New data on the nature of fine and ultrafine lamellae in titanomagnetite

Microtextural features and the chemical composition of intergrowths in titanomagnetites from various localities and rock types have been studied by SEM and energy-dispersive microanalysis on samples etched with conc. HCl to allow the analysis of ultrathin ( approximately 1 mu m) lattice intergrowths. All the intergrowths in titanomagnetite fall into the following groups: 1) ilmenite-geikielite, 2) spinel-magnetite and magnesian magnetite, 3) spinel-pleonaste, 4) spinel-hercynite. Compositional data are presented for most of these intergrowths, and in two instances their nature had been confirmed by XRD on lamellar phases isolated from the titanomagnetite by repeated treatment with HCl and magnetic separation. These lamellar phases prove to be members of the ilmenite-geikielite group, in disagreement with the conventional view that they consist of ulvoespinel. Some spinel and ilmenite inclusions developed as skeletal or isometric crystals, simultaneously with the growing magnetite grains; most inclusions, however, formed as a result of titanomagnetite exsolution at decreasing T.