Most oolitic forms are in reference to round forms in a matrix of
contrasting material. Ocean jasper is a good example as is Poppy
Jasper from here in California.
Oolites and orbicular jasper aren’t even close to being the same
thing.
Oolites usually occur as limestone made up of tiny spherical
carbonate particles, called ooliths, cemented together. Ooliths have
a concentric structure with a diameter up to 2 mm/0.08 in. They were
formed by chemical precipitation and accumulation on ancient sea
floors. The surface texture of oolites is rather like that of fish
roe. The late Jurassic limestones of the British Isles are mostly
oolitic in nature. (Credit Tiscali Reference).
According to the US Geological Survey and other sources, orbicular
jasper forms when a silica rich rhyolitic ash flow cools quickly.
Quartz and feldspar crystallize in spherulites, radial aggregates of
needle like crystals, that provide the interesting structure seen in
this kind of jasper. Better-known examples of orbicular jasper are
often seen offered as Poppy Jasper or Morgan Hill Jasper from
California or Ocean Jasper from Madagascar.
We seem to have a litle tempest in a teapot going about descriptive
terms regarding circular structures in rocks.
The three terms causing this storm are orbicular, oolitic and
pisolitic. All three terms are meant to describe circular forms in
various rocks. There is very little agreement as to what the size,
origin or mode of occurrence might be. The literal definitions of
each of the terms are
Oolitic…like fish eggs.
Pisolitic…like peas and
Orbicular…having orbs ( circles )
Now, it seems to me that angels might be orbicular inasmuch as they
usually have orbs. The sun might also occaisionally be called
orbicular because it very often has a halo of ice crystals. As for
oolitic, fish eggs seldom are as tiny as 2 mms. or less; more often
than not they are closer to 5 or 6 mm. As for pisolitic, most of us
know that peas are usually around the size of fish eggs. What ! all
this is terribly confusing…
Then there is the statement that orbicular jasper is typically like
the Poppy or Ocean Jaspers and that they are usually derived from
indurated and siliciified volcanic ash. This might be the case for
the Ocean Jasper, but I seriously doubt that it applies to the
several oolitic or pisolitic jaspers that occur in the coastal
ranges of California. These California jaspers are actually
indurated and silicified oceanic sediments that have been
metamorphosed into chert resembling jasper. They are formed as a
result of subduction and involve low temperature/ medium pressure
metamorphism.
There are certain jaspers that readily fall into the realm of
orbicular and I would suggest that Bruneau jasper from Idaho is a
perfect example as are several others that emanate from Mexico. In
these cases one of the characteristic attributes of same is that the
orbs are seldom complete. The so called orbs in Poppy Jasper are
always complete…indeed, since they are supposedly derived from
ocean sediments it is entirely possible they they were formed from
"oolitic" or “pisolitic” limestone. And so we come full circle…to
pea or not to pea !