[meteorite-list] Questions on olivine in meterorites.

From: Warin Roger <warinroger_at_meteoritecentral.com>
Date: Mon Sep 11 10:43:26 2006
Message-ID: <20060911064645.73171.qmail_at_web26406.mail.ukl.yahoo.com>

Hi Suzanne & Jim, all,
 
A rock with olivine never contains quartz. In the presence of olivine, SiO2 produces pyroxenes. Quartz has a low birefringence, as opposed to olivine. Quartz polarizes in the light gray and white of the 1st order. In terrestrial rocks, it is thus easy to control the thickness of the thin section: quartz cannot be coloured in cross polarized light. It is thus necessary to thin the thin section to remove any colour in quartz. That's what's done for terrestrial rocks.
 
In the frequent absence of quartz in meteorites, I think another reference is used, like the colour of feldspars.
  
>From the point of view of the chemical stability of olivine, there is nothing to fear. Under normal conditions, olivine is a very stable mineral. Its reactivity with respect to water or quartz appears only under extreme conditions of metamorphism.
 
Also, do not forget that olivine is a semi-precious stone (= peridot). At room temperature, the stability of an unprotected thin section will depend especially on alteration of the metal, phosphides, etc., not on olivine, feldspars, or pyroxenes.
 
Regards.
Roger Warin
www.agab.be
(thin sections)


----- Message d'origine ----
De : Suzanne and Jim <suzieandjim_at_yahoo.com>
? : Meteorite-list_at_meteoritecentral.com
Envoy? le : Samedi, 9 Septembre 2006, 11h43mn 44s
Objet : [meteorite-list] Questions on olivine in meterorites.


hi all

here is a question for those out there that have
prepared thin sections without the aid of expensive
equipment ( or a university degree in geology ) .

It seems to me to get an appropriate standard 30micron
thickness without the aid of specialist equipment
you have to know exactly what you are looking at in
the slide. If you know you are looking at olivine
in crossed polarized light then a 2nd order orange
would be a good indicator interference colour for the
correct thickness.

Suppose we have an unidentified material trying to
determine if the crystal you are looking at is
olivine but can not be sure of the material thickness.
  You see a good 2nd order orange, so
now the crystal could be olivine and you have reached
30 microns , or the crystal could be quartz and
you have reached 100 microns. ( or one of a number of
other possibilities but lets keep it simple and
assume its either olivine or quartz)

Whats the trick in determining if its olivine or
quartz in this situation? I believe that both olivine

and quartz have fairly similar properties both
fracture the same and have no cleavage both can be
clear in normal polarized light and depending on the
thickness of the section can give good 2nd and 3rd
order colours in crossed polarized light.

Knowing its definitely quartz you are looking at and
that there is a lot of it in there means that you can
be fairly certain that the material is not meteoritic,
well at least in most cases.

While I have your attention and talking about olivine,
anyone have any ideas about the weathering of olivine
in a non-arid climate. I know that olivine may be
altered to serpentine and other things
and that serpentine does not have the high
interference colours of olivine, so its possible that
material containing olivine in a moisture rich
environment may lose the tell-tale high interference
colours we love to look at so much in meteorite thin
sections.

The questions are, can this alteration to serpentine
occur at normal ambient daytime temperature ( say 15
to 30C) and what order of magnitude of time does it
take olivine to alter in a non-arid environment.
10 years, 100 years, 1000 years etc?

Looking at this question another way, if you have a
thin section containing olivine that is exposed to the

environment ( i.e. no cover slip on the slide) how
long will the olivine keep its high interference
colours?
How long will the thin section be good representation
of the orginal material?.


jim





Suzanne & Jim

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