Conodont Alteration Index (nonfiction)
The Conodont Alteration Index (CAI) is used to estimate the maximum temperature reached by a sedimentary rock using thermal alteration of conodont fossils. Conodonts in fossiliferous carbonates are prepared by dissolving the matrix with weak acid, since the conodonts are composed of apatite and thus do not dissolve as readily as carbonate. The fossils are then compared to the index under a microscope.
The CAI ranges from 1 to 6, as follows:
| CAI | Approximate conodont color | Temperature range (Celsius) | |
|---|---|---|---|
| 1 | Pale brown | <50°-80° | |
| 2 | Dark brown | 60°-140° | |
| 3 | Dark grey-brown | 110°-200° | |
| 4 | Dark grey | 190°-300° | |
| 5 | Black | 300°-480° | |
| 6 | Pale grey to white | 360°-550° | |
The CAI is commonly used by paleontologists due to its ease of measurement and the abundance of Conodonta throughout marine carbonates of the Paleozoic. However, the organism disappears from the fossil record after the Triassic period, so the CAI is not available to analyze rocks younger than Template:Ma. Additionally, the index can be positively skewed in regions of hydrothermal alteration.
See also
- Conodont (nonfiction) - extinct agnathan chordates resembling eels, classified in the class Conodonta. For many years, they were known only from tooth-like microfossils found in isolation and now called conodont elements. Knowledge about soft tissues remains limited. The animals are also called Conodontophora (conodont bearers) to avoid ambiguity. Conodonts are considered index fossils, fossils used to define and identify geological periods.
- Foraminiferal Coloration Index (nonfiction) (FCI) - a tool for assessing the thermal alteration of organic matter buried in sedimentary rock. It uses temperature-controlled colour changes in the organic cement of agglutinated foraminifera (microfossils) to estimate thermal alteration. The method is empirical and based on determination of colour by visual comparison of fossil specimens to the Geological Society of America Rock-Color Chart (Munsell colour system) under a binocular microscope.