THE STRUCTURAL CHEMISTRY OF KALIPYROCHLORE, A HYDROPYROCHLORE

The crystal structure of kalipyrochlore from the Lueshe carbonatite, Z aire, Fd3m, a 10.604(1) angstrom, has been refined to an R of 1.62% (w R = 1.65%) for 100 observed reflections (MoKalpha radiation). On the b asis of electron-microprobe and X-ray-diffraction data, the formula 0T i0.20)SIGMA2(O4.06OH1.94)SIGMA6([H2O]0.86K0.14), which results in an i deal empirical formula of Nb2(O,OH)6.pH2O, where p less-than-or-equal- to 1.75. Kalipyrochlore has several unusual features not shown by othe r pyrochlore-group minerals. It is extremely deficient in A cations, s o much so that by far the dominant occupant of the A site is H2O; it h as some OH at the site normally occupied only by O, and has more than 1 H2O group pfu. The maximum amount of H2O in the pyrochlore structure is controlled by the cation occupancy of the A site; the maximum H2O content ranges from 1.00 H2O pfu for ideal pyrochlores (two A cations pfu) to 1.75 pfu for defect pyrochlores (no A cations).