[meteorite-list] Shergotty and Tschermak

From: bernd.pauli_at_paulinet.de <bernd.pauli_at_meteoritecentral.com>
Date: Thu Apr 22 10:31:10 2004
Message-ID: <DIIE.0000003D00001DA1_at_paulinet.de>

TSCHERMAK G. (1885) Die mikroskopische Beschaffenheit der Meteoriten
(Stuttgart E. Schweizerbart'sche Verlagshandlung, E. Koch, 23 pp.).
English Translation: The Microscopic Properties of Meteorites, Vol. 4,
No. 6 (Smithsonian Contributions to Astrophysics, Washington, D.C., 1964).

Translation by J.A. Wood and E.M. Wood

Shergotty, which I described several years ago (Tschermak, 1872b), should also be
included with the eucrites. It is a conspicuously granular rock consisting essentially of
dull yellow-gray grains and prisms, which were found to be augite, and water-clear glassy
grains and laths. The latter resemble no known mineral; I named this material maskelynite.

By incident light, the augite resembles volcanic augite. It is light brown and almost free
of inclusions. Simple twinning ... is common. The augite has an unusual turbid appearance,
visible by both incident and transmitted light. This is caused by very fine irregular cracks,
which indicate that the augite has undergone a mechanical transformation. It differs from
terrestrial augite in this respect, as well as in its chemical composition, being poorer in
lime (CaO). The maskelynite is absolutely colorless and clear. In thin-section it usually
occurs in elongated forms with fine lines parallel to their length, so that by ordinary light
it looks exactly like plagioclase. Its chemical composition corresponds to a plagioclase of
the labradorite series; but by polarized light it remains completely dark, proving that it is
simply refracting. Cracks in the maskelynite are conchoidal*, like fracture surfaces in a glass.
Maskelynite therefore has the properties of labradorite that has been transformed into an
amorphous state by remelting or by a purely mechanical transformation.

Inclusions in the maskelynite are common. They are quite irregular in form and consist of
augite and magnetite. Cloudy white particles, a form of maskelynite, are visible here and
there in Shergotty. A yellow silicate, optically biaxial, is present in small grains,
but is very rare. Two opaque constituents have been recognized. One is magnetite, which
was first found to be a meteoritic mineral in Shergotty; the other is very rare pyrrhotite.

* conchoidal = noting a shell-like fracture form produced
on certain minerals by a blow (Random Webster's Dictionary).


Best wishes,

Bernd
Received on Fri 02 Apr 2004 04:57:45 PM PST