[meteorite-list] Tatahouine:Slickensides vs Shattercones

[E. L. Jones]

Hello Steve, List.

You raise a couple questions about geologic structures so here is the 
scoop and why.  These photos are not an example of  "slickenside".  Zag, 
however, has the definitive example of slickenside found within a 
meteorite. a slickenside appears  polished--hence the term "slick". For 
an enlarged photo of Zag slickenside 
<http://www.caillou-noir.com/TM2/image/Zag128400Aslickbzoomweb.jpg> or 
<http://www.caillou-noir.com/TM2/CGZag.htm> for the main page

Terrestrial slickenside can be very thin or not, undulating, and can run 
for the length of a fault.  In meteorites, they are much thinner, form 
rapidly-- or in theory, by successive slips caused by successive 
collisions, as opposed to forming over time during fault slippages such 
as would be the case on Earth.   It can undulate depending on the axis 
one views it.  It is filled with a micro vein of reconsolidated "rock 
flower"or --debris dislodged in the grinding process of the two adjacent 
surfaces slipping by each other. Meteoritical slickenside such as in Zag 
resembles a very thin vein of coal or carbonaceous shale.  To my 
knowledge it doesn't contain glass such as maskenlite (which would 
indicate flash melting and shock rebounding). It is clearly formed by 
two surfaces slipping by each other.

As to shattercones...
The possibilities of "shattercone-like" structures in Tatahouine as been 
discussed before with no conclusions.  Recently a study was conducted on 
Tatahouine that found blebs of Alpha-crystobalite (a low temperature but 
high temperature polymorph of quartz).  This perhaps indicates a low 
shock stage which tends to rule out shattercones.  On the other hand, 
there were clues to a shock stage 5 followed by "annealing" which would 
have reabsorbed the laminae we would expect to find in sheared surfaces 
which might favor a shattercone effect. If there are shattercones in 
this meteorite they are not in a form as we know them on earth.

What you are likely seeing in your specimen is the striation/laminae of 
the "enstatite twinning law". Striations are superficially similar but 
distinctly different structures to shattercones.  In mineralogy, certain 
minerals tend to follow patterns within a given mineral. A twin is two 
or more crystals growing within each other.(still called a twin even if 
it is a triplet). Sometimes they cross, sometimes they grow interlaced 
like a deck of cards shuffled into each other. This interlaced form is 
what enstatite follows.  This meteorite is composed of an orthopyroxene 
(inosilicate series) at a ratio of Enstatite75%:Mg2 Si2 O6 and 
Ferrosilite25%: Fe2 Si2 O6.   Enstatite is known for a laminate 
twining/striations.

As I understand it-- with cleavage plans at 87° or so, the enstatite 
plates are orientted back to back ( think of stacking playing cards face 
to back and looking at the whole deck from the side.  What you are 
seeing is the truncated cleavage planes edge on.  If you looked down the 
edge, parallel to the laminae,  under a microscope, the edge would look 
like the teeth of a saw blade.  To further complicate the observation, 
Tatahouine appears sort of homogeneous-what might be called in 
mineralogy "massive".  In fact it seems to be composed of large 
intergrown and extensively orientated enstatite crystals. This may 
contribute to the appearance of a shattercone look when trying to trace 
the striation lines from one surface to another.  We don't ordinarily 
see a fragment large enough to see a zig-zag-zig-zag as in a line of 
toppled dominoes. We only see closely spaced "zigs"in different planes 
so they look like they are converging toward a point. 

Regards,
Elton


Steve Witt wrote:

>Greetings List,
>
>Slickensides has been a topic brought up many times on the list. I'm
>still not exactly clear on what this is and I'd like to get a
>clarification. I recently acquired a small fragment of Tatahouine
>from Anne Black at Impactika. Besides having some fusion crust the
>specimen shows striations very similar to those seen in shattercones.
>