[meteorite-list] Primitive Achondrite Question

From: Bernd V. Pauli <bernd.pauli_at_meteoritecentral.com>
Date: Tue, 6 Dec 2011 08:33:08 +0000
Message-ID: <DIIE.00000078000030B7_at_10.0.100.101>

Hi Ruben and List,

As Jeff Grossman already stated the "primitive achondrite"
vs. LL7 isssue is highly controversial but here are some
criteria and examples culled from the literature:

1) Dodd (1981) lists three criteria that can be used to
   distinguish between types 6 and 7 ordinary chondrites:

(a) chondrules are poorly defined in type 6 and relics only in type 7;
(b) low-Ca pyroxenes in type 6 contain <1.0 wt% CaO, while in type 7
    they contain >1.0 wt%;
(c) feldspar coarsens going from type 6 to 7, with those in type 7
    being >100 ?m in size.*

2) WALLACE M. et al. (1991) Classification of ... Crockers Well
   (Meteoritics 26-3, 1991, 250):

- in thin section no visible chondrules
- clasts: recrystallized assemblages of olivine, pyroxene and plagioclase
- olivine: narrow range in composition from Fa29.6 to Fa30.1
- orthopyroxene (Fs23.7 to Fs24.2) is very calcium rich

3) Dar al Gani 1022 (LL7), Met.Bull. 87, 2003 July, p. A194:

- rare chondrule relics are present
- plagioclase grain size up to 200 ?m (see above 1c)*
- mineral phases have homogeneous chemical composition

4) Dhofar 011 (LL7), Met.Bull. 84, 2000 Aug:

- chondrules almost completely absent
- matrix thoroughly recrystallized and relatively coarse grained

5) Sahara 97037 (LL7), classified by M. Bourot-Denise

- very well recrystallized, with no evidence of chondrules
- a large fraction of pyroxenes are Ca-rich
- feldspar* and phosphates make mm-sized associations

6) EET 92012, EET 92013, EET 92016 (LL7)

Thin section description: B. Mason

- a few coarser aggregates appear to be vestigial chondrules
- olivine and pyroxene of uniform composition

Best wishes from rainy,
windy Southern Germany.

Bernd
Received on Tue 06 Dec 2011 03:33:08 AM PST