Dear Rick, et al.
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 !
Ron MIlls, Mills Gem Co. Los Osos, Ca.
