GeologyRocks

Nope, definitely not. Limestones are biogeneic in origin (mostly) and are always precipitated from water. They *may* contain siliciclastic fragments, but this is rare, as siliciclastic sediment tends to inhibit the organisms that precipitate calcium carbonate in enough quantity to make a limestone (e.g. corals).

In addition, a gneiss consists of quartz and mafic minerals - all silicates. There may be bits of Ca in some of the minerals, but calcite would be the first thing to remineralise when making a gneiss.

Out of interest, why do you ask?

Thank you Jon for your quick repsonse!

 The reason I ask is because I am currently doing a geology coursework paper and the question asks me to explain how gneiss exposed at the surface could provide some of the chemical and mineral ingredients for sandstone, mudstone evaporite deposit and limestone.

I was able to answer the first three but was confused when it came to limestone. I posted on this board to get someone's opinion who knew more about the subject than myself. I suspected it was unlikely to be straight forward but now I do not know what they could possibly be looking for as an answer.

My only guess would be that the feldspar is plagioglase and it is calcium that is bonded to balance the charge. Do you not think it would be possible for the calcium to dissolve when in contact with acid rain?

The only way I can see this is, as you suggest, they mean the Ca in the plag providing Ca ions. Acid rain won't affect the Ca bonded in the plag as acid rain only dissolves CaCO3. I suspect it more that Gneiss contains Ca, which will be leached into any water passing over it and hence provide the Ca ions. However, most of the plag will be sodic, rather than calcic.

Some of the mafic minerals might contain Ca too, such as biotite and hornblends.

Best I can come up with!