porcelanite - in the series from diatomite to chert - 5 student specimen unit

Description

If making multiple purchases, please read the note following the
red
text
below!
Porcelanite from the Monterey Formation, Lake Lopez, San Luis Obispo County, California - Unit of 5 student specimens
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, the 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.
We really don't like the tiny finger nail-sized specimens offered to students, as we feel these are too small to see rock textures and too small for examining the identifying characteristics of a mineral. We want the "Oh, Wow!" effect when students see the set of specimens they are going to work with. We collect these specimens so that five specimens, or possibly six, will fit in a small flat rate box, and we keep the price low, as we know these will be used in a classroom. For teachers we count like a baker and a dozen is more than 12, since you will always need a few spares.  If you are a teacher, let us know.
You are buying a unit of five student specimens. The photo is representative of what we ship.
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 mechanical pencil, a silver and black pen, a silver and blue pen, or one of those plus some number of coins, or you can let us make the selection.
Making multiple purchases?
Just add items to your cart. When finished shopping, click on
“request total”
at upper right in the Shopping Cart window.
Do not proceed to checkout.
We will send an eBay invoice with the correct combined shipping.
The combined shipping will often be less than eBay shows. Please wait for that invoice before paying.
There is no obligation to purchase. You can decide after seeing what shipping will cost.
Shipping:
Ships in a Priority Mail small flat rate box unless combined with other purchases. We always combine shipping to get your purchase to you at the lowest cost.
International customers:
We sell and ship internationally. If eBay’s page does not let you enter your country name to determine the shipping cost for something you are interested in, please contact us.  We can tell you what the shipping would cost and how to make the purchase. You can then decide if you wish to complete the transaction. Use the
Contact Seller
link in the Seller Information box on the right side of this page.
Pay securely with your debit or credit card through PayPal! You do not need a Pay Pal account to do this. Yes, we do combine shipping!