Garnet almandine

Manning, P. G. (1967b) The optical absorption spectra of the garnets almandine-pyrope, pyrope, and spessartine, and some structural interpretations of mineralogical significance. Canad. Mineral., 9,237-51. 

Red. The red stones include ruby and both pyrope and almandite garnet (almandine). 

Chakraborty S. and Ganguly J. (1992). Cation diffusion in aluminosilicate garnets Experimental determination in spessartine-almandine diffusion couples, evaluation of effective binary diffusion coefficients, and applications. Contrib. Mineral Petrol, 111 74-86. 

Cressey G. (1978). Exsolution in almandine-pyrope-grossular garnet. Nature, 271 533-534. 

Cressey G, Schmid R., and Wood B. I (1978). Thermodynamic properties of almandine-grossular garnet solid solutions. Contrib. Mineral Petrol, 67 397-404. 

Geiger C. A., Newton R. C. and Kleppa O. I (1987). Enthalpy of mixing of synthetic almandine-grossular and almandine-pyrope garnets from high-temperature solution calorimetry. Geochim. Cosmochim. Acta, 51 1755-1763. 

Isaak D. G. and Graham E. K. (1976). The elastic properties of an almandine-spessartite garnet and elasticity in the garnet solid solutions series. J. Geophys. Res, 81 2483-2489. 

Sato Y, Akaogi M., and Akimoto S. (1978). Hydrostatic compression of the synthetic garnets pyrope and almandine. J. Geophys. Res., 83 335-338. 

Velbel, M.A. (1984) Natural weathering mechanisms of almandine garnet. Geology 12 631-634... 

Example 4.1. Suppose olivine and garnet are in contact and olivine is on the left-hand side (x<0). Ignore the anisotropic diffusion effect in olivine. Suppose Fe-Mg interdiffusion between the two minerals may be treated as one dimensional. Assume olivine is a binary solid solution between fayalite and forsterite, and garnet is a binary solid solution between almandine and pyrope. Hence, Cpe + CMg= 1 for both phases, where C is mole fraction. Let initial Fe/(Fe- -Mg) = 0.12 in olivine and 0.2 in garnet. Let Xq = (Fe/Mg)gt/ (Fe/Mg)oi = 3, >Fe-Mg,oi = 10 ° mm+s, and Dpe-Mg,gt =... 

Ganguly J., Cheng W., and Chakraborty S. (1998a) Cation diffusion in aluminosilicate garnets experimental determination in pyrope-almandine diffusion couples. Contrib. Mineral. Petrol. 131, 171-180. 

Direct observation of immiscibility in pyrope-almandine-grossular garnet. 

Fig. 37. Cubic system. (See also Figs. 16-22.) a. Unit cell type, b and c. Two habits of pyrites, FeSs. Class m3, d. Tetrahedrite, Cu3SbSa. Class 43m. e. Spinel, MgAls04. Class m3m. /. Almandine (Garnet), Fe8Al2(Si04)3. Class mZm.

Figure 5. The CaO vs MgO plot for garnets from various jewels shows 4 provenances pyropes from Bohemia, rhodolites from Ceylon and two sources of almandine in India...

The trace elements content and slight differences in major composition permit to split these three groups in five different sources two sources of pyrope garnets (with and without chromium) and two sources for almandine garnets (distinctive calcium, magnesium and yttrium contents). 

Comparison with the composition of natural garnets published in the literature suggests that almandine garnets observed on archaeological jewels have been mined from India while rhodolite garnets may have been imported from Ceylon (Sri Lanka). The sources of pyrope garnets likely correspond to the Bohemian deposits (Czech republic). 

The second provenance criterion is based on the identification of inclusions in gemstones. Micro-Raman spectrometry was used for this task in almandine garnets. Various inclusions were observed like apatite, zircon, monazite, calcite, and quartz and two of them, curved needles of sillimanite (Al2Si05) and 10-pm metamict radioactive crystals, were specifically found in archaeological garnets. Fig. 6 shows the Raman spectra of a sillimanite needle, which is a mineral formed under a high temperature and high pressure metamorphism. 

Some cubic crystals with faces indexed by Miller indices, (a) Cubic unit cell and labels of axes and angles (b) and (c) two habits of pyrites FeS2, class m3 (d) tetrahedrite Cu3SbS3, class 43m (e) spinel MgAl204, class m3m and (f) almandine (garnet) Fe3Al2(Si04)3, class m3m. 

Figure 5.6 Crystal field spectrum of almandine. The garnet with 70 mole per cent Fe3Al2Si3012 is from Fort Wrangell, Alaska.

Figure 5.7 3d orbital energy level diagram for Fe2+ ions in the eight-fold coordinated site in garnet. Observed transitions in the spectrum of almandine (fig. 5.6) are indicated. 

As noted in 5.4.1 and illustrated in fig. 5.5, Fe2+ ions located in the eight-coordinated distorted cube site in almandine garnets produce CF bands located near 4,400 cm-1,5,800 cm-1 and 7,600 cm-1. However, only the latter two bands have been investigated at high pressures due to experimental difficulties below 5,000 cm-1 (Shankland et al., 1974 Bell and Mao, 1969 Balchan and Drickamer,... 

The spectra of different garnets measured at 400 °C (Parkin and Bums, 1980) show temperature-induced red-shifts in almandine of all three Fe2+ CF bands to 4,150 cm-1, 5,850 cm-1 and 7,620 cm-1. Increased temperature also induces... 

Newman, D. J., Price, D. C. Runciman, W. A. (1978) Superposition model analysis of the near-infrared spectrum of Fe2+ in pyrope-almandine garnets. Amer. Mineral., 63, 1278-81. 

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