Fragments of so-called Dakhla Glass appear in clumps of ancient lake sediment.
 Photo by A. Haldemann

A beautiful large compact piece

My opinion:
The Dakhla Glass (DG) is a volcanic glass. The high capacity of CaO and Al2O3 points to a basaltic eruptiva. The Dakhla Glass (DG) has emerged at the same time like the Libyan Desert Glass. The Libyan Desert Glass (LDG) is probably orthomagmatic-hydrovolcanic of origin and have an age of ~28 Ma.
Both glasses differ however by the chemistry. The DG is volcanic or subvolcanic of origin. (see).
Between the Dakhla region and the Gilf Kebir plateau in the southwestern direction exists an immense amount of examples for subvolcanic activities in the tertiary period (see). This region is rich of craters, basaltic plugs and dykes, which contain melted breccias as well as fractured quartz grains. The occurrence of DG jointly with fractured quartz grains points to it, that the place of the source is to be found in this large crater field of the "Contour" in the southwestern direction. The Dakhla Glass after transport and destruction was deposited in a new position. Now it is found as clumps in younger sediments nearby Dakhla. It is not to be excluded however, that the source is nearby.
Hydrovolcanic events are to be found everywhere in the region, also in the White and Black Desert in the north (see).

http://www.lpi.usra.edu/meetings/volatiles/pdf/3038.pdf

Four main places to find Dakhla Glass

Gilf Kebir Craters Field

THE DAKHLEH GLASS: Product of an impact airburst or cratering event in the Western Desert of Egypt ?

Gordon R. OSINSKI -- Departments of Earth Sciences/Physics and Astronomy, University of Western Ontario, Canada,
Johanna KIENIEWICZ -- Department of Geosciences, Denison University, Granville, USA
Jennifer R. SMITH -- Earth and Planetary Sciences, Washington University, Saint Louis, USA
Mark B. E. BOSLOUGH -- Sandia National Laboratories, Albuquerque, USA
Mark ECCLESTON -- Archaeology Program, La Trobe University, Australia
Henry P. SCHWARCZ -- School of Geography and Earth Sciences, McMaster University, Hamilton, Canada
Maxine R. KLEINDIENST -- Department of Anthropology, University of Toronto at Mississauga, Canada
Albert F. C. HALDEMANN -- European Space Agency, ESTEC HME-ME, Noordwijk ZH, Netherlands
Charles S. CHURCHER -- Department of Zoology, University of Toronto, Canada

Source: Meteoritics & Planetary Science 43, Nr 12, 2089–2107 (2008)

"Dakhleh Glass was first observed during regional archaeological and palaeoenvironmental surveys in the 1980s and 1990s (Kleindienst et al. 2006), with 6 known occurrences (Osinski et al. 2007). In 2007, we carried out a systematic search and discovered DG at 140 different locations throughout the Dakhleh Oasis, covering an area of 40 × 10 km. Based on this new work, it is clear that DG occurs in 4 main settings: (1) as a lag deposit on the deflated surfaces of Pleistocene lacustrine CSS; (2) in situ within the same sediments; (3) as lag deposits on Taref Formation sandstone surfaces in close proximity to the Pleistocene lacustrine sediments, but in areas where these sediments have been completely removed; and (4) redeposited on, or into, Holocene pan sediments. Searches of both older Quaternary formations and younger alluvial terraces yielded no DG fragments. Thus, an important observation is that the DG glass appears to be spatially associated with Middle Pleistocene lacustrine sediments. Recent fieldwork shows that these sediments were deposited in a lake with a maximum estimated extent of 1735 km2."
"DG is typically black when fresh and greenish-grey when weathered. Our new field observations indicate that individual specimens of DG vary markedly in terms of vesicularity, which is often a reflection of the size of the individual specimens. Smaller samples are typically highly vesicular; whereas larger masses, up to several kg and 30–40 cm across, are partially to fully crystalline. The majority of the large masses are flattened. Several of these flattened masses possess irregular but moulded lower surfaces and flat, vesicular upper surfaces, with an increase in vesiculation upwards. This gradient in vesicularity represents a clear way up criteria and suggests that these larger samples ponded in small topographic depressions with enough time to crystallize and release a volatile component. Many of the larger pieces are whole masses and not fragments broken from larger blocks consistent with lack of significant movement/erosion following deposition. As noted previously, an unusual characteristic of the DG is that approximately one-third of DG specimens studied display impressions or “pyromorphs” of reed-like stems or leaves on the underside—or more rarely on other surfaces or internally—of the large flattened masses".
"Several of us collected over 200 samples of DG and surrounding sedimentary lithologies over numerous field seasons in the Dakhleh Oasis. We performed optical microscopy on polished thin sections from 45 DG and 6 cultural glass samples. Quantitative analyses and investigation of micro-textures were then carried out using wavelength dispersive X-ray (WDS) techniques on a JEOL JXA-8900 L electron microprobe. The beam operating conditions for the electron microprobe were 15 kV and 20 nA during analysis of glasses. The standards used consisted of natural and synthetic mineral and glass phases."
"Approximately 165 analyses of DG and 40 analyses of cultural glasses were collected. We determined the bulk chemistry of 20 samples of Dakhleh Glass and 6 samples of sediments and sedimentary rocks from the Dakhleh Oasis region were obtained using X-ray fluorescence (XRF) techniques. This augments the XRF analyses of 8 DG samples and 42 sedimentary lithologies presented in Osinski et al. (2007)."

"The Dakhleh Glass comprises a highly vesicular, glassy groundmass containing primary crystallites (clinopyroxene, with minor plagioclase), spherules, and lithic and mineral clasts. Glass Optical and scanning electron microscopy observations mirror the field and hand specimen observations and show that the DG bodies vary markedly in terms of vesicularity. Despite its name, the Dakhleh Glass is typically rich in crystallites. Hypohyaline (i.e., >80% glass) samples are rare and hypocrystalline samples (i.e., mixtures of glass and crystals) predominate so that actual glass contents range from 35 to 10 vol%. Completely crystalline samples are also present but are rare. Such samples may, therefore, be best termed “clast-poor glassy impact melt rocks” or just “impact melt rocks”; however, we use the term Dakhleh Glass here because this name has been associated with this material for over 25 years. In thin section, crystallite-free glassy areas within DG glass samples are colorless and transparent. The bulk of the DG samples, however, have a grainy and “spotty” appearance. This dark coloration can be explained by the interaction of incident light with the crystallites, resulting in reduced light transmission through the sample. The majority of DG specimens investigated display evidence for flow in the form of elongated and irregularly-shaped vesicles and intermingling of glasses of different composition. X-Ray Fluorescence data for 24 individual samples of DG are presented. Given the bulk nature of these analyses (i.e., it was not possible to completely separate clasts, spherules, globules, and secondary alteration products from the glass), the data should be interpreted with caution; however, these analyses are useful for assessing the major geochemical properties of the DG. It is notable that the DG is typically CaO- and Al2O3-rich, although there are considerable variations (e.g., from 8 to 21 wt% CaO) between individual samples and between different locations. There is no systematic difference in composition between DG found as a lag and that found in situ. Alkalis are typically <2 wt%. Loss-on-ignition (LOI) data suggest the presence of variable amounts of volatiles. Some of these volatiles are undoubtedly bound in secondary phases such as calcite caliche and anhydrite; however, the systematically low WDS totals (typically 97–100 wt%) in several samples suggest that the relatively high volatile contents may be original. The lack of perlitic fractures—which form due to the accommodation of strain following volume increases associated with the diffusion of meteoric water into a solid glass—also suggests that these volatile contents may be original. This is consistent with the pristine condition of the DG and the current hyper arid environment. Trace element data for DG show persistent amounts of Ni , Co, and Cr; however, there is no systematic enrichment relative to values for regional geological units. WDS analyses confirm the XRF findings that DG is typically CaO- and Al2O3-rich. Glass areas adjacent to crystallites were avoided during WDS analyses. Element maps confirm that compositional variations in the glass due to crystal fractionation are only important immediately adjacent to the crystallites, such that variations in spot analyses within individual samples reflect actual variations in glass composition. These data show that, including variations between DG samples from different locations in the Dakhleh Oasis and at single locations, there are substantial internal compositional variations; the greatest deviation is in SiO2, Al2O3, CaO and MgO contents. An important sub-type of DG occurs as small irregular-shaped enclaves of glass devoid of crystallites that appear darker, in BSE mode, than the ‘host’ glass. WDS analyses and element maps indicate that these irregular glasses are highly silica rich, with SiO2 contents of 90–100 wt%."

Remarks:
The new investigations confirm for me, that the DG's are more than once redeposited, therefore they do not occur in situ. With it the age of the glasses is not clarified. The glasses can have like by me postulates also a tertiary age. The volcanic or subvolcanic origin of the glasses in the context with the spread volcanic activities in the region before ~28 Ma is probable. Interesting is the presence of plant impressions on the underside of many DG masses. That indicates that the differently DG melt flow was in a semi-liquid state — likewise as the LDG. Perhaps the plant impressions can be used for the determination of the age.