The ZTR index is a quantitative measure used in sedimentary geology to assess the degree of mineralogical maturity of sediments and sedimentary rocks. Introduced by John F. Hubert in 1962, the index expresses the percentage of three resistant heavy minerals—zircon, tourmaline, and rutile—relative to the total population of transparent, non-micaceous detrital heavy minerals.
Definition and Method
The ZTR index is calculated by summing the abundance of zircon, tourmaline, and rutile grains and dividing by the total count of transparent heavy minerals, then expressing the result as a percentage. These three minerals are selected because of their exceptional resistance to both chemical and physical weathering. As sediments undergo repeated transport, reworking, and weathering cycles, less durable heavy minerals are progressively eliminated, while the resilient zircon, tourmaline, and rutile grains become relatively concentrated.
Applications
The ZTR index serves as an indicator of how weathered a sediment or sedimentary rock is, both mechanically and chemically. A high ZTR index (often exceeding 90% in ortho-quartzite sandstones) implies a greater degree of mineralogical maturation, which typically results from prolonged transport distances, multiple sedimentary cycles, or intense weathering conditions. The index is commonly elevated in beach and littoral-zone depositional environments, where high-energy conditions and long transport distances favor the survival of these stable minerals.
ZTR minerals are typically abundant in beach, stream, and river sediments due to their high specific gravity and resistance to weathering. The index can be used alongside other minerals—such as garnet, magnetite, sphene, and minerals from local provenance sources—to refine assessments of sediment maturity and provenance.
Citation and Origin
The concept was formalized by John F. Hubert in his 1962 paper, "A zircon-tourmaline-rutile maturity index and the interdependence of the composition of heavy mineral assemblages with the gross composition and texture of sandstones," published in the Journal of Sedimentary Research (vol. 32, no. 3, pp. 440–450).
