[meteorite-list] Orbicular granite and chondrites

From: Elton Jones <jonee_at_meteoritecentral.com>
Date: Sat Sep 10 22:00:43 2005
Message-ID: <43238FC9.3060700_at_epix.net>

Impactika_at_aol.com wrote:

> But could one (orbicular granite) be mistaken for the other (chondritic
> meteorites)?

Only if going by written descriptions like is found on your typical
meteorite identification page.(wink) If one isn't familiar with the
scales/magnitudes between a typical "og" zenolith vs a chondrule then a
black and white photo of orbicular granite looks a lot like a
non-polarized, thin-section photo of a chondritic meteorite.

The typical size of the nodules/zenoliths range between 1/4(rarely) to
3/4(more common) to 3 and 4 inches.

Radiating kind:
<http://www.tucsonshow.com/reports/tucson2004/images/Mvc-148x.jpg>
<http://www.tucsonshow.com/reports/tucson2004/p5.shtml>

<http://www.atacamatravel.cl/fotos/03400/kugelgranit2.jpg>
<http://www.cst.cmich.edu/users/dietr1rv/granite-a.JPG>
<http://www.cst.cmich.edu/users/dietr1rv/granite.htm>
Extract from above:In addition, granites (etc.) of certain overall
colors or exhibiting certain relatively distinct appearances have been
given names that relate to those characteristics, and those names are
used along with or instead of the formal geographically based names.
And, a few of these rocks have been and continue to be used as gemrocks.
  Two examples are:

     * Orbicular granite - Typical orbicular granites contain orbicules
-- i.e., spheroidal masses the components of which are arranged in
roughly concentric layers -- surrounded by a continuum of "normal"
granite. The layers consist of concentrations of the major light versus
dark components of the rocks. In some orbicular granites, at least some
of the orbicules have cores of the "normal" rock. (Figures D & E).
     * Rapakivi granite (also called "Baltic brown") - Typical Rapakivi
granites consist of ovoid masses, with chiefly alkali feldspar cores and
relatively thin chiefly off-white plagioclase feldspar mantles, within a
so-to-speak continuum of "normal" granite. The alkali feldspars in
most of Rapakivi granite are salmon colored orthoclase whereas the
plagioclases tend to be off-white oligclase; the other common
constituents are biotite and/or hornblende. Some petrographers consider
these rocks to be porphyries with the feldspar ovoid masses the
phenocrysts and the surrounding "normal" granite the groundmass.



more mineralogy: (yawn)

Ingo mentioned diorite, "orbicular granite" in trade spans granite,
diorite and gabbro classes of granitoid . "Orbicular granite" is more of
a trade name and covers possibly two or three different mineralogical
processes. I mentioned before this was a remelt but other studies
suggest this is a cumulate around the margins of a batholith. You know
at the edges in what is called the chilled zone where heat is lost to
the surrounding rock. I tend to discount the idea that this is a
cumulate in the sense of meteorite cumulates.

I hate to call it a zenolith because 90% of what I've seen has "spheres"
aka zenoliths are predominately pink plagioclase feldspar nodules which
are 60-90% plagioclase and some mixture of hornblende/amphibole,
mica(biotite?),and silica (quartz). The nodules are surrounded by a
halo of quartz. The more expensive stuff and what was likely the
original orbicular granite from Finland has radiating patterns in cross
section--which for me pretty much mandates an entirely different
mineralogical process between what is sold in the jewelery trade vs the
dimension stone trade.

I use a slab of orbicular granite as a door stop. The zenoliths range
from 1/4 inch to 2 inches in diameter . They have a halo of quartz
followed by another halo of hornblende. This is consistent with a
Bowen's reaction and temperature of solidification/fusion. Bowen's
Reaction with examples
<http://csmres.jmu.edu/geollab/Fichter/RockMin/RockMin.html>


Elton
Received on Sat 10 Sep 2005 10:00:41 PM PDT