[meteorite-list] Re: TEKTITES!! ....METEORITES OR JUST GLASS Obsidian

From: EL Jones <jonee_at_meteoritecentral.com>
Date: Thu Apr 22 10:08:31 2004
Message-ID: <3D917B0A.9070804_at_epix.net>

  Let me try to get this one right the first time.

This doesn't address the Lunar meteorite vs Ejecta debate but is a
discussion on how volcanic glass-specifically "Apache Tears" forms. See
the previous discussion on tektite and impact glass for comparison.

First some terms:
Felsic- Light-colored low temperature rock/mineral from the bottom of
Bowen's Reaction Series (BRS) These are the last minerals to form from a
magmatic source as the temperature drops towards 800°C e.g. muscovite,
quartz. The rocks obsidian and scoria are the exceptions to the light
colored felsic rocks.

Malfic- Dark-colored high temperature rock/minerals from the top of the
BRS, These are the top end which start to solidify below 1400°C e.g.
olivine, pyroxene, plagioclase.

Intermediate- middle range rock/mineral having characteristics of both
malfic and felsic e.g. orthoclase, biotite.

 Scoria/Pumice/Tuft/Ash- all pyroclastic (extrusive volcanic) rocks
which are compositionally the same which have subtle
texture/structure/cooling differences which grade into each other.
They are often used interchangably in general discussion but are
distinct technically.

Bowen's Reaction Series (BRS)- A predictable, idealized model of magma
evolution first developed around 100 years ago( important in the study
of meteorites!*). It describes a succession of mineral combinations
which occur as a melt cools to solid; forming mineral/crystals given a
sufficiently slow cooling period. It is frequently depicted on a "Y"
shaped diagram. Key to the understanding of BRS as it relates here is
1) the higher temperature minerals get "first dibs" on compatible
elements and compounds and leave the party in a pecking order such that
the last in line is Silica/quartz. 2) If the temperature doesn't get
high enough, the high (melting) temperature compounds don't even come to
the party and Silica Rules!

Apache Tears as common term for nodular obsidian.

The bulk content of the rock obsidian is silica, silicon dioxide-- SiO 2
which is a light colored, "felsic" mixture. Notice I did not say
"mineral". It is the relatively-fast cooling which forms glass, without
allowing a crystalline structure to form nor, for other mineral-forming
compounds to segregate to form a mineral--a subtle but technical point.
The trace material's found in obsidian are from the "felsic to
intermediate" BRS "pre- mineral compounds/ molecules". Bulk obsidian
composition will be from the bottom (felsic) end of the BRS. and one
would think it would be light in color. Obsidian is dark because, while
it is a glass, it has many impurities which absorb light and make it
appear dark.

Apache tears are frequently found in stream beds were they have eroded
from a softer "felsic" pumice/scoria-ash in which they form. So they may
on first observation look as it they were spewed from a volcano high
into the air. They are not ejected in the form they are found. They
form after hot ash accumulates. The ash retains enough heat to allow
low end BRS actions to occur, while too cool for anything above molten
Silica and volcanic gasses to migrate. It is hot enough but barely long
enough for those two things to happen. Molecules actually remelt and
droplets of silica glass accrete into developing gas pockets. This
allows most gas to escape from within the glass as it pools and grows
in non-crystaline fashion. Obsidian has a relatively high water content,
interestingly enough . The "tear" is more dense than the scoria/pumice/
tuft which is still highly gaseous. So it settles by this short-lived
sorting action in bands within the ash. If the mass remained hotter
longer the obsidian would consolidate into veins of obsidian towards the
bottom of the ash layer. If you crushed the scoria to eliminate the gas
pockets the obsidian would float to the top, but because of the gas
content scoria- specifically pumice will float on water.

 Apache Tears, therefore are a post-ejection form of obsidian which
form rapidly and cease growing into larger veins of obsidian because the
temperature of the scoria/ pumice ash falls below the threshold of the
mineral formation in the lower end of Bowen's reaction series. When one
looks at ash containing Apache Tears under a microscope tiny droplets of
obsidian may be seen. Change any of the variables such as hotter ash,
longer cooling times, or more malfic content and other wonderful things
happen like augite crystals and muscovite in scoria from Vesuvius or in
one case topaz crystals from ash deposits in Utah. Apache tears can
only form when the conditions are just right as Goldie Locks might
say....not too hot, not too long, not too much ash accumulation to keep
all that heat in --just right!.

Now if we only understood so well or could duplicate the conditions
under which tektites formed! One thing is for certain. Apache Tears,
while glass, do not share a common history with mainstream tektites.

Regards,
Elton

* BRS: Interpreting the composition and size of crystals in Achondrites
disclose the temperature, size, and composition history of the
Achondrite parent body. Interpreting the coarseness or fineness of
Irons also disclose the history and size of the Iron's parent body. The
matrix of common chondrites is malfic; being the olivine, bronzite,
hyperstene and amphibole discontinuous branch of the BRS.



thornysahuaro wrote:

>Greetings all,
>Would any of you please explain the differences between Tektites and
>Apache Tears. Apache Tears are rounded bits of smokey grey obsidian
>(volcanic glass) found all over the Southwest US. They have a dull black
>surface and often have perfect regmaglypts when found. To me this would
>indicate that they were ejected quite high before they fell. When
>polished they range from nearly opaque black to a nearly clear light gray
>color. Some have lines or swirls of darker glass that can be seen if held
>up to a light.
>Thanks,
>
Received on Wed 25 Sep 2002 04:59:54 AM PDT