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Minerals Why Do Geologists Bother?

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Minerals Why Do Geologists Bother? EARS 1 Lab 2 - Minerals Why do geologists bother? • Humans use minerals. Lots of minerals. – Based on current consumption, it is estimated that the average American will use more than a million pounds of rocks, minerals and metals during their lifetime • Minerals make up rocks, rocks are records of earth processes Minerals: What are they? • Naturally occurring • Inorganic • Solid • Definable chemical composition • Crystalline (orderly arrangement of atoms) The building blocks of all Earth materials, and the source of most physical resources used by humans Mineral Identification • Minerals have certain easily detectable properties that are invariable, e.g. quartz in an Indian granite is equally as hard as quartz in a West Virginian sandstone. • These properties include: – Hardness - (Color) – Cleavage - Luster – Crystal habit - “Special Properties” – Streak • Notice that color is in parentheses. Color is a “tricky friend” when it comes to mineral identification. It can be very helpful but also misleading Mineral Properties - Hardness • Hardness – A measure of the strength of the mineral’s chemical bonds – A harder mineral will always scratch a softer mineral – Measured on the 1-10 Mohs scale (log scale) – Can be deceptive; make sure the mineral is leaving a scratch on the surface, not scratching off onto that surface Mineral Bonding • Minerals often have a mixture of bond types. • Covalent bonds generally make minerals harder, and less soluble (less likely to dissolve) • Minerals will “cleave” along planes of weak bond type • Dominant Ionic Bonds: Halite, Calcite • Mixed Ionic/Covalent: Olivine • Mixed Ionic/Covalent/Van Der Waals: Micas • Covalent: Quartz, Diamond Mineral Properties - Cleavage • Cleavage – The pattern of fracture which occurs when a mineral breaks – Determined by heterogeneities in the microscopic arrangement of molecules, “planes of weakness” Cleavage: a tendency to break “clean” along planes 1 Dir a 0° 2 Dir a 90° Cleavage: a tendency to break “clean” along planes 2 Dir a 60°/120° 3 Dir a 90° 3 Dir rhombohedral Cleavage: Conchoidal Fracture no specific planes of weakness e.g., quartz e.g., obsidian Mineral Properties – Crystal Habit • Crystal Habit – The shape in which a crystal will grow in an unencumbered space (rarely the case) – Crystals grow as atoms attach to mineral surfaces, starting from a central seed crystal and expanding outward as atoms accumulate. Cubes Octahedra Blades Hexagonal Prisms Dodecahedra Compound Forms Rhombohedra Te t r a g o n a l P r i s m s Well developed quartz crystals vs. Poorly developed quartz crystals Qtz Crystal Form (Habit) vs. Cleavage Crystal form: Crystal Shape when GROWING in an unconfined space Cleavage: Tendency of crystals to BREAK along planes Mineral Properties – Luster and Streak • Luster – The appearance of a mineral surface in reflected light. – Some minerals have some variance between samples, e.g. quartz. Two subdivisions: – Metallic (Looks like a metal). – Nonmetallic. • Vitreous (glassy) • Greasy • Waxy • Pearly • Earthy (dull). • Streak – Color of powder made when a mineral is scratched against a ceramic streak plate. Metallic luster Vitreous luster Greasy luster Waxy luster Pearly luster Earthy luster Mineral Properties - Color • Color – For some minerals, color is diagnostic. – For many minerals, color varies from sample to sample. – Variance caused by trace elements in the crystal structure Cr subs in for Al Ti/Fe subs in Al Both Corundum Al2O3 Be wary of color… Rhodochrosite- MnCO3 Pink tourmaline “Diagnostic” colors are not always so diagnostic… • Kyanite Useful Special Properties • Calcite fizzes in HCl CaCO3 + 2HCl CaCl2+CO2+ H2O • Magnetite, Fe3O4, is……magnetic. • Specific gravity- minerals with Fe, Mg, and Pb are usually much denser than others. Galena, PbS, is particularly dense. • Plagioclase feldspar has striations on its crystal faces. These striations are called “polysynthetic twinning” Summary • Minerals can be roughly identified using a number of visible properties: – Hardness - (Color) – Cleavage - Luster – Crystal habit - “Special Properties” – Streak • A single mineral property is often not diagnostic – multiple properties are often needed for ID. • A given mineral’s chemical formula and crystal structure are ultimately what determines its macro-scale physical properties..
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