[meteorite-list] Wbar Pearls or Beads

From: Bernd Pauli HD <bernd.pauli_at_meteoritecentral.com>
Date: Thu Apr 22 10:08:32 2004
Message-ID: <3D94BC03.EBE82053_at_lehrer.uni-karlsruhe.de>

Hello Anne, Hi Harald and List,

Anne wrote:

> I hope they are good enough but I am not so sure.
> 2 are of a Wabar pearl with black grainy interior.
> And 2 of another pearl showing WHITE interior.

They are extraordinarily beautiful - especially the ones with the
jet-black coating and the shining white interior - what an aesthetic
contrast!!! Thank you for the pics!

Anne also wrote:

> And what exactly do you mean by "de-vitrify" ?

a) Random House Webster:

devitrify = undergo a change in texture
                 from glassy to crystalline

to vitrify = to be converted into glass

b) Norton O.R. (2002) CEM, Glossary, p. 342:

devitrification = the conversion of a glass to a crystalline
                       texture while in the solid state

Harald wrote:

> Wabar impact glass is fortunately not devitrified yet.

No, not yet. But according to our dear, late friend and list member
Daryll Futrell, this will happen sooner or later. Here is an excerpt
from his and J.A. O'Keefe's article "A brief discussion of the
petrogenesis of Libyan Desert Glass" (FUTRELL D. et al. (1997)
Silica '96 - Meeting on Libyan Desert Glass and related desert
events, ed. MICHELE V., July 18, 1996, Bologna University,
pp. 116-117):

All natural glasses formed on earth from terrestrial materials
devitrify eventually. This includes impactite melts, volcanic
glasses, and fulgurites ...

On the other hand, all tektite glasses, with the exception of the
high K2O - low MgO type such as are found at the KT boundary
apparently will not devitrify unless heated for hours or days by
humans or in fires such as in the cases as described by Barnes
and Russell (1966).

Harald wrote:

> There are two type of impact glasses there
> - white pink fused sand
> - black blueish glass with NiFe spherules.

In the same paper, p. 117, Daryll and O'Keefe even present four
varieties of glassy qualities in Wabar crater impactite melts:

- slaggy black, brown, blue, green, grey and white specimens,
- specimens with an interior of vesicular snow-white melt and
   a black and brown opaque glazed coating,
- vesicular opaque grey melt with patches of vesicular
   snow-white melt
- opaque glassy splashform melts.

Harald wrote:

> As Shoemaker found out, the fused sand was coated
> with melt (black glass) while still in the air !

Right! In their dramatic description of the first seconds
after the Hiroshima bomb-like blast, the authors say :

Glowing fluid has coated the white boulders with a splatter that first
looks like white paint but then turns progressively yellow, orange, red
and finally black as it solidifies - all within the few seconds it takes

the rocks to hit the ground. Some pieces of the white rock are fully
coated by this black stuff; they metamorphose into a frothy, glassy
material so light that it could float on water, if there were any water
around.

(Reference: The Day the Sands Caught Fire - A desert impact site
demonstrates the wrath of rocks from space (by Jeffrey C. Wynn
and Eugene M. Shoemaker)

and: "objects > 5 mm in size invariably seem to possess dusty coatings"

Reference:

MITTLEFEHLDT D.W. et al. (1992) Dissemination and fractionation
of projectile materials in the impact melts from Wabar Crater, Saudi
Arabia (Meteoritics 27-4, 1992, pp.361-370).

Anne originally wrote:

> I found that they are composed of a glassy outer shell packed
> with tiny black grains of sand (I suppose) tightly packed and
> glued together. It looks as if the vitrification process was
> incomplete, only the outer layer turned to glass.

Tiny black grains:

As Harald already stated those tiny, black grains should be minute
spherules of metallic nickel-iron. These are ballistically dispersed
melt samples having aerodynamical shapes (> 10% meteoritic component).
These small melt samples (generally < 1 cm) can be found as droplets,
spheres, dumbbells and, according to Mittlefehldt et al. (1992) seem
to be abundant on the crater rim.

But the late Gene Shoemaker wrote in the above-mentioned paper:

Near the rims of the Wabar craters, the black glass looks superficially
like Hawaiian pahoehoe, a ropy, wrinkled rock that develops as thickly
flowing lava cools. Farther away, the glass pellets become smaller and
more droplike. At a distance of 850 meters northwest of the nearest
crater, the pellets are only a few millimeters across ...

and, last but not least, Anne also wrote:

> And 2 of another pearl showing WHITE interior.

Specimens with a white interior:

The highly vesicular, black and white melts (< 5% meteoritic
component) represents material from inside the crater.

Why two types with a different amount of "meteoritic contamination":

Mittlefehldt et al. write:

Therefore, the melt-spray samples ... are assumed to have been
derived from a location stratigraphically higher than those that
yielded the more massive black and white melts. The upper level
of the target zone is expected to experience higher shock stresses
and temperatures, and to be physically closer to the penetrating
meteorite, leading to increased contamination by meteoritic materials.


OK, that's all I can contribute presently. Maybe I
was able to shed at least some light on this topic.

Best wishes,

Bernd
Received on Fri 27 Sep 2002 04:13:55 PM PDT