Chapter 8 M i n e r a l o g y o f M e t e o r i t e s Revealed by Cathodoluminescence Ian M. Steele Department of the Geophysical Sciences, University of Chicago, Chicago, IL 60637 Within meteorites enstatite, forsterite, plagioclase and glass are the major phases which show cathodoluminescence (CL). For enstatite and forsterite, Cr and Mn both produce CL with Cr emission in the infrared (about 740nm) but extending into the visible and Mn emission in the red (about 660nm). Both phases show a blue emission near 450nm of unknown origin. The visual color is a function of these two elements, the blue intensity and the Fe content which effectively quenches the blue emission in forsterite at the 0.75 wt.% level and the Cr emission at the 2.0 wt.% level. Iron quenching affects enstatite but levels are not known. The wavelength of the Cr emission is not constant and may reflect the distribution of Cr between the two non-equivalent sites of these minerals. Spectral intensity measurements can provide a means of rapidly determining Cr and Mn concentrations i f Fe levels are constant. Feldspar is the most common phase showing CL but the activator is not known. The intensity of blue emission in anorthite correlates inversely with the Mg and Na content and allows recognition of zoning in these minor elements. Visual observations of plagioclase luminescence in the carbonaceous chondrites provides evidence of extensive alteration of anorthite to garnet-melilite assemblages. All three silicates show a blue emission to which no activator element has been recognized and strengthens arguments that it is associated with the tetrahedral portion of the structures. Brief observations are made for CL features of melilite, spinel, garnet, fassaite, hibonite, corundum, oldhamite and diopside but systematic studies of these phases have not been made. Cathodoluminescence (CL), or the emission of light upon electron irradiation, has long been recognized. Initially observations were 0097-6156/90/0415-0150$06.00/0 © 1990 American Chemical Society
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made v i s u a l l y w i t h p o o r o p t i c s b u t w i t h t h e advent o f e l e c t r o n beam a n a l y t i c a l i n s t r u m e n t s i t s u s e f u l n e s s was r e c o g n i z e d f o r r e v e a l i n g t e x t u r e s a n d c o m p o s i t i o n a l changes on a m i c r o s c a l e i n m i n e r a l s il.tZ) . I t was r e c o g n i z e d t h a t f o r t e r r e s t r i a l m i n e r a l s CL was l i m i t e d t o t h o s e m i n e r a l s t h a t a r e Fe-poor w h i c h r u l e d o u t CL f r o m many o f t h e r o c k - f o r m i n g m i n e r a l s . Most s t u d i e s were made on q u a r t z , common c a r b o n a t e s , f e l d s p a r s and a p a t i t e w h i c h a r e common i n many t e r r e s t r i a l s e d i m e n t a r y r o c k s a n d p r o v i d e v a l u a b l e i n f o r m a t i o n r e g a r d i n g d e p o s i t i o n and subsequent m o d i f i c a t i o n o f these r o c k s . Other s t u d i e s p e r t a i n t o r e l a t i v e l y rare minerals including c a s s i t e r i t e Q ) . Of t h e s e o n l y t h e f e l d s p a r s a r e r e l a t i v e l y common i n meteorites. W h i l e t h e r e a r e many t y p e s o f m e t e o r i t e s , from t h e p o i n t o f v i e w o f CL t h e most i n t e r e s t i n g t y p e s a r e t h e p r i m i t i v e m e t e o r i t e s w h i c h formed e a r l y , about 4.5 b i l l i o n y e a r s ago, and have r e m a i n e d e s s e n t i a l l y u n a f f e c t e d b y t h e r m a l o r c h e m i c a l p r o c e s s e s . These m e t e o r i t e s c o n t a i n some m i n e r a l s w h i c h a r e v e r y l o w i n Fe a n d p o t e n t i a l l y w i l l show CL; i n c o n t r a s t , m e t e o r i t e s w h i c h may have f o r m e d e a r l y b u t s i n c e have b e e n a l t e r e d now c o n t a i n m o s t l y F e - b e a r i n g phases and o n l y r a r e l y show CL. The s p e c i f i c g r o u p s o f m e t e o r i t e s w h i c h show CL a r e : 1) t y p e 1, 2, 3 c a r b o n a c e o u s chondrites ( C l , C2 and C 3 ) ; 2) e n s t a t i t e c h o n d r i t e s ; 3) e n s t a t i t e achondrites (aubrites); 4) t y p e 3 ordinary chondrites ( u n e q u i l i b r a t e d o r d i n a r y c h o n d r i t e s o r UOC). W h i l e n o t s t r i c t l y m e t e o r i t e s , o t h e r e x t r a t e r r e s t r i a l samples which may show CL a r e : 1) deep s e a p a r t i c l e s w h i c h a r e samples o f e x t r a t e r r e s t r i a l m a t e r i a l g e n e r a l l y l e s s t h a n about 1mm i n maximum d i m e n s i o n w h i c h c a n be r e c o v e r e d f r o m deep s e a s e d i m e n t s ; 2) s t r a t o s p h e r i c p a r t i c l e s w h i c h are minute, u s u a l l y f l u f f y aggregates which e n t e r t h e upper atmosphere a n d r e m a i n i n t h e s t r a t o s p h e r e t o be c o l l e c t e d by h i g h f l y i n g p l a n e s ; 3) p a r t i c l e s f r o m G r e e n l a n d l a k e sediment w h i c h a r e s i m i l a r t o deep s e a p a r t i c l e s b u t a r e c o n c e n t r a t e d i n l a k e s formed on t h e G r e e n l a n d i c e s h e e t . C a t h o d o l u m i n e s c e n c e o b s e r v a t i o n s by t h e m s e l v e s r e v e a l d e t a i l s o f t e x t u r e among m i n e r a l s a n d s u g g e s t c h e m i c a l o r s t r u c t u r a l v a r i a t i o n w i t h i n i n d i v i d u a l g r a i n s . Seldom c a n t h e CL o b s e r v a t i o n s be i n t e r p r e t e d w i t h o u t a d d i t i o n a l i n f o r m a t i o n such as c h e m i c a l o r structural analysis. W i t h t h i s i n mind, d a t a o b t a i n e d f r o m t h e e l e c t r o n m i c r o p r o b e (EMP), i o n m i c r o p r o b e (IMP), s c a n n i n g e l e c t r o n microscope (SEM), o r b y t h e r m o l u m i n e s c e n c e s t u d i e s complement CL o b s e r v a t i o n s and many o f t h e CL examples f o r m e t e o r i t e s c a n be b e s t i n t e r p r e t e d o r r e l a t e d t o mineralogy through these d a t a . I n s t r u m e n t a l t e c h n i q u e s f o r CL s t u d i e s
The most common method o f o b s e r v i n g CL i s b y v i s u a l o b s e r v a t i o n i n a l u m i n o s c o p e w h i c h f l o o d s an a r e a o f s e v e r a l mm w i t h 10-30 kV e l e c t r o n s p r o d u c i n g CL w h i c h c a n be o b s e r v e d t h r o u g h a m i c r o s c o p e . A simple m o d i f i c a t i o n t o the o p t i c s allows photographic recording i n color. The main drawbacks a r e t h e q u a l i t a t i v e d e s c r i p t i o n s w h i c h may v a r y among o b s e r v e r s and t h e w a v e l e n g t h range w h i c h i s l i m i t e d t o t h a t d e t e c t e d by t h e e y e ; w i t h p h o t o g r a p h i c r e c o r d i n g , c o l o r i s o f t e n a f u n c t i o n o f f i l m , exposure, and d e v e l o p i n g and t h e c o l o r may not match t h a t o b s e r v e d by t h e eye. However, t h e c o l o r photographs 2
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clearly show t e x t u r e s , c o n t r a s t i n g m i n e r a l s and o f t e n subtle features within i n d i v i d u a l minerals. They f o r m a permanent r e c o r d b u t show d e t a i l s o f c o l o r v a r i a t i o n o n l y a s a sum o f i n d i v i d u a l emissions. The dynamic r a n g e o f t h e f i l m may r e q u i r e g r a d e d e x p o s u r e s t o r e p r e s e n t t h e e n t i r e r a n g e o f CL f r o m t h e m i n e r a l s p r e s e n t ; l i k e w i s e , because e x p o s u r e t i m e o f t e n i s a t r i a l and e r r o r determination e s p e c i a l l y f o r weak CL, g r a d e d e x p o s u r e s may be r e q u i r e d t o r e p r e s e n t t h e e n t i r e i n t e n s i t y range. The e x p o s u r e s u s u a l l y r e q u i r e seconds t o t e n s o f seconds b u t weak CL may r e q u i r e e x p o s u r e s o f t e n s o f m i n u t e s and some CL f e a t u r e s c a n be a f f e c t e d b y the long i r r a d i a t i o n times. The main advantage o f t h e l u m i n o s c o p e i s t h e a b i l i t y t o r a p i d l y r e c o g n i z e t e x t u r e s on a c o a r s e s c a l e . I n c o n t r a s t t o a b r o a d e l e c t r o n beam u s u a l l y u s e d i n t h e l u m i n o s c o p e , a narrow beam o f a s c a n n i n g e l e c t r o n m i c r o s c o p e o r an e l e c t r o n m i c r o p r o b e c a n be r a s t e r e d o v e r an a r e a o f s e l e c t e d s i z e . The CL i n t e n s i t y p r o d u c e d a t e a c h p o i n t c a n be m e a s u r e d b y a p h o t o m u l t i p l i e r t u b e a n d u s e d t o m o d u l a t e a n image i n t e n s i t y on a CRT; t h e s e images a r e a n a l o g o u s t o c o n v e n t i o n a l s e c o n d a r y e l e c t r o n (SE) o r b a c k s c a t t e r e d e l e c t r o n (BSE) images. The d i s p l a y e d image i n t e n s i t y i s a sum o f a l l c o l o r s b u t b e c a u s e t h e p h o t o m u l t i p l i e r t u b e may be most s e n s i t i v e t o a p a r t i c u l a r w a v e l e n g t h r a n g e , t h e image may n o t a c c u r a t e l y r e f l e c t t h e i n t e n s i t i e s seen b y t h e e y e o r r e c o r d e d on c o l o r f i l m . An advantage i s t h a t t h e d e t e c t i o n c a n e x t e n d b e y o n d t h e v i s i b l e o r be r e s t r i c t e d t o a c o l o r r a n g e determined by t h e a c t i v e p h o t o m u l t i p l i e r m a t e r i a l . The r e s u l t i n g image c a n be photographed. S e l e c t i v e f i l t e r s c a n be i n s e r t e d i n t h e l i g h t p a t h b u t a t t h e expense o f i n t e n s i t y . D i s p e r s i n g e l e m e n t s s u c h a s g r a t i n g s o r p r i s m s c a n be u s e d t o s e l e c t a c e r t a i n wavelength i n conjunction w i t h a s c a n n i n g beam o r t o s c a n a wavelength range f o r a fixed electron beam. Mechanical spectrometers r e q u i r e a f i n i t e time t o scan t h e spectrum, and f o r m i n e r a l s s e n s i t i v e t o t h e e l e c t r o n beam, damage a n d c o n s e q u e n t m o d i f i c a t i o n o f t h e CL i n t e n s i t y may o c c u r d u r i n g t h e s c a n . Optical multichannel analyzers (OMA) w i t h l i g h t i n t e n s i f i e r s a r e now a v a i l a b l e which a l l o w an e n t i r e s p e c t r u m t o be r e c o r d e d a t one t i m e and s t o r e d i n a memory. E l e c t r o n i c s a r e a l s o a v a i l a b l e t o r e a d a s e l e c t e d p o r t i o n o f t h e spectrum, s y n c h r o n i z e d w i t h a s c a n n i n g beam t o d i s p l a y t h e i n t e n s i t y o f a s e l e c t e d wavelength range as a f u n c t i o n o f beam p o s i t i o n . The image c a n be d i g i t i z e d , s t o r e d , a n d computer m a n i p u l a t e d b y s t a n d a r d image p r o c e s s i n g t e c h n i q u e s . An i m p o r t a n t c o n s i d e r a t i o n i n c o m p a r i n g images f r o m t h e s e d i f f e r e n t t y p e s o f o b s e r v a t i o n s , o r t h e same t y p e o f o b s e r v a t i o n s from d i f f e r e n t instruments, i s t h a t t h e t r a n s m i s s i o n o f t h e s p e c t r a l i n f o r m a t i o n may be m o d i f i e d b y t h e o v e r a l l o p t i c a l p a t h o f t h e instrument. F o r a c c u r a t e a n d p r e c i s e work, a c a l i b r a t e d l i g h t s o u r c e s h o u l d be r e c o r d e d a n d a n y s p e c t r a a d j u s t e d f o r the transmission of the instrument. F o r s p e c t r a l measurements t h e w a v e l e n g t h o f t h e e m i s s i o n s must be r e l a t e d t o an e m i s s i o n s t a n d a r d s u c h a s a Hg-Ne s o u r c e b e c a u s e s m a l l changes may r e f l e c t u s e f u l m i n e r a l o g i c a l parameters. O b s e r v a t i o n s c a n a l s o depend on t h e a c c e l e r a t i n g v o l t a g e , c u r r e n t d e n s i t y , a n d d w e l l t i m e o f t h e beam a t a point. B e c a u s e some o f t h e s e v a r i a b l e s may n o t be r e p o r t e d , o b s e r v a t i o n s may be d i f f i c u l t t o r e p r o d u c e .
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Mineralogy of Meteorites Revealed by Cathodoluminescence
M e t e o r i t i c M i n e r a l s showing C a t h o d o l u m i n e s c e n c e
The o c c u r r e n c e o f m i n e r a l s w h i c h show CL i s h i g h l y dependent on t h e type of m e t e o r i t e . P o s s i b l y t h e most common phase w h i c h o c c u r s i s feldspar. B e c a u s e t h i s m i n e r a l a c c e p t s v e r y l i t t l e Fe i n t o t h e s t r u c t u r e , quenching i s not a problem; however, b e c a u s e t h e f e l d s p a r s t r u c t u r e i s q u i t e open, t h e Na- and K - r i c h f e l d s p a r s a r e e a s i l y damaged by e l e c t r o n beams. I n c o n t r a s t a n o r t h i t e , t h e Ca rich variety, i s quite stable. P y r o x e n e and o l i v i n e a r e common phases i n m e t e o r i t e s but because they both u s u a l l y c o n t a i n i r o n , most do n o t l u m i n e s c e . O n l y i n t h e p r i m i t i v e m e t e o r i t e s do n e a r l y p u r e e n s t a t i t e and f o r s t e r i t e o c c u r and b o t h show b r i l l i a n t CL. O t h e r m i n e r a l s a r e r a r e but i n c l u d e : CL
observed
corundum (AI2O3) d i o p s i d e (CaMgSi206) hibonite (CaAli20i9) l o n s d a l e i t e (C) nepheline (Na3K(AlSi04)4) p e r o v s k i t e (CaTi03) s i n o i t e (S12N2O) s p i n e l (MgAl204)
diamond (C) fassaite (Ca(Ti,Al)(Al,Si)2Ο8 g r o s s u l a r (Ca3Al2Si30i2) m e l i l i t e (Ca2Al2Si07) o l d h a m i t e (CaS) q u a r t z (S1O2) s o d a l i t e (Na4Al3Si30i2Cl) w o l l a s t o n i t e (CaSi03)
CL not d e s c r i b e d but l i k e l y b a d d e l e y i t e (Zr02) c a r b o n a t e s (e.g. CaC03) m o i s s a n i t e (SiC) phosphates (e.g. C a s ( P 0 4 ) 3 C l ) z i r c o n (ZrSi04) The l i s t e d c h e m i c a l f o r m u l a e a r e i d e a l and most o f t h e s e m i n e r a l s c o n t a i n t r a c e and m i n o r e l e m e n t s w h i c h u n d o u b t e d l y a f f e c t t h e CL. S e v e r a l o f t h e s e m i n e r a l s have p o l y m o r p h i c or compositional v a r i e t i e s w h i c h a l s o may, o r do, show CL (e.g. the silica polymorphs: q u a r t z , c r i s t o b a l i t e , t r i d y m i t e ; phosphate c o m p o s i t i o n a l varieties: apatite, whitlockite, farringtonite, buchwaldite; carbonate compositional v a r i e t i e s : c a l c i t e , dolomite, magnesite). G l a s s and m a s k e l y n i t e (shock m o d i f i e d f e l d s p a r ) , a l t h o u g h n o t strictly m i n e r a l s , a r e r e l a t i v e l y common. Below a r e d e s c r i b e d t h e CL o b s e r v a t i o n s f o r t h e most common p h a s e s i n c l u d i n g e n s t a t i t e , f e l d s p a r and f o r s t e r i t e and t h e y a r e r e l a t e d t o t h e i r use f o r i n t e r p r e t i n g t h e m i n e r a l o g y o f m e t e o r i t e s . The o b s e r v a t i o n s f o r t h e o t h e r m i n e r a l s a r e s p o r a d i c and many d e t a i l s have y e t t o be s t u d i e d . E n s t a t i t e The f i r s t e x t e n s i v e o b s e r v a t i o n s o f CL i n m e t e o r i t e s were f o r e n s t a t i t e (MgSi03) p r o b a b l y b e c a u s e i t shows p a r t i c u l a r l y b r i l l i a n t CL c o l o r s and i t i s t h e m a j o r m i n e r a l i n t h e e n s t a t i t e achondrites. The v i s u a l CL i s commonly d e s c r i b e d as b l u e , r e d o r l e s s commonly p u r p l e and t h e e a r l y s p e c t r a , m a i n l y f r o m powdered samples and u s i n g p r o t o n i r r a d i a t i o n , c l e a r l y showed t h e p r e s e n c e o f a b l u e and r e d e m i s s i o n ( 4 - 6 ) . These e m i s s i o n s were c o n f i r m e d u s i n g e l e c t r o n i r r a d i a t i o n and s p e c t r a showed a b l u e peak near 400-420nm
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and a r e d peak n e a r 660-680nm. N e u t r o n a c t i v a t i o n and e l e c t r o n p r o b e a n a l y s e s showed t h a t t h e i n t e n s i t y o f t h e r e d e m i s s i o n o r t h e v i s u a l redness showed some c o r r e l a t i o n w i t h t h e Mn c o n c e n t r a t i o n ; i n a d d i t i o n , t h e w a v e l e n g t h o f t h e r e d peak f o r t h e n a t u r a l e n s t a t i t e m a t c h e d c l o s e l y t h a t o f s y n t h e t i c M n - b e a r i n g e n s t a t i t e (2) . The c a u s e o f t h e b l u e e m i s s i o n was n o t e x p l a i n e d a l t h o u g h i t was o b s e r v e d t h a t b l u e e m i s s i o n i s common t o many s i l i c a t e s and was s u g g e s t e d t h a t i t i s i n t r i n s i c t o t h e s i l i c o n t e t r a h e d r o n (&). The b l u e peak, however, was n o t o b s e r v e d f o r s y n t h e t i c M n - e n s t a t i t e (Mn«4.5 wt%) (£L,1,2JL). NO c o r r e l a t i o n o f v i s i b l e b l u e i n t e n s i t y w i t h t h e o r d e r i n g o f o r t h o e n s t a t i t e (lfi.) s u g g e s t e d t h a t t h e CL c o l o r i s n o t a f u n c t i o n o f d e f o r m a t i o n which has a f f e c t e d most e n s t a t i t e achondrites. W h i l e p a s s i n g m e n t i o n o f e n s t a t i t e CL i n m e t e o r i t e s c a n be f o u n d i n t h e l i t e r a t u r e , n o t u n t i l 1980 was i n t e r e s t r e v i v e d (11-13) . i n t h e s e t h r e e p a p e r s t h e v i s u a l c o l o r s o f e n s t a t i t e i n c h o n d r u l e s o f t h e e n s t a t i t e c h o n d r i t e s were d e s c r i b e d and t h e apparent b i m o d a l o c c u r r e n c e o f e i t h e r r e d o r b l u e CL w i t h i n i n d i v i d u a l c h o n d r u l e s was a t t r i b u t e d t o m e c h a n i c a l m i x i n g o f two t y p e s o f e n s t a t i t e . I n (JJL) t h e m i n o r element c h e m i s t r y o f t h e r e d and b l u e e n s t a t i t e s was c o m p a r e d and t h e f o l l o w i n g average c o n c e n t r a t i o n s ( o x i d e wt. %) were r e p o r t e d f o r e a c h : o x i d e - r e d , b l u e ; CaO - 0.24, 0.7; MnO - 0.16, 0.04; Cr23 - 0.34, 0.04; T i 0 2 0.05, 0.005; AI2O3 - 0.24, 0 . 0 8 ; Na20 - 0.05, 0.12; FeO 0.93, 0.93. W h i l e no e s t i m a t e was made o f t h e v a r i a t i o n i n r e d o r b l u e CL i n t e n s i t y , t h e s i m p l e t w o - f o l d c l a s s i f i c a t i o n showed a g o o d c o r r e l a t i o n w i t h t h e m i n o r element l e v e l s . F o r b o t h e n s t a t i t e s , t h e FeO l e v e l s were e s s e n t i a l l y i d e n t i c a l , w h i l e f o r a l l o t h e r e l e m e n t s e x c e p t Na r e d e n s t a t i t e showed c o n s i s t e n t l y h i g h e r l e v e l s t h a n b l u e e n s t a t i t e . No c o n c l u s i o n can be drawn as t o what element ( i f any o f t h o s e a n a l y z e d ) causes t h e d i f f e r e n c e i n CL c o l o r because o f s t r o n g g e o c h e m i c a l c o r r e l a t i o n s , b u t t h e h i g h e r Mn o f t h e r e d g r a i n s i s c o n s i s t e n t w i t h t h e o b s e r v a t i o n s n o t e d above t h a t r e d CL i s a s s o c i a t e d w i t h Mn. I n c o n t r a s t t o t h e s e o b s e r v a t i o n s , C r and Mn d a t a f o r e n s t a t i t e o f a n o t h e r e n s t a t i t e c h o n d r i t e (1A) were shown t o b e a r no u n i q u e r e l a t i o n t o t h e v i s u a l l y e s t i m a t e d CL c o l o r . These d i f f e r e n t o b s e r v a t i o n s might be e x p l a i n e d i n s e v e r a l ways i n c l u d i n g : the l i m i t a t i o n of v i s u a l l y determined c o l o r , e n s t a t i t e i n d i f f e r e n t m e t e o r i t e s being d i f f e r e n t , or a s e l e c t i o n of d i f f e r e n t types of e n s t a t i t e i n t h e two s t u d i e s . To o b t a i n a q u a n t i t a t i v e measure o f t h e CL i n t e n s i t y o f i n d i v i d u a l CL e m i s s i o n s , an o p t i c a l m u l t i c h a n n e l a n a l y z e r was c o u p l e d t o t h e o p t i c a l s y s t e m o f an e l e c t r o n m i c r o p r o b e a l l o w i n g s i m u l t a n e o u s c o l l e c t i o n o f CL s p e c t r a and m i n o r element d a t a f r o m a s i n g l e p o i n t ( S t e e l e , I.M. M e t e o r i t i c s s u b m i t t e d ) . F o r CL s p e c t r a o b t a i n e d w i t h a 15 kV f o c u s e d beam, e n s t a t i t e f r o m b o t h e n s t a t i t e c h o n d r i t e s and a c h o n d r i t e s showed t h r e e d i s t i n c t p e a k s ( F i g . l ) c e n t e r e d a t about 742, 664, and 483nm. To a l l o w assignment o f t h e s e p e a k s , s p e c t r a f r o m s y n t h e t i c Mn and C r doped e n s t a t i t e a r e shown i n F i g . 2 and t h e e m i s s i o n s f r o m t h e s e two samples c l o s e l y match t h e two r e d p e a k s o f m e t e o r i t i c e n s t a t i t e ; n e i t h e r s y n t h e t i c sample shows a b l u e p e a k . The peak p o s i t i o n s o f C r and Mn a r e n o t c o n s t a n t f o r d i f f e r e n t m e t e o r i t i c e n s t a t i t e s and a r e not t h e same as f o r t h e C r and Mn doped s t a n d a r d s . The v a r i a t i o n i s about 20nm r
8. STEELE
Mineralogy of Meteorites Revealed
880
700
Wavelength
500
(nm)
155
300
F i g u r e 1. T y p i c a l CL s p e c t r u m f r o m e n s t a t i t e i n t h e e n s t a t i t e c h o n d r i t e ALHA 77295. Peak p o s i t i o n s f o r t h i s and o t h e r s p e c t r a a r e e s t i m a t e d b y eye and t h e s p e c t r a a r e n o t c o r r e c t e d f o r instrumental transmission.
880
700
500
Wavelength F i g u r e 2. C a t h o d o l u m i n e s c e n c e Mn-doped e n s t a t i t e .
300
(nm)
spectra
from
s y n t h e t i c C r - and
156
SPECTROSCOPIC CHARACTERIZATION OF MINERALS AND THEIR SURFACES
and m i g h t be e x p l a i n e d by s i t e occupancy o f t h e C r , t h e h i g h e r C r l e v e l i n t h e s y n t h e t i c samples o r other structural differences. This v a r i a t i o n i n peak p o s i t i o n among m e t e o r i t i c e n s t a t i t e s may c a r r y a d d i t i o n a l i n f o r m a t i o n regarding d e t a i l s of the s t r u c t u r e . The C r peak o f t h e s y n t h e t i c Cr e n s t a t i t e i s w e l l i n t o t h e i n f r a r e d but the sample shows visual red CL, and because the intensity-concentration r e l a t i o n i s n o t known, t h e r e d CL of e n s t a t i t e i s a f u n c t i o n o f b o t h Cr and Mn ( S t e e l e , I.M. Meteoritics. s u b m i t t e d ) . For e n s t a t i t e of the Cumberland F a l l s e n s t a t i t e a c h o n d r i t e , t h e r e i s a v e r y good p o s i t i v e c o r r e l a t i o n between Mn and t h e i n t e n s i t y o f t h e peak n e a r 664nm ( S t e e l e , I.M. Meteoritics. s u b m i t t e d ) , b u t b e c a u s e C r c o r r e l a t e s p o s i t i v e l y w i t h Mn, both e l e m e n t s a c t u a l l y show t h i s c o r r e l a t i o n w i t h CL. The c o r r e l a t i o n o f C r c o n c e n t r a t i o n w i t h t h e peak n e a r 742nm i s n o t y e t p r o v e d and would r e q u i r e a s t u d y u s i n g peak d e c o n v o l u t i o n t e c h n i q u e s t o e x t r a c t t h e C r i n t e n s i t y f r o m s p e c t r a s u c h as shown i n F i g . 1. In the n a t u r a l samples t h e b l u e peak i s c o n s p i c u o u s but no c o r r e l a t i o n w i t h any element d e t e r m i n e d w i t h t h e e l e c t r o n p r o b e (Na, A l , Ca, T i , C r , Mn, Fe) has been f o u n d . I t s i n t e n s i t y i s highest f o r the p o i n t s w i t h the lowest t o t a l minor element c o n t e n t but because of the s t r o n g c o r r e l a t i o n s between e l e m e n t s due t o g e o c h e m i s t r y t h i s may have l i t t l e s i g n i f i c a n c e . The apparent d i s c r e p a n c i e s ( I X , 2Λ) i n the c o r r e l a t i o n of v i s i b l e CL c o l o r w i t h m i n o r e l e m e n t c o n c e n t r a t i o n points to d i f f i c u l t i e s i n i n t e r p r e t i n g and c o m p a r i n g CL d a t a . In t h i s case t h e FeO c o n t e n t ranges f r o m 0.1 t o 1.5 wt% (1A) and, a l t h o u g h t h e q u e n c h i n g e f f e c t i s not known, d a t a f o r f o r s t e r i t e (see l a t e r ) show t h a t t h e b l u e peak w i l l be quenched a t l o w e r v a l u e s o f Fe t h a n t h e r e d . Moreover, t h e two r e d peaks may be a f f e c t e d d i f f e r e n t l y . Thus c o l o r i s a f u n c t i o n o f s e v e r a l v a r i a b l e s and a good c o r r e l a t i o n between any two can not n e c e s s a r i l y be e x p e c t e d . Other e f f e c t s m i g h t be s i g n i f i c a n t i n c l u d i n g the o p t i c a l t r a n s m i s s i o n and d e t e c t i o n of the near i n f r a r e d l i g h t from Cr. The CL , however, can provide s p a t i a l chemical data e i t h e r q u a l i t a t i v e l y through o b s e r v a t i o n o r q u a n t i t a t i v e l y . S p e c t r a s u c h as t h a t i l l u s t r a t e d i n F i g . 1 can be c o l l e c t e d i n s e v e r a l seconds and u s i n g a c a l i b r a t i o n o f CL i n t e n s i t y vs Mn ( o r C r ) , a c o m p o s i t i o n a l map a t t h e s e low l e v e l s can be o b t a i n e d much f a s t e r t h a n w i t h a m i c r o p r o b e . F e l d s p a r U n l i k e e n s t a t i t e and f o r s t e r i t e , f e l d s p a r i s p r e s e n t i n most m e t e o r i t e s . The meteoritic f e l d s p a r s are dominated by p l a g i o c l a s e which are those f e l d s p a r compositions i n the continuous s o l i d s o l u t i o n s e r i e s r a n g i n g from a l b i t e (NaAlSi308) t o a n o r t h i t e ( C a A l 2 S i 2 0 8 ) ; c o m p o s i t i o n s n e a r t h e ends o f t h i s s e r i e s a r e most common. The e a r l i e s t CL s t u d i e s o f f e l d s p a r were prompted by t h e l u n a r program about 1970 and a s e r i e s o f p a p e r s compared t h e CL o f lunar, meteoritic and doped s y n t h e t i c plagioclase. Later o b s e r v a t i o n s were made f o r a n o r t h i t e i n t h e carbonaceous c h o n d r i t e s . Because most work has been done on l u n a r samples and b e c a u s e some m e t e o r i t e s o r i g i n a t e f r o m t h e Moon and some p r o c e s s e s a f f e c t i n g b o t h t y p e s o f sample a r e s i m i l a r , a b r i e f d e s c r i p t i o n i s g i v e n o f these observations. The CL s p e c t r a a r e d o m i n a t e d by two bands c e n t e r e d n e a r 450 ( d e v i a t i o n o f +16 t o -20nm) and 559nm ( d e v i a t i o n o f +16 t o -6.5nm), w h i l e a peak n e a r 730nm ( d e v i a t i o n o f +20 t o
8. STEELE
Mineralogy ofMeteorites Revealed by Cathodoluminescence 157
-20nm) f o u n d i n t e r r e s t r i a l p l a g i o c l a s e i s o n l y r a r e l y p r e s e n t , and o n l y w e a k l y (12). L i k e l u n a r p l a g i o c l a s e , p l a g i o c l a s e from t h e b a s a l t i c a c h o n d r i t e m e t e o r i t e s ( e u c r i t e s ) h a s no CL e m i s s i o n a t 730nm ( 1 £ , 1 2 ) · o r m e t e o r i t i c p l a g i o c l a s e which shows shock damage, CL shows b r o a d e r , l e s s i n t e n s e peaks w i t h t h e 559nm peak s h i f t e d t o longer wavelengths (15., 12) . F o r l u n a r (15.) and m e t e o r i t i c m a s k e l y n i t e o n l y a b r o a d f e a t u r e l e s s band o f CL i s p r e s e n t . Using p r o t o n bombardment o f l u n a r p l a g i o c l a s e , two bands a t 450 and 560nm and a v e r y weak peak a t about 770nm c a n be o b s e r v e d (lfi) . The 560nm peak was a t t r i b u t e d t o Mn a t t h e s e v e r a l h u n d r e d ppm l e v e l , a n d b a s e d on c i r c u m s t a n t i a l and weak c o m p o s i t i o n a l d a t a i t was s u g g e s t e d t h a t t h e Mn i s p r o b a b l y s u b s t i t u t i n g i n t h e Ca s i t e (lil) . M e t e o r i t i c p l a g i o c l a s e does n o t show a 770nm peak (IfL), a n d t h e 450nm peak i s a t t r i b u t e d t o a l a t t i c e d e f e c t common t o most s i l i c a t e s (&,1£). The peak o f t e n seen n e a r 780nm i s r e p o r t e d t o be due t o F e 3 i n t h e Ca s i t e (IB.) and i s u s u a l l y a b s e n t o r v e r y weak due t o t h e l a c k o f Fe+3 i n m e t e o r i t e s a n d on t h e Moon. For s y n t h e t i c p l a g i o c l a s e doped w i t h Mn and Fe+3, t h e a s s i g n m e n t s o f t h e 560nm and 770nm peaks were c o n f i r m e d ( 1 £ ) . W i t h i n carbonaceous c h o n d r i t e s some s p h e r i c a l ( c h o n d r u l e s ) and other i r r e g u l a r (aggregates) features a r e p r e s e n t and a r e c h a r a c t e r i z e d b y a r e f r a c t o r y m i n e r a l o g y d o m i n a t e d by c a l c i u m , aluminum, magnesium, t i t a n i u m and s i l i c o n . The m a i n m i n e r a l s a r e m e l i l i t e , a n o r t h i t e , s p i n e l , and f a s s a i t e . I t was r e c o g n i z e d t h a t t h i s t y p e o f m i n e r a l o g y i s c o n s i s t e n t w i t h c o n d e n s a t i o n f r o m a gas o f s o l a r c o m p o s i t i o n (2Û) and i n t e n s i v e s t u d y on t h e s e o b j e c t s has been made t o d e r i v e p o s s i b l e p r o c e s s e s o p e r a t i n g i n t h e n e b u l a . I t was r e c o g n i z e d t h a t samples o f t h e s e h i g h t e m p e r a t u r e i n c l u s i o n s c o n t a i n e d e x c e s s 26Mg (21) and i t was p o s t u l a t e d t h a t t h i s magnesium was d e r i v e d f r o m t h e decay o f 26A1 v e r y e a r l y a n d t h a t t h e A 1 e n t e r e d t h e m i n e r a l s t r u c t u r e and d e c a y e d in s i t u (22.) The r e c o g n i t i o n o f t h e s i t e o f e x c e s s 2 6Mg a n d i t s c o r r e l a t i o n w i t h t o t a l A l w o u l d show t h a t t h e s e i n c l u s i o n s had n o t been a p p r e c i a b l y a l t e r e d s i n c e f o r m a t i o n and would p r o v i d e e v i d e n c e f o r a t i m e s c a l e f o r the formation of these i n c l u s i o n s r e l a t i v e t o the production of 26 1. The a n o r t h i t e w i t h i n t h e s e i n c l u s i o n s was an e x c e l l e n t c a n d i d a t e f o r s e a r c h i n g f o r a r e l a t i o n between Mg a n d A l as t h e Al/Mg r a t i o i s v e r y h i g h and e x c e s s M g c o u l d be e a s i l y d e t e c t e d s i n c e n o n - r a d i o g e n i c Mg i s a t t h e 500ppm l e v e l . Examination of p o l i s h e d s e c t i o n s o f t h e s e i n c l u s i o n s u s i n g a CL m i c r o s c o p e showed t h a t most phases h a d b r i l l i a n t CL and i n p a r t i c u l a r t h e a n o r t h i t e had e x t r a o r d i n a r y t e x t u r e s w i t h i n s i n g l e c r y s t a l s as r e v e a l e d by CL (21)· A l l p l a g i o c l a s e shows b l u e CL b u t w i t h i n s i n g l e c r y s t a l s t h e i n t e n s i t y ranges by a f a c t o r o f about 5 w i t h g e n e r a l l y sharp, s t r a i g h t b o u n d a r i e s s u g g e s t i n g c r y s t a l l o g r a p h i c c o n t r o l (see 22 f o r c o l o r photographs i l l u s t r a t i n g these f e a t u r e s ) . These v a r i a t i o n s a r e s u p e r p o s e d on t h e normal t w i n n i n g o f p l a g i o c l a s e . Using c o l o r CL p h o t o g r a p h s a s a g u i d e , e l e c t r o n m i c r o p r o b e a n a l y s e s f o r p o i n t s s h o w i n g d i f f e r e n t b l u e i n t e n s i t i e s w i t h i n one c r y s t a l showed a q u a l i t a t i v e n e g a t i v e c o r r e l a t i o n between Mg and CL i n t e n s i t y , as w e l l a s between Na and CL i n t e n s i t y , i . e . t h e b r i g h t e s t CL a r e a s had t h e l o w e s t Mg and Na (21)The range i n Mg c o n c e n t r a t i o n was about a f a c t o r o f 5 (200-1000ppm) f o r t h e extreme CL i n t e n s i t i e s . No F
+
2 6
A
2 6
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SPECTROSCOPIC CHARACTERIZATION OF MINERALS AND THEIR SURFACES
c o r r e l a t i o n o f CL i n t e n s i t y w i t h T i c o n c e n t r a t i o n (37, i . e . i n t e r m e d i a t e between h i b o n i t e a n d corundum. Grossular garnet occurs a s s o c i a t e d with a n o r t h i t e as an apparent a l t e r a t i o n p r o d u c t . I t shows y e l l o w CL b u t no d e t a i l s have been r e p o r t e d . F a s s a i t e w i t h i n t h e h i g h t e m p e r a t u r e i n c l u s i o n s does n o t show CL b u t r a r e g r a i n s a t t h e b o u n d a r i e s o f c h o n d r u l e s do l u m i n e s c e (22); a g a i n no d e t a i l s a r e known. Diopside i n t h e e n s t a t i t e a c h o n d r i t e s shows y e l l o w t o p a l e g r e e n CL a n d p r o v i d e s e a s y r e c o g n i t i o n ; no d e t a i l s o f s p e c t r a a r e known. One o f t h e most common phases i n m e t e o r i t e s i s g l a s s w h i c h i s i n t e r s t i t i a l t o grains, f i l l s cracks penetrating into discrete f e a t u r e s , o r forms a m a t r i x f r o m w h i c h c r y s t a l s have grown. These g l a s s e s a l l show b l u e CL o f w h i c h t h e i n t e n s i t y i n c r e a s e s as t h e m e t e o r i t e i s a f f e c t e d by thermal processes. A good c o r r e l a t i o n has been d e m o n s t r a t e d w i t h t h e r m o l u m i n e s c e n c e a s a r e c o r d e r o f t h e r m a l e f f e c t s a s d i s c u s s e d e l s e w h e r e i n t h i s volume ( S e a r s , DeHart a n d Hasan). Of t h e m i n e r a l s f o r which b r i l l i a n t CL i s known f o r t e r r e s t r i a l o c c u r r e n c e s , d i a m o n d i s p o s s i b l y t h e most i n t e r e s t i n g . Some m e t e o r i t e s ( u r e i l i t e s ) c a r r y f i n e - g r a i n e d diamonds a n d l o n s d a l e i t e b o t h o f which show b r i l l i a n t CL ( 3 2 ) ; however, r e c e n t o b s e r v a t i o n s have r e c o g n i z e d m i n u t e diamonds w h i c h c a r r y e x t r a o r d i n a r y i s o t o p i c c o m p o s i t i o n s (21) w i t h i n c a r b o n a c e o u s c h o n d r i t e s . These diamonds a r e c o n c e n t r a t e d b y d i s s o l v i n g a s s o c i a t e d phases p r o d u c i n g a s m a l l volume o f r e s i d u e . The r e c o g n i t i o n o f diamonds w i t h i n t h i s r e s i d u e has been made u s i n g e l e c t r o n d i f f r a c t i o n b u t m i g h t be as e a s y w i t h CL i f t h e s p e c t r u m i s d e f i n i t i v e . L i k e w i s e a s e a r c h f o r in s i t u diamond o c c u r r e n c e s i n m e t e o r i t e s m i g h t be p o s s i b l e e s p e c i a l l y i f t h e diamonds o c c u r c o n c e n t r a t e d i n c l u s t e r s . M o i s s a n i t e from c a r b o n a c e o u s c h o n d r i t e s has been c o n c e n t r a t e d b y t e c h n i q u e s s i m i l a r t o t h o s e u s e d f o r diamond, a n d h a s a l s o b e e n shown t o c a r r y extraordinary i s o t o p i c compositions. These g r a i n s a r e up t o 10 m i c r o n s i n t h e l a r g e s t d i m e n s i o n , and because t e r r e s t r i a l S i C shows CL, in situ g r a i n s might be found u s i n g CL o b s e r v a t i o n .
8. STEELE
Mineralogy ofMeteorites Revealed by Cathodoluminescence
Acknowledgments Financial support was derived from NASA NAG 9-47 (J.V. Smith) and instrumental support through NSF EAR-8415791 and NSF EAR-8608299 Literature Cited 1. 2. 3. 4. 5. 6.
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