If iron is coloring calcite, of what color? This should give a major clue.

It is golden brown with titanium in equal ratio to iron.

As Stefano mentioned coloring would give a clue. Iron II would be TYPICALLY (I am emphasizing typically, to be certain you need laboratory test though) red-brown and iron III would be darker with coloring towards more blackish

How much is trace? If 1% or more Fe, Mossbauer spectroscopy would be the best method. If less than 1%, XAFS would be useful down to about 20 ppm Fe. But it would need careful calibration.

Hello

is it possible that you have ilmenite micro inclusions? and @Shahnewaz: Fe2+ gives black in its oxides, greenis in silicates and perhaps (occasionally) dark brown in carbonates - if I look to some siderite crystals on my table. But Fe+3 gives "ochre" meaning yellow to brownish yellow if it is oxy-hydroxide like goethite or amorphous and colours the calcite also to yellow or pale brown. If it occurs as oxide, like Fe2O3 what we call hematite (from hematos->blood) it has a typical red color, and may colour carbonates, silicates and glass to red, pink or pale pink as its amount decreases. And if Shannon has Fe2+ substituting calcite than upon heating it will probably turn into black (wustite) oxide. If she has goethite than it will turn into pink, due to red (hematite) oxide formation. And if she has ilmenite inclusion the color would not change. On a fine grained powder ~450 grades C would do it...

But if need analytical proof, go for Mössbauer.

Good luck!

Frank Huggins presented the two best methods for determining iron oxidation state and speciation. There are a number of papers in the literature on the topic as well.

Within my knowledge, these colors indicate Fe III. The high titanium amount would indicate a basic Ilmenite composition, and the golden-brown material could well be called Leucoxene, a generic term for Ti oxides weathering products. Precipitation of Fe II in association with calcite would require a very very anoxic environment

Thank you all so much for your help. Because this is only an undergraduate thesis, I don't necessarily need analytical proof (although I may if I can get approval). However, if anyone knows of any specific publications that address the coloring issue, I am eager to look deeper into this topic.

I would advise that analysis of the calcite sample be carried out using a XRD or Microprobe analyzer. Color alone is not a good proof.

I love XAFS as much as anyone, but there are simpler ways of determining Fe valence. For a start, try this paper: J. A. D. Dickson, Carbonate identification and genesis as revealed by staining, Journal of Sedimentary Research, June 1966, v. 36, p. 491-505. There are later papers on this topic as well.

Ferrous Fe can be titrated or measured photometrically, but that is only necessary if you need a quantitative determination.

Color is a perilous thing to go by! Nontronite contains mostly Fe(III) but is usually green, and celadonite containing mostly Fe(III) is either the green of its name or a beautiful blue. But despite that, golden brown sounds more likely to be ferric.