Abstract Phase relations in the system KAlSiO4-MgAl2O4 were determined up to 28 GPa and 1500 °C. A hexagonal aluminous (NAL) phase is stable above 16 GPa with a narrow compositional range of (1-x)KAlSiO4·xMgAl2O4, x ≈ 0.61–0.65. The stability field of NAL phase in the CaAl2O4-MgAl2O4 system was also determined up to 29 GPa and 1200 °C, resulting in a compositional range of NAL phase of (1-x)CaAl2O4·xMgAl2O4, x ≈ 0.67–0.74. Compared with the above compositional widths of NAL phases, the stability field of NAL phase in the NaAlSiO4-MgAl2O4 system is much wider, (1-x)NaAlSiO4·xMgAl2O4, x ≈ 0.47–0.70 (Ono et al., 2009). The difference may be caused by the fact that both of Na+ and Mg2+ with similar cation sizes enter 6-fold trigonal prism sites in the NAL structure. In the system KAlSiO4-MgAl2O4, KAlSiO4 kalsilite and MgAl2O4 spinel coexist below ~8 GPa, above which an assemblage of MgAl2O4 spinel, corundum, K2Mg2Si2O7-rich phase X and pyrope becomes stable. The assemblage further changes into NAL phase at 16 GPa. These results and compositional similarity suggest that composite inclusions consisting of MgAl2O4-rich spinel and kalsilite-nepheline solid solution found in diamonds in the Juina-5 kimberlite, Brazil, were formed below ~8 GPa presumably by decomposition of K-rich NAL phase trapped in diamonds in the lower mantle conditions.

中文翻译：

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KAlSiO4-MgAl2O4 和 CaAl2O4-MgAl2O4 系统中的高压和高温相关系：NAL 相的稳定性场

摘要 在高达 28 GPa 和 1500 °C 的条件下，确定了 KAlSiO4-MgAl2O4 体系中的相关系。六方铝 (NAL) 相在 16 GPa 以上时是稳定的，其组成范围很窄，为 (1-x)KAlSiO4·xMgAl2O4，x ≈ 0.61–0.65。CaAl2O4-MgAl2O4 体系中 NAL 相的稳定性场也被确定为高达 29 GPa 和 1200 °C，导致 NAL 相的组成范围为 (1-x)CaAl2O4·xMgAl2O4，x ≈ 0.67–0.74。与上述 NAL 相的组成宽度相比，NaAlSiO4-MgAl2O4 体系中 NAL 相的稳定场要宽得多，(1-x)NaAlSiO4·xMgAl2O4，x ≈ 0.47–0.70 (Ono et al., 2009)。这种差异可能是由于具有相似阳离子尺寸的 Na+ 和 Mg2+ 都进入了 NAL 结构中的 6 重三角棱柱位点。在 KAlSiO4-MgAl2O4 体系中，KAlSiO4 kalsilite 和 MgAl2O4 尖晶石在~8 GPa 以下共存，高于此值时，MgAl2O4 尖晶石、刚玉、富含 K2Mg2Si2O7 的 X 相和辉石的组合变得稳定。该组合在 16 GPa 时进一步变为 NAL 相。这些结果和成分相似性表明，在巴西 Juina-5 金伯利岩的钻石中发现的由富含 MgAl2O4 的尖晶石和钾长石-霞石固溶体组成的复合夹杂物形成于约 8 GPa 以下，这可能是由于富钾 NAL 相的分解而形成的。下地幔条件下的钻石。