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Learn about the physical properties of minerals such as color, streak, luster, hardness, and density. Understand how these properties are important in identifying and distinguishing different minerals. Explore crystal forms, polymorphism, and more.
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Physical Testing II From: http://webmineral.com/data/Rhodochrosite.shtml
1) The Physical Properties of Minerals • Color • Streak • Luster • Hardness • External Crystal Form • Cleavage
The Physical Properties of Minerals (cont.) • Fracture • Specific Gravity • Special Properties • Other Properties • Chemical Tests
Physical properties of minerals • Minerals are composed of atoms, arranged in a specific order, with a well defined chemical composition. • the microscopic variations in bond environment, will also be manifested in macroscopic physical and chemical properties..
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: • glassy, pearly, shiney, 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 carats http://www.nmnh.si.edu/minsci/hope.htm
Important 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 soils
More Zoning From http://www.geo.wvu.edu/~lang/Geol284/Min8IgFels
Zoning in Plagioclase Feldspar Na-rich rim Ca-rich core
Important 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 Asbestos Belongs to the Serpentine mineral family - hydrated ferromagnesian silicate.
Quartz Interfacial Angles Misshapen Crystals Perfectly Proportioned Crystals Steno’s Law (1669): Crystal face internal angles remain constant!
Macroscopic Forms and Microscopic Blocks Cubes Macroscopic Crystal Forms Rhombs
Unit Cells and Crystal Structure Cubic unit cell: smallest repeatable unit
Important Physical Properties • 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) todiamond (hardest).
Polymorphism 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.
Polymorphism 3 mm Natural Octahedral Diamond Graphite & Calcite From: http://www.phy.mtu.edu/~jaszczak/diamond.html
Diamond vs. Graphite Crystal Structures Hardness: 10 Hardness: 1-2 From: http://www.molecules.org/elements.html#diamond
Important Physical Properties VI • Cleavage - Orientation and number of planes of weakness within a mineral. Directly reflects the orientation of weak bonds within the crystal structure. This feature is also highly diagnostic. • Fracture - This describes how a mineral breaks if it is not along well defined planes. In minerals with low symmetry and highly interconnected atomic networks, irregular fracture is common.
Special and Other Properties • Striations - Commonly found on plagioclase feldspar. Straight, parallel lines on one or more of the cleavage planes caused by mineral twinning. • Magnetism - Property of a substance such that it will spontaneous orient itself within a magnetic field. Magnetite (Fe3O4) has this property and it can be used to distinguish it from other non-magnetite iron oxides, such as hematite (Fe2O3). • Double Refraction - Seen in calcite crystals. Light is split or refracted into two components giving rise to two distinct images.
Bragg Relationship • William Lawrence Bragg and William Henry Bragg From: http://www.geology.wisc.edu/~g203/xray.htm
Laue X-ray photograph of Vesuvianite From: http://www.geology.fau.edu/course_info/fall02/gly4200/X-RAY.htm
Family Nobel Prizes • 1915 William Lawrence Bragg (son) and William Henry Bragg (father) • Only father and son to win together • 1903 Marie Curie shared the Nobel Prize in Physics with her husband Pierre Curie • 1911 Marie Curie Nobel Prize in Chemistry. (First woman and only woman to win two noble prizes) • Irène Joliot-Curie, (Marie’s daughter) received the Chemistry Prize in 1935 with her husband Frederic • Frédéric Joliot-Curie Chemistry Prize in 1935
Family Nobel Prizes • Gerty Radnitz Cori and Carl Ferdinand Cori shared the 1947 Prize I • J. J. Thomson the Physics Prize in 1906 His son, George Paget Thomson, received the same prize in 1937 • Niels Bohr the Physics prize in 1922, and his son, Aage Bohr, in 1975 • Manne Siegbahn, Physics Prize in 1924, and his son oKai Siegbahn, Physics Prize in 1981. • Hans von Euler-Chelpin, Chemistry Prize in 1929, was the father of Ulf von Euler, the Physiology Prize in 1970 • C.V. Raman Physics Prize in 1930 was the uncle of Subrahmanyan Chandrasekhar, Physics Prize in 1983
Physical tests and procedures may include: • precise measurement of position, orientation and dimensions: • inclinometers, verniers and laser • thickness using vernier, X-ray and gamma ray • particle size using sieving and laser • dimensional stability involving expansion, contraction and weathering • movement using strain gauge and accelerometer
Law of distance and absorption of gamma rays • To measure the impulse counting rate as a function of the distance between the source and the counter tube.
