Other Gems

FIGURE 36.25 Amethyst crystals from a section of a cathedral The crystals were inially inside the geode. 

FIGURE 36.26 Topaz. Examples of Irradiated tough, natural imperial topaz and a huge natural single crystal. 

Garnet crystals are one of the older popular gemstones and a special class of mineral (much like spinel). The crystal structure is able to accommodate many different cations, which then produce different colors. All those shown in the box (on the next page) are natural silicates, although several synthetic garnets are now available synthetic garnets produced for gemstones are usually doped YAG. 

Natural garnets are rarely pure. Natural alman-dines usually contain Ca, Mg, and Fe , so these denote the end-member garnets. We then use whichever name most closely matches the composition we have. There is then a whole extra array of names. Rhodolite lies 

Pyralspites are named according to the dominant R cation present  

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The extreme instability and explosive behaviour of this diazide caused work on other gem-diazides to be abandoned. 

The impetus for the development of gem-bimetallics was initially to discover alkylidene-transfer reagents akin to Tebbe s reagent [14]. Schwartz prepared bimetallic aluminum—zirconocene derivatives by the hydrometallation of various vinyl metallic compounds [15—17]. Knochel has developed zinc—zirconium gem-bimetallics by hydrozircona-tion of vinylzincs and has used them as alkylidene-transfer reagents [18], More recently, other gem-bimetallics have been developed that exhibit different reactivities of the two carbon—metal bonds. Thus, Normant and Marek have reported the allylmetallation of vinyl metals to afford zinc—magnesium and zinc—lithium gem-bimetallics, which react selectively with various electrophiles such as ClSnBu3, H20, etc. [19, and references cited therein]. However, selective and sequential cleavage of the two carbon—metal bonds... 

Reactions with other gem-diiodoalkanes under these conditions proceed in low yield however, addition of DMF (and ultrasonic irradiation) provides a very general method for alkylidenation of aldehydes with good (E)-selectivity (equation II). 

The cis/trans isomerization of l,l-difluoro-2,3-dimethylcyclopropane (2) has an activation energy about lOkcal mol-1 lower than that of the parent hydrocarbon.5 Unlike the parent hydrocarbon, competing cyclopropane to propene rearrangement is not observed, a generalization that extends to other gem-difluorocyclopropanes studied in the Dolbier laboratories.1... 

The first term, representing acid-"catalyzed" hydrolysis, is important in reactions of carboxylic acid esters but is relatively unimportant in loss of phosphate triesters and is totally absent for the halogenated alkanes and alkenes. Alkaline hydrolysis, the mechanism indicated by the third term in Equation (2), dominates degradation of pentachloroethane and 1,1,2,2-tetrachloroethane, even at pH 7. Carbon tetrachloride, TCA, 2,2-dichloropropane, and other "gem" haloalkanes hydrolyze only by the neutral mechanism (Fells and Molewyn-Hughes, 1958 Molewyn-Hughes, 1953). Monohaloalkanes show alkaline hydrolysis only in basic solutions as concentrated as 0.01-1.0 molar OH- (Mabey and Mill, 1978). In fact, the terms in Equation(2) can be even more complex both elimination and substitution pathways can operate, leading to different products, and a true unimolecular process can result from initial bond breaking in the reactant molecule. 

Spinel is a colorless magnesium aluminate (MgAl204) of cubic structure. It is hard and durable, but, like white sapphire, it is not a good diamond substitute because it has a low refractive index and lacks brilliance. However, it is readily doped to produce other gems of various colors. Artificial ruby, for example, is often natural red spinel, and most synthetic blue sapphires on the market are actually blue spinel. 

Alternatively, we could attempt to re-estimate the enthalpy of formation of 1,1-dipiper-idinobutane, the purported contaminant in l-(l-butenyl)piperidine. However, since this gem-diamine is present in a thermochemically ill-defined amount (4-6%) and the only other gem diamines for which there are chronicled enthalpies of formation are bis(dimethylamino)methane, hexamethylenetetramine (1,3,5,7-tetraazaadamantane) and the fluorinated CFn(NF2)4 (n — 0,1 and 2), this re-estimation hardly seems advisable. Our prejudice is thus to disregard the enthalpy of formation data for l-(l-butenyl)piperidine altogether. 

Unlike other gem materials, it is common to see swirls and bubbles in amber and copal as the material does not form in the same way as mineral crystals. It is occasionally possible, though rare, to see double bubbles of gas and liquid inside these resins. 

In 1663 Boyle observed that diamonds become luminous if rubbed in the dark they become luminous, too, after exposure to light or to cathode rays. When exposed to ultraviolet light some diamonds yield a blue glow. They are more transparent to X-rays than other gems and after prolonged exposure to radium a colourless diamond becomes green. 

The Brazilian aquamarine is found either in primary or secondary gem deposits. In primary deposits, aquamarine is found in gem pockets hosted by granitic pegmatites. In secondary deposits, aquamarine along with other gem varieties of beryl, topaz, and tourmaline is found in the weathered material coming from the parent pegmatite and they can be subdivided into ... 

Heliodor is the golden yellow gem variety of beryl due to the isomorphous substitution of aluminum by ferric cations. It is also known as yellow beryl if the golden tint lacking. Good colored material is relatively rare. As other gem varieties of beryl it is found in granitic pegmatites in Brazil in the Ural Mountains (Russia), and Namibia. 

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