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.
 

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 (JUX, 1983). Inclusions of sedimentary fragments (e.g. clayey drops) are not rare. That are clear proofs for it, that the glass mass in unhardened condition had contact with the unaffected sediment. That is no good proof for an impact event.

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.

Some remarks for new considerations and field researches
 

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. 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 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 Gilf-Kebir plateau, basalts of Djebel Uweinat etc.).

• The age of this basaltic magmatism 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 (Bretonian event ?).

• 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.

• 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 (Also already by Clayton, 1932 observes).

• To clear up the things further, BARAKAT (2001) and KLEINMANN et al. (2001) 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.

Are the stony fields - between the western dune tracks -
 remains of a dome structure at the fault line ?
Location: 25° 36' E and 25° 20' N

What is still relevant

            
By heat transformed sandstones at the eastern borderline of the northern part of Gilf Kebir (nearby Aqaba passage).
Position: Longitude 25° 38' E; Latitude 23° 36' N
 

"The following 1/2 kg "impact glass" is a bit odd. It is not the usual translucent yellow that tektite fanciers expect to see. It is probably unlike any specimen of this type that you have ever seen."
Source: Calvin Shipbaugh

Crystalline microstructures in Libyan Desert Glass: Effect of microgravity environment
C. Patuelli, R. Serra, S. Coniglione, M. Chiarini
Source: 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 polymorphs.
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.
                                                                                                                                              

Evidence for shock metamorphism in sandstones from the Libyan Desert Glass strewn field
B. Kleinmann, P. Horn and F. Langenhorst
Meteoritics & Planetary Science, vol. 36, no. 9,  2001

In five hand specimen from the LDG strewn field do not show signs of faulting or shattering or other disturbances. The samples are whitish to yellow in colour, compact and hard and present a quartz-rich fine-grained sandstone (orthoquartzite).
Although these sandstones do not seem to be shattered or otherwise disturbed, the microscopic analysis presented here reveals evidence for all degrees of shock metamorphism in the quartz grains.
The microscopic study of the five samples shows that this, mainly orthoquartzitic, sandstone is more or less brecciated.
From their textures, samples S2 and S4 are relatively similar to each other, but differ from S3 in that they show clear evidence for in situ crushing. Quartz grains exhibit conchoidal fractures, which dismember the grains into elongated splinters with extremely sharp edges (sample S2). It can be seen that this sandstone is equally very dense with a low porosity. These samples S2 and S4 are best described as a monomict breccia.
A higher degree of deformation can be observed in samples S1 and S5. In both samples, the quartz grains are mainly irregular in shape, intensely fractured and have an undulatory extinction. The matrix, in a higher proportion than in the other samples, is the product of extensive brecciation into quartz splinters and minute dust-like debris.
In both samples, distinct rhombohedral cleavages are present in quartz (S5). Furthermore, the quartz grains frequently show undulatory extinction, mosaicism and patchy isotropization (S1). Single or multiple sets of planar deformation features (PDF) are quite common in the larger quartz fragments (S1).
Neither glass phases nor high-pressure polymorphs of quartz have been detected in any of the thin sections. Some 90% of the larger quartz grains in the most affected samples (S1 and S5) showed at least one of the features.
The transmission electron microscope (TRM) was used to characterize the planar defects. Conventional bright-field and dark-field imaging and electron diffraction were performed on a field emission gun (FRG) Philips CM 20 STKM. The TEM observation focused on quartz grains in sandstone (S-1), which according to optical inspection shows the strongest shock signature. Quartz grains from this sample display various signs of shock-metamorphism; planar deformation features (PDFs), mechanical Brazil twins, and shock-fused silica glass.
The thin and straight PDFs contain numerous dislocations and some voids. The latter occur preferentially at intersections of crossing PDF sets. The PDF spacing is variable. The PDFs are predominantly oriented parallel to planes of the rhombohedral form {1011}, which is the typical PDF orientation in quartz from sedimentary rocks.
Mechanical Brazil twins occur exclusively parallel to the (0001) planes of quartz. In TEM images, they are visible as fringes that are interrupted by partial dislocations.

Remark: Further proofs for it, that subvolcanic and hydrovolcanic activities in the area of the LDG strewn field occured, like in the area eastern of the Gilf Kebir. Similar sets of PDF's in quartz grains were found also in this large crater field.