.
Questions and phone orders: 10 AM - 10 PM Pacific (760) 920-6465
Sold Out

porcelanite - formed from the reprecipitation of silica during the conversion of siliceous sediments to sedimentary rock - Monterey Formation, CA - display specimen

$ 9.50

Back in stock soon. Inquire.

Porcelanite from the Monterey Formation, Lake Lopez, San Luis Obispo County, California

The Monterey Formation is a diatomaceous accumulation of sediments in basins that were adjacent western North America between 15 and 5.5 million years ago. It is a part of a circum-Pacific ring of diatomaceous sediments that resulted from major changes in paleoclimate, coastal upwelling and areas of biosiliceous productivity that occurred at that time. Intensified coastal upwelling resulted in high plankton productivity and rapid deposition of siliceous ooze from the constant rain of dead diatoms and radiolarians falling to the sea floor. 

The sediments that accumulated farthest from the coast were primarily calcareous and siliceous. These sediments were rapidly and deeply buried. During diagenesis, biogenic silica dissolved and reprecipitated as more stable opal and eventually, with time and increased temperature, forming porcelanite, chalcedony and chert.  Diagenesis refers to the conversion of sediments into sedimentary rock. Where clay is a larger component of the sediments, diagenesis forms porcelanite, which has a silica content of 50 to 85%. With a silica content of 90% to 95%, the rock would be chert.

The Monterey Formation is the source rock for most of California’s petroleum production. The field photo shows tilted beds of porcelanite in the Monterey Formation near Lake Lopez.

 

 

Select a specimen: When more than one specimen is shown, you can select a particular specimen by telling us what is in the photo with it, a black and silver pen, a black mechanical pencil, a blue and silver pen, or one of those plus some number of coins, or you can let us make the selection.

Making multiple purchases? Click on the "combine shipping" button in the shopping cart. We'll send an invoice with combined shipping. A link in that invoice will bring you back to checkout, no hassle.

Shipping: By Priority Mail. Click > here < for shipping rates. See comment at Note! Use back button to return to this page.