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Sponge Porosphaera F2127
Minerals in fossils
Fossils always largely consist of minerals. Usually, however, they are not recognizable as crystals, but they are present in the material. Most fossils consist of the mineral Calcite (we call these calcified), other fossils consist largely of Quartz (which we call silicified) and also fossils of phosphate (phosphatized) are common. But fossils may also include more exotic minerals. During the fossilisation proces, the original material is completely or partially replaced by other minerals which are dissolved in the water in the sediment.
In some cases, the mineral is visible in a crystalline form in a fossil. This may be due to recrystallization of the material in the fossil, or because crystals are grown in a hollow space.
In some cases, the original material has been replaced by other minerals. In the Middle Miocene deposits of the 'Broken Cliffs' locality in South Australias Riverlands District for example, marine shells have been replaced by gypsum. Gypsum is in itself not a rare material, but this kind of fossilisation is.
Gastropod Livonia hannafordi (McCoy, 1866) from the Broken Cliffs locality, completely replaced by gypsum. Collection C. Ah Yee.
Fossils are usually filled by massive sediment which leaves no space. However, the space sometimes remain hollow or the fossil is only partially filled. By deposition of dissolved minerals in groundwater slow crystallization may occur over geological time. The walls of the cavity are thus coated with crystals. Depending on the geological conditions, this is usually Calcite, but Quartz also occurs.
If the crystallization stops after some time, a partially filled space is left which can be found later as a geode. However, most geodes are of volcanic origin. Geopetals as geodes in fossils are called are relatively rare. If the crystallization continues the cavity will become completely filled over geological time. Individual crystals are generally not recognizable as such.
Fossils with a shell or scale with only small openings are most likely not to filled with sediment, and therefore most likely to crystal growth in the cavity. Crystal growth is relatively common in cephalopods (Orthocones, goniatites, Ceratites, Ammonites), the inside of a sea urchin, brachiopods and bivalves. These fossils must have been buried alive or very fast after death to leave open spaces in its body.
Geologists are delighted with partially filled cavities in fossils. Even though later the rest of the cavity was filled by crystallization, often the filling plane can easily be traced. This shows the original top and bottom of the layer in the field. Partially filled cavities in fossils in the sediment is called a to -bottom criterium. Layers are of course more or less deposited horizontally, but may be put upright or tilted by tectonics. Therefore it is very important to know what the original top and bottom in a given layer has been. The starting point is always that the filling level has been more or less horizontally in the cavity, and the filled portion is on the bottom.
Thanks to C Ah Yee (Aussifos) for contributing to this page
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