[meteorite-list] Jeff's Response with regard to type 7

From: Bernd Pauli HD <bernd.pauli_at_meteoritecentral.com>
Date: Thu Apr 22 09:47:11 2004
Message-ID: <3BF2CE73.11805726_at_lehrer1.rz.uni-karlsruhe.de>

Subject:Type 7 Chondrites
Date: Wed, 14 Nov 2001 14:32:38 -0500
Sender: Jeff Grossman <jgrossman_at_usgs.gov>

Hello Jeff and List!

Thank you, Jeff, for this response. As I think it is not a private
message, I take the liberty to pass it on to our List. Jeff wrote:

> The definitive discussion of type 7 chondrites is in Mittlefehldt
> and Lindstrom (2001), MAPS 36(3) p. 439. Just because some-
> thing is called type 7 in the literature (including MetBull) doesn't
> mean that's a good classification. Here's part of their discussion:

"Petrologic grade 7 chondrites were first defined by Dodd
et al. (1975) based on a study of the Shaw L chondrite.
Subsequent petrologic study showed that Shaw is an impact-
melt breccia (Taylor et al., 1979) and thus should not really be
considered an extension of the chondrite petrologic grid
originally defined by Van Schmus and Wood (1967). This
petrologic grid presumes a metamorphic sequence (i.e., a
sequence of solid-state changes caused by increasing
temperature). Brearley and Jones (1998) have suggested that
the petrologic grade 7 chondrite classification is not well
described in relation to grade 6, and that the classification is
not generally accepted. However, there is a clear definition
for the petrologic characteristics of type 7 chondrites (Dodd,
1981). Petrologic grade 7 chondrites are known from the LL
group. Heyse (1978) classified the Uden LL chondrite as type
7, and found type 7 clasts in the St. Mesmin LL chondrite
breccia. Takeda et al. (1984) classified Yamato (Y) 74160 as
an LL7 chondrite. However, these authors concluded that
Y-74160 was formed by localized partial melting of a
metamorphosed chondrite as a result of impact, and for this
reason we argue that Y-74160 should not be classified as type 7.

"Dodd (1981, Tables 2.3 and 4.2) lists three criteria that can
be used to distinguish between types 6 and 7 ordinary
chondrites:

(i) chondrules are "poorly defined" in type 6 and
"relics only" in type 7;

(ii) low-Ca pyroxenes in type 6 contain
<=1.0 wt% CaO, while in type 7 they contain >1.0 wt%;

(iii) feldspar coarsens going from type 6 to 7, with those in type 7
being >=100 µm in size. For typical L-chondrite low-Ca
pyroxenes, 1.0 wt% CaO translates to about 1.9 mol% Wo.

"The distinction between the relative states of preservation
of chondrules is subjective. Nevertheless, we believe that in
LEW 88663 the chondrules are best described as being only
relics as for a type 7 designation. Our petrographic microscope
observations did not reveal chondrules or their remnants, and
this was also noted in the initial description (Score and
Lindstrom, 1992a). It was only in backscattered electron
imaging (Fig. 2b) that we identified textures that indicate that
relic chondrules are present. In type 6 ordinary chondrites,
even highly weathered specimens, chondrules are easily found,
although their borders with the host chondrite are indistinct."
Received on Wed 14 Nov 2001 03:05:07 PM PST