Biological origin of minerals in pisoliths in the Darling Range of Western Australia

Bauxitic pisoliths are widespread and abundant in the Darling Range of Western Australia. They occur as lag and within soil, range in size from a few millimetres to over 10 mm and largely consist of gibbsite with various amounts of hematite, maghemite, goethite, quartz, anatase and corundum. Scanning electron microscopy (SEM) revealed various morphologies throughout the pisoliths resembling fungal hyphae that are composed of gibbsite and Fe oxides. The fungal hyphae exhibit significant differences in size and shape, and they are gibbsitised in different ways and at different stages of development. Our interpretation of these observations led to the conclusion that the pisoliths, in part, are formed by the active precipitation of Al and Fe by fungi, rather than by subsequent replacement of fungi by Al and Fe.