orbicular

7

The Mysterious Rediscovery of Ocean Jasper

Ocean jasper is such a unique stone, its found in only one place in the world – a small site along the northwest coast of the island of Madagascar. Ocean jasper is a variety of orbicular jasper, a type of jasper named for the spherical shapes that pattern the stone. The term jasper comes from the Greek word, iaspis, meaning “spotted stone.” Various forms of this jasper can be found in many areas around the world, however the ocean jasper of Madagascar is unique due its beautiful colors and markings.

Another aspect that lends mystery to the stone is the story of how it was found, or actually “found again”. Ocean Jasper was first written about in 1922, but the location was lost for the next 75 years. All that was known was that it came from somewhere in Madagascar, and that the location of the quarry had been lost. In the 1950s, a sample specimen was brought to the Museum of Sciences in Paris, France. But with the sample came a mystery – no one knew where the source was located and Madagascar is a very large island (approximately 1,000 km long and 300 km wide).

The stone made its reappearance to the world at the 2000 Tucson Gem and Mineral Show, and the story behind its rediscovery was the talk of the crowd. After 45 days of tirelessly searching along the Madagascar coast, an exploration group from the mining company, Madagascar Minerals, located the ocean jasper deposit. The reason the site was lost for so long is that it is only visible at low tide.

sources 1, 2, 3, 4, 5

Orbicular granite

We recently shared a photo of a Rapakivi granite, which contains grains of pinkish potassium feldspar surrounded by pale or white rims of plagioclase feldspar (http://tmblr.co/Zyv2Js1Zz1JTG). This is a different kind of granite called orbicular granite but it shows a similar feature – minerals with rims.

Orbicular granites form when different minerals crystallize from a magma in alternating patterns. Here, the first mineral to form was probably plagioclase feldspar. A small grain of it formed, but then something about the magma changed, possibly a temperature increase, causing the edges of the grain to re-melt and rounding it into a spherical shape. Crystals don’t grow spherical on their own, so the shape must imply some sort of erosion of the grains.

The magma then began crystallizing a different mineral, the dark layer, presumably in this case an amphibole mineral like hornblende, as is common in rocks that are crystallizing plagioclase. As the amphibole formed, the magma composition changed and the temperature decreased until again plagioclase was able to crystallize. 

From there, the alternating pattern continued, possibly due to arrival of new, hotter magma, or possibly due to movement of the crystal up and down inside a churning, convecting magma chamber. Finally, after this grain formed, it was broken and cross-cut by a dike of a different, later magma.

The sample is likely a few centimeters across and is about 400 million years old. It comes from Sout Island on New Zealand’s west coast and is on display at the Auckland War Memorial Museum.

To be completely accurate, this probably isn’t a true “granite”. True granites have a lot of potassium feldspar in them and this one doesn’t seem to, probably making it closer to a diorite or a quartz diorite. Commonly, the mining industry will call almost any igneous, crystalline rock “granite” even though geologists break these rocks apart into different groups depending on composition.

-JBB

Image credit: https://flic.kr/p/iQyXa

Read more:
http://www.aradon.com.au/orbicular_granite.html
http://www.kristallin.de/orbiculite/orbicular_rocks1.htm

2

The Featured Creature: Orbicular Batfish: This Leaf Looks a Little Fishy…

Juvenile orbicular batfish (Platax orbucularis) resemble dead leafs to a T – from the rusty brown color of its scales right down to the very way it moves in the water, it puts up quite the skilled mimetic performance.

Make sure to watch the video here: http://buff.ly/1lmDezw !

photo(top): vimeo still, (bottom): Flickr user dachalan

26 pound Ocean Jasper Sphere complete with fossils all through one side. Ocean Jasper, also known as Orbicular Jasper, assists in accepting responsibility and instills patience. A talisman for those with a fear of water. #divine #oceanJasper #jasper #oceanPlanet #fossil #orbicular #fibonacci #goldenSpiral #nautilus #sacredGeometry #talisman #openSea #shell #ocean #sphere #adornyourselfinquartz