Dissolution of weddellite, calcium oxalate dihydrate, in Pyxine subcinerea

On the thalline lobes of Pyxine subcinerea there are delimited parches of p ruina consisting of bipyramidal prisms, bipyramids and druses of weddellite , i.e. calcium oxalate dihydrate (COD), as revealed by X-ray analysis and S EM observations. The disappearance of the crystals from older thalline area s is preceded by an increase in the frequency of corrosion figures, in the form of etch square pits, and a decrease in the number of crystals occurrin g on the thalline surface. These features follow a dissolution gradient fro m the lobe apex rewards the proximal edges of the patches. The corrosion fi gures are linked to the thermodynamic metastability of GOD. Partial dissolu tion of the crystals, performed in water and in EDTA, has confirmed this la tter assumption. Ir is thought that the disappearance of pruina, which inte rferes with the transmission of light to the algal layer, may allow the for mation of soredia in mature thalline portions. (C) 2001 The British Lichen Society.