Opaque in transmitted light, white in reflected light. Brassy-yellow in reflected light. Shows black to deep red colour in reflected light. Hydrated iron oxides, limonite is amorphous, and goethite forms micro-crystals. Limonite is generally translucent and red-brown in reflected and transmitted light.
Goethite forms red pseudo-hexagonal crystals a few microns across.
These are generally alteration products of mafic minerals and impart red colour to rocks, often coating grains. Most igneous and metamorphic rocks, and many sedimentary rocks, are composed largely of silicate minerals. However, one important group of rocks consists largely of carbonate minerals. These are the limestones and their metamorphic equivalents, marbles . Additionally, carbonate minerals are often an important component of silicate-dominated sandstones.