Abstract

Ilmenite–pyrophanite crystals from a garnet pegmatite dyke from the Upper Codera Valley (Sondrio, Italian Alps) showing exsolutions of titanohematite and columbite–tantalite were investigated by scanning and transmission electron microscopy. The titanohematite precipitates share the same crystallographic orientation of the ilmenite–pyrophanite host, are bean-shaped when observed on sections inclined to the pinacoidal section, and are elongated when observed on sections closer to the prism section, possibly because of their discoidal shape parallel to (001). The columbite–tantalite precipitates form a hexagonal network of needles elongated along 〈110〉 of the ilmenite–pyrophanite and titanohematite host. The following crystallographic relationship was established: [100]Col//[001]Ilm; [001]Col//〈110〉Ilm; [010]Col//〈110〉Ilm, which can be explained in terms of preservation of the oxygen close packing between the ilmenite and columbite structures. The interfaces between any two of the three different phases are coherent but show lattice strain contrast and sometimes dislocations because of their different unit-cell dimensions. On the basis of textural observations, titanohematite is supposed to exsolve first, followed by columbite–tantalite at temperatures below 500°C. The addition of MnO to the Fe2O3–FeTiO3 system is supposed to considerably influence the topology of the related TX phase diagram and the solubility of Nb2O5 and Ta2O5 in this system.

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