Obsidian is an uncommon igneous rock that varies in chemistry from felsic to intermediate to mafic to alkaline.  Chemical analysis can show whether an obsidian is rhyolitic, dacitic, etc.  The rock shown below is a trachyte obsidian from a small volcano in the East African Rift Valley called Ol Doinyo Nyukie (several variant spellings exist).  Eastern Africa is being ripped apart by the Afar Hotspot and quite a few volcanoes having unusual lava chemistries are present along the rift zone.  Ol Doinyo Nyukie is a Quaternary-aged volcano that has principally erupted phonolites and trachytes - both lithologies are intermediate extrusive igneous rocks noticeably high in alkali content (sodium & potassium).  This rock is almost 100% glass, but there are some small, scattered phenocrysts of feldspar.


Locality: Ol Doinyo Nyukie (01 09 South latitude, 36 21 East longitude), a small volcanic center on the southern side of the Mt. Suswa caldera, southern Kenya, eastern Africa.


Trachyte obsidian (7.9 cm across) from the Quaternary-aged Ol Doinyo Nyukie Volcano, southern Kenya.



Some info. provided by Tony Peterson.



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