[MSA-Talk] Substitution in Siderite

[ja...@ktgeo.com]

Hello All,

I am looking at a set of XRD traces taken on shale samples from a well. I
have identified magnesite (MgCO3) in one of the samples. Some of the
other samples appear to have siderite. The main siderite peak and the
other siderite peaks I can see are shifted to higher 2 theta angles
(smaller d-spacings). In the past, I have seen a shift of siderite peaks
to lower 2 theta angles and have always assumed this to be a substitution
of Ca and or Mg into siderite. My question is, what element(s) commonly
substituted into siderite would cause this shift to higher 2 theta angles?

These samples also contain Dolomite (2-20%), Calcite (<1%), Pyrite,
Marcasite, occasional chlorite and of course Quartz and Illite.

Thanks in advance for your help.

James Talbot
K-T GeoServices, Inc.
www.ktgeo.com


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[Barry Bickmore]

Hi James,

Higher 2-theta means shorter repeat distances, so you are looking for something smaller than Fe2+ that would lead to smaller unit cell dimensions.  Mg2+ substitution might be the culprit.  See this paper:

http://www.minsocam.org/msa/ammin/toc/Articles_Free/1998/Zhang_p280-287_98.pdf

Barry Bickmore
Professor of Geological Sciences
Brigham Young University

[Barry Bickmore]

Also, I don't have time to look up papers on it, but it seems reasonable to suppose that Ca2+ (which is larger than Fe2+) would make the unit cell dimensions larger, and hence make the 2-theta angles smaller.  

Barry