EXPERIMENTAL-EVIDENCE FOR THE EXSOLUTION OF ILMENITE FROM TITANIFEROUS SPINEL

Microintergrowths of titaniferous spinel and ilmenite have been experi mentally produced in the system Fe-Ti-Cr-O by annealing in vacuo at co nstant temperatures in the range 700-1100 degrees C of spinel + ilmeni te assemblages previously synthesized at 1300 degrees C under controll ed low f(O2). Depending on the original f(O2) of synthesis at 1300 deg rees C, annealing produces three types of microintergrowths: Type I (l owest f(O2): 10(-11)) has rims and trellis-like lamellae of ilmenite a s well as small blebs of metallic iron; type II (f(O2): 10(-10)-10(-9) ) shows spinel with rims and a few lamellae of ilmenite; type III (hig hest f(O2): 10(-8.5)) displays both spinel with rims and lamellae of i lmenite and ilmenite with rims and lamellae of spinel. On the basis of microprobe analysis, including precise determination of the O content s, it is shown that the synthetic microintergrowths did not form by a redox process. Rather, the textures represent exsolution caused by vac ancy relaxation in spinel (and, for the most reduced samples, in ilmen ite), according to the substitution scheme Fe(2)(2+)Ti(-1)(4+)square(- 1) (1). The presence of metallic iron in the most reduced samples (typ e I) results from the shift of the iron saturation surface toward more Fe-rich compositions with decreasing temperature. The rims and lamell ae of spinel around and within ilmenite in the type-III samples result from the combination of Exchange 1 and the well-known substitution Fe 2+ + Ti4+ = 2Fe(3+). These experimental results show that natural spin el + ilmenite microintergrowths do not always result from either an ox idation or a reduction process. In particular, textures similar to tho se of type I, which were observed in lunar basalts, may simply record cooling under O-2-conserving conditions. In most terrestrial rocks, ho wever, spinel + ilmenite intergrowths are certainly best interpreted a s resulting from oxidation of at least a subsystem of the rock.