Diabase (Dolerite)
Archae Solenhofen (solenhofen@hotmail.com)
(Last modified April 3, 2001)

   Diabase is a fine-grained intrusive igneous rock of a composition similar to basalt, but is slightly more coarse-grained than basalt. The rock consists of the minerals calc-plagioclase (bytownite to labradorite) and pyroxene (commonly augite, pigeonite, and hypersthene) with apatite, magnetite, and olivine commonly present. Modal rock composition of mafic minerals is less than 90% of the rock by volume. Diabase is associated with sill and dike intrusions in the hypabyssal (upper few km of the Earth's crust). The term diabase is the North American word for dolerite, which is commonly used in Europe to describe the same rock type (in Europe the term diabase sometimes refers to a weakly metamorphosed dolerite). The diabase deposits of Egypt can also be called coarse-grained basalts because of their fine grain size.

   Diabase occurs in several locations in Egypt mainly in the Eastern desert, the Wadi Esh region, at Wadi Atolla, at Gebel Dokhan, in the Sinai, and at Fayum. The quarries range in age from the Old Kingdom to Roman periods (Lucas and Harris, 1962). A slightly greenish diabase dike of Tertiary age, with black aphanitic groundmass and medium gray plagioclase phenocrysts (up to 15 mm) was also quarried by the ancient Egyptians during the Late period in the Rod el-Gamra region. A polished rock slab image of this porphyritic diabase can be seen at the Ancient Egyptian Quarries website (section #28).

   Diabase was used by the ancient Egyptians in ways similar to basalt. However, its main use was as pounders for the working of hardrocks. Pounders can be seen in hardrock quarries as roughly spherical masses and are found in large quantities in the ancient granite and granodiorite quarries at Aswan, and at the siliceous sandstone quarry at Gebel Ahmar, near Cairo (Lucas and Harris, 1962). The spherical diabase pounders observed in the quarries are similar to cobble to bolder sized naturally occurring sediments present in the cataract regions of the Nile and in the Eastern desert, presumable the result of erosion of diabase and rounding as the coarse sediments were transported down the Nile during floods. 

   The reason why diabase was used for pounders by the ancient Egyptians was due to its very high fracture strength. As presented in Table 2 of the section on Rock Properties a typical diabase has an unconfined compressive strength that can be considerably higher than that of a typical granite or other coarse-grained igneous rock. This means that a solid diabase sphere is usually more durable than that of a granite block, and when pounded against granite, the granite will mechanically breakdown faster than that of the diabase. This is the same basic principle of  Boucharde hammers, which are today used in percussion carving of hardrocks (Rich, 1970); cast iron is used in these hammers because it has a strength greater than that of any rock. 

References

Lucas, A. & Harris, J.R. (1962) Ancient Egyptian materials and industries. E. Arnold, London, 523 p.

Rich, J. C. (1970) The materials and methods of sculpture. Oxford Univ., New York, 416 p.

Links to examples of diabase usage

a) Bowls

b) Vases

c) Statues

d) Misc objects

Old Kingdom period diabase pounders found at Sphinx temple
 
 




 
 
 
 
 
  1