Possible compounds for the 9.7 micron spectral feature and the 2175A spectral bump are examined. Results from electron microscopy and high resolution transmission electron microscopy are discussed

Buseck, P. R.

English

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"CLEAN" VS. "DIRTY" SILICATE GRAINS AND THE STATE OF CARBON 
CRYSTALLIZATION IN INTERSTELLAR AND CIRCUMSTELLAR DUST 
Peter R .  Buseck, Departments of Geology and Chemistry, Arizona S t a t e  
Universi ty,  Tempe, Arizona 85287 
The 9.7p-n spect ra l  f e a t u r e  of i n t e r s t e l l a r  dust  has comn~only been 
ascribed t o  rock-forming s i  1 i c a t e s  such as  01 iv ine  [(Mg ,Fej,SiO4] and 
e n s t a t i  t e  pyroxene [ ( ~ g  ,Fe)SiO,]. However, i t  only approximately 
matches t h e i r  spect ra .  To explain th'e d i s t i n c t i o n ,  astronomers r e f e r  t o  
"d i r ty"  s i l i c a t e s ,  although the  meaning of t h i s  term i s  l e f t  vague. A 
common explanation i s  t h a t  these  minerals a re  "disordered" ,  b u t  without 
specifying what i s  meant. However, t e r r e s t r i a l ,  meteori t i c ,  and 
in terplanetary  o l iv ine  and e n s t a t i t e  a r e  well ordered (as ide  from the 
Mg-Fe d isorder  ' t h a t  i s  typica l  of these  minerals)  and, moreover, the re  
a re  no l i k e l y  crystal-chemical ways in which they could d isorder .  lnie 
have used e lec t ron d i f f r a c t i o n  t o  study many in te rp lane ta ry  dust  
p a r t i c l e s  t h a t  contain o l i v i n e  and pyroxene (e .g . ,  Tomeoka and Buseck, 
1984), and in none of them have we detected any s o r t s  of anomalous 
disorder .  I t  i s  not a t  a l l  apparent what s o r t s  of "d isorder"  could 
account f o r  the  "d i r ty"  s i l i c a t e s  in the  i n t e r s t e l l a r  dus t .  I t  thus 
behooves astronomers t o  attempt t o  speci fy  more c l e a r l y  what. s o r t s  of 
s i l i c a t e s ,  i f  indeed they a r e  s i l i c a t e s ,  a r e  indicated by the  9 .7  pm 
spectral  f ea tu re .  Terminology as  used a t  present  i s  not compatible with 
standard mineralogical usage. o 
In another pa r t  of the  spectrum, the  2175 A spect ra l  "bump" appears 
t o  be ubiquitous f o r  i n t e r s t e l l a r  dus ts .  I t  i s  commonly assigned t o  
amorphous graphite  o r  some s imi la r  ma te r i a l ,  although concerns have been 
expressed t h a t .  the wavelength of the  bump should s h i f t  s l i g h t l y  
depending on grain s i z e ,  and such s h i f t s  in  pos i t ion  have not been 
observed (e .g . ,  Mathis, 1985). Recently, r e s u l t s  have been obtained of 
high-rescl ution transmission e lec t ron  microscopy of both natural 
carbonaceous niaterials a n d  organic molecules t h a t  have been heated and 
carbonized in the laboratory under cont ro l led  condit ions (Buseck and 
Huang, 19852,b). These r e s u l t s  ind ica te  t h a t  there  a re  a wide range o f  
recognizable s tages during the  t r a n s i t i o n  from t c t a l l y  disorganized arid 
amorphous carbon t o  good c r y s t a l l i n e  graphi te .  
Degree of polymerization i n t o  individual sub-planar and then planar 
polycyclic aromatic sheets  of hydrocarbons grade in to  poorly stacked 
s e t s  of sheets  f u l l  of many edge d i s loca t ions  and bonding vacancies. 
These, in t u r n ,  grade in to  the  regular ly  stacked and l a t e r a l l y  extensive 
sheets  t h a t  charac ter ize  graphi te .  Work i s  required t o  determine 
whether these s teps  each h a v ~  t h e i r  own IR and UV s i ana tu res  a n 6  how 
they may r e l a t e  t c  the  2175 A bump. In the meantime, however, i t  i s  
important t o  be aware t h a t  the  c r y s t a l l i z a t i o n  of carbon t o  graphi te  i s  
con~plex and t h a t  i t  would be des i rab le  t o  specify the  s t a t e  of c r y s t a l -  
l i z a t i o n  in the i n t e r s t e l l a r  medium more prec ise ly  than j u s t  whether or 
root i t  i s  "amcrphous". 
https://ntrs.nasa.gov/search.jsp?R=19860014036 2020-03-20T15:34:52+00:00Z
B , B .  -J.  (1985a) Conversion of carbonaceous 
material t o  graphite during metamorphism. Geochim. Cosmochim. 
kcta,  in press. 
Buseck, P,, R.  and Huang , B .  -J .  (1985b) A n  electron-microscope 
investigation of the structures of annealed carbons. I n  prep. 
Mathis, J .  S . ,  1985, Observations and theories of in t e r s t e l l a r  dust. 
Proceedings, p .  8 ,  NASA Workshop on the Inter-re1 ationships Among 
C i  rcurnstell a r ,  In te rs te l la r  and Interplanetary Dust Grains, Aspen 
Insti t .ute,  W-ye, Mar-yland. 
Tomeoka, K. and-~useck;  P.  R., 1984, Earth Planet. Sci. Lett. - 69,  
243-254. 


Clean versus dirty silicate grains and the state of carbon crystallization in interstellar and circumstellar dust

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