Mineralogical Investigation of an Enterolith from a Grant's Zebra

An enterolith surgically removed from a Grant's zebra was investigated by standard mineralogical techniques including X-ray diffraction, scanning electron microscopy and electron microprobe analysis. It was predominantly composed of a suite of phosphate minerals which have precipitated on and between aggregates of undigested plant material. It was apparently cored by a small nidus of granitic rock which served as a site for heterogeneous nucleation for the larger phosphatic overgrowth. Phosphate was deposited by at least two distinct mechanisms. In the enterolith's core, phosphate minerals replaced or rimmed silicate grains (e.g. feldspars and quartz) which are relics of the disaggregated granitic nidus. As the enterolith subsequently accreted outwards from the nidus, struvite was precipitated directly onto fibres of plant material, or within the pore spaces between them. The generally fine grain size of struvite in enteroliths, relative to that in urinary calculi, may be due to the ubiquitous presence of the undigested plant fibres. Struvite, close to (NH₃)_0.9K_0.1MgPO₄ · 6H₂O in composition, is the only phosphate mineral in the outer portions of the enterolith. Within the enterolith's interior, struvite occurs subequally with Mg-vivianite, close to Mg_2.9Fe_0.1(PO₄)₂ · 8H₂O in composition. Textural relations suggest that the mineralogical transformation of struvite to Mg-vivianite is a consequence of diagenesis, presumably related to a decrease in pH and/or (NH₄)⁺ activity, within the core of the enterolith. If enterolith growth is mediated by the accumulation of a particular form of undigested plant fibre (e.g. lucerne hay), the identification and dietary removal of this material may prevent or at least impede the development of enteroliths.