ATi Index
Definition
Apatite to Tourmaline index: [apatite/(apatite + tourmaline)] x 100. Low values indicate advanced weathering or sediment recycling because apatite is dissolved by weak acids while tourmaline is stable.
- Formula
- [apatite / (apatite + tourmaline)] x 100
- Value range
- 0 to 100
- Low value means
- Advanced chemical weathering or sediment recycling
- Unstable mineral
- Apatite, dissolved by weak acids
- Stable mineral
- Tourmaline, resistant to chemical weathering
Common questions
Why does apatite drop out of a sediment before tourmaline does?+
Apatite is chemically unstable in acidic soil water and dissolves relatively quickly, while tourmaline is one of the most weathering-resistant heavy minerals. As a sediment ages or is reworked, apatite is progressively removed and the ATi ratio falls.
How is the ATi index applied in forensic soil comparison?+
It is one of several heavy-mineral stability indices used alongside grain-size and mineralogical profiling to compare a questioned soil sample against a known source. A large ATi mismatch can support excluding a proposed source, but it is interpreted together with other indices rather than alone.
Related terms
- Bromoform (CHBr3)
- Tribromomethane, density 2.89 g/cm3, historically the standard heavy-separation liquid. Now largely displaced by sodium polytungstate because of its toxicity and regulatory requirements,...
- Heavy Minerals
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- Provenance
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- Sodium Polytungstate (SPT)
- An aqueous inorganic salt solution used as a non-toxic alternative to bromoform for density separation. Density is tunable by concentration; the 2.85-2.90...
- ZTR Index
- Zircon-Tourmaline-Rutile index: [(zircon + tourmaline + rutile)/total transparent non-opaques] x 100. High values mark texturally and chemically mature sediments that have lost...