Tourmaline occurs as alluvial crystals in weathered pegmatites 25 miles from Ibadan. The locality (http://www.mtlilygems.com/mineinfo/nigerinfo.html) was discovered in 1998.
To date, rossmanite from this locality has only been identified as zones in faceted gemstones. It occurs in pink, red and green stones.
Tourmalines of mixed species from Ibadan, Nigeria
In a given crystal, only part of the crystal may have the rossmanite composition. An individual crystal may be mostly liddicoatite or elbaite with a rossmanite zone in it.
A representative composition of a rossmanite from Ibadan (triangles in the plot below) is:
(0.38Na0.35Ca0.27) [Al1.53Li1.39Mn0.03Fe0.03] Al6 (BO3)3 [Si6.00] [(OH)3.53F0.47]
A representative composition of a fluor-rossmanite from Ibadan (circles in the plot below) is:
(0.48Na0.41Ca0.11) [Al1.56Li1.44] Al6 (BO3)3 [Si6.00] [(OH)3.46F0.54]
In each of these cases, the X-site vacancy dominates the X-site leading to the classification of the species as rossmanite.
Gem News (2002) Gems & Gemology W02, 356-357.Giller BS (2003) An overview of tourmaline mineralogy from gem tourmaline producing pegmatite districts in Africa. Master of Science Thesis, University of New Orleans, LA.
Mountain Lily Gems (2005) http://www.mtlilygems.com/mineinfo/Ibadan,%20Nigeria/tourmalinefromib.html (PDF file)
last updated 17-Nov-2012