[Rockhounds] dolomite/Fe-dolomite/ankerite ??

[Pmodreski at aol.com]

Dennis, I'll take a stab at this one, not that I have an answer.
 
I don't know of any simple foolproof way to distinguish these.  There  is a 
continual series of iron content between dolomite, ferroan dolomite  
(considered just a variety of dolomite) and ankerite.  One could measure  the exact 
refractive indices and that would allow one to distinguish them with  reasonable 
accuracy (r.i. increases with iron content), but only a petrographic  
microscopy lab with a set of calibrated r.i. oils has the equipment to do  that.  
Likewise a precise measurement of specific gravity might allow one  to distinguish 
them (likewise, sp. gr. increased with iron content), but that  would require 
a reasonable-size, pure matrix-free piece of the mineral and a  balance to 
make accurate sp. gr. determinations.
 
A visual guideline would of course be that if the mineral is very light  
colored, white or off-white or pale pink, it is probably dolomite.  If it  is dark 
tan or brown and shows a lot of limonite oxidation staining on exposed  
surfaces, it is probably iron-rich and hence is probably ankerite.   "Somewhere in 
between" on those characteristics, is where it would always remain  hard to 
judge.  That's as much as I can offer--probably not anything new to  you.
 
Pete Modreski
 
In a message dated 1/24/2008 6:17:11 AM Mountain Standard Time,  
buff1 at ptd.net writes:

Ok,
Here's my next "stupid" question... Is there a non-lab process  for 
determining the differance between dolomite, Ferroan Dolomite, and  
ankerite?? Other than the historical ID of a mineral I would think it  
might be difficult. On the basis of rock type found in, I would say this  
is a very thin vein of crystallized "?" in a triassic age red bed shale  
apparently influenced or altered by a triassic age diabase. Also at what  
point does ferroan dolomite become simply dolomite or ankerite. I am not  
in dispute with any historical ID but would like to know the answer that  
has been posed by my never ending inquisition.

Dennis  Buffenmyer







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