geo10 - :. Madison .: 2/5/09 6:18 PM Lecture 10: Oxides;...

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2/5/09 6:18 PM :. Madison .: Page 1 of 13 Lecture 10: Oxides; Corundum, Chrysoberyl, Spinel, Rutile, Hematite I. Corundum Ruby crystal. Courtesy the F. John Barlow mineral collection, MH photo. 1. Basic Data: Chemical Formula: Al2O3 Mohs' hardness 9 Crystal System Hexagonal Color Red (ruby) Blue (sapphire - also see text) Fracture Irregular Specific Gravity High (around 4.0) Refractive Index 1.77 Luster: Adamantine to Vitreous Interesting Property: 1/400 the hardness of a diamond
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2/5/09 6:18 PM :. Madison .: Page 2 of 13 Bracelet with cut rubies and diamonds. Note: some texts list the crystal system of ruby as trigonal (or rhombohedral). Trigonal (rhombohedral) is more simply considered a subdivision of the hexagonal crystal system for this course. Corundum has two of the most valuable gemstone varieties, ruby (orange-red to purple-red) and sapphire (greenish blue to violet blue). The name sapphire refers to the blue color variety, but the other colors are generally termed "fancy" sapphires and are described by that color. Pink sapphire and golden sapphire are common examples. These are all varieties of the same mineral - trace impurities give rise to all the various colors as pur corundum is colorless. Common corundum is a fairly abundant mineral, being comprised of aluminum and oxygen. What's also interesting is that aluminum and oxygen are fairly light elements, yet the SG of corundum is fairly high (4.00). This has to do with the close packing arrangement of the atoms - aluminum and oxygen are closely packed and strongly bonded together - which also accounts for its high hardness. Keep in mind though, that Mohs' hardness scale is not linear. Diamond has a hardness of 10, but while corundum (the 2nd hardest natural gemstone) has a hardness of 9, it is only 1/400th as hard (granted, that's still really hard). Silicon carbide (SiC) is a synthetic material that has a hardness of about 9.5. 1. Ruby: The name Ruby comes from the Latin for red, but it may also be pinkish or brownish- red; (absorbs blue, transmits and fluoresces red). Ruby can have strong pleochroism. Red is from Cr+3, Brown arises from Fe+3. Ruby can fluoresce weakly in LWUV, the chromium ion is the activator.
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2/5/09 6:18 PM :. Madison .: Page 3 of 13 Large (3+ carat) rubies. 2. Sapphire: Color range for sapphires collected from gem gravels in the Missouri River, Montana. Stones range in size from 0.1 to 2.6 carats. The blue color is due to charge transfer involving Fe-Ti. Different concentrations of
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geo10 - :. Madison .: 2/5/09 6:18 PM Lecture 10: Oxides;...

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