[meteorite-list] Re: Petrologic types and point values

From: Jeff Grossman <jgrossman_at_meteoritecentral.com>
Date: Thu Apr 22 10:23:48 2004
Message-ID: <5.1.0.14.2.20030313142501.023d0428_at_gsvaresm02.er.usgs.gov>

No, it's simple. The decimals divide petrologic type 3 into ten finer
divisions. They are meant to convey a sense of relative metamorphic grade,
just as the original 3-4-5-6 numbers did. The reason this was done for
type 3 and not for 4-5-6 is that the differences between a low-type-3 (now
3.0) and a high-type-3 (now 3.9) are as important as the differences
between a type 4 and a type 6. So it was decided that subdivision was
needed to make these distinctions (I was a coauthor on that original Nature
paper proposing this scheme back in 1980). You are correct that the scale
should not be thought of as linear.

jeff

At 02:18 PM 3/13/2003, E.L. Jones wrote:
>Hello Gents, List
>
>To my best recollection, the decimal value is not a scale from 3 to 4,
>such that 3.5 is "half way" would be between the two grades. It is an
>index for another characteristic peculiar to feldspathic minerals , I believe.
>
>The value after the decimal (e.g. 3."X", 3.5, 3.7 etc.) is a index of the
>degree to which some feldspar content fluoresces, indicating another,
>specific type of metamorphic process beyond the equilibration(
>comingling/co-melting) of all types of chondrites as they move from
>3-4-5-6 etc. I infer that the process is lost after stage 3 and may be
>insignificant(???)
>
>Perhaps someone else would offer the specifics as to who designed the
>index and what its significance is. We have discussed this before perhaps
>it is in the archives.
>
>Regards,
>Elton
>
>Lars Pedersen wrote:
>
>><snip>?
>>But what is the "tecknical" background for 3.2 - 3.8 ?
>>I would like to dig a litle bit deeper.
>>
>>Thanks
>>Lars
>>
>>
>>
>>>Hei Lars,
>>>Even finer grading, lowest value of 3.x means the chondrules
>>>are the most distinct, higher values that they become
>>>more 'blured'. But even 3.8 are quite distinct since
>>>it's 3, not 4,5,6 in the first place.
>>>
>>>
>>>
>
>
>
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Dr. Jeffrey N. Grossman phone: (703) 648-6184
US Geological Survey fax: (703) 648-6383
954 National Center
Reston, VA 20192, USA
Received on Thu 13 Mar 2003 02:31:33 PM PST