Lecture 7 Notes (Geoscience 331)
Lecture 7 Notes (Geoscience 331) Geoscience 331
Popular in Gems: The Science Behind the Sparkle
Popular in Geology
This 3 page Class Notes was uploaded by Hannah James on Wednesday October 14, 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 87 views. For similar materials see Gems: The Science Behind the Sparkle in Geology at University of Wisconsin - Madison.
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Date Created: 10/14/15
Silicates Part 2 Olivine Pvroxene and Amnhibole Groups Lecture 7 I Olivine Peridot 0 Basic Data 0 Chemical Formula Mg Fe2SiO4 Mohs39 hardness 657 Crystal System Orthorhombic Color Yellowgreen to green Fracturecleavage Conchoidalpoorvery poor Specific Gravity 327437 Refractive Index 169 Luster glassy 0 Interesting Property Major component of the mantle 0000000 I Olivine is a name for a range of chemical compositions and colors however it is often a pale green higher iron content darker color 0 Peridot is the gem form of olivine 0 Origin and occurrence 0 Found mostly in igneous rocks olivine is common in basalts and peridotites peridot rich which contain very little if any quartz I Dunite is a rock made almost entirely of olivine I Peridot is common in Hawaii 0 Olivine has a high melting point and is one of the first minerals to crystallize from molten rock I Mgrich olivine crystallizes first the Ferich ones 0 Olivine can exhibit color zonation tend to have Mgrich cores which are pale colored and Ferich rims which are more brown 11 Pyroxenes 0 Basic Data 0 Chemical Formula LiAlSi206 Mohs39 hardness 657 Crystal System Monoclinic Color variable white gray pink yellow green Fracturecleavage Conchoidalgood Specific Gravity 31532 Refractive Index 167 Luster Vitreous 0 Interesting Property Crystals up to 40 feet in length found in the black hills of SD 0 Pyroxenes are silicate minerals with a wide variety of compositions O The simplest pyroxenes are MgSiO3 I The silica tetrahedrons form long chains which are made of only one row of silica each unlike amphibole 0000000 0 Pyroxenes are not resistant to Earth s physical and chemical processes III IV 0 They react to rain weak acids and even physical abrasion O Pyroxenes combine with carbonic acid to form clay Pyroxenes usually crystallize from an igneous melt but can form in hightemperature metamorphic rocks 0 Pyroxene and olivine both have high melting points and are often found together Spodumene or Kunzite 0 Lithium Li bearing pyroxene Li is the lightest metal 0 Color is light pinkpurple 0 Some crystals are large usually found in pegmatites I Irradiation causes them to go yellow green unstable color Diopside Calcic magnesian monoclinc pyroxene 0 Common mineral less commonly used in jewelry 0 Sometimes displays asterism Augite calcic clinopyroxene 0 Another common pyroxene whose composition is similar to diposide Hiddenite O Chromium Cr rich pyroxene I Strongly pleochroic 0 Good cleavages makes gems difficult to cut Amphiboles Basic Data 0 Chemical Formula Ca2Mg Feg5Si8022OH2 Mohs39 hardness 56 Crystal System Monoclinic Color white to light green Fracturecleavage Conchoidalgood Specific Gravity 3032 Refractive Index 161 Luster vitreous 0 Interesting Property amphibole group includes minerals that form asbestos Amphiboles have similar chemical structure to pyroxenes but are more complex 0 Unlike pyroxenes the silica tetrahedrons are arranged in doublewide chains I This affects the physical properties of amphiboles 0 Not many important gem forms of amphiboles exist but several varieties of asbestos are amphiboles 0 When quartz replaces crocidolite a form of asbestos fibers the fibrous shape results in a highly sought after effect in quartz I Ex Tiger39s eye quartz exhibits chatoyancy silky sheen and is a result of the quartz retaining the fibrous structure of crocidolite OOOOOOO Jade Jade refers to two separate minerals jadeite a pyroxene and nephrite an amphibole 0 Both often occur as a semitranslucent green mass of small interlocking crystals I This forms a very tough aggregate I The tightly interlocking crystals makes is resistant to fracture it was often used as ancient toolsweapons I J adeite 0 Sodium NAAluminum Al silicate often used for carvings O Moh s hardness 657 0 Specific Gravity 34 0 Crystal System Monoclinic I Formed of small interlocking crystals 0 Cleavage at 90 degrees not seen in massive form 0 Color due to Cr 3 deep green I Fe produces a paler color 0 Formed by highpressure metamorphism of Narich rocks I Found in North Burma Guatemala Japan etc 0 Imperial or gem jade is highly valued vivid green variety I Nephrite 0 Amphibole not pyroxene double chain silicate I Unlike pyroxenes amphiboles are hydrous materials contain water 0 Used for carvings in places like China and Central America 0 Composition calcium Ca and Magnesium Mg silicate 0 Green color is due to Fe I Greatly resembles jadeite in color and structure 0 Moh s hardness 665 I Harder than steel used in Neolithic tools 0 Specific gravity 295 0 New Jade serpentine MH 556 SG 25 o Styrian Jade chlorite