1 left

glass - display specimen of natural glass formed by combustion metamorphism of phosphate rich shale

$ 25.50

The most commonly encountered geologic glass is obsidian. Occasionally a lightning strike on a sand dune fuses the sand into a glassy tube called a fulgurite. Vitrophyre, another geologic glass, forms in a volcanic ash bed when the ash layers are thick enough to form an insulating blanket, and the temperature of the extruded ash is high enough to melt it. This glass, formed here in phosphatic layers in shale of the Monterey Formation, is uncommon, though deposits are also known from Israel, Jordan, Russia, Iran, India, Australia and Canada.

Combustion metamorphosed shales in the Monterey Formation form a belt roughly 3 km wide and 20 km long in Ventura County, California. Spontaneous subsurface combustion, probably occurring in the late 19th century, occurred when naturally occurring organic matter, likely petroleum, served as a fuel. Temperatures reached as high as 1600˚C and melted phosphatic layers in the shale that alternate with layers poor in P2O5. This specimen is particularly glassy and an excellent example.

The phosphate-poor layers became sintered, but did not melt. The melted phosphatic layers formed pseudomagmas that formed small-scale intrusions such as dikes, sills and laccoliths. The glass is either slaggy and vesicular or more dense and resembling obsidian. These specimens were collected from Grimes Canyon, just south of Fillmore, California and show the alternating glassy and stony laminae well. An excellent display specimen. The pencil is 5 inches long for scale.



Ships in a Priority Mail medium flat rate box and can be combined in that box with other purchases.  Click > About Shipping < for shipping rates. Use back button to return to this page. 

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.