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.
  

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. clay 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
 

Fused (?) and  polished debris from the stony field

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 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 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 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 (Also already observes by CLAYTON in 1932.

• 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 a fault line ?
Location: 25° 20' N / 25° 36' E

Qaret el Hanash: Hill of fused sandstone south of Silica
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)

Quartz clumps, were found at the presumed main fault at the eastern site
 of the southern plateau
Position: 23° 39' N / 26° 23' E

What is still relevant

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.
Summary
The chemical composition of silica glass (LDSG) from the Western Desert of Egypt (Great Sand Sea) is characterized by the high content of SiO2 (~ 98%), considerable concentrations of Al2O3 (~ 1 %) yet only small amounts of alkalies and earth alkalies (0,01-0,03 %). New analytic results confirm a rather uniform composition of all LDSG samples studied. As a rule water contents are rather high (~ 0,1 %). Most of the corraded or otherwise polished desert glass has a greenish-yellowish, slightly opalescent transparency. Except some weak reflexes of Cristobalite and Tridymite the x-ray diffraction pattern reveals an amorphous composition that lacks definite crystalline structures. There are just a few dark or light varieties with varying amounts of included Cristobalitic spheres (~ 1 mm), thus showing tendencies for ordered internal arrangements. Brownish or dark LDSG includes both saturated and unsaturated hydrocarbons which are marked by a noteworthy share of isoprenoid compounds. Neogene microfossils, mainly plant tissues and sporomorphs, could be identified from macerated samples as well as in chips (artefacts!) 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. Geologically this conclusion is confirmed by primary depositional occurrences of LDSG. With its coarse, sandy crusts, desert glass is found in fissures (desiccation cracks) of neogene, lacustrine deposits (gravel, sands, clays). The adjacent rock shows no zone of reaction, however, it coined the original sculpture of LDSG, its shape being defined by the crevices.

1

2

3

4

1) This rarely clumps of "dark" LDG documents, that Silica was original in viscous to liquid condition. There evidently are included parts of sediments.
2) Large piece of LDG. Source: Roman Gozdzikowski
3) "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
4) Laminated LDG !, Source: exotica.com

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 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.

Liquid immiscibility and gas content in dark schlieren of Libyan Desert Glass
M. C. Bölitz  & F. Langenhorst, Bayerisches Geoinstitut, Universität Bayreuth, Germany
http://www.lpi.usra.edu/meetings/lpsc2009/pdf/2018.pdf 

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.