Peridots

A variety of minerals are prized for their exquisite beauty, rarity, and exceptional durability. These extraordinary materials are classified as gemstones. One such mineral, silica, with a chemical composition of SK>2 (silicon dioxide), exhibits several crystal structures. Several gemstones are crystalline forms of silica, including amethyst, aquamarine, emerald, garnet, peridot, topaz, tourmaline, and zircon J l... 

By increasing the proportion of chromium or oxide of coppor, and adding oxide of iron to tho mixture, one may vary the green shade, and imitata peridot or dark emerald. 

Peridot is the gem variety of olivine, a magnesium silicate containing iron (about 9 Mg atoms for every Fe atom). Peridot is usually transparent,... 

August peridot (olivine) (Mg.Fe SiO, yellow-green Fe2 Crystal field transitions in Fe" in two distorted six-coordinated sites. 

Fe2+ many green ferromagnesian silicates such as peridot, diopside and actinolite, gillespite,... 

Farrell, E. F. Newnham, R. E. (1965) Crystal-field spectra of chrysoberyl, alexandrite, peridot and sinhalite. Amer. Mineral., 50, 1972-81. 

The materials most commonly used as gems and ornamental stones are listed in Table 2.9. This is by no means a complete listing of all materials ever used in jewelry or for decorative purposes. There are many worked specimens that are one-of-a-kind, made from unexpected materials that were opportunistically obtained. These often pose problems of identification and consequently of conservation, since once a stone has been worked it loses its natural luster and form. Stones have been altered with dyes and heat for thousands of years, so it does not hold true that just because something is in an old artifact or Grandma s necklace that it cannot be dyed or otherwise not natural. Synthetics are relatively new, but imitations are as old as the stones themselves. If someone wanted a red gem, and there were no rubies available, then a garnet or spinel could be used instead. No emeralds Use an olivine (peridot) or green sapphire. A synthetic must have the same composition and internal structure as the natural material, but an imitation just has to look like the natural stone. 

Cobalt Orthosilicate, Co2Si04, is obtained by heating to bright redness a mixture of amorphous silica, and the chloride and oxide of cobalt. Excess of silica is removed by treatment with concentrated soda solution, the residue consisting of the orthosilicate in the form of a violet crystalline powder. The crystals are isomorphous with peridote density 4-63. They are decomposed by hydrochloric acid.3... 

Gaetani G. A. and Grove T. L. (1999) Wetting of mantle olivine by sulfide melt implications for Re/Os ratios in mantle peridot te and late-stage core formation. Earth Planet. Sci. Lett. 169, 147-163. 

Mantle garnets have Fe /J Fe ratios of 0.02-0.15 (Luth et al, 1990). The Fe " " content in garnet correlates with T of equilibration and Fe203 of the bulk rock (Canil and O Neill, 1996) and is also a useful oxygen barometer for garnet peridot tes (Luth et al, 1990 Gudmundsson and Wood, 1995). 

Jamtveit B., Carscoell D. A., and Mearns E.W. (1991) Chronology of the high-pressure metamorphism of Norwegian garnet peridotities/pyroxenes. J. Metamorph. Geol. 9, 125-139. 

Another problem with the transport models is that they assume complete equilibrium between melt and solid throughout the melting regime. This implies that the solid residues at the top of the column (e.g., near the Moho), should be in chemical equilibrium with MORE. However, another key observation of MORBs is that they are out of equilibrium with abyssal peridotities near the Moho for both major and trace elements. A quick fix for the transport models is to assume that melts remain in chemical equilibrium up to some depth and then melt fractionally for the remaining distance (e.g., see Kelemen et al., 1997). However, different attempts to explain both U-series and stable elements in melts and residues in the single-porosity transport models has motivated much of the development of the two-porosity models in next section. 

Brown JM, Slutsky LJ, Nelson KA, Cheng L-T (1989) Single-crystal elastic constants for San Carlos Peridot An apphcation of impulsive stimulated scattering. J Geophys Res B94 9485-9492 Carpenter MA, Salje EKH, Graeme-Barber A (1998) Spontaneous strain as a determinant of thermodynamic properties for phase transitions in minerals. Em J Mineral 10 621-692... 

Rocks and Minerals. Absorption and extinction coefficients for several rocks and minerals- - — were found to increase apprecijably at high temperatures, eg a (peridot) increases from 0.5 cm at 25 °C to 4.3 cm at. 1240 °C. Values.of k. . (or a ) have been recorded by Kingery by Kawada (comparative linear flow... 

Few systematic Sr-Nd isotope studies have been performed on ocean island xenolith suites. Ducea et al. (2002) analyzed clinopyroxenes from plagioclase-spinel and spinel peridot tes from Pali, (Oahu, Hawaii) and found relatively depleted strontium and neodymium isotope systematics that they interpret as representing their evolution as residues from the extraction of Pacific Ocean crust. Consistent with this is a 61 20Ma errorchron defined by the pyroxene separates that is within error of the 80-85 Ma age of Pacific lithosphere beneath Hawaii. 

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