The geochemistry and cryptic zonation of pyrochlore from San Vicente, Cape Verde Islands

N. A. Hodgson, M. J. Le Bas
1992 Mineralogical magazine  
Zr-rich pyrochlore crystals in carbonatite from San Vicente (Cape Verdes Islands) show cryptic, concentric and rhythmic chemical zonation with Ca increasing and Ti, U and Zr decreasing towards the rims. In one carbonatite, taken from the Camile dyke, previously undocumented cryptic sector zonation of Ti, U and Zr is also observed in these crystals. The chemical variation is investigated by wavelength-dispersive electron microprobe, with both single spot and crystal map analyses. The concentric
more » ... es. The concentric and rhythmic zonation, marked by element substitution, was generated by magma heterogeneity and/or element diffusion kinetics, but it is suggested that the sector zonation, marked by differential site substitution, was governed by protosite variation between octahedral and cubic faces.
doi:10.1180/minmag.1992.056.383.06 fatcat:6nw3vn2o5nbuli6kxuun2l7o4u