The geochemistry and mineralogy of parent materials and soils in two catchments in Laikipia district, Kenya.

Abstract

Rock and subsoil samples were taken for geochemical and mineralogical studies from Sirima and Mukogodo catchments in Laikipia District. Rock samples from Sirima catchment showed an average content (in per cent) of 52.4 silica, 15.5 aluminium, 13.8 iron, 6.1 magnesium, 7.6 calcium, 3.2 sodi¬um and 1.9 potassium. The soils showed an average value (in per cent) of 49.5 silica,14.8 aluminium, 20.7 iron, 1.7 magnesium, 2.9 calcium, 3.1 potassium, 2.5 sodium, 0.2 manganese and, 18.1 ppm titanium. The rock samples from Sirima area had an average mineralogical composition (in per cent) of 63.0 calc-plagioclase, 18.0 magnetite, 10.0 pyroxene and 9.0 olivine while the soil mineralogy was dominated by calcic-pla¬gioclase, magnetite and hematite although other minerals like quartz, limonite, ilmenite, hornblende and calcite occurred in some samples. The rock samples from Mukogodo showed an average composi¬tion (in per cent) of 77.9 silica, 14.1 alumini¬um, 2.3 iron, 1.25 calcium, 3.0 sodium, 3.0 potassium and 0.15 magnesium while that of the soils was 64.2 silica, 13.1 aluminium, 9.3 iron, 1.3 calcium, 0.8 magnesium, 2.7 , potassium, 3.0 sodium, 0.12 manganese and 7.0 titanium. The mineralogical compo¬sition of the rock samples showed an aver¬age (in per cent) of 62.0 K-feldspars, 25.0 quartz, 7.0 calc-plagioclase,' 4.0 biotite, 1.0 magnetite and 0.7 iron ores while that of the sand fraction was dominated by quartz, K.feldspars and hematite but muscovite, biotite, magnetite, calc-plagioclase, limonite, calcite, jadeite and sericite occurred in smaller amounts in some samples.