Fibrous beamed Calcite

A rock born in a sea rich in carbonate

01

Sea water is a complex solution that has a high concentration of salt or sodium chloride and almost all known elements in equal proportions, even if only present in trace amounts.

02

Dissolved carbonate is one of the most important components of seawater: it is an ion, i.e. a charged portion of a molecule, made up of carbon and oxygen.

03

But while the sodium chloride content of the sea has always been the same, the dissolved carbonate content was up to four times higher in the past than it is now.

04

And so it must have been during the Jurassic period, approximately 200-175 million years ago. Calcium carbonate was formed spontaneously in the sea. 

05

You could say that the sea was like a huge bathtub with considerable limescale build-up. The calcite in this photograph is in fact one of these build-ups: calcium carbonate that spontaneously precipitated from seawater.

Name: Calcite cement in limestone
Classification: Chemical sedimentary rock, bioclastic floatstone
Mineral composition: Calcite
Fossils: Molluscs, algae (not visible in the photograph)
Location: La Varella Refuge, Fanes Alps (46°36’48.35″N12°0’24.42″E)
Formation: Group of grey limestones
Era: Lower Jurassic (approximately 200-175 million years ago)
Depositional environment: Carbonate substructure