Primarily, it is likely to be an Fe-bearing calcite or an Fe-bearing dolomite which under the influence of oxygen has been altered as to the valence state of Fe. Since trivalent Fe cannot be accommodated into the lattice of carbonate minerals it is expelled from the structure and concentrated along the cleavage plains as a mixture of Fe oxide hydrates which suitably are denominated as "limonite" (rarely pure goethite). You are on the right track with your assumption thta the entire carbonate mineral is tarnished by this in crease of the Eh value within the environment.

H.G.Dill

Sir, thank you so much

Your answer is greatly appreciated.

You are welcome !

H.G.Dill

what about titanite as a resistate detrital mineral??

Dear Vina: I agree with Dr. Dill's answer in that it definitely is a carbonate mineral, somewhat oxidised to "limonite" in its borders and fractures, but it is NOT the golden one with XPL, since it has the usual pastel green-pink color of carbonate under XPL! The golden interference color is due to another mineral, with granular habit, high relief and moderate-high birrefringence, which I think is epidote. There's also some biotite and/or green chlorite, some titanite and colorless quartz. Perhaps you have noticed there are some remainings of microfossils in this marble, they look round, so might be foraminifers... Anyway, it is a metamorphic, low grade, rock in greenschist facies, as far as I can tell by looking at fixed images... Regards, Sebastián.

the photos indicate a grain mount; place the grain mount in a SEM under low vacuum get the EDS spectral image and interpret definitively

I agreed with Prof. G.Dill answer. There is more point i want to say here, When calcite is interact with aqueous fluids under oxidised environment, you can find some Fe rich secondary minerals, due to alteration. So the colour is become yellowish.

Dear all: the photomicrographs shown are not filtered with a blue cobalt glass, so everything looks yellowish. Moreoever, if you look closely you'll find out that it is not the carbonate the "golden" mineral under XPL, but a species with high relief and lower birrefringence than the carbonate, which could be epidote, or something else. Regards, Sebastian.