Complex exsolution microstructures in ilmenite-pyrophanite from the Garnet Codera dike pegmatite (Central Italian Alps): An electron microscopy investigation

Ilmenite-pyrophanite crystals from a garnet pegmatite dike from the Upper Codera Valley (Sondrio,Italian Alps) showingexsolutions of titanohematite and columbite-tantalite were investigated byscanning and transmission electron microscopy. The titanohematite precipitates share the same crystallographic orientation of the ilmenite-pyrophanite host, are bean-shaped when observed onsections inclined to the pinacoidal section, and are elongated when observed on sections closer to theprism section, possibly because of their discoidal shape parallel to (001). The columbite-tantaliteprecipitates 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 observation, titanohematite is supposedto exsolve first, followed by columbite-tantalite at temperature below 500 °C. The addition of MnO tothe Fe2O3-FeTiO3 system is supposed to influence considerably the topology of the related T-X phasediagram and the solubility of Nb2O5 and Ta2O5 in this system.