How are chert nodules formed?
How are chert nodules formed?
Chert and flint nodules are often found in beds of limestone and chalk. They form from the redeposition of amorphous silica arising from the dissolution of siliceous spicules of sponges, or debris from radiolaria and the postdepositional replacement of either the enclosing limestone or chalk by this silica.
Where is nodular chert found?
limestone
Nodular chert is most common in limestone but may also be found in shales and sandstones.
What are nodules in geology?
Geologically speaking, nodules are defined as round or spherical concretions of rock. In marine geology, more specifically, the term usually refers to manganese nodules, a mineral type that forms by accreting in concentric layers around a fragment of some other rock or biological material, called a nucleus.
What are the two types of chert?
Two major varieties of chert deposits exist—namely, bedded chert and nodular chert.
How do you tell if a rock is a nodule?
A nodule rock is a solid rock with no hollow center. So unlike geodes, the interior of a nodule rock contains no empty space. It does, however, contain minerals. Nodule rocks are harder than the rocks that surround them.
How do flint nodules form?
As it compacted, it squeezed out the seawater containing dissolved quartz, or silica (which comes from the skeletons of tiny sponges, a very simple animal). This silica was pushed out into gaps, cracks and burrows in the chalky mud to form nodules or layers of flint.
What environment is chert formed in?
carbonate environments
Cherts typically form in places where clastic influx is low (i.e. clastic starved), or in carbonate environments. Both of these generally require tectonic quiesence. The very clean nature of this specimen (indicated by its color) tells us it was probably formed in a very clastic-clean carbonate environment.
How do minerals form nodules?
Nodules form due to chemical reactions in the sediment often very soon after burial, whilst the rest of the sediment is still soft. Localised chemical conditions cause the rock to harden (cementation).
How do you identify chert rocks?
Chert has four diagnostic features: the waxy luster, a conchoidal (shell-shaped) fracture of the silica mineral chalcedony that composes it, a hardness of seven on the Mohs scale, and a smooth (non-clastic) sedimentary texture. Many types of chert fit into this categorization.
How many types of chert are there?
Several varieties are included under the general term chert: jasper, chalcedony, agate (qq. v.), flint, porcelanite, and novaculite.
How do you tell the difference between a geode and a nodule?
“A geode is a round rock which contains a hollow cavity lined with crystals. Rocks which are completely filled with small compact crystal formations such as agate, jasper or chalcedony are called nodules. The only difference between a geode and a nodule is that a geode has a hollow cavity, and a nodule is solid.”