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Petrology and geochemistry of selected nepheline syenites from Malawi and their potential as alternative potash sources

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posted on 2022-11-03, 16:42 authored by Annock Chiwona, Joaquín Cortés, Rachel Gaultona, David A.C. Manning

Abstract

The Sub Saharan Africa agricultural sector is one of the most disadvantaged regions, partly due to high fertiliser  import costs from the northern hemisphere. Malawi is one such country  which faces these fertiliser challenges for the agricultural sector  growth and food crop production. However, Malawi has numerous intrusive alkaline rocks, nepheline syenites, especially within the Chilwa alkaline province. This study was therefore conducted to assess these nepheline syenites for their potential as potassium sources. We used Malawi's new airborne geophysical gamma ray data acquired in 2013, coupled with satellite remote sensing,  to identify nepheline syenites suitable as possible sources for  alternative silicate K-fertiliser, and carried out geochemical analysis  of whole rock samples. Results show that the K2O content for the nepheline syenites varies from 3.17 wt % to 9.14 wt % with an average of 5.22 wt %. The K2O/Na2O  ratio for Malawi's nepheline syenites ranges from 0.41 to 1.28 with an  average of 0.65 showing that the nepheline syenites are mostly sodic but  with variable composition. In addition to nepheline, the calcium  feldspathoid davidsmithite ((Na,Ca)AlSiO4) was  identified in the syenites using scanning electron microscopy with  energy dispersive analysis. Although the different intrusive complexes  are not homogenous, our results show that, generally, the nepheline  syenites from Malawi have similar geochemistry and mineralogical composition to those which have been used as crushed-rock fertilisers in other parts of the world.

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