Zebra Stone

Zebra Stone - how the zebra got it's stripes

Let the rocks tell their story
This is what is known of zebra stone...
Some suggestions for the stripes and "blebs" in Zebra stone ...

The Story

Zebra stone is a very strange looking rock and there is lots of controversy regarding it's age and how it was formed.
Zebra stone looks like zebra stripes and stripes with blobs -  a very unusual stone!
This specimen shows the dark hematite (haematite) rich layers as both stripes and regular "blebs"
The quick answer is that we do not know for sure how these patterns were formed so long ago.
 And that is a good thing.
There are still lots of mysteries to solve in geology and the more we look the more mysteries we discover!
There are still lots of exciting things to explore and understand in the world of earth sciences!
To explain the patterns in zebra stone geologists have suggested some interesting and let's face it some pretty wild ideas...

This is what is known of zebra stone...

• zebra stone is a unique very fine grained siltstone / claystone
• first discovered in 1924 by T. Blatchford, near the old Argyle Downs homestead, Western Australia.
• the rock is composed largely of extremely fine grained quartz and sericite, with minor alunite, kaolinite clay and its polymorph dickite.
• it shows regularly spaced, ferruginous (rusty iron) brown bands on a white to pale brown clay rich matrix.
• the width of the bands vary depending on the thickness of the beds they come from. Patterns also include rods and irregular "blebs", extending through the rock in parallel rows, similar to the near vertical alignment of bands. 
• both the light and dark bands contain some iron minerals otherwise the composition is the same
•  the light bands contain Limonite Fe₂O₃ + H₂O
• while the dark bands contain Hematite (Haematite) Fe₂O₃.
• Hematite (Haematite) is generally denser than Limonite
• Hematite (haematitie) is paramagnetic (slightly magnetic)
• focus seems to be on why the dark bands are irregular rather than why the light bands are regular
• the rock readily absorbs water and currently is found only below the level of the local water table
• it is old; some say 1.2 billion years old, others say 600 million years old
• atmospheric oxygen levels at this time would be much lower than present
• the zebra stone is located near the Argyle diamond deposits which tap rocks very deep in the earth

Some suggestions for the stripes and "blebs" in Zebra stone ...

• the original layers seem to have been laid down by some periodic or rhythmic process in very very calm water over a great deal of time to form layers though the extreme fine grained nature of the minerals grains prevents clear  identification of these layers
• if the layers remain then... these clay-like layers retained a high proportion of water while subject to gentle pressure and/or heating  and/or density settling that are he clay-like layers are over-saturated with regard to some chemical compounds
• the patterns are caused by cyanobacterial mats infilling subtle ripple marks in low energy shallow marine environments perhaps with periodic drying to remove water
• the minerals grains are so fine as to act in the manner of a non-Newtonian fluid (think of a thick mixture of cornstarch and water  - solid if left untouched , liquid if disturbed) put under pressure the slightly denser dark layers segregate to form layers or "blebs"
• a "sloshy" clay-like fluid, very high in water facilitates each "layer" to settle out in density levels with the only variables being Hematite and Limonite
• if the coloured bands can be shown to cut across the layers then...  the colour banding is thought to have resulted from the rhythmic precipitation of well defined, hematite rich liesegang bands( ie coloured bands that cut across the layers) in selected parts of the siltstone, caused by the percolation of warm hydrothermal fluids or other chemical activity during a period of alteration of the rock
• the slight magnetism (paramagnetism) of Hematite (haematite) may be enough to attract mineral grains together but not enough to cross the boundary of a layer resulting in accumulations
• in the nearby diamond deposits, the diamonds are though to have come up from extreme depths (maybe 200km down) at high speeds and their presence nearby may account for some very unusual  and as yet undescribed geological processes

In short then we do not know for certain what happened.

Learn more about ORIGINS OF SEDIMENTARY ROCKS

source: https://www.mindat.org/loc-251210.html
             from the Earth Science Australia collection
             Loughman F.and Roberts F.,1990, Composition and origin of the zebra rock from the East Kimberley region of Western Australia, Australia Journal of Earth Sciences, v37, p201-205
             https://www.researchgate.net/post/How_are_liesegang_rings_formed_in_sandstone


TOP