Proceedings of the XIth International Congress and Exposition
June 2-5, 2008 Orlando, Florida USA
©2008 Society for Experimental Mechanics Inc.
Obsidian - Natural Nanostructured Glass: Preliminary Research Results
B.F. Dorfman, Dr of Sc, Clarkson University, 8 Clarkson Avenue, Potsdam, NY 13699,
[email protected]; 229, 19th Ave., #9, San Francisco, CA, 94121; L.C. Zhang, Ph.D.,
Clarkson University; C.E. Skinner, Ph.D., Northwest Research Obsidian Studies Laboratory;
S.C. Zunjarrao, R.P. Singh, Ph.D., Oklahoma State University
ABSTRACT
Correlated Mechanical-Structural-Chemical Analysis of obsidians (from 15 American localities) by micro- and
nano-indentation, XRD, SEM, TEM (2-5 nm resolution), SEM-EDS, XRF revealed distinguished microfracture
behavior of volcanic glass vs. artificial glasses, fused quartz, and nano-crystalline silica, as well as fused
homogenized obsidians of the same origin, while the crack propagation threshold in obsidian is superior to these
reference materials in order of magnitude, and the fracture pattern in obsidian is predominantly two-dimensional
(2D) superficial vs. quasi one-dimensional (1D), deep radial cracks dominating in artificial glass. Compositional
profiling of obsidian detects sharp change in relative content of oxygen across inter-domain borders and across
deflected micro-cracks thus revealing correlation ‘oxygen distribution - domain structure - fractural mechanics’. It
suggests that relatively slight structural-chemical variation resulting with hierarchical network of inter-domain
borders may produce essential mechanical reinforcement of glass. If proofed, the last conclusion represents
particular interest for glass industry. Beyond of that, the obsidian formation covers a gap between geological and
technological timescales; its study may lead to new findings useful for various fields of materials development.
However, due to extreme complexity of obsidian structure and mechanisms of formation, the further research
needs cross-disciplinary efforts.
1. Introduction
Obsidian as natural volcanic glass is commonly assumed possessing a fracture behavior
identical to conventional artificial glass albeit it employed from the pre-historic tool and arm and up to
contemporary surgical tool in the manner, which would be unfeasible for various artificial glasses. In spite of some
early remarkable researches [1-4], the prevalent present definition of obsidian is not strongly differentiated from a
definition given over decades ago and structurally equalizing volcanic glass with common glass. Of about 3200
publications on obsidian referred in the literature up to 1998 [5], only 8 papers [1-4, 6-9] concern with mechanical
properties of natural glasses. Although archeologists and anthropologists were aware about the clearly
distinguishing mechanics of obsidian long ago [2], correlation between such a unique mechanics and specific
structural or compositional features of volcanic glasses remains unknown. Recent researches in the obsidian’s
structure and composition are extensive (see some examples: [10-14]), but predominantly associated with
geological knowledge of its formation, devitrification and age, as well as archeological tracing with rather
“axiomatic” acceptance of its unique mechanical properties underlying its ancient use and archeological
significance. This article is narrowly focused in search for structural/chemical features which may be responsible
for mechanical properties generally distinguishing volcanic glasses from artificial glasses. Any analysis of
obsidian may encounter the problem of extreme complexity: variation of basic chemical composition of glass
matrix combined with diversity of crystalline inclusions depend in turn on geophysical and thermochemical
conditions of formation and cooling of volcanic mass. To make the task at least in principle solvable, we selected
certain set of samples assuming that they represent sufficiently broad diversity of these volcanic materials but
avoiding any analysis of actual mechanisms of those rocks formation. Certainly, as soon as the responsible
features will be defined, the respective analysis of their genesis will be absolutely necessary.
2. Experimental Brief description of 15 examined samples is shown in Table 1, including the samples’ origin,
geological age and content of bulk elements. Since the trace elements are generally used for identification of the
obsidian’s sources [15], concentration of Fe, Ti, Mn, Zn, Ga, Rb, Sr, Y, Zr, Nb, Ba, Pb, Th was analyzed by the
XRF and exhibited in Table 2 for all the examined samples from various localities, as shown in Table 1. Numbers
of standard deviations are also indicated. The tables mark all the samples with numbers, which used in the
following description of experimental results and discussion. Some of samples like #6, #12 and #13 display
Table 1. List of examined samples and Overall Chemical contents (in at. %) of bulk elements
#
Origin
Age
O
Si
Al
Na
K
1
Beatys Butte: OR
10.36 ±0.53 my 47.5 ±5.3
40.5 ±4.5 6.1 ±1.3 3.1 ±1.0 2.8 ±0.7
(K-Ar)
2
Big Obsidian Flow: OR
1,300
50.5 ±5.0
35.3 ±3.5 7.2 ±1.4 4.1 ±1.3 2.9 ±0.8
yrs BP
3
Burns Butte:OR
7.54my (K-Ar)
56.0 ±6.4
29.8 ±3.3 7.3 ±1.5 4.1 ±0.9 2.8 ±0.6
4
Cougar Mountain: OR
4.31±0.3my
51.4 ±5.2
35.0 ±3.2 7.4 ±1.0 3.5 ±0.8 2.7 ±0.3
5
Glass Buttes V.1: OR
4.91±0.7my
54.5 ±4.3
30.5 ±2.4 7.9 ±1.3 3.7 ±0.7 3.4 ±0.5
6
Glass Buttes V. 3:OR
46.0 ±3.5
40.0 ±3.3 6.7 ±0.8 3.6 ±0.6 3.7 ±0.3
7
Glass Mountain: CA
900 yrs BP
55.0 ±4.6
30.2 ±3.5 6.9 ±1.0 4.4 ±0.8 3.5 ±0.5
8
Horse Mountain: OR
6.9±0.14 my
57.6 ±5.3
28.5 ±3.4 6.9 ±1.3 4.2 ±0.5 2.8 ±0.3
9
Mckay Butte: OR.
0.58±0.10 my
44.3 ±3.3
40.7 ±3.1 7.4 ±0.9 3.4 ±0.4 4.2 ±0.3
10
Interlake Obs. Flow:OR 7,300 yrs. BP
43.6 ±4.5
40.8 ±4.3 7.8 ±1.1 4.3 ±0.7 3.5 ±0.5
11
Glass Buttes
47.4 ±4.7
38.5 ±3.3 7.5 ±0.8 3.5 ±0.3 3.1 ±0.3
12
Glass Buttes 6
46.5 ±4.9
39.3 ±4.4 7.6 ±0.7 3.4 ±0.3 3.2 ±0.3
13
Buck Mountain, N. CA
46.1 ±5.0
38.8 ±3.3 7.7 ±1.0 3.9 ±0.6 3.5 ±0.5
14
Brazil. Details unknown 37.5 ±4.3
48.8 ±5.0 6.5 ±0.7 3.4 ±0.4 3.8 ±0.5
15
Mexico. (“Rainbow”)
42.4 ±4.1
43.7 ±4.2 7.6 ±0.8 3.7 ±0.3 2.6 ±0.2
Table 2 Overall Chemical compositions. Trace elements (min/max, unit: ppm except Fe in wt.%)
##
Ti
Mn
Fe
Zn
Ga
Rb
Sr
Y
Zr
Nb
Ba
Pb
Th
1
728/
349/ 0.97/ 24/
13/
123/ 165/ 13/
159/ 10/
863/
16/
4/
993
442
1.24 41
18
132
176
19
164
13
1021 23
10
2
1149/
425/ 2.03/ 57/
16/
118/ 53/
44/
353/ 20/
776/
13/
9/
1389
514
2.40 70
21
127
60
48
367
24
882
21
14
3
908/
234/ 1.20/ 43/
16/
120/ 26/
39/
244/ 28/
475/
17/
8/
1253
349
1.69 60
21
131
30
44
274
31
605
22
15
4
250/
226/ 0.76/ 68/
14/
92/
36/
52/
127/ 10/
1123/ 15/
4/
390
346
1.20 85
22
102
41
57
140
14
1377 19
13
5
301/
251/ 0.67/ 19/
10/
65/
22/
42/
90/
9/
1106/ 8/
1/
621
414
0.84 44
25
82
49
50
117
15
1225 19
13
6
461/
237/ 0.64/ 30/
11/
99/
66/
25/
109/ 6/
1108/ 15/
9/
628
328
0.97 39
25
108
72
29
115
11
1273 20
12
7
1327/
209/ 1.47/ 37/
15/
146/ 107/ 25/
219/ 7/
719/
19/
12/
1844
280
1.90 45
21
163
126
28
234
14
773
25
22
8
801/
437/ 2.81/ 177/ 13/
128/ ND/ 100/ 618/ 34/
42/
33/
7/
1073
593
3.23 205
25
143
5
106
640
41
62
40
16
9
833/
258/ 1.41/ 54/
17/
127/ 55/
37/
202/ 7/
968/
18/
9/
1077
436
1.76 117
27
139
62
42
214
15
1072 24
20
10
1150/
323/ 1.76/ 43/
43/
133/ 60/
40/
282/ 16/
766/
17/
8/
1350
372
2.03 60
60
141
66
45
290
21
843
21
17
11
Assumed same as #5
12
501
608
0.71 23
21
108
69
18
99
9
644
21
5
13
556
327
0.74 36
15
111
62
24
98
8
1004 20
10
14
596
770
0.77 52
20
184
24
26
131
37
51
30
16
15
482
252
1.42 114
25
159
10
61
510
59
27
16
9
ER
~90
~35
0.11 6-7
2-3
2-3
6-7
~2
6-7
1-2
~20
~3
~3
the bi-color or multi-color features. For purpose of the sample identification, analysis of trace elements (Fe, Ti,
Mn, Zn, Ga, Rb, Sr, Y, Zr, Nb, Ba, Pb, Th) in obsidian was conducted using a Spectrace 5000 energy dispersive
X-ray fluorescence (XRF) spectrometer. The system is equipped with a Si(Li) detector with a resolution of 155 eV
2
FHWM for 5.9 keV X-rays (at 1000 counts per second) in an area 30 mm . Signals from the spectrometer are
amplified and filtered by a time variant pulse processor and sent to a 100 MHZ Wilkinson type analog-to-digital
converter. The X-ray tube employed is a Bremsstrahlung type, with a rhodium target and 5 mil Be window. The
tube is driven by a 50 kV 1 mA high voltage power supply, providing a voltage range of 4 to 50 kV. The overall
structures of the samples were analyzed by obtaining X-ray diffraction (XRD) patterns from Bruker D8-02 X-ray
diffractometer. Morphologies of obsidians were characterized by JEOL JSM-6300 scanning electron microscope
(SEM) and JEOL JEM-2010 Scanning transmission electron microscope (TEM). The SEM samples were firstly
grinded and polished. In order to reveal the real morphology of the inclusions hidden within the obsidians, some
areas of SEM samples were etched by 10vol.% HF solution. To avoid the electrical charge effects, the thin
coating of vaporized Au had been applied. For TEM sample preparation, the broken pieces of samples were
grinded into fine powders. A suspension containing these fine powders was made in an ethanol medium. A drop
of the suspension was deposited on to carbon coated TEM grid and dried under vacuum. The analyses of the
bulk elements Si, O, Al, Na and K in obsidian were carried out by means of the same SEM served with energy
dispersive spectroscopy (EDS) systems capable of
detecting light elements down to boron. The analyses in
this research include: overall chemical analysis; local
micro-chemical analysis for identifying various inclusions;
a linear EDS scanning, in particularly cross the internal
borders of obsidian structures. The SEM-EDS
microanalyses can give rather comparable data. The
errors due to different surface geometry can be negligible
because of very small and practically flat surface of the
analyzed samples. Worth to note, determination of the
oxygen concentration is still challenging since oxygen
generates very soft X-ray subject to strong absorption by
either the surface or the detector window. To assure
convincing results, the SEM-EDS microanalysis on the
uncoated (less-conductive) samples was also performed
with comparisons between the corresponding EDS
spectra. For standard quantification, the fused quartz, i.e.
stoichiometric SiO2 (of semiconductor purity) was used as
the standard reference sample. The comparative EDS
spectra were collected from the obsidian samples and
standard fused quartz, with all of them simultaneously
mounted on the SEM sample stage in order to maintain
the similar beam and surface conditions for a series of
spectra collections.
Combined nano- & micro- indentation covering the load
range from a few mN to 24 N (except the gap
100<L<150mN) was found as the most effective for
obsidian study. Nanoindentation had been conducted with
standard nanoindenter in the following conditions: Load
controlled experiments; Indentation using a diamond
Berkovich tip; Peak load of 100 mN; Poisson’s ratio
assumed to be 0.3. Samples were cut by high speed tile
saw, mounted in epoxy and polished to obtain a mirror
finish before indentation.
Table 3 Nano-indentation on Obsidian (GPa):
Sample
E
H
∆E
∆H
Fig. 1
Typical Load-Displacement plots for
nanoindentation of obsidians 1, 12, 13
1
67.307
0.217
7.849
0.138
12/Red
12/black
13/red
69.123
69.795
67.423
0.688
0.880
0.696
8.662
8.8
9.003
0.049
0.172
0.356
13/pink
65.832
0.723
8.576
0.034
A standard manually operated Vickers microindenter was
employed in manual mode allowing investigation of
fracture behavior in exceptionally wide range of available
loads and convenient direct observation of post-load fracture phenomena. Maximum loads up to 24.0 N were
employed in this research. Because such extraordinary loads could possibly damage the diamond pyramid, the
indenter was periodically examined and calibrated by the hardness measurements of reference samples
(aluminum, steel, glass and single crystal silicon). For a comparison, conventional lime glass (“window glass”),
Hv= ~5.5 GPa; optical polished glass, Hv=~6.0 GPa, and fused quartz (synthetic polished substrate of
semiconductor industry quality), Hv=~9.0 GPa had been examined with the same technique.
2b
2a
2b
3b
3a
4a
5a
4b
5b
Fig.2 Typical fracture pattern in Obsidian (a) at
L=13N and in Lime glass at 12N (b)
Fig.3 a,b. Typical fracture pattern in Lime glass
at Loads 16N (a) and 24N (b)
Fig.4 Obsidian. L=16N. 4a Typical fracture
pattern. 4b EDS profile across the deflected
crack
Fig.5. Obsidian. L=24N 4a Typical fracture
pattern
5b EDS Compositional
profile across the
deflected crack
In addition, three kinds
of obsidian were fused
using CARLISLE bench
burner up to clear,
homogenized,
virtually
stress-free state. Two
fusing methods tested:
Low temperature fusing
with
propane/oxygen
0
flame at ~1000 C and
high temperature fusing
with
hydrogen/oxygen
0
flame at ~1500 C. At
0
~1000 C, a complete
homogenization reached
an essentially longer
fusing time (up to about
1 hour total time of multistep
fusing
and
annealing), but the final
material is virtually free
of gas bubbles. In the
higher
temperature
range,
the
optically
homogeneous state may
be produced during a
few minutes, but an
extensive gas emission
results
with
microbubbles formation, and
even more essential time
is required to produce
uniform, clear, stressfree glass. A simple
polaroscope was used to
check
the
residual
thermally
generated
stress. The mechanical
behavior
of
homogenized
fused
obsidians was evaluated
qualitatively.
3b
3. Experimental results Typical results of nano- and micro-indentation shown on Fig. 1-5 and Table 3. It was
found that the fracture of obsidians (Figures 2a, 4, 5) is strongly differentiated from both common glasses and
fused quartz. The differences are reflected in four aspects as follows: 1) Fracture pattern: Up to extremely high
loads of 13-15 N, the fracture in obsidian is predominantly two-dimensional (2D) superficial vs. quasi onedimensional (1D), deep radial cracks dominating in artificial glass (Fig. 2b, 3 a, b). In detail, 2D cracks in obsidian
usually propagate on ≤ 100µm level only and then arrested by certain micro-irregularities. Alternatively in glass,
the 1D cracks nucleate at loads <<1.0 N and propagate in radial directions from indent, evidently becoming
deeper with the load increase. 2) Fracture threshold: Two subsequent thresholds were found in obsidians: a)
~4.2N (mean value) for occurrence of superficial micro-chipping. No cracks propagation beyond of crackarresting stressed area found below or beyond of this threshold and up to the second threshold. b) ~14N for
radial crack propagation – over an order of magnitude exceeding that of artificial glass (~0.6N). 3) Microhardness: A complex micro-hardness behavior of obsidian revealed over the entire range of loads: at loads <
~1N, the Vickers micro-hardness approximately corresponds to mineralogical data 5.0– 5.5 GPa; at the higher
loads ≥ ~2N, the Vickers technique shows two differentiated features: superficial imprint corresponding to Hv=5-6
GPa, and deep imprint leading to hardness of Hv=6.5-9.5 GPa, thus approaching the quartz values. Accordingly,
the measured Hv values increase with the load increase, contrary to the normal behavior of glass. 4) Fracture
toughness: Based on the crack indentation pattern by the Lawn or Pharr methods [16, 17], the fracture
toughness of obsidian may be evaluated as Kk > 1.6 - 3.9 MPa√m. For a comparison, the fracture toughness of
artificial glass and fused quartz with the same indenter was also evaluated: a) Polished optical glass: Kk = 0.84
MPa√m, which reasonably corresponds to referred value Kk = 0.70 MPa√m [18]. b) Fused quartz: Kk > 0.64
MPa√m which corresponds to Kk = 0.58 MPa√m [18]. Fracture toughness of obsidian essentially exceeds the
values for polished optical glass and fused quartz. Distinctive mechanics of obsidian implies existence of some
topological hierarchy in its structure. In this research, the structures, intrinsic morphologies and compositional
distributions in various obsidians had been examined from over 10 certified locations in the USA, as well as two
random samples from Mexico and Brasilia. Certain intrinsic particularities of obsidian, which may be responsible
for its unique mechanics, are considered below.
Typically, obsidians are rich with inclusions, and their possible contributions in obsidian mechanics must be
clarified first. Figures 6-9 show represent a brief summary of examined varieties. Most inclusions (Fig. 6,7)
identified as the silica phases (such as quartz, cristobalite and moganite) and the silicate phases (like sodium
silicate), although some minor inclusions of complex silicate were not determined yet. The degree of crystallinity
in the a-matrix can be roughly estimated in term of the respective heights of XRD peaks from the a-matrix and
crystalline inclusions. Accordingly, the obsidians are arbitrarily separated into three categories: (1) Fig. 6(a): six
with low population of inclusions; (2) Fig. 6(b): six with medium population of inclusions; (3) Fig. 6(c): three with
high population of inclusions (note: the only purpose of this classification is analysis of possible correlation
‘structure vs. mechanics’; general classification of structure and inclusions of obsidian is out of scope of this
research).
A series of SEM images in Fig. 6 which were obtained from etched obsidians, reveal the abundant
features regarding both a-matrix and inclusions, including frequent morphological evidence of self-organizing
phenomena. Figs. 7 (a)-(c) exhibit common features of the nearly semi-transparent obsidians (like the smoky #14,
the jet-black #11 and the smoky area of the red/black #12). Fig. 7(a) displays small inclusions with typical size of
<0.2µm. The small inclusions are randomly distributed. Besides, some of inclusions display semi-clustering
patterns (see Fig. 7(b)). Some snowflake inclusions of alkali chloride crystals were also observed, as displayed in
Fig. 7(c). Figs. 7(d)-(f) are typical images from the black and dark-gray obsidians. Fig. 7(d) displays a high-density
of large inclusion clusters in the Rainbow #15. These large clusters with random distribution are virtually selforganized structure in the a-matrix. In other black obsidians, the similar clusters but in lower populations were
observed, as illustrated in Fig. 7(e). In addition, the rod-like inclusions, usually co-oriented in one direction often
found in the obsidian #7 - Fig. 7(f) give an example (chemical composition - Fe, Mg and Ca-rich – shown below
on Fig 9(d)). Figs. 7(g)-(l) are some SEM images from the red area of the black/red #12, the brown #6 or the pink
#13. Figs. 7(g), (h) and (j) reveal typical lined-up feature of large inclusion clusters, and Fig. 7(i) displays some of
clusters adopting the snowflake pattern. Fig. 7(k) shows the hierarchical structure of one large cluster and
different scales in its hierarchical arrangement. Fig. 7(l) shows groups of clusters, each consisting of even smaller
inclusions in turn. TEM observations further confirmed the hierarchical structure of inclusions down to several
nanometers in size. Fig. 8 shows an example: each inclusion embodied in one of numerous clusters detectible by
SEM is explored by TEM as actually a hierarchical cluster as well, embracing several of even smaller particles.
Chemistry analysis of obsidians was carried out through the SEM-EDS method. The standard fused quartz was
simultaneously inserted with the studied obsidians. The overall EDS spectra were collected over very large area
of samples, so reliable mean chemical compositions could be obtained from the analyzed samples with
inhomogeneous features. Fig. 9(a) shows an overall EDS spectrum of the rainbow #15 in comparison with that of
fused quartz standard. It is clearly seen that the O:Si ratio in peak heights as well as peak integral intensities of
the rainbow #15 is nearly half of that of fused quartz, implying the severe oxygen-deficit of silica in the rainbow
#15. A series of similar results of comparative overall EDS spectra were also obtained from other obsidian
samples. Table 1 summarizes the average concentrations of bulk elements Si, O, Al, Na and K respectively
according to standard quantification of more than 5 overall EDS spectra from each individual kind of obsidians. All
of the EDS results in Table 1 display that almost all of the 15 obsidians (except of #3, #8) are in different degrees
of oxygen- deficit with the lower O:Si ratios than that of fused quartz, whereas the other bulk elements - Al, Na, K
Fig. 6 Series of XRD patterns in the range of 19°-39° for all the 15 obsidians. In the XRD patterns, the
broadening peaks between 19° and 29° originate from the amorphous matrix (a-matrix), and the individual sharp
peaks from crystalline inclusions in a-matrix #1-15 classified in three groups: (a) low population of inclusions; (b)
medium population of inclusions; (c) high population of inclusions. Notes: 1) label q - quartz, c - the cristobalite; sthe sodium silicate phases. Note: this
classification is only for illustration of
varieties of specimen in this work.
Fig. 7 SEM Images from obsidians after
etching: (a), (b), (c) semi-transparent
obsidians: small inclusions; snowflakes; (d),
(e), (f) -black or dark-grey color obsidians:
large inclusion clusters;(g), (h), (i) – brown,
red or pink obsidians: lined-up of large
inclusion clusters; snowflake clusters; (j),
(k), (l) - brown, red, pink obsidians: semiordered
inclusions
and
hierarchical
structure on a large scale. Note:
“Snowflakes” on Fig. 7(c) is not silica
devitrification
but
chloride
crystals,
apparently, formed during lava solidification
100nm
Fig 8 TEM. Obsidian #12.
- in different obsidians are at the similar concentration levels. Worth to note, the absolute oxygen concentration by
SEM-EDS may need further verification by more precise analytical methods [19], but data for relative variation,
especially in the O:Si ratios between obsidian samples and standard fused quartz, may be considered as reliable.
The values of O/Si ratio fluctuate even between structural domains of the same obsidians, as demonstrated in
SEM-EDS line-scanning analysis of Fig. 9(b). SEM image in the left reveals feather-like domains in dark contrast.
Fig. 9 (a) Comparison of the overall
EDS spectra between the fused
quartz standard and the rainbow #15;
(b) The SEM-EDS line-scanning
analysis of the pink #13: Left SEM image reveals domains in
various contrasts; Right-top - the
SEM image with a scanning line
across the dark-contrast domain;
Right-bottom - profiles of the
counts of Si and O during line
scanning. (c) The EDS spotanalysis of the black #11: Spot 2 –
a-matrix; Spots 1, 3 and 4 –
inclusions (with Au coating to
avoid the charge effect) (d) The
EDS spot-analysis of the obsidian
#7: Spot 1 - a-matrix; Spot 2,4 the needle-like inclusions of MgCa-Fe-rich sodium silicate phase.
Fig. 9a,b clearly reveals a sharp
change of the O:Si ratio in counts
across the intrinsic border. This is
a common feature for all
examined silica matrices where
the contrast between certain
structural domains is observable.
Apparently, this phenomenon is
general
chemical-structural
feature
of
obsidian.
The
comparable chemistry of the amatrix and the inclusions in
obsidians was also investigated
by SEM-EDS spot analysis.
Although the inclusions exhibit varieties in different obsidians, they can be basically classified into two types in
st
term of their chemical analysis: (1) The 1 type of crystalline inclusions (most of the observed inclusions virtually
in every obsidian) is the silica or silicate phase in the oxygen-deficit state compared with the a-matrix. Such an
example can be seen in Fig. 9(c) obtained from the black #11. It displays that the inclusions in spots 1,3,4 have
similar chemistry as the a-matrix in spot 2, but the values of the O:Si ratio for inclusions are dramatically reduced
nd
to about half of that for the a-matrix. (2) The 2 type of crystalline inclusions (some inclusions adopted, but varied
in different obsidians) is so-called ‘foreign inclusion’ containing high concentrations of the foreign elements. The
“foreign inclusion’ could be the Fe, Ca, Mg-rich inclusions as shown in Fig. 7(f) or alkali chloride as shown in Fig.
9(c). Such an example of chemistry analysis can be seen in Fig.9 (d) obtained from the obsidian #7. It displays
that the needle or rod-shaped inclusions in spots 2, 4 are rich in the foreign elements like Fe, Mg and Ca in
comparison with the a-matrix in spot 1.
4. Discussion
It is important to point out prior to this discussion: all the presented results concerning the structure, morphology
and chemical composition of examined specimens of volcanic glasses are considered strictly at the aspect of
possible influence of the respective features on glass’ mechanics. Correspondingly, it is not assumed that any
revealed feature has independent significance until its mechanical influence is proved, and on the contrary:
features that are proved not to be significant for glass mechanics removed from the further consideration. Initially,
we considered two principle classes of particularities of obsidian which may be responsible for mechanic behavior
differentiating it from artificial glasses:
1. Innate properties of silica matrix, i.e. chemical variations and/or structural features; 2. Foreign inclusions.
Foreign inclusions. The foreign inclusions taking alone cannot be responsible for all revealed mechanical
particularities of obsidian, but at least in principle, they can work as crack-arresting features thus resulting with
higher threshold values of observable fractural phenomena and their predominantly 2D topology. Therefore, such
a possibility should be verified and proved or excluded first. The analyzed 15 kinds of obsidians expressed
2
broadly varied density, structure, chemistry, scale (from ~10nm to ~10 µm grain size) and distribution patterns of
foreign inclusions (Fig. 6-9) thus providing sufficiently strong base for a reliable verification. In fact, the
characteristic fracture behavior and pattern as described in the introduction were preserved in all of the examined
obsidians irrespectively to specific forms of inclusions (Figs. 7 and Fig. 9 show examples), while their density
varied from very high to nearly zero, i.e. some of examined obsidians were virtually free from foreign inclusions
observable by SEM and TEM techniques (the featureless images are not shown). Furthermore, both nano- and
micro- indentation in good consistency show that basic mechanical features of volcanic glasses have no
correlation with foreign inclusions (with possible exception for relatively small deviations in E and H values). In the
other words, the basic quantitative features of obsidians do not show any dependence on foreign inclusions. A
possible role of crystalline silica inclusions cannot be completely excluded; however, the nano-crystalline silica
(not shown here) expresses extreme brittleness with very low fracture threshold. Although the possibility of slight
dependence of specific values of mechanical properties on micro- and nano-crystals incorporated in silica matrix
should not be excluded, it is unlikely that the crystalline inclusions is common responsible for characteristic
mechanics of volcanic glass, and the further discussion will be limited with the intrinsic properties of matrix itself.
Innate properties of silica matrix. Nanoindentation shows the values of elastic modules of obsidian slightly inferior
to glass; hence, the E value cannot be responsible for the unique mechanics of volcanic glass. The higher value
of hardness cannot be responsible as well: quartz is harder than obsidian, but demonstrates much lower fracture
threshold. Hence, it is unlikely that the mean bulk characteristics are the cause of specific fractural behavior of
volcanic glasses. Basic chemical composition and related structural features of silica matrix as a probable cause
is discussed below. The content of both trace and bulk elements shown in Tables 2 and 4 basically correspond
to well known reference data for volcanic glasses, except that the average O:Si ratios in concentration were found
essentially lower than the stoichiometric value of SiO2, and this ratio is essentially and sometimes sharply varied
even inside the silica a-matrix of a given sample. In contrast, variation of other bulk elements such as Al, Na and
K is slight (except of the foreign inclusions). Basically, the oxygen deficient centers and clusters are very common
even in crystalline quartz, including crisotbalite in rhyolite and obsidian [20]. Indeed, formation of absolutely
uniform stoichiometric SiO2 during the magma solidification would be thermodynamically improbable. In nonstoichiometric SiOx (x<2), the entire range of O:Si ratio between 0:1 to 2:1 is possible, but most stable forms are
apparently grouped in three ranges in vicinities of stoichiometric values: at 0<x<<1; at x1, and at x2.
Practically, non-stoichiometric SiOx in proximity of x=1 is commonly used in electronics over half a century, and
more recently extensively developed for optimization of Si/SiO2 structures in the course of nano-scaling of
semiconductor devices [21-23]. Metallurgically, non-stoichiometric SiOx (x<2), particularly in combination with
alumina and other oxides, is able to produce numerous structures differentiating in glass transition temperature,
viscosity and diffusion coefficients as well as solid mechanics. In summary, this systematic study revealed two
common features for all of the 15 obsidians: (1) Structurally, the obsidians contain many hierarchical levels of
structural arrangement: from a few hundreds microns and up to several nm level (See SEM and TEM images in
Figs. 7-9); (2) Chemically, all the examined obsidians are characterized with relatively narrow range of aluminum
concentrations (about 7.0±0.9 at.%; Table 1) but generally oxygen deficient vs. pure fused quartz, and the O:Si
ratio essentially varied cross the observable intrinsic borders (like those separating various domains in Fig. 9(b)
or separating inclusion and a-matrix in Fig. 9(c)). Except of one sample shown on Fig. 7d and selected fields of a
few more samples where the density of foreign inclusions is too high and they mask the intrinsic obsidian
structures, the domain are detectable in nearly all specimens by contrast (as on Fig. 9) or by the closed contours
of certain inter-domain borders (Fig. 7 g, j,k,l are the examples). The exact nature of the domains is not finally
identified, but it correlation with oxygen content is evident (Fig. 9b gives an example). In most cases, there is a
clearly detectable hierarchical arrangement, in some instances fractal-like pattern of the domains; it may be
observed by changing the scale of observation (the sequence shown of Figures 7h7l9 gives an example; in
fact, many steps of magnification had been used to observe and actually revealed such hierarchical arrangement)
or observed even in one image obtained from selectively etched obsidian surface (Fig. 7k). Apparently, this
structural hierarchy is due to complex, stepwise post-volcanic processes occurring during the lava’ cooling. As
soon as the critical importance of this domain structure for the obsidian mechanics will be proved, the analysis of
its correlation with geological circumstances of obsidian formation will be the necessary following step.
The fractal geometry exists in numerous natural and synthetic materials [24]. The fractal structures in natural
rocks [25] and sandstones [26], manmade glass-ceramics [27] and polymers [28] were also studied by methods
of small-angle scattering and SEM, and moreover, fractal materials science has recently became a new
promising direction in the field of materials science and engineering. It is likely that such a hierarchical (fractal)
structure in obsidians is responsible for their unique fracture pattern since this kind of structure seems to be
particularly effective in arresting or deflecting the propagating micro-cracks. So far, some preliminary
observations support this assumption; however much more research is still needed to prove it.
The nature of this domain structure is not defined yet. The concept of coexisting structural units with fixed Si/Al or
even individual Si/Al ratio [29] may suggest a clue, especially combined with the fact that some properties of
silica-alumina glasses and melts can show minima or maxima in relationships to Al/(Al+Si) as well as pronounced
minimum or maximum values in certain range of the nonbridging oxygen per tetrahedrally coordinated cations
ratio (NBO/Si+Al); besides, some narrow range of aluminum content may correspond to the maximum values of
specific properties of glass [30, 31]; this may well match our observation that the fused obsidian does not express
any mechanical particularities of volcanic glass, even though the only small portion of aluminum lost as the result
of fusing. On the other hand, the domain structure is completely dissolving under such high temperature
homogenization, and this may be the key reason of drastic mechanical change.
The evidence of certain structural domains and correspondingly – inter-domain borders in obsidian is definite, and
their crucial influence on mechanical properties of volcanic glass is possible.
5. Conclusion
Preliminary examination of micro-mechanical features of obsidian reveals its strongly distinctive behavior vs.
various kinds of artificial glass, including common glass, fused quartz, as well as artificial glass produced by
fusing the same obsidian. In this research, characterizations of 15 kinds of obsidian by XRD, SEM, TEM, EDS
and XRF methods were carried out to define plausible cause of unique mechanics of volcanic glasses.
Structures and morphologies from various obsidians are varied, but in many cases the observable intrinsic
features are the internal borders dividing areas of certain inborn segregation, at least in some samples clearly
hierarchical on 3 to 5 levels, from a few millimeters or larger areas and up to clusters consisting of 3 or 4 nanograins of 2-5 nm size each. While the aluminum concentrations is only narrowly varied from sample to samples
and rather uniform in each of them, the O:Si ratio is generally lower than that of stoichiometric SiO2, and it is often
varied even in single sample of obsidian. In all samples scanned by SEM EDS technique across the deflected
micro-cracks, sharp change of the O:Si ratio was detected. The hierarchy of intrinsically inhomogeneous structure
of obsidian, associated with segregation of its major chemical components O/Si at the moderate level of
aluminum content, seems to have an explicit correlation with the unique mechanical particularities of obsidian.
Being proved by the further research, this would means that a weak domain structure produces strong
mechanical reinforcement of volcanic glass - a possible prototype for novel artificial glass.
Acknowledgements
At the final phase, this research was supported in part with NYSTAR. It is pleasure to express our appreciation to
Professor S.V. Babu for his inspiring encouragement. Authors are thankful to Mr. Brian Doran for help in
experiments with fusing obsidian.
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