Summary: A hydrated barium pyrochlore (pandaite) has been found at Mrima Hill (Kenya) in a residuum consisting mainly of goethite/limonite and gorceixite. The pandaite occurs as pale yellow or white octahedral crystals, which have a micro-crystalline interior. It is isotropic, the refractive index varies from 2·07 to 2·11. The calculated specific gravity is 4·07, and the unit cell dimension is 10·59 Å. After deduction of impurities the following formula was obtained (Na_0·03 K_0·02 Mg_0·05 Ca_0·09 Ba_0·36 Ln_0·05 Th_0·01 Pb_0·004) (Ti_0·21 Nb_1·78 Ta_0·01) (H₂O)_1·64 O_5·52.

Comparison of the calculated formula, essentially A_0·614 B₂ (H₂O)_1·64 O_5·52 with the ideal pyrochlore formula A₂B₂O₆F indicates important deficiencies in the structure. Differential thermal analysis shows a large endothermic peak at 340° C and moderate exothermic peaks at 480 and 780° C. In its physical, chemical, X-ray, and thermal properties the Mrima pandaite is similar to pandaites described from other localities.

The nature of the alteration of primary pyrochlore to pandaite has been studied using an electron probe.