Zebra rockI picked up a couple of pieces of intriguing rock in a market in Sydney a few years ago, a
Zebra rockI picked up a couple of pieces of intriguing rock in a market in Sydney a few years ago, and finally got around to looking it up. Made up of fine grained silt sized particles, it displays a quite astonishing variety of patterns all picked out in red banding on a creamy tan background. It turns out to come from the Kimberleys in Western Australia, where it outcrops in lenses in the Johnny Cake Shale member of the late Precambrian Randford Formation (what a great name), a succession of brown shales that settled, maybe seasonally, in a quiet body of water.It formed as a siltstone some 670 million years ago, composed of mixed fine grains of, quartz and sericite (a kind of white mica), with a smaller amount of various clay minerals, while the reddish brown bands are rust stained by the iron oxide haematite (see http://on.fb.me/1QvqdSH and http://on.fb.me/1WK4JC5), that may have precipitated out rhythmically from pulses of passing fluids at some later point in geological history, or reflect layers of iron rich volcanic ash sedimentation. Some of the seams run for many kilometres, retaining uniform patterns throughout. Carved into a wide variety of ornaments, zebra stone is now getting established amongst the world’s ornamental rocks.LozImage credit: 1: Western Australian Museums 2: Fine Art Hahttp://bit.ly/20oOegJ -- source link
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