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

From: Lars Pedersen <lars.b_at_meteoritecentral.com>
Date: Thu Apr 22 10:23:48 2004
Message-ID: <006c01c2e999$86013790$7a21a150_at_star>

Hi Jeff and list
>You are correct that the scale
>should not be thought of as linear.

Ok ...
But when it is not linear, there must be specific definitions for each
subdivision ?.

Best
Lars

----- Original Message -----
From: "Jeff Grossman" <jgrossman_at_usgs.gov>
To: <meteorite-list_at_meteoritecentral.com>
Sent: Thursday, March 13, 2003 8:31 PM
Subject: Re: [meteorite-list] Re: Petrologic types and point values


> 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
>
>
>
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Received on Thu 13 Mar 2003 02:48:29 PM PST