geo331 lect7 - Lecture 7 Silicates part 2 Olivine Pyroxene...

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Lecture 7, Silicates part 2: Olivine, Pyroxene and Amphibole Groups Before we go on, I should make a comment about the order in which we are reviewing gemstones. Minerals are often grouped together by their dominant anion group: silicates contain the silicate anion: SiO4 (-2 charge), oxides contain oxygen (without silicon), carbonates have the CO3 (-2 charge) anion and so on. Lecture 7 covers olivine (a very simple silicate) and the Pyroxene Amphibole groups (some of the more complex silicates). The remaining lectures will also cover gemstones that are grouped together primarily by chemical structure (and also by time/space constraints). While we will cover most of the "important" (read: popular) gemstones, there are many more that we don't have the time or space to include. I. Olivine (Peridot) 1. Basic Data: Chemical Formula: (Mg, Fe)2SiO4 Mohs' hardness 6.5-7 Crystal System Orthorhombic Color Yellow-green to green Fracture Conchoidal Specific Gravity 3.27-4.37 Refractive Index 1.69 Luster: glassy Interesting Property: Major component of the mantle Olivine is a name for a range of chemical compositions and colors from: Mg2SiO4 ==> (Mg, Fe)2SiO4 ==> Fe2SiO4 pale green ==> brown green ==> black Most often, however, it is a pale green. As you should be able to tell from the diagram above, the higher the iron content, the darker the color. Peridot The gem form of Olivine.
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Cut peridot - note the variation in green hues 2. Origin and Occurrence: Olivine is found throughout the world, most commonly associated with igneous rocks. Olivine is common in Basalts and peridotites (peridot-rich) which contain no quartz. "Dunite" is a rock made almost entirely of olivine. Peridot is common in the basalt lavas of Hawaii. The black sand beaches contain tiny rounded grains of olivine. Olivine-rich nodules may occur in some basalts. These nodules   were small pieces of mantle rocks carried to surface by
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geo331 lect7 - Lecture 7 Silicates part 2 Olivine Pyroxene...

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