Texture And Composition Of Ferrian Ilmenite From Hornblende Andesites Of The Timok Magmatic Complex, Serbia

Ferrian ilmenite, representing an intermediate member of the hematite-ihnenite solid solution, is frequently present as an accessory mineral in hornblende andesites of the Timok Magmatic Complex (Serbia), which host world-class porphyry copper systems of the Bor Metallogenic Zone. In Krivelj, a type locality of this rock, ferrian ilmenite occurs in homogenous crystals without exsolutions present. Its unit cell parameters are a = 5.062(1) A, c =13.895(4) A, V= 308.4(1) angstrom(3) and according to the dependence of unit volume with composition, studied ferrian ilmenite corresponds to an intermedi-ate member of the series with 53 mole% ilmenite (X-ilm = 0.53). Transmission electron microscopy investigations show that this mineral is homogenous in terms of hematite-ilmenite solid solution and presence of exsolutions, even at the nanoscale level. Fenian ilmenite from other localities (Majdanpek and Nikolidevo) shows various patterns of exsolution texture. Thus, ferrian ilmenite from Majdanpek shows very fine exsolutions of hematite-ilmenite phases. X-ray diffraction analysis confums that initial ferrian ilmenite is broken down to two phases: a predominant host phase (ilmenohematite) with X-ilm = 0.31 and a second exsolved phase (hemoilmenite) with X-ilm = 0.78. Average grain compositions of this decomposed ferrian ilmenite correspond to a large degree to the previous homogenous ferrian ihnenite from Krivelj. However, due to slower cooling of andesite from the Majdanpek locality, a subsequent breakdown of the initial ferrian ihnenite was played out resulting in the formation of exsolution textures, that also caused changes in the magnetic properties of this mineral. In general, differences in ferrian ilmenite textures can imply the local variability in the conditions of the forniation of the host volcanic rock and/or indicate that subsequent alteration processes have occurred.