A REINVESTIGATION OF BOLIVARITE AND EVANSITE

The type specimen of bolivarite from Campo Lameiro, Pontevedra, in Spa in, is shown to have physical and optical properties similar to those of the type specimen of evansite from Mt. Zeleznik, Gomor, Slovakia. B oth 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 deg rees C. IR spectra show absorption peaks at 3500, 1600 and 1000 cm(-1) , which are attributed to OH, H2O and PO4, respectively. NMR spectra g ive 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 octa hedral coordination. Chemical analyses give the empirical formula Al-2 (PO4)(0.92)(OH)(3.25). 4.03H(2)O for bolivarite and Al-3(PO4)(1.09)(OH )(5.73). 7.77H(2)O for evansite. Analyses of other specimens of hydrou s aluminum phosphates from the Pontevedra area give results that indic ate a range of Al:P atomic ratios varying between 2.44 and 3.58. Becau se of the amorphous nature of these materials, it is difficult to know if these analytical data pertain to mixtures of hydrous aluminum phos phates or if bolivarite and evansite represent intermediate members of a wide solid-solution series in which PO4 radicals are replaced by 3( OH).