The ortoclase-microline inversion

8/18/2019 The ortoclase-microline inversion

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354

The orthoclase microcline inversion.

B y WM. SCOTT MAcKENzIE, M.A., B .Sc., Ph .D .

Geophysical Laboratory, Carnegie Ins t i tu t ion of Washington,

Washington, D.C.

[Taken as read January, 1954.]

INTRODUCTION.

H E association of orthoclase 1 and microcline in the same roc k has

been comm ented on by ma ny wri ters , and f rom these occurrences

at te mp ts have been m ade to deduce the re la t ive s tab i li ty o f these two

low-te mpe rature forms of potash-felspar. Ahnost al l invest igators hav e

concluded that n l icrocline is the lower-temperature modificat ion, but

difficulties in explaining the relationship of adutaria to orthoclase and

microcline ha ve been encountered.

The identif icat ion o f the monoclinie potash-felspar in association with

microcline is almost always based on optical propert ies , v iz. s traight

extinction in the zone [010] or the absence of visible multiple twinning.

These are necessary conditions, but are not in themselves sufficient

evidence of a monoclinic felspar lat t ice. X- ra y powder diffract ion

pa tte rn s are some times effective MacKenzie, 1952), bu t detectio n of

very s l ight departures from monoclinic symmetry may require s ingle-

crys ta l X-ra y m ethods .

In discussing the lattice and twin ning o f nlicrocline, La ve s 1950)

s tates that the relat ionship between the pericl ine and albi te twinning

can be explained only on the assum ption th at the microcline crys tal lized

with monoclinic symmetry and subsequently acquired tr icl in ie sym-

me try. M any petrologis ts may f ind difficulty in accepting this hypo -

thes is from their experience of the occurrence of microclines , but the

evidence given by Lave s fro m the m aterial which he invest igated is ver y

good. In one of the specimens to be described here, orthoclase is actua l ly

present along with microcline, and the relat ionship between pericl ine

and albi te twinning in the microcline appears to be identical with that

described by Lave s 1950). Ther e seems no firm reason, however, to

extend the hypothesis to untwinned microclines .

1 The term orthoclase is used throughout this paper to indicate low-temperature

monoclinic potash-felspar.


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oRTHOCLASE MICROCLINE INVERSION 355

VARIATIONS IN THE MICROCLINE LATTICE.

I n 1 9 5 0 L a v e s c o n s i d e r e d t h a t m i c r o c li n e a n d t r i cl in i c a d u l a r i a s h o u l d

b e r e g a r d e d a s s e p a r a t e m o d i f ic a t i o n s o f p o t a s h - r i c h f e ls p a r, a n d h e g i v e s

l a t t ic e a n g l e s ' c h a r a c t e r i s t i c ' o f e a c h f o r m e v e n i n a m o r e r e c e n t p a p e r

( 1 9 5 2 ) , i n w h i c h h e s u g g e s t s t h a t t h e r e m a y b e a c o n t i n u o u s g r a d a t i o n

f r o m a t r u l y m o n o c l i n i c l a t t i c e t o a t r i c l i n i e l a t t i c e w i t h a n g l e s a

9 0 ~ 3 9 ' a n d 7+ 8 7 ~ 4 7 '. T h e s e v a l u e s a r e , h o w e v e r , v e r y c o m m o n i n

m i c ro c li n es . T h e y r e p r e s e n t t h e l a rg e s t d e p a r t u r e f r o m m o n o c l i n ic s y m -

m e t r y s o f a r f o u n d , a n d f o l l o w i n g a s u g g e s t i o n b y D r . L a v e s ( p e r s o n a l

c o m m u n i c a t io n ) , t h e t e r m ' m a x i m u m ' m i c ro c li n e is u s e d h er e t o d e n o t e

a p o t a s h - f e l s p a r h a v i n g a p p r o x i m a t e l y t h e s e l a t t i c e a n g l e s .

F i g . 1 s h o w s t h e X - r a y s p e c t r o m e t e r p a t t e r n s o f f iv e s a m p l e s o f l o w -

t e m p e r a t u r e , p o t a s h - r i c h fe lsp ar~ P a t t e r n A is o f a t r u l y m o n o c l i n i c

f e l s p a r , a n d t h e r e m a i n i n g p a t t e r n s a r e p r o d u c e d b y t r i c l i n i c f e l s p a r s .

T h e p e a k s j o i n e d b y b r o k e n l in e s r e p r e s e n t r e f le c t io n s f r o m t h e (1 30 )

a n d (1 50 ) p l a n e s b e t w e e n 2 0 v a l u e s o f 2 3 ~ a n d 2 5 ~ ( C u - K a ) , a n d f r o m

t i le ( 1 31 ) a n d (1 51 ) p la n e s b e t w e e n 2 0 v a l u e s o f 2 9 ~ a n d 3 1 ~ T h e p a t t e r n s

a r e a r r a n g e d i n o r d e r o f i n c re a s in g d e p a r t u r e f r o m m o n o c l in i c s y m m e t r y .

T h e l a tt ic e p a r a m e t e r s o f s p e c im e n s C , D , a n d E , m e a s u r e d f r o m t h e s e

p a t t e rn s b y t h e m e t h o d o f D o n n a y a n d D o n n a y (1 95 2), a r e s et d o w n in

t a b l e I . T h e r e l a ti o n b e t w e e n t h e s e p a r a t io n o f t h e 1 3 0 a n d 1 5 0 p e a k s

a n d t h e a n g l e y * is g iv e n b y t h e f o l lo w i n g e x p r e s s io n :

1

Q l ~ o - Q l a o = 1 2 a ' b * c o s ~ * , w h e r e

Q h k l - -d ~ 2 h k l

T h e v a l u e o f 201~o- 201ao is t h u s d i r e c t l y d e p e n d e n t o n v a r i a t i o n s i n ~ * ,

s in c e v a r i a t io n s i n a * a n d b * a r e v e r y s m a l l. T h e a n g l e s ~ a n d $ c a l c u l a te d

f r o m s p e c i m e n E c o r r e s p o n d f a ir ly c l o se ly t o t h e a n g le s o f a ' m a x i m u m '

m i c ro c l i ne . Th e va l ue o f 201~o- -201ao ( C u - K ~ ) f o r t h i s m i n e r a l i s 0 8 4~

T h e c o m p o s i t i o n s o f t h e s e f i ve m i ne r a ls , c a l c u l a t e d f r o m t h e c h e m i c a l

a n a l y s e s , a r e g i v e n i n fig . 1 . I n s a m p l e s B , D , a n d E t h e p e a k a t a b o u t

2 0 = 2 8 ~ ( C u - K a ) i n d i c a t e s t h e p r e s e n c e o f a lb i t e a s a s e p a r a t e p h a s e .

T h i s is t h e s t r o n g e s t p e a k i n t h e p o w d e r d i f f ra c t io n p a t t e r n o f l ow -

t e m p e r a t u r e a l b i t e 1 a n d is m a d e u p o f r e f le c t io n s f r o m 0 4 0 , 0 0 2 , a n d 2 2 0.

T h u s t h e c o m p o s i t i o n s d o n o t r e p r e s e n t t h e p o t a s h p h a s e a l o n e a n d

c a n n o t b e u s e d i n a s t u d y o f t h e m i c r oc li n e l a tt ic e w i t h o u t c o r r e c t in g

f o r t h e a m o u n t o f a l b it e p r e s e n t a s a d i s ti n c t p h a se . I f a l a rg e a m o u n t

o f l o w - t e m p e r a t u r e a l b i t e is p r e s e n t , t h e 1 11 r e f le c t io n o f t h e a l b i te m a y

1 From this pea k alone it is impossible to distinguish between high and low

temperatur e albite.


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3 ~ 6 W . S . M A CK EN ZIE O N

obscu re the 130 re f l ec tion o f o r thoe lase , bu t in these pa t t e r ns th i s d iff i-

cu l ty does no t a r i se .

T h e l a t ti c e p a r a m e t e r s o f sa m p l e B h a v e n o t b e e n d e t e rm i n e d , b e c a u s e

D I

~

B

I I

15 2 2 5 3 O 3 5 4

F r o . 1. P o w d e r X - r a y d i f f r a c t io n p a t t e r n s o f l o w - t e m p e r a t u r e a l ka l i- f el s p ar s .

P a t t e r n A i s o f e m o n o c l i n i c f e l s p a r a n d p a t t e r n s B , C , D , a n d E a r e o f m i e r o c li n e s

w i t h v a r y i n g d e p a r t u r e f ro m m o n o e l i n ie s y m m e t r y . T h e p e a k a t a b o u t 2 8 ~ C u - K a )

i n p a t t e r n s B , D , a n d E i s d u e t o t h e p r e s e n c e o f a l b i t e a s a s e p a r a t e p h a s e .

T h e c o m p o s i t i o n s o f t h e s e m i n e r a l s f r o m c h e m i c a l a n a l y s e s a r e : A - I ) , S p e n c e r ,

1937) A , Ors4 .sA bl3 .sAnl .7 ; B , Ors l . sA bls . lAn0 .4 ; C , Ors~ .gAbl2.~Anr4 ; D, Orsa. a

Ah la .~ A n r s ; E B lu e M r . m ic r o c l i n e , t a b l e I I ) , Or g0 .~ Ab g.2 An 0.a . S e e p . 3 6 6 , 5 ote.)


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O R T H O C L A S E M I C R O C L I N E I N V E R S I O N 57

t h e r e f le c t io n s w h i c h s h o w t h e t r i c li n ic c h a r a c t e r o f t h e l a t ti c e a r e n o t

c l e a r l y r e s o l v e d f r o m o n e a n o t h e r . I t i s p r o b a b l e t h a t t h e l a t ti c e a n g l e s

o f t h i s m i n e r a l a r e c lo s e t o t h o s e g i v e n b y L a v e s (1 95 2) a s c h a r a c t e r i s t i c

TABLE I. Cell para met ers of microclines.

Specimen C. Specimen D. Specimen E.

a . . . . . . . . . 8.566 8.55~ 8.574

b . . . . . . . . . 12-964 12.976 12.981

c . . . . . . . . . 7.211 7.202 7.222

. . . . . . . . . 90 ~ 17' 90 ~ 23' 90 ~ 41

. . . . . . . . . 115 58 115 54 115 59

y . . . . . . . . . 89 11 88 47 87 30

c~* . . . . . . . . . 90 05 90 09 90 18

y* . . . . . . . . . 90 46 91 09 92 22

The revised orientation suggested by Laves (1951) h as been adopted. Dr. S. W,

Bailey, o f the Univer sity o f Wisconsin, has determined cell para meter s of a specimen

of the sa me material as sample C by single-crystal X- ra y methods. These are given

for compar ison with the above values (Bailey, personal communi cation): a 8.5784,

b 12-9600, c 7.2112, a 90 ~ 18', fi 116 ~ 02' , y 89 ~ 08 , a* 90 ~ 05', fl* 63 ~ 58[, y* 90 ~ 50'.

T B L E II . Analyses of 'ma xi mum ' microclines. (Analyst, J. H. Scoon.)

SiO 2 ...

A12Os

F%Os

MgO ...

CaO

Na20

K2

...

H 2 0

M n

o ~ w t .

) ...

Ab ...

A I 1 . . .

1. 2. 3.

64.46 64.58 64.40

18.55 18-68 19.09

0.14 0.12 0.31

nil nil 0.04

0.17 0.20 0.05

0'49 0.63 1 09

16.07 15-68 15.26

0'06 0-07 nil

nil nil 0.01

99.94 99.96 100.25

95.0 93-6 90.5

4-2 5.4 9-2

0.8 1.0 0.3

1. Microcline from ap eg ma ti te in grennai te, No rra Kgr r, Sweden. Or 95.0Ab4.2An0.s

2. Amazonston e fro m Pikes Peak, Colorado. The analysed sample was estimated

optically to contain 2 % of albite as a s eparate phase ; thu s the composition of the

po ta sh phase is Org>6Aba.4Anl. 0.

3. Microcline from the nepheline-syenite of Blue Mountain, Ontario. The analysed

sample was estimated optically and by X -ra ys to contain 2 % of albite. The cor-

rected composition of the potash phase is Or92.sAbT.2An0.a.

o f t ri c li n ic a d u l a r ia , b u t f e w p e t r o l o g i st s , o n s e e i n g t h e u n i f o r m c r o ss -

h a t c h t w i n n i n g c o v e r i n g t h e e n t i re a r e a o f t h e c r y s t a l s, w o u l d c a ll t h i s

m i n e r a l a d u l a r ia .

S p e c i m e n s A , B , C , a n d D a r e f r o m D r . E . S p e n c e r ' s co l l e c ti o n o f

o r t h o c l a s e - a n d m i c r o c l i n e - m i c r o p e r t h i te s ( S p e n c e r, 1 9 37 ), a n d t h e


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3 5 8 W S M A C K E N ZI E O N

writer is indebted to Drs . N. L. Bowen and O. F. Turt le for permiss ion

to use them . Specimen E is from the nepheline-syenite of Blue Mountain ,

Ontario , and i ts composit ion was calculated from a chemical analys is

by Mr. J . H. Scoon ( table II) .

FELSPAR FROM THE BEARPAW MOUNTAINS MONTANA.

The fe lspars f rom the nepheline-syeni te pegmat i tes o f Ro cky Boy s tock

in the Bearpaw Mountains , Montana, have been descr ibed by Pecora

(1942). He suggested th at the felspars in m an y of the pe gma ti tes had

been hydro ther ma lly al tered and n oted tha t , in some cases , grains which

appea r homogeneous in p lane po larized l igh t have a sp l o tc hy tex tur e

unde r crossed nicols. Professor C. E. Tilley not ed a conside rable va ria -

t ion in the size of the optic axial angle of wh at ap pe are d, in plane

polarized l ight , to be a homogeneous felspar crys ta l from one of these

pegm ati tes . The specimen described here is not identical with an y of the

mater ia ls descr ibed by Pecora , b u t i s o f the same genera l type and /ru der

crossed nicols i t shows the sp lo tc hy text ure m entioned by him.

Fig. 2 shows a photo mic rograp h of part of one of these felspar crys tals

under crossed nicols; a suggestion of nmltiple twinning can be seen in

this photograph . The small included crys tals are aegir ine. The sect ion

is app rox im atel y normal to the acute bisectr ix. The optic axial angle of

the dark areas is mo dera te (30 40 ~ ; in the l ighter areas i t is much larger

(70-75~ The optic plane is com mon to the whole crys tal , b u t the posi-

tion of the acu te bisectrix differs with t he size of the optic a xial angle.

I t was no t poss ib le to m ake op t ica l determinat ions o f the sym me try o f

the various p arts of the crys tal , s ince the cleavages are not well enough

defined in th in sect ion to obtain measurements more accurate than

• 1 ~ The (001) and (010) cleavages ap pe ar to be continuous thro ug ho ut

the whole crystal , thus em phasizing i ts appare nt optical hom ogeneity in

p lane po larized hght . An X-r ay spect rometer powder pat te rn o f th is

mineral shows that i t is r ich in potash and consis ts of microcline and

orthoclase with no soda-felspar phase. Fig. 5a (p. 362) shows pa rt of

the X- ra y s pec t romete r pa t t e rn .

Zero layer-l ine a- and c-axis Weissenberg photographs were made of

crys ta l f ragments o f bo th the mater ia l o f moderate op t ic ax ia l angle and

th at of larger optic axial angle. The patte rns of the form er show only

one set of reflections and are consis tent with monoclinic symm etry . The

pattern obtained from the fragment of larger optic axial angle (f ig . 3)

shows groups of three and five reflections instead of single reflections.

The centre spot in every case corresponds with the appropriate spot on


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ORTIIOCLASE I~IICROCLINE INVERSION

359

the photographs of the monoclinie crystal, while the addi tio nal spots

represent four triclinic components.

The s tereogram in fig. r shows the relat ions between these five uni ts as

deduced from the X-ray photographs by Dr. J. V. Smith. The axes of

the monoclinic compone nt are denoted by the subscript M and those of

:FIG. 2. Photomicrograph of a felspar from a nepheline-syenite pegmatite of

Rocky Boy stock, Bearpaw Mts., Montana. The crystal is cut approximately

normal to the a crystallographic axis. The dark ~reas are monoclinie and the lighter

areas are trictinic. Crossed nicols: • 96.

the triclinic component s by the subscript s 1, 2, 3, and 4. The c -axis

appears to be co mmon to bo th monoclinic and trielinic parts. Equa lly

inclined to the a -axis of the monoclinic compon ent are two ad dition al

a -axes which lie in the a b* plane of the monoclinic material. Two

addi tional b -axes are equally inclined to the b -axis of the monoelinic

material , and lie in the plane at 90 ~ to the comm on c -axis. There are

thus five separate orientations, of which one is monoclinie and four are

triclinic with a common c -axis. Stu dy of the stereogram reveals th at it

R 997 A a


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FIG. 3. Ze r o l aye r - l ine We i s senbe r g pho t og r aph t ake n ab ou t t he c - ax i s o f a f rag -

m e n t o f t h e fe l s p ar f r o m t h e B e a r p a w M r s . M o n t a n a u s i n g C u - K ~ r a d ia t i o n . ho

r e f l ec t i ons f r om t he l a t t i c e p l anes o f t he f ive d i f f e r en t l y o r i en ted u n i t s i n t h i s c r ys t a l

a r e c l e a r ly s h o w n a n d f o r m s m a l l c r o ss e s i n t h e p h o t o g r a p h .


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O R T H O C L A S E - M I C R O C L I N E I N V E R S I O N 3 6 1

m u s t b e o n l y a n a p ] ? r o x i m a t i o n , a s t w o o f t h e t r i c l i n i c c o m p o n e n t s

e v i d e n t l y h a v e b * i n c o m m o n w i t h t h e m o n o c l i n i c m a t e r i a l a n d t h e r e f o r e

c a n n o t h a v e a c o m m o n c * - a x i s w i t h i t u n l e s s t h e t r i c l i n i c a * i s e x a c t l y

9 0 ~ T h e m a x i m u m d e p a r t u r e f r o m 9 0 ~ o f t h e a n g l e a * i n a m i c r o c l i n e ,

a b o u t 2 5 a c c o r d i n g t o L a v e s ( 1 9 5 0 ) , c a n n o t b e r e l i a b l y d e t e c t e d i n t h e s e

W e i s s e n b e r g p h o t o g r a p h s .

L a v e s ( 1 9 5 0 ) h a s d e s c r i b e d t h e r e l a t i o n s h i p b e t w e e n t h e s e p a r a t e

C M , I ,- 2 , 3 ; 4 _ } .b M ,3 ,

FIG. 4. Stereogram showing the approximate relation between the axes of the

reciprocal lattices of the five different units of the felspar from the Bearpaw Mrs.,

:Montana. The subscript M refers to the monoelinie part and the subscripts 1, 2, 3,

and 4 refer to the four trielinie parts. The angular relations in the stereogram are

very much exaggerated for clarity.

triclinic units of a twinned microcline as due to albite and perieline

twinning, the albite-twinned parts retaining the plane of symmetry of

the original monoclinic material and the pericline-twinned parts retain-

ing the axis of symmetr y. From the stereogram in fig. 4 it can be seen

th at the triclinic parts 1 and 2 are in pericline twin relation and parts

3 and 4 are in albite twi n relation. There is no evidence to suggest tha t

the twinni ng arr ang ement differs from tha t proposed by Laves 1950),

with the exception that the original monoclinic mater iali s still preserved,

and the close relationship between the orientations of t h e triclinie

material and the monoclinic pare nt material can be seen in the X-ray

photographs.

The lattice angles a and y of this triclinic material, measured from the

X-ray spectrometer pattern, do not show as large a departure from 90~


9/13

3 6 2

w s MACKENZIE ON

a s i n ' m a x i m u m ' m i c r o c l i n e , t h e v a l u e o f 2 81 ~0 --2 81 30 b e i n g 0 7 1 ~

Cu-K~).

FELSPAIt FROM PARAGNEISS, GLEN URQUI=IART SCOTLAND

T h i s f e l s p a r o c c u rs i n a n a r e a o f p a r a g n e i s s e s w h i c h , a c c o r d i n g t o

F r a n c i s (1 95 2) , h a v e b e e n s u b j e c t e d t o p o t a s h m e t a s o m a t i s m . A l m o s t

201

A A

2 0 21 2 2 2 3 2 4 2 5

2 0

C u - K

o

FIG. 5 . Pa r t s o f t h e X - ray s p ec t ro m e t e r

pa t te r ns o f the fel spars f rom (a) the B earpaw

M ts. , Montana, and (b) Glen U rqu har t , Scot-

land . In pa t te rn (a ) the p resence o fo r thoc lase

is indic ated by the reflection between the 130

an d 130 reflections of microcline. In pa tte rn

(b) th e var iab le l a t t i ce o fmicroc l ine is shown

by the b ro ad peaks rep resen t ing the 130 and

1 3

reflect ions and the 111 and l l l ref lec-

t ions . By co n t ras t the 205 peak i s sharp and

w ell defined.

a l l t h e g r a i n s s h o w v e r y u n -

e v e n e x t i n c t i o n , p a r t s o f i n -

d i v i d u a l c r y s t a l s s h o w c r o s s -

h a t c h t w i n n i n g t y p i c a l o f

m i c r o cl in e , a n d o t h e r p a r t s

a r e u n t w i n n e d . T h i s i s q u i t e

c o m m o n f e a t u r e o f m i c ro e l in e ,

a n d , o n t h e b a s i s o f e x t i n c -

t i o n - a n g l e m e a s u r e m e n t s ,

m a n y a u t h o r s h a v e a t t r ib u t e d

t h i s t o t h e a s s o c i a t i o n o f

o r t h o e l a s e a n d m i e r o e l in e . A

s m a l l a m o u n t o f m i c r o p e r t h -

i t i c a l b i t e i s i r r e g u l a r l y

d i s t r ib u t e d i n m a n y o f th e

c r y s t a l s .

L a v e s ( 1 9 5 0 , p . 5 5 3 ) h a s

d i s c u s s e d t h i s q u e s t i o n o f u n -

e v e n e x t i n c t io n a n d o p t i c a l ly

m o n o c l i n i c a r e a s in a m i c r o -

e l i n e f r o m M a d a g a s c a r , a n d

h a s p r e s e n t e d t w o p o s s i b l e

e x p l a n a t i o n s ; v i z . o p t i c a l

d e v i a t i o n f r o m m o n o c l i n i c

s y m m e t r y i s a f u n c t i o n o f

e i t h e r ( a) t h e l a t t i c e a n g l e s

a n d y , o r (b ) t h e r a t i o o f l e f t -

h a n d t o r i g h t - h a n d a l b i t e t w i n

p o s i t io n s i f t h e m a t e r i a l i s

s u b m i c r o s e o p i e a l l y t w i n n e d .

S i n c e h e f o u n d d i ff e r e n c es i n

t h e i n t e n s i t i e s o f t h e X - r a y

r e fl e ct io n s f r o m t h e t w o p a r t s

o f t h e t w i n , h e a d o p t e d t h e


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ORTIIOCLASE-MICROCLIN E INVERSION 363

l a t t e r e x p l a n a t i o n a s th e c o r r e c t o n e f o r t h e M a d a g a s c a r m i c r o -

cl ine.

P a r t o f a n X - r a y s p e c t r o m e t e r p a t t e r n o f t h e G l en U r q u h a r t f e ls p a r

i s r e p r o d u c e d i n fig . 5 b. T h e s in g l e b r o a d p e a k s h o w n b y t h i s m i n e r a l f o r

t h e 1 3 0 a n d 1 ,~ 0 r e f l e c ti o n s i n d i c a t e s t h a t , a l t h o u g h m o n o e l i n i e o r v e r y

n e a r l y m o n o c l i n i c m a t e r i a l i s p r e d o m i n a n t , m i e r o c l i n e i s p r e s e n t a n d

t h e r e i s c o n s i d e r a b l e v a r i a t i o n i n i t s l a t t ic e . I n t h i s p a r t i c u l a r c a s e , t h e n ,

t h e o p t ic a l d e v i a ti o n f r o m n m n o e li ni c s y m m e t r y m a y w e ll b e a m e a s u r e

o f t h e s t r u c t u r a l d e v i a t io n . A l t h o u g h i t s e e m s q u it e l ik e ly , it c a n n o t b e

e s t a b li s h e d f r o m t h i s p a t t e r n t h a t a t r u l y m o n o e l in i c f e l sp a r is p r e s e n t ,

s in c e a v e r y s l ig h t d e p a r t u r e f r o m n m n o e l in i c s y m m e t r y w i ll n o t c a u s e

s u t ii c ie n t s e p a r a t i o n b e t w e e n t h e 1 3 0 a n d 1 :~ 0 p e a k s i n t h e p o w d e r

p a t t e r n f o r t h e s e t o 1)c c l e a r l y r e s o l v e d f r o m e a c h o t h e r .

1 ISCUSSION.

F r o n t t h e s e t w o e x a m p h ; s i t w o u h l a p p e a r t h a t t h e r e a r e t w o d i f f e r e n t

w a y s i n w h i c h t r ie l in i c a n d m o n o c l i n i c p o ta s h- fi ~ l sp a r n l a y c o e x i s t : ( a )

t h e n m n o c l i n i c m a t e r i a l i s a s s o c i a t e d w i t h t r i c l i n i c m a t e r i a l h a v i n g a

f i x e d a n d f a i r l y l a r g e d e p a r t u r e f r o m m o n o c l i n i e s y m m e t r y , o r ( b ) t h e

m o n o e l i n i c m a t e r i a l is a s s o c i a t e d w i t h tr i c l in i c m a t e r i a l w h i c h s h o w s a ll

g r a d a t i o n s i n i ts d e v i a t i o n f r o m n m n o c l in i c s y m m e t r y .

B a r t h (19;~4) f i rs t

suggeste

t h a t ~ h e d i f f e r e n c e s b e t w e e n o r t h o c l a s e

a n d m i c r o c l in e a r e t h e r e s u l t o f d i s o r d e r i n g a n d o r d e r i n g o f t h e S i a n d A I

a t o m s , m i e r o c li n e b e i ng t h e f o r m i n w h i c h o r d e r i n g is m o r e n e a r l y c o m -

p l e te . L a v e s (1 9 50 , 19 52 ) e l a b o r a t e d o n B a r t h s h y p o t h e s i s a n d s u g g e s t e d

t h a t b e lo w 7 0 0 ~ C . t h e v a l u e s o f t h e a x i a l a n g l c s a a n d y i n p o t a s h -

i ~ ls p a r i n c r e a s i n g ly d e v i a t e f r o m 9 0 ~ t h e d e v i a t i o n b e i n g a f u n c t i o n o f

t h e d e g r e e o f o r d e r i n g o f t h e S i a n d A 1 a t o m s . F e l s p a r s .~ ho w in g i n t e r -

m e d i a t e s t r u c t u r a l s t a t e s b e t w e e n a m o n o c l i n ic l a t ti c e a n d t h a t o f a

m a x i m u m m i c ro c li n e m i g h t b e d e sc r ib e d b y L a v e s as a d u l a r ia ( 19 5 2,

p . 4 4 5) . I n a n a t t e m p t t o e x p l a in t h e r e l a ti o n s h i p o f a d u l a r i a t o o r t h o -

c l a se a n d n f i c r o c ]i n e , h e s u g g e s t e d t h ~ t , a l t h o u g h a d u l a r i a is g e n e r a l l y

c o n s id e r e d t o h a v e b e e n f o r m e d a t v e r y lo w te m p e r a t u r e s , i t m i g h t b e a

p a r t i a l l y d i s o rd e r e (t m e t a s t a b l e f o r m r e s u l t in g f r o m t h e r a t e o f g r o w t h

b e i n g s u f fi c ie n t ly h i g h t h a t t h e o r d e r i n g f o rc e s a r e o v e r w h e l m e d d u r i n g

c r y s t a l l i z a t i o n . L a v e s h im s e lf s ta t e s , h o w e v e r , t h a t n o n a t u r a l a l b i te s

h a v e b e e n r e p o r t e d t h a t d i s p la y a b e h a v i o u r t h a t c o u l d b e c o n si d e re d

t o b e d u e t o a n i n t e r m e d i a t e s t a t e b e t w e e n a l b it e a n d a n a l b i t e u ( 19 5 2 ,

* Although petrologists may nol~ have reportcwl intermediate states between high-

and low-temperature albite, these probably do exist in volcanic and hypabyssal


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364 w . s . MACKENZIEON

p . 4 46 ). A l b i t c s f r o m l o w - t e m p e r a t u r e e n v i r o n m e n t s c h a r a c t e r i s t ic a l l y

s h o w v e r y s h a r p e x t i n c ti o n . T h e p re s e n c e o f a n y h i g h - t e m p e r a t u r e

a l b i te w o u l d b e e a s y t o d e t e c t o p t i c a l l y b e c a u s e o f t h e c o n s i d e r a b l e

d i ff e re n c e s in t h e e x t i n c t i o n a n g le s a n d o p t i c a n g le s o f t h e t w o f o r m s .

A l t h o u g h L a v e s ( 19 52 ) h a s i n s e r t e d a f o o t n o t e t o t h e e f fe c t t h a t t h e

c e l l a n g l e s w h i c h h e g i v e s f o r m i c r o c l i n e m a y b e s o m e w h a t v a r i a b l e

d e p e n d i n g o n t h e s o d a c o n t e n t , n o n e o f t h e d i sc u s s io n s o f t h e r e l a t io n -

s h ip b e t w e e n m o n o c l i n i c a n d t r ie li n ie p o t a s h - f e l s p a r a p p e a r t o h a v e

t a k e n f ul l a c c o u n t o f t h e p o s s i b il it y t h a t t h e s y m m e t r y m a y b e c o n -

t r o ll e d b y th e c h e m i c a l c o m p o s i t io n . T h i s o m i s s io n c a n p r o b a b l y b e

e x p l a i n e d b y t h e d i f fi c u lt y o f o b t a i n i n g a c c u r a t e l y t h e c o m p o s i t i o n o f

t h e p o t a s h p h a s e i n m i c r o p e r t h i t e s . 1 t s e e m s p o s s i b l e

th t

v a r i a t i o n s

i n t h e l a t t ic e o f m i c r o c l in e s m i g h t b e d i r e c t l y r e l a te d t o t h e i r c h e m i c a l

c o m p o s i t io n , a s h a s b e e n s h o w n t o b e t h e e a se i n t h e a n o r t h o c l a s e s e ri es

( D o n n a y a n d D o n n a y , 1 9 52 ). S i n ce m i e r o c li n e s a r e a h n o s t a l w a y s m i c r o -

p e r t h i t i c , i t i s d i ff i c u lt t o f i n d m a t e r i a l f r o m w h i c h t h e s o d a p h a s e c a n b e

c o m p l e te l y s e p a r a t e d a n d t h e c o m p o s i ti o n o f t h e p o t a s h p h a s e d e t e r -

m i n e d c h e m i c a ll y . A s o lu t i o n t o t h i s p r o b l e m m i g h t b e f o u n d i n a n

e s ti m a t e, f r o m t h e X - r a y s p e c t r o m e t e r p a t t e r n , o f t h e a m o u n t o f a l b it e

p r e s e n t a s a s e p a r a t e p h a s e i n e a c h a n a l y s e d s p e c i m e n .

M i x tu r e s c o n t a i n i n g 1 0, 5 , 4 , a n d 2 % o f a l b i t e w e r e p r e p a r e d f r o m

n a t u r a l s a m p l e s o f a r a t h e r p u r e h o m o g e n e o u s o r th o c l a s e a n d a l o w -

t e m p e r a t u r e a l b it e , p r e v i o u s l y s ie v e d t o t h e s a m e g r a in - s iz e . X - r a y

s p e c t r o m e t e r p a t t e r n s o f t h e s e m i x t u r e s s h o w e d t h a t t h e h e i g h t o f t h e

p e a k r e p r e s e n t i n g t h e 0 0 2 , 0 4 0 , a n d 2 0 0 r e f le c t i o n s o f a l b i t e is a f u n c t i o n

o f t h e c o n c e n t r a t i o n o f a l b it e . T h i s p e a k w a s e a s i ly d e t e c t e d i n th e

m i x t u r e c o n t a i n i n g o n l y 2 % o f a lb it e. B e c a u s e o f t h e u n c e r t a i n t y

r e g a r d i n g t h e m i n i m u m s iz e o f t h e u n i t s o f a l b i te i n m i e r o c l i n e - p e r th i t e s ,

t h e a c c u r a c y o f t h is m e t h o d f o r d e t e r m i n i n g t h e a m o u n t o f a l b it e in a

p e r t h i t e h a s n o t b e e n i n v e s t i g a t e d i n d e t a i l . '

I n t h r e e n e w a n a l y se s o f ' m a x i m u m ' m i er oe lin e s f ro m w h i c h m o s t o f

t h e a l b i te w a s s e p a r a t e d ( t a b l e I I ) , t h e a m o u n t o f s o d a - f e ls p a r in s o l id

s o l u t io n d o e s n o t e x c e e d 7 5 % a f t e r c o r r e c t i n g f o r a l b i te v i s ib l e u n d e r

t h e m i c ro s c o p e . F e l s p a r s li k e t h o s e d e s c r i b e d b y L a v e s ( 19 50 ) a s t r i -

c lin ic a d u l a ri a h a v i n g o n l y a s m a ll d e p a r t u r e f r o m m o n o c li n ic s y m m e t r y

rocks. Optical inhomogeneity generally would be attribute d to chem ical zoning,

which, o f course, m ay Mso be present.

Vogt (1926), Andersen (1928), and Spencer (1937) have remarked on the limited

range of composition of mieroeline-mieroperthites, but Spencer states th at more

extreme compositions could possibly have been fou nd in material which was dis-

carded as not suitable for his accurate optical investigations.


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ORTHOCLASE-MICI:tOCLINE IN ~ ERSION

365

are frequently non-perthitic. In the literature, chemical analyses of

adularias show that these have between 10 and 20 ~o of soda-felspar in

solid solution. Specimen C (fig. 1) probably has less than 1 of albite

as a separate phase, so it will have 13 To of soda-felspar in solid solution.

Returning to a consideration of the two specimens described in this

communication, it appears tha t the felspar from the Bearpaw Mountains

has changed from a monoclinic lattice directly to a triclinic lattice of

fixed lattice parameters with very little material of intermediate struc-

tural states. There is no evidence of a soda phase either under the micro-

scope or in the X-ray diffraction patterns. According to the argument

proposed here, this type of change is possible only because the original

monoclinic felspar has a very low soda content. The Glen Urquhart

felspar, on the other hand, shows gradational lattice parameters, and the

presence of small blebs of albite indicates that some exsolution has taken

place. It is conceivable that these two factors'are related and that only

by exsolution of albite to reduce the amount held in the potash phase

can the lattice angles approach those of a 'maximum' microcline.

One piece of evidence which may conflict with the hypothesis of

chemical control of the lattice of microelines was given to me recently by

Dr. 0. F. Tuttle (personal communication). An X-ray spectrometer

pat tern was made of a mierocline-microperthite and the distance between

the (130) and (150) peaks was measured on the chart; the presence of

albite lines in the pattern was noted. The specimen was heated for

different periods of time until albite lines could no longer be detected in

the X-ray pattern, and it was found that the separation of the (130)

and (150) peaks had not altered within the limits of error in measure-

ment. The albite had been taken into solid solution in the potash phase.

Dr. Tuttle has remarked, however, that the homogeneous phase pro-

duced may be metastable, so this cannot be considered as evidence

against the hypothesis proposed above.

The writer believes that both the character of the lattice and the very

existence of the lattice of microeline are controlled chiefly by the chemical

composition of the potash phase, which, in most cases, is controlled by

the extent to which the exsolution of soda-felspar has proceeded. If

metamorphism is capable of promoting exsolution, and many petrolo-

gists have expressed the view that it is, a possible reason for the occur-

rence of microcline as the stable form of potash-felspar in rocks which

have been subjected to regional metamorphism is that the mineral is

almost pure KA1Si30s.

The hypothesis of chemical control of the lattice angles of microclines


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36 6 w . s . MACK ENZIE O1~ ORTHOCLA SE-MICROCLINE INVEI SION

m a y n o t b e c o m p l e t e l y a t v a r i a n c e w i t h t h e id e a t h a t t h e c h a n g e f r o m

m o n o c l i n i c t o t r ic l in i c s y m m e t r y is c o n n e c t e d w i t h t h e o r d e r i n g o f t h e

S i a n d A 1 a t o m s . P e r h a p s t h e S i a n d A 1 a t o m s c a n n o t o r d e r b e y o n d a

c e r t a in l i m i t u n t i l t h e s o d a a n d p o t a s h p h a se s h a v e u n m i x e d t o t h e

e x t e n t t h a t e a c h is a l m o s t a p u r e e n d - m e m b e r o f t h e s o li d so l u t io n se r i es .

T h i s l i m i t o f t h e o r d e r i n g o f S i a n d A 1 m a y b e r e p r e s e n t e d b y o r t h o c l a s e

o c c u r r i n g a s a d is c r e te p h a s e o r b y t h e m o n o c l i n i c p o t a s h p h a s e i n

o r t h o c l a s e - m i c r o p e r t h i t e s .

A c k n v w l e d g e m e n t s . T h e

w r i t e r i s g r a t e f u l t o P r o f e s s o r C . E . T i l l e y ,

D r s . N . L . ] ~ ow e n , J . V . S m i t h , O . F . T u t t l e , a n d W . t t . T a y l o r f o r t h e i r

a d v i c e a n d e n c o u r a g e m e n t . D r s . N . L . B o w e n a n d F . C h a y e s k i n d l y r e a d

t h e m a n u s c r i p t a n d o f f er ed v a l u a b l e c r i t i c i sm . T h e W e i s se n b e r g X - r a y

p h o t o g r a p h s w e r e m a d e b y M r . K . R i c k so n , o f t h e D e p a r t m e n t o f

M i n e r a l o g y ~ n d P e t r o l o g y , C a m b r i d g e U n i v e r s i t y , E n g l a n d .

References.

A~DERSEN O. ), 1928 . T he genes is o f some types o f f e ldspa r f rom g ran i t e peg m a t i t e s .

Norsk Geol . Tidsskr . , vol . 10 , pp . 116407.

BA~TH (T . F . W. ) , 1934 . Po lym orph ic phenom ena and c rys ta l st ruc tu re . Am er .

Jo ur n. Sci ., ser. 5, vol. 27, pp. 273-286. [M.A. 6-64 .]

D o ~ A x ~ (G . ) a n d D O l ~ A Y ( J . D . H . ) , 1 9 5 2 . T h e s y m m e t r y c h a n g e i n t h e h i g h -

t em pera tu re a lka l i- fe ldspa r s er ie s. Am er . Jou rn . Sc i., Bow en vo lum e , pp . 115-

132 . [M.A. 1~ 96 . ]

FaA ~c i s (G . H . ) , 1952 . Unp ub l i shed Ph .D . Thes is , Cam br idge Un ive rs i ty , Eng land .

LAV]~S (F. ) , 195 0. Th e la t t ice and twin ning of m icroc l ine and o th er po tash fe ldsp ars .

Jo ur n. G eol. , vo l. 58, pp. 548-571. [M.A. 11-327.]

1951 . A rev i s ed o r i en ta t ion o f mie roc line and i t s geome t r i ca l r e l a t ion to a lb i t e

and cr ypto per th i tes . Jou rn . G eol. , vol . 59 , pp . 510 511. [M.A. 11-490.]

1952. Pha se re la t ions of the a lka l i fe ldspars . Jou rn . Geol. , vol . 60 , pp . 43 6-

450, 549-574. [M.A. 12-136.]

MA cKenz IE (W. S .) , 1952 . T he e f fec t o f t em pera tu re on the sy m m et ry o f h igh -

t em pera tu re soda - r ich fe ldspa rs . Am er . Jou r n . Sc i., Bow en vo lum e , pp . 319-

342. [M.A. 12-135.]

:PEcoRA (W. T . ), 1942 . Nephe l ine syen i t e pegma t i t e s , R ock y B oy s tock , Bea rpaw

M ounta ins , Mon tana . Am er. Min., vol . 27 , pp . 397-424. [M.A. 9-173. ]

SP ~C E~ (E . ) , 1937 . The po ta sh -soda - fe l spars . I . The rm a l s t ab i l i ty . Min . Mag . ,

vol. 24, pp. 453-494.

VoGT (J . H . L . ) , 1926 . The ph ys ica l chemis t ry o f the m agm at ic d i f fe ren t i a tion o f

igneous rocks, I I . On the fe ldspa r d iag ram Or : Ab : An . Skr i f t e r Norske Vidensk .

Ak ad. , Mat . -na tur . KI . , no . 4 , pp . 1-101. [M.A. 8-379. ]

Note. Fig.

1 , B a n d D : l a t t i c e p a r a m e t e r s a n d a m o u n t o f e x s o l v e d a lb i t e a r e

v a r i a b l e i n d i f f e r e n t f r a g m e n t s . A , B , C , D a r e S p e n c e r ' s D , W , U , E ; h i s

ana lyse s a re reca lcu la ted to 100 .

The ortoclase-microline inversion

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