Organic matter? Oxide coating? Ferromagnesian character of the dolomite? Insoluble residue? Any other than these? Check with multiple thin sections and magnifications and you may find the answer. If you have access to CL, it is better.

Dolomite is brown or even black when Fe is present in the rock. Please, have a look at http://www.galleries.com/dolomite

Hello,

dissolved sulfate is an inhibitor of dolomite precipitation. Under sulfate-reducing conditions, sulfate concentration decreases, allowing for dolomite precipitation. Sulfide (especially iron sulfide) thus released may be trapped within dolomite crystals, thence the dark color. I hope it may help.

NT

Dear Mr. Massoud,

dolomite used to contain little bivalent Fe which is able to substitute for bivalent Mg and as such being conducive to a s.s.s. from dolomite through ferridolomite to ankerite. In case of oxygen entering into the chemical system Ca(Mg, Fe) (CO3)2 bivalent Fe is going to be oxidized and no longer capable of being accommodated into the crystal lattice of dolomite. The resultant hematite or goethite is either disseminated in the crystal or concentrated along its cleavage planes, leading to a reddish or brownish outward appearance.

Organic matter or inclusions of opaque minerals (ore minerals like pyrite) can be made accountable too but are second and third in the row as producers of such tarnishing effects.

The effect of oxidation of bivalent Fe in carbonate carbonates is the stronger the higher the Fe content is as it is the case with siderite and Fe-bearing smithsonite. Siderite may show up, in places, as a black carbonate. The lowest effect of this color changes may be observed in Fe-bearing calcite, which stand for reducing environments and may show a faint tint of brown being brought into the reaches of an oxidizing environment.

Best regards

H.G.Dill

Thanks for all