I am working on a project for school on identifying rocks and I have a rock that is dark brown, has impressions of shells in it but does not seem to actually contain shells, it reacts a little bit with HCl but not very much and it is softer than glass.What is it? Thanks!
my guess is dolostone. dolomite does not act as readily with acid as calcite does. it has to be powdered. so scratch it to create a powder then put acid on the powder. if it bubbles vigorously then it's a rock made of dolomite, thus dolostone. ----------- I don't think it could be a sandstone if there are imprints of fossils in it unless it's a very fine-grained sandstone. Even then, it's very doubtful. Could be a shale with limestone or some calcite in it. Just depends on the grain size of the sample. But I don't think it could be a shale considering it has shell imprints and shales are usually associated with very calm waters in lake environments or on the continent shelf in deep ocean waters.
It could be a sandstone or shale that has a small amount of limestone in it.
