State of Alteration of Ilmenite in Teris, Southern Tamil Nadu

The early-modern red dune sands (teris) in the erstwhile Tirunelveli District of southern Tamil Nadu are typified by the large content of heavy minerals (HM), particularly ilmenite. Size-specific studies show that HMs dominate the -80 to +100 mesh, and magnetic susceptibility (X) studies demonstrate the presence of three fractions of ilmenite, viz., a (X= 286×10^-6), b(127×10^-6) and c (72×10^-6),which the fraction b is most abundant (range = 52-85%). Optical studies show a variety of exsolution features between ilmenite and haematite dominantly in fraction a, less in b and almost absent in fraction c. Chemical and XRD data of ilmenite fractions, indicate that a and b are relatively unaltered and belong to the ferrian ilmenite state, whereas c is altered and reaches the hydrated ilmenite state with notable development of pseudorutile and rutile phases. Further, the b fraction is composed of relatively pure ilmenite compared to a and c. Based on this investigation, the boundary between altered and unaltered ilmenite has been drawn at a magnetic susceptibility of 127×10^-6 cgs units. Hence, a physical separation of bulk ilmenite in terms of grain size and magnetic susceptibility can provide commercial ilmenite concentrates of relatively homogeneous nature in respect of composition and crystallinity.