Bahariya Oasis

Western Desert

Egypt

February 24, 2020

The "Crystal Mountain" in Egypt

A subvolcanic vault, filled with crystals of a hydrothermal event

Norbert Brügge, Germany
Dipl.-Geol.

(I have taken the liberty to edit this essay for syntax and punctuation.  jp)

The crystals from the Crystal Mountain between the Bahariya Oasis and Farafra Oasis, in the northern section of the White Desert, are not Quartz crystals. They are Barite crystals (BaSO4) and/or Calcite crystals (CaCO3). 

To ascertain the hardness of the crystals, Quartz (SiO2) has the hardness 7, Barite and Calcite the hardness 3.5-3.0 (Mohs-scale). Quartz crystal can scratch glass, Barite or Calcite cannot.

At first glance, the crystals of Calcite and Quartz are similar, but a closer look reveals the differences.  Quartz crystals have six sides, mostly with a pyramid-shaped crystal tip.  The crystals of Calcite, on the other hand, can be rhombohedral (crystal with six rhombus-shaped sides) or scalenohedral (triangles with different lengths and with different numbers of sides). Every specialist will confirm that what we see here are not Quartz crystals.

The discovery of this Crystal Mountain is noteworthy. By chance, the hill was opened during road construction from Farafra to Bahariya and partly destroyed at that time. Some of the material was installed into the road.   Today the Crystal Mountain is a popular stop for tourists.

Still more fascinating is the geological context. The hill is not a paleokarst cave with speleothems. It is a subvolcanic vault which emerged probably during the Oligocene age. The visible layers are limestones of the Khoman Formation (late Cretaceous age), as well as a coal seam and reddish to brownish ferruginous layers above. The strata are broken or brecciated and intensely folded with each other by intense heat. The coal seam was transformed to anthracite?.

The crystals have formed out of rising hydrovolcanic solutions. The hot solutions were highly concentrated with BaSO4 and/or CaCO3, which had been dissolved from the sediments. The solutions have penetrated all the cavities. After cooling of the solutions, the crystals could grow in cavities.  Columns or domes with crystals also formed within the cavities.