A re-investigation of bolivarite and evansite

CSIC Museo Nacional de Ciencias Naturales, Departmento de Geologia, Madrid, Spain
Universidad Autonoma de Madrid, Spain
CSIRO, Australia

The type specimen of bolivarite from Campo Lameiro, Pontevedra, in Spain, is shown to have physical and optical properties similar to those of the type specimen of evansite from Mt. Zeleznik, Gomor, Slovakia. Both minerals are X-ray amorphous. DTA spectra of both minerals show a strong endothermic effect at 120 degrees C and a weaker one at 399 degrees C. IR spectra show absorption peaks at 3500, 1600 and 100 cm (super -1) , which are attributed to OH, H ₂ O and PO ₄ , respectively. NMR spectra give a P signal centered at -10.7 ppm, typical of amorphous phosphates, and an Al signal centered at -4.2 ppm, which is typical of Al in octahedral coordination. Chemical analyses give the empirical formula Al ₂ (PO ₄ ) (sub 0.92) (OH) (sub 3.25) .4.03H ₂ O for bolivarite and Al ₃ (PO ₄ ) (sub 1.09) (OH) (sub 5.73) .7.77H ₂ O for evansite. Analyses of other specimens of hydrous aluminum phosphates from the Pontevedra area give results that indicate a range of Al:P atomic ratios varying between 2.44 and 3.58. Because of the amorphous nature of these materials, it is difficult to know if these analytical data pertain to mixtures of hydrous aluminum phosphates or if bolivarite and evansite represent intermediate members of a wide solid-solution series in which PO ₄ radicals are replaced by 3(OH).

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