Dolomite or High Magnesium Calcite?

Question

Dolomite is easily recognised from its shape and chemical composition. I have a problem in determining and differenciating crystals thought to be dolomite and/or high Mg calcite (HMC). These are very old carbonates (Late Cret age) and most of the crystals show inclusions and are either euhedral or subhedral-anhedral, they also show high peaks of Mg on EDX. No doubt, these are dolomites....BUT! other crystals (which are cements) are very clean, no inclusions, are planar subhedral-anhedral and show some high Mg peaks on EDX; some are nonplanar too. As old as they are, I would hardly think they are HMC's. Is it possible to find clean sparry crystals (as cements) with the above features to be dolomite? Any ideas? Thanks

Thank you all for the wonderful answers to my question. They were helpful in aome way.
Mr. Scientist, I agree XRD is still a good technique and that is how I got peaks of Mg of different levels. None of the crystals show only MgCo3; very low peaks of Fe, Si and ocassionally K, occur. I think HMC is metastable and cannot resist diagenesis for such a time.
Actually, it's not a single mineral but a lake carbonate rock with lots of microbial structures in the matrix. From the planar subhedral crystals and inclusions in them, I thought they were originally calcites that got dolomitized during diagenesis. But these inclusions also show high Mg peaks from XRD...I wonder?
Thanks again for the wonderful answers....I am enjoying this exchange of knowledge and welcome input from anybody.

Thank you all for the wonderful answers to my question. They were all helpful in some way.
Mr. Scientist, I agree XRD is still a good technique and that is how I got peaks of Mg of different levels. None of the crystals show only MgCo3; very low peaks of Fe, Si and occassionally K, occur. I think HMC is metastable and cannot resist diagenesis for such a time.
Actually, it's not a single mineral but a lake carbonate rock with lots of microbial structures in the matrix. From the planar subhedral crystals and inclusions in them, I thought they were originally calcites that got dolomitized during diagenesis. But these inclusions also show high Mg peaks from XRD...I wonder?
Thanks again for the wonderful answers....I am enjoying this exchange of knowledge and welcome input from anybody.

Answer 1

I suggest you using XRD (X-ray diffraction). Calcite and dolomite belong to different mineralogical groups, so you can distinguish them by the old, but still the best technique for identifying and characterizing minerals, XRD.
You can even model how much magnesium your calcite has based in XRD.

You didn't mention the results for Ca. If you are just getting MgCO3, then it is not dolomite, but magnesite.

Do not use the age of your rock to speculate that they can not be HMC. Use independent evidence like XRD.

PD: There are secular variations of the Ca/Mg ratio in the shells of marine organisms. May be that is related with your rock. Check the composition of the cement against the shells that you describe.

Answer 2

The birefringence may help if you are able to take an R.I or Refractive Index Calcite 1.72 where Dolomite is 1.79 increasing with iron

hello are you familiar with webmineral.com a mineralogy database this site has a search engine the link is below

In gemmology we use Spectrascope, Refractive Index, Birefringence, Specific Gravity, Crystalography, Polariscope, Optic Sign and Character, Ultraviolet light, Dichroscope,Chelsea Filter, Microscope Inclusions .

But these are usually well facetted specimans mineralogy deals with other than well cut stones . The refractive Index may diferentiate the two. If you have access to these instruments

http://www.webmineral.com/index.shtml

Search
http://www.webmineral.com/cgi-bin/search/search.pl

Dolomite This link is quite good
http://www.webmineral.com/data/Dolomite.shtml

http://www.webmineral.com/cgi-bin/search/search.pl?Match=1&Realm=All&Terms=dolomite

Answer 3

I am a geologist, and I would love to help you, but I sucked so bad at mineralogy (not to mention that I'm color blind so that wasn't a help either).

If I remember correctly, isn't dolomite a chemical alteration of limestone/mudstone/silstone over time? If correct, couldn't you look at the stratigraphic column for the site and look at the amounts of limestone present.

Again, if I remember correclty; isn't high Mg Calcite a low grade metamorphic rock, whereas everything else is sedimentary.