Electron Probe Micro Analyzer (EPMA) studies of basalt rock from fluoride rich groundwater areas of Pandharkawada Taluka, Yavatmal district, Maharashtra

Abstract

Groundwater from the study area contained as high as ∼5 ppm fluoride from shallow aquifers and 17 ppm from deeper aquifers during the pre-monsoon season of 2004. In order to find out the source mineral contributing to high fluorine in the groundwater, two fresh rock samples of basalt were collected from a flow (compound pahoehoe) occurring at 260–280 and 240–260 above mean sea level (amsl). One of the samples is vesicular in nature with amygdales and the other one is massive. Fluorine content in the two types is up to 6.3 ppm and 7.4 ppm respectively. A combined microscopy and Electron Probe Micro Analyzer (EPMA) study was carried out to identify the fluorine bearing phase in the massive basalt sample. The Back Scattered Electron (BSE) image techniques and qualitative analysis indicate that plagioclase feldspar and pyroxene (augite and diopside) are the major modal minerals (95 vol%). No primary fluorine bearing mineral such as fluorite and topaz were found. However, fluorine is contributed by secondary minerals such as apophyllite, zeolites and hydrous mafic minerals like amphiboles and biotite where it is present in the crystal lattices indicating that the high concentration of fluoride in groundwater from the area is not related to fluorine bearing mineral phases. Other factors such as moderate to high alkalinity, high residence time, semi arid climate and sluggish movement of groundwater in compound pahoehoe flows etc. are likely to have played a significant role in the fluoride-rich nature of groundwater in this terrain.