Texture, average. observations rock name Rock Type size of minerals, clasts or grains Felsic - Light colored pumice Frothy Mafic - Dark colored (“none”) Dark to black - felsic Glassy obsidian (DOES NOT follow normal color index) Felsic - Light colored rhyolite Fine grained Intermediate andesite Extrusive, volcanic IGNEOUS ROCKS Mafic - Dark colored basalt Rock formed from a molten amygdaloidal Modifier for extrusive rocks with trapped gas bubbles that might fill in with minerals state or vesicular Interlocking homogenous Felsic - Light colored (“none”) crystalline texture – no Medium grained Intermediate dacite apparent preferred Dikes, sills, etc. orientation to the mineral Mafic - Dark colored diabase grains Felsic - Light colored granite Coarse grained Intermediate diorite Generally intrusive or plutonic Mafic - Dark colored gabbro Ultramafic peridotite porphyry or Modifier for extrusive rocks with fine matrix and well developed crystals porphyritic Rounded clasts conglomerate Coarse Fragments Angular clasts breccia “Clean” quartz (may be other minor minerals as well) sandstone SEDIMENTARY Sand sized fragments ROCKS “Dirty” with rock fragments & clay graywacke Rocks formed from other Fine grained - cannot see Nonfoliated siltstone rocks or existing materials. individual clasts or grains Foliated shale Consolidated detrital clasts Fossiliferous Mostly shell fragments coquina (grains), chemical Softer - reacts with dilute acid limestone precipitates, and or Chemical - fine grain biological residue Harder - does not react with dilute acid chert bedded to massive salt which is the mineral halite rock salt Evaporates Bedded to massive gypsum rock gypsum Dull – metallic sound when tapped with metal object slate Foliated, very fine grained-no Foliated, shiny due to increased size of METAMORPHIC visible minerals phyllite ROCKS micaceous minerals (almost see them) Foliated - medium to coarse Individual mineral grains visible. Major mineral(s) Rocks changed by heat schist grain included as name modifiers (ex. Biotite Schist) and or pressure but not melted. Alternating layers of light (felsic) and dark (mafic) Color banded gneiss Interlocking, mixed minerals crystalline textures, Distinct layering - often highly Alternating layers of felsic igneous rock (light) and migmatite commonly with an folded and contorted mafic gneiss (dark) orientation to the minerals. Soft – reacts with dilute acid marble Rocks can be folded or Non-foliated with non-oriented Hard – does not react with dilute acid quartzite crenulated. grains Interlocking amphibole / hornblende crystals amphibolite Items in italics are not present or are very rare in Michigan It is important to note that there are many, many intermediate steps or subdivisions of these main divisions provided here. Geology is full of "shades of gray," and the naming of rocks is not an exception. If you have a comment, correction, addition or deletion, please contact, Steven E. Wilson at [email protected]. THANKS! .