Optical Features, Microstructure and Microanalysis of Botswana Agates

Abstract

In Botswana agates are found in the north-eastern region of the Central District in the basalt from the 180 Ma Karoo volcanic rocks. Predominantly they are wall-lining agates but some vein agates can also be found. There is another form of microcrystalline quartz with green streaks of impurity, locally known as green-moss agate or gorgonzola, found in the same area. Both the wall-lining agates and the gorgonzola are coated with a green-clay which is easily eroded on weathering. In this paper the optical features of polished Botswana agates and gorgonzola are presented, and their microstructure is studied using a scanning electron microscope. From the microstructure it is revealed that gorgonzola is not an agate, as it is commonly known, but it is microcrystalline quartz. Compositions of the green-earth coating on the agates and the green impurity in gorgonzola are determined from Energy Dispersive Spectrum (EDS) analysis. Both the green-earth coating on agates and the green impurity in gorgonzola are characterized by x-ray diffraction (XRD), and both are found to be a loosely bound clay mineral, Celadonite-1M, aluminian.