The non-impact origin
of the Libyan Desert Glass (LDG)
Norbert Brügge, Germany
LDG with Cristobalite
The strewn field of the Libyan Desert
Glass (LDG) is located in the Western Desert of Egypt nearby the Libyan
border (part of the Great Sand Sea). The area is occupies with high
parallel sand dunes, which extend from north to south direction more
as hundred kilometers in length. As centre is assumed an area, which
is expands about 20 km from W to E and about 50 km from N to S around
the position at 25° 25' N and 25° 30' E. The occurrence of silica-glass
was documented for the first time by Patrick A. Clayton in 1932.
It is supposed, that on a plain of about 6500 km2 a mass of ~1400 tons
of LDG is distributed. The most productive locations therefore are directly
in the north of Gilf Kebir plateau.
The Libyan Desert Glass (LDG) is in its
chemical and physical characteristics absolutely unique and with no
other natural glass comparable (volcanic glass, Tektites and impact
glass). Nevertheless should be evidences for an impact origin the presence
of schlieren and partly digested mineral phases, and Lechatelierite
(a high-temperature mineral melts of Quartz, however at slight pressure),
and Baddeleyite, a high-temperature breakdown product of Zircon. But
the so characteristic inclusions of small crystals of alpha-Tridymite
and alpha-Cristobalite are missing in impact glasses. Also typical for
tektite are spherical or drops - formed aerodynamic forms.
There are however also differences between the LDG and the "classical"
impact glasses, mainly by the chemistry. LDG is a very silica-rich natural
glass with about 95.5 - 99 wt.% SiO2, and shows a limited variation
in major and trace element abundances. The degree of hardness (MOHS)
is 6, the specific weight is 2.2 g/cm3, the refractive index is 1.46.
The viscosity is essentially greater than at tektites. The melting point
is with 1727-1713° C more as 500° higher, than which other natural glasses.
The Desert Glass differs from the Tektites also by higher capacity of
water inclusions (0.050 - 0.200 wt.%). The colours of the LDG's varies
from light-yellow, honey-yellow, green-yellow, milky-white to black-grey.
At the surface deposited fragments are
polished often by the wind erosion, in sediment sticking fragments have
sharp corners, are not gleaming, and sand grains are stuck. In macroscopic
examination, the glass shows no impact tracks (JUX,1983), in some fragments
are however schlieren, which point to internal movements in the material
at high temperatures.Tiny, irregularly formed bubbles, and light- and
dark-brown bands can enforce the homogeneous glassy mass. Is the concentration
of bubbles high, the Desert Glass appears milky - white and is opaque.
Beside the bubbles different kinds of further inclusions are to be recognized.
To this (e.g.) belong smallest crystal-grains of Quartz similar minerals
alpha-Tridymite and alpha-Cristobalite.
MURALI et al. 1997; ROCCHIA et al. 1996; KOEBERL 1997 have found
that the contents of siderophile elements, such as Co, Ni and Ir, are
significantly enriched in some rare, dark bands that occur in some LDG
samples. KOEBERL (1997) studied such dark bands and found that the contents
of Fe, Mg, and Ni are high in the dark zones and low in the "normal"
LDG. TEM investigation of the dark streaks (PRATESI et al. 2002) also
revealed the presence of small amorphous Fe-rich silicate spherules,
within the silica-glass matrix, resulting from silicate-silicate liquid
Aggregats of alpha-Cristobalite
in the LDG
If the very hot SiO2-melt cools down, amorphous lechatelierite (Silica)
be produced at 1500-1000°C. During further cooling existing beta-cristobalite
be converted to alpha-cristobalite at about 272°C.
LDG with many
LDG with many unidentified
Inclusions of gas bubbles
and a drop of dark glass.
The elongated shape of the inclusions indicates a flow
of the glass during the gel state. Source: Richard de Nul
Flowing dark stripes (Köberl,
Brownish streaks and unidentified
inclusions (Köberl, Univ. Wien)
Dark streaks and bands
Very dark LDG
of LDG. Source: Roman Gozdzikowski
LDG. Source: exotica.com
dating of the Libyan Desert Glass by BIGAZZIi & De
This dating with a weighted
mean of 28.5 +/- 0.8 Ma
confirm the age (25.9 +/-0.4 Ma), the size corrected age (28.5
+/- 2.3 Ma), and the plateau age (29.4 +/- 0.4 Ma)
quoted by STORZER & WAGNER, 1977 in their list
A plausible thesis for the origin
of Libyan Desert Glass
(in sense FELLER, 1996)
On base of the not plausible sediment-hypothesis
(in sense JUX, 1983), whereupon LDG should have emerged in a sol-gel-process
at low temperatures in a lake, developed FELLER his hydrovolcanic-hypothesis.
He agrees with JUX therein, that LDG is a silica gel and not a melted
glass (tektite, volcanic glass), but the origin of LDG is to be explained
with hydrovolcanic processes*.
FELLER supposes, that faults have emerged in the area, which were expanded
up to 4000 m under the earth's surface. On fissures sour magma penetrated
at the earth's surface. In the cooling- and hardening-phase the magma
were produced great quantities at water, in which the silica from the
magma were accumulated.
At high concentration of SiO2 a gel-mass could develop. At lower concentrations
of SiO2 resulted first a precipitation. It could ripen then the gel
by water-secretion and contraction to the LDG. This theory becomes confirms
by most different minerals in the LDG, similarly like them also in volcanic
An agreement exists in expert
circles to the age of the LDG. Measurements based on the fission-track
method determined an age of 29 - 28 Ma (Oligocene).
The hydrovolcanic hypothesis must be specified. The process can
be described as an orthomagmatic hydrotherme. There are
magmatic-fluide solutions which have separated themselves from
the magma body. They are concentrated on top of the pluton. The
solutions can contain magmatic water, solved volcanic gases and
minerals. Because of the pressure conditions is this water also
with temperatures far over 100°C still liquid. Supercritical water
exists at conditions above its critical temperature (374°C) and
pressure (22 GPa).
For example can be also higher quantities of SiO2 in solution.
The crystallization is prevented during a rapid cooling (for example
during a quick ascent and outflow at the earth's surface). Modifications
of Quartz (e.g. Lechatelerite, beta-Tridymite, beta-Cristobalite,
which emerge in conditions of higher temperatures between 870
and 1470°C) and further high - temperature minerals (e.g. Baddeleyite)
are entries from the plume.
It had their root in the basaltic magma concentration in about
400-700 km depth with temperatures of about 1800°C.
This Basalt source also contains molten material from the subduction
During an ascent and the slow cooling of the hydrovolcanic solution
it comes then normally to the remaining crystallization in veins,
fractures and cavities.
Almost pure SiO2-rich hydrovolcanic solutions can harden consequently
under loss of water to an natural glass.
intermediate between purely hydrothermal and purely magmatic end
members and is best referred to as hydrovolcanic and/or subvolcanic
eruptions. Hydrovolcanic vent breccias are variably clast to matrix
supported, suggesting transport in a fluidized medium. Vent breccia
fragments are angular to subrounded, depending on distance and
velocity of transport and on the character of prior alteration
On the other hand, the term hydrothermal eruption refers to an
eruption, which appears to have been driven wholly by the energy
contained in the geothermal reservoir.
Hydrothermal eruption products show no evidence for a direct contribution
of magmatic energy.
remarks for new considerations and field researches
Marks this breccia with abundant
quartz the suspected main fault ?
The origin of the LDG is up to now unsolved
despite all efforts. The majority of workers favor an origin by impact.
There are however some differences to "classical" impact glasses. There
is also no credible references for an impact event in the region.
It is also not plausible, that a mass of ~1400 tons of clean glass is
produced by an impact event (or an air burst). It is not conceivable,
that for a so great quantity of LDG an ground-reservoir of pure silicium
- sand was available, which then was melted by an extraterrestrial event.
Besides the Sahara not exist
in the Oligocene age (!). The surface was dominated by consolidated
Devonian or Nubian sandstones.
The context between the localities of LDG, age of the Desert Glass and
the hydrovolcanic origin, proposed by FELLER, is it perhaps possible
to solve the mystery around the LDG:
There is definitive no significant
impact-structures in the region.
In contrary, in the Tertiary period
is a subvolcanic and/or orthomagmatic hydrovolcanic activity in the
region far spreading (Clayton
Craters, crater-field Gilf Kebir, Basalts on the Gilf-Kebir plateau,
crater and dykes in the Djebel Uweinat etc.).
The age of this volcanic activities
has been indicated to be 28.2 to 26.7 Ma. This age is conform to the
indicated age of the LDG.
The high plateau of the Gilf Kebir
probably is flanked by
A such fault is marked by Quartz clumps
of fissures (not documented unfortunately) and blocks by heat transformed
sandstones, flanked by an intensive red colored zone, at the eastern
borderline of the northern part of Gilf Kebir as well as
at the eastern borderline of the southern part of Gilf Kebir (documented).
Probably the eastern main-fault is
extended to the Jabal Zalmah (Dalma) in Libya and have then a great
proximity to the "strewn field" of the LDG. Within the strewn field
of Silica is an distinctive area. Here many blocks of sandstones and
breccias are to be found, which were subjected a great heat . A prominent
locality also is the Qaret el-Hanash with finds of
To clear up the things further, BARAKAT
as well KLEINMANN et al. found some shocked Quartz-bearing breccias
in the LDG strewn field. With it is postulates a connection to the
subvolcanic and orthomagmatic hydrovolcanic structures at the Gilf
Kebir and in the eastern direction of it.
Discoveries of micro-diamonds in rocks
nearby (BARAKAT) or rare elements such as Os, Ir, etc. in LDG (KOEBERL),
are no evidence for an extraterrestrial event. They can occur in the
earth crust or below of it.
Jasper from the Qaret-el -Hanash
Qaret-el-Hanash: Hill of fused
sandstone south of Silica
a subvolcanic gap with outflow of silica solutions (Jasper)
Location: 25° 04' 30'' N / 25° 56' 12'' E
Striking hill with fused sandstones
at the presumed Aqaba fault line
Place with fused sandstones at
the presumed Aqaba fault line
Position: 23° 58' N / 25° 37' E
Red volcanogenic zone with the
hill of fused sandstone
Baked and burnt sandstones in
the eastern direction of the Gilf Kebir (Crater field)
It is well possible, that an orthomagmatic-hydrovolcanic SiO2-rich
gel have climbed the earth's surface along an older main-fault in
the Tertiary period. This outflow was then hardened to Silica. These
SiO2-rich hydrovolcanic solutions were hardened consequently (under
loss of water) to an almost pure natural glass. Because of the fast
cooling was prevented further crystallization.
Partly digested mineral phases in the glass, the presence of high-temperature
minerals of Quartz, as well as Baddeleyite, a high-temperature breakdown
product of Zircon and other minerals
are entries from a
It had their root in the basaltic magma concentration in about 400
- 700 km depth with temperatures of about 1800°C.
The findings of "sub-micrometer diamonds in an amorphous, carbon-dominated
matrix" are a clear indication for the origin of the glass melt
at great depths.
The glass flow in unhardened condition had an unaffected contact
with the sediment. The glass contain organic (?) and sedimentary
remains. That is no good proof for an impact origin of Silica. The
significant contents of Fe, Mg, Ni an Ir in some dark bands
of Silica is no proof for the presence of a meteoric component.
The trace element Iridium in the LDG is also no problem. In the
Paleocene (including the K/T boundary) now worldwide in sediments
prolonged Iiridium concentrations were detected with three peaks.
The origin by volcanism is very likely.
What is still relevant
Dakhla Glass. The DG is not a glass, it is a volcanic andesitic
lava and rich of CaO and Al2O3, what normal is
by Norbert Brügge, Germany, Dipl.-Geol.
Research results of LDG
Donald Kasper, Lancaster, California,
about in this sense:
"I have master references of some volcanic
and "impact" glasses as well of the Libyan Desert Glass (LDG)
from the mid-infrared reflectance (Magna 560 spectrometer). The
result is that the LDG is not glass, not agate, opal, or jasper.
It does, however, very precisely match hyalite in infrared (right).
tektites - all
the glass and tektites match volcanic rocks.
For proper volcanic
identification, first we have to see a suite of additional 1200
cm region spectral bands not found in the literature due to the
limitations of using transmission infrared instead of reflectance
The LDG is a vapor-deposition of silica in volcanics. The comparison
hyalite specimen coming from the Czech Republic. The hyalite is
a glass-like opal, usually found associated with volcanic rocks,
has characteristics different from other types of opal."
A.G. Smallwood, P.S. Thomas and A.S. Ray, 2008
"Opaline silica that is deposited in crusts on volcanic rocks
by quenching from high temperature silica fluids. Opaline
silica, although a natural hydrous silica, has a much lower water
content and has properties that are much more silica glasses.
Examples of this type of silica are hyalite or the Libyan Desert
and dark streaks in Libyan Desert Glass
A. Greshake, Ch. Koeberl, J. Fritz and U. Reimold
Meteoritics & Planetary Science 45, Nr 6, 973–989 (2010)
Dark streaks and different types of inclusions in Libyan Desert
Glass (LDG) collected from the LDG strewn field in Egypt were
"The spherules generally occur as distinct single objects; only
rarely two or more are found in close proximity to each other.
They are separated from LDG matrix by concentric cracks and contain
a high density of internal cracks, which do not extend into the
surrounding glass. Sometimes these cracks are curved and delimit
rounded subgrains resembling some features of ballen silica, although
the rounding of subgrains is not as regular as normally observed
in ballen silica. Tiny bubbles are abundant in the spherules and
cause partial brownish staining. Viewed with cross polarized light,
the spherules appear completely birefringent and show pronounced
twin lamellae. Electron microprobe analyses indicate that the
spherules have, on average, higher SiO2 (approximately 99.8 wt%)
and lower major oxide concentrations (e.g., Al2O3, MgO, FeO, and
TiO2) than the surrounding LDG glass. Accordingly, they appear
slightly darker in BSE images compared to LDG matrix. Raman spectra
recorded on various spherules show pronounced peaks at 230 and
418 cm-1 and smaller peaks at 780 and 1076 cm-1,
which are diagnostic for alpha-cristobalite.
In some spectra the peak at 780 cm-1 is shifted to
793 cm-1, which might be attributed to the intense
"Brownish inclusions are irregularly shaped, elongated objects
that exhibit smooth contacts to the surrounding glass, with schlieren
extending from the inclusions into the glass matrix. Optical microscopy
reveals that the inclusions are composed of numerous very small
bubbles causing the brownish color and of subrounded to rounded
mineral grains displaying higher refractive indices than the surrounding
glass. While some of these phases appear as small roundish to
elliptical droplets, a few larger and more angular grains exist.
Generally, all phases within the inclusions appear isotropic when
viewed under cross polarized light.
Electron microprobe analysis revealed that angular grains and
droplets have very similar chemical compositions. All these phases
contain considerable amounts of silica (approximately 53–54 wt%
SiO2) with approximately 5–6 wt% Al2O3, approximately 23–24 wt%
MgO, approximately 14–15 wt% FeO, and minor concentrations of
TiO2, Cr2 O3, MnO, and CaO. Analyses also indicate poor stoichiometry
of the phases attesting to a low degree of crystallinity. These
compositions most closely resemble that of an Al-rich orthopyroxene.
According to X-ray elemental mapping, the angular grains and droplets
are compositionally homogeneous and do not show any chemical zoning.
Due to the high porosity of the two birefringent grains, their
electron microprobe analyses are also nonstoichiometric and yielded
totals between 95.5 and 99.1 wt%. Although containing slightly
less Al than the droplets, the overall composition of the birefringent
grains closely resembles that of the isotropic grains and droplets.
Raman spectra obtained from the different isotropic grains and
droplets are characterized by two broad features at approximately
570 and 960 cm-1 attesting to their amorphous nature
or very low degree of crystallinity.
In contrast, Raman spectra collected from the birefringent grains
show several distinct peaks, which are clearly recognizable despite
a strong fluorescence. The most prominent Raman bands are found
at ~340, ~385, ~464, ~655, ~674, ~839, ~935, and ~1001 cm-1.
According to systematic Raman studies of pyroxenes, these bands,
especially the splitting of the doublet in the 650–680 cm-1
region, are typical for orthopyroxene.
Compositions and Raman spectra clearly indicate that all pyroxene-like
phases observed in the brownish inclusions were at least partially
molten....and that no low-Ca, Al-rich pyroxene phenocrysts are
present in the glassy groundmass. Hence, the pyroxene-like phases
are clearly not co-genetic."
"The dark streaks investigated in this study appear as 0.3 to
about 1 mm wide wavy bands that continue for about 1–3 cm almost
parallel within the light greenish glass fragment. Intensity of
the brownish staining varies locally from weak to strong over
several millimeters. Optical microscopy revealed that these bands
resemble turbulent flow structures that do not contain any discernable
mineral phases at the millimeter scale. Differently shaped bubbles
are quite abundant within the streaks and sometimes appear as
nucleation sites for the bands. In high contrast BSE images the
streaks are clearly discernable and appear as curved, variably
gray-colored bands or as lenticular regions with pronounced flow
textures. Electron microprobe analyses and X-ray elemental mapping
of several regions within streaks show a strongly heterogeneous
distribution of Al, Si, Mg, and––more rarely and less pronounced––of
Fe and Ti. Generally, the Si-rich regions appear darker in BSE
imaging, whereas those with higher Al and Mg contents are brighter.
Variations in Al abundances allow tracking the flow structures
observed in BSE mode.
The Micro-XRF mapping clearly shows enrichments and good correlation
of the abundances of Mn, Cr, Fe, and Ni in the dark streaks. The
Ti-distribution also shows pronounced schlieren structures, which
appear, however, not related to the Fe maximum intensity. The
Zr distribution map shows the presence of distinct Zr-rich spots,
probably zircon or baddeleyite.
The concentrations of the elements Cr, Mn, and Ni were extrapolated
using the linear correlation of peak intensity and element concentration
determined in different standards and should be taken as semi-quantitative.
The concentrations for dark and light areas, respectively, are
for Ti 920 and 780 ppm and for Fe 4900 and 1160 ppm. In the dark
streaks the concentration of Cr is 170 ppm, that for Mn 74 ppm,
and for Ni 80 ppm; the concentrations of these elements are below
the detection limit in the light regions.
Raman spectra recorded from the dark streaks are featureless and
confirm that no crystalline phase is present in the bands."
Desert Glass: new filed and Fourier Transform InfraRed data
F. Fröhlich, G. Poupeau, A. Badou, F. X.
Le Bourdonnec, Y. Sacquin, S. Dubernet, J. M. Bardintzeff, M. Veran,
D. C. Smith, and E. Diemer
Meteoritics & Planetary Science, Volume 48, Issue 12, pages 2517–2530,
"All of LDG IR spectra exhibit the three IR
active absorption bands typical for condensed (SiO4) tetrahedra that
are predicted by theoretical considerations. The bands near 1100 and
471 cm-1 are
assigned to degenerated vibration modes of the (SiO4) tetrahedral
unit, respectively, stretching (n Si-O) and bending (d Si-O) (Lecomte
The n Si-O band is quite stable at 471–472 cm-1
for amorphous phases. The wave number of the m Si-O band is affected
by the surroundings of the unit tetrahedron (Lecomte 1949); for crystalline
silica phases (e.g., quartz, cristobalite, opal-CT), it is regarded
as the envelope of several, poorly resolved bands; then delta
n and I are less significant than for amorphous phases. For
LDG samples and fulgurite, the wave number of this band is not very
variable around 1103 cm-1,
whereas the wave number values for fused quartz and amorphous biogenic
silica are lower, respectively, 1101 and 1100 cm-1.
The absorbance, the half-width, and the integrated intensity vary
rather widely, whereas they are stable for crystalline phases.
The third, weaker band, near 800 cm-1,
is assigned to a vibration of the bridging oxygen atom in the plane
of the Si-O-Si bonds, between the adjacent tetrahedra (Lecomte 1949).
The relative motion results in a deformation of the Si-O-Si angle
in the direction of its bissectrix (Parke 1974)."
"In polished thin sections, LDG appears as a
homogeneous glass with spherical bubbles and mainly tubular cavities.
Sometimes they are at least partly filled with sand grains and/or
hardened silt. The millimeter-sized (and smaller) whitish polycrystalline
spherical inclusions visible inside the LDG volume were identified
by FTIR (and also by Raman spectroscopy) as cristobalite concentrations.
Other mineral inclusions were detected by SEM in BSE mode (backscattered
electrons) and determined by EDS. These included melted crystals (deduced
on the basis of their elliptical form) of monazite, a rare earth element
(REE) phosphate depleted or totally lacking in phosphorus as seen
on their EDS spectra, according to the three domains analyzed (in
wt%: Al = 0.2–0.8%, Si = 15.0–26.0%, O = 36.2–37.0%, P = <0.1–9.0%,
Ca = <0.1–0.4%, La = 8.4–9.9%, Ce = 14.0–16.9%, Pr = <0.1–1.6%, Nd
= 5.9–6.5%, Sm = <0.1–1.0%, Gd = <0.1–0.8%, Th = 3.8–5.8%), and zircon
(ZrSiO4) transformed into a mixture of baddeleyite (ZrO2) and lechatelierite;
both of these mineralogical processes require great heating."
Wadi Qubba LDG Occurrence
During a stop the expedition discovered some small centimeter-sized
LDG specimens of pink color. This new occurrence (N 24° 52.189´,
E 25° 27.044´) is located in the Wadi Qubba, about 50 km south of
the main LDG occurrences in the Great Sand Sea corridors. The molecular
structure is slightly different to specimen from the main occurrece.
Wadi Qubba specimens of pink color (!?)
in Libyan Desert Glass: Effect of microgravity environment
C. Patuelli, R. Serra, S. Coniglione, M. Chiarini
Microgravity and Space Station Utilization, vol. 3, no. 4, 2002
Samples of Libyan Desert Glass were analyzed
by X-ray micro-diffraction technique. It was identified fourteen nano-sized
crystalline LDG phases with different colours: Coesite, tridymite,
stishovite, baddeleyite, huttonite, yttrium, moissanite, platinium,
polymorphs of diamond and graphite.
The four praphite polymorh phases found in LDG samples can be explained
by taking into account that the graphite came from the earlier history
of the material. The element platinum is extremely scarce in most
crustal rocks. The origin of platinum is from ultra-mafic igneous
rocks. Its melting point is 1775 °C. The zircon oxide mineral Baddeleyite
is the product of the decomposition of zircon at 1775° - 1900°C. Moissanite
is a natural silicon carbide (SiC). Huttonite is a low-temperature
and low-pressure Thorite-polymorph (ThSiO2).
The identification of nine highbaric phases, the presence of hexagonal
diamond with four phases of graphite polymorphs, as well as huttonite
and baddeleyite, confirm that LDG formed by "shock metamorphism" at
very high pressure and temperature. The nano-sized crystalline phases
revealed point out that LDG rapidly solidified.
Preliminary X-ray micro-diffractometry analyses were presented at
the “Silica 96” workshop (Patuelli, 1997). High pressure and high
temperature phases were identified: including samarium, germanium
12T, thorium beta (this beta phase occurs only at a temperature above
1350°C) and stishovite, which is a high temperature and pressure form
Silicate-Silicate liquid immiscibility
and graphite ribbons in Libyan Desert Glass
G. Pratesi, C. Viti, C. Cipriani, M. Mellini
Geochimica et Cosmochimica Acta, March 2002
Transmission electron microscopic (TEM) investigation of the dark
(brown or bluish) streaks occurring in Libyan Desert Glass reveals
the common presence of small glass spherules. The spherules,
mostly 100 nm in size, are homogeneously dispersed within the silica-glass
matrix. The complete absence of electron
diffraction effects confirms their amorphous nature. The spherules
are Al-, Fe- and Mg-enriched with respect to the surrounding silica
matrix and their (Mg+Al+Fe) : Si ratio is close to 1. The silica-glass
matrix and amorphous spherules form an emulsion texture (i.e., globules
of one glass in a matrix of another glass), which originates from
silicate-silicate liquid immiscibility.
The silica glass also contains carbonaceous inclusions consisting
of 5–50 nm thick, polygonalized graphite ribbons that form
closed structures up to 200 nm in diameter.
3.3. Graphite inclusions
Rare carbon-bearing inclusions randomly occur within the silica-glass
matrix. Their presence is ubiquitous and not limited to the regions
with dark streaks. C-bearing inclusions produce an evident C peak
in the EDS spectra. The inclusions consist of ring-shaped polygonalized
ribbons, 5–50 nm thick, typically forming closed structures with an
overall diameter of ~200 nm. Their SAED patterns consist of rings
with d-spacings of 3.35, 2.09 and 1.67 A: these values correspond
to graphite (3.36, 2.13-2.03, 1.678 A, JCPDS 23-64). Lattice imaging
shows 3.35 A polygonized (002) lattice fringes. The fringes are quite
regular, thus indicating good structural order with no evidence of
deformation or defects. Dark vertical bands are evident at the polygonal
edges and are interpreted as Moire textures.
and gas content in dark schlieren of Libyan Desert Glass
M. C. Bölitz &
F. Langenhorst, Bayerisches Geoinstitut, Universität Bayreuth, Germany
In this study we have focused on the chemical
and textural characteristics of dark schlieren. Our investigation
aims at obtaining further information on the cooling history and precursor
material of LDG.
Backscattered electron (BSE) images were taken on the SEM and microprobe
in order to monitor chemical variations in LDG and to detect tiny
inclusions. Schlieren-free, bulk LDG samples show only slight variations
in chemistry, with two different grayscales in BSE images. Dark grey,
lens-like areas consist of almost pure SiO2 and thus represent lechatelierite,
the melt product of quartz. Bright grey areas have on average a SiO2
content of 98 – 99 wt.%; additional minor elements are Mg, Fe, Al,
Ca, K, and Na.
Compared to the bulk glass, the brownish-black schlieren in LDG are
distinctly enriched in Mg, Fe, and Al; the concentrations of measured
trace elements such as Ti, Ni, Cr, and La are equally enhanced. In
some parts of schlieren the SiO2 content decreases down to 86 wt %.
Within the dark schlieren we observe furthermore distinctly larger
chemical variations than in the bulk glass sample.
BSE images indicate that there are two types of dark schlieren. One type of
dark schlieren consists exclusively of tiny, mostly 100 nm in size,
glass spherules. This type of schlieren have been previously described
in a transmission electron microscopy study, as well. In comparison
to the glass matrix, the spherules are enriched in Al, Fe, Mg, and
Ni and depleted in Ca.
The other type of dark schlieren displays flow structures and large,
up to 25 μm diameter glass spherules. The overall texture of these
schlieren indicates an immiscibility of two silicate liquids. To detect
the miscibility gap, the chemical compositions of spherules and the
surrounding groundmass in dark schlieren have been measured with the
microprobe and are plotted in a ternary MgO-Al2O3-SiO2 diagram. The
analytical data define a trend that deviates from the known stable
miscibility gap along the MgO-SiO2 join. Instead, the data points
follow closely the Al2O3-SiO2 tie line, along which a metastabile
miscibility gap has been described.
Microprobe analyses of spherules and ground-mass in dark schlieren
of LDG. The ternary plot shows also the known stable miscibility gap
along the MgO-SiO2 join.
LDG samples with and without black schlieren were stepwisely heated
up to 1450°C. In both samples, bursting bubbles released mostly H2O
and CO2. The black schlieren contain however one order of magnitude
more H2O and CO2 than the bulk silica glass. Another difference between
black schlieren and bulk glass concerns the temperatures of gas release.
For example, CO2 is released from black schlieren in two temperature
intervals between 250° - 300°C and 450° - 550°C. In the bulk sample,
CO2 is however only liberated in the upper temperature interval between
420° - 650°C.
The data presented here provide hints to
the cooling history of LDG and the precursor material of dark schlieren.
Microprobe data of glass spherules and surrounding matrix in dark
schlieren indicate that the compositions of the two immiscible silicate
liquids are close to the Al2O3-SiO2 join. According to experimental
studies, this binary system displays only metastable immiscibility
for very rapid cooling of the melt, as it is expected for LDG.
DEGAS analyses reveal that dark schlieren are
distinctly richer in volatiles, particularly in H2O, than bulk LDG.
It is thus likely that the precursor material might have contained