LITMIR.BIZ

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of the organism is held in place by a cement layer in the host sediment. This can be either as a complete block, in which case infiltration by depositing solutions will tend to take longer, or as coating laid down on the remains of the organism and fused into a cement case within a looser sediment, under these conditions calcite deposition is faster.

      The second method by which aragonite can be replaced by calcite takes place across a thin film, with the aragonite being dissolved on one side of the film and calcite being deposited on the other. This process is often referred to as calcitisation and is a process which can retain considerable skeletal detail in the final fossil. It also tends to reuse more of the original calcium carbonate, although this is not necessary if ambient conditions are correct, for example, if the water is a saturated calcium carbonate solution. With the replacement of one form of calcium carbonate with another, there is an almost inevitable reduction in fidelity since the original structure would have had other biomolecules present, such as scaffold proteins and the crystal structure would be very small indeed, controlled by the cells of the living organism. So laying down a calcite copy by simple chemical crystallisation will follow the original mould but will have larger crystals.

      Even though we refer to calcite as a uniform substance, the nature of calcite is such that it is described as having two forms itself. These are Low Magnesium Calcite (LMC) and High Magnesium Calcite (HMC). The difference between these two is quite small. LMC has between 1 and 4 mol% MgCO_3, while HMC contains 11–19 mol% MgCO_3. This small difference between the two is enough to make HMC less stable than LMC. Consequently, it is possible for skeletal material that was originally laid down by the organism as LMC to be retained through geological time and consequently retaining structural details down to the micro level. This situation is found in the brachiopods, which start with an LMC structural composition and consequently are sometimes little changed during fossilisation. With the original material remaining in situ it can be used to give a good basis for analysis of carbon and oxygen isotope values from their LMC calcite shells. Similarly, trilobites are also well represented in the fossil record, not just because they were a common part of the ecology, but also because their shells were mostly calcite with some phosphates present.

Structural crystalline minerals other than calcium carbonate can occasionally be found to have replaced calcite in some fossils, although this generally only happens in very specific circumstances. It is not always known in taphonomy what these specific circumstances are, but the fact that the substitutions do not take place commonly would imply specific chemical needs for the process to go to completion. It may also require unusual pressure and temperature to complete the conversion. Silification, that is, substituting silicate minerals in place of the calcium based calcite, is something which happens occasionally. In certain circumstances, silification may have spectacular results, one such is when fossils become converted to opal.

      Hydrated amorphous silica (SiO₂ nH₂O) in the form of opal can form fossils which are so decorative that they are often found being broken up for use in jewellery. If the silicates enter a body cavity and precipitate from solution, the external structure can be very well preserved, but not the internal details. As an alternative, should it infiltrate the organic material before decomposition, then good internal details can be preserved. Because deposition of silicon depends on the ground water and basal rocks, it is a very rare combination of events which results in opalised fossils. Some of the very best sites for these opal fossils are found in Australia, usually in active opal mining areas, such as Lightning Ridge in New South Wales and Coober Pedy in South Australia. Although there are other sites in the world where opalised fossils can be found, Australia is the most renowned area. It was in 1987 at Coober Pedy in South Australia that what has become known as Eric the Pliosaur was discovered. This reptile, Umoonasaurus demoscyllus, is one of the most complete opalised vertebrate skeletons known. Not only is it of interest and value as a fossil, it is also of considerable financial value for its opal content alone. The history of the fossil after being discovered is quite convoluted. It was originally sold to a dealer who went bankrupt, at which point it was very nearly sold to a jewellery consortium which had planned to turn it into jewellery at which point it would have been opal with the enhanced value of being a fossil. With help from Akubra Hats and money raised by school children, the fossil was bought for the Australian Museum, where it is now on show.

      The formation of opal fossils is a rare event, just as the formation of opals themselves is. Although opalised fossils must have a quite complicated process of formation, we can gain some insight from the way that gem‐stone opals are formed. Opal formation starts as silicon oxide spheres in a silica‐rich solution. These spheres settle under gravity and build up to form the gemstone opal. This process is very slow and to produce the precious colours of opal, the spheres need to be uniform in shape and between 150 and 400 nm in diameter. Although opal is generally made from even‐shaped and sized spheres, there is no long‐range or short‐range order in the stacking of the spheres. There are broadly two forms of opal, opal‐A (sometimes AG or AN) and opal‐C (sometimes CT). Opal‐A can transform into opal‐C under high pressure from overlying sedimentary deposits. This form of opal can sometimes contain as much as 10% by weight of H₂O.

Another mineralisation process which replaces calcium in fossilisation is pyritisation in which iron salts are laid down instead of calcite. This takes place when the water in which the organism died is high in iron sulphides. When surrounding water is high in iron, deposition of FeS₂ is affected by sulphides originating from decaying organic matter, of which there will be considerable amounts in a corpse. Pyritic fossils can be difficult to store because under humid or damp conditions, the iron pyrites of which they are made up, that is FeS₂, will chemically decay into iron oxide and sulphides, sometimes sulphur itself. This is a process which can be hastened by some types of bacteria. If this does start to happen to a pyritic fossil, it is possible to find sulphur deposits on the surface and the fossil may expand as iron oxide is a much larger molecule. This is exactly the same process which causes rust on ferrous metal to flake as the iron oxide pushes itself apart. Iron pyrite is not the only iron containing mineral which can create a fossil impression, but it is by far the commonest. Pyritisation has been shown to be a process capable of recording considerable morphological detail of soft‐bodied organisms of the Ediacaran (Smith 2019).

      It is possible for rapid precipitation to occur around a subject being fossilised which will then form a nodule retaining considerable detail in the organism at the centre of the nodule. A mineral that is often found involved in this process of nodule formation is siderite, that is iron (II) carbonate, FeCO₃. The unusual thing about siderite is that it is approximately 48% iron and consequently a valuable iron ore for commercial production of steel. Siderite is a diagenetic mineral in shales where it creates authigenic moulds of fossils as nodules, which have to be split to discover their content. One of the most famous sources of these fossil nodules is the Mazon Creek fossil beds in Illinois, USA, where it is even possible to find fossil sharks. These are rare fossils because although shark teeth are common, their skeletons are cartilaginous and therefore do not normally fossilise before decaying.

      Interestingly there is a single species of living gastropod, which is the only animal known to use an iron‐based mineral for its shell. Chrysomallon squamiferum is a gastropod from deep sea thermal vents at depths of 2500 m and greater, where the outer shell contains iron sulphides including the mineral greigite, Fe₃S₄.

      While there is a general procedure of one mineral replacing another, or sediment developing in a systematic way over the top of a form which will become fossilised, this is, perhaps, an oversimplification of a dynamic process. It has been shown that the linear progression model may not be so common as a simultaneous process, depending upon tissue type. It is quite likely that local chemistry within the cadaver will alter ion concentrations and influence specific precipitation events. These chemical changes will also be influenced by the state of decay, and it has been suggested that rapid biodegradation can enhance the detail which is eventually left behind (Jauvion et al. 2020). The rapid deposition of some preservative minerals will then be replaced at a slower pace by more robust minerals such as calcite.

As can be appreciated, the formation of fossilised material is a rare chance event which unless interpreted carefully gives

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