The non-impact origin of the Libyan Desert Glass (LDG)

March 2006
Norbert Brügge, Germany

last update: 10.10.2015

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 single 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 (STORZER & KOEBERL,1991). But the so characteristic inclusions of small crystals of Tridymite and 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 (KOEBERL,1994). 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. To mention are Al2O3, MgO, Na2O, K2O, CaO, FeO and TiO2. All other elements (e.g. the group of the rare earth elements) occur only as trace. 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 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 Tridymite and Cristobalite.

Cristobalite and Tridymite have high and low forms. Low Tridymite is orthorhombic and pseudohexagonal, low Cristobalite is tetragonal and pseudo-cubic.
The equilibrium formation temperature of Cristobalite is 1470°C. The equilibrium formation temperature of  Tridymite is 870°C.

Cristobalite aggregats in the LDG

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 immiscibility.

Inclusions of organic substances have been found (Ulrich JUX, 1983).

Zusammensetzung und Ursprung von Wüstengläsern aus der Großen Sandsee Ägyptens.
Zeitschrift der Deutschen Gesellschaft für Geowissenschaften, Band 134. p. 521-553, 4 fig. , 2 tab. , 4 pl.

"Brownish or dark LDSG includes both saturated and unsaturated hydrocarbons which are marked by a noteworthy share of isoprenoid compounds. .... microfossils, mainly plant tissues and sporomorphs, could be identified from macerated samples as well as in chips and thin sections. From this follows a terrestrial origin of LDSG. This agrees well with other analytical results, especially of the gas extracted from bubbles in milky glass."

: Unfortunately, the findings of Prof. Dr. Jux were not accepted, and thus the archived material was still no further testing has been undertaken. Various further authors have claimed to find organic inclusions within the LDG (e.g. Adolphe et al. 1997)


Cristobalite ?


LDG with many unidentified inclusions

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

LDG with many unidentified inclusions


Flowing dark stripes (Köberl, Univ. Wien)

Brownish streaks and unidentified inclusions (Köberl, Univ. Wien)

Fluidal structure

Dark streaks and bands


Mysterious inclusion

  Large piece of LDG. Source: Roman Gozdzikowski

Laminated LDG. Source:             



Fission-track dating of the Libyan Desert Glass by BIGAZZIi & De MICHELE, 1995


Sample 1

Sample 2 Sample 3 Sample 4
Heating Age (Ma) +/- Age (Ma) +/- Age (Ma) +/- Age (Ma) +/-
ambient 26.3 1.4            
ambient 27.0 2.0 26.0 1.8 29.0 1.8 28.8 1.9
220°C 28.0 1.9 28.2 1.4        
270°C 30.2 2.0 25.3 1.3     28.7 1.9
320°C     29.4 2.0     29.4 1.8
370°C 28.5 2.5 27.9 2.2     28.4 2.2

 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


Hydrovolcanic  Hypothesis (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 waters occur.

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 (first crystal growth of beta - Quartz at approx. 500°C). Other modifications of Quartz (e.g. alpha - Quartz, Lechatelerite, Tridymite, Cristobalite, which emerge in conditions of higher pressures and temperatures of up to 1500°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 zone.
During an ascent and the slow cooling of the hydrovolcanic solution it comes then normally to the remaining crystallization in veins, fractures and cavities.
The further crystallization is prevented however during a rapid cooling (for example during a quick ascent and outflow at the earth's surface).
Almost pure SiO2-rich hydrovolcanic solutions can harden consequently under loss of water to an natural glass.

*Eruptions 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 and silicification.
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.

Some 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 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 old fault-lines.

  • A such fault is marked by Quartz clumps of fissures (not documented unfortunately) and blocks by heat transformed sandstones at the eastern borderline of the northern part of Gilf Kebir as well as Quartz clumps 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 Jasper.

  • 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

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

Hill detail 1

Hill detail 2

Hill detail 3

Hill detail 4

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 of Quartz.
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 (e.g. hexagonal Diamond with four phases of Graphite polymorphs)
are entries from a basaltic 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 zone.
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 and 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.

Note: The origin of the Chicxulub crater in the Yucatan is also misinterpreted. The proof of an andesitic magma in the crater indicates a supervolcano event.

 What is still relevant


An other sample is the Dakhla Glass. The DG is a dark volcanic variant of the Libyan Desert Glass and rich of CaO and Al2O3.


New: A not credible message about the discovery of cometary material in the Egyptian desert (Libyen Desert Glass area)
        by Norbert Brügge, Germany, Dipl.-Geol.


New: Research results of LDG genesis

Donald Kasper, Lancaster, California, wrote me 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).
Volcanic glass (obsidian etc), alleged impact glass, 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 used here.
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 Glass."


Libyan 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, December 2013

"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 1949).
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 (!?)

Crystalline microstructures 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 as a result of an "impact event". 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 of SiO2.

Graphite polymorphs

Liquid immiscibility 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 hydrous phases.

Investigation of inclusions trapped inside Libyan Desert Glass by Raman microscopy
Marcel Swaenen, Elzbieta Anna Stefaniak, Ray Frost, Anna Worobiec and Rene Van Grieken
Analytical and Bioanalytical Chemistry, 397 (7), pp.2659-2665  (dead link)

Several specimens of Libyan Desert Glass (LDG), an enigmatic natural glass from Egypt, were subjected to investigation by micro-Raman spectroscopy. The spectra of inclusions inside the LDG samples were successfully measured through the layers of glass and the mineral species were identified on this basis. The presence of cristobalite as typical for high-temperature melt products was confirmed, together with co-existing quartz. TiO2 was determined in two polymorphic species, rutile and anatase. Micro- Raman spectroscopy proved also the presence of minerals unusual for high-temperature glasses such as anhydrite and aragonite.

As it was mentioned before, MRS is very convenient to analyse inclusions in transparent samples without damaging them. First we measured Raman activity of glassy matrix.
Fig. 2 shows a piece of LDG together with a Raman spectrum of its bulk material. Such a spectrum has been previously published. The glassy matrix does not give a distinct Raman spectrum, just irregular background. However, a closer look at the spectrum in Fig. 2 allows us to distinguish some vibrational bands, also described elsewhere. Two typical broad bands – one around 480 cm-1 and the other at 820 cm-1 – are typical for glassy silicate materials. The range 400-600 cm-1 is ascribed to bending in and between the SiO4 tetrahedra associated with cationic motions; the range near 800 cm-1 is the symmetric motion of adjacent Si atoms with respect to a bridging oxygen (Si-O-Si).......
The Raman band marked with a red arrow (Fig. 2) should be emphasised, hence this sharp (although not intense) peak would indicate the presence of fullerene. It was a small black spot deep under the surface; however we managed to record the spectrum. Although it seemed quite incredible for us that fullerenes could be found inside LDG, it has already been mentioned in the literature.
Figs. 3 to 8 present the other inclusions analysed by MRS. Cristobalite is the silica form most often reported as a component of LDG. It is widely assumed that its presence proves the temperature of glass quenching as above 1470°C. The shape of cristobalite inclusion presented in Fig. 3 is typical for LDG, with an almost ideally spherical form. However, we also found cristobalite as crushed, irregularly shaped inclusions (Fig. 3). Moreover, its Raman spectrum is fairly disturbed by luminescence, probably due to a presence of transition metals or REE. As for other polymorphs of silica, tridymite is stable below 1470°C and above 870°C, while quartz – below 870°. However, the presence of one form does not exclude the other one, which is clearly shown in Fig. 4. The Raman spectrum gives an undeniable confirmation of quartz being present in LDG; the question remains whether it is a primary or secondary mineral. In a cristobalite structure, each SiO4 tetrahedron shares its oxygen atoms with adjacent tetrahedra; how likely it is then to convert cristobalite into quartz within 29 million years? A similar system of two TiO2 polymorphs is presented in Fig. 5 and Fig. 6. Rutile is a common accessory mineral formed in high-temperature and high-pressure metamorphic rocks. Anatase (and brookite) are less stable and revert to rutile at low temperatures (anatase at 915°C and brookite at 750°C). Considering that both rutile and anatase can be found as LDG inclusions, it leads to a doubt whether the temperature of cooling down was indeed so extremely high.
The spectrum shown in Fig. 6 belongs to zircon – a mineral also recognized as inclusion of LDG, however more often as a precursor of baddeleite (ZrO2), which is a product of thermal decomposition of ZrSiO4. In our investigation we didn't find any traces of baddeleite, but we definitely confirmed the presence of zircon, which would speak in favour of rather low or medium temperature of quenching.
The two inclusions pictured in Fig. 6 are even more mysterious than all described so far. Here we have two representatives of minerals which usually form sedimentary rocks..... It would be very reasonable if these two soft minerals were found in the holes or cracks of our LDG specimens. However, these minerals were detected inside LDG objects, through a glass layer. In other words, these minerals must have appeared during the formation of LDG.

The inclusion in Fig. 7 was quite surprising as well. It is composed of at least two different phases – the white one, which MRS spectrum was only a big fluorescence hump, and the brownish one, which apparently contains amorphous carbon. MRS spectra in Fig. 7 represent different spots within the brown area. The two characteristic D and G band shape proves doubtlessly that carbon was there, but it doesn't give any explanation how organic matter could be preserved in hot molten silica.
There were also some inclusions which were difficult to determine by MRS; an example is given in Fig. 8. The white objects occurred a few times in the specimens; however, each time MRS spectrum was compromised by the fluorescence.

Fig. 3
Raman spectrum of Cristobalite and a disturbed Cristobalite

Fig. 4
Raman spectrum of Quartz within two inclusions

Fig. 6
Raman spectrum showing inclusions of
Anatase, Zirkon, Anhydrite and Aragonite

Fig. 5
Raman spectrum of a Rutile inclusion

Fig. 7
Raman spectrum showing amorphous Carbon
in one of the inclusions

Fig. 8
Raman spectrum of an unidentified inclusion

Spectroscopic integrated method in the way towards the search of the origin of Libyan Desert Glass
Leticia Gomez-Nubla et al, Department of Analytical Chemistry, University of the Basque Country
Revista de la sociedad española de mineralogía, macla nº 16. junio ‘12

"Macroscopically analyzed LDG are translucent glassy matrix with brownish spherical inclusions located all over the external surface before slicing. However, after the lamination of the samples, it could be observed that in the inner part the brownish inclusions were not discerned, whereas whitish inclusions could be easily perceived. Microscopically, it could be noticed little dark inclusions.
Raman spectroscopy allows determining the mineral phases of the inclusions. Comparing the results obtained from the inclusions of the surface and those obtained from the inner inclusions, it could be noticed important differences.
Apart from the silica mineral phases such as cristobalite and quartz, other minerals were found in the inclusions distributed through the matrix of the LDG. In the inclusions located on the surface of the samples iron compounds were not detected, whereas they are the main component of the inner ones. In contrast, compound such as corundum is only present at the surface.
Besides, they were identified mineral characteristic phases of the LDG impact (high pressure) and of LDG settled state (low pressure). The presence of calcite suggests a possible transformation of high pressure aragonite to the low pressure calcite both detected in the present study.
Other couples of minerals similar in composition are also detected. In this way, sulphates as anhydrite and gypsum were detected. Anhydrite corresponds to the anhydrous phase of gypsum. During weathering, hematite can be transformed into a mixture of hydrated iron oxide-hydroxide known as limonite.
In the case of silicates, the identified cristobalite could be formed during high temperature, since it is a hightemperature mineral; in contrast quartz is the cristobalite low-temperature polymorph. High temperature mineral phases such as microcline and anatase were distinguished.
By Raman spectroscopy respective low temperature polymorphs were not found. However, by optical microscopy of fine rutile microlites can be observed. This phase can correspond to the transition of the anatase with high temperature.
In order to complement the previous results it was carried out an element mapping using SEM-EDS. In this way it could be seen how the elements are distributed in the inclusions. In Fig. 1 the first image is corresponded with the SEM image of an inclusion of LDG. The other images are elemental maps done with EDS over the same inclusion. In those maps, the white colour represents the presence of an element and the black the absence.
It can be seen that the inclusions are not formed by only one compound. The inclusion analyzed in the image, around the big cavity, is composed mainly by silicon but some elements are concentrated in certain areas, such as: C, O, Mg, Al, K, Ca, Ti and Fe.
On the one hand, calcium and carbon maps are very similar. The place where the carbon and the calcium are more concentrated is the same. This fact supports the presence of calcite and/or aragonite, both determined by Raman.
On the other hand, silicon, iron and aluminium compounds appear similarly place and are in concordance the mineralogy detected by Raman spectroscopy. This chemical and mineralogical heterogeneity was observed in all the analyzed samples.
This work is an approach to the knowledge of the origin of the Libyan Desert Glass. It can be concluded that the silica is the main component in LDG glassy matrix. Inclusions are composed by different compounds and therefore are chemically more heterogeneous.