Characterisation of Vermiculites from the Neoproterozoic Mozambique Belt of Tanzania for Agricultural Applications

The study assessed the mineral phases and properties of five Tanzanian vermiculites in response to the temperature used to expand them with respect to their potential suitability for agricultural applications. One sample from Palabora Mines in South Africa was included for comparison. X-ray diffraction was used to establish their mineralogy. Water release characteristic, cation exchange capacity (CEC) and pH were among the agronomic properties assessed after heating the samples at 15, 200, 400, 600 and 800o C in a muffle furnace. The XRD studies revealed that not all the studied samples were vermiculites; some were hydrobiotites with a wide variety of accessory minerals, which cannot preclude them for agricultural applications. Heating decreased the CEC and exchangeable Mg, Ca and Na while increased exchangeable K particularly in hydrobiotites. Vermiculites had high CEC and thus, they are suitable as a growing medium with ability to retain plant nutrients from leaching. However, heating above 600 oC should be avoided as it reduced the CEC of vermiculites by more than 80% and made some of the exfoliated products strongly alkaline and, thus, unfavourable for crop production. Although samples from Tanzania expanded on heating, their capacity to retain plant available water was relatively low in comparision to that from Palabora. Disintegration on heating and the presence of a high amount of iron could be among the factors affecting their water release characteristic.