Ethiopian Natural Zeolite: An unusual occurrence of zeolitic volcaniclastic sediment

A recent discovery of clinoptilolite in the North West Highlands of Ethiopia is a new locality for this mineral. The North West Highlands occur in the north of the East African Rift Zone which is well known for its geologically recent volcanism. In this area diatomaceous sediments were found in1968. These rocks occur over a wide area and are known to be of lacustrine origin of Tertiary to Pleistocene age. The Main Ethiopian Rift is known for its widespread silicic volcanism and it is thought that the presence of lacustrine basins containing high concentrations of silica favour the accumulation of diatomaceous oozes which, on uplift, compaction and dewatering form diatomite (Ministry of Mines, Geological Survey Of Ethiopia).

Power XRD analysis of some of these rocks show clinoptilolite to be present as a well formed crystalline phase, whereas the siliceous exoskeletons of the diatoms, being made of amorphous silica, scatter x-rays and do not show well defined reflections. The occurrence of clinoptilolite presents the question of whether the zeolite occurs by the alteration of volcanic glass falling into the lacustrine basins or is formed from the alteration of diatomite during compaction. Recent laboratory studies have shown that relatively little energy is required to synthesise a zeolite from diatomite, without an organic template, which raises the controversial question as to its origin.

Further fieldwork is being undertaken to map the location of the natural zeolite bearing rocks which occur within the diatomite formations. However, at this time as the clinoptilolite bearing diatomite is only known to occur in some localities it would appear that the alteration of volcanic glass falling into water and undergoing alteration is the most likely explanation