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The relationship between optical properties and occurrence of some black tourmalines from Northern Nigeria

Published online by Cambridge University Press:  05 July 2018

Patricia McCurry
Patricia McCurry*
Affiliation:
Department of Geology, Ahmadu Bello University, Zaria, Nigeria.
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Summary

Seventy-nine black tourmalines collected over a 12 000 km2 area of Precambrian to lower Palaeozoic rocks in northern Nigeria show a close correlation between maximum R.I., absorption colour, and occurrence in non-feldspathic micaceous schists and schorl rocks on the one hand, and in gneisses, migmatites, and feldspathic pegmatites on the other. Pleochroism is known to depend upon the total iron content and the Fe2+Fe3+ interaction. Colour variations may thus provide useful indicators of the geochemical environment during crystallization.

Limited published data, supplemented by additional measurements on four tourmalines from outside the original area, suggest that similar correlations would be obtained for the rest of Nigeria.

Selective processes of boron metasomatism may have been important in the association of different coloured tourmalines with feldspathic and non-feldspathic rocks, and the apparent lack of biotite in most tourmaline-rich rocks. The source of the boron probably lay within the upper Proterozoic to lower Palaeozoic metasedimentary belts with which the tourmaline is most strongly associated.

Type
Research Article
Information
Mineralogical Magazine , Volume 38 , Issue 295 , September 1971 , pp. 369 - 373
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1971

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Footnotes

1

Present adress: Dept. of Earth Şciences, The Open University, Walton Hall, Bletchley.

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