Lecture 10 Notes (Geoscience 331)
Lecture 10 Notes (Geoscience 331) Geoscience 331
Popular in Gems: The Science Behind the Sparkle
Popular in Geology
This 6 page Class Notes was uploaded by Hannah James on Friday November 6, 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 38 views. For similar materials see Gems: The Science Behind the Sparkle in Geology at University of Wisconsin - Madison.
Reviews for Lecture 10 Notes (Geoscience 331)
Don't know yet because I just realized I think I have been paying for this but not getting the full notes...
Report this Material
What is Karma?
Karma is the currency of StudySoup.
You can buy or earn more Karma at anytime and redeem it for class notes, study guides, flashcards, and more!
Date Created: 11/06/15
Oxides: Corundum, Chrysoberyl, Spinel, Rutile, Hematite (Lecture 10) I. Corundum Basic Data: o Chemical Formula: Al2O3 o Mohs' hardness: 9 o Crystal System: Hexagonal o Color: Red (ruby) Blue (sapphire also see text) o Fracture: Irregular o Specific Gravity: High (around 4.0) o Refractive Index: 1.77 o Luster: Adamantine to Vitreous o Interesting Property: 1/400 the hardness of a diamond Corundum has two of the most valuable gemstone varieties, ruby (orangered to purplered) and sapphire (greenish blue to violet blue) o 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 pure corundum is colorless Common corundum is an abundant mineral, being comprised of aluminum and oxygen o 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 o Keep in mind though, that Mohs' hardness scale is not linear Diamond has a hardness of 10, and corundum (the 2nd hardest natural gemstone) has a hardness of 9 But Corundum 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. Ruby (name Ruby comes from the Latin for red) o But it may also be pinkish or brownishred; (absorbs blue, transmits and fluoresces red) o Ruby can have strong pleochroism Red is from Cr3+, Brown arises from Fe3+ o Ruby can fluoresce weakly in LWUV (long wavelength UV light) The chromium ion is the activator Sapphire o The blue color is due to charge transfer involving FeTi Different concentrations of various impurities produce a range of colors from quite pale, due to low concentrations, to quite deep blue o Other colors include: purple and pink, yellow, orange, green, etc. Some varieties of sapphire are fluorescent (weakly under LWUV) Origins of sapphires and rubies o In order to form, corundum requires a high concentration of aluminum o Clay minerals are rich in aluminum metamorphosed clayrich limestones are often the primary source of corundum crystals Also, some igneous rocks that are rich in aluminum, but poor in silica (Si and O) can form corundum: if there is abundant silica, more common feldspars or sillimanite will form instead o Rubies and sapphires are often concentrated in alluvial deposits since they have both a high specific gravity and are quite hard Value enhancement of sapphires o Undesireable Inclusions: Many cut rubies and sapphires contain inclusions Gems that have inclusions which are visible to the naked eye (termed "eye visible") are less desirable than "eyeclean" stones In some cases, inclusions can make the stone more vulnerable to breakage Obviously, the fewer inclusions, the more valuable o Desirable Inclusions: These give rise to asterism (star stones) The "fabric" of these stones is often referred to as silk This is due to the sheen produced by the fine rutile inclusions There are 3 special orientations in which fibrous rutile crystals occur These are parallel to the hexagonal crystal faces (thus at 60 degrees to each other) o The chemical formula of rutile is TiO2, and Ti is one of the elements responsible for color in some sapphires Asterism is strongest when cut "en cabochon" and so most star rubies and sapphires are cut this way The more "centered" a star is within a cabochon, the more valuable it will be The star can change positions depending on where the light hits the stone as one turns the stone, the star will appear to shift along the surface (this can sometimes be quite eerie) Treatments o Heat Treatment of Corundum is done to improve its appearance Many sapphires are heattreated to improve their color In some cases, heating the stone causes changes in the oxidation state of impurities o An especially important example involves reduction of Fe (conversion of Fe3+ to Fe2+) Fe2+ causes color in a variety of sapphires via charge transfer (see lecture 3) o Conversely, stones that are too deep blue may be lightened by oxidation of Fe (conversion of Fe2+ to Fe3+) In some cases, heat treatment will improve the depth of color because heat causes dissolution of inclusions/impurities into the surrounding corundum o Because fine inclusions cause some stones to look cloudy, heat treatment that dissolves the inclusions may also improve the clarity of the stone o Heat treatment may also remove local color concentrations because heat allows the colorcausing impurity (Cr in ruby) to more evenly distribute However, this may require such long times that it is impractical o The conditions for heat treatment vary, depending upon the individual stone How can you tell if a stone has been heat treated? o Detection of heat and diffusion treatment is possible because these treatments modify natural inclusions This may involve rupture of gas/fluid inclusions or partial dissolution of mineral inclusions o For gems that contained needles, the needle margins may become diffuse In any case, it is not easy to see, and can adversely affect the value o Honest disclosure by the gem dealer is sometimes difficult to come by Diffusion treatment of corundum o Color enhancement can be achieved through addition of the colorcausing impurity to the surface of the faceted gemstone o For Ruby, this involves heating the stone to very close to its melting point in the presence of a chromium source (chromium oxide powder: Cr2O3) Chromium enters into the structure of the corundum (diffuses into the corundum) o This is a slow process, so chromium enrichment only occurs in the surface layer This is sufficient to produce a strong color enhancement that is difficult to detect by eye o Diffusion treatment for sapphire is similar to that for corundum To enhance the blue color Fe and Ti oxide powders are placed in contact with the faceted gem and Fe and Ti diffuse into the surface of the stone o Diffusion treatment is done to faceted stones and is probably not obvious by inspection under normal viewing conditions o There are several ways one can determine if a stone has been treated Diffusion treatment will result in concentration of color at facet junctions, and will modify the refractive index of the gemstone Look for a darkening of the blue around the facet edges An excellent, simple, nondestructive method is as follows: Place the faceted gem in methylene iodide Note that color concentrations are apparent at facet junctions (where the gemstone is thin) This color concentration tells us that the deeply colored layer is quite thin Natural verses synthetic corundum o Rubies and sapphire are commonly synthesized by "pulling" a seed crystal out of a crucible of molten Aluminum Oxide It is possible to synthesize both clear crystals and stars How does one tell if the stone is synthetic? o A Natural Origin may be proved by: Inclusions: natural gaseous and fluid bubbles Spectroscopic measurements o SYNTHETIC origin may be indicated by: The presence of flux inclusions and nonnatural gas inclusions Synthetic corundum may contain a visible seed crystal, especially in older gems Wispy white veils within the stone Strain cracks and curved striae on the surface, or visible as lines within the stone II. Chrysoberyl Basic Data: o Chemical Formula: BeAl2O4 o Mohs' hardness: 8.5 o Crystal System: Orthorhombic o Color: Yellow, green and brown o Fracture: Conchoidal o Specific Gravity: 3.653.8 o Refractive Index: 1.75 o Luster: vitreous o Interesting Property: It is the 3rd hardest mineral in the world Varieties: o Cats eye o Alexandrite (Fe produces the yellow color and Cr responsible for the effect) Chrysoberyl is NOT the same as beryl o It is among the world's rarest gems Uncut crystals are commonly found in the shape of "cyclic twins" Chrysoberyl is found only in Be and Cr rich, silicapoor environments (not unlike beryl) It is often formed in pegmatites (but these usually have to be silicapoor since there is no silica in chrysoberyl o It often is found in alluvial deposits in gem gravels Unusual properties: o Color change: chrysoberyl displays the "Alexandrite Effect" where the stone looks green in daylight (rich in blue light) and red in candle light (rich in red light) This effect is most obvious in thick stones, and is due to the substitution of Cr +3 for Al +3 Ruby and spinel also both exhibit this color change on occasion Chatoyant Chrysoberyl, or "Cat's Eye" is due to the presence of inclusions (needleshaped) or tubelike cavities within the stone o These needles are parallel to the base of the cabochon and perpendicular to long axis of the cut gem o This phenomenon is sometimes called "cymophane" (Greek for waving light) III. Spinel Basic Data: o Chemical Formula: MgAl2O4 o Mohs' hardness: 8 o Crystal System: Cubic o Color: varied o Fracture: o Specific Gravity: 3.54.1 o Refractive Index: 1.72 o Luster: vitreous o Interesting Property: Often used as simulants of rubies Spinel is a metamorphic mineral and gem quality spinel is often found in limestones o Because of its hardness, it can be found in gem gravels, very often along with ruby (another source of confusion between the two gems) o It is commonly synthesized IV. Rutile Basic Data: o Chemical Formula: TiO2 o Mohs' hardness: 66.5 o Crystal System: Tetragonal o Color: silver or redbrown (due to Fe impurity) o Fracture: distinct cleavage o Specific Gravity: 4.2 o Refractive Index: 2.61 o Luster: Metallic, can be subadamantine o Interesting Property: Sometimes used as a diamond stimulant Rutile's natural color is usually so dark that it is rarely encountered as a cut gem, but it is important in gemology because of its presence as fine inclusions, often as needle like crystals, in gems o If the needles are very fine, this results in asterism and chatoyancy Rutilated quartz contains many fine rutile needles, often very obvious Rutile can be synthesized and used as a diamond stimulant Because of synthetic rutile's high refractive index and dispersion, it displays exceptional fire o However, this can be too much of a good thing: the dispersion of synthetic rutile is much greater than that of diamond, and is easy to identify as an imposter V. Hematite Basic Data: o Chemical Formula: Fe2O3 o Mohs' hardness: 5.56.5 o Crystal System: Hexagonal o Color: redidsh brown to black o Fracture: irregular o Specific Gravity: 5.26 o Refractive Index: opaque o Luster: metallic (sometimes dull) o Interesting Property: red streak, abundant ore of iron Hematite is not often a gemstone by itself, but polished stones of massive hematite are common Hematite looks steel blue, but when performing a streak test, leaves a red powder o The appearance is due to a change in the way massive crystals reflect light as opposed to smaller particles Hematite is more often an inclusion in other stones o It is an important ore of iron and large deposits of hematite are found in northeastern Minnesota and the Upper Peninsula of Michigan
Are you sure you want to buy this material for
You're already Subscribed!
Looks like you've already subscribed to StudySoup, you won't need to purchase another subscription to get this material. To access this material simply click 'View Full Document'