FMF - Minerals Forum, discussion and message board

[John S. White]

Jordi is now offering some interesting new items on his web site, so-called pseudomorphs of rutile and hematite after ilmenite from Zambia. I asked him how he knew that these were pseudomorphs and he replied that they had been examined microscopically by Dr. Joan Viñals* who made this determination. We are familiar with rutile crystals growing epitaxially on hematite, most famously from Switzerland, but these appear to have had a different history because, according to Dr. Viñals, the hematite is in the form of fine-grained masses occupying the interstices between the rutile crystals. He suggests that the original ilmenite (iron titanium oxide) crystals were altered hydrothermally and from that this mix of rutile (titanium oxide) and hematite (iron oxide) formed, yet the shape of the original ilmenite crystal was preserved. Textures of this type have been observed previously by Ramdohr (The ore minerals and their intergrowths. Pergamon Press, Oxford. Vol. 2, p. 1008) who described the rutile as a trigonal network in the basal sections of former ilmenite with fine-grained hematite in the rutile interstices. The trigonal network of rutile, of course, is polysynthetically twinned (sagenitic) rutile, yet its being oriented with respect to the former ilmenite suggests that it is also epitaxial. I like specimens like these, specimens that make you scratch you head, that have a complex history. They have a story with multiple interpretations and we may never know the perfect one.

(*) Full Professor of Materials Science and Metallurgy Engineering Department, Barcelona University (Spain). Academic referee for many Mineralogical and Metallurgy Science journals. Describer of the new species: Cobaltarthurite, Barahonaite-(Al) and Barahonaite-(Fe) all of them from Pastrana, Murcia, Spain

[Jordi Fabre]

Some images of the Ilmenites pseudo:

[Joan Rosell]

I would like to add a few lines that might be interesting.

Ilmenite is an accessory mineral in igneous and metamorphic rocks. It is found in large concentrations in the ultramafic and mafic intrusions and dikes, which usually occur as conformable layers within layered silicate intrusions. Ilmenite also occurs at the intrusion pyroxene levels.
At higher temperatures it has been demonstrated there is a complete solid solution between Ilmenite and Hematite. At low temperatures there is a point of coexistence of the two minerals, but not the solid solution. This coexistence may result in exsolution lamellae or separation in layers of micro or macroscopic phases of Ilmenite-Hematite, if in the system there is more iron than can be homogeneously accommodated in the crystal lattice.
Alterated Ilmenite forms Leucoxene, an important source of titanium in the heavy mineral sands ore deposits. Leucoxene is a typical component of altered gabbro and diorite and, in general, indicative of ilmenite in the unaltered rock.