Zinc- and Y-group-bearing senaite from St Peters Dome, and new data on senaite from Dattas, Minas Gerais, Brazil

A Zn- and Y-group-bearing senaite, formulated as AM₂₁O₃₈ (Pb_0.63Na_0.46Ba_0.11)Σ1.20(Ti_14.64Zn_1.74Y‒group REE_0.70Mn_0.38Nb_0.20Sn_0.03Zr_0.03Th_0.02)_Σ20.9O₃₈, a member of the crichtonite group, is a newly characterized phase associated with murataite at the St Peters Dome area, Colorado. The Zn- and Y-group-bearing senaite is uranium-free and nonmetamict, but otherwise is comparable to known senaites and davidites in X-ray diffraction pattern, symmetry, and structure. The REE distribution shows a strong dominance of Y and the Y-group REE which are present in M(1). Megascopically, the mineral is black, submetallic, and opaque; in polished section it appears to be white and moderately bright compared with murataite. Cleavage is absent but twinning on {520} is present.

Senaite and hitherto unreported zinc-bearing senaite from Dattas, Diamantina, Minas Gerais, Brazil, have the following respective structural formulas as determined by electron microprobe:

(Pb_0.76Sr_0.20La_0.08Ce_0.07Ba_0.04)_Σ1.15(Ti_12.89Mn_0.88Y_0.59Zn_0.12Mg_0.07Nb_0.03Cr_0.02Th_0.02)_Σ20.61O₃₈

and

(Pb_0.89Ba_0.15Ce_0.02Sr_0.02)_Σ1.08(Ti_14.57Zn_1.76Na_0.63Mn_0.50Y_0.28Nb_0.14Mg_0.04Cr_0.02)_Σ20.83O₃₈