m ) - o A ( r )' o B ( m )0 c ( m )0 D ( m )

at the domain boundaries (Fig. 6b; Korekawa et al.,1978). In this model orientation and spacing of theperiodic APB's are a function of the extent of or-dered bands relative to disordered zones which sepa-rate them. The average structure does not allow ex-traction of quantitative details about the atomicdistribution in the superstructures, but we may implyfurther information from the microstructure of thefour samples, their geological history, and some addi-tional heating experiments.

Microstructure

All four samples appear to be chemically homoge-neous. Apart from minor local lamellar structures in

Table 7. Ca-Na(l)-{a-Na(2) and T-O bond lengths (in A) andtetrahedral bond angles (in ") taken from X-ray refinements.

Estimat€d error in parentheses

S u r t s e y L a k e C o u n t y V e r z a s c a

9 l

clase, where the main superstructure intensity is com-ing from only one pair ofsatellites (e reflections). (Insome calcic labradorites and bytownites additional/reflections are present, satellitic about a positions.)

We have noted above that average structures oflabradorite are very suggestive of stacking of twostructural elements, both -7A. These structural ele-ments are similar but difer in AVSi arrangements,displacements of oxygen atoms, and possibly occupa-tion and position of CalNa. As in NaNOr, A and Bare both acentric and related to each other by a cen-ter of symmetry (co_mpare Fig. 3c). A combination ofthese elements in 1l symmetry @ : l4A) with A on0,0,0 and r/z,lz,r/2. and B on 0,0,r/z and, Vz,rh,O (Fig. 6a)produces the superstructure of body-centered plagio-clase with D reflections (Tagai et al.,1978).

For labradorite with e reflections the same ele-ments may build the superstructure in a periodicantiphase pattern with less-ordered transition zones

T - O distonces

-./-\.-

d e f g

Fig. 4. Plot of average T-O distances of average plagioclasestructures vs. composition. [Samples plotted include Wainwrightand Starkey, l97l (k); Berking, 1976 ();Fleet et al.,1966 (i); Kleinand Korekawa,1976 (E); Phillips et al., l97l (c, d); prev/itt e, 4r.,1976 (b); and Winter et al., 1977 (a), in addition to rhe four newrefinements and unpublished data of threc metamorphicplagioclases (e, f, h).1

s i ssone

(o)

. 7 9 3 ( 5 ) . 8 2 i

] . 6 8 5 ( 2 ) 1 . 6 9 2| . 6 7 8 ( 2 ) 1 . 6 7 9r . 6 6 r ( 2 ) t . 6 6 21 . 5 9 3 ( 2 ) t . 6 9 7

r . 6 8 0 L 6 8 3

t 0 r . 6 ( r ) 1 0 r . 2 7i l 7 . 1 ( r ) i l 7 . 5 51 0 r . 5 ( l ) r 0 l . 3 0r r r . 8 ( t ) i l t . 8 9i l 3 . 9 ( l ) 1 1 3 . 8 9i l 0 . 5 ( r ) | 0 . 4 0

r . 5 8 0 ( 2 ) I . 5 B O1 6 \ 2 ( 3 ) t . 6 \ 9t . 6 6 8 ( 2 ) | 6 7 31 . 6 \ 9 ( 2 ) t . 6 5 2

r . 5 6 0 1 - 6 6 \

l 0 q . l ( r ) r 0 4 . r 2i l 2 . 8 ( 1 ) i l 2 . 8 8l 0 5 . o ( r ) r 0 4 . 8 7i l r . 9 ( t ) n 2 0 l1 1 3 . 2 ( r ) n 3 . 1 2r 0 9 . 6 ( r ) r 0 9 . 5 5

1 . 6 7 9 ( 2 ) r . 6 8 01 . 5 6 8 ( 2 ) 1 . 6 6 6t . 6 5 6 ( 2 ' ) \ . 6 5 6r .5 t '0 (2 ) | .635

1 .551 | .660

1 0 7 . 4 ( r ) r 0 7 . 2 4r 0 2 . 8 ( r ) 1 0 2 . 6 1r 0 8 . 6 ( r ) r 0 8 . 8 91 t 2 . 3 \ t ) 1 t 2 . 2 \1 r i . 2 ( r ) i l 1 . 1 8n 4 . o ( l ) i l 4 . 0 7

| .697 \21 r .580r . 5 5 0 ( 3 ) | 6 5 0t . 6 5 2 ( 2 ) 1 . 6 5 21 . 6 6 9 ( 2 ) 1 . 6 7 0

| . 6 6 2 1 . 5 6 3

r 0 9 . 4 ( r ) 1 0 8 . 9 1' 1 0 5 . 7 ( i ) r 0 5 . 6 5f 0 7 . 4 ( t l t 0 7 . 5 2i l r . l 0 ) i l r . 2 91 r 0 . 4 ( r ) i l 0 . 4 9i . 2 . 6 ( r ) r r 2 . 7 3

r 4 0 . r ( r ) 1 3 9 . 6 3126.3 ( t ) t25 .951 3 6 . 2 ( l ) r 3 5 . 9 81 5 6 . r ( 2 ) t 5 5 . 8 rl 3 l . t ( l ) t 3 r . 3 6r 3 r . 6 ( r ) D r . r 3t 3 3 . 2 ( r ) r 3 r . 8 6r 5 0 . 6 ( 2 ) r 5 r . 0 6

\ 2 ) . 8 r 3 ( 7 ) . 8 2 8 ( 9 )

( r ) 1 . 6 9 1 ( 2 ) 1 . 6 9 3 ( 3 )( r ) r . 6 8 9 ( 2 ) r . 6 8 5 ( 3 )0 ) r . 6 7 r ( 2 ) t . 6 5 6 ( 3 )( t ) t . 7 0 0 ( 2 ) r . 5 8 3 ( 3 )

r . 6 8 8 t . 6 7 9( 6 ) r 0 2 . I o ( 9 ) r o o . j Q )( 6 ) 1 1 6 . 5 0 ( 8 ) 1 r 7 . 5 ( r )( 6 ) r 0 2 . 6 8 ( 9 ) r o r . r ( l )( 6 ) i l r . 9 3 ( 8 ) i l 2 . 2 ( l )( 6 ) | l 2 . 9 4 ( 8 ) 1 1 4 . 3 Q )( 5 ) 1 l 0 . 2 j r ( 8 ) 1 1 0 . 7 ( l )

( r ) 1 .652 t2 ) 1 672 (3 ' )( r ) 1 . 6 2 7 Q ) 1 . 6 \ 2 ( 3 )( r ) 1 . 6 5 2 ( 2 ) 1 . 6 7 7 3 \( r ) r . 6 4 3 ( 2 ) r . 6 4 9 ( 3 )

l . 6 l r q 1 . 6 6 0( 7 ) i 0 5 . 8 9 ( 9 ) r 0 4 . 2 ( 2 )( 6 ) i r 2 . 3 1 ( 9 ) I 1 2 . 9 ( t )( 6 ) r 0 5 . 6 9 ( 9 ) r 0 4 . 6 ( 2 )( 6 ) i l 0 . 9 6 ( 9 ) r r r . 5 ( 2 )( 6 ) l r 2 . 6 5 ( 9 ) I t 3 . 5 Q t( 6 ) 1 0 9 . 2 2 ( 8 ) 1 0 9 . 8 ( 2 )

( r ) r . 6 6 3 ( r ) t . 6 7 3 3 )( t ) 1 . 6 3 7 Q l r . 6 6 r ( 3 )( r ) 1 . 6 3 8 ( 2 \ 1 . 5 4 8 ( 3 )( r ) t . 6 2 6 ( 2 \ r . 5 3 t ( 3 )

t . 6 \ 2 1 . 6 5 3( 5 ) t 0 8 . 8 1 ( 8 ) r 0 7 . 2 ( r )\ 6 ) 1 0 3 . \ 2 ( 8 ) r 0 2 . 3 ( l )( 6 ) 1 0 8 . 0 7 ( 8 ) 1 0 8 . 9 ( t )( 6 ) r 1 . 6 8 ( 8 ) l l 2 . o ( 2 )( 7 ) i l r . 2 i ( 9 ) i l 1 . 4 ( 2 )( 6 ) I 1 3 . 2 1 ( 8 ) t 1 4 . / r ( l )

( r ) 1 . 6 6 7 Q ) r . 5 8 1 ( 3 )( r ) t . 6 3 \ Q ) t . 6 5 0 ( 3 )( r ) t . 6 3 \ Q ) r . 6 1 1 5 ( 3 )( l ) l . 6 1 1 8 ( 2 ) 1 . 6 8 0 ( 3 )

| . 6 \ 6 t . 6 6 4( 7 ) r 0 9 . 0 5 ( 8 ) r 0 8 . 7 ( t )( 6 ) r 0 5 . 6 r ( 8 ) r 0 5 . 8 ( l )( 5 ) t o 7 . 7 3 ( 8 ) t 0 7 . 6 ( l )( 7 ) | 1 0 . 8 1 ( 9 ) | | 1 . 0 ( 2 1( 7 ) I 1 0 . 2 5 ( 9 ) I l o . 8 ( 2 )( 5 ) r r 3 . 1 6 ( 9 ) I r 2 . B ( t )r o ) 1 4 0 . 8 9 ( 1 0 ) t 3 9 . 2 ( 2 1o ) t 2 7 . t 7 ( 6 ) I 2 5 . 8 ( 2 )to ) t37 .92 (7 ) 135.2 (2 )l 0 ) r 5 7 . 4 0 ( l o ) 1 5 5 . 0 ( 2 \f o ) l 3 r . 1 5 ( 8 ) r 3 1 . 6 ( 2 tr o ) 1 3 2 . 9 4 ( 7 ) 1 3 0 . 9 ( 2 )r o ) r 3 3 . 2 4 ( 7 ) 1 3 1 . 6 ( 2 )r 0 ) r 5 0 . 8 5 ( r 0 ) 1 5 t . 3 ( 2 )

Ave rage

T r ( m ) A T B' ATC

ATDBTC8TDC T D

T r ( o ) - o A ( 2 )- o B ( o )0c (m)0D (m)

T ? ( 0 ) A T B

A T DBTCBTDC T D

r , ( m ) - o A ( 2 )' o B ( m )0 c ( 0 )0 D ( 0 )

A v e r a g e

r" ( , ) AT8. ATc

ATDBTCBTDC T D

T t - 0 A 0 ) - T l12-0A(2) -r2

T l - 0 8 ( 0 ) - T 2T l - 0 8 ( m ) - T 2T r - o c ( 0 ) - T 2r r - 0 c ( m ) - T 2T I - 0 D ( 0 ) - T 2T t - o D ( m ) - T 2

92 WENK ET AL; STRI]CTURE OF LABRADORITE

LAKE COUNTY

SISSONE

labradorites An 50-60 and bytownites An 70-85which show chemical exsolution (e.9. Clnff et al.,1976). There are structural homogeneities, though: wedescribed antiphase domains in Lake County sam-ples which probably formed during cooling. Thereare similar boundaries in the Yerzasca crystal (Fig.2e). The presence of two structures in the LakeCounty sample is documented by a diffraction pat-tern with sharp b and diffuse e reflections (Raineyand Wenk, 1978, their Fig. 4b).

In solid solutions either exsolution or ordering willoccur, depending on whether repulsive or attractiveforces dominate between unlike atoms. In plagioclasewith two coupled substitutions both processes takeplace, but the four labradorite samples describedhere appear to be chemically homogeneous represen-tatives from the region between the Bdggild and Hut-tenlocher exsolution gaps in the phase diagram (com-pare e.g. Fig. 3 in Wenk, 1979).

Ordering can take place by different mechanisms.It may start at nuclei and proceed by growth of or-dered domains. Microstructures which are the resultof such a mechanism are often characterized bycurved APB's fike those observed in Lake Countycrystals (Fig. 2b). Alternatively, at larger under-66sling below the critical ordering temperature, or-

D i s o r d e r e d

SURTSEY

VERZASCA

r AFig. 5. Four F(obs) maps of the Ca-Na peak. Reflections with d

> 1.25A are omitted from the summation to increase resolution. yzsection at x : O.2'7. (Compare Table 6).

Surtsey (Fig. 2) and Verzasca (E. Wenk et al., 1975,their Fig. 6c), there is no evidence fot chemical heter-ogeneity. The a reflections in X-ray diffraction pat-

terns obtained with focusing Laue technique aresharp and not split. This is in contrast to "schiller"

TTId

o

B A B A B BIA

A B A B A 8\8tOOll=z

con= 14.2A

rnol

[OOl].2qo,t

c su P"'

Fig. 6. Schematic models for the superstructure of labradoritesas different stackings of units .A and B. (a) Body-centeredplagioclase (Tagai et al.,1978). (b) Intermediate plagioclase An-56(Korekawa et al., 1978).

Fig. 7. Schematic diagram illustrating homogeneous ordering of

two atomic species A and B in a one-dirnensional crystal by a

spinodal mechanism (b) with formation of a superstructure with

45 and continuous order (c) resulting in periodic APB's.Horizontal axis x is a crystal coordinate. Vertical axes give

amounts ofA (solid) and B (dashed) in each site.

B

a

Coniinuous Ordering

APBI

,s\,

oa-

t, t

b

Spinodol Ordering

B A x X A B x X B

A B x X B A x x Ao,o

WENK ET AL: STRUCTURE OF I.IIBRADORITE

dering may take place homogeneously throughoutthe crystal.

De Fontaine (1975) distinguishes between twomodes of homogeneous ordering which are illus-trated schematically in Figure 7.In spinodal orderingthe new site distribution in the superstructure is at-tained gradually as the transformation proceeds. Or-dering generally results in a superstructure with a de-crease in symmetry-such as in Figure 7---or adoubling of the unit cell. In continuous ordering alarge wavelength compositional modulation is con-voluted with the short wavelength of ordering. Thislatter mechanism produces modulated structureswith periodic APB's.

The thermal history of samples and accompanyingstructural changes can be traced in a time-temper-ature-transformation diagram (TTT) which has beenused to illustrate chemical decompositions of geolog-ical samples (e.g. Champness and Lorimer, 1976).The analogy to ordering is not strictly applicable.Structures in the regions labeled ',nucleation andgrowth" and "continuous ordering" could be difer-ent. Volcanic samples with b reflections may onlyhave a short-range ordered AllSi distribution adjust-ing for non-stoichiometry in a random pattern.

The very schematic and empirical diagram in Fig-ure 8 shows fields for heterogeneous (or possibly ho-mogeneous) nucleation of an ordered equilibriumstructure (11 structure with 6 reflections) and for con-tinuous ordering at conditions of undercooling (inter-mediate plagioclase with e reflections). Beginningand end of the ordering reaction are indicated. In thecase of nucleation and growth the end product is apattern of ordered domains separated by ApB's. Incontinuous ordering a modulated structure formswhich decomposes during prolonged annealing bypairwise recombination of APB's. Changes in micro-structure and particularly the high mobility oft/z[001]APB's are further elaborated by Wenk andNakajina (1979), who document a whole range oftransitions between e and b plagioclase simply due todifferent arrangements of APB's. Temperatures inFigure 8 are rough estimates, and the time coordinateis based on empirical observations.

Surtsey labradorite (Su) was rapidly quenchedfrom temperatures close to the solidus. The coolingpath barely passes through the "continuous', noseand leaves it again before ordering in a periodic pat-tern is complete. Lake County (LC) cooled moreslowly, allowing nucleation of ordered 1T domains. Asmall volume fraction is still disordered when thecurve passes through the fleld ofcontinous ordering,

T i m e

Fig. 8. Empirical TTT (time-temperarure-transformation)diagram illustrating cooling histories and microstructural changesin various An 66 labradorites.

giving rise to additional weak e reflections. The twometamorphic samples Verzasca (Vz) and Sissone (Si)crystallized at low temperature and cooled veryslowly, forming fully-ordered periodic structures. Sis-sone, the higher-grade plagioclase, attains a largerwavelength (5lA) than Verzasca (27A) and decom-poses partially by recombination of e APB's to Dplagioclase.

We have done heating experiments which seem toconform with such a model (see also Tagai et al.,1977). Heating Lake County crystals in a furnace atll80"C for I week erases b reflections and b APB's(Fig. 9b). Our model leaves two alternatives. Eitherheating produces true disordering of Si and Al on tet-rahedral sites or it merely causes a random stackingof still-ordered units. We prefer the latter ex-planation unless crystals are heated very close to themelting point. Such a mechanism requires only mo-bility of antiphase boundaries rather than structuralrearrangements, which is in better agreement withobserved changes during s66ling. Consecutive slowcooling to 900oC at 25"C per day produced 6 reflec-tions again, and darkffeld images show a micro-structure with small ordered domains (Fig. 9c). Allthese experiments were in the field of 1T structures.Hydrothermal annealing of Surtsey at 870oC pro-duces sharp e satellites (Wenk, 1978a), while afterlonger annealing at 900"C e reflections are replaced

-;- \ - 9'-^: ' j - ---\{-Prosiocrose

WENK ET AL.: STRI]CTURE OF LABRADORITE

Fig. 9. Changes in microstructure of Lake County labradorite after heating (compare also Tagai el al., 1971). Darkfield photographs

taken with D reflections (a) APB's in untreated crystals. (b) APB's disappear and D reflections b€come extremely weak and di-ffuse after

heating at l200oC for I week. (c) Upon slow cooling to 900"C at 25oC per day b reflections appear again and small ordered domains

become visible.

by D reflections (McConnell, 1974). The upperboundary of the field for continuous ordering is esti-mated to be around 900-950'C. The geological his-tory of a sample which established the state of orderhas a decisive influence on changes during heat treat-ment, and results presented here cannot be general-ized and applied to other labradorites.

c reflections

A last point to mention is the presence of diffuse creflections in volcanic labradorite. Streaking is mostpronounced parallel to b*, but more exactly intensityis distributed in a disk normal to [001]. [001] is theimportant direction in the plagioclase structure,along which chains of 4-fold tetrahedral rings extend.In anorthite this is the direction of antiphase Al-Sistacking. We interpret these streaks as due to faultswith a displacement vector Vrlllll and a fault plane(010) but a poor periodicity. It is interesting that c re-flections are weaker in metamorphic than in volcaniclabradorite.

The ideas presented here, which attempt to synthe-size evidence from both structural and physical fea-tures of labradorite plagioclase, are speculative andneed to be corroborated further by experimental in-vestigations of stability relations and also com-parative refinements of both average and super-structures of heat-treated crystals. We hope that we

can provoke other feldspar researchers to participate

in this fascinating endeavor.

Acknowledgments

H. R. W. is most indebted to Professor Korekawa and the

Frankfurt Institute for the hospitality during a sabbatical leave in

1976 which started this program. He also acknowledges support

from NSF grants EAR76-14'156 arld EAR77-00127, and valuable

discussions with D. de Fontaine and G. Nord. W. J., T. T., and

M. K. are indebted to the Bundesministerium fiir Forschung und

Technologie, F. R. G. and the Deutsche Forschungsgemeinschaftfor financial support. Computations were carried out at the Com-puter Centers of the University of California' Berkeley, and the

University of Frankfurt am Main' We thank Dr. H. Kroll' Miin-

ster, for determinations of precise lattice param€ters of Surtsey

and Lake County crystals. A. Ross cheerfully typed the various

versions of the manuscript.

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Manuscript received, September 18, 1978;

acceptedfor publication, July 30, 1979.