NATROPHOSPHATE FROM THE ARIS PHONOLITES, WINDHOEK, NAMIBIA

Natrophosphate has been identified in the Aris phonolites between Wind hoek and Rehoboth, Namibia. At this, the third occurrence of this rare species, the natrophosphate forms a glassy vein filling in a single 4 0 cm large boulder found in the Aris East quarry in September 1995. It forms aggregates that were colourless and transparent when found. Mai nly due to dehydration, they are now milky white to greyish white and translucent and are covered by a layer of powdery white material consi sting of villiaumite and secondary hydrophosphates. Associated mineral s include makatite, villiaumite and tuperssuatsiaite. The chemical ana lysis of the natrophosphate from Aris corresponds well with those prev iously published for natrophosphate from Khibiny and Lovozero alkalic massifs, Kola peninsula, Russia, and with that of the corresponding sy nthetic compound. The empirical formula calculated by normalizing the sum (Na + P + S) to 9.00 is: Na-7.04((PO4)(1.87)(SO4)(0.09))(Sigma 1.9 6)(F1.28Cl0.39)(Sigma 1.67). 19.29H(2)O. For the first time sulphur an d chlorine were detected in natrophosphate. The high amounts of fluori ne plus chlorine result in a surplus of negative charges and the prese nce of molecules of PO3F, PO3Cl, SO3F and/or SO3Cl is tentatively offe red as a way of overcoming this problem. The unit cell parameter of th e natrophosphate from Aris was found to be: a = 27.93 +/- 0.05 Angstro m and corresponds reasonably with values given for both natural natrop hosphate and the corresponding synthetic material. The refractive indi ces of the natrophosphate from Aris, n = 1.450 +/- 0.005 is low but wi thin the range given by FLEISCHER et al. (1984).