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1.
An end member of the tourmaline series with a structural formula □(Mg2Al)Al6(BO3)3[Si6O18](OH)4 has been synthesized in the system MgO-Al2O3-B2O3-SiO2-H2O where it represents the only phase with a tourmaline structure. Our experiments provide no evidence for the substitutions Al → Mg + H, Mg → 2H, B + H → Si, and AlAl → MgSi and we were not able to synthesize a phase “Mg-aluminobuergerite” characterized by Mg in the (3a)-site and a strong (OH)-deficiency reported by Rosenberg and Foit (1975). The alkali-free tourmaline has a vacant (3a)-site and is related to dravite by the □ + Al for Na + Mg substitution. It is stable from at least 300°C to about 800°C at low fluid pressures and 100% excess B2O3, and can be synthesized up to a pressure of 20 kbars. At higher temperatures the tourmaline decomposes into grandidierite or a boron-bearing phase possibly related to mullite (“B-mullite”), quartz, and unidentified solid phases, or the tourmaline melts incongruently into corundum + liquid, depending on pressure. In the absence of excess B2O3 tourmaline stability is lowered by about 60°C. Tourmaline may coexist with the other MgO-Al2O3-B2O3-SiO2-H2O phases forsterite, enstatite, chlorite, talc, quartz, grandidierite, corundum, spinel, “B-mullite,” cordierite, and sinhalite depending on the prevailing PTX-conditions.The (3a)-vacant tourmaline has the space group R3m with , , and . However, these values vary at room temperature with the pressure-temperature conditions of synthesis by , , and , probably as a result of MgAl order/disorder relations in the octahedral positions. Despite these variations intensity calculations support the assumed structural formula. Refractive indices are no = 1.631(2), nE = 1.610(2), Δn = 0.021. The infrared spectrum is intermediate between those of dravite and elbaite. The common alkali and calcium deficiencies of natural tourmalines may at least partly be explained by miscibilities towards (3a)-vacant end members. The apparent absence of (3a)-vacant tourmaline in nature is probably due to the lack of fluids that carry boron but no Na or Ca. 相似文献
2.
Enthalpies of solution in 2PbO· B2O3 at 712°C have been measured for glasses in the systems albite anorthite diopside, NaAlO2-SiO2, Ca0.5AlO2-SiO2 and albite-anorthite-quartz. The systems albite-anorthite and diopside-anorthite show substantial negative enthalpies of mixing, albite-diopside shows significant positive heats of mixing. For compositions up to NaAlO2 = 0.42 (which includes the subsystem albite-silica) the system NaAlO2-SiO2 shows essentially zero heats of mixing. A negative ternary excess heat of mixing is found in the plagioclase-rich portion of the albite-anorthite-diopside system. The join Si4O8-CaAl2Si2O8 shows small but significant heats of mixing. In albite-anorthite-quartz. ternary glasses, the ternary excess enthalpy of mixing is positive.Based on available heat capacity data and appropriate consideration of the glass transition, the enthalpy of the crystal-glass transition (vitrification) is a serious underestimate of the enthalpy of the crystal-liquid transition (fusion) especially when the melting point, Tf, is many hundreds of degrees higher than the glass transition temperature, Tg. On the other hand, the same heat capacity data suggest that the enthalpies of mixing in albite-anorthite-diopside liquids are calculated to be quite similar to those in the glasses. The enthalpies of mixing observed in general support the structural models proposed by Taylor and Brown (1979a, b) and others for the structure of aluminosilicate glasses. 相似文献
3.
S.K. Saxena 《Geochimica et cosmochimica acta》1981,45(6):821-825
Using the model of fictive ideal components, Gibbs free energies of formation of pyrope and Al2O3-enstatite have been determined from the experimental data on coexisting garnet and orthopyroxene and orthopyroxene and spinel in the temperature range of 1200–1600 K. The negative free energies in kJ/mol are:
TK | 1200 | 1300 | 1400 | 1500 | 1600 |
Pyrope | 4869.92 | 4747.05 | 4614.26 | 4462.63 | 4311.00 |
Al2O3-enstatite | 1257.25 | 1244.28 | 1191.93 | 1158.67 | 1125.64 |