[meteorite-list] Re: Chondrules Hardness

From: Elton Jones <jonee_at_meteoritecentral.com>
Date: Tue May 16 00:53:33 2006
Message-ID: <44695AC5.5010107_at_epix.net>

Hello Rob, Pete

This has surfaced on the list a few times but long ago. Your assessment
is seems reasonable to me. One thought was that the chondrule would
have the hardness of its mineral of specific composition. (e.g an
Olivine chondrule would be the same as Fayalite 6.5-7 or feldspar as
Orthoclase 6) with the matrix on the order of
Pyroxene/Bronzite/Hyperstene/Enstatie 5-6 Mohs etc. I don't know that
anyone actually tested that.

There are microhardness testers which should be capable of doing these
measurements and I , too would like to know. Perhaps someone with an
academic subscription could research any quantative work done on not
just chondrules but meteorite matrices in general.

Elton

Mohs and Hardness Trivia: " Hardness is the property of a material that
enables it to resist plastic deformation, usually by penetration.
However, the term hardness may also refer to resistance to bending,
scratching, abrasion or cutting."
<http://www.gordonengland.co.uk/hardness/> Most folks doing basic
mineral identification are familiar with the Mohs(Mohs') Scale. We take
the published hardnesses for granted, it is usually a range of values
which that are just relative after all.

Several words of caution regarding the Mohs scale or any hardness scale
for that matter. The Mohs scale was developed in the early 1800s and
based on common minerals. Be it remembered that this is a scale using
ordinal numbers but the actual hardness increases at an increasing rate.
(e.g gypsum-2 isn't twice as hard as talc-1 and corundum-9 isn't 9/10ths
the hardness of diamond-10) As technology progressed, somewhere, someone
added common items for reference. Glass being 5.5 was based on
non-tempered, soda-lime, plate glass-- and all glasses aren't the same.
Some volcanic glasses are in the 6+ range. There is some difference of
opinion now that glass is 5.5 Mohs. Plate glass has traditionally been
5.5 Mohs/143 Brinell but some glass's tensile can be 325
Brinell_at_1000psi/ 700 Vickers: which is in the mild tool-steel range.
Some new references put the hardness at 6 Mohs. As to knife blades and
nails this reference was established long before cheap pot-metal , 99
cent knife blades came into vogue. Files are harder than normal steel.
Normal nails not the panel brads are around 5, but I am hard pressed to
scratch glass with it most of the time. The new US penny is no longer
copper and I don't know if the English penny is/was comparable to the US
Alloy but "the penny" is considered to be a 3 Mohs.

I have a set of Mohs Index
minerals<http://teach.fcps.net/trt20/projects/EKU/Minerals/ID_Tests/MOHMIN.GIF>
at <http://teach.fcps.net/trt20/projects/EKU/Minerals/ID_Tests/> mounted
on rods with epoxy in a nice little case. Always remember when doing a
hardness test to start higher then you think it is because you'll wear
out your softer points a lot faster!

 Ergo, when checking for hardness, insure your reference
material/tool/point is the hardness you believe it to be, that the
hardness you are measuring is the hardness definition you intend. Start
with the harder points first.

Finally, the streak test is another common field test for mineral ID but
it is of little value in identifying most silicates, which is the
primary composition of chondrites and related clans. This is owing to
the fact that most all the silicates do not leave a colored streak even
thought their hardness is 5-6 and a streak plate is in the 7.5 Mohs
range. Minerals over 6 in hardness tend to break into little grains
along clevage planes rather than smear. So we tend to call the streak
"white" rather than clear. There are also black streak plates for the
purist.

Finally, a resource page for measuring hardness values and converting
scales, and etc.
<http://www.gordonengland.co.uk/hardness/brinell_conversion_chart.htm>

Rob McCafferty wrote:

>Well, My understanding is that normal glass is at
>about 5.5 and steel around 6.5 on the Moh scale.
>
>My own experience is that a chondrule rich meteorite
>will dull a steel file fairly quicky and I'd imagine
>that despite chondrules being supposed to be glass,
>they are much harder than terrestrial glass and it is
>the silicates which allow meteorite friability.
>
>I have not heard of a chondrule study but I don't
>think they'd be less than 6.0 on the Moh scale.
>
>Rob McC
>
>
Received on Tue 16 May 2006 12:53:25 AM PDT