By  Stephen E. Pierce

 

A NEW METHOD TO DETERMINE THE DIFFERENCE BETWEEN TEKTITES
AND OTHER NATURAL (VOLCANIC) GLASSES 

 

Tektites are among some of the world’s most curious natural glasses.  They have been gathered by collectors the world over for their uniqueness and scientific value in learning more of the world around us.  Charles Darwin first recorded tektites in 1840 (Darwin, 1891).   But the word tektite was first applied to these unique glasses was by F. Suess in 1900 (McCall, 1973).

 

Physically, they are composed mainly of glass containing high amounts of SiO2 having relatively few mineral inclusions and contain no crystallites.  Tektites generally have aerodynamically formed shapes including the form of buttons, teardrops, and dumbbells.  And they range in size from less than 0.01 mm (microtektites) to masses greater than twenty- two pounds (King, 1976).

 

Tektites are associated with strewn fields that have distinct ages ranging from the North American strewn field that is 34 million years before present (mybp), 15 mybp (Moldavites), 1.3 mybp (Ivory Coast), and 0.7mybp (Austalasian).  

 

When found with the typical aerodynamic shape tektites are easily distinguished from

obsidian, the nearest appearing natural glass.  The intimate association of aerodynamically shaped tektites with strewn fields and obsidian with volcanic provinces make identification between the two a simple matter.  However when abraded or only a fragment is present it is difficult to differentiate the similar colored opaque glasses of tektites from obsidian, or even artificial glass.  

 

The more common natural glasses that are most likely to be encountered are shown in Table A.  Glasses including lightning produced cylindrical fulgurites and deviritrified basaltic palogonite are so characteristic that little confusion should result differentiating these from tektites.  Impactite, though probably having a similar origin as tektite is invariably associated with meteorites and meteor craters. 

   

TABLE A

SOME NATURAL AND ARTIFICIAL GLASSES

Category

Object

Origin

Volcanic

Obsidian

Silica rich volcanic

 

Perlite

Silica rich volcanic

 

Pitchstone

Silica rich volcanic

 

Tuff, pumice, lapilli, etc

Silica rich volcanic ash

 

palogonite

Silica poor volcanic

 

Tachlyte or sideromelae

Anhydrous silica poor volcanics

Tektite

Tektite

Meteoroid Impact

 

Microtektite

Meteoroid Impact

Impactite

Impactite

Meteoroid impact

Miscellaneous

Fulgurite

Lightning

Artificial glass

Furnace and industrial slag, and artifacts

Man made activities

 

 

 

 

Because obsidian is the most likely glass that is usually confused with tektites the relationship between these two will be examined, with only a cursory look at artificial glass.

 

In general, there are three methods that I am aware of to distinguish tektites from obsidian, physical, chemical, and optical.  No one method that I am aware of can distinguish between tektite strewn fields and distinguish tektites from obsidian in one measurement.  However, a new method, spectrophotometry May provide new insights to the characterization and differentiation of these glasses.  Meteorite Exchange has kindly provided a tektite and obsidian for spectrophotometric examination (Table B).  The tektite is an Australasian tektite from the Khorat Plateau, Thailand.  The obsidian is from California. 

 

TABLE B

TEKTITIE, OBSIDIAN and GREEN GLASS

 INDEX OF REFRACTION and SPECIFIC GRAVITY

Material

Index of Refraction

Specific Gravity

TEKTITE

1.505

2.43

OBSIDIAN

1.485

2.36

GREEN GLASS

1.52

2.48

Tektite- Khorat Plateau, Thailand, Australasian strewn field.  Obsidian is from California Provided by Meteorite Exchange.   

 

Physical methods.

 

These methods commonly employ morphology (aerodynamic shape) and specific gravity (SG).  With samples of tektites showing the familiar aerodynamic shape identification is easy (Table C).  The SG is a common measurement that expresses the weight of the sample to an equal volume of water.   As Table B illustrates the Meteorite Exchange tektite and obsidian have SG’s of 2.43 and 2.36 times the equivalent volume of water.  This measurement can be useful to catalog known tektites and their associated strewn fields (McCall, 1973).  However, tektites and obsidian have similar overlapping SG’s (Table C).  With doubtful (abraded or fragmental) specimens physical methods May prove frustrating.  Measuring the SG is a simple inexpensive measurement requiring only a balance and does not destroy the specimen.

 

TABLE C

OPTICAL AND PHYSICAL PROPERTIES OF SOME NATURAL GLASSES

Material

Index of refraction*

Specific Gravity*

Water content**

Characteristics

Obsidian

1.48-1.51

2.13-2.42

< 1%

Vitreous luster

Perlite

1.49-1.51

2.23-2.30

1-4%

Perlitic cracks

Pitchstone

1.49-1.51

2.22-2.51

>4%

Dull resinous luster

Palogonite

1.51-1.61

2.5-2.99

?

Partially devitrified

Tektite

?-1.505-?

2.27-2.46

Very, very low

Aerodynamic shape

Fulgurite

??

??

??

Thin opaque cylinders

*Johannsen, 1939.  **Williams, et al, 1954

 

Chemical analysis.

 

Chemical analyses of tektites showing the common cations measured as oxides are found frequently in the literature.  Comparison between tektites of the four known strewn fields, obsidian, and microtektites are shown in Table D.  The chemistry of the tektites is similar to obsidian with SiO2 concentrations between 71% to 80%.  Microtektites however, have much lower SiO2 amounts (Table D) and the microtektites associated with the North American strewn field have only 64% silica.  Note the great difference between the microtektites and tektites of the same North American strewn field having silica amounts of 65% and 76% respectively.  As can be seen from the table, chemical analyses can distinguish tektite-strewn fields but the tektite and obsidian oxides overlap.  These volcanic glasses can also be differentiated by their water content (Table C).  Chemical analyses can be expensive especially when comparing many samples.

 

TABLE D

MAJOR ELEMENT COMPOSITION OF TEKTITES AND OBSIDIAN (percent)

OXIDES

MICRO

ASIA

USA

EUROPE

AFRICA

OBSIDIAN

SiO2

64.15

73.06

76.37

80.07

71.05

76.78

TiO2

0.88

0.68

0.76

0.80

0.70

0.08

Al2O3

14.15

12.23

13.78

10.56

14.60

12.09

MgO

2.41

2.04

0.63

1.46

3.29

0.1

CaO

2.89

3.38

0.65

1.87

1.67

0.57

Na2O

1.63

1.27

1.54

0.51

1.71

3.79

K2O

3.09

2.20

2.08

2.95

1.53

4.93

Fe2O3

8.37

0.60

0.19

0.15

0.18

5.60

FeO

*

4.14

3.81

2.29

5.51

2.61

P2O5

0.72

NA

0.19

0.15

0.18

NA

MnO

NA

NA

0.04

0.11

0.08

NA

H2O

NA

NA

NA

NA

NA

0.2

MICRO=Microtektites, N.A. strewn field, (Varekamp, 1982):Asia=Australites, (McCall, 1973) USA=North America, Europe=Moldavites,  Africa=Ivory Coast, (King, 1976): Obsidian=Obsidian Yellowstone Park, (Hatch et al, 1972).  *All iron reported as Fe2O3 :NA=No data available

 

Optical methods.

 

Index of refraction.  The index of refraction (IR) of a glass measures the amount of light that is bent (refracted) passing through it to, for example air or water.  Normally, IR is measured either by a refractometer or the comparison of the sample using liquids of differing indices of refraction under the microscope.  The varying degrees of IR of the tektite glass reflect the amount of silica (SiO2) in the tektite.  When the variation of silica is determined for individual strewn fields and compared to the IR’s of the tektites used in the chemical analyses, a correlation of increasing IR with decreasing silica will result.  However, by comparing the value of the Meteorite Exchange tektite (Table B) with the published values of IR for obsidian they overlap and IR alone cannot distinguish a tektite from obsidian.

 

Microscopic observations.  This is the generally the most used method to distinguish a tektite from obsidian.  The volcanic glass obsidian and its relatives, perlite and pitchstone contain gas bubbles, crystallites (embryonic crystals) and spherulites (radial fibrous crystals) either separately or in trains.  The mineralogy of tektites is much different.  Minerals within tektites include shock produced coesite, baddelyite, and lechatelierite (King, 1973).   However, to recognize the minerals associated with tektites and obsidian knowledge of optical mineralogy and possession of a petrographic microscope is usually required.  And I am not aware of microscopic differences between tektite-strewn fields.  

 

Spectrophotometric signatures.

 

Preliminary spectrophotometric evaluation suggests that not only can the differentiation between tektites and obsidian be made but also the differentiation between the tektite-strewn fields.  This can be performed in a simple straightforward procedure on a single measurement.  However, the sample needs to be prepared by cutting and polishing parallel faces.  A small sample is all that is needed.   As mentioned above, Meteorite Exchange provided a sample of a tektite and obsidian for examination.  A varying wavelength spectrophotometer was utilized to produce distinct spectrophotometric signatures for the tektite and obsidian.  These absorption curves are shown in Fig 1. 

 

 

The tektite has an absorption curve that is a broad transmission band from 465 to 800 nanometers (nm).  Also, around 410 and 450 nm there are two absorption peaks.  Obsidian has an absorption curve that is much different, absorbing the spectra more or less the same.  As can be seen, the absorption curve is not linear and contains occasional transmission bands.  At a greater scale these differences become much clearer.  A green glass (7-UP) bottle is shown as well to compare an example of an artificial glass.  As can be seen the absorption curves of the tektite, obsidian, and artificial glass are very distinctive.      

 

To ascertain if the tektite signature was unique to that particular sample or more suggestive of all tektites, I compared the Australasian tektite with a Moldavite (Slovika) on the U.S.G.S. website: http://minerals.gps.caltech.edu/FILES/Visible/Tektite/Tektite_Slovika.gif.  It is the only spectrophotometric signature of a tektite that I am aware of but unfortunately I know of no obsidian absorption curves other than what I have produced.

 

The comparison of the Thailand and Moldavite tektites are shown in Fig. 2.  They have very close similarities.  Both curves have broad transmission bands in the visible (400-700 nm) range.  The Moldavite absorption curve has a distinct asymmetry.  The minimum absorption of the Thailand tektite (Australasian strewn field) is around 640 nm whereas the Slovokia sample (Moldavite strewn field) is at 582 nm.  This May be related to the chemistry of one or more of the elements making up the tektite.  As can be seen it Table D many of the oxides are characteristic of a strewn field.  This suggests an additional line of investigation.  The comparison of the maximum transmission values of tektites from different strewn fields might provide an additional tool in studying tektite distributions.  Also, there is an absorption peak at 460 nm or the Thailand tektite that is not apparent on the U.S.G.S. sample which would be interesting to investigate.  It must be kept firmly in mind however, that these comparisons are based upon only two samples.  More work including examining tektites from different strewn fields clearly needs to be taken.

 

 

The instrument used for the tektite examination was a variable wavelength spectrophotometer Odyssey DR/2500.  The instrument has a resolution of 1 nm and a wavelength range from 365 to 880 nm.   The tektite was cut and polished to a thickness of 1.5 mm and the obsidian was cut and polished to a thickness of 1 mm.

      

Further spectrophotometric enquires into these curious objects.

 

1.  Determine if absorption curves could be utilized to determine tektite-strewn fields and their associated sub- fields.  The same method May be applied to studying different volcanic provinces using obsidian.

 

2.  The literature associates microtektites with tektites.  For example tektites in the North American strewn field are associated with microtektites in the equivalent stratigraphic section in marine sediments offshore.  But at least to me the relationship between tektites and microtektites is not that clear.  The microtektites have a different chemistry than tektites (Table D) and are found only in marine sediments.  It might be interesting to compare the spectrophotometric signature of a microtektite to a tektite.

 

3.  Most students of tektites including myself consider that impactites are related to tektites.  The primary difference between them is that tektites show an atmospheric history whereas impactites are associated with impact craters.  What does the absorption curve of an impactite look like? 

 

Tektites are curious objects that to me still require much work.  Their association with distinct strewn fields and possible affect on the earth’s biota and climate (Glass, 1982) makes them a fascinating subject.

 

Acknowledgements.  I would like to thank Paul Harris and Jim Tobin of Meteorite Exchange for the opportunity to examine the tektite and obsidian samples.   Jim Tobin for cutting and polishing them.  And to Red Enright for his encouragement and his technical ideas on future equipment that May be applicable to studying these unique glassy rocks.

 

 

References

 

Bowen, N.L., 1928, The evolution of Igneous Rocks, Dover Publications, Inc.

 

Darwin, C., 1891, Geological Observations on the volcanic Islands of and Parts of South America during the Voyage of H.M.S. Beagle, Rebublication, Appleton & Co., New York.

 

Glass, B.P, 1982, Possible Correlations between tektite events and climate changes?  Geological Society of America Special Paper 190.

 

Hatch, F.H. et al, 1975, Petrology of the Igneous Rocks, Thomas Murby & Co.

 

Heinrich, E. W., 1956, Microscopic Petrography, McGraw-Hill Co.

 

Johannsen, A., 1939, A Descriptive Petrography of the Igneous Rocks, Allied Pacific Private, Ltd.

 

King, E., 1976, Space Geology an Introduction, John Wiley & Sons, Inc.

 

McCall, G.J., 1973, Meteorite and their Origins, Wren Publishing PTY LTD

Varekamp, J., Thomas, E., 1982, Chalcophile elements in Cretaceous/Tertiary boundary sediments: Terrestrial or extraterrestrial?, in GSA Special Paper 190, Geological Society of America.

Williams, H., et al, 1954, Petrography, W.H. Freeman and Co.