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.
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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.
Composition:
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.
References:
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