WHY DO IRIDESCENT AMMONITE FOSSILS APPEAR IN MADAGASCAR?

 

Cretaceous (Albian ~ 110 Million years old) Aioloceras aff besairiei (Collingion, 1949) from Madagascar..Discovered in the Mahajanga province of Madagascar, these ammonites have taken on a brilliant, rainbow iridescence (Photo by Jordi Sanchez).

The appearance of iridescent ammonite fossils in Madagascar is due to a unique combination of geological factors and the specific conditions that occurred during their fossilization. Here are the main reasons:

1. Preservation of the Nacre (or Mother-of-Pearl):

• The shells of ammonites were primarily composed of aragonite, a type of calcium carbonate that also forms the iridescent inner layer of many living mollusk shells, known as nacre or mother-of-pearl.
• In the particular sedimentation conditions of certain areas of Madagascar during the Cretaceous period (approximately 100 to 66 million years ago), this nacre layer was preserved during the fossilization process in some ammonites.

 Aioloceras aff besairiei (Collingion, 1949) with a rainbow iridescense (Photo by Jordi Sanchez).

2. Mineralization and Chemical Replacement:

• Over time, the sediments that covered the ammonites compacted and hardened, forming rocks. During this process, the ammonite shells underwent mineralization.
• In some cases, the original aragonite of the shell was replaced by other minerals such as calcite, silica, or pyrite. However, when the conditions were right, the laminar microstructure of the nacre could be maintained despite the mineral substitution.

3. Light Interference:

• The iridescence we observe in these fossils is not due to color pigments, but to an optical phenomenon called interference.
• Nacre is composed of thousands of microscopic layers of aragonite separated by thin layers of organic material. When light strikes these layers, part of it is reflected from the surface of each layer.
• Due to the different distances traveled by the reflected light waves, some interfere constructively (reinforcing each other and producing bright colors), while others interfere destructively (canceling each other out).
• The color we observe depends on the thickness of the aragonite layers and the angle from which the fossil is viewed. Thicker layers tend to produce red, orange, and yellow tones, while thinner layers generate greens, blues, and violets.

 Aioloceras aff besairiei (Collingion, 1949) with a brillant red iridescense (Photo by Jordi Sanchez).

4. Geological Conditions of Madagascar:

• Madagascar is known for having Cretaceous sedimentary deposits that provided the necessary chemical and pressure conditions for the preservation of nacre in some ammonites.
• The specific composition of the sediments, the presence or absence of certain minerals, and the rate of sedimentation played a crucial role in this process.

In summary, the iridescence of the ammonite fossils from Madagascar is the result of the preservation of the aragonite microstructure of their original nacre during fossilization, which allows for the interference of light and the creation of the characteristic bright colors. The particular geological conditions of Madagascar during the Cretaceous period favored this type of preservation in certain specimens.