[meteorite-list] Ibitira in thin section

From: bernd.pauli_at_paulinet.de <bernd.pauli_at_meteoritecentral.com>
Date: Mon Mar 21 13:24:59 2005
Message-ID: <DIIE.0000000900003565_at_paulinet.de>

Hello Listees, Listoids, List,

The noncumulate, unbrecciated, monomict Ibitira eucrite with its
mm-sized gas holes or vesicles has always been on my wish list
(see O.R. Norton, p. 153 Cambridge Encyclopedia of Meteorites
for a breathtaking slice measuring about 10 by 10 cm in size).

So I was more than happy when Michael Cottingham offered a small
but very characteristic slice of 0.21 grams for sale some time ago
that I immediately purchased from him.

Not long ago, Michael Blood offered 3 Ibitira thin sections, one of
which I acquired, as this would be the ideal complement to my little
slice and it even looked so similar -- almost as if it had been cut
from the same piece.

Today, when I looked at my Ibitira thin section under crossed polars and
with several different magnifications, there were many "oohs" and "aahs"
at the spectacular, colorful Ca-poor pyroxenes (pigeonite) most of which
are crossed by countless parallel, fine laths of Ca-rich augite (so-called
exsolution lamellae).

The pyroxenes account for about 60% of my thin section, the second most
abundant phase is, of course, plagioclase (grayish-white because it is
chemically zoned). These plagioclase crystals show undulatory extinction
when you rotate the section under your microscope with polars crossed.

There are also several opaque inclusions throughout the section (they look
black both in transmitted light and under crossed polars). According to the
literature, they are ilmenite (CaTiO3), chromite (FeCr2O4), FeNi-metal and
troilite.

Ibitira is also said to contain the silica polymorph tridymite (SiO2) but I
haven't been able to locate these laths yet as I've never looked for tridymite
before. These laths can be as long as 7 mm (in NWA 1181) but are usually
about 1 mm in length.


Best Eucritic
Wishes,

Bernd
Received on Sat 19 Mar 2005 04:41:49 PM PST