How Do We Identify Minerals? Earth Science Ms. Kurtzweil Spring 2010

Identification by Careful Inspection…

Color ► First and most easily observed mineral property. ► Some minerals have very characteristic colors that help identify them..  Cinnabar  Malachite  Sulfur  Azurite

BUT… Color is also the least useful property for mineral identification ► Many different minerals have similar colors  Orthoclase feldspar, calcite, others… milky white ► Traces of impurities can change a mineral’s color  Traces of impurities can change a mineral’s color… Ex: Quartz / Corundum ► + Fe… amethyst / +Cr… ruby ► + Ti… rose quartz / + Co, Ti… sapphire

► A mineral may change color when exposed to air ► Weathered surfaces may hide true color…  So always inspect freshly-exposed surfaces!

Luster ► Luster: how light is reflected from the surface of a mineral ► Are either metallic or nonmetallic ► Metallic- shines like polished metal  Galena  Pyrite

► Nonmetallic lusters: ► Glassy (vitreous)  Quartz ► Pearly  Mica, talc ► Waxy  Sulfur, sphalerite ► Brilliant (adamantine)  Diamond ► Earthy  Gypsum ► Others (greasy, oily, dull)

Crystal Shape ► When minerals have enough time and space to form, their atoms or ions always arrange themselves into patterns ► Form 1 of 6 basic crystal shapes  Flat-faced  Regularly shaped  RARE to find

Identification by Simple Tests…

Streak ► Streak: the color of a mineral in powdered form ► Although the color of a mineral may vary, the streak rarely does. Much more reliable! ► Examples:  Iron pyrite (brass-yellow) streak: greenish-black  Hematite (brown,red,silver) streak: reddish brown

► To test streak:  Rub specimen across an unglazed ceramic tile, called a streak plate (white or black) ► Streak rule of thumb  Metallics: streak will be at least as dark as hand specimen  Nonmetallics: streak is usually colorless or white

Cleavage ► Cleavage: a mineral’s tendency to break evenly or separate along a flat surface. ► Ability is related to the type of bond in atomic structure- will separate at weak bonds ► Can be observed in tiny grains; therefore very useful

► Micas (muscovite, biotite)  1 perfect plane of cleavage  Parallel sheets ► Calcite, galena  3 planes of cleavage  Right angles

► Fracture: when minerals with no cleavage planes break unevenly into curved or irregular pieces  Uneven/irregular ► Ex. Cinnabar, some quartz  Splintery/fibrous ► asbestos  Conchoidal- like broken glass ► Obsidian

Density ► Depends on the kind of atoms present (more massive?) and how closely packed they are ► Heft  Galena “feels” heavier than quartz ► Density is more specific: d=m/v ► Most common minerals in Earth’s crust range from ► Best for identifying heavier metals (Au, Pb, U, Ag) which range from

Hardness ► Hardness: the ability of a mineral to resist scratching ► Mohs’ Scale of Hardness  Used as a practical test for determining a mineral’s hardness  1 (softest) Talc  10 (hardest) Diamond

Mohs Knows Best ► Don’t confuse a streak with a scratch!  Rub away streak to feel and look for the scratch ► Ask:  Which is the hardest mineral on the scale that it can scratch?  Which numbers does it fall between?  If neither scratch each other, they have the same hardness ► Graphite and diamond are both pure C  Graphite: 1, Diamond: 10  Difference is in their bonds!

Special Properties…

Magnetism ► Mineral can be picked up by a magnet, or will pick up small metal objects ► Examples:  Magnetite  Lodestone

Fluorescence/Phosphorescence ► Fluorescence: ability to glow under ultraviolet (UV) light  Some fluorite, calcite samples ► Phosphorescence: ability to glow after UV light is cut off  Some willemite, sphalerite samples

Double Refraction ► Refraction: the bending of light rays as they pass through a translucent substance ► Double refraction: the splitting of light rays into two parts, causing a double image ► Calcite

Effervescence ► A mineral is said to display effervescence if it fizzes in a chemical reaction with cold, weak HCl ► Limestone and Marble  Calcite (CaCO 3 ) is the principle mineral  Bubbles released are carbon dioxide

Radioactivity ► Certain atoms have unstable electron and/or nuclei arrangements ► An unstable nucleus becomes stable over time by releasing particles and energy (radiation) ► Most common elements: Uranium and Radon ► Minerals: carnotite and uraninite found in U