Lecture 8 Notes (Geoscience 331)
Lecture 8 Notes (Geoscience 331) Geoscience 331
Popular in Gems: The Science Behind the Sparkle
Popular in Geology
This 5 page Class Notes was uploaded by Hannah James on Thursday October 22, 2015. The Class Notes belongs to Geoscience 331 at University of Wisconsin - Madison taught by Huifang Xu in Fall 2015. Since its upload, it has received 29 views. For similar materials see Gems: The Science Behind the Sparkle in Geology at University of Wisconsin - Madison.
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Date Created: 10/22/15
Feldsnars Garnets and Tonaz Lecture 8 I Feldspars I Basic data 0 Chemical Formula Potassium Sodium Aluminosilicates Mohs39 hardness 6 Crystal System Monoclinic Orthoclase Triclinic Plagioclase Color see varieties Fracture Irregular good cleavage in two directions Specific Gravity 257 orthoclase Refractive Index 152 orthoclase Luster vitreous 0 Interesting Property Most abundant mineral in the Earth39s crust I Feldspars area framework silicates because of the tetrahedra silicon surrounded by four oxygen ions being linked together in three dimensions to form a complex framework 0000000 I Plagioclase feldspars 0 Rich in Na and Ca sodium calcium and aluminum silicates I Ex Labradorite anorthite I Akali feldspars 0 Sodium potassium aluminum silicates I Ex Moonstone amazonite etc I Where are feldspars formed 0 Feldspars make up 5060 of the volume of crustal rocks and are found in many different rock types I Common in igneous rocks and sometimes form huge crystals indicating that they have cooled slowly far beneath the surface of the earth 0 Feldspars are very important and interesting minerals but less important as a source of gemstones O Relatively susceptible to weathering and react with mild acids to produce clay I Other interesting characteristics 0 Plagioclase feldspars I Commonly twinned twin planes produce parallel striations on mineral39s surface I Gem varieties Labradorite dark grey color iridescent I Play of color is referred to as 39labradorescence39 I Some wavelengths are amplified and some are cancelled I Net result is that different spacingsorientations produce rainbowlike effects I Iridescence is mostly blue but often with rainbowlike appearance 0 Akali feldspars I Gem quality orthoclase yellow stones colored by Fe impurities I Microcline I Amazonite is a light greenblue form of microcline II Garnet Basic data 0 Chemical Formula varies see below Mohs39 hardness 6575 Crystal System Cubic Color Any except blue see below Fracture Conchoidal Specific Gravity 3543 Refractive Index 1 7 1 1 89 Luster vitreous to resinous 0 Interesting Property from Latin Granatum quotseed likequot after pomegranate seeds Garnets are silicate minerals with diverse compositions 0 All garnets have almost identical atomic structures The term garnet applies to fifteen distinct minerals five of which are common gemstones O Garnets can appear nearly every color except blue 0 Garnets do not have cleavage The generalized chemical formula of garnets is X3Y2 Si043 where 0 X indicates a divalent cation such as ironFe2 magnesium Mg2 calcium Ca2 or manganese Mn2 O Y is a trivalent cation such as aluminum Al3 iron Fe3 or chromium Cr3 O The SiO4 indicates silica tetrahedrons Garnets belong to the Cubic crystal system 0 Garnets often grow in a distinctive welldeveloped crystal form known as a dodecahedron 12 sided usually have triangular or rhomboid shaped faces 0 Because the atoms are tightly packed garnets are relatively hard and dense Physical Properties vary with composition they are all different 0 Garnets break down into two main groups Alrich garnets and Carich ones I Most have compositions that involve complex mixtures of cations OOOOOOO Because of their variable composition garnets may have almost any color 0 Al rich garnets PyralspiteCa rich garnets Ugrandite I The ugrandites are rarer than the pyralspites Grossular In its purest form this calcium aluminum silicate is colorless but displays a wide range of color depending on the impurities mainly Fe present 0 Two main color varieties are Tsavorite and Hessonite I Tsavorite a green grossular second only to Demantoid in value I Hessonite is a variety of Grossular greenyellowbrown Andradite 0 This calcium iron silicate is often yellow black or brown and is not commonly used for gems except the green variety Demantoid I Demantoid Crandradite rare vivid green garnet color most valuable I Colored by Cr V thus Ca AlV 0 It has more fire than diamond but it39s much softer 0 Sometimes chatoyant may contain quothorsetailquot inclusions mostly from Urals I Spessartine Orange colored garnets Fe Mn Al 0 Pyrope Brownred quotcape rubyquot 0 Almandine Violetred I Uvarovite Emeraldgreen garnet 0 Rhodolite Purple red 0 Geological locations 0 Garnets can be found in a variety of rocks igneous metamorphic and sedimentary 0 Ironrich almandine the most common garnet is widespread in metamorphic rocks such as schists and gneisses and may occur in granitic igneous rocks 0 The magnesium garnets are formed in high pressure environments and are found in magnesium rich metamorphic rocks formed at great depth I They may be an important mineral in the mantle of the Earth 0 Spessartine is found in manganeserich gneisses and in coarse grained igneous rocks pegmatites O Grossular containing calcium and aluminum is found in clay rich limestones that have been metamorphosed to marble and in contact metamorphic deposits skarns formed when an igneous rock intrudes and reacts with limestone 0 The calciumiron garnet andradite and the rare calciumchromium garnet uvarovite are also usually found in skams I They are also formed by regional metamorphism especially of sedimentary rocks such as limestone 0 Other interesting facts 0 Garnets have been a prized gemstone for over 5000 years I The Romans used them for carvings I They were a particularly popular gemstone in the 19th century 0 Limited availability of natural gem quality garnets keeps them from being commonly used today I Nongem quality almandine garnets are sometimes used as abrasives especially in garnet sandpaper 0 Demantoid is so named from the Old German word Demant which means Diamond I Demantoid has a higher dispersion than diamond and adamantine luster I Gems larger than a few carats are quite rare and it is still a valuable gem today I The other color varieties of andradite garnets are brown yellow and black but these do not often become gemstones III Topaz I Basic data 0 Chemical Formula AlZSiO4FOH2 O Mohs39 hardness 8 Crystal System Orthorhombic Color varies see below Fracture Conchoidal perfect basal cleavage Specific Gravit 3436 Refractive Index 161 Luster vitreous 0 Interesting Property Largest faceted gem weighs over 10 pounds 0 Color reddishyellow quotimperialquot orangebrown quotsherryquot clear pinkish natural blue light and treated blue darker 0 Geologic conditions of formation 0 Topaz crystallizes from uorinebearing vapor in last stages of solidification of igneous rocks 39 Occurs within cavities in lavas and granitic rocks primarily in pegmatites 000000 0 Here large cavities and slow growth can produce huge crystal 0 Secondary concentrations of topaz occur in stream beds and other alluvial deposits I They don39t form in the stream but are concentrated in stream gravels by the moving water 0 Topaz is the most common irradiated gem on the market today 0 This is primarily due to consumer demand for the deep blue shades O Particles emitted by radioactive decay or electromagnetic rays ionizing radiation have enough energy to produce color centers within the gem39s crystal structure 0 It is likely that all dark blue topaz has been irradiated and often subsequently heat treated Most pink topaz has been quotpinkedquot heat treated to remove a yellowbrown color center IV Radiation and Gemstones 0 The use of radiation has only begun relatively 0 Artificial radiation involves bombarding the gem material with a great deal of high energy radiation the type found in linear accelerators nuclear reactors and xray machines 0 The growing public concern over excess radiation as it can damage cell tissue DNA etc prompted the government to step in with regards to irradiated gems 0 Radiation 0 Radiation is just energy emitted either as particles or as electromagnetic radiation photons 0 When we think of quotbadquot radiation we are usually referring to ionizing radiation I This is radiation that has enough energy to ionize 0 These alterations in the molecules of the crystal lattice cause color centers or lattice defects plus a few other minor effects that result in altered color 0 The problem lies not with the creation of the color center but rather the residual radiation 0 Nuclear reactors produce a great deal of neutron radiation any object exposed to this radiation stays quothotquot radioactive for quite a long time 0 Linear accelerators and other radiation sources that produce a higherenergy radiation such as gamma rays do their damage and leave the crystal I Neutrons however are heavier and are sometimes quotcapturedquot by atoms in the material called neutron capture 0 This can create unstable isotopes isotopes that at some later point will release radiation 0 This is one reason why nuclear weapons are so destructive O The explosive force is immediate but the neutron radiation that remains can stay around for thousands of years Examples of types of radiation 0 Alpha particles high speed helium He nucleus with no electrons 0 Beta particles high speed electrons O Gamma rays high energy photons of electromagnetic radiation similar to Xrays but with shorter wavelength and more energetic O Neutrons neutral subatomic particles same mass as proton Treatment is controversial because 0 Swamp out naturally colored material reducing its value 0 Irradiation can not be detected 0 Color may not be stable 0 Safety concerns rarely a problem as the amounts of radiation are relatively low Heat treatment after irradiation 0 Topaz irradiation by gamma rays often produces a greenishbrown color blue yellow and red tinge I Each of the components of the color is due to a color center I The least stable are the yellow and red controlled heating removes these leaving blue color centers Heat Treatment of Topaz O Topaz color can be altered by heating them I Heating at 450C of orangebrown Crbearing topaz produces a pinkpurple termed 39pinking39 O A color center is often an electron trapped in an atomic site a form of structural damage often caused by radiation I Heat allows crystal to heal itself and release electron from trap removing brownish tinge leaving pink