Annual Report 1961 - BEEP

-46 --The results for the aluminium silicateminerals have been more precisely defined thanis the case with the iron and aluminium oxides,as these latter materials present some difficultiesin quantitative X-ray work.Apart from the dark magnesium clay greatsoil group, which 8,S is well known, is a predominantlymontmorillonitic clay, 2: 1 latticeclays apparently do not occur III the freedraining soils of Mauritius.The typical titration curve? of soils fromthe Mapou area gives an inflexion around pH7and in view of the fact that the pH of thesesoils is around neutrality and the solurn containsnumerous particles of the parent dolcriticbasalt, it was felt that, even though the soilsare free draining, possibly some montrnorilloniticclay had been formed. This opinion wassupported by the work of SHERMAN and UEHARA 6who found that montrnorillonite was formed incavities in the parent rock and particul.irlyimmediately underneath the boulders wherethe soil is protected to some extent fromexcessi ve leaching.The X-ray analyses of the clay fraction ofthe !VI apou soil however have not demonstratedthe presence of montmorillonite.In Table 12 the soils have been dividedinto two groups. immature and mature and thenarranged in order of inc.casiug rainfall. Chemicaldata obtained during the soil survey and theestimated average mineralogical compositiontaken from the data in Table 11 arc given.The table shows clearly the rapid declineIn silica content with increasing rainfall and thecomplementary decrease in the ratio of silicato aluminium oxide. The most important pointfrom the mineralogical data is the increase ofthe aluminium oxide - gibbritc - at the expenseof kaolin; tile goctuite content also tends toincrease with iucrc.ising rainfall but not somarkedly as gibbsitc.These Xvr.ty analyses are only of an exploratorynature and the whole subject of themineralogical composition of local soils willhave to be studied in detail later. The dataobtained so far, however, are interesting in thatthey confirm tile broad interpretations whichhave been made of the chemical data.Table 12.The effect of increasing rainfall and age 011 the chemical composition of the wholesoil and the estimated mineralogical composition of the clayMATURESOILS.Average yearly Soil ClaySire Rainfall Si0 2 % Si0 2/A1203 Kaolin Goethite GibbsiteSolitude 50" 32 2.0 > 50 50 35 50IMMATURESOILS.Average Yearly Soil ClaySire Rainfall Si0 2 % Si0 2/A 1 2 ° 3 Kaolin Goethite GibbsitePetite Case40" ')Noyale 1- 32 2.0 > 50 < 25