Libyan desert glass
The as Libyan desert glass, also briefly LDG ( Libyan Desert Glass) or LDS ( G ) ( Libyan Desert Silica ( Glass) ), known amorphous silica glasses are in the south west of the Great Sand Sea, extending from Libya to Egypt, found.
Formation
Mostly it is assumed that the glasses were taken at high probability when a meteorite that is to be descended before about 28 to 30 million years ago in North Africa. At high pressures and temperatures of the then upcoming superficial sandstone was melted and the liquid melt thrown. With rapid cooling in the flight phase as might arise glass. An impact crater but have not been found.
An alternative theory suggests that passed through a hydrovulkanische explosion SiO2 gel to the surface. Volcanic activity, as they are known from the so-called Clayton craters are occupied in much of the north-eastern Sahara.
The desert glass is 98 % of Lechatelierite, a natural quartz. There were inclusions of baddeleyite arising from zircon sand at temperatures of over 1700 ° C, as well as traces of the meteorite ( up to 0.5% ) was detected, ruling out an also discussed emergence as deposit enriched in dissolved silica lake. The desert glass is attributed to the Impaktgläsern, since it differs from the tektites by up to 30 times higher content of water inclusions of up to 0.16%.
History
The Libyan desert glass was used as early as the Neolithic period as a tool or as an arrowhead, a twelve -centimeter-long hand-ax from this era is national d' histoire naturelle in Paris Muséum exhibited, among other things. The relatively unspectacular appearance of the rare mineral led over time again and again to confusion with conventional glass or ceramics. Howard Carter in 1922 held the material of the scarab in the pectoral of Pharaoh Tutankhamun nor chalcedony, one of quartz.
It was the English geologist Patrick A. Clayton and Leonard James Spencer, who collected 1932 LDSG first scientifically recorded, larger quantities and the study results published in 1933.
Finally, in 1998 succeeded the Italian mineralogist Vincenzo de Michele by means of a refractometer examination of the evidence, that the scarab in a pectoral of Tutankhamen ( Carter Fund number 267d, Egyptian Museum, JE 61884 Inv ) is a polished piece of Libyan desert glass.
Features and Description
The color of the Libyan Desert glass varies from pale yellow, honey yellow, green yellow, milky white to dark gray. Specimens found at the earth's surface is often polished from the desert sand (wind cut ) in the soil plug end, however, corrode and dull. The Mohs hardness of 6 to 7, the density is 2.2 g / cm ³, the fracture is conchoidal and the melting temperature is approximately 1700 ° C.
Chemical composition:
Occurrence
The scattered field in the Libyan- Egyptian desert covers about 6500 km ², and lies between the great sand sea near the oasis Koufra in southwestern Egypt and the N'Giffel - Khabir Plateau ( Gilf el- Kebir ) in the Egyptian- Libyan border area, which is partially closed military zone is. Dunes enter again free single copies of the desert glass. The overall incidence is estimated at about 1400 tons.