Meteorite/Planetary Science Quiz

Question #8

Is the petrographic grade 7 valid?

Van Schmus and Wood originally defined the petrologic grid in 1967 (to grade 6). The petrologic grid presumes a metamorphic sequence ( solid-state changes caused by increasing temperature).

Some researchers have suggested that the petrologic grade 7 classification is not well described in relation to grade 6, and should not really be considered an extension of the chondrite petrologic grid. However, there is a clear definition for the petrologic characteristics of type 7 chondrites. They were first defined by Dodd in 1975.

In a later publishing, Dodd, (1981) list three criteria that can be used to distinguish between type 7 and 7 ordinary chondrites:

1. Chondrules are "poorly-defined" in type 6 and "relics-only" in type 7.

2. Low-Ca-pyroxenes in type 6 contain <1.0 wt% CaO, while in type 7 they contain >1 wt%.

3. Feldspar coarsens going from type 6 to 7 with those in type 7 being >100mm in size.

Recently, March 2001, LEW 88663 was classified as an L7 by Mittlefehldt and Lindstrom.

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