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Minerals and Rocks Study Aid Most photos and tables courtesy of Katryn Wiese City College of San Francisco Minerals (A rock is an aggregation of one or more minerals) To be a mineral, it will meet all these requirements: - solid - naturally occurring - inorganic - has a characteristic chemical composition has a definite internal atomic arrangement (crystal structure). Thus it will have distinctive physical properties: -Crystal Habit (Shape) -Cleavage and Fracture -Hardness -Color (often the least useful diagnostic tool) -Luster (how light plays off the surface) -Specific Gravity (Density—how heavy it will feel) -Taste (not recommended since some are toxic) -Reacts or not to Hydrochloric acid -Twinning -Exsolution Lamellae -Magnetic -Double Refraction double refraction Using this diagnostic tools in combination will help you identify mineral hand samples in the field. Sphalerite Bauxite Igneous Rocks Sedimentary Rocks Metamorphic Rocks We use mineralogy and texture in a metamorphic rock to determine the agents that caused the rock to form (the highest temperature and pressure and/or types of fluids to which a rock was exposed). We call the intensity of metamorphism to which a rock was exposed metamorphic grade. GRAPHIC OF BASALT METAMORPHISM BY METAMORPHIC SETTING Textural changes Major textural changes occur as metamorphic grade increases, due to increased pressures and temperatures. If the pressures are uniform in all direction (confining pressure), the results are different than if the pressure is high in only one direction (directed pressure). In the latter case, pressure is released if minerals align themselves perpendicular to the direction of pressure. • Density increases (volume shrinks) – Grains/crystals pack closer together under confining pressure. • Foliation increases – Minerals align when under directed pressure. • Crystal size increases – Grain boundaries migrate, enlarging crystal size as pressure (any kind) placed on crystal boundaries. Other Textural Changes found in metamorphic rocks: • Veins – Fractures filled by minerals that precipitated from hydrothermal fluids. • Porphyroblasts – Unusually large crystals set in a finer-grained groundmass. • Folds, lineations, stretched or sheared grains – Clasts or layers in the original rock are stretched out or folded under directed pressure. • Slickensides – Smoothed, grooved surfaces – formed when two rocks move across one another, like along faults or cracks. Mineral changes Minerals can change and grow in metamorphic rocks, without melting. The chemically active fluids and pressure at crystal grain boundaries can cause the ions in the solid rock to migrate as though they were in a fluid. For this reason, metamorphic minerals tend to show some of the most perfect crystal faces. In addition, as metamorphic grade increases, minerals change to more stable ones and crystals get larger. Polymorph example: These three minerals have the same chemical formula, but are stable at different pressure and temperature conditions. Setting: B=Burial, C=Contact, R=Regional, and S=Subduction. More Stuff • Practice Sheets • Answer Keys • Flash Cards
