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Charnockites and their Colour


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1 Department of Geology, King's College, London, United Kingdom
     

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The characteristic dark colour of charnockite is due to thin pale greenish or brownish yellow veins and stringers throughout the rock. These veins are seen particularly in the feldspars but also occur in quartz and other minerals. The colour is removed from charnockite if it is leached as chips or thin slices with warm hydrochloric acid, when the rock becomes almost identical in appearance with a normal Ieucocratic granite. In thin section the yellow veins can then be seen to have disappeared. Chemical analyses show that the material leached out contains appreciable iron, and comparison of analyses of the rock before and after acid treatment shows that the leaching has decreased the amount of iron present. From this and other evidence obtained on heating, the colour of charnockite is attributed to the presence of a yellowish mineral containing ferric iron, which occurs in veins and wisps throughout the light minerals giving them their characteristic dark olive-brown appearance in hand specimen. The exact nature of this mineral is still problematical.

Although recent work has demonstrated that some charnockitic rocks have almost certainly been derived by the metamorphism of rocks laid down contemporaneously with the surrounding sediments and probably have originated as sediments, it is maintained that the charnockite series of the type area around Madras represents a regionally metamorphosed suite of rocks of original igneous parentage.


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  • Charnockites and their Colour

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Authors

R. A. Howie
Department of Geology, King's College, London, United Kingdom

Abstract


The characteristic dark colour of charnockite is due to thin pale greenish or brownish yellow veins and stringers throughout the rock. These veins are seen particularly in the feldspars but also occur in quartz and other minerals. The colour is removed from charnockite if it is leached as chips or thin slices with warm hydrochloric acid, when the rock becomes almost identical in appearance with a normal Ieucocratic granite. In thin section the yellow veins can then be seen to have disappeared. Chemical analyses show that the material leached out contains appreciable iron, and comparison of analyses of the rock before and after acid treatment shows that the leaching has decreased the amount of iron present. From this and other evidence obtained on heating, the colour of charnockite is attributed to the presence of a yellowish mineral containing ferric iron, which occurs in veins and wisps throughout the light minerals giving them their characteristic dark olive-brown appearance in hand specimen. The exact nature of this mineral is still problematical.

Although recent work has demonstrated that some charnockitic rocks have almost certainly been derived by the metamorphism of rocks laid down contemporaneously with the surrounding sediments and probably have originated as sediments, it is maintained that the charnockite series of the type area around Madras represents a regionally metamorphosed suite of rocks of original igneous parentage.