黑铜矿的高温高压稳定性及其地学意义探讨

Study on the stability of tenorite at high pressure and high temperature environments and its geological implications

与其他大多数一氧化物晶体结构不同,黑铜矿(CuO)结构中+2价铜离子与氧形成四方面状配位,这是由它的3d^9价电子构型产生强烈的Jahn-Teller畸变所致。了解地球深部高温高压环境中+2价铜离子的这种晶体化学性质是否改变从而影响其地球化学行为,是探讨地球深部铜在矿物、金属、熔体间分配行为及其微观机制的重要基础。本研究利用金刚石压腔装置和激光双面加热技术模拟地幔温压环境,通过高压原位Raman光谱测试技术,开展了黑铜矿高温高压稳定性的实验研究工作。研究结果表明,黑铜矿直到30GPa压力范围其结构保持稳定,反映直到地球内部800 km深处,黑铜矿结构中+2价铜离子仍受Jahn-Teller效应制约。在30 GPa压力下对样品进行激光加热(温度1500~2000K),黑铜矿经过热处理后,在总体上仍保持稳定(不发生结构相变)的同时,发生部分分解反应,生成了少量的金属铜单质并释放出氧气:2CuO?2Cu+O_2。上述实验结果表明,一方面,+2价铜离子在地球内部直到800 km深度很可能仍保持CuO_4四方面状态配位形式,而主要的硅酸盐矿物和氧化物结构(包括它们的高压相)中都没有这种配位位置,显示+2价铜离子仍不相容于这些矿物中;另一方面,在地球深部较还原的环境中,黑铜矿分解反应产生的氧气被缓冲,+2价铜离子很可能大部分还原成金属铜单质,并在核-幔分离过程中,随金属铁一起进入地核。

Unlike most of the monoxides, tenorite crystallizes in a distinct monoclinic structure, in which the copper atom has a square planar coordination with four oxygen atoms. This kind of coordination of Cu2＋ cation in the structure is attributed to the strong Jahn-Teller distortion characteristic of the d9 valence electronic configuration. Understanding whether the unique crystal chemical properties of the Cu2＋ cation change or not at high pressure and high temperature environments is an interesting topic because it is the structural foundation to explore the copper partitioning behaviors between the various minerals, metals and melts in the deep earth. In this study, the diamond anvil cell apparatus and the laser double-sided heating system were used to model the pressure and temperature environments to probe the stability of tenorite through the in situ high pressure Raman spectroscopic measurements, and therefore, to uncover the crystal chemical behaviors of Cu2＋ cation in the earth＇s mantle. The experimental results indicate that the tenorite is stable at high pressures up to 30 GPa, revealing the Cu2＋ cation isstill constrained by the Jahn-Teller effect in the earth＇s interior at least to a depth of 800 km. At this pressure of about 30GPa, the tenorite sample was heated by the infrared laser beam to around 1500-2000 K. After heating, while the tenorite is still stable totally, it is also decomposed partially according to the chemical reaction： 2CuO 2Cu ＋ 02, which is evidenced by the detection of the released free oxygen and elemental copper. The experimental results may imply the geochemical behaviors of copper in the deep earth. On one hand, the Cu2＋ cation remains as the rare CuO4 square planar coordination in the deep earth at least to the depth of 800 km. However, the structures of the main silicate and oxide minerals, including their high pressure phases, have no this kind of coordinated position. The special requirement for the structural position of Cu2＋ cation makes it difficult to enter the main minerals in the deep earth, and therefore, the main silicate and oxide minerals are incompatible with the Cu2~ cation in the earth＇s mantle. On the other hand, the earth＇s lower mantle has a fairly low oxygen fugacity. In this kind of reduced environment, as the released free oxygen generated by the decomposed reaction of tenorite, which can be considered as the representative of the oxidative copper species, is generally buffered, more elementary copper is formed. The metal copper enters possibly the core with iron in the process of the core-mantle segregation.