9.3" Polished Mookaite Jasper Slab with Belemnite Cast - Australia

This is a gorgeous, 9.3" wide, polished slab of vibrantly colored mookaite. The hole at the middle of the specimen was formed by a belemnite that decomposed, resulting in the circular cast. One side has been polished to a glossy finish and it's accompanied by an acrylic display stand.

Mookaite jasper is a stunning, multi-colored stone with vibrant hues of reds, yellow and purples, formed from the fossilized remains of microscopic radiolarians. It is found in the 110 million-year-old Windalia Radiolarite formation, which formed on a shallow, marine shelf. Mookaite is found as vibrantly colored nodules in certain beds of soft white clay within the formation.

Radiolarite is a fine-grained, chert-like sedimentary rock composed predominantly of the microscopic remains of radiolarians. Radiolaria are tiny (100 micrometers!), free-floating, zooplankton that produce intricate mineral skeletons made of silica. When the radiolarians died, their tiny silica based skeletons settled on the bottom of the ocean and became covered in sediments like mud and clay.

As this sediment turned to sedimentary rock in a process known as diagenesis, the silica from the radiolarian skeletons pinched and coagulated into ribbons, nodules and other irregular concretions. These ribbons, nodules, and concretions are the mookaite within the clay layers. The coloration is due to other (mostly iron based) mineral impurities present along with the silica. Thus, mookaite can be considered as a fossiliferous sedimentary rock made out of the tiny skeletons of radiolarians.

Jasper is a term that can be applied to an opaque variety of chalcedony. The opaqueness is due to a higher concentration of impurities mixed with silica/quartz compared to other varieties of silica, such as quartz or agates. Like agate it may form in a wide variety of colors, and is often multi-colored. In most cases, jasper forms when silica-rich fluids permeate throughout a soft sediment or volcanic debris deposit. The fluids then crystallize around the particles/impurities, resulting in a cementation process. Most often, the impurities present determine the coloration of the deposit following solidification, but other factors can play a role in the color of what is now considered a jasper.
Chalcedony var. Jasper
Mooka Creek, Gascoyne Junction, Western Australia
Windalia Radiolarite
9.3 x 3.8", .35" thick