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Not necessarily, but the preparation and the specific scans you run are very important to being able to detect the mineralogy, particularly when clays are involved.  Without all this information it is not possible for me to say whether you have handled the sample correctly to identify all the important components. 

If the gouge was collected from outcrop or near surface exposures, it is also possible that the macroscopic properties you observe are due to the presence of organic material, which may have been destroyed or lost in your preparation (washing, drying or grinding) or might not be well characterized by XRD.

The short answer is, I don't know any reason why a 90% quartz sample would have the appearance you describe.  If you wish to send me some more detailed information (directly, not to the whole list) I would be happy to see if I can give you some more specific advice.

Good luck,
Christie

2011/11/14 ÕÅÀ× <[log in to unmask]>
Hi Prof Christie. Rowe,
 
The sample was grinded into powder and passed through the 300 mesh size seive(the sample particle size is less than 50 micron).
Is there any problem?
 
Date: Mon, 14 Nov 2011 10:18:49 -0500
From: [log in to unmask]
Subject: Re: Anyone can give me some advice?
To: [log in to unmask]


How was the sample prepared for XRD?

2011/11/14 ÕÅÀ× <[log in to unmask]>
Hi,
Now I am doing the natural gouge experiment using the triaxial appratus.
and I am confused by the  samples' XRD data.
The black natural gouge sample(water-saturated) is very soft and smoothly when you touch it, and it looks very like cohesive soil.
but the XRD data showes the major mineral content is quartz (90%)and the clay mineral is little (3%).
anyone can give me some advice why the sample(containing very high quartz content) looks like cohesive soil, and its fricitonal strength
is  low to 0.3-0.35?
 
 
 



--
***********************************************
Christie Rowe
Asst. Prof & Wares Faculty Scholar
Earth & Planetary Sciences, McGill University
3450 University St., Montreal, QC H3A 2A7
Office: 514 398 2769
http://eps.mcgill.ca/~crowe




--
***********************************************
Christie Rowe
Asst. Prof & Wares Faculty Scholar
Earth & Planetary Sciences, McGill University
3450 University St., Montreal, QC H3A 2A7
Office: 514 398 2769
http://eps.mcgill.ca/~crowe