Role of calcium oxalate biomineralization by fungi in the formation of calcretes; a case study from Nazareth, Israel

Biomineralized fungal filaments (often referred to as "calcified filaments") are of interest because of their role in the calcium cycle in calcretes. The mineralogy of the crystals associated with these filaments and the evolution of their numerous crystal habits in a calcrete from Nazareth, Israel, are the subject of this study. Removing the Be window from an energy dispersive spectrometry (EDS) microprobe coupled with a scanning electron microscope (SEM) permitted the measurement of the light elements C and O, us well as Ca. Comparison of crystal spectra to those of known standards demonstrated that the great majority of needles associated with the fungal filaments are calcium oxalate. Based on these results, a model of fungal filament mineralization is proposed that describes the evolution of weddellite (polyhydrate calcium oxalate) into whewellite (monohydrate calcium oxalate) in a sequence that includes all of the crystal morphologies observed by SEM, followed by a less well understood diagenetic transformation of monohydrate calcium oxalate into calcium carbonate.