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 µm 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 µm (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
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