THE INTERNAL STRUCTURE OF TEKTITES: PETROGRAPHY
OVERALL STRUCTURE
Homogeneity of tektite glass
If a typical splash-form tektite is examined in an ordinary thin section, no structure whatever is seen apart from a few small bubbles (vacuoles). Rankama (1965), after a study of 33,000 australites, remarked that the lack of inclusions is highly significant; the point was made earlier by Scrivenor (1933) from scanty data. An unmistakable inclusion of terrestrial soil or rock would of itself settle the question of the origin of tektites; such inclusions are always found in undoubted impact glasses, such as those from Wabar, Henbury, or the Ries Kessel (Chao and Littler, 1962). It is also of great importance to theories of the formation of tektites that they consist for the most part of glass which is homogeneous at the level of a percent or two (Dixon and Meadows, 1968; Hawkins, 1963; Walter, 1965; see Figs. 18 and 19), and has so few vacuoles that it is not ordinarily necessary to crush a tektite in order to get an accurate measurement of its density (at the level of 0.001 g/cm³; Chapman et al., 1964). It is particularly remarkable that there should be marked homogeneity in individual tektites, and yet striking differences in composition between tektites found side by side, or even welded together (King and Bouška, 1968). The homogeneity is strikingly illustrated by Askouri et al. (1973), who compare the macroscopic homogeneity of tektites with that of obsidian, and contrast it with the inhomogeneity of impact glasses. Under the microscope it is seen that obsidians have tiny crystals throughout (microlites) which are absent in tektites.
The significance of the lack of crystals was studied by Beall et al. (1965) and Wosinski et al. (1967); they found that it required cooling in less than about 10 hours for an indochinite; they deduced that the indochinites could not have cooled as parts of a sphere of glass with a radius greater than 90 cm. Yagi (1966) noted that at 900° C, quartz is the end-product of devitrification experiments on tektites, while at 1000° C, after 48 hours, he obtained cristobalite and a calcic plagioclase.
Strain polarization
If a whole splash-form tektite, for example a moldavite (Wright, 1915; Barnes, 1960; Soukeník, 1971a), is put in a dish of light machine oil and put between crossed polaroids, then if the tektite is sufficiently transparent, an overall pattern will be seen, due to a condition of internal strain in the tektite as a whole. For a spherical tektite, the pattern is a black cross (see Plate 18A).
The explanation is that the tektite appears to have cooled rapidly, as a complete body. The outside cooled first, of course; it formed a rigid outer sphere, which fitted over the still-molten interior. Then the interior cooled and shrank. This put the whole interior into a state of tension (of negative pressure). Since this is isotropic, it does not affect the polarized light.
The exterior is pulled inward; accordingly it is in a state of tension in the radial direction. The tension is resisted by compression in the tangential direction, as in a masonry arch, or the dome of a basilica. Hence light which goes through the center of the sphere retains its original direction of polarization, because in the outer part of the sphere it encounters glass which is in compression in both of the two possible directions of the light vector (perpendicular to the direction of propagation). But for a ray which grazes the sphere, passing only through the outer part, the two light vectors, in the radial and tangential directions, encounter different conditions in the glass. Since the glass atoms are pulled apart in the radial direction, the glass behaves, for the radial vector, like a medium of reduced density and hence reduced index of refraction; and vice versa in the tangential direction. If the ray which is coming up through the dish from the lower polaroid (the polarizer) is polarized in either the radial or the tangential direction, there is no net effect on the state of polarization, and hence the ray is blocked by the second (analyzing) polaroid; this makes the dark arms of the cross. But when the direction of polarization of the incoming ray is intermediate, then the glass resolves the ray into radial and tangential components, which get different lags,and which combine on emerging from the glass to form elliptically polarized light, which does, in part, get through the analyzing polaroid, and makes the bright spaces between the arms of the cross.
The phenomenon is important because it means that the splash-form tektites cooled as separate objects from a temperature above the annealing temperature (600° C) and did not afterwards lose the outer, compressed shell (e.g. by ground chemical attack). For example, Rost (1967) showed that the pattern disappears if the outer 3 mm are ground off. The same phenomenon was seen by Wright (1915) in Icelandic obsidians from Hrafntinnuhryggur. Hammond (1950) estimated the strains at 735-1220 pounds per square inch (51-84 bars). He remarked that cooling at a rate of about 50° C per minute or more was required, especially between 700° C (the strain point) and 600° C (the annealing point).
Von Koenigswald (1963a) reports a large philippinite (648 g) which broke into at least 124 fragments on cutting, the fragments being pyramidal with the points directed inward. He attributed this to the same kind of stresses. He remarked that in some cases bubbles seem to have formed at the center of the tektite as a result of these stresses; when the bubble is found, the stresses appear to be less.
Chapman (1964) found that there is a strain pattern, visible in polarized light, apparently associated with the heating of australites and javanites in their descent through the atmosphere. He was able to duplicate the patterns by ablating tektite models in a wind tunnel. His calculations and experiments led to the conclusion that the strain pattern should vary in thickness from 2 mm for vertical descent to 4 mm for descent at a shallow angle. The observed patterns indicate that the javanites descended at a steeper angle than the australites. The same stresses are responsible for the frequent spallation of the front surfaces of australites and javanites (see also Fenner, 1949). Chapman was not able to trace the effects of strong one-sided heating in other tektites; he attributed this to prompt spallation of the affected zone, combined with attack by ground chemicals.
Barnes (1964c) has pointed out that this structure is not observed in the Muong Nong tektites; it follows that they cooled more slowly, presumably because they were parts of even larger masses.
On the other hand, in Muong Nong tektites one does observe a strain pattern around each individual grain of lechatelierite. This is clearly due to the fact that the lechatelierite has a smaller coefficient of thermal expansion than the glass; hence the glass shrinks all around it and compresses it
Structural anisotropy
Raman (1950a) drew attention to a different kind of birefringence which he first observed in Libyan Desert glass. Under very strong illumination (full sunlight) Raman found weak birefringence which followed a set of plane parallel layers in the glass. He considered that this could not be strain birefringence because, given the very low thermal coefficient of expansion of silica glass such as Libyan Desert glass, it is hard to see how strain birefringence could be set up. He found similar birefringence in artificial silica glass, clearly related to the directions along which the silica glass had been worked. Raman considered that this birefringence was due to the fact that the glass yields to the stresses of working, not only by breaking oxygen bonds, but also by deforming the SiO4 tetrahedra. He called the result "structural anisotropy", distinguishing it sharply from strain anisotropy. In later papers (Raman, 1950b, c) he found structural anisotropy weakly manifested in commercial rolled plate glass; he attributed it to deformation of the glass as it passed through the rollers. The implication of Raman's finding is that the Libyan Desert glass was sheared, while being cooled, along a set of parallel planes. The pressures used between the rollers in a typical plate-glass factory are of the order of 20 N/m², or for thin sheets, 70 N/m² (J.F. Wosinski, personal communication, 1975).
SMALLER STRUCTURES
Lenticules
From the petrographic standpoint the most important tektites are those of the Muong Nong type, which have been reported from Indochina by Lacroix (1935b), Barnes (1964b), Fontaine (1966); from the Philippines by Barnes (1964b); from Czechoslovakia by Barnes (1964b) and Rost (1966); and from Texas by Barnes (1964b). In relatively thick sections, around 500 μm, they show a phenomenon which Barnes calls the shimmering structure. The comparison is with the shimmering of a distant landscape seen through hot, turbulent air. It is clearly due to regions of varying index of refraction, for which I propose the name lenticules because in general the small regions are drawn out into lens-like shapes, as seen under the microscope, a few tens of micrometers in width, and hundreds of micrometers in length. They are most clearly seen when the diaphragm is stopped down, or when an occulting object is moved through the light-path. Under these circumstances, the boundaries of the lenticules appear quite sharp, as if the Muong Nong tektites were really composites of a large number of small glass bodies.
In some Muong Nong tektites, the lenticules are bounded in part by void spaces (Plate 18B); a similar thing is seen in the Libyan Desert glass in some parts.
Dr. L.S. Walter kindly polished some Muong Nong tektites, and then demonstrated that the boundaries of the lenticules can be shown by attack with HF (Plate 19A). He also showed, in an unpublished work which he kindly permits me to quote, that the boundaries of the lenticules can be traced by variations in the chemical composition.
Barnes (1964b) suggested that the lenticules, which he was the first to recognize, were formed by the shock melting of mineral grains in a hypothetical parent rock. He noted the cuspate (spiny) form of the voids which bound the lenticules in some Muong Nong tektites, and suggested that these were relics of incompletely rounded voids between the original sand grains of the parent rock.
The suggestion of Barnes was taken up by O'Keefe and Adler (1966) who noted that the hypothetical parental sandstones would be expected to consist of grains of distinct minerals, chiefly quartz; they showed, however, that the lenticules do not have the composition of any mineral, but instead, as found by Walter (Figs. 18 and 19), they have the composition of homogeneous glasses, at least qualitatively like the bulk composition of the tektite.
They suggested that the Muong Nong tektites are similar to terrestrial
welded tuffs. A welded tuff is a deposit of volcanic ash, often
largely glassy, which was laid down hot (above 800° C, according to
Boyd, 1961). Under pressure from overlying layers, the particles of
ash (shards) have deformed
plastically so as to close the voids. The result, under certain
conditions of pressure and temperature, is a solid mass of glass, in
which, however, the outlines of the original shards may be visible.
The suggestion that the Muong Nong-type tektites might be a sort of
welded tuff was laid before the late Hoover Mackin, who remarked that
the lenticules do not really resemble the shards in a welded tuff,
because they do not have the Y-shapes which are a prominent feature of
terrestrial welded tuffs. These Y-shapes result from the spiny form of
the terrestrial shards. Their absence in the lenticules seemed to us
(O'Keefe and Adler, 1966) to mean that the parent bodies of the
lenticules were rounder than the terrestrial volcanic shards.
A
few months after this remark was published, the microtektites were
discovered by Glass (1967). They correspond in size and shape to the
hypothetical parent bodies of the lenticules; in addition, Glass has
shown that the lenticules from a given tektite show a sequence of
compositions like that observed in the corresponding microtektites (see
Chapter 6). It is difficult to doubt that there is some connection
between the microtektites and the lenticules.
The structural
anisotropy noted by Raman (1950a) might then be plausibly explained as
the result of liquid flow in a cooling unit of welded tuff. The
pressures implied seem too large for the idea of Barnes and Pitakpaivan
(1962) that Muong Nong tektites formed as puddles of liquid glass.
Underneath
the flanges, on the original outer surface of the australites, Barnes
(1962b) detected some small objects of higher index of refraction.
B.P. Glass (verbal remarks) says that these turn out to be the denser
glasses which he finds in tektites.
Striae
In
splash-form tektites, the lenticules as such are rarely seen (but note
Glass, 1969b, a lenticule in an australite); instead, one sees striae
or schlieren (see Plate 8B), i.e. contorted layers of varying
refractive index. These seem to grade into the parallel systems of
layers which are seen in Muong Nong tektites. The striae ordinarily
meet the surfaces of the tektites abruptly (Hubbard et al., 1956); this
is significant because it does not fit the idea that the splash-form
tektites were formed by some kind of condensation process from the
gaseous state.
On the front surfaces of australites, the striae
do not usually meet the surface abruptly, except at the center.
Instead, they curve aside and meet it at a very shallow angle. The
obvious explanation is that given by Chapman, namely that the front
surface was molten, and was dragged to the rear by aerodynamic forces
to form the flanges. This explanation is generally satisfactory; but
Van der Veen remarked that in the photographs of Dunn (1912) it is
clear that in some cases the surface sculpture cuts through the striae,
particularly toward the edge of the tektite.
Within the
australite flanges, the striae are coiled in a way which clearly
reveals how the flange has formed. It is curious that the flanges of
australites show evidence in this way of being much more tightly coiled
than either the models of australite ablation produced by Chapman et
al. (1962) or the flanges of javanites (Von Koenigswald, 1963b).
Within the flanges the striae often become wavy; the same appears in
the models.
Many tektites, particularly indochinites, have long
tails. The striae in the tails are drawn out parallel to the axis of
the tail.
Bubbles
Statistics
on bubbles in tektites were put together by Aghassi (1962). Lacroix
(1931) noted that the bubbles are rounded in the round portions of the
splash-form indochinites, and are drawn out into ellipsoidal forms in
the stretched tails of indochinites. He remarked that bubbles in
terrestrial obsidians are normally drawn out if the silica content is
greater than 70%; he inferred that the temperatures of formation of
indochinites must have been higher than those of obsidians, so that the
viscosity could be lower.
Hawkins (1963) found by furnace
experiments that bubbles in simulated tektite glass belong to two
types: the small bubbles, which show a steep gradient on a
size-frequency plot, correspond to the original intergranular spaces;
while the large bubbles, with a shallow gradient, correspond to
escaping gases. He traced this to the effects of the critical bubble
size; since surface tension exerts more pressure as the bubble gets
smaller, it follows that for any given pressure in the bubbles, say
that due to vapor pressure of some constituent, there is a size below
which the bubbles are unstable and shrink, while above it they grow.
Hawkins found only the small regime in most tektites; in the Darwin
glass he found both.
INCLUSIONS
Lechatelierite
The
commonest inclusions in tektites are small bodies of low-index glass
which were shown by Barnes (1939) to be lechatelierite (silica glass;
see Plate 19C). They are of silt size, i.e. somewhat smaller than
typical sand grains. In Muong Nong tektites they tend to be
accompanied by clusters of bubbles whose size varies greatly from one
tektite to another. The lechatelierite bodies themselves tend, in
Muong Nong tektites, to be nearly equant, and brownish in color. In
splash-form tektites, the lechatelierite is no longer equant; it tends
to have long tails, drawn out parallel to the striae and often
contorted; the length may be up to half a millimeter (Barnes, 1939).
Around
the lechatelierite particles the glass shows a pattern of strain
birefringence, which results from the difference in thermal coefficient
of expansion, and cannot be removed by annealing.
Barnes (1962,
1963b) has drawn attention to the fact that on many tektites, and
especially on the posterior surfaces of australites, the lechatelierite
inclusions are more numerous than elsewhere in the glass; and the flow
structure around them looks as if they had been pushed down into the
glass. He calls these "fingers".
Walter (1965) found coesite in the lechatelierite of a Muong Nong tektite from Phaeng Dang. He studied the shapes of the crystals further (L.S. Walter and C. Sclar, 1967, unpublished) under a scanning electron microscope. P. Pellas (private conversation) found none in tektites from Muong Nong itself. Chantret et al. (1967) reported that the brown inclusions contained α-quartz, calcic plagioclase (?) and possible hypersthene. They stated that the quartz is not detrital. Relative to the matrix, the inclusions are enriched in silicon, depleted in aluminum and potassium; iron, magnesium and calcium are unchanged.
Quartz
Quartz
is found in some Muong Nong tektites (Barnes, 1963a; see Plate 19B).
It is anhedral, with some rounding. In Aouelloul glass, it is often
cracked; the fragments, however, retain the same crystallographic
orientation; evidently the crystals were broken in their present
locations, and there was relatively little flow thereafter.
Barnes
(1964b) notes that the layered structure observed in Muong Nong-type
tektites is sometimes interrupted by miniature faults. The faults are
usually welded shut, and occasionally are filled with tektite glass.
Along the faults, crystals, usually of quartz, are observed. From
their shape, the crystals re judged to be detrital, in most cases;
there is some evidence of very small euhedral quartz crystals (5 µm in
diameter) which seem to have resulted from devitrification of the glass
along the fault. Barnes reports another unidentified mineral, which
forms triangles and rosettes, up to 50 µm across,with weak or no
birefringence.
Rays
In
some tektites, Barnes (1964a) has noted some long narrow structures,
which he calls "rays", protruding from tektite bubbles. They are
usually of lower index of refraction than the matrix glass.
Heavy minerals
By
crushing Muong Nong tektites and using heavy liquids to separate the
denser fractions, Glass (1970b, c) has found crystals of zircon,
chromite, corundum, rutile, and monazite. The crystals are in general
fractured, although enclosed in a complete, unbroken envelope. The
corundum is associated with silica glass and appears shocked. Glass
considers that it results from the decomposition of sillimanite.
Zircon and baddleyite
Baddeleyite (ZrO2)
was noted in the Martha's Vineyard tektite by Clarke and Henderson
(1961); it was found in Aouelloul glass by El Goresy (1965), and in a
Georgia tektite by King (1966). El Goresy stated that the baddeleyite
was the product of the disintegration of zircon (ZrSiO4); in
fact, zircons are also seen in tektites (Glass, 1970b). El Goresy
noted that the transition from zircon to baddeleyite plus silica takes
place at 1676° C, and concluded that the tektites had reached this
temperature. Kleinman (1969) quoted El Goresy on this point when she
found baddeleyite in Libyan Desert glass. The conclusion was criticized
by Clarke and Wosinski (1967) who furnished clear evidence that zircon
can be attacked by liquid glass at temperatures as low as 1500° C, with
the formation of baddeleyite. The contrast between melting and solution
in the disappearance of solid phases is a recurrent theme in
glass-making.
Nickel-iron spherules
Chao
and co-workers (Chao et al., 1962) discovered nickel-iron spherules in
philippinites from Santa Isabela. (The earlier claim of Spencer
(1933c) to have found them in australites and Darwin glass has never
been confirmed.) Others were found in indochinites from Dalat (Chao et
al., 1962). The spherules were accompanied by inclusions of troilite
and schreibersite, which are typical of iron meteorites. Further
studies were reported by Chao et al. (1964); the sequence of trace
elements corresponds to that in iron meteorites. Subsequently, Chao et
al. (1966a) reported nickel-iron spherules in Aouelloul crater glass.
Brett
(1966, 1967) has drawn attention to a law which governs the appearances
of these spherules, and which seems to distinguish between tektites and
impact glasses such as Wabar and Henbury, as follows:
Nickel-iron spherules formed by meteorite impact in the earth's atmosphere are very rich in nickel, and are surrounded by an iron-rich halo. Those formed in tektites have normal nickel abundance and no halo.
The
meaning of Brett's Law is discussed in Chapter 6. Chao et al. (1966a)
noted an iron-rich region around the Aouelloul nickel-iron spherules,
whose nickel content, however, goes only up to 9%. O'Keefe pointed out
that not only iron but also other cations were enriched in this region,
as if it were a local patch of relatively low-silica glass.
Reid
et al. (1964) showed that although small iron spherules can be produced
in tektite glass by heating under reducing conditions, the synthetic
spherules do not have the same sequence of trace elements as meteoritic
iron.
Magnetite
Magnetite
has been reported by Vorob'yev (1959a), Kleinman (1967), and by Fechtig
and Kleinman (1967). Vorob'yev's particles were on the outside, and
Kleinman found hers by grinding, so that it was hard to be sure that
they were really inside. A.S. Doan (personal communication, 1975) has
recently tentatively identified the black particles in Libyan Desert
glass mentioned by Clayton and Spencer (1934) as titano-magnetite,
confirming a remark of Kleinman. The crystalline form could not be
traced. Although the particles were deep inside the glass, Doan found
evidence that atmospheric gases penetrate the Libyan Desert glass;
hence it is possible that the iron was oxidized on the earth. In a
number of cases, the magnetite bodies have a core of metallic iron.
SUMMARY
Tektites
are generally homogeneous glasses usually without crystalline inclusion
or bubbles amounting to as much as 0.1% of the total volume.
Muong
Nong tektites in particular and to a lesser extent other tektites
appear to be composed of small glass particles (lenticules) which have
been welded together. The lenticules may have been microtektites
before welding.
Tektites cooled as separate bodies of at least roughly their present dimensions.
There are inclusions in some tektites of minerals which appear detrital.
Coesite is reported but disputed.
Nickel-iron spherules are found, resembling meteoritic spherules.
