MICHAEL SZOENYI / SCIENCE PHOTO LIBRARY MICHAEL SZOENYI / SCIENCE PHOTO LIBRARY
Ancient Roman columns. This shows several types of rock used to make architectural columns. In the foreground is fine-grained The colours in this column indicate different minerals in the matrix: quartz is white, plagioclase feldspar is grey and mica is black. Behind it is a far coarser-grained mineral, with travertine and marble behind that.
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