Mark A. Stegner and Protip K. Ghosh
 
M.U. SEM Facility
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Fig. 2 Sample after cutting, polishing, and acid etching
    
     
Fig. 1 Outside surface before preparation
            
 
   

 

 

Abstract:

...In early 2004 the Marshall University Geology Department obtained a sample of a meteorite found by Mr. Skip Spencer of Charleston, WV. It was found near Blacksburg, VA. The purpose of this study was to confirm that the sample was a meteorite and to classify it as to its type. The sample was highly magnetic, had a black ‘fusion crust’, and showed the presence at the surface of ‘regmaglypts” (thumbprint shaped atmospheric burn marks).
The meteorite was cut in two with a rock-saw and then polished to a high sheen with 1u alumina paste. The polished surfaces were etched using a solution of ethyl alcohol (90%) and nitric acid (10%). The etched samples were then studied with a polarizing microscope and with a Scanning Electron Microscope (SEM-EDS).
Based on preliminary chemical, mineralogical, and structural studies, this meteorite can be classified as a coarse to medium Octahedrite, type IAB. Minerals identified are the two Fe-Ni minerals kamacite (Ni,< 6%) and taenite (Ni, > 25%), schreibersite ((Fe, Ni)3 P), and a silicate mineral (probably olivine ((Mg, Fe)2 Si O4 ). Etching clearly showed the Widmanstätten pattern, formed by the intergrowth of kamacite and taenite lamellae. Also seen at places are Neumann Lines, fine sets of parallel lines created within kamacite due to the development of twinning.
Further analyses of trace elements (Ge, Ga, Ir) is being conducted to determine the exact classification and also to determine whether this sample is related to an octahedrite (Dungannon) previously reported from Virginia.

   
               
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Fig. 3a Kamacite under SEM Magnified x 100
    
     
   
 

Objective:

This study was conducted to determine if this sample is a meteorite. Finding that it is a meteorite an attempt will be made to identify the minerals present and classify the meteorite into its chemical and structural classes.

    
     
   
     
   
     
Fig. 3b Kamacite SEM X-ray spectrum analysis (Iron - Nickel)
    
     
   
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Fig. 4 Neumann Lines under S.E.M. X 500 appear as closely spaced parallel groves.
  
 
 
  Materials and Methods:  
       
  Meteorite sample collected by ‘Skip’ Spencer (retired geologist)
Rock saw Lapidary polishing wheel
120 to 800 grit paper & alumina polishing paste (1 u)
Ethyl alcohol, Nitric acid, Hydrochloric acid 		Standard solutions of Ni, Co, Ga, Ge, Cu, and Cr
Scanning Electron Microscope (SEM - EDS)
Inductively Coupled Plasma Spectrometer (ICP-AES)
Petrographic Microscope
  
     
  o The meteorite was first tested for magnetic strength as a general test.

o The surface was then observed for atmospheric ablation characteristics with a petrographic microscope.

o The meteorite was then cut on a rock saw. This presented great difficulty due to the strongly crystallized metal and the frictional heat of sawing causing expansion pinching of the blade.

o The cut sample was polished using grit paper on a lapidary wheel; the final polishing was done with the alumina paste. The resulting surface had mirror quality reflectance.

o The polished surface was etched for approximately thirty minutes with a mixture of ethyl alcohol (90%) and nitric acid (10%).

o Once etched, the sample was prepared for the S.E.M. by immersion in alcohol for dehydration, and then the alcohol volatized at 150o C. The sample was placed directly into the S.E.M. with landmarks referenced with graphite paint. A piece of 100% copper was used as the EDS standard.

o For chemical analysis, a small slice of the meteorite was dissolved in acid.

o The ‘stock’ solution thus obtained was analyzed for Ni, Co, Ge, Ga, Cu, and Cr by the ICP-AES. ‘Standard Addition’ method was used to avoid matrix effects.

o The remaining part of the meteorite has been preserved using a coating of paraffin wax and stored with silica gel.

  
 
 
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Fig. 5a Taenite Composition under SEM X-ray analysis.

Note that lighter areas mark higher concentration.
    
     
   
      Results:

1. Strong magnetic response
2. Atmospheric ablation marks and regmaglypts (Fig. 1)
3. Glassy rind (Fig. 1)
4. Metallic core (95%) composed of iron and nickel (Fig. 2)
5. Minerals identified:

Kamacite Fe (93-96%) Ni (4-7%) iron rich (Fig. 3 A,B)
Taenite Fe(60-80%) Ni (20-40%) nickel rich (Fig. 5 A,B,C)
Schreibersite Fe (55%) Ni (14%) P (31%) accessory (Fig. 6 A,B)
Unidentified Silicate Mg, Si, P, S silicate (Fig. 7)
Troilite Fe S    
Other small minerals embedded in Taenite and Schreibersite.

6. Structures seen:
Widmanstätten pattern (Very coarse) (Fig. 2)
Neumann Lines (Fig. 4)
7. Chemical analysis:

Ni (mg/g) Co (mg/g) Ga (u g/g) Ge (u g/g) Cu (u g/g) Cr (u g/g)
67.3 6.3 168 293 123 5.3
   
     
   
     

Fig. 5b Taenite Analysis

Note the significantly higher nickel.
   
             
Fig. 5c Taenite S.E.M. X 350

Note the scale bar’s size. The light colored areas are Schreibersite inclusions.
    
                 
 
                   
     
   
     
Fig. 6a Schreibersite band: S.E.M. X 200
    
     
   
      Discussion:

1. The sample is a meteorite collected near Blacksburg, Va.
2. The magnetic strength separates this sample from slag material that may have similar appearance.
3. The presence of regmaglypts (thumb-shaped indentations caused by melting on atmospheric entry) along with the smaller ablation pits, demonstrate that this sample is extra-terrestrial.
4. The glass rind on the outside surface also points to fusion during atmospheric entry. This rind also prevented weathering.
5. The freshness of the iron and nickel, seen on the inside, also points to a non-terrestrial origin. It is very rare to find such un-oxidized terrestrial material.
6. The Widmanstätten pattern, which is a pattern produced between the kamacite and taenite interface, confirms this is a meteorite. Widmanstätten patterns develop in a zero-G environment and on very slow cooling.
7. The presence of Neumann Lines, which appear as fine parallel lines running in (up to) six directions, indicates impact with another body. These lines are caused by twinning due to post-consolidation compression.
8. The high Fe-Ni constituents identify this as an Iron meteorite.
9. The band-width of the kamacite within the Widmanstätten pattern (average > 2 mm), and the content of Ni around 6.7%, indicates that this is a coarse octahedrite.
10. The Ga-Ni and Ge-Ni concentrations, when compared to other reported octahedrite values, points to the fact that this is a IAB type iron. (Fig. 8) The Ga content is rather high, but high Ga values are known in some octahedrites.
11. Schreibersite, and minor amounts of silicate minerals, are known to occur in octahedrites. 		   
     
   
     
   
     
Fig. 6b Schreibersite analysis

Note the presence of phosphorous.
    
                   
                   
                   
     
   
     
Fig. 7 Silicate under SEM X 100

Note texture.
    
     
   
      Conclusions:

1. This sample is certainly a meteorite.
2. The Widmanstätten band-width, and the Ni, Ga, and Ge contents, all classify this as an IAB octahedrite.
3. The chemical composition of this meteorite is markedly different from the only other iron meteorite (Dungannon) found in Virginia

  Ni (mg/g) Co (mg/g) Ga (u g/g) Ge (u g/g) Cu (u g/g) Cr u g/g)
This sample 67.3 6.3 168 293 123 5.3
Dungannon 69.5 4.72 78.8 332 153 27

4. Although the analytical method (ICP) used for this study is somewhat semi-quantitative, there is a strong chemical indication that this meteorite found near Blacksburg is distinctly different from the Dungannon meteorite.
5. A strong case can be made that this is a previously unreported meteorite find.
6. Further work will involve chemical analysis by a better quantitative technique, and a closer control on the exact location and date of collection.

Final Classification: Iron Coarse Octahedrite Type 1AB		    
                   
                   
                   
     

References

New England Meteoritical Services;
http://www.meteorlab.com/

Weir, David. Meteorite Information;
www.geocities.com/CapeCanaveral/9278/menu2.htm

Darling, David. Classification of Iron Meteorites;
www.daviddarling.info/encyclopedia/I/iron_meteorite.html

McDonough, W.F. Comp. Model for the Earth’s Core. Treatise on Geochemistry, Vol. 2, p.547-568, 2003

     

Acknowledgments

‘Skip; Spencer (Retired Geologist) Who kindly supplied the original specimen.

Dr. Aley El-Shazly (MU Geology) For help with analyses.

Dr. Mike Norton (MU Chemistry) Standars for analysis.

    
     
     
                   
 


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