marți, 18 aprilie 2017

Cum se formează fosilele opalizate?

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It is extremely rare for conditions to be right for formation of fossils; and even more rare for opalised fossils to form. Usually, only the hard parts of living things fossilise – for example seed pods, wood, teeth, bones and shells. This often happens after the plant or animal (or a part of it) is buried in sand or other sediments that slowly turn to stone.

Opal forms in cavities within rocks. If a cavity has formed because a bone, shell or pinecone was buried in the sand or clay that later became the rock, and conditions are right for opal formation, then the opal forms a fossil replica of the original object that was buried. We get opalised fossils of two kinds:


1. Internal details not preserved: Opal starts as a solution of silica in water. If the silica solution fills an empty space left by a shell, bone etc that has rotted away – like jelly poured into a mould – it may harden to form an opalised cast of the original object. Most opalised shell fossils are ‘jelly mould’ fossils – the outside shape is beautifully preserved, but the opal inside doesn’t record any of the creature’s internal structure.


2. Internal details preserved: If the buried organic material hasn’t rotted away and a silica solution soaks into it, when the silica hardens it may form an opal replica of the internal structure of the object. This happens sometimes with wood or bone.






Moroccan Ammonite with a calcite replacement of its cellular structure and inlaid sections of synthetic Opal. Height: 5 in. Photo Copyright © Bonhams




Some ammonites were formidable animals more than 2 feet in diameter, such as this spectacular 75-million-year-old specimen. Credit: AMNH/C.Chesek




Shell Opal. Locality: Wyoming Opal Fields, Lightning Ridge, Finch Co., New South Wales, Australia. Largest Crystal Size: 14 mm. Photo Copyright © Matthew Goodwin




Opalised theropod dinosaur tooth. Lightning Ridge, New South Wales. Photo Copyright © Carl Bento/Australian Museum




Opalized Ammonite, Diameter 11 cm. Photo Copyright © Claude/flickr




 Opalized Snail




Opalized Pine Cone Fossil - Lightning Ridge, NSW, Australia




Opalised Belemnite




Opalized bivalve (dorsal view of hingeline) from the Coober Pedy Opal Field, South Australia. Photo Copyright © James St. John




Precious opal after belemnite. Locality: Coober Pedy, Central North, South Australia, Australia. Field of View: 12 cm. Photo Copyright © Dmitry Tonkacheev


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