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
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...
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
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
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
atmospheric oxygen levels at this time would be much lower than
the zebra stone is located near the Argyle diamond
deposits which tap rocks very deep in the earth
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
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.
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