Slide 1The Physical Properties of MineralsThe Physical Properties of Minerals (cont.)Physical properties of mineralsImportant Physical Properties IImportant Physical Properties IIHope Diamond: 44.5 caratsImportant Physical Properties IIIDensity and Specific GravityColor and DensityCrustal MineralsMore ZoningZoning in Plagioclase FeldsparImportant Physical Properties IVChrysotile AsbestosCrystal Forms: QuartzFeldsparIntergrown cubic crystals of fluoriteQuartz Interfacial AnglesMacroscopic Forms and Microscopic BlocksUnit Cells and Crystal StructureImportant Physical Properties VMohs’ Hardness ScalePolymorphism and polymorphsPolymorphismDiamond vs. Graphite Crystal StructuresFingernail Hardness (2.5) Scratches Gypsum (2)Important Physical Properties VIPlaner Cleavage in MicaWeak Bonding Yields Planer CleavageAmphibole Cleavage ~120/60°Rhombohedral Cleavage in CalciteConchoidal Fracture in GlassSpecial and Other PropertiesPlagioclase striationsCalcite Double RefractionX-ray diffraction: Laue photographic methodBragg RelationshipLaue X-ray photograph of VesuvianiteVesuvianite: Contact Metamorphic MineralMinerals II: Physical Properties and Crystal FormsFrom: http://webmineral.com/data/Rhodochrosite.shtmlThe Physical Properties of Minerals•Color•Streak•Luster•Hardness•External Crystal Form•CleavageThe Physical Properties of Minerals (cont.)•Fracture•Specific Gravity•Special Properties•Other Properties•Chemical TestsPhysical properties of minerals•Based on the principles discussed during the last lecture and above, we now know that minerals are composed of atoms, arranged in a specific order, with a well defined chemical composition. We might expect then that the microscopic variations in bond environment discussed above, will also be manifested in macroscopic physical and chemical properties. This is indeed the case.Important Physical Properties I•Luster - This property describes the appearance of reflected light from the mineral's surface. Nonmetallic minerals are described using the following terms: vitreous, pearly, silky, resinous, and earthy.Important Physical Properties II•Color - Although an obvious feature, it is often unreliable to use to determine the type of mineral. –Color arises due to electronic transitions, often of trace constituents, in the visible range of the EM spectrum. For example, quartz is found in a variety of colors. •Color of a mineral may be quite diagnostic for the trace element and coordination number of its bonding environment.Hope Diamond: 44.5 caratshttp://www.nmnh.si.edu/minsci/hope.htmImportant Physical Properties III•Streak - The color of a mineral in its powdered form; obtained by rubbing the mineral against an unglazed porcelain plate. –Streak is usually less variable than color.–Useful for distinguishing between minerals with metallic luster.Density and Specific Gravity•Density - Defined as the mass divided by the volume and normally designated by the Greek letter, rho, mass/volume; SI units: kg/m3 or kg m-3, but geologists often use g/cm3 as the unit of choice.•Specific Gravity - Ratio of the mass of a substance to the mass of an equal volume of water. Note that water = 1 g cm-3. S.G. is unitless.•Examples - quartz (SiO2) has a S.G. of 2.65 while galena (PbS) has a S.G. of 7.5 and gold (Au) has a S.G. of 19.3.Color and Density•Two broad categories are ferromagnesian and nonferromagnesian silicates, which simply means iron and magnesian bearing or not. The presence or absence of Fe and Mg strongly affects the external appearance (color) and density of the minerals.•Ferromagnesian silicates - dark color, density range from 3.2 - 3.6 g/cc–Olivine - high T, low silica rocks; comprises over 50% of upper mantle–Pyroxenes - high T, low silica rocks–Amphiboles - esp. hornblende; moderate T, higher silica rocks–Mica - esp. biotite; moderate T, higher silica rocks–Garnet - common metamorphic mineral•Nonferromagnesian silicates - light color, density close to 2.7 g/cc–Mica - exp. muscovite; moderate T, higher silica rocks–Feldspars - plagioclase and orthoclase; most common mineral in crust; form over a wide range of temperatures and melt compositions–Quartz - low T, high silica rocks; extremely stable at surface, hence it tends to be a major component in sedimentary rocks.–Clay - esp. kaolinite; different types found in different soilsCrustal MineralsFrom http://www.geo.wvu.edu/~lang/Geol284/Min8IgFelsMore ZoningZoning in Plagioclase FeldsparCa-rich coreNa-rich rimImportant Physical Properties IV•Crystal form or habit - The external morphology of crystals generally reflect the internal arrangement of their constituent atoms. This can be obscured, however, if the mineral crystallized in an environment that did not allow it to grow without significant interaction with other crystals (even of the same mineral).Chrysotile AsbestosBelongs to the Serpentine mineral family - hydrated ferromagnesian silicate.Crystal Forms: QuartzFeldsparIntergrown cubic crystals of fluoriteQuartz Interfacial AnglesPerfectlyProportionedCrystalsMisshapenCrystalsSteno’s Law (1669): Crystal face internal angles remain constant!Macroscopic Forms and Microscopic BlocksCubesRhombsMacroscopicCrystal FormsUnit Cells and Crystal StructureCubic unit cell:smallest repeatable unitImportant Physical Properties V•Hardness - This is the resistance of the mineral to abrasion or scratching. This property doesn't vary greatly from sample to sample of the same mineral, and thus is highly diagnostic. It also is a direct reflection of the bonding type and internal atomic arrangement. A value is obtained by comparing the mineral to a standard scale devised by Moh, which is comprised of 10 minerals ranging in hardness from talc (softest) to diamond (hardest).Mohs’ Hardness ScalePolymorphism and polymorphs•Substances having the same chemical composition but different crystal structures.–e.g. diamond and graphite•Both minerals are composed of pure carbon, but diamond is the high pressure polymorph of graphite.•This gives rise to extremely different physical properties.PolymorphismGraphite & CalciteNatural Octahedral Diamond3 mmFrom: http://www.phy.mtu.edu/~jaszczak/diamond.htmlDiamond vs. Graphite Crystal StructuresFrom: http://www.molecules.org/elements.html#diamondHardness: 10 Hardness: 1-2Fingernail Hardness (2.5) Scratches Gypsum (2)Important