A Hydrated Barium-Strontium Pyrochlore in a Biotite Rock from Panda Hill, Tanganyika1

E. Jäger, E. Niggli and A. H. van der Veen
Min.-Petr. Inst., University of Berne, Switzerland
Min. Petr. Dept., Research Laboratory of the Billiton Maatschappij N.V., Hollandsche Metallurgische Bedrijven N.V., Arnhem, Netherlands.
1Published by permission of the N.V. Billiton Maatschappij and the Colonial Development Corporation.

Summary: A hydrated barium-strontium pyrochlore with only subordinate amounts of Ca and Na has been found in a weathered biotite rock (contact-rock of a carbonatite) from Panda Hill, Tanganyika, as small euhedral yellowish-grey cubic crystals (showing the combination of the octahedron and the cube) in a rock containing euhedral biotite, some orthoclase, and several other minerals. Hydrated Ba-Sr pyrochlore is isotropic, the refractive index varies from 2·07 to 2·10. The reflectivity (vertical illumination) is 13·2 %. H. 4 1 2 −5 –5 Poor {111} cleavage. The pyrochlore structure (space-group Oh7−Fd3m) and the unit-cell dimensions (a 10·562 Å.) are derived from X-ray powder and Weissenberg photographs. The calculated specific gravity is 4·01 (observed, 4·00 on dried material). Chemical analysis gives BaO 12·5, SrO 6·4, Na2O 0·28, K2O 0·25, CaO 1·35, rare earths (mainly Ce2O3) 2, ThO2 0·6, FeO 0·45, TiO2 3·9, Nb2O5 67·0, Ta2O5 0·22, H2O+ 4·0, other constituents 2·21, total 101·16. After deduction for impurities the following formula resulted: (Ba0·30Sr0·22Ca0·05Ce0·04Na0·03Fe0·02K0·01Th0·01)(Nb1·83Ta0·004Ti0·17)O5·61(H2O)0·80.

The ideal formula for pyrochlore is A2B2O6(F,OH). In the mineral described only a third of the A-positions are occupied by Ba, Sr, etc. Infra-red spectrophotometry does not indicate hydroxyl-groups. When the mineral is treated with TINO3 solution the intensities of the X-ray diffraction lines 333/511, and 444 are changed; hydrated Ba-Sr pyrochlore shows a certain exchange-capacity.

The name pandaite, from Panda Hill, is proposed for the new mineral.

Mineralogical Magazine; March 1959 v. 32; no. 244; p. 10-25; DOI: 10.1180/minmag.1959.032.244.03
© 1959, The Mineralogical Society
Mineralogical Society (www.minersoc.org)