Petrology And Geochemistry Of Selected Nepheline Syenites From Malawi And Their Potential As Alternative Potash Sources

The Sub Saharan Africa agricultural sector is one of the most disadvantaged regions, partly due to high fertilizer import costs from the northern hemisphere. Malawi is one such country which faces these fertilizer 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-fertilizer, 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 fertilizers in other parts of the world.

撒哈拉以南非洲农业领域是全球最弱势的区域之一，部分原因在于自北半球进口化肥的成本居高不下。马拉维便是其中之一，其农业发展与粮食作物生产均面临此类化肥供应困境。然而，马拉维境内分布有大量侵入型碱性岩，其中尤以奇尔瓦碱性岩省（Chilwa Alkaline Province）内的霞石正长岩（nepheline syenites）最为富集。为此，本研究旨在评估此类霞石正长岩作为钾源的应用潜力。研究团队采用2013年获取的马拉维航空地球物理伽马射线数据，结合卫星遥感技术，筛选出可作为替代性硅酸盐钾肥（silicate K-fertilizer）原料的霞石正长岩，并对全岩样品开展地球化学分析。分析结果显示，霞石正长岩的氧化钾（K₂O）含量介于3.17 wt%至9.14 wt%之间，平均含量为5.22 wt%；其氧化钾/氧化钠（K₂O/Na₂O）比值介于0.41至1.28之间，平均比值为0.65，表明此类岩石以钠质为主，但成分具有不均一性。除霞石外，研究团队通过扫描电子显微镜-能谱分析（scanning electron microscopy with energy dispersive analysis）技术在霞石正长岩中识别出钙似长石类矿物戴维史密斯石（davidsmithite，化学式(Na,Ca)AlSiO₄）。尽管不同侵入杂岩体的均一性欠佳，但整体而言，马拉维产出的霞石正长岩与全球其他地区已用作碎岩肥料（crushed-rock fertilizers）的同类岩石具有相似的地球化学特征与矿物组成。