It may help you in better understanding of ZTR index
A zircon-tourmaline-rutile maturity index and the interdependence of the composition of heavy mineral assemblages with the gross composition and texture of sandstones
John F. HubertJournal of Sedimentary Research (1962) 32 (3): 440-450.DOI: https://doi.org/10.1306/74D70CE5-2B21-11D7-8648000102C1865DPublished: September 01, 1962