What make fossils Colourful?
We get used to the wonderful and numerous colors of some fossils, as if they were magic, featuring a range of bright and beautiful tonalities. But it is not magic... And science has the answer.
Before we go into the specifies of what colours the fossils, we must first understand how fossils are formed.
The Fossilisation Process
Visual representation of the sequence of events leading to the fossilisation of organic material and the subsequent discovery of the fossil. (i) Death of the organism, (ii) decay of soft tissues and burial, (iii) sediment deposition and fossilisation, (iv) uplift, erosion and exposure and (v) discovery and extraction.
The fossilisation processed are covered by the taphonomy, a very interesting branch of Paleontology that studies how the rests of the "living being" remain intact even today.
Not all fossils are generated in the same way. It all depends on its original composition the physical, chemical properties of the sedimentation environment among other conditions. Some of the quarries with exceptional preservation are called laggerstatten sites. Examples being the Solnhofen, Messel Pits and the Jehol Biota where we can find entirely articulated rests, with even soft parts preserved - including skin, hair, feathers and other delicate structures. These can make us think twice how such a whimsical preservation was even possible. However, under specific environmental conditions, the preservation is relatively easy to achieve.
The main factor that makes the rests of a living being decompose and disappear after death is oxygen. This element is the biggest destruction agent of organic matter and can greatly hamper the fossilisation process. However, in specific environment, the concentration of oxygen is low and in addition, there are low energy environment (weak current and winds). These are the conditions where the rests of dead animals do not suffer complete decomposition, favoring the preservation of structures which would otherwise have disappeared forever. The fossils we have today despite being well preserved are obviously not the organic matter of the animal. Through the process of mineralisation, permineralisation, substitution (where organic matter is replaced by inorganic matter), the rests are fossilised and maintains the original shape and structure.
Mineralisation
It is mainly due to the fluids that circulated between the sediments before their lithification. These aqueous fluids sometimes had high concentrations of certain elements which interacted with each other and with the medium, causing great mineralogical changes in the processes of sedimentary petrogenesis. The mineralization of the preserved elements can be by addition of new mineral components (cementation) or by substitution of the minerals existing in them (neomorphism or replacement).
Minerals With Greatest Influence
The main minerals that have intervened in the mineralization of the preserved elements, and currently found in fossils, are:
Calcite (calcium carbonate)
Internal chambers of an ammonite is filled with Calcite. Photo by Pandawithacamera ©
Dolomite (magnesium carbonate)
Pyrite and marcasite (iron sulfide)
Iron pyrite fossil ammomites found in blue lias deposits, Charmouth, along the Jurassic Coast, Dorset, UK. Alex Hyde ©
Limonite (hydrated iron oxide)
Hematite (iron oxide)
Argonite (crystal calcium carbonate)
Planticeras Meeki permineralisaed with Aragonite that refracts light and causes the iridescence. Photo by Clarevoiyant ©
Opal (hydrated siliceous oxide)
Opalised Shell from Lightining ridge, Australia. Photo by Clarevoiyant ©
Gypsum (hydrated calcium sulphate)
Glauconite (hydroxy-silicate of Fe, Al, Mg)
Vivianite (hydrated iron phosphate)
Apatite and hydroxyapatite (calcium phosphates)
Quartz (silicon oxide)
In some exceptional sites, fossils consisting of baritine, uraninite, and other radioactive minerals. Siderite, rhodochrosite, chalcopyrite, diopside or tremolite are also known. And yet rarer are the fossils constituted by azurite, malachite, anglesite, fluorite, galena, blenda, or native metals like silver and copper.
In the next posts we will be exploring on Dinosaur tooths and their colours. Also, why even in one location and formation, the colours can still vary!
