Cupriferous pseudobrookite in a Tertiary basalt from the Faeroe Islands

Forty-five electron microprobe analyses have been carried out on pseudobrookite occurring in a basalt from the Faeroe Islands. It is shown that pseudobrookite formed after ilmenite contains between 1 and 3% CuO, whereas pseudobrookite formed after titanomagnetite does not contain Cu. This difference in Cu content is not inherited from the original ilmenite and titanomagnetite, but arises during the formation of the pseudobrookite. The pseudobrookite in this basalt, regardless of whether it formed from ilmenite or from titanomagne­tite, is richer in Ti than in the formula Fe2TiO5, the surplus Ti4+ being balanced by the presence of divalent ions such as Mg, Mn, Fe and Cu. Mg and Cu dominate in pseudobrookite after ilmenite, Fe and Mg domi­nate in pseudobrookite after titanomagnetite. Pseudobrookite after titanomagnetite is richer in Ti than pseudobrookite after ilmenite. The pseudobrookite is not homogeneous. Both pseudobrookite formed from ilmenite and that formed from titanomagnetite contain small blebs of hematite and rutile, and furthermore pseudobrookite after ti­tanomagnetite is intergrown with larger coherent areas of hematite. The hematite blebs in pseudobrookite after ilmenite can contain up to more than 5% CuO, but there is virtually no copper in either type of he­matite in the pseudobrookite after titanomagnetite, nor do the rutile blebs contain copper.